Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of

Review Cite This: Chem. Rev. 2018, 118, 8983−9057

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Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures Dario Pasini*,† and Daisuke Takeuchi*,‡ †

Department of Chemistry and INSTM Research Unit, University of Pavia, Viale Taramelli, 10-27100 Pavia, Italy Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori, 036-8561, Japan

Chem. Rev. 2018.118:8983-9057. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/02/18. For personal use only.

‡

ABSTRACT: Monomers possessing two functionalities suitable for polymerization are often designed and utilized in syntheses directed to the formation of cross-linked macromolecules. In this review, we give an account of recent developments related to the use of such monomers in cyclopolymerization processes, in order to form linear, soluble macromolecules. These processes can be activated by means of radical, ionic, or transition-metal mediated chain-growth polymerization mechanisms, to achieve cyclic moieties of variable ring size which are embedded within the polymer backbone, driving and tuning peculiar physical properties of the resulting macromolecules. The two functionalities are covalently linked by a “tether”, which can be appropriately designed in order to “imprint” elements of chemical information into the polymer backbone during the synthesis and, in some cases, be removed by postpolymerization reactions. The two functionalities can possess identical or even very different reactivities toward the polymerization mechanism involved; in the latter case, consequences and outcomes related to the sequence-controlled, precision synthesis of macromolecules have been demonstrated. Recent advances in new initiating systems and polymerization catalysts enabled the precision syntheses of polymers with regulated cyclic structures by highly regio- and/or stereoselective cyclopolymerization. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures, which can hardly be obtained by conventional polymerization methods.

CONTENTS 1. Introduction 2. Radical Cyclopolymerization 2.1. 5- and 6-Membered-Type Systems 2.2. Acrylate Based-Systems Providing Larger Ring Formation 2.3. Larger Systems Obtained Using Difunctional Styrene-Like Monomers 3. Ionic Cyclopolymerization 3.1. Cyclopolymerization of Vinyl Ethers and Phthalaldehydes 3.2. Cyclopolymerization of Diepoxides, Diepisulfides, and Triepoxides 4. Transition-Metal Catalyzed Cyclopolymerization of Dienes and Trienes 4.1. Cyclopolymerization of Dienes Using Metallocene Catalysts 4.2. Cyclopolymerization of Dienes Using Ti, Zr, Hf, and Sc Catalysts 4.3. Cyclopolymerization of Dienes Using Fe, Co, Pd, and Ni Catalysts 4.4. Pd-Catalyzed Polymerization of Dienes and Trienes with Concurrent Isomerization and/ or Double Cyclization 5. Transition-Metal Catalyzed Cyclocopolymerization of Dienes © 2018 American Chemical Society

5.1. Cyclocopolymerization of Dienes Using Ti, Zr, Hf, and Sc Catalysts 5.2. Cyclocopolymerization of Butadiene with Ethylene 5.3. Cyclocopolymerization of Dienes with Olefins by Co, Pd, and Ni Catalysts 5.4. Cyclocopolymerization of Dienes with CO 6. Cyclopolymerization of Diynes, Eneynes, and Bis(cycloolefin)s 6.1. Cyclopolymerization of Diynes Using Mo and W Complex Catalysts 6.2. Cyclopolymerization of Diynes Using Mo- or W-Based Three- or Four-Component Catalysts 6.3. Cyclopolymerization of Diynes Using Ru Catalysts 6.3.1. Cyclopolymerization of 1,6-Heptadiynes Using Grubbs Catalysts 6.3.2. Cyclopolymerization of 1,7-Octadiynes and 1,8-Nonadiynes 6.3.3. Synthesis of Block and Random Copolymers

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9008 Received: May 4, 2018 Published: August 27, 2018

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DOI: 10.1021/acs.chemrev.8b00286 Chem. Rev. 2018, 118, 8983−9057

Chemical Reviews 6.3.4. Cyclopolymerization of Diynes Using Modified Hoveyda−Grubbs Catalysts 6.4. Cyclopolymerization of Diynes Using Rh Catalysts 6.5. Bergman Cyclopolymerization of Diynes 6.6. Metathesis Polymerizations Associated with Ring Opening and Ring Closing of Cycloalkenes 6.6.1. Cyclopolymerization of Biscycloalkenes via Ring-Opening and Ring-Closing Metathesis 6.6.2. Cyclopolymerization of Cycloalkenylalkynes via Ring-Opening and Ring-Closing Metathesis 7. Transition Metal-Catalyzed Cyclopolymerization of Monomers Other than Dienes, Diynes, and Enynes 7.1. Cyclopolymerization of Bisallenes 7.2. Cyclopolymerization of Bis(diazocarbonyl) Compounds 7.3. Cyclopolymerization of Diisocyanides 7.4. Cyclopolymerization of Bisthiophenes 8. Kinetic and Theoretical Studies on Cyclopolymerizations 9. Conclusions and Outlook Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments Abbreviations References

Review

The archetypal architectures for linear polymers could be roughly structured into three broad categories: random, block, and alternating. Anionic, cationic, and controlled free radical polymerizations4−7 have become very successful in the synthesis of well-defined block copolymers with low polydispersity, controlled molecular weight, and chemical properties, allowing access to well-defined materials. Random copolymers can also be obtained with the above-mentioned techniques. Controlled polymerization methods have emphasized the possibility of sequence regulation in chain-growth polymerization processes, when in combination with time-controlled monomer additions, the copolymer composition can be tuned in terms of sequence.8,9 Cyclopolymerizations have aroused great interest in the past in conjunction with the design of innovative macromolecular architectures. In most of the early systems, the discovery of efficient cyclopolymerization processes has been serendipitous: difunctional monomers, designed, in fact, for cross-linking purposes, turned out to be able to afford linear, soluble polymers.10−12 Cyclopolymerization processes provide the unique ability to perform two things at the same time: make polymers efficiently and generate cyclic structures in the polymer backbone. As such, they offer the control of polymer rigidity, packing and solubility. Figure 1 (top) describes the typical scheme for a cyclopolymerization process: two functionalities, A, reactive toward chain-growth polymerizations (e.g., carbon-carbon double bonds in free radical polymerizations) are joined through a suitable covalent bond network (a “tether” T in Figure 1). The tether can be an essential part of the final polymers, or, in some instances, it can be removed at a later stage. In this latter case, the tether “imprints” information onto the polymer chain (e.g., stereochemical) and is removed once its “template” role has been achieved. The combination of the peculiar polymerization process and of the possibility of introducing and directing stereochemical information (in the template, in the catalyst, in the initiator and/or in the polymerization media) makes cyclopolymerizations particularly suitable for imparting stereoregularity in the final polymer backbones. It is possible to obtain alternation in condensation polymerizations using monomers with suitable, complementary functionalities at both ends (e.g., A-A vs B-B monomers, with A ends reacting with B ends). In chain-growth polymerization techniques, instead, a general strategy for alternation in stepgrowth polymerizations is still very much in demand. Regularly alternating structures are possible only in very specific cases, in which the organic monomers, as a consequence of their peculiar structure and electronic properties, possess a negligible rate of reaction for their free-radical induced homopolymerization, but they possess a high reactivity toward other selected functional monomers. For example, maleic anhydride or maleimides do not homopolymerize but easily form alternating copolymers with reactive addition monomers (styrene and acrylates) via free radical polymerization. The peculiar alternating intraintermolecular chain mechanism of cyclopolymerization has the ability to warrant, when using properly designed monomers, a third thing (Figure 1, bottom): together with the above-described efficient cyclization and propagation, alternation between two functionalities with very different reactivities toward the propagating chain can occur. Butler’s first cyclopolymeric system,13 poly(diallyldimethylammonium chloride) and its copolymers with acrylamide, were commercial products within 10 years of their discovery, and they

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1. INTRODUCTION Organic polymers have revolutionized everyday life in the last century: they are used so much industrially because they offer physical properties (processability in thin films, thermoelastic behavior, etc.) which are not achievable in discrete organic low molecular weight compounds. A very small number of commodity polymers still nowadays constitute the vast majority of product sold yearly worldwide; more than 80% of synthetic polymer production involves only six major commercial materials.1 Their structural properties are not particularly refined; this is in sharp contrast to the wide structural complexity of natural polymers, such as proteins, polysaccharides, or polynucleotides, which, as the result of millions of years of evolution, display several levels of complexity, at their primary, secondary, and tertiary structures, and such complexity is essential for their function. Modern polymer science is a very much “alive and kicking” scientific discipline. The transition from commodity polymers to specialty polymers and, more recently, precision polymers, will likely characterize polymer science in the next few decades.2 As polymer chains naturally belong to the nanometer scale length and nanodevices are designed with increasing sophistication, specialty polymers have been exploited in recent years in various directions. The tools available for the precise construction of macromolecules have gained increasing levels of sophistication, as the merging and synergies with advanced organic chemistry have become more and more stringent.3 8984


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are still marketed throughout the world, mainly for water treatment and personal care formulation. The most recent review dealing comprehensively with the topic “cyclopolymerization” is a book and dates back to 1996.10−12 Several other reviews have covered sectorial aspects of this topic up to 2004.14−20 Since then, the field has grown steadily, with more than 900 papers published in the last 15 years (Source: Scifinder, with keyword “cyclopolymerization” or “cyclocopolymerization” in the title). The quality of the papers published has also grown steadily. In this review, we will address fundamental and most exciting progresses made in the field of cyclopolymerization in the last 15 years. This field is somehow intermixed or can be at first sight confused with the field of main chain cyclic topologies in macromolecular architectures, certainly of great and current interest; the two topics would have been too ample to be treated together, and we refer the readers to recent reviews on the latter subject.21,22 The review is divided according to the chain-growth polymerization mechanism used for the cyclopolymer synthesis.

Mecerreyes and co-workers have reported the polymerization of diallyldimethylammonium ionic liquids (monomer 1 in Figure 2 having a [(CF3SO2)2N]−, BF4−, PF6−, or SCN− counteranion).27 The free-radical polymerization (in H2O) followed a similar ring-closing cyclopolymerization mechanism as the one observed before in diallyldimethylammonium halides. Copolymerization using different ionic liquid monomers was also realized, leading to a new family of random copolymers bearing the same poly(diallyldimethylammonium) backbone and a mixture of counter-anions. In a subsequent paper, novel cyclopolymers containing different cyano-functionalized counter-anions were realized.28 Their films, blended with different amounts of free ionic liquids, show increased CO2 permeability and also CO2/N2 permselectivity, with the C(CN)3−anion as the best performing. The archetypal skeleton of polymer P1 has been modified by means of the introduction of side chains for several purposes. Photochromic spiroxazine functionalities have been introduced by postopolymerization functionalization on a copolymer containing a small quantity of diallylamine comonomer, in order to introduce photoswitching capabilities into the watersoluble polymer backbone. The resulting polymer was found to be photochromic in solution and in thin solid films.29 Dealing with device-based solid-state photopolymerization, Hall and coworkers described a new family of nonacrylate UV curable threedimensional polymeric networks for coatings and adhesives based on the photoinitiated cyclopolymerization of diallylamine salts and diallylamides.30 A series of 1,6-heptadienes, bearing a nucleic acid base as the substituent in the 4 position, either a carbon or a quaternary nitrogen atom (in this latter case making the system analogous to monomer 1), have been studied in terms of stereoselective cyclization and their tendency to cyclocopolymerize with SO2. Although thorough 13C NMR studies have been conducted, no GPC characterization could be performed, due to the scarce solubility of the obtained macromolecules.31 In a similar attempt, the incorporation of adenine functionalities as side chains in monomers similar to 1 have also been carried out, although with poor results related to their cyclopolymerization.32 Ali et al. have reported negatively charged substituents on the ammonium center, thereby generating zwitterionic monomers (such as 2−4) and their corresponding cyclopolymers as poybetaines (P2−P4). 33−37 They are poly(electrolytezwitterion)s with structural characteristics common to both conventional anionic polyelectrolytes and polyzwitterions. In particular, polymer P4 has a phosphonate, a multivalent anionic portion, so that the resulting polymer is pH responsive, and contains the residue of the osteoporosis drug alendronic acid, making this macromolecular system interesting for applications in nanomedicine. Monomer 4 could also be copolymerized with SO2 to afford a pH-responsive copolymer with “antipolyelectrolyte” behavior.38 In order to investigate the possible participation of propargyl group in Butler’s cyclopolymerization process, monomers containing allyl and propargyl units were prepared and investigated. With two propargyl units no cyclopolymerization or cyclocopolymerization with sulfur dioxide could be observed. With one allyl and one propargyl, groups could be cyclopolymerized with sulfur dioxide to give linear and water-soluble macromolecules, with a five-membered cyclic structure embedded in the polymer chain.39 Amide-based systems, in a slight variation of the main design reported in

2. RADICAL CYCLOPOLYMERIZATION 2.1. 5- and 6-Membered-Type Systems

Free-radical cyclopolymerizations were the first to be discovered, and early work has been well reviewed by Butler, Kodaira, Mathias, and others, covering advances in the field up to the late 1990s.12,13,15 To highlight the serendipity often involved in the discovery of the early systems, we report a few paragraphs by Mathias:12 “during vacuum distillation of the crude monomer reaction mixture (of monomer 5c in Figure 3) in an effort to increase yield, the student increased the temperature until, at approximately 125°C, spontaneous polymerization took place to give a hard rock mass in the distillation pot. The student assumed this was a highly crosslinked and intractable material, suitable only for disposal. At my suggestion (professors do have some uses), the student attempted to soften this mass by adding CHCl3 so we could save the flask. To our surprise, the material completely dissolved.” In the past decade, theoretical contributions have elucidated fundamental aspects of the polymerization process for poly(diallyldimethylammonium chloride) P1 (Figure 2). Allyl monomers are in fact generally considered as poor monomers for radical polymerization, since chain transfer reactions take place readily by abstraction of allylic hydrogens of monomer by the propagating radical. Aviyente and co-workers23,24 have given a precise picture of the process by comparing computationally the reactivities of N-diallylamine and N,N-dimethyl-N,Ndiallylammonium (monomer 1 in Figure 2). In the former, propagation and cyclization have almost the same activation barriers. In the latter, cyclization is much more facile than propagation, leading to higher cyclopolymerization efficiency and to the highly regioselective formation of five-membered ring structures in P1. In recent work, Aggarwal and co-workers have successfully demonstrated reversible addition-fragmentation chain transfer (RAFT) controlled free radical methods to the polymerization of 1. They also demonstrated that RAFT using trithiocarbonate agent in aqueous solution at 60 °C is possible under microwave irradiation, leading to relatively high molecular weight cyclopolymer P1 (up to Mn = 30.000) with narrow polydispersities (approaching 1.05). Chain extension with the same monomer, demonstrating the living character of the process, was also successfully carried out.25,26 8985


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Figure 1. Schematic representation of a cyclopolymerization process involving a difunctional monomer with identical reactivities (top) and a cyclopolymerization process with a difunctional monomer with differing reactivities (bottom). On the right: otucomes and advantages obtained using the cyclopolymerization technique.

Figure 2, have also been reported to cyclopolymerize, to afford five- or six-membered rings repeating units.40−42 Difunctional acrylic-type systems have also been realized. Acrylates are much more reactive than allyl systems toward free radical polymerization; much of the early work focused on the elucidation of the mechanistic and thermodynamic parameters characterizing their cyclopolymerization and demonstrated that the propagation mechanism form exclusively six membered rings in the polymer backbone. Moreover, sterically demanding moieties placed either as the tether X or as the ester substituents R1 (Figure 3) favor cyclization, propagation, and efficient

Figure 3. Top: mechanism of cyclopolymerization starting from the radical initiator (I). Bottom: difunctional acrylate-type cyclopolymerization.

hydrogen bonding. Using them as comonomers with 5a in a low feed ratio (10 mol%), cyclocopolymers could be obtained; 5d and 5e in any case demonstrated promise as reactive diluents or cross-linkers in dental applications.43 In recent years, activities to systems schematized in Figure 3 have been focused on their controlled free radical polymerization, on the formation of block copolymers, and on the use of such cyclopolymeric backbone to orient and direct complex superstructures. The ATRP controlled free radical cyclopolymerization of 5c was successfully demonstrated by Acar et al.44 with high cyclization efficiencies in xylene using CuBr/ PMDETA at 70 °C. Polydispersities as low as 1.05, for degrees of polymerization of approximately 50, could be demonstrated. The livingness of the propagating cyclopolymers was shown through successful block copolymerization with t-butyl acrylate where the former was used as macroinitiator. RAFT techniques have also been applied with success to the same monomer 5c.45 The physical properties of the cyclopolymers were investigated, and it was found that ring microstructures and polymer polydispersities affected the glass transition temperatures of the polymers obtained. The cyclopolymerization of difunctional, acrylic type monomers, such as 6b in Figure 3, and their cocyclopolymerization with maleic anhydride, have been used by Fréchet et al. in the design and synthesis of robust, carbon-rich polymeric

Figure 2. Top: mechanism of cyclopolymerization starting from the radical initiator (I). Bottom: Butler’s original cyclopolymeric skeleton (P1) and its more recent structural modifications (P2−P4).

cyclopolymerization.12 In 5c and 6a, the matching between thermodynamically favored ring closure and proper steric hindrance at the reactive positions afforded fully cyclized structures even by bulk polymerization, e.g., without any dilution in solvents, to obtain fully characterized, six membered ring containing polymeric structures P5c and P6a. Small changes in the functionalities involved can bring about rather large effects in the cyclopolymerization efficiency. Avci and coauthors have reported two novel bis(methacrylamide)s (5d and 5e) containing phosphonate groups; the corresponding cyclopolymer, however, could not be obtained, due to the high cross-linking tendency of these monomers which may be due to a lower ‘‘effective bulkiness’’ of the substituents and/or 8986


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backbones for use as photoresists for 193 nm lithography.46,47 Polymers for 193 nm lithography require good resistance to the etching process involved in the microlithographic process, and this property had been demonstrated to be associated to a high carbon to hydrogen ratio in the molecular composition (that is, a high number of unsaturations, typical of aromatic polymers, largely used with success in 248 nm lithography). Cyclopolymerizations were in the case of such materials efficient in a dual way: generating rings (e.g., unsaturations) during the polymer process and producing suitable polymeric materials for use as chemically amplified resists. Further enrichment of the carbon to hydrogen ratio has been introduced with the use of tricyclic adamantyl moieties as the malonate esters (monomer 6b) while preserving t-butyl acrylate esters with the dual purpose of promoting efficient cyclization and to insert the needed (photogenerated acid)-labile functionalities needed for imaging. Pasini et al. have implemented the cyclopolymerization of diacrylate-type monomers decorated with elements of supramolecular recognition, in the form of malonate crown ethers (Figure 4). In the design of polymers P7 and P8, cyclo-

polymerization was used as a tool in order to force neighboring crown ethers to adopt preferred “stacked” conformations, orthogonal to the polymer chain, so that cooperativity effects could be developed between neighboring crown ethers for recognition and transport of alkaline cations.48,49 Indeed, in the transport of alkaline cations across a liquid membrane, P7 showed selectivity for K+, which shifted the selectivity observed for the corresponding monomer (centered on the smaller cation Na+). The same group published more recently more complex systems, such as cyclopolymer P9, in which “push−pull” chromophores were attached to the rigid cyclopolymeric backbone by means of a bis-malonate crown ether fragment.50 The embedded chromophores maintained the ability to bind supramolecularly Eu3+ cations, through coordination to the 1,3dicabonyl units, demonstrating promise as polymeric scaffolds for such supramolecular chromophores.51−54 Malonate-based monomers incorporating two functionalities possessing different reactivities toward free radical polymerization have been also the subject of in-depth investigation. In early studies, both Mathias55 and Yamada56 reported simple systems able to undergo efficient cyclopolymerization and alternation of the differing reacting groups on the monomer. Mathias was able, with the aid of cyclization experiments on model compounds and sophisticated 13C NMR structural elucidation, to demonstrate that monomer 10 cyclopolymerize efficiently to afford exclusively five-membered repeating units (P10 in Figure 5), according to the mechanism highlighted in Figure 5, top. Yamada studied in detail monomers 11 and 12, which were found to yield polymers P11 and P12 readily via cyclopolymerization. The methallyl group in the α-substituent facilitates more significantly the cyclopolymerization than the allyl group, although they were not able to substantiate which one of the two forms (either five or six membered rings in Figure 5) was predominant in cyclopolymers P11 and P12. In a recent example, Mathias cyclopolymerized in both bulk and ethyl acetate a nonsymmetric divinyl monomer, obtained from the reaction of ethyl a-hydroxymethylacrylate with maleic anhy-

Figure 4. Cyclopolymers with embedded crown ethers obtained from difunctional acrylate-type monomers.

Figure 5. Top: mechanism of cyclopolymerization starting from the radical initiator (I). Bottom: difunctional mixed acrylate/olefin-type cyclopolymerization. 8987


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Figure 6. Large ring formation through cyclopolymerization of suitably designed bisacrylic monomers.

reaction onto the neighboring allyl group to form five membered ring systems. The degree of cyclization was between 70 and 80% determined by 19F NMR, and a bimodal molecular weight distribution having 105 as the higher molecular weight further corroborated a not optimal cyclopolymerization outcome.61

dride, to afford, similarly to P10, exclusively five-membered ring structures in the repeating unit.57 In back-to-back reports,58,59 Fréchet and co-workers expanded on their initial studies45 by implementing carbon and cage-rich, cyclopolymerizable monomers 13 and 14, for use as photoresists in 193 nm lithography. With the aid of cyclization studies on model compounds and refined NMR spectroscopic techniques, they were able to rationalize that, whereas allyl systems cyclopolymerize to from exclusively five-membered repeating structures (P13), the methallyl systems (14) cyclopolymerize to give both the five- and the six-membered structures (Figure 5). In the case of P10−P14 a perfect alternation within the polymer main chain of acrylic-like and olefin-like double bonds, possessing very different reactivities toward free radical polymerization, is observed. The authors were able to demonstrate that monomers 13 and 14 (and others, structurally related) were able to polymerize efficiently with alternation with maleic anhydride, exactly like all (not covalently tethered) acrylate or olefin systems do. Other, peculiar cyclopolymeric systems afforded repeating ring structures even smaller than five or six-membered rings. For example, Niu and Fréchet published regioregular 2-alkoxycarbonylnortricyclene polymers by controlled cyclopolymerization of norbornadiene-derived monomers; the cyclopolymerization process afforded in this case a cyclopropane (three-membered ring) moiety in the repeating unit, to produce a carbon-rich, robust polymeric system to be used as photoresist for 193 nm photolithography.60 An unusual system was recently proposed wherein the cyclopolymerization between an allyl group and a fluorinated vinyl group was performed. In perfluoroisopropenyl vinylacetate [CF2C(CF3)OCOCH2CHCH2] the addition of hydrocarbon radical to perfluoroisopropenyl group, which shows scarce homopolymerization reactivity, to produce a fluorinated carbon radical, is followed by the intramolecular addition

2.2. Acrylate Based-Systems Providing Larger Ring Formation

The compact rings described in the previous examples provide rigidity to the resulting polymers; larger rings can recognize atoms or molecules inside. Most of more recent contributions in the field of radical cyclopolymerization dealt with the ability of generating larger rings, in order to develop their host−guest properties, perhaps as a consequence of the more and more stringent cross-fertilization between materials and supramolecular chemistry. Jana and Sherrington have published systems in which the “tether” anchoring two reactive methacrylate fragments is based on an optically active metal complex, possessing suitable free binding sites for chelating the carboxylate units of the methacrylates. Several methacrylate metal complexes have been prepared and tested in free radical copolymerization reactions, primarily with electron rich styrene-type monomers. In selected Zn(II) complexes, cyclopolymerization under the stereochemical control of the (−)-sparteine-derived ligands followed by reaction with an electron rich styrene-type molecule yielded asymmetric triads along the polymer backbone after removal of the metal complex template.62,63 Kakuchi and co-workers have recently reported the enantiomer-selective cyclopolymerization of monomer 15 (Figure 6) using ATRP in combination with a chiral amine ligand.64 The leading author has been very active in the field of cyclopolymerization in the late 1990s to the early 2000s, especially for synthesizing optically active polymers using divinyl monomers having a chiral template,65,66 as recently reviewed.17 The ATRP of a racemic mixture of (RR)- and (SS)-15 was carried out using compound 16/CuBr/chiral amine ligands as 8988


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Figure 7. Using supramolecular recognition for preorganization and efficient generation of crown-ether containing cyclopolymers. Adapted with permission from ref 72. Copyright [2013] Springer Nature.

the intiating systems. In P15, as demonstrated by 1H and 13C NMR studies, the extent of the cyclization was ca. 100%, generating cyclic repeating units consisting of a 10-membered ring structure. One enantiomer was predominantly polymerized, and the enantiomeric excess of the recovered monomers increased with increasing monomer conversion. The diacrylate monomer 16, incorporating a 1,2,3-triazole was reported by Liu et al. to cyclopolymerize very efficiently by free radical initiation, affording polymer P16 by the formation of 11-membered rings in the repeating units, as shown by detailed NMR analysis. High cyclization efficiency and unimodal GPC traces were further indicative of fidelity of the process.67 The capability of generating even larger rings in the repeating units has been recently further extended. Hu et al.68 reported the cyclopolymerization using ATRP of dialkylsilylene-tethered bismethacrylates 17, generating fourteen-membered cyclic units. The polymerizations were carried out in very diluted conditions (250 μM in DMSO) using standard ATRP conditions. In such conditions the precursor 2-hydroxyethylmethacrylate did not polymerize, whereas monomer 17a reacted slowly. Instead, monomers 17b−d could be efficiently polymerized with DP up to 60 and D̵ down to 1.2. The presence of bulky substituents on silicon facilitates the cyclopolymerization process, by virtue of the Thorpe-Ingold effect, to favor the syn conformation so that the cyclization process is facilitated. Endo et al. have reported the synthesis of two different, yet very efficient, cyclopolymerization protocols on amide systems. In the first example,69 the monomer was built around α pinene, optically active moiety, yielding a chiral cyclopolymer with an 11-membered ring repeating unit. In the second,70 the cyclopolymerization of 18a proceeded via a 19-membered ring formation, either via free-radical or controlled RAFT polymerizations. The ring-closing efficiency is imputed to the two directing groups: the cyclohexane ring and the urethane groups acting through steric directioning and hydrogen bonds cooperatively. In s more recent contribution, some of the authors demonstrated the viability of such an approach even by using the less reactive bisacrylate 18b.71 Sawamoto et al. have recently reported a cation templateassisted cyclopolymerization of poly(ethylene glycol) dimethacrylates as an efficient strategy directly yielding polymeric pseudocrown ethers with large in-chain cavities (up to 30membered rings) for selective molecular recognition.72 Their system is a perfect example of how the cross-fertilization between the fields of supramolecular chemistry and polymer science can be utilized to make precision polymers. The key in

their design is to select a size-fit metal cation for the spacer unit of the divinyl monomer, to form a pseudocyclic conformation where the two vinyl groups are suitably positioned for intramolecular cyclization. The authors examined a series of monomers, differing about the length of the ethylene glycol chains. Figure 7 shows the template-assisted cyclopolymerization of monomer 19, in which the most efficient alkaline cation bound is potassium. The polymerization is efficiently carried out with monomer 18 in the presence of K+ (in a 1:1 ratio) with a Ru catalytic system, with an alkyl chloride compound as the initiator; the process has a controlled/living character, yielding water soluble cyclopolymer P19 with narrow polydispersities ( Cp2ZrCl2 > Ph2C(Cp)(Flu)ZrCl2. The cyclization selectivity becomes lower in the cyclopolymerization at lower temperature or increased concentration of the monomer. Cp*2ZrMe2 can promote cyclopolymerization of 1,6-heptadienes having OSiMe3 group on the 4 position (45a) to give selectively the corresponding polymer with 6-membered rings. rac-Et(Ind-H4)2ZrMe2 is more easily poisoned by the siloxy group and inactive for the cyclopolymerization of the dienes having OSiMe3 group (45a) but can polymerize the monomer with more sterically hindered siloxy group (OSitBuMe2) (45b).144

1,5-hexadiene in the presence of methylaluminoxane (MAO) to give the polymer rich in trans- and cis-cyclopentylene groups, respectively. Cyclization efficiency is over 99%. The active species of the polymerization is cationic alkylzirconocenes. In the case of Cp2ZrCl2 or Cp2ZrMe2, the intermediate with transchair conformation is energetically favored, which leads to transfused cyclopentylene groups (Figure 24 (i)).134 In the case of Cp*2ZrCl2, in contrast, the intermediate with trans-chair conformation becomes disfavored with respect to that with cistwist conformation, and the unit with cis-fused cyclopentylene group forms predominantly (Figure 24 (ii)). The molecular weight of the polymer is much higher than that of poly(1-hexene) obtained under identical conditions. This is due to the slow termination of the growing terminal unit in the cyclopolymerization, because the β-hydrogen elimination of the growing terminal unit leads to the strained and thermodynamically unstable exocyclic double bond. Thus, chain transfer to MAO, rather than β-hydrogen elimination, is the major chain transfer reaction.135 Zirconocene catalyst with ferrocenyl groups ((FcSiMe2Cp)2ZrCl2) promotes selective cyclopolymerization of 1,5-hexadiene to give a polymer with high content of the trans-unit (up to 98% trans selectivity).136 The chiral C2 symmetric zirconocene catalyst (R,R)-Et(IndH4)2Zr(BINOL)/MAO catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Figure 25).137−139 The resulting polymer is composed of isotactic trans-fused cyclopentylene units (trans = 68%), and it is optically active.

Figure 25. Enantioselective cyclopolymerization of 1,5-hexadiene by the optically active metallocene catalyst ((R,R)-Et(IndH4)2Zr(BINOL)/MAO).137−139

The cyclopolymerization of 1,5-hexadiene by various stereospecific metallocene catalysts in the presence of Al(i-Bu)3 and [Ph3C][B(C6F5)4] as cocatalysts was examined at various temperatures.140 Both the isospecific C2 symmetric catalyst rac-Et(Ind)2Zr(NMe2)2 and the syndiospecific Cs symmetric catalyst Me2C(Cp)(Flu)ZrMe2 afford trans-diisotactic rich polymer (trans = 55−68%), whereas the aspecific C2v symmetric catalyst Cp*2ZrMe2 produces atactic polymer rich in cis structure (cis = 57−84%). The polymer obtained by the catalyst

4.2. Cyclopolymerization of Dienes Using Ti, Zr, Hf, and Sc Catalysts

In addition to metallocene catalysts, metal complexes with one or no cyclopentadienyl ligands (postmetallocene catalysts) have been utilized as catalysts for the cyclopolymerization of nonconjugated dienes.145 8998


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selectivity (trans = 82% when replacing N-Et of I into Ncyclohexyl). Successive polymerizations of 1-hexene and 1,5hexadiene enable synthesis of diblock and triblock copolymers containing a poly(methylene-1,3-cyclopentylene) block and isotactic poly(1-hexene) blocks. Microphase separation of the triblock copolymer into a cylindrical morphology with hard cylinders of poly(methylene-1,3-cyclopentylene) block surrounded by more elastic poly(1-hexene) block was confirmed by AFM imaging. I/[PhNHMe2][B(C6F5)4] system also promotes living cyclopolymerization of 1,6-heptadiene.148 The produced polymer has six-membered ring repeating units and its stereochemistry and tacticity is controlled resulting in cis-diisotactic structure (Figure 27). Hafnium amidinate complex (II) is also effective for the cyclopolymerization but the produced polymers possess a low degree of stereoregularity, because of its Cs symmetric structure. When the catalyst with less than 1 equiv. of [PhNHMe2][B(C6F5)4] with respect I is used, the polymerization induces epimerization of the chiral complex, and a stereoblock copolymer is formed (Figure 28). The length of the block in the stereoblock copolymer can be controlled by tuning the amount of [PhNHMe2][B(C6F5)4] with respect to I, which also enables fine-tuning of thermal properties of the polymer. 1,6Heptadiene-1-hexene block copolymer with stereoregular isotactic structure can be synthesized by using I/[PhNHMe2][B(C6F5)4], and microphase separation of the block copolymer was again observed by AFM imaging. II/[PhNHMe2][B(C6F5)4] enables the synthesis of 1,6heptadiene-propylene-1,6-heptadiene triblock copolymers with different lengths of the blocks.149 The produced triblock copolymers have hard stereoirregular poly(methylene-1,3cyclohexylene) block and soft atactic polypropylene block. They undergo microphase separation as confirmed by AFM and TEM. The triblock copolymers with 9 and 23 mol % of methylene-1,3-cyclohexylene unit show spherical and cylindrical morphologies, respectively. The triblock copolymers behave as thermoplastic elastomer, where the copolymer with 9 mol % of methylene-1,3-cyclohexylene unit shows superior elastic recovery (94 ± 1%). I/[PhNHMe2][B(C6F5)4] is also effective for the cyclopolymerization of diallyldimethylsilane (46a).150 Similarly to the cyclopolymerization of 1,6-HPD, the polymer having a sixmembered ring with a cis-diisotactic structure is obtained, and the stereoblock copolymer can be synthesized by lowering the amount of borate with respect to the zirconium. Both the cisdiisotactic polymer and the stereoblock copolymer show high glass transition temperatures (Tg > 120 °C), and the latter is amorphous and transparent. Polymerization of 1,5-hexadiene by half titanocene complex with aryloxide ligand III in the presence of MAO afford an insoluble gel (Figure 29).151,152 NMR analysis of the soluble polymer obtained by the polymerization at low concentration of 1,5-hexadiene indicates the presence of 25 mol % of the butenyl side chain in addition to atactic methylene-trans-1,3-cyclopentylene repeating unit. The insertion of another diene monomer takes place preferentially compared to the cyclization. Polymerization of 1,7-octadiene by III/MAO under high concentration of the monomer affords soluble polymer containing uncyclized units as the major repeating unit (74 − 92 mol %) in addition to the minor units with 7-membered ring.153 Optically active Zr complexes with [ONNO]-type salen ligands having (Δ,R,R) stereochemistry promote the cyclo-

Figure 26. Cyclopolymerization of 1,6-heptadienes138,144 and 1,7octadienes138,143 by metallocene catalysts.

Charts 1 and 2 show early transition metal complex catalysts and nonconjugated dienes recently utilized in cyclopolymerization. Table 1 lists representative results of the cyclopolymerization of nonconjugated dienes by the early transition metal catalysts. The constrained-geometry catalyst Me2Si(C5Me4)(N-tBu)TiCl2/MAO is effective for the cyclopolymerization of 1,5hexadiene to give the polymer with methylene-1,3-cyclopentylene repeating units.146 Although the amount of uncyclized units is less than 5% in the cyclopolymerization with the monomer concentration lower than 0.84 mol/L, 22% of the uncyclized unit is included in the produced polymer at higher concentration of the monomer (2.10 mol/L). cis/trans selectivity of the cyclopentylene groups is not controlled (almost equal amount). Zirconium amidinate complexes (I) promote the cyclopolymerization of 1,5-hexadiene in the presence of a borate coinitiator ([PhNHMe2][B(C6F5)4]).147 The cyclopolymerization proceeds in living fashion to give the polymer rich in trans structure (trans = 64−82%) (Figure 27). NMR analysis also suggests the high selectivity of the synthesis, which is a consequence of the C2 symmetric structure of the catalyst. Increasing the steric bulk leads to a polymer with increased trans 8999


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Chart 1. Early Transition Metal Complexes Used As Catalysts for Cyclopolymerizations

polymer is rich in trans-diisotactic structure (Figure 27). The polymer obtained by IVa contains noncyclized vinyl pendant, and the insoluble nature of the polymer is due to the crosslinking of the vinyl group. However, the polymerization at higher temperature (55 °C) under dilution condition in toluene afforded soluble polymers.155 The produced polymer is optically active ([α]D = −24°) as a result of the high isospecificity of the catalyst. Optically active Zr complexes with [OSSO]-type ligand having (Δ,R,R) or (Λ,S,S) stereochemistry (V) are also effective for the enantioselective cyclopolymerization of 1,5-hexadiene in the presence of dried MAO.156 Their activity is very high (up to 1960 g mmol Zr−1 h−1). The product contained a CHCl3insoluble fraction, owing to the cross-linking of pendant olefin fragments. NMR analysis of the CHCl3-soluble fraction indicates that the cyclization ratio is >99%, and predominantly trans-diisotactic structure of the polymer (86−79% trans, isoselectivity (α) = 75−78%). The produced polymers are optically active ([α]D = +28 to +32° from (Λ,S,S)-V and −26 to −34° from (Δ,R,R)-V). The Hf complex with a tridentate phenoxyamine ligand (VI) brings about cyclopolymerization of 1,5-hexadiene in the

Chart 2. Nonconjugated Dienes Used As Monomers in the Cyclopolymerization Catalyzed by Early Transition Metal Complexes

polymerization of 1,5-hexadiene in the presence of B(C6F5)3.154 The complexes with electron-withdrawing phenolate groups such as IVa show higher activity, whereas those with bulky substituents such as IVb show lower reactivity. Although most of the produced polymers are highly insoluble, a polymer obtained by IVa was more soluble in organic solvents and allowed characterization by 13C NMR. It revealed that the produced 9000


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Table 1. Cyclopolymerization of Dienes by Early Transition Metal Catalysts Mn

PDI

Tg, Tm /°C

1,5-HD

I (Zr)

1,3-MCP

64% trans

20 000

1.09

98 (Tm)

1,6-HPD

I (Zr)

1,3-MCH

cis-diisotactic

8200

1.08

1,6-HPD

II (Hf)

1,3-MCH

cis

16 800

1.02

92.2 (Tg) 209 (Tm) 72.2 (Tg)

46a

I (Zr)

1,3-MCH

cis-diisotactic

14 800

1.52

1,5-HD

IVa (Zr) (Δ,R,R)

1,3-MCP

trans-diisotactic (75% trans)

1,5-HD

IVb (Zr) (Δ,R,R)

1,3-MCP

1,5-HD

V (Zr) (Λ,S,S)

1,3-MCP

1,5-HD

VI (Hf)

1,3-MCP

1,6-HPD

VI (Hf)

1,3-MCH

1,5-HD

VII (Hf)

1,3-MCP

1,7-OD

VII (Hf)

1,3-MCHP

47

VII (Hf)

1,3-MCP

46a

VII (Hf)

1,3-MCH

1,6-HPD

VIII (Ti)

1,5-HD

VIII (Ti)

1,6-HPD

IX (Sc)

1,3-MCH + 1,2-ECP 1,3-MCP + VTM 1,3-MCH + 1,2-ECP

trans-diisotactic (66% trans) trans-diisotactic (79% trans) cis-diisotactic (74% cis) cis-diisotactic (97% cis) cis-diisotactic (67.8% cis) cis-diisotactic (100% cis) cis-diisotactic (93% cis) cis-diisotactic (100% cis) 80% cis (CH)

1,6-HPD

X (Sc)

1,3-MCH + 1,2-ECP

1,6-HPD

XIa (Sc)

1,3-MCH + 1,2-ECP

1,6-HPD

XIb (Sc)

1,3-MCH + 1,2-ECP

1,6-HPD

XIc (Sc)

1,3-MCH + 1,2-ECP

1,5-HD

IX (Sc)

1,3-MCP + VTM

monomer

catalyst

ring

microstructure

86% cis (CH) 66% trans (CP) 88% cis (CH) 61% trans (CP) 81% cis (CH) 53% cis (CP) 85% cis (CH) 64% trans (CP) 90% cis (CH) 72% trans (CP) 86% trans (CH)

123 (Tg) 264 (Tm) ca. 5 (Tg) ca. 100 (Tm)

remarks

ref

living polymerization block copolymer with 1-hexene living polymerization block copolymer with 1-hexene stereoblock copolymer

147

living polymerization block copolymer with propylene living polymerization block copolymer with 1-hexene stereoblock copolymer

149

148

150

α (enantioselectivity factor) = 0.75

154

α ≥ 0.99, optically active ([α]D = −24°)

155

11 500

1.7

91.7 (Tm)

α = 0.78, optically active ([α]D = +26°)

156

211 000

1.66

α = 0.95

157

87 000

1.38

α > 0.97

157

8300

2.96

α = 0.987

158

13 300

2.79

16.1 (Tg) 119.7 (Tm) 103.9 (Tg) 179.0 (Tm) −6 (Tg) 117 (Tm) 51 (Tg)

α > 0.99, cyclization ratio = 84.9%

158

509 600

1.87

160 000

1.85

268 000

1.27

4000

1.99

3000

99.8 (Tg) α = 0.928 223.7 (Tm) 126.0 (Tg) α > 0.99 262.8 (Tm) 1,3-MCH: 1,2-ECP = ca. 1:1

159 159 160

1,3-MXΠ: VTM = 63:37

161

52 (Tg)

1,3-MCH: 1,2-ECP = 91:9

165

6.21

57 (Tg)

1,3-MCH: 1,2-ECP = 91:9

165

27 000

1.69

70 (Tg)

1,3-MCH: 1,2-ECP = 90:10

165

22 000

1.73

83 (Tg)

1,3-MCH: 1,2-ECP = 85:15

165

15 000

2.47

67 (Tg)

1,3-MCH: 1,2-ECP = 80:20

165

16 000

2.57

−7 (Tg)

1,3-MXΠ: VTM = 83:14

165

presence of B(C6F5)3 to afford a polymer rich in cis-diisotactic structure (cis = 70−74%, α = 0.93−0.96; Figure 27).157 The catalyst also promotes the highly isotactic cis-selective cyclopolymerization of 1,6-heptadiene (cis-diisotactic = 97%). The tridentate pyridylamide Hf complex VII in combination with [Ph3C][B(C6F5)4] promotes the cyclopolymerization of 1,5hexadiene and 1,7-octadiene (i-Bu3Al is used as scavenger).158 The cyclopolymerization of 1,5-hexadiene affords the polymer with the cis structure (cis up to 69.5%) with high isotactic selectivity (α up to 98.7%; Figure 27). The cyclopolymerization of 1,7-octadiene by the catalyst produces highly cis-diisotactic polymer (cis = quant., α > 99%), although a small amount of the repeating units with an uncyclized structure is also present. The cis-selective cyclization in the cyclopolymerization of 1,5hexadiene was also supported by much smaller activation energy in cyclization via the intermediate with cis-twist boat conformation (3.53 kcal/mol) compared to that via the intermediate with trans-chair conformation (7.09 kJ/mol) on

DFT calculation. VI/[Ph3C][B(C6F5)4] is also effective for cyclopolymerization of 3,3-dimethyl-4-sila-1,5-hexadiene (47) and 4-sila-1,6-heptadienes (46).159 The cyclopolymerization of 47 by VII/[Ph3C][B(C6F5)4] produces a polymer with cis-fused silacyclopentane ring (cis > 90%) and with high isotacticity (>90%). The polymerization proceeds via first 1,2-insertion of the vinyl group followed by cyclization via 1,2-insertion of the allyl group (Figure 30). DFT calculation shows smaller activation energy in cyclization via cis-twist boat conformation (43.94 kJ/mol) than that via the intermediate with trans-chair conformation (53.47 kJ/mol). The produced polymer shows high Tg (up to 99.8 °C) and high Tm (>200 °C). The cyclopolymerization of 46 by the Hf catalyst also affords the polymer with highly cis-diisotactic structure, but the cyclization ratio is not quantitative (80.0−93.2%). The early transition metal catalysts, mentioned above, afford the polymer containing 1,3-cyclopentylene and 1,3-cyclohexylene groups, respectively, in the cyclopolymerization of 9001


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Figure 27. Selected examples of selective cyclopolymerization of 1,5-hexadiene and 1,6-heptadiene catalyzed by early transition metal catalysts.

alternatingly. The polymerization proceeds in living manner at −78 °C. Rare earth metal complexes also have been utilized as catalysts for the cyclopolymerization of nonconjugated dienes. Earlier reports on the cyclopolymerization of 1,5-hexadiene by [Cp*Y(OC6H3tBu2-2,6)H]2 revealed that the resulting polymers possess methylene-1,3-cyclopentylene repeating units.163 The catalyst is not active for the cyclopolymerization of 1,6heptadiene. Half-sandwich Sc complexes IX, X, and XI bring about the cyclopolymerization of 1,6-heptadiene in the presence of [Ph3C][B(C6F5)4], affording the polymer containing methylene-1,3-cyclohexylene and ethylene-1,2-cyclopentylene units as the major (80−91%) and minor (9−20%) repeating structures, respectively.164,165 The stereochemistry of the cyclohexylene group is mostly controlled in cis (81−90%), whereas that of the cyclopentylene group is less controlled (trans = 47−72%). The Sc complexes are also effective for the cyclopolymerization of 1,5-hexadiene, but most of them afford cross-linked polymers.144,166 Only IX affords a soluble polymer, which has methylene-1,3-cyclopentylene units (83%) and 2vinyl-1,4-butylene units (17%). The cyclopentylene group is

1,5-hexadiene and 1,6-heptadiene, because of the 1,2-selective insertion of the first vinyl group of the dienes. In contrast, the polymer obtained by the cyclopolymerization of 1,6-heptadiene catalyzed by titanium complex with phenoxyimine ligand (VIII) in the presence of MAO contained an equal amount of methylene-1,3-cyclohexylene unit and ethylene-1,2-cyclopentylene unit (Figure 31).160 Stereochemistry of the cyclohexylene group is predominantly controlled in the cis structure (80%). In the cyclopolymerization of 1,5-hexadiene by the catalyst VIII/ MAO, the produced polymer has a methylene-1,3-cyclopentylene unit (MCP) and a 2-vinyl-1,4-tetramethylene repeating unit (VTM; 63 and 37 mol%, respectively).161 The latter repeating structure is formed by 2,1-insertion of the vinyl group, cyclization via 1,2-insertion of the other vinyl group, and ring opening of the strained cyclobutane group by β-alkyl elimination. Similar formation of a polymer containing methylene-1,3-cyclopentylene units and 2-vinyl-1,4-tetramethylene repeating units has been reported in the cyclopolymerization of 1,5-hexadiene catalyzed by V(acac)3/AlEt2Cl.162 The resulting polymer is composed of those two repeating units 9002


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Figure 28. Formation of stereoblock copolymer in cyclopolymerization of diene by I or II/[PhNHMe2][B(C6F5)4].148,150

Figure 29. Polymerization of 1,7-octadiene by III/MAO.151,152

obtained in a controlled way, mostly in a trans structure (86%). The Sc complex with N-heterocyclic carbene ligand XII also brings about the cyclopolymerization of 1,5-hexadiene with a cyclization ratio of 97.8%.167 The produced polymer contains methylene-1,3-cyclopentylene units, but its stereochemistry is not clear.

Figure 30. Cyclopolymerization of 4-sila-1,5-hexadiene (47) and 4-sila1,6-heptadiene (46) by VII/[Ph3C][B(C6F5)4].159

Table 2 lists representative results of the cyclopolymerization of nonconjugated dienes by the late transition metal catalysts. Fe and Co complexes with bis(imino)pyridine ligands (such as XIII and XIV)168,169 promote polymerization of 1,6heptadiene in the presence of MAO.170 The resulting polymers are synthesized with selectivity toward the 1,2-cyclopenthylene group, in contrast with the cyclopolymerization of 1,6heptadiene by early transition metal catalysts, which tend to give selectivity toward the 1,3-cyclohexylene group. Although both Fe and Co complexes have ligands with similar structures, the trans−cis stereoselectivity in the products is the opposite: the Fe catalysts afford polymers rich in cis-1,2-cyclopenthylene groups, whereas the Co catalysts afford polymers with trans-1,2-

4.3. Cyclopolymerization of Dienes Using Fe, Co, Pd, and Ni Catalysts

In addition to early transition metal complexes, late transition metal complexes also have been used as catalysts in ethylene and olefin polymerization. They often show unique properties such as different regioselectivity in olefin insertion, isomerization of the growing terminal group, tolerance toward polar groups, and copolymerization with polar monomers. The cyclopolymerization of nonconjugated dienes by the late transition metal catalysts has been also reported recently. Chart 3 shows late transition metal catalysts used as the catalyst for cyclopolymerization. 9003


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Figure 31. Cyclopolymerization of 1,6-heptadiene and 1,5-hexadiene by VIII/MAO160,161 or IX, X, and XI/[Ph3C][B(C6F5)4].165

Chart 3. Late Transition Metal Complexes Utilized As Catalysts for the Cyclopolymerization of Dienes

cyclopenthylene structure exclusively (Figure 32). The cyclopolymerization using Fe complexes with bulky substituents on the ligands leads to decreased cis selectivity (cis = 70% for the complex with N-2,6-diisopropylphenyl group). In contrast, Co complexes afford polymers with high cis selectivity regardless of the size of the substituents on the N-aryl group. The polymer

produced by XIIIa/MMAO is mostly atactic, but that obtained by XIV/MMAO is controlled in moderately isotactic structure. The isoselectivity of the polymer is further improved by using the Co complex with more bulky substituents on the ligand (N2,6-dicyclohexylphenyl group; isotactic triad = 62%). Kinetic studies indicate that the cyclopolymerization of 1,6-heptadiene 9004


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Table 2. Cyclopolymerization of Dienes by Late Transition Metal Catalysts monomer

catalyst

Mn

PDI

Tg, Tm /°C

1,6-HPD

XIIIa (Fe)

1,2-ECP

95% cis

14 000

2.4

1,6-HPD 48 48 51

XIV (Co) XIIIb (Fe) XIV (Co) XVb (Pd)

1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP

>99% trans >99% cis >99% trans >99% trans

9300 2700

1.7 3.5

0.4 (Tg) 130 (Tm) 3.2 (Tg)

9200

1.75

52 53 54 59 62 73 74 76 50 50 50 81

XVb (Pd) XVb (Pd) XVb (Pd) XVa (Pd) XVa (Pd) XVb (Pd) XVb (Pd) XVb (Pd) XVa (Pd) XVIa (Ni) XVIb (Ni) XVII (Pd)

1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,2-ECP 1,3-MCH 1,2-ECP (major)

>99% trans >99% trans >99% trans >99% trans >99% trans >99% trans >99% trans >99% trans >99% trans trans cis trans (major)

9400 23000 2800 9200 7500 7100 8300 8200 17 800 18 200 6100 4350

1.71 1.22 1.72 1.12 1.67 1.56 1.12 1.17 1.95 2.05 1.36 1.7

ring

microstructure

remark

ref 170

moderately diisotactic (rr = 35%)

isotactic and syndiotactic polymer can be obtained by 9c and 9d, respectively. 20 (Tg) 143 (Tg) living polymerization 90 (Tg) 90 (Tg) 101 (Tg) 89 (Tg) 154 (Tg) tacticity is highly controlled. no Tg in the region −50 to 300 °C

170 170 170 179,186 179 180 180 180 182 180 180 180 180 188 188 193

oligomers (especially dimers) rather than high molecular weight polymers.171,172 α-Olefins undergo 2,1-insertion into the catalyst to give the species with secondary alkyl-metal bonds, which lead to β-hydrogen elimination rather than insertion of other olefin molecules. 1,6-Heptadiene also undergoes 2,1insertion into the catalyst, but the species with the secondary alkyl-metal bond, thus formed, is transformed into the reactive species with the primary alkyl-metal bond via the subsequent cyclization. The Fe and Co catalysts are also applicable to the polymerization of 1,6-heptadienes with functional groups on the 4 position, such as 4-siloxy-1,6-heptadiene (48), 4-phenyl1,6-heptadiene (49), and 9,9-diallylfluorene (50).173 Similarly to the cyclopolymerization of 1,6-heptadiene, the cyclopolymerization of 48 and 49 using the Fe catalyst (XIIIb/ MMAO) affords the polymer with high cis selectivity (cis > 99%), and that using the Co catalyst (XIV/MMAO) produces the polymer with trans structure selectively (trans > 99%). The cyclopolymerization of 50 by XIIIb/MMAO proceeds to give the polymer having cyclopentylene repeating units, but its trans−cis selectivity is not controlled (cis = 65%). The Co catalyst (XIV/MMAO) is not active for the cyclopolymerization of 50. Although Fe and Co catalysts are effective for the cyclopolymerization of 1,6-heptadienes, the cyclopolymerization of 1,7-octadiene by the Fe catalysts only reaches low molecular weight oligomers. Diimine Ni and Pd catalysts have been known to promote the polymerization of ethylene, propylene, and α-olefins.174−177 In contrast to bis(imino)pyridine Fe and Co catalysts and early transition metal catalysts, diimine Ni and Pd catalysts afford polyolefins with unique branches originating from frequent isomerization of the growing terminal units via β-hydrogen elimination and reinsertion during the chain growth (the chain walking reaction). Such chain walking reaction during the reaction of nonsubstituted dienes ends up with stable π-allyl metal species, which are very slow to undergo further insertion of olefins. Thus, the nonsubstituted dienes usually poison the olefin polymerization using diimine Ni and Pd catalysts.178 Nonconjugated dienes having quaternary carbons, in contrast, undergo cyclopolymerization using diimine Ni and Pd catalysts,

Figure 32. Cyclopolymerization of 1,6-dienes by Fe and Co catalysts.

by the Co catalyst (XIV/MMAO) obeys zeroth order kinetics with respect to the monomer concentration, whereas that by the Fe catalyst (XIV/MMAO) is dependent on the concentration of the monomer. These results indicate that the cyclopolymerization by the Co catalyst involves cyclization of the polymer end via intramolecular insertion of the vinyl pendant group as the rate determining step and the polymer is built with thermodynamically stable trans-cyclopentylene repeating units. It is worth noting that the reaction of α-olefins with Fe and Co complexes with bis(imino)pyridine ligands generally affords 9005


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groups, and chain walking reaction to give the stable chelate complex (Figure 34). The chelate intermediate A and the intermediate B are in equilibrium, and B reacts selectively with the monomer. Smooth polymerization of the monomers without carbonyl groups in the 4 position can be ascribed to the absence of such stable intermediate during the cyclopolymerization. The polymerization of 51 by XVa obeys first-order and zeroth-order kinetics at low ([51] = 0.50 M) and high ([51] = 1.0 M) concentrations of the monomer, respectively. The formation of the intermediate B from A is the rate-determining step of the cyclopolymerization at high monomer concentration, but the coordination of the monomer to the Pd complex becomes the rate-determining step at low monomer concentration. In both case, the cyclization takes place rapidly and selectively. The selective cyclopolymerization of 51 by XVa takes place even under bulk conditions without formation of cross-linking or the incomplete cyclization of the diene. As for the Pd catalyst, XVb generally enables faster cyclopolymerization of dienes than the more bulky XVa. The pseudo-C2 symmetric Pd complex (XVc) promotes isotactic polymerization of 51 to afford polymer with trans-diisotactic structure (isotactic triad = 83%).179 In contrast, syndiotactic polymerization of 51 can be achieved by a pseudo-Cs symmetric Pd complex with a cyclic diimine ligand (syndiotactic tetrad = 60%).186 The isotactic and syndiotactic stereoisomers of polymer 51 adopts helical and barb-shaped structure, respectively. The syndiotactic polymer undergoes self-assembly into nanofibers, which leads to 3D-network formation to give a physical gel. The nanofibers from the syndiotactic polymer were used for soft lithography with controlled volume shrinkage via heat-induced cross-linking.187 Diimine Ni complexes (XVI) in combination with MAO catalyze the cyclopolymerization of 9,9-diallylfluorene (50) and 4,4-diallyl-4H-cyclopenta[def]phenanthrene (79).173,188,189 The size and stereochemistry of the cycloalkane group in the repeating units is largely affected by the diimine ligand of the catalyst. The Ni complex with N-2,6-diisopropylphenyl groups (XVIa) affords the polymer with trans-1,2-cyclopentylene groups, whereas that with N-2,6-dicyclohexylphenyl groups (XVIb) gives the polymer with cis-1,3-cyclohexylene groups (Figure 35). 2,1-Insertion of the first vinyl group of 50 takes place selectively for the reaction by XVIa, similarly to the diimine Pd catalyst, whereas XVIb with a sterically hindered structure brings about selective 1,2-insertion, giving cyclopolymers with six-membered rings. Phosphinesulfonate Pd complex catalysts enable copolymerization of ethylene with various polar comonomers.190−192 The catalyst XVII (L = dimethyl sulfoxide) also promotes cyclopolymerization of diallyl ether (81) in toluene at 80 °C (Figure 36).193 The major repeating unit is ethylene-1,2-trans-4oxacyclopentylene, but ethylene-1,2-cis-4-oxacyclopentylene units, methylene-1,3-(5-oxacyclohexylene) units (both trans and cis), and uncyclized units are also present in minor amounts. The successful homopolymerization of diallyl ether is noteworthy, because its radical and cationic polymerization is problematic due to the formation of stable π-allyl radicals or cations. Actually, the radical and cationic polymerizations of diallyl ether by AIBN and [Ph3C][B(C6F5)4] afford low molecular weight (Mn < 500) in low yield, and the structure of the product is completely different from that obtained by XVII. The transition-metal-catalyzed polymerization of allyl monomers is limited.194 Allyl ethyl ether does not polymerize smoothly using the catalyst, because it mainly undergoes 1,2-

because the intramolecular chain walking reaction is suppressed by the quaternary carbon. The diimine Pd complex (XV) in combination with NaBARF (BARF = [B{C6H3-(CF3)2-3,5}4]−) is effective for the cyclopolymerization of 1,6-heptadienes with functional groups at the 4 position, giving selectively the polymer with trans-1,2disubstituted cyclopentylene groups (Figure 33).179−181 Chart 4 summarizes the 1,6-dienes that can be polymerized by Pd catalysts.182

Figure 33. Cyclopolymerization of 1,6-dienes by XV/NaBARF.179−181,183,184

Through the cyclopolymerization of the 1,6-dienes, a large variety of functional groups such as ester (51, 52, 68, 69, and 78), imide (53−55), cyclic diketone (57−61), ether (62−67), and sulfonamide groups (73−77) can be incorporated into the polymer backbone. The substituents on the monomers greatly affect their polymerizability. The polymerization of the dienes without carbonyl groups attached to the 4 position proceeds more smoothly than that of the monomers with ester groups or imide groups in the 4 position. N,N-Diallylsulfonamides with fluorinated aryl or alkyl groups (74−76) undergo faster cyclopolymerization than those without an electron-withdrawing group (73 and 77). The monomer with an indanedione group (59) undergoes living polymerization to give the polymer with narrow molecular weight distribution. The Pd-catalyzed cyclopolymerization allows the incorporation of a bromoisobutyrate group (69), which can be applied as an initiator for atom transfer radical polymerization (ATRP). The polymer obtained by the cyclopolymerization of diene with terthiophene group (61) showed intramolecular interactions between terthiophenes in the excited state, and electron affinity of the polymer makes it a candidate for n-type organic fieldeffect transistor (OFET) materials.183 The Pd catalyst also enables cyclopolymerization of a diene with redox-active anthraquinone moieties (78).184 It is worth noting that the diene without quaternary carbon affords the corresponding polymer efficiently. The ester group may suppress the formation of π-allyl Pd species via the intramolecular chain walking reaction and promote cyclization. The polymer, thus formed, has potential for application as the active material in secondary batteries. Monomers such as 1,6-heptadiene, diallyl ether, diallylmalononitrile, 46 and 48 do not undergo smooth cyclopolymerization using the Pd catalysts.185 The reaction of 51, 53, or 55 with XVa/NaBARF produces the corresponding chelate intermediate A, as confirmed by X-ray crystal structure analysis. The plausible mechanism of the cyclopolymerization includes selective 2,1-insertion of diene, trans-selective cyclization via 1,2-insertion of the other vinyl 9006


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Chart 4. Nonconjugated Dienes Utilized in Cyclopolymerization by XV/NaBARF

Figure 34. Mechanism of cyclopolymerization of 1,6-dienes by XV/NaBARF.180

insertion into the Pd-carbon bond to form an unreactive fivemembered chelate complex. The second CC bond of the

diallyl ether enhances 2,1-insertion of the allyl group, which leads to fast cyclization without stable chelate formation. XVII is 9007


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Figure 35. Cyclopolymerization of 9,9-diallylfuluorene (50) by XVI/ MMAO.173,188

Figure 36. Cyclopolymerization of diallyl ether (81) and divinyl formal (82) by XVII.193,195

also effective for the cyclopolymerization of divinyl formal (82) to give the corresponding polymer with Mn = 2670.195 This result is in contrast with the reaction of n-butyl vinyl ether using XVII, where low molecular weight oligomers form in low yield. Thus, the cyclization of divinyl formal is of key importance for the efficient polymerization of the monomer.

Figure 37. Isomerization polymerization of 7-alkyl-1,6-dienes by XV/ NaBARF.196,197

α,ω-alkylene group into the main chain of the polymer. Thus, the resulting polymer contains trans-1,2-cyclopentylene groups and α,ω-alkylene groups in alternating sequence along the polymer chains. The intervals between neighboring cyclopentylene groups can be controlled by the length of the alkyl group of the monomer. Similarly to the Pd-catalyzed cyclopolymerization of 1,6-heptadienes, various functional groups can be introduced into the polymer. The thermal properties of the polymer can be tuned by the length of α,ω-alkylene group incorporated into the polymer backbone. The alkyl-substituted 1,6-dienes with cyclic acetal group (84) afford the corresponding polymer with narrow molecular weight distribution. Block

4.4. Pd-Catalyzed Polymerization of Dienes and Trienes with Concurrent Isomerization and/or Double Cyclization

Although many examples of the cyclopolymerization of α,ωdienes have been reported, the cyclopolymerization of monoterminal dienes has been rare because of the lower polymerizability of 1,2-disubstituted olefins with respect to αolefins. Recently, however, diimine Pd catalysts XV were reported to bring about the cyclopolymerization of 1,6-dienes having alkyl groups in the 7-position (Figure 37).196 The polymerization proceeds via quantitative cyclization of the monomer. The terminal alkyl group of the diene is introduced as 9008


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Figure 38. Mechanism of cyclopolymerization of 7-alkyl-1,6-dienes by XV/NaBARF.196,197

Figure 39. Isomerization cyclopolymerization of 1,7-diene 80 by XVa/NaBARF.182

The cyclopolymerization of trienes is expected to produce polymers with two cycloalkane groups in a repeating unit via double cyclization during the chain growth, but examples are limited. 1,1,1-Triallylethane was reported to undergo cyclopolymerization in the presence of Ziegler catalysts to yield a polymer with a bicyclic group in the repeating unit via insertion of a vinyl group of the monomer into the metal−polymer bond and subsequent double cyclization.198 The resulting polymer was not fully characterized, but pendant vinyl groups were identified, due to incomplete cyclization. Recently, Pd diimine catalysts XV/NaBARF were used to promote controlled double cyclization polymerization of linear trienes possessing two vinyl and one vinylene groups (Figure 40).181,199 1,6,11-Dodecatrienes undergo double cyclizative polymerization using a Pd catalyst (Figure 40 (i)). The resulting polymers contain two trans-cyclopentylene groups in a repeating unit, and the biscyclopentylene group is produced in a stereocontrolled manner. Trienes with acetal groups (93) or ether groups (94) cyclopolymerize efficiently, whereas the triene with cyclic ester groups (92) cyclopolymerizes in lower yield. The trienes with acyclic ester groups, imide groups, and sulfonamide groups do not undergo cyclopolymerization using the Pd catalyst. The cyclopolymer 92 is produced with stereocontrolled tacticity and narrow molecular weight distribution. Similarly to the cyclopolymerization of 1,6-dienes, 1,6,11trienes having alkyl group on 12-position undergo double

copolymers with different oligomethylene fragments can be synthesized by the sequential polymerization of those dienes with different alkyl lengths. In addition to the dienes with linear alkyl groups, dienes with a branched alkyl group can be also used.197 Dienes with isopropyl groups (89) and isobutyl groups (90) afford polymers with methyl branches on the α,ω-alkylene groups. Dienes with sec-butyl groups (91) produce polymers containing partially uncyclized units, owing to an insufficient cyclization efficiency. The α,ω-alkylene groups of the polymers contain methyl branches rather than ethyl branches. Figure 38 illustrates a plausible mechanism of the polymerization. 2,1-Insertion of the vinyl group of the diene and cyclization via insertion of the vinylene group forms 1cyclopentylalkyl Pd species (C). The intermediate C does not allow the reaction with the diene monomer but undergoes isomerization via chain walking reaction. The diene monomer selectively reacts with the intermediate D possessing a −CH2-Pd bond, incorporating the alkyl group of the diene as α,ω-alkylene in the polymer main chain. As for the diene with sec-butyl group, the chain growth takes place via -CH(CH3)-CH2-CH2-Pd terminal units rather than via the more sterically hindered -CH(CH2CH3)-CH2-Pd terminal unit. The cyclopolymerization of 1,7-diene 80 also proceeds in the presence of XVa/NaBARF.182 The major repeating unit is the trans-1,2-cyclopentylene group, which is formed by the first 2,1insertion of the butenyl group and then chain walking before cyclization (Figure 39). 9009


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cyclizative isomerization polymerization (Figure 40 (ii)).200 The resulting polymers contain biscyclopentylene groups (with controlled stereochemistry) and α,ω-alkylene group in an alternating manner. Monoterminal 1,6,11-triene with different functional groups on 4 and 9 positions (96) undergoes cyclopolymerization with those groups in alternating sequence. Double cyclizative isomerization polymerization of 1,6,13heptadecatriene having functional groups at 4 and 11 positions (97−100) produces the polymers containing two functionalized trans-1,2-cyclopentylene groups in every repeating unit (Figure 40 (iii)).201 The two neighboring five-membered rings of the polymer are separated by ethylene and pentamethylene spacers alternatingly. Figure 41 shows the mechanism of the double cyclizative isomerization. The chain-walking of the growing polymer end occurs smoothly until the formation of the fivemembered ring or insertion of a vinyl group of the new monomer into the terminal Pd−CH2 bond. The trienes having the cyclic acetal group at the 4-position and the cyclic acetal, cyclic ester, acyclic ester, or sulfonamide groups at the 11 position undergo smooth cyclopolymerization, whereas the monomer with ester groups at 4-position gives polymers in low yields. Thus, the cyclizative isomerization polymerization of dienes and trienes enable precise control of the density and distribution of the functionalized cyclopentylene units in the polymer, and allows the fine-tuning of the their thermal properties.

5. TRANSITION-METAL CATALYZED CYCLOCOPOLYMERIZATION OF DIENES The catalysts utilized in the cyclopolymerization of nonconjugated dienes are also active for the polymerization of ethylene and/or α-olefins. Thus, the copolymerization of nonconjugated dienes with ethylene or propylene has been examined by using those catalysts.146 The resulting polymers are comprised of cycloalkane groups, formed by the cyclization of the dienes. Ethylene and propylene can undergo insertion before the cyclization of the diene takes place, which leads to

Figure 40. Isomerization cyclopolymerization of (i) 1,6,11-dodecatrienes,181,199 (ii) 12-alkyl-1,6,11-trienes,200 and (iii) 1,6,13-heptadecatrienes by XV/NaBARF.201

Figure 41. Mechanism of double cyclizative isomerization polymerization of 1,6,13-heptadecatrienes.201 9010


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Figure 42. Cyclocopolymerization of ethylene with 1,5-hexadiene by C1 symmetric zirconocene catalysts.212

the amount of incorporated 1-hexene in ethylene/1-hexene copolymerization under similar conditions. The ethylene/1hexene copolymer obtained by the C1 symmetric catalyst Me2Si(Ind)(Flu)ZrCl2 is rich in the alternating sequence, but the ethylene/1,5-hexadiene copolymer contains a significant fraction of methylenecyclopentylene−methylenecyclopentylene sequence (Figure 42). Ethylene/1,5-hexadiene copolymers obtained by C2 and C2v symmetric metallocenes Me2Si(Cp)(Flu)ZrCl2 and Me2Si(Flu)2ZrCl2 and ethylene/1-hexene copolymers obtained by C1 symmetric metallocene Me2Si(Ind)(Flu)ZrCl2 contain predominant odd-numbered ethylene sequences, whereas ethylene/1,5-hexadiene copolymers obtained by Me2Si(Ind)(Flu)ZrCl2 are rich in even-numbered ethylene sequence. The catalyst Me2Si(Ind)(Flu)ZrCl2 has an α-olefin-selective site and an ethylene-selective site. The above polymerization results are ascribed by the dual-site copolymerization mechanism. The coordination/insertion of the C C double bond takes place on the two sites of the catalyst alternatingly. When one of the CC bonds of 1,5-hexadiene undergoes coordination/insertion at the α-olefin selective site, the cyclization follows via coordination/insertion of the other CC bond at the ethylene-selective site. The catalyst after the cyclization of 1,5-hexadiene has vacant α-olefin selective site, which leads to methylenecyclopentane−methylenecyclopentane sequence. Zirconocene catalysts also promote the cyclocopolymerization of propylene with 1,5-hexadiene.213,214 Isospecific C2 symmetric zirconocenes (rac-Me2Si(Ind)2ZrCl2/MAO and rac-Et(Ind)2Zr(NMe2)2/i-Bu3Al/[Ph3C][B(C6F5)4]), and syndiospecific Cs symmetric zirconocenes (Ph2C(Cp)(Flu)ZrCl2/ MAO and Me2C(Cp)(Flu)ZrMe2/i-Bu3Al/[Ph3C][B(C6F5)4]) enable incorporation of methylenecyclopentylene repeating units in the stereoregular polypropylene (0.8−72.5 mol %). The cyclopentylene groups of the copolymer are predominantly controlled in the trans structure (58 − 65% for the C2 symmetric zirconocenes and 62−71% for the Cs symmetric zirconocenes). The cyclization ratio of the 1,5-hexadiene is >88.6%, which decreases as the 1,5-hexadiene feed ratio increases. Detailed structural analysis of the produced polymer indicates that the

copolymers with pendant olefins, which can cross-link, giving insoluble fractions, or react again after the insertion of ethylene and α-olefins to form cyclic repeating unit with larger ring size. Some dienes, which do not undergo homopolymerization smoothly, can be copolymerized with ethylene or α-olefins efficiently. 5.1. Cyclocopolymerization of Dienes Using Ti, Zr, Hf, and Sc Catalysts

Ziegler−Natta catalysts have been utilized for the cyclopolymerization of 1,5-hexadiene. The stopped-flow polymerization of 1,5-hexadiene, catalyzed by MgCl2-supported Ziegler catalyst, was reported to proceed in a quasi-living manner.202−204 This methodology was applied also for the synthesis of a block copolymer of propylene and 1,5-hexadiene, polypropylene-block-poly(methylene-1,3-cyclopentylene-copropylene) (PMCP-co-PP).205 Each block length can be controlled by changing the polymerization time. The block copolymer has a crystalline polypropylene part linked with noncrystalline PMCP-co-PP part.206 Copolymerization of ethylene with 1,5-hexadiene has been examined by using metallocene catalysts with various structures.207−209 The amounts of incorporated 1,5-hexadiene, of the solvent-insoluble fraction, and of the catalytic activity are largely dependent on the ligand structure of the catalyst and on the reaction temperature. Bridged metallocenes tend to incorporate more amount of 1,5-hexadiene than nonbridged metallocenes.210 C2 symmetric metallocenes such as racEt(Ind)2Zr(NMe2)2 and Cs symmetric metallocenes such as Me2C(Cp)(Flu)ZrMe2 catalyze isotactic and syndiotactic propylene polymerization, respectively, but both of them afford the polymer containing cyclopentylene groups rich in trans structure in the cyclopolymerization of 1,5-hexadiene.211 On the other hand, the use of Cp*2ZrCl2 affords a polymer rich in cisfused cyclopentylene groups. Ethylene/1,5-hexadiene copolymerization catalyzed by C1 symmetric metallocenes affords a copolymer with a unique sequence of the monomers.212 The amount of incorporated 1,5hexadiene in its copolymerization with ethylene is larger than 9011


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1,2-insertion of 1,5-hexadiene by the isospecific and syndiospecific zirconocenes takes place in enantiomorphic site control. Cyclocopolymerization of propylene with 2-methyl-1,5hexadiene by stereospecific zirconocene catalysts has been also reported.215,216 Smooth polymerization of 2-methyl-1,5-hexadiene is remarkable because 1,1-disubstituted olefins generally show very low activity toward metallocene catalyzed polymerization. C2 symmetric catalyst XVIII affords isotactic polymer containing methylene-(1-methyl)-1,3-cyclopentylene unit formed by the cyclization of the diene (Figure 43). The

The copolymerization of diallylsilanes with ethylene catalyzed by zirconocenes allows the corresponding copolymers to have silacyclohexane units (Figure 44).230 Bridged zirconocenes

Figure 44. Cyclocopolymerization of ethylene or propylene with diallylsilanes (46).230,231

showed higher activity than nonbridged zirconocenes. The copolymer containing up to 26 mol% of the repeating units derived from 46a can be synthesized by using Cs symmetric Ph2C(Cp)(Flu)ZrCl2/MAO. The cyclization selectivity is quantitative. In contrast, the copolymerization of ethylene with 46c obtained by Me2Si(Ind)2ZrCl2/MAO affords uncyclized repeating units (7−10 mol %). The melting point of the copolymer decreases as the incorporation of the diallylsilanes increases. The copolymerization of propylene with diallylsilanes also proceeds in the presence of the zirconocene catalysts.231 Compound 46a is incorporated as the repeating unit with silacyclohexylene structure, whereas the propylene/46c copolymer contains uncyclized units. Similarly to the ethylene copolymerization, the maximum incorporation of the diene is 28 mol%, which is attained in the copolymerization with 46a using Ph2C(Cp)(Flu)ZrCl2/MAO. The stereochemistry of the silacyclohexylene group is mainly controlled in the cis structure, regardless of the stereoselectivity of the catalysts. The copolymer with 46c shows higher Tg than that with 46a. The constrained-geometry catalyst Me2Si(C5Me4)(N-tBu)TiCl2/MAO is also effective for the copolymerization of ethylene with nonconjugated dienes. The copolymerization of ethylene with 1,5-hexadiene affords the corresponding copolymer having up to 52 mol% of the methylene-1,3-cyclopentylene unit from diene.146 Copolymerization of ethylene with 1,7octadiene by the catalyst at 40 °C affords the polymer having the repeating unit from diene (up to 30.6 mol %).232 The copolymer obtained in low concentration of the monomer (lower than 0.10 mmol/L) is totally soluble in o-dichlorobenzene, but that obtained with monomer concentration of 0.20 mmol/L contained 75 wt % of the insoluble fraction. 1,7-Octadiene is predominantly incorporated into the copolymer as methylene1,3-cyclononylene repeating units, in addition to minor repeating units having the methylene-1,3-cycloheptylene structure. This is in contrast to the homopolymerization of 1,7-octadiene, which affords only methylene-1,3-cycloheptylene repeating units in the macromolecular chain. The molar ratio of methylene-1,3-cyclononylene units and methylene-1,3-cycloheptylene units is almost 7:3. The stereochemistry of the cycloheptylene group is moderately controlled in the cis structure (61−80%). The polymer also contains uncyclized repeating units (ca. 30 mol %). The formation of the ninemembered rings in the copolymerization is ascribed by the

Figure 43. Zirconocene catalysts used in propylene/2-methyl-1,5hexadiene copolymerization.215,216

stereochemistry of the cyclopentylene group is quantitatively controlled in the trans structure. C1 symmetric catalyst XIX and Cs symmetric catalyst Ph2C(Cp)(Flu)ZrCl2 afford isotactic and syndiotactic polymer with the methylene-(1-methyl)-1,3-cyclopentane group, but the stereochemistry of the cyclopentane group is not controlled. Crystalline structure of polyethylene and isotactic polypropylene containing 1,3-cyclopentylene unit, obtained by ethylene/1,5-hexadiene and propylene/1,5-hexadiene copolymerization catalyzed by the zirconocene catalysts, has been reported.217−222 For the ethylene/1,5-hexadiene copolymer, the 1,3-cyclopentylene unit is incorporated into the crystalline phase of polyethylene by partially changing the trans zigzag chain into a gauche conformation and inducing a transformation of orthorhombic crystal to pseudohexagonal crystal. Copolymerization of ethylene or propylene with 1,7octadiene213,223−227 and 1,9-decadiene223,228,229 by zirconocene catalysts has been investigated. The product obtained in the copolymerization of ethylene with 1,7-octadiene by various zirconocenes contains some fraction insoluble in orthodichlorobenzene owing to the cross-linking of the uncyclized unit. C2 symmetric zirconocenes rac-Et(Ind)2ZrCl2 and racMe2Si(Ind)2ZrCl2 afford the product rich in ortho-dichlorobenzene-soluble fraction (68−99%).223 1,7-Octadiene is incorporated as a methylene-1,3-cycloheptane structure, quantitatively, in the soluble fraction, and its stereochemistry is predominantly controlled in the cis structure (83.8−92.0%). Propylene/1,7-octadiene copolymerization by Me2Si(2-Me-4Ph-Ind)2ZrCl2/MAO afford isotactic polypropylene with long chain branches owing to the uncyclized 1,7-octadiene unit.227 1,9-Decadiene is incorporated in the copolymer without cyclization and sometimes leads to cross-linking. Me2Si(Flu)2ZrCl2/MAO brings about alternating copolymerization of ethylene with 1,9-decadiene without cyclization or crosslinking to give polymer having pendant octenyl group, which can be used for polymer reaction such as hydrosilylation.228 9012


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Figure 45. Cyclocopolymerization of ethylene with 1,7-octadiene by Me2Si(C5Me4)(N-tBu)TiCl2/MAO and the mechanism for the formation of 7and 9-membered rings.232

cyclization selectivity is largely affected by the substituents on the silicon atom in the monomer. High cyclization selectivity is observed for the less bulky 47. The cyclocopolymerization of 1,5-hexadiene with 46 affords the copolymer containing the repeating units from those monomers in varied ratio.238 The copolymer with more of the repeating unit derived from 46 shows increased hydrophobicity and modulus and decreased dielectric constant. Propylene/1,5-hexadiene copolymerization by titanium complex with phenoxyimine ligand (VIII) affords syndiotactic polypropylene containing the repeating units from 1,5hexadiene (4−13 mol %).160 Similarly to the homopolymerization of 1,5-hexadiene, the diene is incorporated as methylene1,3-cyclopentylene and 2-vinyl-1,4-butylene repeating units. Their ratios are affected by the propylene pressure in the copolymerization reactor. The preference for the methylene-1,3cyclopentylene units, similarly to the homopolymerization of 1,5-hexadiene, is observed under lower propylene content, while the copolymerization carried out at higher propylene content favors the formation of 2-vinyl-1,4-butylene repeating units. The methylene-1,3-cyclopentylene units and 2-vinyl-1,4-butylene units form via 1,2- and 2,1-insertion of the diene to the polymer chain end, respectively. The increased amount of 2-vinyl-1,4butylene units at higher propylene content is accounted for by a higher probability of a 2,1-insertion of the monomer to the bulky secondary alkyl titanium species. By utilizing the living character of the propylene polymerization using VIII, block copolymers of propylene and 1,5-hexadiene have been also synthesized. The vinyl groups on the side chain the copolymers can be functionalized by cross metathesis reaction catalyzed by second generation Grubbs catalyst.239 The dinuclear dititanium complex (XX; Figure 46) promotes the cyclocopolymerization of ethylene with 1,5-hexadiene, giving the copolymer with methylene-1,3-cyclopentylene repeating units.240 Compared to

insertion of ethylene after the 1,2-insertion of one CC bond of the diene, followed by the cyclization via 1,2-insertion of the other CC bond (Figure 45). The cyclopolymerization of ethylene with 1,7-octadiene at 140 °C also affords the copolymer containing both 7- and 9-membered rings in almost one to one ratio.233 The half titanocene complex with the bulky phenoxy ligand III/MAO promotes the copolymerization of 1,7-octadiene with 1-octene151 and the terpolymerization of 1,7-octadiene, ethylene, and styrene.234 In these cases, no cyclization of the 1,7octadiene takes place during the polymerization, the corresponding copolymer having pendant olefins. The hydroboration/oxidation of the vinyl groups on the side chain leads to a polymer having hydroxy alkyl side chains, which can be further utilized as the initiating groups for the ring-opening polymerization of ε-caprolactone. The pyridylamidohafnium complex (VII) brings about the cyclocopolymerization of propylene with 1,5-hexadiene in the presence of i-Bu3Al and [Ph3C][B(C6F5)4], to afford the copolymer containing up to 43.4 mol % of the repeating unit from diene.235 The resulting copolymer is essentially isotactic polypropylene, containing methylene-1,3-cyclopentylene units with predominantly cis structure (72%), which are located randomly on the macromolecular chains. No uncyclized repeating units are included. The Hf catalyst also catalyzes cyclocopolymerization of propylene with Si-containing dienes 46 and 47.236,237 The cyclocopolymerization of propylene with 46 by VII/i-Bu3Al/[Ph3C][B(C6F5)4] proceeds with quantitative cyclization of 46. The resulting copolymer contains up to 25.3 mol% of the repeating unit derived from 46. The copolymer with 10.4 mol % of the unit derived from 46 shows Tg and Tm at 1.8 and 100.0 °C, respectively. The cyclocopolymerization of propylene with 47 by VII/i-Bu3Al/ [Ph3C][B(C6F5)4] affords the copolymer containing partly uncyclized units. The 9013


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homopolymers. In contrast, XIc promotes cyclocopolymerization of 1,6-heptadiene with styrene in the presence of [Ph3C][B(C6F5)4] (Figure 47).241 The resulting copolymer is composed of polystyrene and polydiene blocks, but the syndiotacticity of the latter is not very high. 1,6-Heptadiene is mainly incorporated as forming methylene-1,3-cyclohexylene repeating units, respectively, in addition to minor amounts of ethylene-1,2-cyclopentylene repeating units. The scandium catalysts X, XIa, and XIc have also been applied for the copolymerization of isoprene and 1,6-heptadiene or 1,5hexadiene (Figure 47).165 The copolymer of isoprene and dienes obtained using X or XIa have the uncyclized random repeating units. In contrast, the copolymerization of dienes with isoprene using the catalyst XIc affords alternating copolymers. The cyclohexylene repeating units in the isoprene/1,6-heptadiene copolymer is obtained with high stereocontrol (cis structures up to 99%). The alternating copolymer showed better mechanical properties than the corresponding random copolymer. The copolymer with cyclohexylene repeating units showed much higher stiffness and strength than that with cyclopentylene repeating units. The N-heterocyclic carbene scandium complex (XII) brings about the cyclocopolymerization of 1-hexene with 1,5-hexadiene to afford atactic copolymers containing 26.6− 98.6 mol % of the repeating units derived from 1,5-hexadiene.167

Figure 46. Dinuclear dititanium complex XX.240

the corresponding mononuclear titanium complex (VIII; incorp. diene = 3.2 mol %), the dinuclear complex incorporates more amount of the repeating unit derived from 1,5-hexadiene (incorp. diene = 8.8 mol %). The nuclearity of the catalyst slightly affect the cis/trans selectivity of the cyclopentylene units (cis:trans = 48:52 for XX, 30:70 for VIII). IX/[Ph3C][B(C6F5)4] is also effective for the cyclocopolymerization of 1,5-hexadiene with styrene (Figure 47).166 The

5.2. Cyclocopolymerization of Butadiene with Ethylene

1,3-Butadiene does not undergo cyclopolymerization, but copolymerization of ethylene with butadiene can lead to the polymer containing cycloalkane groups as the repeating units in the macromolecular chains. Ethylene/butadiene copolymerization catalyzed by the C2 symmetric zirconocene catalyst (rac)CH2(Ind)2ZrCl2/MAO affords the polymer with 1,2-cyclopentylene groups.242 The cyclopentane group is formed via 1,2insertion of butadiene and ethylene insertion followed by cyclization via 1,2-insertion of the vinyl group. Ethylene/ butadiene copolymers obtained by bulky C2 symmetric catalyst XXI contain 1,2-cyclopropylene in addition to 1,2-cyclopentylene repeating units (Figure 48). The stereochemistry of both of the 1,2-substituted cycloalkylene groups are controlled in trans-structure predominantly. The cyclopropane group is formed by the cyclization before the ethylene insertion. The mechanism for the formation of five- and three-membered ring in the ethylene/butadiene copolymerization is supported by DFT studies.243 The predominant 1,2-insertion of 1,3-butadiene in these zirconocenes is noteworthy because it generally undergoes 2,1-insertion to give π-allyl metal species, which leads to 1,4- or 1,2- repeating units without cyclization. The cyclopropanes group in the polymer can be used for thermal cross-linking.244 Ethylene/butadiene copolymerization catalyzed by C1 symmetric zirconocene Me2Si(Ind)(Flu)ZrCl2/MAO affords the polymer with trimethylene-1,2-trans-cyclopentylene repeating units predominantly (Figure 48).245 The repeating structure is produced by the dual-site polymerization mechanism, which consists of alternating reactions of vinyl groups of 1,3-butadiene and ethylene with the active chain end. Less bulky (rac)Me2Si(Ind)2ZrCl2 or more bulky (rac)-Me2Si(Flu)2ZrCl2 incorporate much less amount of butadiene. Thus, the bulkiness of the catalyst is of key importance for the alternating cyclocopolymerization. The cyclocopolymerization of ethylene with butadiene by neodymium catalysts has bee also reported. The structure of the produced copolymers is quite different from those obtained by

Figure 47. Cyclocopolymerization of styrene or isoprene with 1,5hexadiene or 1,6-heptadiene by Sc catalysts.165

resulting styrene/1,5-hexadiene copolymer has an unique structure composed of syndiotactic polystyrene blocks and polydiene blocks. 1,5-Hexadiene is predominantly incorporated as methylene-1,3-cyclopentylene repeating units but 2-vinyl-1,4butylene units are also contained. The terpolymerization of the dienes, styrene, and ethylene is also possible. The composition of the repeating units from the monomers can be varied widely by changing the monomer feed ratio. Application of IX/ [Ph3C][B(C6F5)4] to the cyclocopolymerization of 1,6heptadiene with styrene leads to the mixture of both 9014


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the zirconocene catalysts. Me2Si(Flu)2NdCl promotes the cyclocopolymerization of ethylene with butadiene in the presence of n-BuLi/i-Bu2AlH, n-BuMgCl, or (n-Bu)(n-Oct)Mg.246,247 The produced copolymer contains 6−11.8 mol % of the repeating units derived from butadiene, and over 50% of the butadiene is incorporated as 1,2-cyclohexylene repeating units (Figure 49). The copolymer obtained using XXII/(n-Bu)(nOct)Mg contains 1,4-trans-cyclohexylene (9.6−15.9 mol %) in addition to cis- and trans-1,2-cyclohexylene repeating units (cis: 4.2−5.2 mol %, trans: 34.3−41.9 mol %).248,249 The 1,2cyclohexylene repeating units are formed by 2,1-insertion of butadiene and the insertion of two ethylene molecules, followed by the cyclization via 1,2-insertion of the vinyl groups. When βhydrogen transfer of the intermediate takes place before the cyclization, the subsequent 1,2-insertion of the vinyl group leads to 1,4-cyclohexylene repeating units. 5.3. Cyclocopolymerization of Dienes with Olefins by Co, Pd, and Ni Catalysts

The bis(imino)pyridine cobalt catalyst XIV/MMAO is effective for the cyclocopolymerization of ethylene with 1,6-heptadiene, giving the copolymer with ethylene-trans-1,2-cyclopentylene repeating units derived from 1,6-heptadiene.250 The content of the repeating units from 1,6-heptadiene in the copolymer can be varied 3−50 mol%. The copolymer with 3 mol% of the ethylenetrans-1,2-cyclopentylene unit shows Tm at 116 °C, and that with 50 mol% of the unit does not show only Tg at −25 °C. In contrast, the attempted copolymerization of ethylene with 1,6heptadiene using the bis(imino)pyridine iron catalyst XIII/ MMAO produces a mixture of both homopolymers. This result

Figure 48. Cyclocopolymerization of ethylene with 1,3-butadiene by zirconocene catalysts and the mechanism for formation of three- and five-membered rings.242,245

Figure 49. Cyclocopolymerization of ethylene with 1,3-butadiene by nedodymium catalysts and the mechanism for formation of 1,2- and 1,4cyclohexylene groups.246−249 9015


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mer of ethylene with 1,6-diene is highly branched, but its degree of branching is lower than that of the homopolyethylene obtained using the same catalyst.252 In fact, the trans-1,2cyclopentylene units in the copolymer effectively block chainwalking of the Pd center along the polymer chain. The phosphine-sulfonate Pd complex XVII (L = pyridine) promotes the cyclocopolymerization of ethylene with allyl acrylate in toluene at 85 °C (Figure 51).253 The amount of the allyl acrylate unit in the copolymer is up to 6.4 mol %, and the cyclization efficiency of 41−61%. The resulting polymer contains five- and six-membered lactone structures in the main chain, with a selectivity for the five-membered with respect to the six-membered lactone of 58−67%. Both allyl and acrylic groups of the monomer can insert into the polymer chain end. Allyl groups are considered to undergo 1,2-insertion, whereas acrylic group undergoes 2,1-insertion. Cyclization takes place via 1,2-insertion of acrylic or allyl groups. Thus, when allyl and acrylic groups of the monomer insert first, six- and fivemembered lactone structure form, respectively. Phosphine-sulfonate Pd complex XVII (L = DMSO) promotes cyclocopolymerization of ethylene with acrylic anhydride,254 diallyl ether,193 and divinyl formal (Figure 51).195 The cyclocopolymerization of ethylene with acrylic anhydride by XVII proceeds in toluene at 65−95 °C. The incorporation of the monomer reaches up to 25.1 mol %, and the monomer is predominantly introduced as the repeating unit with trans-fused five-membered cyclic anhydride groups (selectivity = 84.7−89.0%), which is formed by the 2,1-insertion of an acrylic group followed by cyclization via 1,2-insertion of the other acrylic group. It is noteworthy that the molecular weight of the resulting polymer is much higher than that of ethylene/methyl acrylate copolymers obtained in similar conditions: the β-hydrogen elimination is suppressed after the

is ascribed by the heterogeneity of the active species in the polymerization. The phenoxyimine dinickel complex XXIII (Figure 50) does not bring about the cyclopolymerization of nonconjugated

Figure 50. Dinuclear dinickel catalyst XXIII.251

dienes, but it is active for the cyclocopolymerization of ethylene with 1,6-heptadiene and with 1,7-octadiene.251 The resulting polymers have 1,2-trans-cyclopentylene repeating units, regardless of the diene used. In the case of 1,7-octadiene, migration of the Ni center may take place before the cyclization to form the five membered rings. As the ethylene/1-hexene copolymerization using the dinuclear complex or the ethylene/diene copolymerization using the corresponding mononuclear complex does not proceed, the combination of the dinuclear complex and a nonconjugated diene is essential for the efficient incorporation of the comonomer into the resulting cyclopolymer. Diimine Pd complexes/NaBARF promote the cyclocopolymerization of 1,6-dienes with ethylene, α-olefins, and cyclopentene.179−182 The copolymerization proceeds with complete cyclization of the dienes and the resulting polymers contain ethylene-trans-1,2-cyclopentylene repeating units. The copoly-

Figure 51. Cyclocopolymerization of ethylene with allyl acrylate, acrylic anhydride, diallyl ether, and divinyl formal by XVII.193,195,253,254 9016


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cyclizative insertion of acrylic anhydride. Diallyl ether undergoes cyclocopolymerization with ethylene by XVII to give the polymer having 2.3−20.4 mol % of the repeating units derived from diallyl ether.193 Similarly to its homopolymerization, the monomer is mainly introduced in the polymer as ethylene-1,2trans-4-oxacyclopentylene repeating units (56.5−82.6% of the total units from diallyl ether). XVII is also effective for the copolymerization of ethylene with divinyl formal (up to 12.5 mol % of the total incorporation). The divinyl formal is introduced as repeating units with 1,3-dioxolane groups or 1,3dioxane groups. The stereochemistry of the 1,3-dioxolane group is stereocontrolled toward the trans isomer exclusively (>98%), whereas the cis stereoisomer is favored for the 1,3-dioxane group. NMR studies reveal that the divinyl formal units mainly undergo 2,1-insertion to Pd−H bonds and the subsequent cyclization leads to the 1,3-dioxolane group. On the other hand, 1,2-insertion of the divinyl formal is favored for Pd-alkyl species, which leads to the 1,3-dioxane group. Thus, the units with 1,3dioxolane groups mainly locate on the initiating ends. The 1,3dioxane is the more abundant structure in-chain, and the ratio between 1,3-dioxane and 1,3-dioxolane units increases as the incorporation of divinyl formal increases. Recently, Ni complexes with IzQO ligand (XXIV; Figure 52) were reported to catalyze the copolymerization of ethylene with

Figure 53. Copolymerization of nonconjugated dienes with CO by Pd catalysts.259,261

propane (dipp) produces a polymer having 6-membered cyclic ketone repeating units predominantly. The cis/trans ratio of the cyclic units is approximately 1:3, and treatment of the polymer with 4-(dimethylamino)pyridine (DMAP) in refluxing CHCl3 renders epimerization of the less stable cis isomer into the thermodynamically stable trans isomer. [Pd(dppp)(CH3CN)2](BF4)2 is also effective for the cyclocopolymerization of 1,5hexadiene with CO to give the polymer having both 5- and 6membered cyclic ketone repeating units.260 The alternating copolymerization of 1,7-octadiene and 1,6-heptadien-4-ol with CO by the Pd catalyst proceeds without cyclization to give the polymer having pendant olefins. The cyclocopolymerization of 1,4-pentadiene and 1,5hexadiene with CO proceeds in the presence of Pd complex catalyst with (R,S)-BINAPHOS ((R)-2-(diphenylphosphino)1,1’-binaphthalen-2′-yl (S)-1,1′-binaphthalene-2,2′-diyl phosphite) ligand (XXV).261 The resulting copolymers have the 5and 6-membered cyclic ketone repeating units, respectively, owing to the high regioselectivity in insertion of the olefin in the catalyst. cis/trans ratio of the cyclic ketone units is almost 1:1. Both of the polymers are optically active with the molar optical rotation of the 1,5-hexadiene/CO copolymer ([Φ]20D = 13.8 (c = 0.50, CH2Cl2) being much larger than that of the 1,4pentadiene/CO copolymer ([Φ]20D = 0.37 (c = 0.50, CH2Cl2).

Figure 52. Ni complexes with IzQO ligand (XXIV).255

various polar comonomers. The catalysts are also effective for the copolymerization of ethylene with diallyl ether, affording the copolymer having repeating units from the comonomer (1.9 and 0.98 mo l%, respectively, for XXIVa and XXIVb).255 Diallyl ether is incorporated as methylene-1,3-(5-oxacyclohexylene), ethylene-1,3-(4-oxacyclopentylene), and uncyclized repeating units, and their molar ratio in the copolymer is approximately 2:1:1. 5.4. Cyclocopolymerization of Dienes with CO

Pd catalysts have been known to promote copolymerization of olefins with CO to give the corresponding polyketones.256−258 The cyclocopolymerization of nonconjugated dienes with CO has been also reported. The Pd catalyst composed of Pd(OAc)2, 1,3-bis(diphenylphosphino)propane (dppp), 1,4-naphthoquinone, and Ni(ClO4)2·6H2O bring about the cyclocopolymerization of 1,5-hexadiene with CO to give a polymer containing 5and 6-membered cyclic ketones in the polymer backbone (Figure 53).259 Those 5- and 6-membered cyclic ketone repeating units are formed via 2,1- and 1,2-insertion of the diene, insertion of CO, and cyclization via 1,2-insertion of the other CC bonds. The Pd catalyst with optically active (S,S)DIOP ((2S,3S)-(+)-2,3-O-isopropylidene-2,3-dihydroxy-1,4bis(diphenylphosphino)butane) or (S)-BDPP (2,4-bis(diphenylphosphino)pentane) also affords polymers with similar microstructures, which show small optical rotation. In contrast, the Pd catalyst with 1,3-bis(diisopropylphosphino)-

6. CYCLOPOLYMERIZATION OF DIYNES, ENEYNES, AND BIS(CYCLOOLEFIN)S The cyclopolymerization of diynes is noteworthy because the resulting polymers have π-conjugated structures on their main chains and interesting physical, electrical, and optical properties. The cyclic structure of the repeating units can enable higher effective conjugation length compared to polyalkynes obtained by the polymerization of acetylene derivatives.262 Similarly to the cyclopolymerization of dienes, cyclopolymerization of 9017


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Figure 54. Microstructures of poly(1,6-heptadiyne).

Figure 55. Mechanism for cyclization of 1,6-heptadiynes in their cyclopolymerization.

Chart 5. Molybdenum and Tungsten Imido Alkylidene Complex Catalysts for Cyclopolymerization of Diynes

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Chart 6. Diynes Used in Cyclopolymerization Catalyzed by Schrock-Type Molybdenum and Tungsten Imido Alkylidene Complex Catalysts

Table 3. Cyclopolymerization of Diynes by Schrock-Type Molybdenum Imido Alkylidene Catalysts monomer

catalyst

101a 101a 101a 105a 106 107 108 109 111 111 114 115 118 101a 116 117a

XXVIa XXVIIb XXVIIIa XXVIb XXIX XXIX XXVIIIa XXVIIIa XXVIIIa XXXa XXXa XXXa XXXa XXXIb XXXb XXXII

additives

quinuclidine

quinuclidine quinuclidine quinuclidine quinuclidine

solvent

ring (insertion)

Mn

PDI

remark

ref

DME toluene CH2Cl2 CH2Cl2 CH2Cl2 THF THF THF THF 1,2-dichloroethane 1,2-dichloroethane 1,2-dichloroethane 1,2-dichloroethane CH2Cl2 CHCl3 CHCl3

5 and 6 6 5 5 and 6 6 (α = 90%) 6 (α > 94%) 6 6 6 5 (α > 96%) N.D. N.D.

11 600 99 470 30 420 19 500 16 000 57 000 20 000 25 000 19 700 13 200 6500 1100 2600 84 000 6200 20 300

1.16 1.15 1.35 1.05 1.6 1.3 1.4 1.2 1.9 1.9 1.3 1.15 1.3 2.3 1.5 1.6

λmax = 552 nm, living polymerization λmax = 534 nm λmax = 591 nm λmax = 556 nm λmax = 458 nm λmax = 485 nm, living polymerization

265 267 271 275 276 276 277 277 277 278 278 278 278 279 279 279

5 (α ≥ 99%) N.D. 5 (α ≥ 96%)

diynes can afford polymers containing cycloalkenes with different ring sizes as repeating units, depending on the regioselectivity in the addition of the carbon-carbon triple bonds during propagation. In the case of 1,6-heptadiyne, for example, polymers containing 5- and/or 6-membered rings are formed (Figure 54). Figure 55 shows the mechanism of formation of the 5- and 6-membered ring repeating units in the cyclopolymerization of 1,6-heptadienes using metal− alkylidene catalysts. α- and β-additions of the metal−alkylidene to the carbon-carbon triple bonds of the monomer afford 5- and

λmax = 484 nm λmax = 554, 593 nm λmax = 432

6-membered ring repeating units, respectively. trans-cis selectivity in CC bond formation is another issue to be considered in the cyclopolymerization of diynes. Previously reported catalysts for the cyclopolymerization of diynes include Ziegler-type catalysts, MoCl5, WCl6, and Schrock-type alkylidene catalysts which, however, lacked in selectivity in ring size formation of the repeating units. Recent design of new complex catalysts enabled regio- and stereoselective cyclopolymerization of diynes.263,264 9019


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6.1. Cyclopolymerization of Diynes Using Mo and W Complex Catalysts

Recently, Buchmeiser and Nuyken reported that only 5membered ring repeating units can be selectively formed in the cyclopolymerization of 101a by the Schrock-type molybdenum catalysts having tert-butoxy ligands (XXVIII), in combination with quinuclidine at 25 °C (Figure 56).270−274 In the absence of quinuclidine, the catalyst 3a selectively affords polymers with 5membered ring repeating units at −30 °C, but the polymer obtained at 25 °C contains 9% of 6-membered ring repeating units. The polymer obtained using the Mo complexes having less hindered hexafluoroisopropoxy or isopropoxy group such as XXIX in the presence of quinuclidine at 25 °C also contained the 6-membered ring repeating units (15−39%). 1,6-Heptadiyne with only one ester group, 102a, affords the polymer containing 77% five membered rings using XXVIIIa/quinuclidine at −30 °C.271 Selective polymerization of 1,6-heptadiyne having a chiral menthyloxycarbonyl group (103) can be achieved by XXVIIIa in the absence of quinuclidine at −30 °C, whereas the polymerization of 103 does not proceed in the presence of quinuclidine. This result shows the importance of the steric balance between the monomer and the catalyst. The low selectivity of 102 compared to 101 can be ascribed by the low steric demands of the monomer. The polymer of 1,6-heptadiyne having chiral menthyloxycarbonyl group and ethyl group (104), obtained by XXVIIIa/quinuclidine, allows determination of microstructure of the polymer. Detailed NMR analysis of the polymer obtained by XXVIIIa/quinuclidine shows a quantitative trans structure of the vinylene groups of the polymer. The tacticity of the polymer is also controlled, although the assignment of the tacticity is not clear (isotactic stereochemistry is speculated). Attempted cyclopolymerization of dipropargylammonium salt 105a using XXVIIIa does not afford a polymer, whereas XXVIb promoted the cyclopolymerization of 105a to give the corresponding polymer containing both 5- and 6membered ring repeating units.275 The molybdenum catalysts XXVIII and XXIX, in the presence or absence of quinuclidine, promote the cyclopolymerization of 1,7-octadiynes (106 and 107), with quantitative cyclization (Figure 57).276 The polymer of 106 obtained using XXIX is

Charts 5 and 6 show Schrock type molybdenum and tungsten imido alkylidene complex catalysts utilized in the cyclopolymerization and diynes that can be polymerized by the catalysts. Representative examples are summarized in Table 3. Schrock-type molybdenum imido alkylidene catalysts, such as XXVI, have been known to promote the cyclopolymerization of 1,6-heptadiynes.262,263 The polymerization proceeds in a living manner, and polymers with narrow polydispersity can be synthesized.265,266 The degrees of polymerization of the resulting polymers are slightly higher than the monomer-toMo molar ratio, and a faster rate of propagation relative to initiation was suggested. The resulting polymers contain both 5and 6-membered ring repeating units, as a result of a low selectivity between α- and β-addition of the active species to the monomer. It was later found that similar Schrock-type molybdenum catalysts, having bulky triphenylacetate ligand, such as XXVII, promote the cyclopolymerization of diethyl dipropargylmalonate (101a), to give the polymers containing only 6-membered ring repeating units (Figure 56).267 The cis/trans ratio of the

Figure 56. Selective cyclopolymerization of 101a.267,269−271

exocyclic double bonds and s-cis/s-trans conformation of the backbone single bonds could not be determined by NMR analysis. Narrow molecular weight distribution of the resulting polymers and a linear relationship between monomer-tocatalyst/initiator ratio and the molecular weight of the polymers indicate living character of the polymerization process. Although the molecular weight of the polymers estimated by GPC (polystyrene standard) are much higher than the calculated value by a factor of 4, the values estimated by MALDI-TOF almost agree with the calculated values. This inconsistency in the molecular weight estimated by GPC and MALDI-TOF is probably due to the rigid structure of the polymer compared to polystyrene. By utilizing the Mo-catalyzed living polymerization of 1,6-heptadiyne, very long polyenes (up to 1100 carboncarbon double bonds) could be synthesized.268 Third-order polarizabilities (γ) of the polymer saturate around 60 double bonds. The molybdenum catalyst also promotes copolymerization of 1,6-heptadiyne with acetylene.269

Figure 57. Stereoselective cyclopolymerization of 1,7-octadiynes.276

mainly composed of 6-membered repeating units (90%). The content of the 7-membered ring repeating units increases to 20% in the polymerization using XXIX/quinuclidine, with polydispersities in the range 1.5−1.9. The polymer having 6membered ring repeating units can be selectively synthesized (>94%) by the cyclopolymerization of 107 using XXIX, XXIX/ quinuclidine or XXVIII/quinuclidine. The polymerization of 9020


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Table 4. Cyclopolymerization of Diynes by Mo- or W-Based Three- or Four-Component Catalysts monomer

catalyst

ring (insertion)

Mn

PDI

remark

ref

101a 101a 101b 101b 120 121 122 123

MoCl5-n-Bu4Sn-EtOH-qunuclidine (1:1:5:1) MoOCl4-n-Bu4Sn-EtOH-qunuclidine (1:1:2:1) MoCl5 MoCl5-n-Bu4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn

5 (>95%) 5 (>95%) 5 (>95%) 5 (>95%) 6 6 6 6

20 300 19 600 12 500 13 500 26 500 6400 11 500 26 900

1.68 1.75 2.05 1.95 2.3 2.5 2.0 3.1

λmax = 587 λmax = 587 sc CO2 as solvent sc CO2 as solvent

282 282 283 283 285 285 285 285

molecular weight of the polymers estimated by GPC (Mn = 22 000 (Mw/Mn = 1.5) and Mn = 26 000 (Mw/Mn = 1.6), respectively) is higher than the theoretical value (Mn(theo) = 7100), due to the rigid rod structure of the polymer.

107 using XXIX/quinuclidine proceeds in living fashion to give polymers with narrow molecular weight distributions (1.3). Similar 1,7-octadiynes having ester groups (108−110) also undergo cyclopolymerization using XXVIIIa in the presence or absence of quinuclidine.277 Narrow molecular weight distribution of the polymers (1.2−1.9) and linear relationships between Mn and monomer-to-Mo ratio demonstrate the living nature of the polymerization. The higher molecular weight of the polymer estimated in GPC compared to the value calculated from monomer-to-Mo molar ratio is ascribed by the larger propagation rate compared to initiation rate and to the rodlike structure of the polymer. The polymerization obeys first order kinetics with respect to the monomer concentration. Block copolymers of 1,7-octadiyne (110) with 1,6-heptadiyne (101a) was synthesized by utilizing the living polymerization. NMR and IR analysis of the polymers of the 1,7-octadiynes obtained using XXVIIIa/quinuclidine indicate that the polymers are selectively synthesized with the six-membered ring repeating units, and the vinylene groups are also stereoselectively achieved in trans structure. 13C NMR of the polymer obtained from the 108, which is a mixture of meso- and racemo-diastereomers 109 and 110, shows almost statistical incorporation of the diastereomers. Enantiopure chiral 1,7-octadiyne (112) with menthyl groups affords the polymer having highly controlled trans-disyndiotactic structure, as confirmed by 13C NMR, whereas that of the polymer obtained from rac-113 is trans-atactic. Molybdenum imido alkylidene N-heterocyclic carbene complexes (XXX−XXXII) show high functional group tolerance in the cyclopolymerization of 1,6-hepta- and 1,7octadiynes.278,279 For example, the complex XXXa promotes the cyclopolymerization of the monomers having unprotected hydroxy groups (114) or carboxy groups (116 and 118). No high-quality 13C NMR spectrum of those polymers with hydroxy or carboxy groups is available, probably because of the severe aggregation of those functional groups. 1,6-Hepta- and 1,7octadiynes with wide varieties of functional groups can be polymerized using the Mo catalysts. The polymerization generally proceeds via selective α-addition of the catalyst to the monomer, and the polymers containing 5- and 6-membered rings are obtained from 1,6-hepta- and 1,7-octadiynes, respectively. NMR analysis of the polymer formed from 112 using XXXb reveals a highly regular trans-disyndiotactic structure of the polymer (α > 96%, syndioselectivity > 96%).280 Quite recently, Buchmeiser reported that cationic tungsten imido alkylidene complexes with N-heterocyclic carbene ligands (XXXIII−XXXVII) bring about the cyclopolymerization of functionalized 1,6-diynes such as 104, 105, and dipropargyl sulfide.281 The tolerance of the cationic tungsten catalyst toward the protic groups is remarkable. The resulting polymers are mostly insoluble, but the polymer formed from 115 using XXXIII and XXXVI is soluble in N,N-dimethylacetamide. The

6.2. Cyclopolymerization of Diynes Using Mo- or W-Based Three- or Four-Component Catalysts

The cyclopolymerization of diynes by binary/ternary Mo- and W-based catalysts has been known to give the insoluble polymers and/or with less-controlled repeating structure. Recently, some binary/ternary Mo- and W-based catalysts were reported to be effective for the stereoselective cyclopolymerization of diynes (Table 4). Buchmeiser reported that MoCl5-n-Bu4Sn-EtOH-quinuclidine systems and MoOCl4-n-Bu4Sn-EtOH-quinuclidine systems can promote the selective cyclopolymerization of 101a to afford five-membered ring repeating units.282 The polydispersity of the produced polymer is moderately narrow (1.13−1.95), and a quasilinear relationship between Mn and monomer-to-initiator molar ratio is observed. The initiation efficiency of the MoOCl4based catalyst is high (91%). The selective cyclopolymerization of dimethyl dipropargylmalonate (101b (R = Me)) can be also achieved using MoCl5 or MoCl5/n-Bu4Sn as the catalysts and supercritical carbon dioxide as the polymerization medium.283 The resulting polymers contain >95% 5-membered ring repeating units, which is in contrast with the lower selectivity (65−73%) achievable in the polymerization in organic solvents (1,4-dioxane, CH2Cl2, and CF3CH2F). CO2 molecules are proposed to act as the ligand to the Mo atom, which enables selective α-addition of the alkyl group to the polymer chain end. Binary Mo- and W-based catalysts bring about the cyclopolymerization of unique monomers containing internal alkyne groups. MoCl5-EtAlCl2 promotes the double cyclizative polymerization of 4,10-bis(diethyl malonate)-1,6,11-dodecatriyne (119) (Figure 58).284 The resulting polymer is soluble in

Figure 58. Double cyclizative polymerization of triyne.284

solvents such as CHCl3, chlorobenzene, and DMF, and the cyclization takes place quantitatively, as confirmed by the absence of pendant terminal triple bonds in the polymer (NMR). The polymer is proposed to have fused bicyclic groups with conjugated double bonds. The polymer shows a photoluminescence peak at 530 nm, corresponding to a band gap 9021


Chemical Reviews

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6.3. Cyclopolymerization of Diynes Using Ru Catalysts

energy of 2.34 eV, and exhibits irreversible electrochemical behaviors. The cyclopolymerization of 1-phenyl-7-aryl-1,6-heptadiynes (120 − 123) proceeds by using WCl6-Ph4Sn as the catalyst (Figure 59).285 The resulting polymers have six-membered rings

Molybdenum catalysts show high activity for stereoselective cyclopolymerization of diynes, but their air- and moisturesensitive character often limits their application to monomers with wide varieties of functional groups. Ruthenium catalysts have been known to be air- and moisture-stable, and extensively used for olefin metathesis.286−288 An earlier attempt to use commercially available Grubbs catalysts (XXXVIII and XXXIXa) for polymerization was not successful, because of severe side reactions (vide infra).289 Modification of the catalyst and/or the reaction condition enables stereoselective cyclopolymerization of diynes by Ru catalysts. Chart 7 shows the Ru catalysts utilized in the cyclopolymerization of diynes. 6.3.1. Cyclopolymerization of 1,6-Heptadiynes Using Grubbs Catalysts. Although classical Grubbs catalysts have been believed not to be effective as catalysts for the cyclopolymerization of diynes, Choi found that the third generation Grubbs catalyst (XLa) efficiently promotes the living polymerization of various 1,6-heptadiyenes such as 101c.290 Chart 8 shows 1,6-heptadiynes used in the cyclopolymerization using Grubbs catalysts. Table 5 lists representative results of the cyclopolymerization. The reaction solvent is critical in order to achieve smooth polymerization. The polymerization rate is slow in CH2Cl2, whereas fast polymerization takes place in THF, which is a

Figure 59. Cyclopolymerization of 1-phenyl-7-aryl-1,6-heptadiynes by WCl6/SnPh4.285

exclusively. The monomers with longer oligomethylene spacers (1,10-undecadiyne and 1,11-dodecadiyne) do not afford polymers. The poly(1-phenyl-7-aryl-1,6-heptadiyne)s are soluble in organic solvents and thermally stable (no decomposition takes place up to 350 °C). Chart 7. Ru Catalysts Used for Cyclopolymerization of Diynes

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Chart 8. 1,6-Heptadiynes Used in Cyclopolymerization by Grubbs Catalysts

CH2Cl2 or in CHCl3, and it is slow in THF. A radical mechanism is proposed for the transition. The successful cyclopolymerization of 1,6-heptadiene in THF is ascribed to the stabilization of the active species by the weak coordination of THF to the Ru center, to suppress the decomposition of the growing terminal chain. The reaction of 1,6-heptadiyne using the second generation Grubbs catalyst (XXXVIII) or the second generation Hoveyda−Grubbs catalyst (XXXIX) in CH2Cl2 leads mainly to [2 + 2+2] cycloaddition of the diyne, probably due to a Ru species formed by the decomposition of the Ru carbene.293 It was found that such [2 + 2+2] cycloaddition can be suppressed, and the cyclopolymerization of 1,6-heptadiyne takes place using the second generation Hoveyda−Grubbs catalyst (XXXIX) in the presence of weakly coordinating 3-chloropyridine or 3,5-dichloropyridine even in CH2Cl2 as the solvent. This result confirms the importance of the stabilization of the growing species by the coordination of a weak Lewis base for the efficient cyclopolymerization of 1,6heptadiynes. The first generation Grubbs catalyst (11) has been believed to be inactive for the cyclopolymerization of 1,6heptadieynes. Recently, however, it was demonstrated that the

weakly coordinating solvent. The living character of the polymerization was confirmed by low polydispersities (Đ < 1.44), a linear relationship between monomer-to-initiator and Mn, and a relatively large ki/kp value (0.84). Later, it was demonstrated that the addition of weakly coordinating additives such as 3,5-dichloropyridine in the cyclopolymerization using third generation Grubbs catalyst in CH2Cl2 is also effective for achieving living character.291 The use of CH2Cl2 as solvent is beneficial because it can dissolve the produced conjugated polymer better than THF. The vinylene group of the polymer of 101c obtained by XLa in THF is mostly controlled in trans (84%). Due to the presence of a small amount of cis-vinylene group, the conjugated backbone of the polymer is partly twisted and the coplanarity of the polymer is relatively low. Upon standing the polymer solution, however, cis-to-trans isomerization of the vinylene group takes place, and the polymer undergoes transition from a coil-like structure to a rod-like structure. 292 Increased conjugation length of the all-trans polymer was confirmed by UV/vis spectrum. The rod-to-coil transition is very fast in 9023


Chemical Reviews

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Table 5. Cyclopolymerization of 1,6-Heptadiynes by Grubbs Catalysts monomer

catalyst

101c 117b 124 126a 101a 117c 125 127 128a 128b 101e 101c 102b 117b 125 126b 126c 128a 129a 130 131 132 133 134a 134c 136 137 138a 138b 139a 139b 140a 140b 141

XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XLI XLI XLI XLI XLI XLI XLI XLI XLI XLb XLb XLb XLa XLa XXXIX XXXIX XLa XLa XLb XLb XLb XLb XLb

additives

3,5-dichloropyridine 3,5-dichloropyridine 3,5-dichloropyridine 3,5-dichloropyridine 3,5-dichloropyridine 3,5-dichloropyridine sodium benzoate sodium benzoate sodium benzoate sodium benzoate sodium benzoate sodium benzoate sodium benzoate sodium benzoate sodium benzoate

3,5-dichloropyridine

solvent

Mn

PDI

remarks

ref

THF THF THF THF CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 THF/[BMIm][PF6] THF THF THF THF THF THF THF THF THF THF THF THF CH2Cl2

40 600 46 700 34 900 39 600 49 900 40 700 39 800 28 500 28 300 42 200 15 700 23 300 7000 9000 11 500 8200 8700 7400 9800 7200 31 000 185 700 32 100 4600 15 900 424 000 583 000 57 700 66 600 58 300 67 500 62 700 47 000 135 100

1.16 1.28 1.15 1.20 1.16 1.15 1.13 1.17 1.26 1.18 1.38 1.53 2.13 1.71 1.77 1.59 1.55 1.57 1.58 1.32 1.05 1.21 1.40 1.17 1.18 1.47 1.42 1.18 1.15 1.27 1.27 1.38 1.15 1.42

living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization living polymerization

290 290 290 290 291 291 291 291 291 291 291 294 294 294 294 294 294 294 294 294 295 296 296 297 297 299 299 290 290 301 301 301 301 302

first generation Grubbs catalyst (XLI) actually promotes the cyclopolymerization of the monomers in the presence of benzoic acid or sodium benzoate.294 Benzoic acid accelerates phosphine dissociation and stabilizes the growing terminal chain, whereas sodium benzoate exchanges with the chloride ligand to give more active species. The third generation Grubbs catalyst (XLa) enables the living cyclopolymerization of 1,6-heptadiynes having one or two functional groups on the 4 position. 1,6-Heptadiynes having various ester (101, 117, 124, and 125) and ether (126−128) functional groups can be utilized.290,291 The cyclopolymerization of 1,5-heptadiynes with amide groups (129 and 130) can be achieved by first generation Grubbs catalyst (11) in the presence of additives.294 Cyclopolymerization of 101e having bulky dimethyl-substituted carbon by XLa proceeds in the absence of benzoic acid or sodium benzoate in CH2Cl2.291 The polymerization takes longer reaction times when compared to the cyclopolymerization of 101a. The narrow molecular weight distribution of the polymer indicates that the chain transfer is suppressed by the bulky chain end. The cyclopolymerization of imidazolium-functionalized 1,6heptadiyne (131) was achieved by second or third generation Grubbs catalyst (XXXVIII and XLa).295 The polymerization proceeds smoothly when THF/1-butyl-3-methylimidazoium tetrafluoroborate ([BMIm][PF6]) (1/2) is used as the solvent.

living polymerization, λmax = 536 λmax = 595 λmax = 605 living polymerization living polymerization

living polymerization living polymerization

The vinylene group of the polymer is controlled in trans (>95%). The molecular weight of the polymer increases linearly to the monomer-to-initiator molar ratio, and its distribution is narrow (D < 1.10). By utilizing the cyclopolymerization 1,6-heptadiynes, rigid linear conjugated polymers with various π-conjugated side groups have been synthesized, which demonstrate unique selfassembled structures. A polymer with triphenylamine moieties can be synthesized by cyclopolymerization of the corresponding 1,6-heptadiynes (132 and 133), catalyzed using third generation Grubbs catalyst (XLb).296 The polymer exhibited good solubility, excellent optoelectronic properties (λmax = 605 nm (CHCl3), HOMO level = −5.04 eV, and energy bandgap = 1.77 eV), stability toward oxidation, and a high fluorescence quantum yield (12.3%). The polymer forms nanocylinder architectures via π−π interaction of the triphenylamine side groups. Polymers having fluorene moieties, obtained by the cyclopolymerization of dipropargylfluorene derivatives 134, undergo self-assembly to form large-area two-dimensional fluorescent semiconducting nanostructures.297 2D nanosheets with different shapes (rectangle, raft, and leaf) can be obtained when dissolving the polymer in different solvents. An orthorhombic crystalline packing model is proposed for the formation of the 2D nanostructures. The third generation Grubbs catalyst-mediated cyclopolymerization of bis(1,6-heptadiyne) having perylenedii9024


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Figure 60. Successive cyclopolymerization and ADMET to form bridge-like polymer.298

with AFM. It revealed the rod-like conformation of the polymer and the structure of a single molecular wire surrounded by insulating dendrons. Polymers with branched or dendronized triazole groups and oligo(ethylene oxide) chains can be synthesized by the cyclopolymerization of the corresponding 1,6-heptadiyne (139−141) using the third generation Grubbs catalyst (XLb).300,301 The polymers of 139b and 141 doped with LiTFSI displayed ionic conductivities with values of 4.3 × 10−6 and 2.5 × 10−6 S cm−1, and further doping of the latter polymer with iodine showed improved ionic and electronic conductivities with values of 1.3 × 10−5 and 2.1 × 10−7 S cm−1, respectively. Quite recently, third generation Grubbs catalyst was applied for double cyclizative cyclopolymerization of tetraynes (142; Figure 61).302 The polymerization proceeds via olefin metathesis and metallotropic 1,3-shift, and the resulting polymers have fully conjugated main chain structures. The polymerization rate of 142a is comparable to the cyclopolymerization of 1,6heptadiynes, a strong indication that the metallotropic 1,3shift is a fast transformation. The polymerization proceeds in a living manner in the presence of 3,5-dichloropyridine to give the polymer with controlled molecular weight and narrow molecular weight distribution. 6.3.2. Cyclopolymerization of 1,7-Octadiynes and 1,8Nonadiynes. Compared to 1,6-heptadiynes, the cyclopolymerization of 1,7-octadiynes and 1,8-octadiynes by Grubbs catalysts is more difficult to proceed, because the longer interval between the two alkyne groups results in slow cyclization. Choi found that the introduction of quaternary carbons with bulky substituents in 1,7-octadiyne enables its smooth cyclopolymerization as a result of an enhanced Thorpe−Ingold effect.303 Chart 10 shows the 1,7-octadiynes that undergo cyclo-

mide spacers (135) in the presence of 1,4-bis(undecenyloxy)cis-2-butene in THF at 30 °C affords double-stranded polymers having two terminal alkenyl groups (Figure 60).298 The produced polymer is further subjected to acyclic diene metathesis (ADMET) polymerization in the presence of third generation Grubbs catalyst in chlorobenzene at 50 °C to give bridge-like polymers. The double-stranded polymers and the bridge-like polymers can assemble into a ladder-like architecture and a fence-like ribbon morphology. Cyclopolymerization of 1,6-heptadiynes allows synthesis of graft polymers299 and polymers having dendrons on side chains.300,301 4-Hydroxymethyl-1,6-heptadiyne was utilized as the initiator for the ring-opening polymerization of L-lactide and ε-caprolactone in the presence of Sn(Oct)2 and CH3SO3H, respectively.299 The produced macromonomers (136 and 137) undergo cyclopolymerization by second generation HoveydaGrubbs catalyst (XXXIX). Brush polymer with narrow molecular weight distribution can be obtained by the polymerization of 136 by third generation Grubbs catalyst (XLa; monomer-to-initiator molar ratio is 50). The polydispersity of the polymer becomes larger when the monomer-to-initiator molar ratio is over 50. Similarly to the polymer obtained from 101a, the resulting polymers in this case undergo coil-to-rod transition in solution. Extended conformation of the single chains of the brush polymers can be visualized with AFM. 1,6-Heptadiynes with dendrons can be subjected successfully to the Ru-catalyzed cyclopolymerization (Chart 9). The visualization of a single polymer chain by AFM can be also achieved in the polymer having ester-dendrons or polyesters as side chains, obtained by the cyclopolymerization of the corresponding 1,6-heptadiynes (138).290 In this case, a third generation dendron was necessary for observation capability 9025


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Chart 9. 1,6-Heptadiynes with Dendrons Used in Cyclopolymerization by Grubbs Catalysts

isomer shows slightly higher reactivity than that of racemo isomer, which is similar to the polymerization using Mo catalyst.277 The polymer having second generation ester dendrons on 4 and 5 positions can be prepared by the cyclopolymerization of the corresponding 1,7-octadiyne (150). AFM imaging of the polymer reveals rod-like conformation of the polymer. This is in contrast to the polymer of 1,6-heptadiyne with a second generation ester dendron (139a), in which the AFM imaging was unsuccessful. The cyclopolymerization of N-containing 1,7-octadiynes (151−154 and 156) also takes place smoothly in the presence of the third generation Grubbs catalyst (XLa) or second generation Hoveyda-Grubbs catalyst (XXXIX).306 In all cases, the resulting polymers have nitrogen-containing six-membered rings. The introduction of a bulky group on the nitrogen atom leads to more rapid polymerization due to the enhanced Thorpe−Ingold effect. The initial polymerization rate of 154 having a carbamate group is faster than that of an ester group 155, which is ascribed to the C-N bond length being shorter than the C−C bond length. The cyclopolymerization of hydrazide type monomers (156) proceed in a living manner using third generation Grubbs catalyst (XL).

polymerization by the Grubbs catalysts. Results of the cyclopolymerization are summarized in Table 6. The cyclopolymerization of 1,7-octadiyne with one ester substituent (143) or two less bulky methoxycarbonyl substituents (144a) using the third generation Grubbs catalyst (XLa) only affords low molecular weight polymer in low yield (20−25%). In contrast, a monomer with bulky tert-butyldimethylsiloxymethyl groups (145a) undergoes cyclopolymerization to give the corresponding polymer in high yield (>99%). The resulting polymer possesses six-membered ring repeating units, as a result of selective α-insertion of the monomer. The polymerization proceeds in a living fashion, and the molecular weight of the polymer can be controlled by changing the monomer-to-initiator molar ratio (Mn = 6000−28 000). The introduction of dimethyl substituents (147, 145b, and 146a) results in a more rapid cyclopolymerization without spoiling its living character.304 This result is also accounted for by the Thorp-Ingold effect. 1,7-Octadiynes having substituents on 4 and 5 positions (107, 148, and 149) also show higher polymerizability compared to the monomers having two substituents on the 4 position.305 The polymerization of 107 using the third generation Grubbs catalyst shows first-order kinetics with respect to the monomer concentration. The meso 9026


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Figure 61. Double cyclizative cyclopolymerization of tetraynes and the mechanism of the polymerization.302

generation Grubbs catalyst enables synthesis of block copolymers of various diynes. Charts 12 and 13 show the examples of block copolymers. In addition to the block copolymerization of 1,6-heptadiynes with different substituents, the block copolymerization of 1,6heptadiynes with 1,7-octadiynes and/or 1,8-nonadiynes is also possible. In the case of the block copolymerization of 1,8nonadiynes with other diynes, a proper choice of the concentration in each stage of the copolymerization seems to be important.307 As first and third generation Grubbs catalysts are effective for living ring-opening metathesis polymerization (ROMP) of norbornene derivatives, block copolymers of diynes with norbornene derivatives have been also synthesized. Random/statistical and block copolymers of 1,6-heptadiyne with Meldrum’s acid moieties (101d) and of 1,6-heptadiyne with malonate groups (101b) or norbornene derivatives can be obtained using third generation Grubbs catalyst.308 In the second-stage cyclopolymerization of 101d after the first-stage ring-opening polymerization of norbornene, the color of the solution changes to dark red and its viscosity increases. The NMR analysis of the block copolymer does not show the signals due to the block containing Meldrum’s acid, and only the signals due to norbornene unit is observed. This result indicates that self-assembly occurs during the polymerization to form supramolecules having insoluble poly(101d) blocks at the core and soluble polynorbornene blocks as the shell. The formation of the block copolymer micelles was confirmed by

The cyclopolymerization of N- or O-containing 1,8-nonadiynes (157−161; Chart 11) is also possible using third generation Grubbs catalyst (XLa) or second generation Hoveyda−Grubbs catalyst (XXXIX) and polymers containing 7-membered ring repeating units can be synthesized.307 Compared to the cyclopolymerization of 1,6-heptadiynes and 1,7-octadiynes, a low concentration of the monomer is required to achieve high conversion. Zeroth-order kinetics were observed for the cyclopolymerization of 1,8-nonadiynes, indicating that the cyclization step is the rate-determining step. The smooth polymerization of the N- or O-containing monomers is ascribed to the shorter C−N and C−O bond lengths and low rotational barriers. The cyclopolymerization of the acetal monomer is faster than that of the aminal monomer, due to the shorter C−O bond length compared to the C−N bond. However, the molecular weight of the polymer is higher for the cyclopolymerization of aminal monomer because the stabilization of the propagating chain end by the bulky carbamate group suppresses its decomposition to Ru−H species. In contrast to those monomers, the cyclopolymerization of 162 or 163 does not proceeds smoothly: the most stable dihedral angle of CO− N−N−CO is ca. 90° and the rotation of the N−N bond is unfavorable because of the relatively high rotational barrier (ca. 19 kcal/mol). 6.3.3. Synthesis of Block and Random Copolymers. Achievement of living polymerization of diynes by third 9027


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Chart 10. 1,7-Octadiynes Used in Cyclopolymerization by Grubbs Catalysts

AFM and DLS. Heating the micelles to 220 °C leads to thermolysis of the Meldrum’s acid moieties to ketene structures, which undergo dimerization to afford cross-linked cores. Block copolymers of 1,6-heptadiyne with pentafluorophenyl ester (102c) with norbornene derivatives, synthesized by third generation Grubbs catalyst, form a giant helically twisted nanotubular structure through hierarchical self-assembly in solution.309 The bilayer wall of the self-assembled superhelical nanotube consists of π-stacked poly(102c) blocks enwrapped by densely packed polynorbornene blocks aided by pendant biphenyls. 6.3.4. Cyclopolymerization of Diynes Using Modified Hoveyda−Grubbs Catalysts. As mentioned above, second generation Hoveyda−Grubbs catalyst is usually not effective for cyclopolymerization of diynes. Buchmeiser reported that a modified Hoveyda−Grubbs catalyst having electron-withdrawing trifluoroacetate groups (XLII and XLIII) efficiently promotes cyclopolymerization of diynes such diethyl dipropargylmalonate (101a).289,310 Chart 14 and Table 7 show the diyne monomers used in the cyclopolymerization catalyzed by modified Hoveyda-Grubbs catalyst and the results of the polymerization. The resulting polymer has exclusively 5-membered ring repeating units, and the vinylene groups of the repeating units

adopt a highly controlled trans structure. Linear relationships between monomer to catalyst molar ratio and molecular weight of the resulting polymer demonstrate the living character of the polymerization. The broad molecular weight distribution of the resulting polymer (D = 1.10−1.63) is ascribed to the larger propagation rate compared to the initiation rate (kp/ki > 3). The catalyst also promotes the cyclopolymerization of dipropargylmalonates with optically active menthyl groups (104a). The 13C NMR spectrum of the polymer revealed the high stereoregularity of the polymer. A ruthenium catalyst immobilized on an amphiphilic poly(2-oxazoline)-based block copolymer enables the aqueous polymerization of 101a to give a polymer with low D (95%), and the repeating units of the resulting polymer are exclusively 6membered rings. The dicationic ruthenium catalyst (XLVI) promotes the cyclopolymerization of dipropargyl malononitrile (115) and N,N-dipropargyl trifluoroacetamide (166), in addition to 101a, 105c, and 128a.315 This result again indicates the importance of the electronegativity of anionic ligand and the polarization of Ru alkylidene on the smooth cyclopolymerization of diynes by the catalysts. The polymer obtained from 115 is insoluble, and the polymers obtained from 105c and 128a heavily aggregate in solution, which prevents analysis of their microstructure.

Dipropargyl alkylamines do not undergo polymerization using either catalysts, due to the intramolecular coordination of amino group to the Ru center followed by back-biting reactions to give a dimer.275,312 N,N-Dipropargylammonium salts (105), in contrast, undergo cyclopolymerization in the presence of XLIIa or XLIII to give the polymers with 5-membered ring repeating units, although the yield is low. The cyclopolymerization of 165 by XLIII takes place smoothly to give the polymer containing 5-membered ring repeating units in good yield.312 A linear increase in molecular weight of the polymer with respect to the monomer feed is observed. New addition of fresh monomer after the consumption of the monomer resumes the polymerization to give the polymer with larger molecular weight, which indicates living character of the polymerization. A quantum chemical investigation of the 1,6-hepdadiynes with Hoveyda−Grubbs type catalysts with various anionic ligands indicates that the formation of five-membered ring via α insertion of diyne is energetically favored over six-membered ring formation via its β-insertion.313 A lower energy barrier of the reaction with the catalyst with more electron withdrawing 9030


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Chart 13. Block Copolymers of Norbornene/Triynes with Diynes Synthesized by Grubbs Catalysts

The mechanism for the selective formation of six-membered rings in the cyclopolymerization of 1,6-heptadiynes by XLVII and XLVIII was discussed by DFT study.317 Third generation Grubbs catalyst (XLa) selects the 5-member ring repeating units because α-addition of the terminal alkyne takes place. It adopts octahedral structure and is an electrophilic Fischer carbene. In contrast, XLVII and XLVIII adopt a trigonal-bipyramidal structure, which reduces the electrophilicity of the carbene by stronger π-backdonation and results in high selectivity in βaddition of the terminal alkyne.

Most of the cyclopolymerizations of 1,6-heptadiyne catalyzed by Ru-based Grubbs catalysts afford polymers having predominantly five-membered ring repeating units. Recently, it was shown that catalyst XLVII, which is known to be selective toward the Z-stereoisomer of the newly formed double bonds, promotes the cyclopolymerization of 1,6-heptadiynes (101a, 101g (R), 101g, 102, and 129b) to afford polymers containing mainly 6-membered ring repeating units (Figure 62).316 Thus, β-insertion of the alkyne is favored over its α-insertion, owing to the sterically hindered NHC ligand of XLVII. The cyclopolymerization of 1,6-heptadiynes with more bulky substituents and at lower temperature increase the selectivity for the formation of six-membered ring repeating units. UV−vis spectra in solution and in thin films state of the polymers formed from 101g showed lower λmax (513 and 484 nm, respectively) than those of the polymer with 5-membered ring repeating units obtained using the third generation Grubbs catalyst (588 and 515 nm, respectively). This result is ascribed to the less planar structure of the former polymer because of the presence of more sterically demanding Z-double bonds. Cyclic voltammetry revealed that the HOMO level of the 6- and 5-membered ring polymers is −4.94 and −5.14 eV, respectively. The cyclopolymerization with highest selectivity for 6-membered ring repeating units was recently achieved by using the Z-selective Hoveyda catalyst XLVIII.317 The cyclopolymerization of 126c, 128c, and 167 by XLVIII afford exclusively the 6-membered ring repeating units.

6.4. Cyclopolymerization of Diynes Using Rh Catalysts

Rhodium catalysts have been known to promote the polymerization of 1-alkynes and to enable living polymerizations for arylacetylenes.318,319 The produced polymers show unique properties originating from their dynamically chiral behavior.320 The polymerization proceeds via a coordination-insertion mechanism, rather than a metathesis mechanism. Kakuchi reported the Rh-catalyzed cyclopolymerization of bis(ethynylbenzene) tethered by ethylene glycol spacers with or without pyridine unit (168; Figure 63).321−324 [Rh(nbd)Cl]2/ amine and Rh(nbd)BPh4 were utilized as the catalyst. The cyclopolymerization was conducted in highly dilute condition ([monomer] = 0.01−0.05 mol L−1), in order to avoid gel formation. The cyclization takes place quantitatively for the cyclopolymerization of the monomer with penta- and hexaethylene glycol spacers (168a and 168b), whereas the extent of cyclization was 0.89−0.99 for the cyclopolymerization of the monomer with the pyridine-containing spacers (168c and 9031


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Chart 14. Diynes Used in Cyclopolymerization by Modified Hoveyda−Grubbs Catalysts

polymerization of the corresponding bis(ethynylbenzene)s (169) catalyzed by Rh(nbd)BPh4 (Figure 63).325 The cyclopolymerization was performed in highly diluted conditions ([monomer] = 0.02 mol L−1) in order to obtain gel-free polymer. CD analysis of the polymers confirmed their helical structures with an excess single screw sense, which is induced by the chiral binaphthyl units. The addition of achiral ammonium perchlorates to the polymers lead to a chiroptical change of the signal due to the fluctuations in the main chain conformation caused by the host−guest complexation. Chirality responsive helicity change was observed in the complexation of the polymers with chiral amino acids. In those cases, the chiroptical change depended on the size of the crown cavity. Polymers with crown cavities having isophthalamide moieties were obtained by the cyclopolymerization of the corresponding bis(ethynylbenzene)s (170) using [Rh(nbd)Cl]2/Et3N in highly diluted conditions ([monomer] = 0.03 mol L−1; Figure 63).326 The cyclopolymerization in the presence of 3,5bis(carbamoyl)pyridinium chloride proceeds through a chloride anion templated rotaxane formation to give main chain-type

168d). In all cases the resulting polymer is soluble in organic solvents and gel-free. Laser Raman spectra of the polymer with penta- and hexaethylene glycol spacers indicate highly cistransoidal stereoregularity of the polymer. The produced polymers have crown cavities and their cation-binding property was investigated. The polymer with pentaethylene glycol spacers showed increased cation-binding property for the alkali metal with larger cation radius (Li+ < Na+ < K+ < Rb+ < Cs+, extraction yield for Cs+ = 15.6%). The polymer with pyridine-containing spacers, which has azacrown-type cavities, binds efficiently with Ag+ (extraction yield for Ag+ = 96.9 − 98.6%). 1H NMR analysis indicates two azacrown cavities involved in a sandwich-type complex with Ag+. In the presence of amino acids, the polymers with crown cavities showed characteristic induced CD signals, and the sign of the CD spectra depends on the absolute configuration. This result indicates induction of on-handed helicity of the main chain through the host−guest interaction of the crown cavities of the polymer with the chiral guests. Polymers with 29−41 membered crown cavities containing chiral binaphthyl groups were synthesized by the cyclo9032


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Table 7. Cyclopolymerization of Diynes by Modified Hoveyda−Grubbs Catalysts monomer

catalyst

solvent

1,6-heptadiynes 101a XLIIa 114 XLIIa 102 XLIIb 164 XLIIb 117a XLIIb 117d XLIIb 117e XLIIb 105a XLIIa 105d XLIIa 105e XLIIc 105b XLIII 105b XLIIa 105c XLIII 165 XLIII 101a XLIVa 101a XLIVb 101a XLVa 101a XLVb 101a XLVI 127a XLVI 166 XLVI 101a XLVII 101g XLVII 101h XLVII 102a XLVII 129b XLVII 101h XLVIII 117f XLVIII 126c XLVIII 128c XLVIII 167 XLVIII

CH2Cl2 CH2Cl2/MeOH CH2Cl2 CH2Cl2 CHCl3 CHCl3 CHCl3 [1,2-Me2-3-BuIm][NTf2] [1,2-Me2-3-BuIm][NTf2] [1,2-Me2-3-BuIm][NTf2] DMF DMF DMF CHCl3 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 toluene toluene toluene toluene toluene CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

106 107

1,2-dichloroethane 1,2-dichloroethane

XLIVa XLIVa

Mn

ring (selectivity)

5 27 500 N.D. 11 000 5 7300 N.D. 75 700 5 16 800 5 27 400 5 17 900 5 22 200 5 20 400 5 20 800 5 10 900 5 39 400 5 10 900 5 12 700 5 75 800 5 40 500 5 13 500 5 12 400 5 11 600 5 21 000 N.D. 185 000 6 (77%) 8600 6 (67%) 6600 6 (83%) 13 100 6 (87%) 15 600 6 (86%) 6900 6 (94%) 16 100 6 (92%) 15 300 6 (>99%) 6700 6 (>99%) 19 600 6 (>99%) 25 800 1,7-Octadiynes 6 12 000 6 11 000

PDI

remark

ref

1.54 1.09 1.27 1.08 1.71 1.52 1.72 1.02 1.04 1.07 1.01 1.14 1.03 1.40 1.33 2.45 2.66 2.09 1.4 1.9 1.03 1.61 1.9 1.03 1.9 1.49 1.71 1.61 1.51 1.79 1.92

trans-vinylene, living character, λmax = 584 λmax = 517 λmax = 574 λmax = 517 λmax = 584 λmax = 586 λmax = 588

289 311 311 311 312 312 312 275 275 275 312 312 312 312 314 314 314 314 315 315 315 316 316 316 316 316 317 317 317 317 317

2.0 1.7

λmax = 523 λmax = 523 λmax = 490 λmax = 510

selectivity = 86% at −40 °C selectivity = 71% at −40 °C selectivity = 92% at −40 °C selectivity = 95% at −40 °C

λmax = 470 λmax = 483

276 276

was able to bind Z-L-Ala-OH much more strongly than Z-D-AlaOH, and extract Li+ efficiently among alkali metal ions. The cyclopolymerization of bis(ethynylbenzene)s having calix[4]arene tether (172) by [Rh(nbd)Cl]2 takes place smoothly even at relatively higher concentration of the monomer (up to 0.4 M) without gel formation (Figure 63).328 Oligomers and dimers also form as side products, but the selectivity for the polymerization can be improved (up to 90%) when PPh3 was added as an additive in the cyclopolymerization. The helix-sense-selective asymmetric cyclopolymerization of achiral nitrogen-bridged 1,8-diynes (173a) was reported (Figure 64).329 The monomer contains a terminal and an internal alkyne, and the cyclopolymerization proceeds via the intermolecular insertion of the more reactive terminal alkyne followed by the second intramolecular insertion of the less reactive internal alkyne, thus alternating the two moieties in the macromolecular sequence. [RhCl(R,R)-diene]2 was used as the catalyst in combination with triphenylvinyllithium and PPh3 as additives. The structure of the monomer plays an important role for smooth cyclopolymerization. 1,6-Diyne and 1,7-diyne afford [2 + 2+2] cycloadducts, and no polymer was obtained. Oxygenbridged 1,8-diyne (173b) cyclopolymerizes efficiently (up to 99% yield), but the optical activity of the resulting polymer is low ([α]D = −5 to +35). In contrast, nitrogen-bridged monomers

polyrotaxanes. The formation of rotaxane structures was confirmed by CD and DOSY measurements. The mole fraction of the rotaxane units to the total cyclic repeating units was estimated to be 18.6−26.3%. The polymers obtained in diluted conditions ([monomer] = 0.03−0.06 mol L−1) were gel-free, but the cyclopolymerizations conducted at [monomer] = 0.1 mol L−1 gave gelification. The cyclopolymerization of diynes with amino acid-based optically active spacers (171) was examined using Rh(nbd)BPh4 (Figure 63).327 The bis(propargyl ester) monomer (171a) affords soluble low molecular weight oligomers (Mn = 400− 500) in CH2Cl2, THF ([monomer] = 0.02 mol L−1), or DMF ([monomer] = 0.02−0.1 mol L−1), whereas in less diluted conditions ([monomer] = 0.1 mol L−1 in CH2Cl2 or THF) gel formation occurred. In contrast, the cyclopolymerization of the bis(propargyl amide) monomer (171b) takes place even at [monomer] = 0.1 mol L−1 in CH2Cl2 or THF to give gel-free polymer with Mn = 8000−15 000. Intramolecular hydrogen bonding of the two N-propargylamide moieties leading to favorable conformations for cyclopolymerization is proposed as a rationale for the smooth cyclopolymerization. The resulting polymer exhibited a much larger optical activity compared to the monomer, indicating higher order chiral structure. The polymer 9033


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Figure 63. Cyclopolymerization of diynes by Rh catalysts.

Figure 62. Cyclopolymerization of 1,6-heptadiynes by Z-selective Ru catalysts.316,317

with various substituents underwent cyclopolymerizations to give the corresponding polymers in 16 − 89% yield (Mn = 3100−6500), which are optically active, with large [α]D values (+318 and −772, respectively) when bulky aromatic rings (otolyl and mesityl groups) on the nitrogen were used. The cyclic structures formed in the cyclopolymerization are important for constructing the helix-sense chirality, because poly(arylacetylene)s obtained by the catalyst are not optically active. Significant racemization of the polymer does not take place at 25 °C even after 1 month. 6.5. Bergman Cyclopolymerization of Diynes

Aromatization of enediynes and 1,2-dialkynylbenzenes upon heating to form benzene and naphthalene derivatives has been known as the Bergman cyclization.330 The reaction takes place via formation of a 1,4-diradical, which undergoes hydrogen abstraction from 1,3-cyclohexadiene. In the absence of 1,3cyclohexadiene, the 1,4-diradicals undergo coupling with each other to form polynaphthalenes or derivatives (Figure 65). This kind of cyclopolymerization of enediynes and 1,2-dialkynylaromatic compounds proceeds via step growth polymerization, but it does not form volatile side products and no catalyst is necessary. The products are either soluble oligomers with fair thermal stability or insoluble high molecular weight polymers with high thermal stability and poor processability. Thus, the good solubility and high molecular weight of the polymers are in tradeoff relationship, and it has been the main problem for their development into applications.

Figure 64. Cyclopolymerization of diynes 173 by chiral Rh catalysts.329

Recently, Smith, Jr. reported the synthesis of polynaphthalene networks utilizing the Bergman cyclopolymerization (Figure 66).331−333 Bis(1,2-dialkynyl)benzene derivatives (174) are first subjected to the Bergman cyclopolymerization to produce meltand solution-processable reactive oligomers with Mw = 3000− 24 000. The oligomer can be coated as a thin film or molded 9034


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6.6. Metathesis Polymerizations Associated with Ring Opening and Ring Closing of Cycloalkenes

6.6.1. Cyclopolymerization of Biscycloalkenes via Ring-Opening and Ring-Closing Metathesis. Bisnorbornene tethered by a flexible linker such as triethylene glycol (176a), tetramethylene (176b), or an azacrown ether (176c) undergoes ring-opening metathesis polymerization in the presence of first generation Grubbs catalyst (XLI; Figure 68).339,340 The polymerization is associated with the cyclization

Figure 65. Bergman cyclopolymerization.

Figure 66. Synthesis of polynaphthalene network by Bergman cyclopolymerization.331−337

using soft lithography techniques. Subsequent curing at 450 °C affords naphthalene networks. Heating of the network to 900− 1000 °C results in semiconductive glassy carbon in high yield.334 Kinetic studies of the cyclopolymerization indicate a first order rate constant of k = ca. 10−5 s−1 at 210 °C and activation energies of Ea = 28.7−33.5 kcal/mol.335,336 1,2-Diethynylbenzene having 11-hydroxyundecyloxy groups can be utilized as amphiphilic surfactant monomers for the synthesis of hybrid nanocomposites.337 The monomers are mixed with tetraethoxy orthosilicate and an acid catalyst in EtOH, and the solution is spin-coated to form thin films. The polymerization of silica during the coating process leads to highly ordered nanocomposites having the surfactant monomers within the hexagonal channels of the nanocomposites. The Bergman cyclopolymerization of the monomer produces the nanocomposites containing polynaphthalenes in its channels. Recently, enediyne 175 was reported to undergo Bergman cyclopolymerization under photoirradiation (470 nm) in diluted toluene solutions (Figure 67),338 whereas the irradiation of the crystal powder of 175 results in [2 + 2] cycloaddition to give a dimer.

Figure 68. Metathesis cyclopolymerization of bisnorbornenes.339,340

between the two norbornene moieties. The concentration of the monomer is important for the controlled cyclization. The polymerization of bisnorbornene (176a) proceeds with the cyclization of the norbornene groups, with a monomer concentration of 1.6 and 8 mM. On the contrary, the polymerization with a monomer concentration of 40 or 80 mM leads to the gelation within few minutes. The flexible linker and N-aryl group of the monomer play an important role for the efficient cyclization of the monomer, since in other cases the formation of ladder-type and/or cross-linking polymers occurs. The Metathesis cyclopolymerization of biscyclopentene monomers (177) was reported by using first generation Grubbs catalyst (XLI; Figure 69).341 Cyclopentenes with substituents on the 3 position show low reactivity toward ring-opening metathesis polymerization, due to their steric hindrance and decreased ring strain. Biscyclopentene monomers instead undergo smooth metathesis cyclopolymerization. For the selective formation of structurally well-defined polymers, there are some issues to be solved, such as the regioselectivity in ring opening of the first cyclopentene group of the monomer, and selective ring-closing metathesis after the ring opening of the cyclopentene. Detailed investigations of the reaction show that the concentration of the monomer is essential for the formation of structurally well-defined polymers. The reaction with a

Figure 67. Bergman cyclopolymerization of 175 under photoirradiation.338 9035


Chemical Reviews

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second generation Hoveyda-Grubbs catalyst allows the insertion of the diacrylate to afford alternating copolymers. 6.6.2. Cyclopolymerization of Cycloalkenylalkynes via Ring-Opening and Ring-Closing Metathesis. Cyclohexene has been known to hardly undergo ring-opening metathesis polymerization because it has almost no ring strain and equilibrium shift toward the monomer formation. Terminal alkynes have been known to polymerize rarely using the metathesis catalysts because the propagating carbene, formed by α-addition to the ruthenium catalyst, is too sterically hindered for further propagation. Recently, Choi reported that cyclohexenylalkyne undergoes efficient ring-opening/ring-closing metathesis polymerization in the presence of a Ru catalyst (Figures 70 and 71).342 Representative examples of ringopening/ring-closing metathesis polymerization of cycloalkenylalkynes are summarized in Table 8. For example, the polymerization of 178a ([178a]/[Ru] = 100) proceeds rapidly at room temperature in the presence of third generation Grubbs catalyst (XLa; Figure 70). At −30 °C, the polymerization proceeds in a living manner, and polymers with Mn ranging from 8000−41 000 and narrow polydispersities (D = 1.17−1.21) can be synthesized. The polymerization proceeds via α-insertion of terminal alkyne followed by ringopening of cyclohexene and formation of five-membered ring

Figure 69. Metathesis cyclopolymerization of biscyclopentenes.341

monomer concentration below 1 M enable selective metathesis cyclopolymerization, whereas the reaction with the monomer concentration above 1 M results in intermolecular ring-opening metathesis polymerization to form cross-linked networks. However, treatment of the cross-linked gels with the catalyst in dilute conditions changes it to soluble polymers. The reaction of the resulting polymers with diacrylate in the presence of

Figure 70. Tandem ring-opening/ring-closing metathesis polymerization of enyens.342,344 9036


Chemical Reviews

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cyclopolymerize, although the polymerization rate is still lower than the monomer with cyclohexyl and propargyl group (178a). Thus, the polymerization forming five-membered ring repeating units takes place more rapidly compared to that forming sixmembered ring repeating units. The monomer with an amide group in place of a sulfonamide group (178b) showed slightly lower reactivity. The monomers with carbon or oxygen spacer (178i, 178j, and 178k), on the other hand, did not polymerize using the second generation Hoveyda-Grubbs catalyst XXXIX at 50 °C. The introduction of the spacer with quaternary carbon (178d, 178e, and 178f), on the other hand, leads to smooth polymerization even using third-generation Grubbs catalyst XLa at room temperature. The polymerization proceeds in living fashion. The synthesis of G2 and G3 ester dendrons (178g and 178h) was also achieved, and AFM analysis of the polymer with G3 dendrons enables observation of single chains of the polymer. A detailed investigation of the polymerization of the monomer with low polymerizability revealed that the reason for the unsuccessful polymerization is the slow cyclization after the reaction of the alkyne, which results in intermolecular metathesis reaction rather than the polymerization. Actually, the polymerization of 178i, 178j, 178k, and 182 proceeds under lower concentration of the monomer, because intramolecular cyclization is more favored compared to the intermolecular metathesis reaction under these conditions. The monomers with internal alkynes (such as 185) can also polymerize using the third generation Grubbs catalyst under dilute conditions.344 The resulting polymers contain 6membered ring in addition to 5-membered ring repeating units, as both α- and β-insertion of the alkyne groups take place. The selectivity is affected by the steric and electronic factors of the substituents on the terminal alkyne. The monomer with methyl group (185a) afforded the polymers rich in fivemembered rings (70%), but the selectivity dropped to 63% in the monomer with ethyl groups (185b). The monomer with electron withdrawing trifluoromethyl groups (185c) yielded exclusively 5-membered ring repeating units. Kinetic studies showed that first order kinetics with respect to the monomer concentration for the polymerization of 178a and 185c. In contrast, the polymerization of 185a and 185g, affording 6membered ring repeating units, obeys zeroth order kinetics. This result indicates that the cyclization step is the rate determining step in the latter case, whereas the cyclization step is fast in the former. The ring-opening/ring-closing metathesis polymerization can be applied for the synthesis of polymers with controlled sequence. Gutekunst and Hawker reported that the enyne monomers having a macrocycle with triester structure (186) undergo ring-opening/ring-closing metathesis polymerization in the presence of third generation Grubbs catalyst (XLa) and 3,5-dichloropyridine (Figure 73).345 The resulting polymer has narrow molecular weight distribution. The polymer contains the three different ester units and five-membered units with sulfonamide groups in controlled sequence along the polymer chain.

Figure 71. Tandem ring-opening/ring-closing metathesis polymerization of enynes.343,344

repeating units (Figure 72). Transformation of the sterically hindered ruthenium carbene formed after the α-insertion of the terminal alkyne to the less hindered ruthenium carbene by the reaction with cycloalkene is one of the reasons for the smooth polymerization of the monomer, together with the thermodynamically favorable formation of a stable conjugated diene. Structural motifs of the cycloalkene, spacer, and alkyne groups significantly affect the reactivity of the monomer.343 The monomer with the cycloheptenyl group 180 showed almost similar polymerizability to that with cyclohexenyl group. In contrast, the polymer with the cyclopentenyl group 179 is obtained in lower yield at room temperature. The smooth polymerization of the monomer can be achieved at 50 °C by using the thermally stable second-generation Hoveyda−Grubbs catalyst XXXIX. The catalyst also enabled polymerization of monomers having trisubstituted cyclohexene or cyclopentene structure (183 and 184; Figure 71). Changing the propargyl group of 178a into homopropargyl group (182) results in no polymerization under similar polymerization conditions. In this case, however, the monomers with cyclopentyl groups (181)

7. TRANSITION METAL-CATALYZED CYCLOPOLYMERIZATION OF MONOMERS OTHER THAN DIENES, DIYNES, AND ENYNES 7.1. Cyclopolymerization of Bisallenes

Ni catalysts have been known to promote the living polymerization of allenes.346 Tomita reported the cyclopolymerization 9037


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Table 8. Tandem Ring-Opening/Ring-Closing Metathesis Polymerization of Enynes monomer

catalyst

solvent

monomer conc./M

temp.

Mn

PDI

178a 178a 178b 178c 178d 178e 178f 178g 178h 178i 178j 178k 179 179 180 181 182 183 184 185a 185b 185c 185d 185e 185f 185g 186a 186b 186c

XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa XXXIX XLa XXXIX XLa XXXIX XXXIX XLa XLa XLa XLa XLa XLa XLa XLa XLa XLa

THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF THF CH2Cl2/MeOH CH2Cl2/MeOH CH2Cl2/MeOH

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.03 0.03 0.01 0.4 0.4 0.4 0.4 0.03 0.4 0.4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4

r.t. −30 °C −10 °C r.t. −10 °C r.t. 0 °C r.t. r.t. r.t. r.t. r.t. r.t. 50 °C −30 °C 50 °C 40 °C 50 °C 50 °C 15 °C 10 °C 15 °C r.t. r.t. r.t. r.t. 0 °C 0 °C 0 °C

26 000 31 000 13 100 20 000 26 000 26 000 34 000 33 000 52 000 3600 7700 6200 11 400 13 500 5000 12 000 26 000 8000 4400 65 000 36 000 42 000 27 000 22 000 34 000 30 000 21 000 19 300 32 600

1.86 1.18 1.23 1.49 1.17 1.11 1.14 1.39 1.17 1.51 2.88 1.34 1.70 1.62 1.28 1.54 1.50 1.93 1.61 1.20 1.10 1.15 1.31 1.25 1.31 1.48 1.26 1.27 1.26

remarks

ref

E/Z = 6/4 E/Z = 6/4, living polymerization E/Z = 6/4

E/Z = 6/4 E/Z = 6/4 E/Z = 6/4 E/Z = 6/4 E/Z = 1/1 E/Z = 1/1 5-/6-memberd ring = 2.3/1 5-/6-memberd ring = 1.7/1 5-/6-memberd ring = 1/0 5-/6-memberd ring = 2/1 5-/6-memberd ring = 2.7/1 5-/6-memberd ring = 1/1 3,5-dichloropyridine as additive, E/Z = 9/1 3,5-dichloropyridine as additive, E/Z = 9/1 3,5-dichloropyridine as additive, E/Z = 9/1

342 342 343 344 344 344 344 344 344 344 344 344 344 343 343 343 344 343 343 344 344 344 344 344 344 344 345 345 345

Figure 72. Mechanism of tandem ring-opening/ring-closing metathesis polymerization of enynes.

of diethyl bis(2,3-butadienyl)malonates (187) by π-allylnickel catalysts (Figure 74).347 The polymerization carried out at a monomer concentration of 0.1 M afforded products soluble in organic solvents. NMR analyses indicated the absence of allenyl groups and the presence of 1,2- and 2,3-units (52:48) in the

polymer. This is in contrast to the nickel-catalyzed polymerization of alkylallenes, where the amount of 1,2-units is generally below 9%. NMR analyses also indicated that the resulting polymers are selectively produced with 6-membered ring repeating units. The polymerization is proposed to proceeds 9038


Chemical Reviews

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Figure 73. Synthesis of polymers with controlled sequence by tandem ring-opening/ring-closing metathesis polymerization of enyens.345

Figure 74. Cyclopolymerization of bis(allenes).347

via intramolecular 2,3-insertion of an allenyl group of the monomer to the growing terminal unit, followed by cyclization via intramolecular 1,2-insertion of the other allenyl group. Although the molecular weight distribution of the resulting polymer is rather broad (PDI = 1.90−1.97), a linear relationship between Mn and monomer-to-Ni molar ratio was observed. The polymerization carried out at the monomer concentration of 0.5 M also afforded soluble polymers, but the dispersity broadened (Đ = 3.2).

Figure 75. Cyclopolymerization of bis(diazocarbonyl) compounds.350

7.2. Cyclopolymerization of Bis(diazocarbonyl) Compounds

ization of the monomer having binaphthyl groups (188a) using [(π-allyl)PdCl]2 under low concentration of the monomer (0.06 M) afforded the corresponding polymer with unimodal molecular weight distribution; however, observation of broad cluster peaks by MALDI-TOFMS indicated a randomly cross-

Diazoacetates have been known to polymerize using transition metal catalysts such as Rh and Pd.348,349 Ihara reported the Pdcatalyzed cyclopolymerization of bis(diazocarbonyl) compounds having various spacers (Figure 75).350 The polymer9039


Chemical Reviews

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Chart 15. 1,2-Diisocyanoarenes Used in Cyclopolymerization

Since then, the polymerization of a wide variety of 1,2diisocyanoarenes has been reported using Pd and Ni complex catalysts (Chart 15). Pd and Ni complexes of the types MePdX(PR3)2,352 ArPdX(PR3)2, and ArNiX(PR3)2353 are often utilized as the catalysts (Chart 16).354 Ni complexes with PPh3 ligands show poor activity, and PPhMe2 and PMe3 (or PEt3) are frequently used as ligands for Pd and Ni complexes, respectively. The resulting polymers have narrow molecular weight distribution, and their molecular weight can be controlled by the monomer-to-catalyst molar ratio, to indicate living character of the polymerization. Block copolymers of 1,2-diisocyanoarenes with different substituents355 and of monoisocyanide with 1,2-diisocyanoarenes356 can be synthesized. The growing species of the polymerization of 1,2-diisocyanoarenes, when using the Pd catalyst, is so stable to enable isolation of the

linked structure due to insufficient cyclization. The polymerization of similar monomers having longer spacer between the diazocarbonyl groups (188b) or the monomers having phenylene or cyclohexylene groups (189 and 190) using [(πallyl)PdCl]2/LiBPh4 under a monomer concentration of 0.05 M proceeded with quantitative cyclization of the monomer to afford the polymer with controlled terminal groups. 7.3. Cyclopolymerization of Diisocyanides

Isocyanides have been known to polymerize in the presence of nickel or palladium catalysts. In 1990, Ito reported that 1,2diisocyanoarenes (191 and 192) undergo cyclopolymerization in the presence of a Pd catalyst (MePdBr(PPhMe2)2).351 The polymerization of 1,2-diisocyanoarenes is accompanied by the aromatization of the monomer and the resulting polymers possess a poly(quinoxaline-2,3-diyl) structure. 9040


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Chart 16. Ni and Pd Complexes Used in Cyclopolymerization of 1,2-Diisocyanoarenes

Chart 17. Chiral 1,2-Diisocyanoarenes Used in Cyclopolymerization

9041


Chemical Reviews

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substituents of the monomer.379 In this case, handedness of the CPL can be switched by changing the solvent. Optically active helical poly(quinoxaline-2,3-diyl)s having diarylphosphinopendants can be synthesized by the random or block copolymerization of 1,2-diisocyanoarenes with phosphine oxide or phosphine sulfide groups, followed by the deprotection via reduction of the functional group.380−382 The polymers with the phosphine-pendant are applied as chiral ligands for asymmetric reactions.383−385 The combination of the solventdependent switchable nature of the helical chirality and highly effective chiral ligand of the helical poly(quinoxaline-2,3-diyl)s enables the highly selective formation of either product enantiomer from a single chiral catalyst.386−391 Optically active helical poly(quinoxaline-2,3-diyl)s having bipyridyl or 4-aminopyridyl-pendants can be obtained by the random copolymerization of 1,2-diisocyanoarene with pinacolborane group and subsequent Suzuki-Miyaura cross coupling.392,393 The copolymer with bipyridyl group is applied as the ligand for coppercatalyzed cyclopropanation, and that with 4-aminopyridyl group acts as organocatalyst for Steglich reaction. Pd and Ni catalysts other than ArPd(PR3)X and ArNi(PR3)X were also examined as the catalyst for the cyclopolymerization of 1,2-diisocyanoarenes. NiCl2·6H2O shows good polymerization activity in MeOH or DMSO, and moderate activity in THF.394 NiCl2 complexes with trialkylphosphine ligands (PMe3, P(nBu)3) show high activity, although the molecular weight distribution of the polymer is broad. The complexes with arylphosphines, NiCl2(PPh3) and NiCl2(dppe) show lower activity. Grignard thiophene monomer has been known to undergo polymerization in the presence of NiCl2(dppp). The polymerization is accompanied by catalyst chain transfer, and proceeds in living fashion with chain-growth mechanism via AryNiCl(dppp) as the growing species.395,396 The living polythiophene having AryNiCl(dppp) active chain end can be utilized as the initiator for the polymerization of 1,2diisocyanoarenes to give the block copolymers (Figure 76).397,398 The resulting rod-rod copolymers containing polythiophene blocks and poly(quinoxaline-2,3-diyl) blocks undergo self-assembly into various supramolecular structures such as nanofibril and spherical nanoparticles depending on the solvents, and display light emissions with the colors varying from red to purple and blue. Air-stable (phenylbuta-1,3-diynyl)palladium(II) complexes are also effective for the cyclopolymerization of 1,2-diisocyano3,4-dimethylbenzene.399 Although the monomer does not have ortho-substituents, it undergoes polymerization in toluene at 90 °C to give a polymer with narrow molecular weight distribution (Mn = 5400, Mw/Mn = 1.19). The polymerization of 1,2diisocyano-3,4-dimethylbenzene also takes place after Pdcatalyzed living polymerization of 1-ethynyl-4-iodo-2,5-bis(octyloxy)benzene to give the copolymer containing poly(aryleneethynylene) block and poly(quinoxaline-2,3-diyl) block with narrow molecular weight distribution (Figure 76).400

growing polymer by chromatography without the degradation of the living chain ends. For example, siloxane groups can be deprotected by BF3OEt2 and the hydroxyl groups thus formed can be transformed into urethane groups via the reaction with isocyanates in the presence of DMAP.357 The living chain ends keep their activity and are able to reinitiate the polymerization when fresh monomer is fed. One of the features of the poly(quinoxaline-2,3-diyl)s is their helical structure owing to their rigid backbone.358 The rigid helical polymers having long alkoxy groups on the repeating unit shows thermotropic liquid crystalline phases.359 In order to obtain a one-handed helical polymer, screw-sense selective polymerization of 1,2-dicyanoarenes has been examined by using optically active catalysts. Screw-sense selective polymerization of 1,2-dicyanoarenes was attempted using a Pd complex with chiral phosphine ligand (bis{(S)-2-methylbutyl}phenylphosphine).360−362 Although the Pd complex does not promote screw-sense selective polymerization, separation of the right-handed and left-handed helical living oligomers could be achieved. Later, Pd complexes with optically active binaphthyl groups were demonstrated to promote screw-sense selective polymerization.363,364 The structure of the binaphthyl group affects the enantioselectivity of the polymerization: the selectivity of the complex with 2′-methoxy-1,1′-binaphth-2-yl group is low (20% ee) but that of the complex with 7′-methoxy1,1′-binaphth-2-yl group is almost quantitative (>95% ee). One of the drawbacks of the optically active binaphthyl Pd catalysts is that they are synthetically challenging. Recently, Suginome reported that Pd complexes with optically active imidazoline groups, which are readily synthesized, are effective for highly screw-sense selective polymerization of 1,2-diisocyanoarenes (up to > 99% screw-sense excess).365 In addition to the Pd complex, the Ni complex with the optically active imidazoline group also brings about the screw-sense selective polymerization of 1,2-diisocyanoarenes.366 Although the activity of the Ni complex is higher than that of the Pd complex, the screw-sense selectivity is lower (up to > 99% screw-sense excess). The Pd and Ni complexes with the optically active imidazoline group have been applied for screw-sense selective polymerization of various 1,2-diisocyanoarenes.367−369 The stability of the helical conformation of the poly(quinoxaline-2,3-diyl)s is largely affected by the substituents on 3- and 6-positions of the monomer (5- and 8-positions of the repeating unit) and by the chain length. The optically active polymer with less-bulky substituents on 5- and 8-positions or the low molecular weight oligomers tend to racemize especially at elevated temperatures.370 By utilizing the flexible nature of some poly(quinoxaline-2,3-diyl)s the induction of one-handed helical structures can be achieved by introducing optically active groups on the terminal of the racemic polymer.370,371 1,2-Diisocyanoarenes with chiral substituents are also used in the synthesis of helical poly(quinoxaline-2,3-diyl)s (Chart 17). Copolymers of achiral 1,2-diisocyanoarenes with the chiral 1,2-diisocyanoarenes adopt one-handed helical structures due to “sergeant and soldiers” effect. The screw-sense of the helical polymer is affected by the solvent, and the screw-sense can be even switched through a solvent effect.372−375 Such switch of the screw-sense of the helical polymer is also observed in the solid state, by the action of vapor.376,377 Reversible pressuredependent helix inversion of the helical polymer also occurs.378 The copolymer of optically active 1,2-diisocyanoarenes with 3,6diaryl-1,2-diisocyanoarenes exhibit circularly polarized luminescence (CPL), and the emission color can be tuned by the aryl

7.4. Cyclopolymerization of Bisthiophenes

Catalyst-transfer polycondensation has been utilized as a powerful method for synthesizing π-conjugated polymers with controlled molecular weight and regioregularity. Ni-catalyzed catalyst-transfer polycondensation was used for the cyclopolymerization of a bis(2-bromo-5-iodothiophene) monomer (Figure 77).401 Insoluble polymers form under the monomer concentration of 100 mM. However, soluble polymers can be obtained under more diluted condition (6 mM). UV/vis spectra 9042


Chemical Reviews

Review

Recently, DFT studies of the cyclization step in the radical cyclopolymerization of various diallyl monomers and allyl acrylates were performed (Chart 18, Table 9).24,87,403−406 The results of the DFT calculation agree with the results of the experimental results. For example, the activation energies for exocyclization of the monomers M-1, M-2, M-3, M-4, M-5, and M-8, which afford polymers containing five-membered rings, are smaller than those for endo-cyclization, and those for exocyclization of the monomers M-7, M-10, and M-11, which afford the polymer containing six-membered rings, are larger than those for endocyclization. The activation energies (exo and endo) of M-12 are high, and indeed the monomer tends to undergo gelation via cross-linking. Lower activation energies for the cyclization of M-15 compared to M-14 also matches the experimental results. Kinetic studies of radical cyclopolymerization systems affording polymers with larger ring sizes were recently reported. The RAFT cyclopolymerization of a diacrylate with bis(carbamoyl)-1,2-trans-cyclohexylene spacers affords the polymer having 19-membered ring and with narrow molecular weight distribution.70,71 A linear relationship is observed in the pseudo-1st-order kinetic plot, indicating smooth cyclization during the polymerization and negligible irreversible termination reactions. ATRP and RAFT-induced cyclopolymerizations of divinyl monomers tethered by a silylene or disiloxane spacer gives the corresponding polymers with 12−20 membered rings with narrow molecular weight distributions.68,90 The cyclopolymerization proceeds with quantitative cyclization of the monomers. First order kinetics with respect to the concentration of the monomer is observed for the cyclopolymerization, which indicate that the intramolecular cyclization step is faster than the intermolecular reaction of the chain end with divinyl monomer. In the ATRP-induced cyclopolymerization of dimethacrylate systems with silyl tethers, the monomers with bulky substituents such as SiiPr2 group polymerize much faster. The result is ascribed by that the two methacrylate moieties interact with each other because they are in close proximity (Thorpe−Ingold effect). It is also worth noting that the monomethacrylate analogue is unreactive under identical reaction conditions, which indicates that the cyclization process enhances the polymerizability of the monomer. The effect of LiTFSI on ATRP-induced cyclopolymerization of dimethacrylate with nonaethylene glycol tether was reported.75 Kinetics of the polymerization are only slightly affected by the concentration of the Li salt, whereas it greatly affect the molecular weight of the polymer. Using a default of LiTFSI leads to gelation via cross-linking, but the addition of 1 equiv of lithium salt with respect to the monomer results in the polymer with the molecular weight close to the value expected from the reacted monomer and the initiator. RAFT cyclocopolymerization of bis-styrene having oligoethylene glycol linker with maleic anhydride yields gel-free copolymers.91 The polymerization proceeds via alternating reaction of styryl unit and maleic anhydride, and the produced polymer contains a 23−28 membered ring. First-order kinetics with respect to the concentration of the bisstyrene is observed, and a constant free radical concentration and the absence of significant terminal reactions are indicated. Kinetic studies of the cyclopolymerization of nonconjugated dienes and diynes have been also reported. Cyclopolymerization of dienes and diynes affording the repeating unit with five- or sixmembered ring generally obeys first-order kinetics of the

Figure 76. Synthesis of block copolymer of poly(quinoxaline-2,3-diyl)s with poly(thiophene) or poly(aryleneethynylene).397−400

Figure 77. Cyclopolymerization of bithiophenes.401

of the polymers with different lengths of the tether indicate the different dihedral angles in the polythiophene structure depending on the size of the macrocycle formed.

8. KINETIC AND THEORETICAL STUDIES ON CYCLOPOLYMERIZATIONS In order to elucidate the mechanistic details of cyclopolymerizations, a number of kinetic studies have been conducted, and several review articles on this topic have been already published.10,13,14,402 Early kinetic studies regarding the cyclopolymerization of methacrylic anhydride indicated higher energy of activation for the cyclization step than that of intermolecular reaction step. However, the rate of cyclization is considerably faster than the intermolecular propagation step, suggesting steric factors favoring cyclization. In general, the above-mentioned reviews concur in indicating that increasing bulkiness, reaction temperatures, conversions, or lowering monomer concentrations, reaction solvent solubilities favor cyclization. Faster cyclization when compared to intermolecular propagation is also observed in the radical cyclopolymerization of o-divinylbenzene, divinylformal, and in the cationic cyclopolymerization of o-divinylbenzene. The ring size of the repeating units formed in cyclopolymerization also shows that the cyclopolymerization proceeds under kinetic rather than thermodynamic control. 9043


Chemical Reviews

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Chart 18. Model of Monomers Used in DFT Study of the Radiwcal Cyclization

Table 9. DFT Study of the Radical Cyclization of Model of Monomers Ea/kcal mol−1 monomer

exo

endo

M-1 M-2 M-3

6.8 6.6 5.41 4.7 4.5 5.2 8.68 8.9 14.02 13.8 5.33 5.7 9.82 10.0 10.54 13.9 15.2 19.69 19.89

10.8 9.8 11.18 10.3 8.8 9.1 12.20 12.5 11.59 11.0 10.25 10.8 12.08 12.9 15.79 8.7 8.7 26.65 27.03 21.9 16.3

M-4 M-5 M-6 M-7 M-8 M-9

M-10 M-11 M-12 M-13 M-14 M-15

product/kcal mol−1 exo

−11.81

endo

−18.65

−2.07

−7.88

3.75

−8.87

−13.51

−20.07

−2.16

−8.49

calculation

experimental results

B3LYP/6-311++G(d,p) B3LYP/6-311++G(d,p) B3LYP/6-31G* B3LYP/6-311++G(d,p) B3LYP/6-311++G(d,p) B3LYP/6-311++G(d,p) B3LYP/6-31G* B3LYP/6-311++G(d,p) B3LYP/6-31G* B3LYP/6-311++G(d,p) B3LYP/6-31G* B3LYP/6-311++G(d,p) B3LYP/6-31G* B3LYP/6-311++G(d,p) B3LYP/6-31G(d) B3LYP/6-311++G(d,p) B3LYP/6-311++G(d,p) B3LYP/6-31G(d) B3LYP/6-31G(d) B3LYP/6-31G(d) B3LYP/6-31G(d)

ref

cyclopolymerization (exo) cyclopolymerization (exo) cyclopolymerization (exo) cyclopolymerization (exo) cyclopolymerization (exo) cyclopolymerization (exo) cyclopolymerization (endo) cyclopolymerization (exo) cyclopolymerization (exo)

cyclopolymerization (endo) cyclopolymerization (endo) cyclopolymerization (exo) or normal polymerization or cross-linking no report of radical polymerization cross-linking cyclopolymerization (endo)

405 405 403 405 405 405 403 405 403 405 404 405 404 405 406 405 405 406 407 87 87

and dimethyldiallylsilane.54 In these cases, the rate determining step resides on the cyclization step rather than the intermolecular propagation. Thus, the produced polymer often

monomer concentration, which is due to faster cyclization compared to intermolecular propagation. Zeroth-order kinetics is observed for the cyclopolymerization of 1,8-nonadiynes202 9044


Chemical Reviews

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The control of polymer rigidity, packing, and solubility, with direct consequences on the materials properties, have been demonstrated for several systems in different sections of this review. The peculiar alternating intraintermolecular chain mechanism of cyclopolymerization has the ability to afford, when using properly designed monomers, efficient cyclization, propagation, and alternation between two functionalities with very different reactivities toward the propagating chain. As such, in several of the presented sections, through the use of a removable “tethers”, the sequence-controlled, precision synthesis of macromolecules have been demonstrated. We believe this aspect can become increasingly important in the design of future, innovative macromolecular systems.

contains an uncyclized repeating unit especially in the cyclopolymerization under high monomer concentration. One exception is the cyclopolymerization of 1,6-heptadiene using bis(imino)pyridine Co catalyst, where the polymerization proceeds with quantitative cyclization of the monomer (vide supra).66 There is a growing interest toward cyclizations under thermodynamic control, as witnessed by the increasing number of contributions dedicated to equilibria between rings and chains, in dynamic combinatorial and supramolecular polymer chemistry. The concept of effective molarity and of equilibrium effective molarity,407 which gives a quantification of the ease of formation of a cyclic compound, has been recently introduced and it is foreseable it will play a major role in the physical organic chemistry of the cyclopolymerization field in the future.

AUTHOR INFORMATION Corresponding Authors

9. CONCLUSIONS AND OUTLOOK Cyclopolymerizations have been the focus of considerable and increasing attention, as elaborated throughout this review. Recent studies have led to considerable understanding of the effect of substituents, structure, and reaction conditions applicable to the monomers in order to tailor the cyclopolymerization efficiency and fidelity. Furthermore, the control on cyclopolymerization procedures has led to the possibility of realizing complex macromolecular architectures, with repeating units not achievable otherwise through conventional polymerization. The summary of the achievements of the last 15 years, divided in sections following the logical order of the chaingrowth mechanism used for the polymerization, show that the field is progressing fast in all directions. In the early systems, obtained by free-radical polymerization and described in section 2, serendipity played a big role, eventually leading in some cases to consolidated commercial products; these systems are indeed still attracting attention. On one side, their cyclopolymerizations have been the subject of in depth computational studies, in order to predict and rationalize the polymerization outcomes. As a second aspect, side chains and substituents have been modified in order to tailor polymer properties for functional materials applications. Furthermore, the application of modern living/controlled methods for radical polymerization have been sometimes directly translated to cyclopolymerization affording increased fidelity of the process, the control target molecular weights and the demonstrated possibility of reinitiating the process to obtain block cyclopolymers. Large ring systems, achieved by using long yet suitably designed tethering units between acrylate and or styrene-like difunctional monomers, have been then used for functional properties, such as supramolecular recognition. The transition-metal mediated chain-growth polymerizations mechanisms of a huge variety of difunctional dienes, diynes, or enyne systems have demonstrated enormous potential and possibilities in the stereoselective/stereospecific addition during propagation, to afford stereocontrolled and sequence controlled polymer backbones. This is the consequence of recent advances in new initiating systems and polymerization catalysts. The design of the transition metal complex catalysts enabled formation of polymers with different ring size in controlled manner from a single diene or diynes, in addition to the synthesis of polymers with highly controlled tacticity. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures. It is easy to foresee that several aspects in this particular field will be targeted in the near future.

*E-mail: [email protected]. Homepage: www.unipv.it/labt. *E-mail: [email protected]. ORCID

Dario Pasini: 0000-0002-8273-3798 Daisuke Takeuchi: 0000-0001-6150-4083 Notes

The authors declare no competing financial interest. Biographies Dario Pasini obtained his first degree from the University of Pavia in 1992 and a Ph.D. degree in Chemistry in 1996 from the University of Birmingham, U.K. (Prof. Sir J. Fraser Stoddart, Nobel Prize in Chemistry 2016). After postdoctoral research at the University of California, Berkeley, U.S.A., in the group of Prof. Jean M. J. Fréchet (1997−1999), he joined the faculty of the Department of Chemistry at the University of Pavia in 2000, where he is now an Associate Professor (with National Qualification to Full Professor, Organic Chemistry, since Nov. 2014). He continues to develop his scientific interests in the fields of organic, supramolecular, and polymeric materials. He spent periods of research as a visiting professor at the University of Geneva (Prof. Stefan Matile, 2005) and at the University of South Carolina (Prof. Linda Shimizu, 2011). He is a Fellow of the Royal Society of Chemistry. He has supervised more than 40 research collaborators (undergraduate and postgraduate students, postdocs, and visiting scientists). Daisuke Takeuchi was born in 1972 in Ishikawa, Japan. He received his B. Eng. degree from the University of Tokyo under the direction of Prof. Shohei Inoue in 1994 and M. Eng. from the University of Tokyo under the direction of Prof. Takuzo Aida in 1996. In 1998, he joined the Chemical Resources Laboratory, Tokyo Institute of Technology as a Research Associate in the group of Prof. Takeshi Endo, where he received a Ph. D. degree in 2000. He joined the group of Prof. Kohtaro Osakada in 2000 as an Assistant Professor and was promoted to an Associate Professor in 2006. He was a visiting professor at University of Strasbourg (Prof. Dominique Matt, 2009) and a visiting researcher at EPFL (Prof. Harm-Anton Klok, 2011). In 2018, he joined the faculty of Hirosaki Univerisy as a Professor. The research in the Takeuchi group focuses on the development of polymerization reactions utilizing metal catalysts as well as the design of metal complex catalysts for controlled polymerization.

ACKNOWLEDGMENTS D.P. wishes to thank past and present members of his research group for excellent contributions. D.T. is grateful to Professor 9045


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Kohtaro Osakada of Tokyo Institute of Technology for helpful discussion.

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