Polymerization of Polar Monomers Mediated by Main-Group Lewis

Review Cite This: Chem. Rev. 2018, 118, 10551−10616

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Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid−Base Pairs Miao Hong,*,†,⊥ Jiawei Chen,‡,⊥ and Eugene Y.-X. Chen*,§ †

Chem. Rev. 2018.118:10551-10616. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/30/18. For personal use only.

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China ‡ Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States § Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States ABSTRACT: The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

CONTENTS 1. Introduction 2. Overview of the Structures and Properties of Lewis Acids, Bases, and Pairs 2.1. Definitions of Lewis Acids and Bases and Their Reactivity Scales 2.2. Classical Lewis Acids, Bases, and OrganoLewis Acids and Superbases 2.3. Classical Lewis Adducts (CLAs) and Frustrated Lewis Pairs (FLPs) 2.4. Boundary and Intermediacy between CLAs and FLPs 3. Polymerization Mediated by Lewis Acids and Bases 3.1. Cationic Polymerization by Lewis Acids 3.2. Anionic Polymerization by Lewis Bases 3.3. Group Transfer Polymerization (GTP) 3.4. Ring-Opening Polymerization (ROP) 3.5. Lewis Acid-Catalyzed Polymerization Involving (Hydro)silanes 4. Polymerization Mediated by Lewis Pairs 4.1. Conjugate-Addition Polymerization 4.1.1. Linear Acrylic Monomers 4.1.2. Cyclic Acrylic Monomers 4.1.3. Monomers Bearing the CCCN Functionality © 2018 American Chemical Society

4.1.4. Vinyl Phosphonates 4.1.5. Divinyl Acrylic Monomers 4.2. Ring-Opening Polymerization 4.2.1. Cyclic Esters 4.2.2. N-Carboxyanhydrides 4.2.3. Copolymerization of CO2 (COS and Anhydrides) with Epoxides 4.3. Solvent Effects on Lewis Pair Polymerization 5. Lewis Pairs and Polymerization in Materials Chemistry 6. Summary and Outlook Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments References

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10573 10577 10577 10577 10584 Received: June 1, 2018 Published: October 11, 2018

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DOI: 10.1021/acs.chemrev.8b00352 Chem. Rev. 2018, 118, 10551−10616

Chemical Reviews

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polymerization (ROP) of lactides and related cyclic esters.17 Nevertheless, significant advances have been achieved since the inception of LPP in 2010, and a comprehensive review on the status of this emerging area is thus needed, which is the purpose and effort of this invited paper. In this review, the scope of monomers is focused on polar monomers, referred to those polymerizable hydrocarbon compounds containing heteroatoms (commonly O, N, P, and S), particularly polar alkenes, such as acrylic monomers and vinyl ethers, and heterocyclic esters, such as lactones, lactides, and cyclic anhydrides. Such monomers can be polymerized by various mechanisms, including cationic polymerization,18−20 anionic polymerization by classical anionic initiators13,14,21−23 or emerging organic initiators,24−35 radical polymerization,36,37 group transfer polymerization (GTP),38−44 and metal-mediated coordination polymerization.15,45−48 Radical polymerization assisted by LAs to modulate polymerization characteristics, particularly stereochemistry, has already been reviewed in the past.49 The concepts of dual catalysis50 and cooperative catalysis51 have also been discussed in the related polymerization reactions. In light of the above comprehensive reviews already published, following an overview of structures and properties of LAs, LBs, and FLPs in section 2, the polymerizations of polar monomers mediated by the related LAs and LBs alone (i.e., not used together as a pair) covered in section 3 are only updated or highlighted where the chemistry is related to that of the LPP, but are not comprehensively reviewed. The main section, section 4, is focused on various LPP processes, followed by section 5 on the application examples of LPs and LPP in materials chemistry. The review is concluded with a summary and outlook (section 6) and other associate contents. When describing a given polymerization system, key characteristics of a polymerization system that are frequently commented on include polymer molecular weight (MW) information, such as number-average molar mass (Mn), weightaverage molar mass (Mw), and molar mass dispersity (Đ = Mw/ Mn), initiator (catalyst) efficiency, as defined by I* = Mn(calcd)/Mn(exptl), where Mn(calcd) = MW(monomer) × [monomer]0/[initiator(catalyst)]0 × conversion (%) + MW(end groups), and activity, as defined by the turnover frequency, TOF = moles of substrate (monomer) consumed per mole of catalyst (initiator) per hour). For the purpose of this review, polymers with Mn (g/mol) values of 105, and >106 are arbitrarily characterized as exhibiting low, medium, high, and ultrahigh MWs; likewise, polymers with Đ values of 2.0 are arbitrarily described as exhibiting low, relatively low, medium, and high dispersity values. Polymerization systems with TOF (h−1) values of 10, >100, >1000, and >10000 are arbitrarily characterized as exhibiting low, modest, high, very high, and exceedingly high activities, respectively.15 The resulting polymer tacticity, corresponding to isotactic (mm triads), syndiotactic (rr), and heterotactic (mr), is sometimes commented on when such information is available from the reviewed papers. Polymers with mr ≈ 50, mm (rr) = 60−79, mm (rr) = 80− 89, and mm (rr) ≥ 90 are arbitrarily termed atactic, iso-rich (syndio-rich) atactic, isotactic (syndiotactic), and highly isotactic (highly syndiotactic) polymers, respectively.15 It is recognized that many published papers used terms of controlled polymerization and living polymerization interchangeably, but this review defines that a controlled polymerization simply demonstrates its capacity to control the MW,

1. INTRODUCTION This review is focused on the emerging area of polymerization catalysis by Lewis pairs (LPs), termed as Lewis pair polymerization (LPP),1 which is mediated by either classical Lewis (acid−base) adducts (CLAs) or frustrated Lewis pairs (FLPs).2−11 LPP exploits the cooperativity of the Lewis acid (LA) and the Lewis base (LB) in an FLP, a CLA, or an interacting Lewis pair (ILP) in which the LA and the LB promote cooperative monomer activation and chain initiation (for active species formation), chain propagation (for polymer chain formation), and chain termination or transfer events (Scheme 1). Hence, LPP differs from the classical zwitterionic Scheme 1. Illustrative Example of LPP of a Functional Group (FG)-Bearing Conjugated Polar Alkene Monomer (M) by a Classical Lewis Adduct (CLA), an Interacting Lewis Pair (ILP), or a Frustrated Lewis Pair (FLP), Showing a Generic Chain Initiation Step To Generate the Zwitterionic Active Species and Subsequent Propagation Steps To Produce Polymer Products

polymerization that is typically initiated by an LB [or nucleophile (Nu)] or an LA [or electrophile (El)] but not by an LA/LB pair or Nu/El pair.12 LPP also departs from the classic anionic polymerization that is typically initiated by a negative charge, Nu−.13 Although various types of additives, including acids or bases, are commonly added to anionic polymerization systems to enhance their control over the polymerization,14 both initiation and propagation mechanisms invoked in classical anionic polymerization are different from those involved in LPP. Also distinguished here is that, unlike the typical coordination polymerization of olefins, polar conjugated monomers or heterocyclic monomers by cationic metallocenium complexes,15 where the polymer chain is growing only from the cationic site and the anion serves as a charge-compensating reagent, in LPP both LB and LA or Nu and El pairs participate in polymerization events such as chain initiation, growth, transfer, or termination. Since the concept of LPP with an FLP and a CLA was first introduced in 2010,16 LPP has attracted an increasing level of interest and achieved remarkable successes in the areas of polymer synthesis and/or polymerization catalysis over the last eight years, thanks to its demonstrated broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as unique application in materials chemistry. In 2013, Chen overviewed LPP to that date in a short commentary mainly focusing on the conjugate-addition polymerization.1 While this paper was under preparation, Wu very recently published a short review on LP-mediated ring-opening 10552


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dispersity, tacticity, or architecture (i.e., it has a specif ic meaning of controlling what specif ic structures or properties). On the other hand, a living polymerization provides the maximum degree of control in polymerization processes; accordingly, a living polymerization is much more rigorously tested, and a set of key experimental criteria52 (including a linear increase of polymer Mn vs monomer conversion and monomer-to-initiator ratio while maintaining low Đ values and thus predictable polymer Mn, high to quantitative initiation efficiency I* (no or negligible side reactions), precision in polymer chain extensions, and ability to synthesize well-defined block copolymers) must be met before a living polymerization system can be claimed. For well-studied polymerization systems with rich information available, polymerization kinetics, mechanistic aspects including chain initiation, propagation, and termination pathways, and chemo/stereoselectivities will also be described.

energies, enthalpies, equilibrium constants, calorimetric measurements, UV−vis spectra, infrared (IR) spectra, NMR signals and shifts, and computational approaches. The extension of the acid/base concept from the proton (H+) donor and acceptor (Brønsted−Lowry) to the electron acceptor and donor (G. N. Lewis) greatly unifies the bonding interactions and reaction patterns between molecules, but at the same time creates inevitable difficulty on quantifying the corresponding properties. There are indeed no full-range, absolute, and consistently quantitative Lewis acidity/basicity scales, because of the complicated nature of the acid−base interaction and the assumption and inherent limitation for selection of the reference. The reliability and accuracy might be in question if the comparison is based on markedly different references. However, by choosing a sensible and representative set of references, one might be able to obtain a reasonably qualitative or semiquantitative ranking of properties. This review covers some commonly used scales that have been employed or have implication for catalyst properties in the LPP systems. As for LAs, a large number of efforts have been directed to measure and rationalize their strength from both experimental and computational aspects. Childs et al. developed a spectroscopic method to quantify the Lewis acidity by measuring the 1H NMR resonance difference of H-3 of the crotonaldehyde (as the LB reference) before and after complexation with an interacting LA.57,58 The relative Lewis acidity of the selected LAs by setting the value of BBr3 to unity follows the increasing order of Et3Al (0.44), SnCl4 (0.52), TiCl4 (0.66), B(C6F5)3 (0.77), BF3 (0.77), AlCl3 (0.82), SbCl5 (0.85), BCl3 (0.93), and BBr3 (1.00).57,59 Similarly, Gutmann and Beckett used triethylphosphine oxide as a referencing probe to parametrize the Lewis acidity by the change of the 31P NMR shifts upon complexation; they converted the 31P shift scale to the so-called acceptor number (AN) by setting two arbitrary points with hexane (AN = 0) and SbCl5 (AN = 100).60−62 The ANs for commonly used LAs in polymerization are as follows: SnCl4 (59) < TiCl4 (70) < B(C6F5)3 (82) < AlCl3 (87) < BF3 (89) < SbCl5 (100) < BCl3 (106) < BBr3 (109) < BI3 (115).60,63 In addition, computational methods for affinity with different references in the gas phase have served as a complementary approach to gauge the Lewis acidity, especially for transient LAs. Among them, the fluoride ion affinity (FIA), negative enthalpy of formation in the reaction of LA + F− → LA−F−, has often been used as a convenient scale.64−70 Krossing et al. suggested that molecular LAs with an FIA greater than that of monomeric SbF5 (489 kJ mol−1) can be regarded as Lewis superacids, as SbF5 represents a conventional, strong, robust, and synthetically useful LA. In this scale, B(C6F5)3 (FIA = 444 kJ mol−1) lies below the borderline of 489 kJ mol−1, whereas its heavier analogue Al(C6F5)3 (FIA = 530 kJ mol−1) can be categorized as a Lewis superacid (Figure 1).67 It is also worth pointing out that the difference in the hydride ion affinities (HIAs) of B(C6F5)3 and Al(C6F5)3 (484 and 483 kJ mol−1) is significantly smaller than that of the FIAs, indicating that the borane stabilizes the softer hydride ion slightly better than the alane does, although the latter is a stronger acid toward the harder F−. This conclusion is also consistent with Pearson’s hard and soft (Lewis) acids and bases (HSAB) theory71 and Drago’s EC ionic/covalent model,56,72 which predict that the considerably softer B(C6F5)3, with a less electron positive metal center and thus less polar B−C bonds, should have a higher affinity toward softer LBs such as hydride.65 More recently, the concept of the global electrophilicity index (GEI, defined as ω

2. OVERVIEW OF THE STRUCTURES AND PROPERTIES OF LEWIS ACIDS, BASES, AND PAIRS 2.1. Definitions of Lewis Acids and Bases and Their Reactivity Scales

In 1923, G. N. Lewis formulated the Lewis concept to describe chemical interaction between an acid and a base as the action of accepting and donating a lone pair of electrons.53 Accordingly, an LA is defined, on the basis of IUPAC terminology,54 as “a molecular entity (and the corresponding chemical species) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base”, and an LB is defined as the opposite. Consequently, the Lewis acidity (Lewis basicity) can be termed as “the thermodynamic tendency of a substrate to act as a Lewis acid (Lewis base). Comparative measures of this property are provided by the equilibrium constants for Lewis adduct formation of a series of Lewis acids (Lewis bases) with a common reference Lewis base (Lewis acid)”. Reaction between an LA and an LB is usually accompanied by neutralization or reduction of the individual reactivity, leading to the formation of a more robust CLA compared to individual parent precursors. For example, the reactivity of pyrophoric, airsensitive BMe3 and Me3P is quenched upon the formation of a stable Me3B ← PMe3 solid (melting point 91 °C, Scheme 2), whose negative enthalpy of formation ΔH(g) is 16.5 kcal mol−1 in the gas phase.55,56 Scheme 2. Reaction between BMe3 (LA) and PMe3 (LB) To Form a Stable CLA

The LA and LB strength, recommended by IUPAC, should be measured in the form of equilibrium constants or the corresponding Gibbs free energies of adduct formation with a specific interacting reference. Historically, acidity and basicity parameters are often quantified by the degree of the donor− acceptor interaction of the formed Lewis adduct in both thermodynamic and spectroscopic scales, including Gibbs 10553


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= μ2/2η = χ2/2η, where μ is the chemical potential, χ is the electronegativity, and η is the chemical hardness)73,74 is utilized by Stephan et al. as a metric for Lewis acidity.75 They computed GEI values for 22 p-block LAs, including boranes, trityl derivatives, phosphonium cations, and sulfoxonium cations. Although GEI measures the electrophilicity of molecules, it provides an expeditious computational approach to a parameter that shows a good correlation with the Lewis acidity measured by FIA in Stephan’s study. As for LBs, the balance of basicity and nucleophilicity, which is very important in the LB-mediated polymerization, there are also different reactivity scales developed by using different acid references to measure generally Brønsted basicity and nucleophilicity. Brønsted basicity is still a widely used concept since its reference acid is the simplest and most encountered proton H+. The equilibrium constant pKa (pK(BH+) of the B− H+ in a specific solvent such as water, DMSO, or acetonitrile) of the conjugate acid is used to quantify the Brønsted basicity of a base.76−79 The pKa value for basicity has been used for the

Figure 1. Lewis acidity scale of some common LAs determined by computational fluoride ion affinity (FIA). FIA values of BF3, BCl3, BBr3, B(C6F5)3, BI3, AlCl3, SbF5, and Al(C6F5)3 were extracted from ref 67.

Table 1. Comparison of Different Scales of Basicity for Selected Lewis Bases (Often Used as Brønsted Bases as Well) entry

base

pK(BH+)a

ref(pK(BH+))

DN(SbCl5)b

DN(BF3)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 14 15 16 17 18

Et3N Et2O THF pyridine acetonitrile DMF DMSO DMAN DMAP DBU TBD IMe IiPr IiPr(Me) ItBu IMes TPT t Bu-P2 t Bu-P4

10.8 [18.7] −3.8 [0.1] −2.05 [1.1] 5.21 [12.33] −10 −1.2 [6.1] −1.8 [5.8] 12.34 (7.47) [18.62] 9.2 [17.95] 11.9 (13.9) [24.34] 14.5 [26.03] 23.0 (22.0) [32.4] 28.2 (22.8) [33.3] (24.0) 25.2 (23.2) [33.9] 20.8 (19.4) 16.8 (21.5) [33.13] (30.2) [42.7]

76−79 76−79 76−79 76−79 76−79 76−79 76−79 80, 81, 85 85 80, 85, 86 85, 86 87, 88 87, 89 87 87−89 88, 89 88 80, 90 80, 90

132.6 80.3 88.0 142.3 61.1 111.3 124.7

135.87 78.77 90.4 128.08 60.39 110.49 105.34

62, 62, 62, 62, 62, 62, 62,

151.55 159.36

84 84

ref(DN) 84 84 84 84 84 84 84

pKa values determined in water (DMSO) [acetonitrile]. Computational values given in italics. bNegative ΔHf (kJ/mol) with SbCl5 in 1,2dichloroethane. cNegative ΔHf (kJ/mol) with BF3 in dichloromethane.

a

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Figure 2. Illustrative examples of the polymeric or dimeric nature of classical inorganic LAs, depicting solid-state structures of polymeric (AlCl3)n, dimeric (AlBr3)2, and dimeric (AlMe3)2. Crystallographic CIF files obtained from refs 92−94.

arbitrary definition of the so-called organic superbase, such as guanidines, phosphazenes, and Verkade’s bases, whose basicity is equal to or higher than that of the proton sponge, 1,8bis(dimethylamino)naphthalene (DMAN; pKa = 12.34 in water).80,81 In terms of Lewis basicity (or nucleophilicity toward a certain electrophile), Gutmann proposed the concept of the donor number (DN) derived from the negative enthalpy of the complexation between an LB and the LA reference SbCl5 determined by calorimetric measurements.62,82 A more recent LA reference introduced by Gal is BF3 to form the construction of a wider scale that contains a wider list of LBs.83,84 The basicity values of some selected LBs as gauged by pKa (in water, DMSO, and acetonitrile),76−81,85−90 SbCl5, and BF3 scales62,84 are summarized in Table 1, including commonly used coordinating solvents (as the polarity/coordinating properties of a solvent typically have great impact on the polymerization behavior) and basic initiators or catalysts involved in the polymerization processes mediated by LBs and LPs. A more detailed discussion on the Lewis acid−base interaction, Lewis basicity, and measurements with different scales can be found in Gal’s book published in 2009.84 Encouragingly, the existing scales for Lewis acidity and basicity and models for LP interactions do serve as useful tools for polymer chemists to rationalize catalyst reactivities and develop more active and/or efficient polymerization systems. The pKa scale of basic catalysts has been utilized very often in the discussion of the basicity and nucleophilicity of catalyst activities. The Gutmann−Beckett scale for measuring the Lewis acidity was indeed first introduced to correlate the Lewis acidity to the catalyst activity of boron-containing acids in the ROP of epoxides.60 HSAB theory has been employed to elucidate that the activity of LAs in the living cationic polymerization is determined by the balance of the interactions between the carbocation growing site and the metal halide as the LA, with the anions shuffling in between, and the added LB.18

environments and interact with monomers differently in the polymerization processes. As the interest of polymerization methods is often shifting more toward the emphasis of livingness or control, these equilibria become challenging issues that are difficult to overcome; for example, the existence of different active sites of polymeric LAs tends to promote polymerization at different rates, hence resulting in formation of polymers with rather broad or even multimodal molecular weight distributions. Nonetheless, classical LAs, including inorganic and simple organometallic compounds, have been widely used in polymerization processes. Perhaps the best-known example is the development of Ziegler−Natta polymerization.95,96 The inorganic, heterogeneous catalyst consisting of transition-metal complex TiCl3 and main-group LA Et2AlCl as the activator or cocatalyst was first applied for the polymerization of olefins and the generation of isotactic polypropylene, and is still widely used in the large polyolefin industry. However, the inherent complexity originated from the heterogeneous and polymeric nature of the catalyst system, as well as many possible activation pathways of such systems, has prompted chemists to develop single-site catalysts based on discrete transition-metal complexes and main-group LAs. The study of such single-site catalysts, both metallocene- and non-metallocene-based, allowed the isolation and characterization of important intermediates or active species at a molecular level, hence greatly enhancing our understanding of mechanistic aspects of Ziegler−Natta polymerization and providing catalyst design principles for better control of the polymerization and stereoselectivity.97−100 As a notable example, the combination of group 4 metallocene alkyl complexes with a strong organoLewis acid, B(C6F5)3, allowed Marks et al. to generate isolable and crystallographically characterizable cationic metallocenium complexes that are highly active in olefin polymerization.101 One of the most widely used organo-LAs in polymer chemistry is arguably tris(pentafluorophenyl)borane, B(C6F5)3,45,102 which was synthesized in the 1960s by Massey et al.103−106 and not exploited in polymerization catalysis until the 1990s.107 The installation of the three electron-withdrawing C6F5 groups on boron creates a highly electron-deficient boron center at the same time with less polar B−C bonds around it. As a result, compared to inorganic boron halides and other (fluoroalkyl)boranes, B(C6F5)3, in addition to being a strong LA, exhibits several unique features, including (a) excellent thermostability, (b) resistance against oxidation by O2 and hydrolysis by water, and (c) facile preparation in a base-free form. While it was mostly utilized as a cocatalyst in olefin polymerization in the past,45 recent years have seen its extensive application in LA and LP catalysis. Especially in the past decade, B(C6F5)3 has shown its versatility in mediating

2.2. Classical Lewis Acids, Bases, and Organo-Lewis Acids and Superbases

LAs have a long and rich history for their ubiquitous applications in organic and polymer synthesis.91 At the early stage of development, simple inorganic LAs are usually employed because they are readily available, conveniently handled, and inexpensive. However, inorganic LAs such as halides and hydrides are typically in oligomeric or polymeric forms in the solid state and even in solution (Figure 2).92−94 In general, extra energy is required to break down the aggregation; thus, the Lewis acidity might be reduced and difficult to assess. In addition, perhaps more detrimentally, complicated equilibria might exist between those widely distributed active centers that are often associated with different chemical reactivity/steric 10555


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Figure 3. Structural comparison between B(C6F5)3 and [Al(C6F5)3]2. Adapted with permission from ref 115. Copyright 2016 Royal Society of Chemistry.

basic molecules such as toluene solvent or its precursor Et3SiH to form Et3Si+(C7H8) or Et3Si+···H−SiEt3.121,122 However, it is worth noting that, from the polymerization viewpoint, the use of the isolated R3Si+ is often unnecessary as long as the substrates or monomers are sufficiently basic enough to rapidly replace the fourth coordination on silicon. Thus, a variety silylium salts, paired with different counterions, have found their applications in polymer synthesis.123−129 For example, the direct use or in situ generation of silylium ions with weakly coordinating anions has been reported in the ROP of chlorophosphazene trimer [N3P3Cl6]123 and GTP.130−133 Anion-bound silyl compounds such as Me3SiOTf and Me3SiNTf2 have been recently been applied as the LA component in the LPP in conjunction with phosphine LBs.129 The rapid and broad development of the FLP chemistry also promoted chemists to develop other organo-LAs with high reactivity and/or selectivity, some of which are based on elements outside groups 13 and 14. For example, Zn(C6F5)2 has been used in the ROP of lactides.134 Some examples of unconventional and highly Lewis acidic phosphonium cations have been developed by Stephan and co-workers since 2013.135,136 In addition, the heavier analogue stibonium cation was reported as well in 2014.137 These new types of group 15 cations utilize their σ* empty orbital as the electron acceptor and show promising application in small molecule activation and catalysis, although their applications in polymerization catalysis have not been extensively explored. Nevertheless, [(C6F5)3PF][B(C6F5)4] and [(C6F5)2PPhF][B(C6F5)4] have been found to mediate cationic polymerization of isobutylene,138 while phosphonium [(SIMes)PFPh2][B(C6F5)4]2139 and stibonium [Sb(C6F5)4][B(C6F5)4]137 have been shown to promote the ROP of tetrahydrofuran, demonstrating their high Lewis acidity as well as potential applications in polymer synthesis. The Lewis acidity of LAs obviously plays an important role in the polymerization activity and other polymerization characteristics. While the LA with a higher Lewis acidity usually offers faster rates of chain initiation and/or propagation with a higher degree of polar monomer activation, it could also induce undesirable side reactions such as chain termination or transfer processes, thus yielding an uncontrolled or nonliving polymerization. However, high-speed living polymerizations mediated by strong organo-LAs such as E(C6F5)3 (E = B, Al) and silylium ions R3Si+ have been achieved, which will be described in the sections that follow. LBs also play important roles in both organic synthesis and polymerization processes.140 During the earlier stage of developing various polymerization methods, anionic and

different polymerization reactions such as Piers−Rubinsztajn polymerization,108−110 tandem hydrosilylation/GTP,39 and LPP,1 which will be discussed in detail in the corresponding sections. Replacing boron with aluminum in B(C6F5)3 led to another widely applied LA, Al(C6F5)3. Al(C6F5)3 can be prepared by ligand exchange between B(C6F5)3 and AlMe3 or AlEt3.111−113 Several lines of evidence, as summarized by Chen in 2012,114 argued that Al(C6F5)3 possesses higher Lewis acidity. More recently, the successful isolation of stable adducts with weak LBs such as Et3SiH and ferrocene with only Al(C6F5)3 further supports this argument.115,116 Suitable synthetic approaches toward LAs with enhanced acidity can be designed, but the challenge to maintain a completely undisturbed electronaccepting site might not be trivial, because such highly acidic species tend to seek stabilization even from their weakly basic environments, such as solvents, counterions, and their own ligands. In comparison, the more Lewis acidic Al-based LAs tend to have higher propensity to form dimeric structures than the analogous B-based LAs. For example, B(C6F5)3 is believed to exist in monomeric form, whereas the transient and monomeric Al(C6F5)3 without any stabilization has never been observed. The single-crystal X-ray diffraction (SC-XRD) analysis of the alane synthesized in a noncoordinating medium revealed that it is indeed [Al(C6F5)3]2, adopting a dimeric geometry with a doubly bridging interaction between the aluminum centers and the o-fluorines of the −C6F5 ring (Figure 3).115 The geometrical feature of the dimeric structure suggests that the exothermic elimination of Al−F and formation of benzyne species might be responsible for the thermal and shock sensitivity of Al(C6F5)3, especially in its unsolvated form. Due to such instability, extra caution should be exercised when handling this LA. Another example comes from the contrasting features between the monomeric (C6F5)2BCl and dimeric (C6F5)2AlCl.117,118 The cations of tricoordinate group 14 elements are isoelectronic to neutral group 13 LAs discussed above. Carbocations are involved in the process of cationic polymerization as the active chain propagating species. In addition, carbon-based cations can serve as a hydride abstractor to give other LAs involved in polymerization processes, such as generation of silylium ions R3Si+ and related species. The isolation and characterization of tricoordinate, highly reactive silylium cations is an area with intense and continuing interest, which had been surrounded by numerous discussions and debates on whether the generated silylium cation is purely tricoordinate and truly free of the fourth coordination.119−121 For example, the Et3Si+ cation tends to interact with weakly 10556


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(MBL), and γ-methyl-α-methylene-γ-butyrolactone (γMMBL) by Al-based LPs, revealing the first application of the FLP chemistry in polymer synthesis.16 They recognized the ambiphilic nature of such monomers, which could act as an LB or an LA to form an adduct with either LA [Al(C6F5)3]145 or LB (IMes NHC, Figure 4),146 and utilized them for cooperativity with LPs, either a CLA or an FLP, to generate active propagating species (zwitterionic species, Figure 5)147 that rapidly polymerize monomers to high-MW polymers.

inorganic bases were usually used. Recent efforts have been devoted toward the development of systems that enable polymerization in more selective and efficient or greener ways. Hence, the selection of basic catalysts or initiators has been shifted to those neutral, less toxic, organic superbases such as N-heterocyclic carbenes (NHCs), amidines, guanidines, and phosphazenes for organopolymerization.24,35 2.3. Classical Lewis Adducts (CLAs) and Frustrated Lewis Pairs (FLPs)

In a classical term, the combination of an LA and an LB, by sharing a pair of electrons, results in a CLA, the formation of which usually quenches the individual reactivities of the acidic and basic precursors (Scheme 2). The reactivities of LAs and LBs, as well as the interactions between them, have been the fundamental rules for main-group and transition-metal chemistry, assisting chemists to rationalize organic transformations and polymer synthesis. The lack of formation of a CLA between an LA and an LB is often considered due to insufficient reactivity (electronic frustration). Steric hindrance to prevent the formation of a CLA was brought up when Brown and co-workers explained the different phenomena between the 2,6-lutidine/BF3 and 2,6-lutidine/BMe3 Lewis pairs as early as 1946.9,141,142 They found that, while 2,6-lutidine readily forms a CLA with BF3, there is no apparent reaction between 2,6lutidine and BMe3 upon mixing (Scheme 3), to which they

Figure 5. Cooperative activation of MMA with the Al(C6F5)3/tBu3P LA/LB pair to generate phosphonium enolaluminate zwitterion active species and the corresponding SC-XRD structure (the E isomer). Structure reprinted with permission from ref 147. Copyright 2012 Royal Society of Chemistry.

Scheme 3. Differences in Interaction between 2,6-Lutidine and BF3 and between 2,6-Lutidine and BMe3

More recently, Stephan and co-workers reported the involvement of single electron transfer (SET) processes in the FLP-type chemistry.148 The combination of Mes3P (Mes = mesityl) with E(C6F5)3 (E = B or Al) gave rise to a pale purple solution without the formation of an LP adduct. However, the mixture of Mes3P with Al(C6F5)3 is EPR active, indicative of the formation of P-center radical [Mes3P•]+ in the SET equilibria between the FLP and [Mes3P•]+ and [•Al(C6F5)3]−. This result implies that the H2 activation with Mes3P/E(C6F5)3 might proceed through the homolytic cleavage of H2 via one-electron processes.

attributed steric crowdedness as the main reason. However, the investigation of the reactivity of such a system toward small molecules was not carried out at that time. In 2006, Stephan reported the seminal work on the construction of a system with sterically demanding, strongly Lewis acidic and basic moieties, in which the interaction between the LA and LB is precluded due to their steric hindrance.143 As a result, this orthogonal LP exhibited unquenched reactivity and is capable of promoting reversible heterolytic cleavage of dihydrogen into hydride and proton, bonded to the corresponding LA and LB, respectively. This is conceptually different from the conventional understanding of the LP chemistry, in which strong LAs form stable adducts with strong LBs and the individual reactivity would be quenched upon complexation. This unique type of activation mode, termed the FLP chemistry, has opened up a new path toward new reactivity development, expanded to a broad range of other substrates, and to the areas of catalytic transformations, which have been reviewed quite extensively.3−11,144 In 2010, Chen et al. reported the polymerization of polar vinyl monomers such as methyl methacrylate (MMA), α-methylene-γ-butyrolactone

2.4. Boundary and Intermediacy between CLAs and FLPs

One obvious question arising from the comparison of CLA and FLP is whether there is a definitive boundary between them. The ability for substrate activation was once considered to be completely shut down upon the complexation between the acid and base. Therefore, CLAs and FLPs were once regarded as two discrete states that are not interchangeable. This existence of this definitive boundary was reexamined and disproved by Stephan in 2009 with the 2,6-lutidine/B(C6F5)3 system.149 They found that such a system favors the isolable CLA at low temperature, but establishes an equilibrium with the free LA/ LB form in solution. More importantly, the dissociated form provides sufficient orthogonal reactivity to heterolytically cleave

Figure 4. An example of polar vinyl monomer MMA as an ambiphile to react with either an LA or an LB to form the corresponding adducts and the SC-XRD structures of the LA adduct and the enamine derived from the LB adduct. 10557


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polymerization compared to the anionic polymerization. In this regard, selecting appropriate Lewis acidic initiators or activators and balancing different LA−LB interactions in a polymerization system become crucial to stabilize the cationic growing site via a rapid equilibrium between the dormant and active states. Vinyl ethers are one of the most studied classes of polar monomers in living cationic vinyl polymerization, and the progress of this field is well documented and reviewed.18,20,151−153 With the growing species being reactive tricoordinate carbocations, the essence for rendering a cationic polymerization living is tuning and balancing the LA−LB interactions between the Lewis acidic carbocations and different LBs in the polymerization systems. In this context, Aoshima et al. categorized the interacting species into three different classes: nucleophilic counteranions, added weak LBs such as esters or ethers, and added salts, typically quaternary ammonium halides. Classifications from a mechanistic standpoint of living cationic polymerization has been sorted into (1) atom transfer type, (2) dissociation−recombination type, and (3) degenerative chain transfer type (Scheme 5). This section will highlight only recent examples of cationic polymerization with the LAs related to LPP since Aoshima’s reviews.18,151 A clear connection can be made between the FLP chemistry and the dissociation−recombination type (Scheme 5) in which the inactive dormant species is the Lewis adduct of the Lewis acidic polycarbocation and the added LB while the active propagating species is the dissociated Lewis pair or the FLP. Likewise, the atom transfer type can also be connected to the FLP chemistry. However, the motivation of developing such mechanistic pathways was to render a living polymerization from the uncontrolled cationic polymerization initiated by common metal halides with too high Lewis acid strength as they failed to establish the reversible dormant−active equilibrium to stabilize propagating sites. Higashimura et al. achieved living cationic polymerization of vinyl ethers by adding LBs such as ethyl acetate, 1,4-dioxane, tetrahydrofuran, and diethyl ether to modulate the structure/Lewis acidity of the active species.154−157 Different metal halides have been investigated, including EtAlCl2, SnCl4,158−160 AlCl3,161 and

dihydrogen and ring-open tetrahydrofuran, resembling the typical FLP-type chemistry (Scheme 4). In a more recent Scheme 4. Interconversion between Lewis Adduct and Lewis Paira

a

The dissociated form has the capability of activating dihydrogen.

report, Stephan et al. found that even a spectroscopically stable CLA could exhibit FLP-type reactivities.150 Specifically, proazaphosphatrane, P(MeNCH2CH2)3N, forms a stable adduct with B(C6F5)3, but the resulting CLA is capable of activating unsaturated substrates such as CO2, PhNCO, PhNSO, and PhCH2N3. These observations have important implications from a polymerization point of view, as Chen et al. noticed that the same LPs might express drastically different polymerization behaviors and activities, which largely depend on the addition sequence for the polymerization.16

3. POLYMERIZATION MEDIATED BY LEWIS ACIDS AND BASES 3.1. Cationic Polymerization by Lewis Acids

Cationic addition polymerization is a polymerization method involving cationic species as active propagating sites to polymerize electron-rich monomers such as vinyl ethers, isobutene, styrene, and N-vinylcarbazole derivatives. Conventionally, LAs with the typical formula of MXn (typically metal halides) have been used, when interacting with a protonogen or cationogen, to generate a proton or cation that can add to the double bond of a monomer and initiate the polymerization. Unlike anion polymerization, the propagating species in cationic polymerization are highly reactive, usually tricoordinate carbocations, and have high tendency to undergo chain termination/transfer via β-proton elimination and other side reactions. This resulted in delayed discovery of living cationic

Scheme 5. Three Types of Living Cationic Polymerization Systems According to Mechanistic Classifications

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Scheme 6. Use of Mixed-Metal Halides with the Added LB for Ultrafast Living Cationic Polymerization

ultrafast SnCl4/EtAlCl2162,163 (Scheme 6) systems. In 2009, Aoshima et al. carried out a systematic investigation on a wide scope of Lewis acidic metal halides (MXn).152 They found that the polymerization rates, in conjunction with ethyl acetate as the added LB, followed the order of GaCl3 ≈ FeCl3 > SnCl4 > InCl3 > ZnCl2 > AlCl3 ≈ HfCl4 > ZrCl4 > EtAlCl2 > BiCl3 > TiCl4 ≫ SiCl4 > GeCl4 ≈ SbCl3. This order is in fact proportional to the strength of the metal halide to extract the chloride anion from trityl chloride. The authors concluded that the activity of metal halides in cationic living polymerization is not determined by the “absolute” Lewis acid strength, but rather by the “relative” strength with respect to the chlorophilicity and oxophilicity, which determines the interaction of the metal halide with the chlorine atom of the chain end and the oxygen atom of the added base.152,164 These results further demonstrated that the polymerization activities of metal halide LAs with LBs are closely related to the balance between the interactions of different LA−LB pairing in a polymerization system.18,151,153 A recently developed unique type of the living cationic polymerization proceeds through a degenerative chain transfer approach, namely, reversible addition−fragmentation chain transfer (RAFT).165 This technique utilizes sulfur-containing RAFT chain transfer agents such as a thiocarbonylthio [R−S− C(S)−Z] or a thioether (R−S−Z) compound to convert an active cationic propagating chain into the dormant state through a covalent bond to sulfur while generating a new cationic growing site (Scheme 5). The livingness of the polymerization is enabled by the rapid degenerative exchange between the cationic active site and the chain transfer agent. This rather active area166−175 was recently reviewed.165 In a broader context, the interaction between the carbocation and the RAFT chain transfer agent is that of a typical LA−LB interaction, but the reaction leads to a new, active carbocation and the chain transfer agent, instead of an inactive or dormant CLA. Several new directions of the LA-mediated cationic polymerization are noted here. The first is the development of stereoselective cationic polymerization. In this context, Aoshima’s group reported the stereospecific cationic polymerization of N-vinylcarbazole (NVC) with different LAs in 2017.176 The in situ reaction between CF3SO3H and nBu4NX (X = halides) generated HX, which then added across the double bond of NVC to generate the corresponding addition product as the initiator. Without the addition of an external LA, the polymerization was very slow at − 78 °C in CH2Cl2 and required 144 h to achieve 85% conversion (in the case of n Bu4NI), with a low isotaticity of mm = 45%. Upon screening with different LAs, including ZnCl2, EtAlCl2, SnCl4, GaCl3, and TiCl4, the authors found that the combination of CF3SO3H and n Bu4NCl with ZnCl2 at − 78 °C in CH2Cl2 afforded poly(NVC) with Mn = 8.5 kg/mol and Đ = 1.3 and, more importantly, with a high isotacticity of mm = 94%. Such stereospecific polymerization is believed to be controlled by the ion pair interaction in the propagating carbocation and the counterion derived by the halide abstraction with a specific LA.

Another direction is the (sequence) controlled cationic copolymerization of vinyl ethers with other types of monomers such as aromatic aldehydes,177−180 cyclic acetals,181 and oxiranes (e.g., isobutylene oxide, IBO).182 In the latter example, the concurrent cationic vinyl addition and ring-opening copolymerization of vinyl ethers with IBO was enabled by using B(C6F5)3 (together with adventitious moisture) as the initiator. The reason for the use of strong organo-LA B(C6F5)3, instead of conventional LAs such as BF3OEt2 or GaCl3, is twofold. First, the reaction of B(C6F5)3 with adventitious water produced H+, which initiates the cationic polymerization: B(C6F5)3 + H2O → H+[HOB(C6F5)3]−. Second, the resulting anion is weakly coordinating, which allows no formation of too stable, covalently bonded growing ends that could result in no cross-propagation between vinyl ether and IBO monomers with substantially different reactivities. Follow-up studies of the scope of the oxirane monomers revealed that the carbocation formation through the ring-opening of the oxirane-based oxonium is crucial.183,184 Thus, oxiranes that can generate substituent-stabilized or tertiary carbocations, such as IBO and isoprene or butadiene monoxides, can be copolymerized with vinyl ethers because the oxirane-derived carbocation propagation sites could be readily formed and cross-react with the vinyl ether monomer. Aoshima and co-workers also examined cationic terpolymerization of vinyl ethers, cyclic ethers, and ketones. Impressively, such terpolymerizations of isopropyl vinyl ether (IPVE)/cyclohexene oxide (CHO)/methyl ethyl ketone (MEK) mediated by B(C6F5)3185 and IPVE/oxetane/ MKE by Ph3CPF6186 proceeded in a one-direction cycle sequence in the order of a concurrent vinyl addition, ringopening, and carbonyl addition mechanism, affording multiblock polymers with the repeating units of (IPVE)x−(CHO)y− MEK1 and (IPVE)x−(oxetane)y−MEK1, respectively (Scheme 7). Noteworthy also is the cationic copolymerization of vinyl Scheme 7. LA-Mediated Cationic Terpolymerization of Vinyl Ethers, Cyclic Ethers, and Ketonesa

a

DTBP = 2,6-di-tert-butylpyridine.

ethers with biomass-derived monomer furfural, which led to the formation of new 2:(1 + 1)-type alternating copolymer structures comprising two vinyl ether units linked by a Diels−Alder dimer of furfural as the repeating unit (Scheme 8).187 The third direction is the development of metal-free polymerization processes or new polymerization reactions using strong organo-LAs without other additives or cocatalysts/initiators. For example, behaving differently from B10559


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Scheme 8. Cationic Copolymerization of Vinyl Ethers with Furfural To Produce a 2:(1 + 1)-Type Alternating Copolymer Structure

Scheme 9. Direct Polymerization of Styrene (A) and Benzofuran (B) by the Highly Lewis Acidic Al(C6F5)3 and Their Corresponding Active Species

(C6F5)3, which requires addition of a protogen (e.g., water), the stronger organo-LA Al(C6F5)3 can directly initiate the cationic polymerization of styrene in the absence of a protogen such as adventitious water (as the monomer and solvent were extensively dried via distillation over (Oct)3Al, the reaction was performed on a high-vacuum line (10−6−10−7 Torr), and the moisture shut down the polymerization).188 On the basis of mechanistic studies such as the use of proton trap DTBP, which did not form a CLA with the unsolvated Al(C6F5)3 but underwent what is later termed FLP-type reactivity by reacting with the alane−toluene adduct189 (or activated toluene when performed in toluene solvent) to form pyridinium−aluminate ion pairs (Scheme 9A), Chen proposed that the polymerization was initiated by proton transfer from the alane-activated toluene and from the zwitterionic adduct of styrene and the alane. In 2017, Cui and co-workers also reported that the highly Lewis acidic Al(C6F5)3 can directly promote the cationic polymerization of benzofuran without addition of protonic or cationic co-initiating reagents.190 It was proposed that the binding of the alane to the oxygen of the furan ring polarizes the adjacent CC bond, which enables the nucleophilic attack of an incoming monomer to the positively charged carbon to

initiate the cationic polymerization (Scheme 9B). According to the mechanism presented in the work, the benzofuran repeat units have the head-to-tail regioselective arrangement in the polymer, which exists in a variety of stereoisomers of cis and trans configurations, thus an amorphous, transparent material. 3.2. Anionic Polymerization by Lewis Bases

Anionic polymerizations by classical initiators have been extensively reviewed in the past.13,14,21−23 In recent years, organic LBs (and Brønsted bases) have emerged as versatile, selective, and powerful initiators for organopolymerization, which has been recognized as a greener or more sustainable synthesis of polymeric materials.191−196 Over the past 10 years, several groups have published a number of critical and comprehensive reviews24−35 on the topic of organopolymerization, covering the utilization of organic LBs for conjugateaddition polymerization, zwitterionic polymerization, ROP, GTP, and anionic polymerization. Thus, in this section, we only highlight the LB-mediated conjugate-addition polymerization,146,197−206 focusing on the polymerization mechanism and monomer scope, summarized in Table 2, which is closely relevant to the discussion of the LPP in section 4. 10560


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Table 2. Summary of Anionic Polymerization of Acrylic Monomers by Organic Bases

formed upon the nucleophilic addition of the NHC to MMA in DMF. In contrast, less nucleophilic IMes led to formation of a single-MMA addition product, an enamine or deoxy-Breslow intermediate, formed through a two-step proton transfer mechanism (Scheme 10B). On the other hand, the much less nucleophilic TPT catalyzes tail-to-tail umpolung dimerization of MMA207−209 in toluene at 80 °C via the TPT-derived enamine intermediate, which subsequently reacts with MMA through a conjugate-addition step, followed by a proton transfer (Scheme 10C). For the more reactive monomer γMMBL, all three NHCs promoted polymerization rather than single-monomer addition or dimerization, with ItBu being the most active catalyst of this NHC series, exhibiting an exceptionally high TOF of 7320 h−1 and producing PγMMBL with Mn = 70−85 kg/mol and Đ = 1.45−2.10. Two chain transfer and termination pathways, including the H transfer/

The LB-mediated direct conjugate-addition polymerization of MMA and its derivatives, such as γMMBL, dimethacrylates, and multi-vinyl-functionalized γ-butyrolactones, has relied mainly on the utilization of strongly nucleophilic NHCs as the effective initiators. In 2012, Chen et al. revealed the first, rapid conjugate-addition organopolymerization of γMMBL by ItBu at room temperature (RT) to produce acrylic bioplastic,197 followed by a combined experimental and theoretical study on the mechanism of this type of conjugate-addition polymerization toward acrylic monomers.146 It was found that ItBu polymerizes MMA in polar donor solvent DMF to syndio-rich atactic PMMA (rr = 55.7%) with a medium Mn of 33.2 kg/mol (Đ = 1.99) and a TOF of 136 h−1 (Scheme 10A), while no polymerization was observed in the relatively nonpolar solvent toluene; this drastically different reactivity was attributed to the increased stability of the active zwitterionic intermediate 10561


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Scheme 10. Different Mechanistic Pathways Involved in the Reaction and Polymerization of Acrylic Monomers with Different NHCsa

a

TOL = toluene.

Scheme 11. Structures of the TPT Catalyst and Its Derivatives, as Well as the Zwitterionic Species and Corresponding Spirocyclic Species Involved in HTP

reaction was proposed to proceed through conjugate addition of IiPr to two molecules of MMA, leading to a zwitterionic species, followed by proton transfer, cyclization, and the release of methanol to furnish the cyclodimerization product (Scheme 10D). Buchmeiser and Naumann et al. employed a series of CO2-protected NHCs as thermally latent precatalysts for MMA polymerization.210 In 2017, Naumann, Falivene, and co-workers

enamine addition termination present only with a high catalyst loading (Scheme 10A1) and the more energetically favored chain transfer to monomer (Scheme 10A2), have been identified. In 2014, Taton et al. also reported that ItBu initiated the polymerization of MMA in DMF at RT and in toluene at 50 °C, whereas IiPr reacted with two molecules of MMA to form an imidazolium−enolate cyclodimer in toluene.198 The latter 10562


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Scheme 12. Oxa-Michael Addition Polymerization of Hydroxyl-Functionalized Acrylate Monomers Mediated by NHCs (A) and Chemoselective Polymerization of Multi-Vinyl-Functionalized γ-Butyrolactones by NHCs To Produce Unsaturated Poly(MBL)s (B)

revealed that highly polarized N-heterocyclic olefins (NHOs; Table 2) were also effective for organopolymerization of N,Ndimethylacrylamide (DMAA), MMA, methacrylate (MA), and tert-butyl methacrylate (tBuMA) in relatively nonpolar toluene.199 However, the polymerization was uncontrolled, which yielded the polymers with broad dispersities (Đ = 1.63∼11.8), presumably due to unfavorable aggregation of active chain ends. The addition of LiCl to DMAA polymerization can break up the aggregates and stabilize the growing chain ends, leading to PDMAA with relatively lower dispersities (Đ = 1.32∼3.80), but the I* decreased sharply from 73.0% to 1.50−15.7%. Dimethacrylates (DMA) with different linkages between the two methacrylic moieties (Table 2) can be polymerized by TPT and its substituted derivatives in a step-growth mechanism into unsaturated polyesters, through a polymerization termed proton (H) transfer polymerization (HTP) (Scheme 11).200,201 A combined experimental and theoretical study suggested that HTP proceeds through the step-growth propagation cycles consisting of repeated fundamental steps, including conjugate addition, proton transfer, and TPT release, where the second conjugate addition of the enamine intermediate to DMA is the rate-determining step, and the subsequent H transfer step follows an intramolecular mechanism via a five-membered transition state (thus formally a 1,4-H shift). The zwitterionic intermediates formed by conjugate addition prefer to adopt the more stable closed spirocyclic structures (Scheme 11), which thermodynamically drives the reaction toward the polymerization products. The role of the suitable phenol (e.g., 4-methoxyphenol and catechol) is critical for achieving an effective HTP, which not only shut down the radically induced vinyl addition polymerization under HTP conditions (80∼120 °C), but also facilitated the proton transfer processes. Among several TPT-based NHCs employed, OMe2TPT is the most active and efficient catalyst which produces the polymer with the highest Mn of 16.7 kg/mol (Đ = 1.64), attributable to its capability to act as both a strong nucleophile and a good leaving group. Hydroxyl- and double-bond-functionalized multivinyl acrylic monomers can be polymerized by NHCs via oxa-Michael addition and chemoselective polymerization pathways, respectively. Matsuoka et al. found that TPT and IDipp brought about oxa-Michael addition polymerization of hydroxyl-

functionalized acrylate monomers, including 2-hydroxyethyl, (Z)-4-hydroxybut-2-enyl, 4-hydroxycyclohexyl, and 4-hydroxybut-2-ynyl acrylates (Scheme 12A), albeit with low polymerization activity (TOF = 0.1−0.3 h−1, monomer/NHC = 10/1), to give a low MW of the corresponding poly(ester ether) (Mn = 0.54−2.4 kg/mol, Đ = 1.2−3.8).202 Analysis of the polymer chain ends revealed that the NHC is covalently linked to the polymer chain, indicating that the NHC acts as an LB to generate the zwitterionic intermediate without directly activating the hydroxyl group of the monomer. Therefore, the polymerization was proposed to be initiated by the reaction of the NHC with monomer to generate the zwitterionic intermediate followed by proton transfer (Scheme 12A). The resulting alkoxide undergoes oxa-Michael addition to the monomer to form a poly(ether ester). During the polymerization, the active alkoxide can also attack the ester carbonyls in the main chain or monomer, resulting in transesterification. In the case of multivinyl acrylic monomers, such as γ-vinyl-αmethylene-γ-butyrolactone (VMBL), γ-vinyl-β-methyl-α-methylene-γ-butyrolactone (VMMBL), and γ-diallyl-α-methylene-γbutyrolactone (DAMBL), Chen et al. disclosed that the NHCmediated polymerization of such monomers is completely chemoselective, affording vinyl-functionalized poly(MBL)s (Scheme 12B).203 Thus, the polymerizations by NHCs proceed exclusively via polyaddition across the conjugated α-methylene double bond while leaving the γ-vinyl double bond intact. For VMBL, ItBu catalyzed fast polymerization, exhibiting an extremely high TOF of 80000 h−1 at an exceptionally low catalyst loading of only 50 ppm, whereas IiPr(Me) was relatively less effective. The double bonds attached to the repeat unit of PVMBL can be either thermally cured or postfunctionalized via the thiol−ene “click” reaction to afford cross-linked materials or thioether polymer products. Many organic bases can function as both an LB and a Brønsted base so that either a nucleophilic or base mechanism, or both, can operate or compete in a polymerization process. In this context, Chen, Cavallo, and co-workers revealed that the high-speed organocatalytic polymerization of γMMBL directly initiated by tBu-P4 proceeded via chain initiation involving abstraction of a proton from the β-carbon of γMMBL by tBu-P4 (Scheme 13) rather than the nucleophilic addition of tBu-P4 to γMMBL, due to the high basicity and low nucleophilicity of t Bu-P4.204 With a low tBu-P4 loading of 0.02 mol%, the γMMBL 10563


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Scheme 13. Outlined Chain Initiation, Propagation, and Termination Events Involved in the High-Speed Anionic Polymerization of MMBL by tBu-P4 Alone, through the Monomer Deprotonation Pathway

Scheme 15. Proposed Two Competing Mechanistic Pathways for the Catalytic Dimerization of Crotonates by an NHC

polymerization in DMF was complete in 1 min, giving rise to a high TOF of 5040 h−1 and affording medium- to high-MW PγMMBL (Mn up to 111 kg/mol). The base mechanism involves a highly reactive anionic monomer initiating species stabilized by the nanosized cation [tBu-P4H]+. On the other hand, Taton et al. disclosed a controlled polymerization of MMA by ItBu in combination with an alcohol (BnOH) where the nucleophilic and base pathways compete (Scheme 14).205 The nucleophilic pathway involves direct 1,4-conjugate addition of ItBu to MMA, and the role of the alcohol is to increase the electrophilicity of MMA through H-bonding, while the base pathway proceeds through conjugate addition of ItBu-activated BnOH to MMA, thanks to the high basicity of ItBu (pKa = 22.8, DMSO). Waymouth et al. also reported that the dimerization of crotonates, a congener of MMA, into diesters by IiPr(Me) involved the competing nucleophilic and base pathways (Scheme 15).206 The nucleophilic mechanism proceeds via the two steps of Michael addition to form the zwitterionic species, which sequentially eliminates IiPr(Me) to generate the head−tail dimerization product, while the base catalysis proceeds through the deprotonation of the vinylic CH3 protons by IiPr(Me) (pKa = 24.7, DMSO), followed by nucleophilic addition of the second monomer and then proton transfer to the monomer, which regenerates the active species (Scheme 15). Both pathways are viable mechanisms for the dimerization, due to the fact that IiPr(Me) is both a good nucleophile and an

excellent base. When the inorganic base tBuOK was used, an even more rapid dimerization (under 15 s to full conversion) was achieved. 3.3. Group Transfer Polymerization (GTP)

GTP was first disclosed by DuPont scientists Webster et al.43,211 in 1983 while seeking a new living polymerization method to replace the conventional low-temperature anionic polymerization for the production of printer inks and automobile finishes from (meth)acrylate monomers. Such technology is operative at room temperature (RT) or even higher temperatures to produce acrylic polymers with controlled structures with predictable Mn and narrow Đ values. The conventional GTP proceeds with a silyl ketene acetal (SKA) initiator and a nucleophilic LB or electrophilic LA catalyst, and the polymerization was termed such on the basis of the initially postulated associative propagation mechanism in which the silyl group remains attached to the same polymer chain and is simply transferred intramolecularly to the incoming monomer through hypervalent anionic silicon species (path a in Scheme 16A). However, this associative mechanism was challenged by a dissociative mechanism,212−214 which involves the ester enolate anion as the propagating species and a rapid, reversible complexation or termination of small

Scheme 14. Proposed Two Competing Mechanisms for ItBu-Mediated Polymerization of MMA in the Presence of BnOH: Nucleophilic Mechanism (A) and Base Mechanism (B)

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Scheme 16. Four Different Types of GTP Systemsa

a

MMA used as the polar monomer for illustration.

terminology “group transfer polymerization” might not be an accurate description of the actual mechanism for these processes anymore, the name has been embedded in most of the literature and used to refer to polymerizations involving SKA species as initiators. Typical monomeric SKA initiators themselves are regarded as inactive, requiring activation by anionic or neutral nucleophilic bases as catalysts through coordination of the catalyst to the silicon center to generate a pentacoordinate silicon species (Scheme 16A).39 Anionic bases such as SiMe3F2−,211,215,216 HF2−,211,215−217 F−,216−218 CN−,211,216−218 N3−,211,217 oxyanions,219,220 and hydrogen bioxyanions219−221

concentrations of the enolate anion with SKA or its polymer homologue (path b in Scheme 16A).38 There have been heavy debates over these two mechanisms, and it has been suggested that the actual mechanism could interchange between the dissociative and associative mechanisms with slight variation in the structures of the applied catalysts, initiators, and solvents. Over the years, several different types of GTP systems have been developed, according to the catalyst type and initiating mechanism, including LB-catalyzed GTP (i.e., SKA/LB), acidcatalyzed GTP (i.e., SKA/H+ and SKA/LA), oxidative GTP (i.e., SKA/carbocation), and tandem GTP (i.e., R3SiH/LA) (parts A−D, respecitvely, of Scheme 16). Although the 10565


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Table 3. Summary of Organic LB-Catalyzed GTP

that such NHC-catalyzed GTP promotes living polymerization of not only MMA but also a bulky acrylate, tert-butyl acrylate (tBA), thus offering an advantage over the conventional GTP catalyzed by anionic bases. Since then, many other neutral organic LBs have been employed to catalyze the GTP. Kakuchi et al. employed superbases such as DBU, P(CH3NCH2CH2)3N) (TMP), P(iPrNCH2CH2)3N) (TiBP), tBu-P2, and tBu-P4 as catalysts for polymerization of MMA.228 Common phosphines such as Bu3P, Cy3P, Ph3P, and tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP) were also screened as catalysts for GTP.227 Other methacrylates such as stearyl methacrylate (SMA), allyl methacrylate (AMA), and 4-(N,Ndiphenylamino)benzyl methacrylate (TPMA) were also effectively polymerized with SKA/tBu-P4 to the corresponding poly(methacrylate)s with narrow Đ values.231,232 More recently, Chen and co-workers found that GTP with SKA

were commonly employed in the early stage of investigations. Such SKA/anionic base GTP systems can effect living polymerization of methacrylates at RT but not for the more reactive acrylate monomers. More recently, neutral organic bases such as NHCs,29,222−226 phosphorus-based nucleophiles,227,228 phosphazene superbases,228−232 and even basic solvents233 have been utilized as well and are summarized in Table 3. NHC-catalyzed GTP was reported in 2008 by Waymouth225 and Taton226 independently, but the two groups proposed different mechanisms (i.e., dissociative vs associative), based on individual experimental observations using only slightly different structures of the applied NHC catalysts, 1,3-diisopropyl4,5-dimethylimidazol-2-ylidene (IiPr(Me)) vs 1,3-diisopropylimidazol-2-ylidene (IiPr) and 1,3-di-tert-butylimidazol-2-ylidene (ItBu). Regardless of the actual mechanism, both groups found 10566


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Table 4. Summary of Acid-Mediated GTP

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Table 4. continued

a

Generated in situ by the reaction between Et3SiH and [Ph3C]+[B(C6F5)4]−.

(EA) or MMA. Kakuchi et al. further studied the GTP of an extensive library of N,N-disubstituted acrylamides using B(C6F5)3 as the LA catalyst and a series of dimethylketene methyl trialkyl(R)silyl acetals (RSKA) as the initiator.241 The kinetic profile of the polymerization indicated the presence of an induction period, attributed to the structural mismatch between the SKA initiator and the acrylamide monomer, as well as a zeroth-order dependence on the monomer concentration as a result of the tendency of B(C6F5)3 to coordinate to the monomer and maintain a constant concentration of the activated monomer, followed by a relatively slow step of C− C bond formation (Michael addition). More recently, Zhang et al. investigated the GTP of MMA, MBL, and γMMBL with the SKA/E(C6F5)3 (E = B or Al) systems.242,243 They observed that the MMA polymerization by SKA/B(C6F5)3 was sluggish but exhibited good activity for the more reactive cyclic acrylic monomers MBL and γMMBL. However, replacing B(C6F5)3 with Al(C6F5)3 led to a higher efficiency for MMA polymerization but, more importantly, rendered living GTP of MBL

initiators can also proceed without addition of an anionic and neutral base catalyst.233 Table 4 summarizes acid (Brønsted acids131,234−236 and LAs133,237−243) catalyzed GTP processes. Brønsted acids are also included in this discussion as they serve as a reagent or precatalyst to generate an LA (R3Si+) as the true catalyst upon protonation of the SKA initiator (vide infra). In the early stage of the GTP development, inorganic LAs such as zinc halides, aluminum halides, and oxides were used to activate the SKA initiator for the polymerization of more reactive acrylates, with higher catalyst loadings (typically 10−20 mol % relative to the monomer).217,244 The living polymerization of acrylates with low catalyst loadings was achieved by the HgI2/Me3SiI system (5.2 mol % HgI2 and 2.1 mol % Me3SiI relative to the initiator).245−248 In 2000, Ute el al. reported that organo-LA B(C6F5)3237 or alkylaluminum bisphenoxides,238 when combined with a silylating agent such as Et3SiOTf, Me3SiOTf, or Et3SiI, are effective for controlled, SKA-initiated GTP of ethyl acrylate 10568


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Table 5. Summary of Tandem GTP

a

Mn determined by end group analysis using NMR methods.

and γMMBL. The SKA/Al(C6F5)3 exhibited a very different kinetic profile when compared to the SKA/B(C6F5)3 system: the polymerization rate is zeroth-order against the monomer concentration, and therefore, a rate-determining C−C-forming Michael addition is involved, following the relatively rapid monomer activation by Al(C6F5)3. This indeed resembles the polymerization behavior and also the mechanism of the silylium-catalyzed GTP,131 the results of which further confirmed that strongly Lewis acidic Al(C6F5)3 and silylium cations131 are more efficient in activating the monomer, as compared to B(C6F5)3.

Strong Brønsted acids have also been shown to promote living GTP processes. In 2009, Kakuchi et al. employed trifluoromethanesulfonimide (HNTf2) to promote living GTP of MMA with MeSKA.234 Recognizing the inability of the conventional GTP to control the polymerization of acrylamides,211,217 Kakuchi et al. also applied the Brønsted acidpromoted GTP to acrylamide monomers and achieved living or controlled polymerization of acrylamides using an amide-based enolate initiator.235 In 2010, Chen et al. developed the highspeed living GTP of polar vinyl monomers promoted by selfhealing silylium catalysts (R3Si+) derived from the reaction of 10569


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Me

generate the SKA initiator in situ and subsequent polymerization. It is well-known that strong organo-LA B(C6F5)3 can activate the monomer in the LA-catalyzed GTP and also activate a Si−H bond and promote its addition across unsaturated double bonds such as CC and CO via the FLP process.250−252 Considering the fact that SKA is a formal 1,4-hydrosilylation product of a conjugated monomer such as MMA, a dual-functional catalyst such as B(C6F5)3 that promotes both 1,4-hydrosilylation and Michael addition of such a monomer could realize a tandem GTP in which the monomer is first converted into the SKA initiator, followed by a typical GTP process. In this context, Kakuchi et al. examined the tandem GTP of nBA using hydrosilanes/B(C6F5)3.253 They further extended the B(C6F5)3-catalyzed tandem GTP method for the polymerization of N,N-disubstituted acrylamides, with Me2EtSiH being the most effective hydrosilane for polymerizing N,N-diethylacrylamide.254 This tandem GTP system is also capable of polymerizing MMA,116,255,256 other methacrylates,255 and acrylamide monomers,254 as summarized in Table 5. For the tandem GTP, a fine balance between the rates of the 1,4-hydrosilylation and propagation has to be reached to achieve good control over the polymerization characteristics such as polymer Mn and Đ values. Polymerization of methacrylates is usually more efficient and controlled as compared to the more reactive acrylates using other types of polymerization methods, but the tandem GTP of methacrylates by R3SiH/B(C6F5)3 is less efficient than that of acrylates, due to less efficient methacrylate 1,4-hydrosilylation.255 Although the tandem Me2PhSiH/B(C6F5)3 system realized controlled MMA polymerization, the efficiency was not high and the MW of PMMA was limited to Mn ≈ 17.6 kg/mol. In addition, for the tandem GTP, it is important to take into account the two seemly contradicting factors of Si−H bond coordination and CO bond coordination in the hydrosilylation and monomer activation steps. As B(C6F5)3 provides limited monomer activation and thus slow chain propagation, it is reasoned that the enhancement of the Lewis acidity of the applied LA, while maintaining the ability to catalyze 1,4-hydrosilylation, would accelerate the overall rate of MMA polymerization.256 In this context, Chen et al. revealed that the tandem GTP system with the stronger LA silylium cation, Et3SiH/[Et3Si(L)]+[B(C6F5)4]−, enables the rapid, efficient (I* > 96%) polymerization of MMA.256

SKA with Brønsted acids with weakly coordinating anions, such as [H(Et 2 O) 2 ]+ [B(C6 F 5 ) 4 ]− and [HNMe 2 Ph]+ [B(C6F5)4]−, or a chiral disulfonimide anion.131 The direct use of silylium-like cations as the catalyst is also effective. Chen et al. showed that the MeSKA-initiated controlled GTP of MMA was catalyzed by the in situ generation of “Et3Si+” cation from the reaction of Et3SiH with [Ph3C]+[B(C6F5)4]−.133 Kakuchi and co-workers compared the polymerization characteristics by Me SKA/HNTf2 and MeSKA/Me3SiNTf2 and found that the polymerization behaviors of the two systems are similar to each other.239 The Me3SiNTf2, when coupled with the bulkier and less active iPrSKA, can also efficiently promote acrylate polymerization.240 Oxidative GTP was termed such since the activation of the SKA initiator involves oxidation of the neutral SKA by a catalytic amount of [Ph3C]+[B(C6F5)4]− into cationic silylium R3Si+ species (the true catalyst), as opposed to the reductive activation of the SKA by anionic reagents to form hypervalent silicate species.133 This system was first reported by Chen et al. in 2008 for high-speed living GTP of alkyl (meth)acrylate as well as γMMBL catalyzed by the in situ generated R3Si+ cations. On the basis of the results of variable-temperature NMR experiments, the authors proposed that the unique “monomerless” initiation involved vinylogous hydride abstraction of SKA by [Ph3C]+[B(C6F5)4]− to generate the silylated MMA or Me3Si+-activated MMA, which reacts quickly with another molecule of SKA via Michael addition to give a bifunctional active propagating species consisting of both the electrophile (Me3Si+) and nucleophile. Further kinetic and mechanistic studies led to a proposed propagation “catalysis” cycle consisting of a fast step of recapturing the silylium catalyst from the ester group of the growing polymer chain by the incoming MMA, followed by a rate-determining step (rds) of the C−C bond formation via intermolecular Michael addition of the polymeric SKA to the silylated MMA. A follow-up study by Chen et al. investigated structure− reactivity relationships in the oxidative GTP of (methy)acrylates with different group 14 (C, Si, Sn, and Ge) ketene acetals and trityl salt activators.132 The scope of the monomers was further extended to include cyclic and renewable acrylic monomers such as γMMBL.249 Again, for such highly reactive monomers, the iBuSKA + [Ph3C]+[B(C6F5)4]− combination is the most active and controlled system, producing well-defined polymers and copolymers with an exceptionally narrow Đ of 1.02. To further enhance the efficiency and control of the oxidative GTP by overcoming the rate-limiting, bimolecular C− C bond formation step, Chen et al. designed and synthesized di-SKA systems by covalently linking electrophilic R3Si+ and nucleophilic SKA active sites into a single dinuclear catalyst/ initiator.130 Such dinuclear catalyst/initiator systems indeed afforded more efficient and selective oxidative GTP processes relative to the MeSKA system, affording a highly syndiotactic polymer with rr = 91.4% at a polymerization temperature of − 78 °C. The polymerization was shown to be first-order in monomer and catalyst concentrations, consistent with an intramolecular Michael addition propagation mechanism with a more facile C−C bond formation step. The first-order dependence on [M] also indicates that the replacement of the penultimate ester group that chelates to the silylium cation with the incoming monomer becomes the rate-limiting step.130 Tandem GTP refers to a GTP process using a hydrosilane, instead of a typical SKA initiator, in combination with a strong LA that catalyzes both hydrosilylation of the monomer to

3.4. Ring-Opening Polymerization (ROP)

The thermodynamics of ROP is driven by the enthalpy of ringopening for strained cyclic monomers or by the entropy of ringopening for strainless large or macrocyclic monomers, while the kinetics and selectivity of the ring-opening process are strongly influenced by the nature of the reactive chain ends, catalysts, and monomers. The ROP can be mediated or catalyzed by LAs and LBs, which is briefly summarized as follows in the context of their relevance to LPP. The ROP promoted by LAs is closely related to cationic ROP,20,257 a method involving positively charged active centers in the process of converting heterocyclic monomers into larger cycles and/or acyclic polymers. The catalysts and active sites in this type of polymerization are acidic and capable of accepting a pair of electrons. Hence, the polymerizable cyclic monomers typically contain electron-rich sites that can be activated by such types of electrophiles, including cyclic ethers, thioethers, amines, lactones, thiolactones, lactides, lactams, carbonates, 10570


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Scheme 17. Two Generalized Mechanisms (ACEM and AMM) Involved in the LA-Mediated Cationic ROP

siloxanes, phosphazenes, and others.258 The mechanistic discussions for the ROP promoted by electrophiles are well documented.20,257−260 There are two generally accepted mechanisms, the activated chain end mechanism (ACEM) and activated monomer mechanism (AMM), which to operate might depend on a variety of factors such as applied catalysts, monomer type, the use of a (co)initiator, and polymerization conditions. Using the ROP of oxiranes as an example, the first mechanism (ACEM) involves a propagating chain end of an electron-deficient site that adds to an incoming monomer through an SN1 or SN2 pathway. In the SN1 pathway, the ringopening of the coordinated monomer occurs before the attack by another monomer. In the SN2 pathway, the attack of the incoming monomer takes place prior to the ring-opening (Scheme 17). Because the active site always stays at the chain end of the growing polymer, it is termed the activated chain end mechanism (ACEM). In the second mechanism, the monomer is activated by the catalyst and attacked by a nucleophile that is typically an added initiator or its polymeric analogues repeatedly. Since the cationic charge resides on the activated monomer but not the end of the polymer chain, it is termed the activated monomer mechanism (AMM). The kinetic and mechanistic aspects of ACEM and AMM have been compared by Penczek.259,260 The scope of electrophilic catalysts used in the cationic ROP is quite broad, including Brønsted acids, alkylating agents, acylating agents, carbenium cations, oxonium, neutral LAs, and others. Neutral LAs have been seldomly employed alone for controlled cationic polymerization. Instead, they are usually combined with a protongen or cationgen to generate the proton or cation as the true catalyst for polymerization. Brønsted acid catalysts have been studied and reviewed extensively.35,261 Main-group cationic catalysts with discrete structures for cyclic ester and epoxide polymerizations have been discussed by Sarazin and Carpentier in their recent review.262 Strong organo-LAs such as boranes and alanes have been employed to catalyze the ROP. For example, B(C6F5)3 was found to be an exceptional catalyst for the ROP of propylene oxide (PO) with mono- and multifunctional alcohol initiators to produce PPO with high primary hydroxyl contents.263,264 This organo-LA was also combined with double metal cyanide (DMC) catalysts to create a cascade sequence that combines the high primary hydroxyl regioselectivity of B(C6F5)3 with the rapid ring-opening rates achievable by the

DMC catalyst in a single process to produce polyetherols with higher MW (Mn > 4.0 kg/mol).265 B(C6F5)3 has also been found to catalyze the ROP of glycidyl phenyl ether (GPE).266 It was shown that B(C6F5)3 can react with glycidyl phenyl ether to form a zwitterionic intermediate that propagates the chain growth via repeating nucleophilic attack of the incoming GPE monomer. The resulting macrozwitterions (active chains) can cyclize by nucleophilic attack of the oxygen (attached to the boron) on the α-carbon of the oxonium ion, with elimination of B(C6F5)3. In the presence of water as the protongen, linear poly(GPE) was formed with −H and −OH chain ends. While B(C6F5)3 is not capable of initiating the ROP of THF, it can promote the copolymerization of GPE and THF to give poly(GPE-co-THF). The zwitterionic intermediate resulting from the interaction between B(C6F5)3 and GPE (and the macrozwitterionic equivalents) serves as the active species to ring-open the THF molecule, thus achieving cross-propagations. The resulting macrozwitterions can undergo termination to form cyclic poly(GPE), cyclic poly(GPE-co-THF), and linear poly(GPE-co-THF) via elimination of B(C6F5)3, reaction with water, or chain transfer.267 In the presence of reduced graphene oxide as a coinitiator, such poly(GPE) and linear poly(GPE-coTHF) can be grafted onto the surface of reduced graphene oxide.268 Poly(ε-caprolactone) (PCL) with narrow dispersity was synthesized by the ROP of ε-caprolactone (ε-CL) using (B(C6F5)3) as the catalyst and benzyl alcohol as the initiator in the bulk at 80 °C.269 Such a system can tolerate functional initiators such as 2-hydroxyethyl methacrylate, propargyl alcohol, 6-azido-1-hexanol, and methoxypoly(ethylene glycol), resulting in end-functionalized PCLs. This B(C6F5)3-catalyzed ROP of ε-CL exhibits controlled/living characteristics, including the successful chain extension with δ-valerolactone (VL) and trimethylene carbonate (TMC) to afford block copolymers PCL-b-PVL and PCL-b-PTMC.269 In addition, when studying the polymerization of ε-CL with the Al(C6F5)3/ phosphine/ROH system, Nakayama and co-workers noted that Al(C6F5)3 along with ROH can also catalyze the polymerization of ε-CL.270 In the LB-mediated ROP, the LB can act as the catalyst or direct initiator, while the exact mechanism depends on the key properties of the LB (Lewis basicity, steric hindrance, etc.) and use of an alcohol initiator, analogous to the LB-mediated 10571


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Scheme 18. Summary of Various Types of Chain Initiation and Propagation Pathways That Have Been Proposed To Be Involved in the ROP of Lactones by LBs for Polymer Chain Formation

was also employed as the direct initiator for the ROP of Llactide, which behaved as a dinucleophile, forming the zwitterionic species and the bis(zwitterion).295 Baceiredo, Mignani, and co-workers reported the utilization of phosphorus ylides as strong nonionic bases for anionic ROP of cyclic siloxanes.296 In the absence of an alcohol, a strong LB can also deprotonate the monomer to form an enolate species for chain propagation, which is determined by the relative Lewis basicity and nucleophilicity of the LB and the acidity of the monomer (Scheme 18B). For the LBs that possess strong basicity but weak nucleophilicity, the ROP usually proceeds through an exclusive deprotonation initiation process. To this end, Chen et al. disclosed that organic superbase tBu-P4 can abstract the α-H of nonstrained γ-BL to generate highly reactive enolate species that mediate the ROP to form PγBL with full chemical recyclability.297 However, in the cases of LBs that exhibit both strong basicity and nucleophilicity, competitive initiation processes via both base and nucleophilic mechanisms can be observed. For instance, Waymouth et al. reported that the ROP of lactide by a cyclopropenimine superbase was initiated not only by deprotonation of lactide by the cyclopropenimine to generate an enolate but also by the nucleophilic attack of the cyclopropenimine to lactide to form zwitterionic species.298 Such competing mechanisms led to broadening of the polymer molecular weight dispersity. When the polymerization is carried out in the presence of an alcohol or other protic compounds [C(O)OH, S−H, and N−H, etc.], two mechanisms have been proposed, including the activated monomer (nucleophilic) mechanism (AMM) and activated initiator/chain-end (base) mechanism (ACEM). A key feature of the former mechanism is the formation of a zwitterionic intermediate generated from nucleophilic attack of the LB on the monomer followed by proton transfer from the initiating or propagating alcohol and subsequent acylation of

conjugate-addition polymerization (section 3.2). Taking the LB-mediated ROP of lactones as an example, in the absence of an alcohol initiator, the polymerization was proposed to proceed via nucleophilic attack of the monomer by the LB (as Nu:) to generate a zwitterionic species for further chain propagation (Scheme 18A). This zwitterionic ROP has been shown to be effective for generating cyclic macromolecules with relatively high MWs.27,271 The monomer that was studied the most for the basedmediated ROP is probably lactide for the synthesis of various types of PLA materials.272−276 Hedrick and Waymouth et al. first showed that the ROP of lactide can be mediated by IMes to generate cyclic PLA with Mn up to 30 kg/mol and Đ = 1.14− 1.31 (Scheme 18A).27,271 ε-CL and δ-VL can also be polymerized with more nucleophilic NHCs such as IiPr(Me) and IMe(Me).277,278 Cyclic polymer formation via NHCinitiated ROP of other monomers, such as cyclic siloxanes, 279,280 cyclic phosphates, 281 and N-carboxyanhydrides,282−284 was also reported. Highly polarized NHOs were also found to be effective for organopolymerization of lactide, lactones, trimethylene carbonate, and nonstrained γbutyrolactone (γ-BL),285 affording polymers with a linear topology or a mixture of cyclic and linear topologies (PγBL), though the polymerization proceeded in an uncontrolled manner.286,287 By analogy with the NHC-mediated ROP, other organic LBs, such as DBU, TBD, and 2-(tert-butylimino)2-(diethylamino)-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP) can bring about the polymerization of lactide,288 a glucose-based cyclic carbonate monomer (methyl 4,6-Obenzylidene-2,3-O-carbonyl-α-D-glucopyranoside),289,290 as well as homopolymerization and copolymerization of βbutyrolactone (β-BL) and benzyl β-malolactone.291−293 Recently, Waymouth et al. found that isothioureas were more selective for producing cyclic PLAs without appreciable linear contaminants, compared to the DBU initiator.294 (+)-Sparteine 10572


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Scheme 19. Generally Accepted Catalytic Cycle for the B(C6F5)3-Catalyzed FLP-Type Hydrosilylation

Scheme 20. Piers Hydrosilylation and Analogous Hydrosilylation Polymerization Approach

the resultant alkoxide, thus leading to the hydroxy-terminated ring-opened monomer/polymer (Scheme 18C). In contrast, the ACEM mechanism involves the interaction of the OH group of the initiating or propagating alcohol with the LB via H-bonding that renders the OH group sufficiently nucleophilic to undergo nucleophilic attack toward the monomer, thus facilitating the ring-opening of the monomer (Scheme 18D). Depending on the pKa difference between the initiating alcohol and the LB, the RO−H proton can be completely transferred to the basic site, thus generating an alkoxide anion initiating species. Because every growing chain has an equal probability of accepting the activated monomer (i.e., all chains would grow at the same rate), the LB/alcohol catalytic system usually has a kinetic characteristic of living polymerization. In this context, extensive work has been carried out to explore a wide structural and electronic diversity of LBs for the ROP of different monomers, including lactide, lactones, carbonates, cyclopropanes, and silyl ethers.299−302 Phosphazenes promote the ROP to proceed via an ACEM, in agreement with their high Brønsted basicity and low nucleophilicity. In the case of the LB being a strong nucleophile and also an excellent base, such as NHCs, alkylamines, amidines, and guanidines, competing AMM and ACEM pathways have been widely proposed in organopolymerization. Generally, at low alcohol concentrations or high ratios of monomer to alcohol, competing initiation mechanisms can occur and can be probed by carrying out polymerizations in the absence of an alcohol, whereas at high alcohol concentrations, the initiation via the ACEM pathway becomes predominant. Furthermore, cooperative dual activation of both the monomer and the OH initiator/chain end (Scheme 18E) can be achieved with specific mono- or dicomponent bifunctional

organic catalytic systems possessing both the LB (i.e., Hbonding acceptor) and H-bonding donor (e.g., thiourea−amine systems and TBD).303 Coordination of the ester functionality of the monomer to the H-bonding donor component of the catalyst should increase the positive polarization on the carbonyl carbon, thus facilitating the rate-determining ringopening step. This dual activation generally renders the good control of the polymerization with reduced or suppressed transesterification. Such a dual activation mechanism was also exploited for developing the kinetic resolution polymerization of rac-lactide by utilizing enantioselective bifunctional organocatalysts.304,305 3.5. Lewis Acid-Catalyzed Polymerization Involving (Hydro)silanes

In 1996, Piers and co-workers reported the hydrosilylation of CO-containing substrates (esters, ketones, etc.) catalyzed by B(C6F5)3.252 The authors suggested such a reduction is mechanistically different from what has been established for the conventional LA-mediated hydrosilylation, in which the more basic CO group is activated by the LA in the first step. The authors proposed that the unfavored but rapid dissociating B(C6F5)3−carbonyl Lewis adduct is capable of generating free B(C6F5)3, which can subsequently coordinate and activate the Si−H bond of the hydrosilane, rendering a more cationic silicon center that becomes vulnerable to nucleophilic attack by the carbonyl of the substrate. The hydride transfer from the resulting hydridoborate to the electron-positive carbon of the silylium coordinated carbonyl species furnishes the catalytic cycle and affords the silyl ether as the hydrosilylation product (Scheme 19). This type of process can be viewed as the cooperative activation of the Si−H bond with the dissociating B(C6F5)3−carbonyl LP, highlighting the analogous FLP-type 10573


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Scheme 21. Bifunctional Diene and Disilane Monomers Employed for LA-Catalyzed Hydrosilylation Polymerization To Synthesize Linear Polysilanes

Scheme 22. Nonhydrolytic Sol−Gel Process, Piers Silylation, and Piers−Rubinsztajn Condensation in Polymer Synthesis

only a few examples of the use of main-group catalysts for hydrosilylation to construct new Si−C bonds to synthesize polymers. As discussed in the GTP section, 1,4-hydrosilylation of (meth)acrylic monomers generates the corresponding silyl enolate species that serve as initiators for the subsequent GTP, but that process involves no repeating formation of the Si−C bonds as part of the polymer backbone. There are very few examples of main-group Lewis acidcatalyzed hydrosilylation for polymer synthesis. Tanaka et al. found AlCl3 catalyzed the reaction between bifunctional alkynes and hydrosilanes, affording a mixture of oligomeric and polymeric materials with a low Mn of 3.3 kg/mol and broad Đ of 6.3.313 In 2015, Chang et al. reported the utilization of Piers hydrosilylation for the polymerization between dienes and dihydrosilanes catalyzed by B(C6F5)3, affording a polymer with Mn = 11.7 kg/mol and Đ = 2.0, characteristic of a step-growth polymerization.314 It was pointed out that 1,4-diisopropenylbenzene itself was readily homopolymerized by B(C6F5)3 through cationic polymerization in the absence of the hydrosilane monomer to give cross-linked poly(1,4-diisopropenylbenzene), but the possibility of adventitious moisture as the protogen was not discussed. The authors further extended the scopes of the dialkene monomers to 1,3-bis(dimethylsilyl)benzene and 1,7-octadiene, as well as the bifunctional silane monomers to 1,3-bis(dimethylsilyl)benzene and Et2SiH2, obtaining a library of the corresponding polymers with Mn = 4.0 to 14.1 kg/mol, Đ = 1.63 to 3.53, and Tg = − 32 to +59 °C (Scheme 21). In addition, the authors also examined the stepgrowth hydrosilylation polymerization with an AB-type monomer bearing both an olefinic group and a hydrosilyl group, namely, (4-isopropenylphenyl)dimethylsilane, and obtained a polymer with Mn = 5.2 kg/mol and Đ = 2.0. A typical approach to manufacture poly(siloxane)s (silicone) is to hydrolyze a chlorosilane precursor such as dichlorodime-

reactivity that was uncovered later by Stephan et al. in the heterolytic cleavage of the H−H bond.143 The Piers-type hydrosilylation was soon extended to the reduction of imines, alkenes, and others, as well as to dealkylhydrogenation of the C−O bonds of silyl ethers and dehydrogenative silylation of alcohols.306 This Si−H bond activation, examined by further mechanistic investigation and the recent success of isolation of a structure featuring the Si−H···B motif by Piers et al.,307 is also endorsed and expanded experimentally and computationally by other groups.250,308−310 The ability of B(C6F5)3 to activate the Si−H bond and facilitate the addition of Si−H across unsaturated carbon− carbon bonds and/or cleavage of C−O bonds with Si−H provide feasible approaches to construct silicon−carbon and silicon−oxygen bonds, which are two major structures in silicon-containing polymers (i.e., polysilanes and silicones) and are thus attractive to polymer chemists (Scheme 20). Indeed, Rieger et al. reported that PhSiH3 can be polymerized to branched polysilanes, with the formation of SiH4 and C6H6 as side products, in the presence of borane catalysts, such as BCl3, B(C6F5)3, HB(C6F5)2, and B(C6H5)3 at elevated temperatures.311 Assisted by a computational investigation, the authors suggested that the activation of PhSiH3 with the Lewis acidic borane catalyst is the key step to the generation of monomerstabilized silylium active species, which undergo further ligand scrambling and branching to form the polymer. Hydrosilylation polymerization, a type of step-growth polyaddition via repeating additions of Si−H across unsaturated carbon−carbon bonds (alkenes and alkynes), has been well investigated and reviewed.312 Almost all the polymerization systems were previously based on transition-metal catalysts, in particular platinum complexes such as Speier’s catalyst and Karstedt’s catalyst. Hydrosilylation has also been utilized in the synthesis of branched and cross-linked polymers. There were 10574


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Scheme 23. Generally Accepted Catalytic Cycle for the B(C6F5)3-Catalyzed Dehydrogenative/Dealkylative Silylation

Scheme 24. Synthesis of Poly(siloxane)s via Piers−Rubinsztajn Reaction Catalyzed by B(C6F5)3

Scheme 25. Polycondensation of Disilanes with Bifunctionalized Alcohol and Phenol

methoxysilane with 0.04 mol % B(C6F5)3 gave the corresponding copolymer in 83% yield, with Mw = 30.3 kg/mol, Đ = 2.78, and Tg = −7.7 °C (Scheme 24). The robustness and tolerance of B(C6F5)3 under protic conditions allowed the use of substrates with acidic protons (i.e., silanols, alcohols, and phenols) as monomers. For example, Kawakami et al. showed that an asymmetric copolymer can be obtained by dehydrogenative coupling between a chiral disilanol and 1,4-bis(dimethylsilylbenzene) monomers;318 they also reported the synthesis of a “double decker” silsequioxane monomer bearing two Si−OH groups that can be copolymerized with a short dimethylsilicon chain with α,ω-dihydrosilyl end groups.319 Rubinsztajn320 described the preparation of poly[(aryloxy)silane]s and poly[(aryloxy)siloxane]s (Mw = 13.7 to 108 kg/mol, Đ = 1.95 to >3.0) by B(C6F5)3-catalyzed etherification between dihydrosilanes and bisphenols. This method allowed the alternating arrangement of a rigid bisphenol segment with either a hard bis(silylaryl) segment or a soft tetramethyldisiloxane moiety with enhanced Tg values ranging from 44 to 98 °C (Scheme 25). Other than enabling the synthesis of linear silicone polymers, the Piers−Rubinsztajn reaction has also proven an effective approach toward highly branched and cross-linked polysiloxanes. The cross-linked structures were generated by using different combinations of M, D, T, and Q subunits (Scheme 26). For example, the reaction between (RO)4Si (R = Me or Et) as a Q monomer and tetramethyldisiloxane as a D

thylsilane into low-MW linear or cyclic oligomers, which can be further converted into higher MW silicone under acid- or basecatalyzed conditions. In this context, the controlled synthesis of silicone has been a challenge since the polymer formed under a kinetic equilibrium, meaning that the backward depolymerization can also occur under the same conditions if water is present.109 Another approach, termed the nonhydrolytic sol− gel process,315 typically involves the formation of M−O−M (B, Si, and Ti, etc.) fragments from the suitable metal halides and alkoxides (Scheme 22).316 The Piers-type Si−H activation and its utilization in C−O bond cleavage (Scheme 23) allows the reduction of silyl ethers to yield not only the corresponding alkanes, but also siloxanes, which essentially are building blocks for silicone polymers. Rubinsztajn and Cella108 realized that this reaction could be employed as a new polycondensation process for the preparation of linear polysiloxane copolymers through the formation of new Si−O bonds with the elimination of alkanes that can be easily removed from the reaction. The topic of Piers−Rubinsztajn polymerization has been discussed in Brook’s very recent review.110 It is a remarkably robust condensation reaction that takes place under mild conditions and low B(C6F5)3 catalyst loadings for the synthesis of siliconcontaining polymers with well-defined architectures and enhanced properties.109,110,316,317 In one of Rubinsztajn and Cella’s earliest investigations,108 they reported the polycondensation between 1,4-bis(dimethylsilyl)benzene and diphenyldi10575


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utilized to make silicone dendrons and dendrimers327 and dendron-branched silicone polymers.328 Another interesting and unique structure generated with the Piers−Rubinsztajn reaction is the structure of silicones with a multicyclic pattern. Chojnowski and co-workers329 studied hydride transfer polymerization of 2,4,6,8-tetramethyltetrahydrocyclotetrasiloxane with B(C6F5)3 as the catalyst (Scheme 28). The polymerization was initiated by the Si−H of one molecule adding across the Si−O bond of another molecule to open the ring. In later stages, a ring-closing step was involved and ring structures with different sizes were formed with the lack of control. More recently, Liu et al. described a more defined strategy to synthesize cyclic poly(siloxane)s with linked cyclotetrasiloxane subunits by Piers−Rubinsztajn-type cyclization (Scheme 28).330 The formation of the macrocyclic structure of the linked cyclotetrasiloxane was confirmed by the absence of end groups from both NMR and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis. For the high-molecular-weight fraction of the polymer sample, which is not suitable for MALDI-TOF analysis, the authors obtained atomic force microscopy (AFM) images to confirm the ring architecture. The diameters of the rings of 200 nm determined by AFM corresponded to an Mn of 323 kg/mol, which is consistent with the Mn of 304 kg/mol measured by gel permeation chromatography (GPC). The tolerance of different functional groups in this Piers− Rubinsztajn process has also been studied quite extensively.331−337 Alkyl halides and alkenes are compatible under the Piers−Rubinsztajn conditions, thus allowing the synthesis of polysiloxanes with halo and alkenyl functional groups, which can be further modified by nuleophilic substitution or hydrosilylation.335−337 Some nitrogen-containing moieties are also tolerated; thus, monomers with triarylamine functional groups can be readily used under Piers−Rubinsztajn conditions to yield polysiloxanes with triarylamine pendants as potential organic hole-transporting materials.331−334 The Piers−Rubinsztajn reaction has been employed to graft siloxanes on different surfaces such as those of carbon nanotubes, graphene oxide, and silica.338,339

Scheme 26. Definition of M, D, T, and Q Basic Units Used in Polysiloxanes

monomer yielded a DQ resin (Mn = 15.3 kg/mol, Đ = 4.4),321 while a more highly cross-linked TQ resin was obtained if (RO)4Si and PhSiH3 were used.322 The DQ and TQ resins generated under such nonhydrolytic conditions are free of undesirable silanol end groups, and therefore exhibit enhanced hydrophobicity, solubility, and stability. In addition, the Piers− Rubinsztajn process toward the cross-linked material can be used in the synthesis of elastomers and foams.323−325 The steric hindrance places a significant influence on the reactivity of Piers−Rubinsztajn reactions, which can be captured for constructing well-defined 3D silicone structures. The assembly of precise and explicit 3D structures of silicone has always been a challenge because of the undistinguishable reactivity of functional groups and propensity to undergo redistribution (depolymerization/repolymerization) under catalytic conditions. In this context, Brook et al. found that, by fine-tuning the steric effect at the silicon centers, the Piers− Rubinsztajn reaction can be utilized for rapid and selective construction of explicit 3D siloxane structures.326 The reaction between 1,1,1,3,5,5,5-heptamethyltrisiloxane bearing an internal Si−H bond and the less sterically hindered Si(OMe)4 yielded the tetrasubstituted siloxane Si(OR)4 in excellent yield at RT (Scheme 27). In contrast, the same hydrosilane only selectively reacts with the more hindered Si(OEt)4 at 60 °C to generate the trisubstituted siloxane Si(OR)3(OEt) with one remaining ethoxyl group that can subsequently be modified by less hindered, more reactive terminal hydrosilanes. By this method, the star siloxane Si(OR)3(OEt) was grafted onto the hydrosilane-capped H−(SiMe2−O)n−SiMe2−H or PhSi(OSiMe2H)3 to create a branch-terminated silicone or a three-arm-decorated highly branched siloxane, respectively, with precise MW and high yield. In addition, the Piers−Rubinsztajn reaction was also

Scheme 27. B(C6F5)3-Catalyzed Polycondensation of Hydrosilanes and Siloxanes To Construct 3D Silicone Structures

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Scheme 28. B(C6F5)3-Catalyzed Polycondensation of Hydrosilanes and Siloxanes To Construct Macrocycles with Cyclotetrasiloxane Subunits

4. POLYMERIZATION MEDIATED BY LEWIS PAIRS

are categorized according to the monomer type, including linear acrylic monomers (e.g., methacrylates, acrylates, and acrylamides), cyclic acrylic monomers, monomers bearing the CCCN functionality, vinyl phosphonates, and polar divinyl monomers, with key results being summarized in Table 6. 4.1.1. Linear Acrylic Monomers. As revealed by Chen and co-workers, MMA can be rapidly polymerized into high-MW polymers by Al(C6F5)3-based CLAs and FLPs.16 The LBs examined in that work included phosphines (PtBu3, PMes3, and PPh3) and NHCs (ItBu and IMes). The control runs using the LA or LB alone for the polymerization of MMA at RT in toluene yielded no polymer formation for up to 24 h, but rapid polymerization took place when a combination of the LA and LB was used with a suitable addition sequence of the LA, the LB, and the monomer. For example, premixing Al(C6F5)3· toluene with MMA (800 equiv) in toluene, followed by the addition of PtBu3 ([Al]/PtBu3 = 2/1), converted all monomer into a high-MW polymer (Mn = 315 kg/mol, Đ = 1.72) in 7 min (TOF = 6840 h−1). When the Al(C6F5)3·MMA adduct was pretreated with PtBu3 in toluene for 10 min to generate the zwitterionic species, followed by addition of MMA, the polymerization became even more rapid, which consumed all monomer in only 4 min to give a high TOF of 12000 h−1 and yield a high-MW polymer (Mn = 397 kg/mol, Đ = 1.52). Switching to the PMes3/Al(C6F5)3 pair, which formed an FLP at − 75 °C and converted to PMes3·Al(C6F5)3 CLA as the major product upon warming to −40 °C, exhibited no activity toward MMA polymerization, because of the inability to generate the zwitterionic species, due to the low nucleophilicity of PMes3 relative to PtBu3. On the other hand, the PPh3/ Al(C6F5)3 pair exhibited an exceptional activity (TOF = 48000 h−1), despite the fact that they formed a stable CLA. However, the resultant polymer had a bimodal molecular weight distribution. Compared to phosphines, IMes as the LB was extremely active for MMA polymerization, giving a high TOF value of >48000 h−1. Switching to ItBu, the polymerization activity was approximately 15 times lower, but the polymer MW was approximately 20 times higher (Mn = 525 kg/mol, Đ = 1.43). Stoichiometric reactions indicated that IMes and ItBu can also react with Al(C6F5)3·toluene at RT in benzene to form clean, stable CLAs, IMes·Al(C6F5)3 and ItBu·Al(C6F5)3, respectively, but the reaction of ItBu and IMes with Al(C6F5)3·MMA readily afforded the corresponding zwitterionic imidazolium enolaluminate active species. The resulting PMMA is a syndio-rich material, with a syndiotacticity of 72.7− 75.8% rr.

4.1. Conjugate-Addition Polymerization

The use of a mixture of Et3Al and PEt3 (in a 2:1 ratio) for the polymerization of MMA with only marginal activity can be traced back to 1960 as described in an internal report by Murahashi and co-workers.340 In 1971, Ikeda et al. also reported that the combination of an LA, such as Et3Al, Et3In, and Et2Zn, with an LB, such as PPh3 and α,α′-dipyridal, was active for the polymerization of MMA and acrylonitrile.341 However, the activity of such LPs was only marginal, with a low overall TOF of ∼2 h−1. An activated Al−C bond of Et3Al, ionized as a result of the adduct formation with the LB, was suggested to be responsible for initiation of the “anionic” polymerization. In 1992, Kitayama and co-workers reinvestigated the polymerization of MMA and other methacrylates342 as well as the block copolymerization of MMA with tBuMA343 at −78 °C by the Et3Al/PR3 pair in toluene. Their study clearly showed that only the binary LP system is active for the polymerization, despite very low activity (TOF = 1.6 h−1), while Et3Al or PR3 alone did not initiate the polymerization of MMA. Interestingly, the Et3Al/PPh3 pair can produce highly syndiotactic PMMA at low temperature with 89% rr or 95% rr at −78 or −93 °C. NMR spectra indicated that both the LA and LB participated in the chain initiation, but the active propagation species was speculated to be a cyclic anion incorporating an exo-phosphorane (Ph3PC 2 (786 h−1) > 1 (224 h−1), but showing no enhancement in tacticity (74−75% rr). The Al(C6F5)3/tBu-P4 superbase pair showed the highest activity among all the LPs evaluated, giving the highest TOF of 96000 h−1 (Mn = 212 kg/ mol, Đ = 1.34). The Al(C6F5)3/tBu-P2 superbase pair displayed a lower activity, but was still highly active with a TOF of 48000 h−1. Additionally, two tertiary amines, NPh3 and NEtiPr2, when partnered with Al(C6F5)3, showed no activity toward polymerization of MMA up to 24 h. For the polymerization of a bulky furfuryl methacrylate (FMA), neither the Al(C6F5)3/PtBu3 pair nor the Al(C6F5)3/IMes pair exhibited noticeable reactivity up to 24 h. The polymerization of nBA by Al(C6F5)3/PtBu3 was active, but encountered with substantial catalyst deactivation, achieving only 33% conversion in 1 h (TOF = 264 h−1), after which no further significant increase in conversion was observed even after 24 h. Moving to electron-rich DMAA, both Al(C6F5)3/PtBu3 and Al(C6F5)3/ItBu exhibited high activity with TOFs of 48000 and 32000 h−1, respectively, affording a high-MW and relatively low dispersity polymer (PtBu3, Mn = 293 kg/mol, Đ = 1.43; ItBu, Mn = 369 kg/mol, Đ = 1.28). In comparison, the polymerization of sterically demanding N,N-diphenylacrylamide (DPAA) by the Al(C6F5)3/PtBu3 pair was much slower, requiring 24 h to achieve quantitative monomer conversion (TOF = 67 h−1) and producing PDPAA with Mn = 357 kg/mol and Đ = 1.31. In another study, Chen and Cavallo et al. examined the chain termination mechanism for the polymerization of methacrylates

by NHC and Al(C6F5)3 LPs via a combined experimental and theoretical study, which led to two chain termination pathways that compete with chain propagation cycles (Scheme 30).344 Scheme 30. Proposed Two Possible Backbiting Chain Termination Pathways That Compete with Chain Propagation Cycles in the LPP of Methacrylates by LPsa

a

Reprinted from ref 344. Copyright 2014 American Chemical Society.

Termination pathway A is proposed to proceed via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the C-ester enolate active chain end to generate a cyclic βketoester-terminated polymer chain, which was observed previously in the polymerization of acrylates by a group 4 metallocenium catalyst345 and is also ubiquitous in the anionic polymerization of acrylates.14 Termination pathway B is proposed to proceed via intramolecular backbiting cyclization involving nucleophilic attack of the activated adjacent ester group of the growing polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (δvalerolactone)-terminated polymer chain. Both pathways were accompanied by the release of the dinuclear anion [Al(C6F5)3− OMe−Al(C6F5)3]−, the crystal structure of which was previously reported by Chen and co-workers.346 Chain-end analysis by MALDI-TOF MS cannot differentiate between the possible cyclic β-ketoester and δ-valerolactone chain ends as the mass difference is one monomer unit. On the other hand, density functional theory (DFT) calculations showed that the formation of δ-valerolactone-terminated chain ends is kinetically favored (lower energy barrier by 12.5 kcal/mol) but thermodynamically disfavored (less stable by 20.1 kcal/mol), as compared to the formation of the β-ketoester-terminated chain end. 10580


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In 2014, Lu and co-workers347 developed a highly active LPP system for polymerizing MMA using an NHO as a neutral nucleophilic LB (Scheme 31), thanks to its strongly polarized

acrylamide monomers. In the polymerization of nBuMA, high yields were achieved within 5 min in [nBuMA]/[NHO] ratios of 400 and 800 (yield 100% and 94%), and even for the ratio of 1600:1, a prolonged reaction time of 15 min afforded a 91% yield. By switching to DMAA (800 equiv), a 100% yield was achieved in 4 min, providing a polymer with Mn = 172 kg/mol and a narrow Đ of 1.05. Owing to the existence of the backbiting termination, the Al(C6F5)3/NHC(NHO) LPP systems were unsuitable for synthesizing block copolymers by sequential additions of two different monomers. Accordingly, Lu, Ren, and co-workers developed a strategy to circumvent this problem by employing the tandem polymerization scheme, which enabled the preparation of PMMA-b-PLA block copolymer (Scheme 32).355 A specifically designed NHC bearing a vinyl group in combination with Al(C6F5)3 was utilized to synthesize doublebond-ended PMMA, which was then converted into the hydroxyl-ended PMMA by the thiol−ene click reaction; this OH-ended PMMA was used to initiate the subsequent organocatalytic ROP of rac-LA mediated by DBU to afford PMMA-b-PLA block copolymer. To overcome the above-described side reactions in the typically fast LPP promoted by the Al(C6F5)3-based CLAs and FLPs and thus develop controlled or even living LPP, four dif ferent strategies have been explored. The f irst is to use a weaker LA to minimize the LA-induced termination reaction (cf. Scheme 30). In this context, in 2017, Taton and co-workers established controlled LPP of MMA using LPs composed of a weak silane-based LA (Me3SiNTf2 or Me3SiOTf) and a simple phosphine LB [PtBu3, PnBu3, or tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP)] (Scheme 33).129 MMA can be polymerized using any of the three phosphines in combination with Me3SiNTf2 in a 1:2 ratio, with the activity (TOF) following the order of TTMPP (14.3 h−1) > PtBu3 (6.1 h−1) > PnBu3 (2.9 h−1) (MMA/LB = 43/1), whereas the attempts to polymerize MMA using the LA or LB alone or Me3SiNTf2/TTMPP in a 1:1 or 0.5:1 ratio were unsuccessful, revealing a bimolecular cooperative activation. The polymerization was controlled in terms of forming PMMA with predictable Mn and low Đ ( IMe (358 h−1) > ItBu (8.3 h−1). The balanced steric and electronic properties of the iPr

Scheme 36. Structures of NHC LBs and the Products Derived from Reaction of NHC and LA B(C6F5)3

group render IiPr(Me) the most active initiator of the series. After an induction period, the polymerization followed zerothorder kinetics with respect to the MMA concentration, and second-order kinetics represents the sum of the reaction order for both the zwitterionic active species and the free LA, in agreement with the bimolecular, activated monomer propagation mechanism. The PMMA materials produced by the NHC/B(C6F5)3 LPs are syndio-rich polymers (74−80% rr), regardless of the NHC used or the [MMA]/[NHC] ratio. Along the lines of the third strategy of using interacting LPs, Xu et al. synthesized cationic rare-earth (RE = Sc, Y, Lu) aryl oxide complexes with phosphorus-tethered β-diketiminate as the ancillary ligand, which can be used as intramolecular interacting rare-earth/phosphorus (RE/P) LPs for MMA polymerization (Scheme 37).366 Upon treatment with a high Scheme 37. Structures of Cationic Rare-Earth Aryl Oxide Complexes and 1,4-Addition Reaction of the RE/P Complex to MMA for LPP of MMA

catalyst loading of 2 mol % at RT, such complexes showed a low activity [TOF: Y (7.63 h−1) ≈ Lu (7.25 h−1) > Sc (1.23 h−1)]. The stoichiometric reaction of the Sc complex with MMA generated the isolated nine-membered addition intermediate, confirming Sc/P LP-type 1,4-addition as the initiating step of the polymerization. Interestingly, this Sc/P addition intermediate itself was capable of polymerizing MMA without an additional LA, which is typically necessary for the maingroup LA-based LP systems, likely due to an inherently higher coordination number for the rare-earth metals. The Mn values of the resultant PMMAs (6.9−20.9 kg/mol, Đ = 1.03−1.15) were higher than the theoretical values, presumably due to the strong RE/P interaction, which impaired chain initiation. More recently, the same group utilized intermolecular homoleptic RE-metal (Sc, Y, Sm, La) tris(aryl oxide) complexes RE(OAr)3 (RE = Sc, Y, Sm, La; Ar = 2,6-tBu2C6H3)367 as LAs and commercially available phosphines as well as NHCs as LBs for LPP of MMA, tBuMA, FMA, and DMAA.368 Notably, increasing ionic radii of the RE metal (La > Sm > Y > Sc) can boost the MMA polymerization activity [TOF: La (>12000 h−1) > Sm (12000 h−1) > Y (2400 h−1) > Sc (267 h−1)]. 10583


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Scheme 38. Structures of NHO LBs That Form Noninteracting, True FLP Catalytic Systems with MeAl(BHT)2 for the Living LPP

and 262 h−1 (PPh3). The MW of PMBL produced by ItBu/ Al(C6F5)3 was approximately 4 times higher than that produced by PtBu3/Al(C6F5)3 (Mn = 44.8 kg/mol, Đ = 2.18), whereas the use of PPh3 as the LB brought about the formation of a bimodal polymer. The γ-Me substitution of the parent MBL renders the good solubility of PγMMBL in CH2Cl2, and thus, the polymerization of γMMBL by Al(C6F 5) 3-based LPs is homogeneous and highly effective in this solvent, achieving high TOFs of 4800 h−1 (PtBu3), 48000 h−1 (PPh3), 48000 h−1 (ItBu), and 48000 h−1 (IMes). The MWs of the resulting PγMMBLs by PtBu3/Al(C6F5)3 (Mn = 192 kg/mol, Đ = 2.28) and ItBu/Al(C6F5)3 (Mn = 139 kg/mol, Đ = 1.15) were much higher than that produced by IMes/Al(C6F5)3 (Mn = 62.8 kg/ mol, Đ = 1.42). Other LP systems based on ClAl(C6F5)2 and methylaluminoxane (MAO) LAs, in combination with PtBu3 or ItBu LB, were much less active (TOF = 3.8−7.6 h−1).147 The stoichiometric reaction of PtBu3 and Al(C6F5)3·MBL afforded the corresponding zwitterionic phosphonium enolaluminate t Bu3P+−MBL−Al(C6F5)3− as a single isomer due to the fixed scis conformation (Figure 6), which is an active propagation species for the chain growth in the bimetallic, activated monomer fashion. Regarding the termination mechanism, different from the LPP of MMA, the LPP of γMMBL was found to be more resistant toward chain termination via backbiting due to its robust, five-membered cyclic structure.344 The MALDI-TOF

The fourth strategy is to utilize a true FLP for LPP so that both LA-induced chain termination and LB-induced reactivity quenching side reactions can be suppressed. Indeed, in 2018, Zhang, Chen, and co-workers utilized this strategy and developed the f irst living LPP of MMA and benzyl methacrylate (BnMA) using a noninteracting, true FLP catalytic system consisting of sterically demanding MeAl(BHT)2 LA and sterically protected but still very reactive NHO LBs (5−8, Scheme 38).369 This NHO/MeAl(BHT)2 LP system possesses a fine balance between the sufficient LA acidity needed for monomer activation and the lowest possible LA acidity to suppress the LA-activated side reaction, as well as a fine balance between the sufficient LP sterics to minimize the LA−LB interaction and the reduced LB sterics to ensure effective initiation of the reaction. The livingness of this FLP-mediated polymerization has been fully and clearly verified by five lines of evidence, including predictable polymer Mn (up to 351 kg/ mol) and low Đ values (∼1.05), high to quantitative initiation efficiencies; a linear increase of polymer Mn vs monomer conversion and monomer-to-initiator ratio, precision in multiple polymer chain extensions, and synthesis of well-defined diblock and triblock copolymers regardless of the comonomer addition order. In contrast, the closely related NHO/Al(C6F5)3 that forms a CLA suffered from irreversible chain termination side reactions due to LA-activated backbiting cyclization. In addition, there were essentially no effects of the activation procedure (i.e., addition sequence) on the polymerization outcome in the case of the NHO/MeAl(BHT)2 LP due to the formation of the FLP catalytic system. At last, the living polymerization of conjugated polar alkenes such as simple, fundamental methacrylates by a noninteracting, authentic FLP has been achieved. 4.1.2. Cyclic Acrylic Monomers. Biomass-derived renewable cyclic acrylic monomers, such as MBL and γMMBL, are of particular interest in exploring the prospects of substituting the petroleum-based methacrylate monomers for renewable specialty chemicals and bioplastics production.370−373 Structurally, MBL and γMMBL can be described as the cyclic analogue of MMA, but they exhibit greater reactivity than MMA, and the cyclic rings in MBL and γMMBL also impart significant enhancements in the materials properties of the resulting PMBL and PγMMBL,249 as compared to PMMA, with increased thermal and optical properties as well as resistance to solvent, heat, and scratching. The first rapid polymerization of MBL and γMMBL into high-MW polymers by LPs was reported by Chen and co-workers in 2010.16 Despite being a heterogeneous process (because of the insolubility of PMBL in CH2Cl2), the Al(C6F5)3-based LPs were still effective for MBL polymerization with TOFs of 7660 h−1 (PtBu3), 704 h−1 (ItBu),

Figure 6. Cooperative activation of MBL with the Al(C6F5)3/tBu3P pair to generate the phosphonium enolaluminate zwitterion species and the corresponding SC-XRD structure. Structure reprinted with permission from ref 147. Copyright 2012 Royal Society of Chemistry. 10584


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Figure 7. Surprising activity trend observed for the γMMBL polymerization by a series of intermolecular and intramolecular P/B FLPs vs ILPs and stable CLAs.

Scheme 39. Different Amine/E(C6F5)3 (E = B or Al) LP Systems Employed for the LPP of γMMBL and Their Corresponding Reactivity

polymerization activity. Moreover, the activity of γMMBL polymerization by a series of CLAs, Ph3P·B(C6F5)3 (24000 h−1) > MePh2P·B(C6F5)3 (6000 h−1) > Me2PhP·B(C6F5)3 (800 h−1) > Me3P·B(C6F5)3 (0 h−1), decreased rapidly with an increase in the adduct strength and completely diminished when the strongest CLA formed with the smallest LB [Me3P· B(C6F5)3] could no longer dissociate in solution. Overall, the most active system is brought about by a good compromise between B site acidity, P site basicity, steric crowding around P, and the strength of the P−B association in solution. Likewise, by tuning the steric bulkiness and electronic properties of NHC LBs, a highly active and efficient LPP system based on NHC[IiPr(Me), IMe, and ItBu]/B(C6F5)3 LPs for polymerization of γMMBL has also been realized.362 The polymerization occurred rapidly, with quantitative monomer conversion accomplished within 1 min, thus reaching a high TOF of 48000 h−1, and produced PγMMBL with high MW and low Đ values (Mn = 33.0−489 kg/mol, Đ = 1.08−1.20). Compared to the widely used LBs such as phosphines and NHCs, amine LBs employed for the LPP are scarce and not well-established. Nonetheless, Chen et al. described γMMBL polymerizations by LPs consisting of bulky E(C6F5)3 (E = B, Al) as strong LAs and common amines [Et3N and 2,2,6,6tetramethylpiperidine (TMP)] as LBs (Scheme 39).375 While mixing of B(C6F5)3 and bulky TMP led to formation of an FLP, the reaction of B(C6F5)3 and Et3N resulted in a disproportionation reaction of the LP to form ammonium hydridoborate and equimolar iminium zwitterion. In contrast, Al(C6F5)3, considered to possess higher Lewis acidity and less steric hindrance relative to B(C6F5)3, forms a CLA with both Et3N and TMP. Nevertheless, the amine/E(C6F5)3 LPs promoted rapid and quantitative polymerization of γMMBL. When combined with Et3N, Al(C6F5)3 (TOF = 24000 h−1) showed a 2-fold higher activity over that of Et3N/B(C6F5)3. Meanwhile, TMP is more efficient than Et3N when paired with either B(C6F5)3 or the Al(C6F5)3. Even at a low catalyst loading (800/ 1/2), TMP/Al(C6F5)3 still exhibited high activity (TOF =

MS spectrum of the low-MW PγMMBL produced by IMes/ Al(C6F5)3 showed three series of mass ions. The major one corresponds to IMes/H chain ends, indicating that the majority of the polymer chains produced in this polymerization are linear, living chains. The other two minor series of mass ions were indicative of side reactions such as chain termination or transfer, one of which gave a structure in the absence of apparent chain ends, consistent with a scenario that chain transfer to the monomer occurs through deprotonation of the γMMBL monomer by the propagating enolate anion, and the resulting anionic monomer initiates new chains, as shown in the related organocatalytic polymerization system by NHC initiator alone. The more user-friendly B(C6F5)3-based LPs were also investigated for the polymerization of MBL and γMMBL.374 Although the true FLP PMes3/B(C6F5)3 was found ineffective for such polymerization, through regulating the LB and LA site cooperativity in both inter- and intramolecular P/B LPs with different steric, electronic, and spatial controls, in 2014, Chen and Xu found that, counterintuitively, CLA Ph3P·B(C6F5)3 (14) is exceptionally active for the polymerization of γMMBL and in fact exhibits the highest activity relative to both the noninteracting FLPs and ILPs in the P/B LP series: 14 (CLA, 24000 h−1) > 13 (ILPs, 5652 h−1) > 12 (ILPs, 1174 h−1) > 11 (ILPs, 382 h−1) > 10 (ILPs, 96 h−1) ≫ 9 (FLP), PMes3/ B(C6F5)3 (FLP, 0 h−1), Figure 7. The intermolecular FLP PMes3/B(C6F5)3 and intramolecular FLP 9 were inactive because, although they have essentially 100% “free” P and B for catalysis, the extremely high steric demand of the P site renders such FLPs incapable of initiating the polymerization. On the other hand, the interacting, tethered intramolecular ILPs (10− 13) exhibit good to high polymerization activity, thanks to the relieved steric stress on the P site. Although Ph3P and B(C6F5)3 form a stable adduct in the solid state, it is “frustrated” in solution, which readily dissociates into free Ph3P and B(C6F5)3 in the presence of donor solvent or monomer for chain initiation and propagation events, thus showing very high 10585


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Scheme 40. Monomers Bearing the CCCN Functionality Employed in LPP and Generation of Active Zwitterionic Intermediates

Figure 8. SC-XRD structures of zwitterionic imidazolium pyridylaluminate ItBu+−CH2CH(C5H4N)Al(C6F5)3− (16) and zwitterionic imidazolium oxazolinylaluminate IMes+−CH2C(Me)(C3H2NO)Al(C6F5)3− (17), derived from cooperative LA/LB activation of 2-VP and iPOx, respectively. Structures reprinted with permission from refs 344 and 381. Copyright 2014 American Chemical Society and 2014 Georg Thieme Verlag KG, respectively.

96000 h−1), affording PγMMBL with a high Mn of 129 kg/mol but a broad Đ of 2.21. In contrast, using intramolecular FLP system 15 (Scheme 39) yielded no polymer formation up to 24 h, attributed to the lack of initiation by the bulky amine site. 4.1.3. Monomers Bearing the CCCN Functionality. The conjugated monomers polymerizable by LPs have been extended beyond the monomers with the typical CC CO functionality to include those containing the CC CN functionality, such as 2-vinylpyridine (2-VP), 2isopropenyl-2-oxazoline (iPOx), and 4-vinylpyridine (4-VP), Scheme 40. The vinyl addition (co)polymerization of iPOx by free radical, anionic, and metal-mediated coordination polymerization methods as well as the controlled polymerization of 2VP has been well established.376−380 In 2014, Chen and coworkers reported the first example of polymerization of 2-VP and iPOx by Al(C6F5)3-based LPs into medium- to high -MW polymers.344,381 LPs based on phosphines are inactive for polymerization of 2-VP and iPOx, due to the inability of

phosphine nucleophiles to perform Michael addition reaction with a monomer−LA adduct, highlighting the large reactivity difference between two classes of conjugated polar alkenes with CCCO and CCCN functionalities toward LPs. On the other hand, the ItBu/Al(C6F5)3 system brought about an effective polymerization of 2-VP with the TOF up to 1490 h−1, producing high-MW polymers (Mn up to 315 kg/mol, Đ = 1.50−2.04, I* ≤ 55%). Compared to ItBu/Al(C6F5)3, IMes/ Al(C6F5)3 was much less active for polymerization of 2-VP, whereas TPT/Al(C6F5)3 was completely inactive. Replacing the strong LA Al(C6F5)3 with MeAl(BHT)2 resulted in a much less active polymerization, while substituting with B(C6F5)3 or AlEt3 led to a completely inactive system. On the other hand, both ItBu/Al(C6F5)3 and IMes/Al(C6F5)3 were active for polymerization of iPOx (TOF ≈ 7 h−1), but the activity was much lower than that for the polymerization of 2-VP, producing the polymer with a medium MW (ItBu, Mn = 73.7 kg/mol; IMes, Mn = 15.0 kg/mol) and a broad Đ of ∼3.0. 10586


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The reaction of monomer adduct 2-VP·Al(C6F5)3 and ItBu generated the isolable zwitterionic imidazolium pyridylaluminate ItBu+−CH2CH(C5H4N)Al(C6F5)3− (16; Scheme 40), and the structures of both the monomer−LA adduct and active zwitterionic intermediate have been confirmed by SC-XRD analysis (Figure 8). Likewise, the reaction of iPOx·Al(C6F5)3 with IMes at RT also cleanly formed the corresponding zwitterionic imidazolium oxazolinylaluminate IMes+−CH2C(Me)(C3H2NO)Al(C6F5)3− (17; Scheme 40) as two isomers (E/Z) in a 2:1 ratio, and the E-isomer has been structurally characterized by SC-XRD analysis (Figure 8). Kinetic studies of 2-VP polymerization by ItBu/Al(C6F5)3 revealed that the polymerization followed zeroth-order kinetics with respect to the monomer concentration, first-order in the LB ItBu concentration, and first-order in the LA Al(C6F5)3 concentration, indicating that the C−C-bond-forming step via intermolecular Michael addition of the propagating species to the LA-activated monomer is the rate-limiting step and the release of the LA catalyst from its coordinated last inserted monomer unit in the growing polymer chain to the incoming monomer is relatively fast (Scheme 41). The LPP also works

P(4-VP) oligomer revealed that the phosphine was directly bound to the chain end, demonstrating the conjugate-addition mechanism via the double-bond activation, which can be achieved even over several bonds (Scheme 40). 4.1.4. Vinyl Phosphonates. The polymerization of vinyl phosphonates affords a straightforward approach to phosphonate-containing polymers with many potential applications, such as in batteries and fuel cells, halogen-free flame retardants, and the biomedical field. Radical and classical anionic polymerizations are typically employed to synthesize such polymers, often in low yields and with not well-controlled structures.40,382−386 In 2012, Chen and co-workers reported the LPP of diethyl vinyl phosphonate (DEVP) by both Al(C6F5)3/ PtBu3 and Al(C6F5)3/IMes CLAs, which were active but sluggish with TOFs of 6 and 10 h−1, respectively.147 Later they utilized the Al(C6F5)3/IMes LP to prepare PDEVP (Mn = 27.9 kg/mol, Đ = 2.11) as an effective kinetic hydride inhibitor for offshore oil and gas production.387 More recently, Rieger et al. reported the relatively more controlled polymerization of DEVP at −30 °C using AlPh3/PEt3 CLA, producing PDEVP with a Đ of 1.33 (TOF = 75 h−1, Mn = 55.0 kg/mol, I* = 45%); increasing the reaction temperature from −30 to −10 °C led to a further increased Đ of 1.42 (TOF = 100 h−1, Mn = 148 kg/ mol, I* = 22%), due to a higher degree of side reactions.358 ESIMS analysis of a 1:1 mixture of the LP and monomer indicated two different series of masses, which were proposed to be caused by conjugate addition pathway A (Scheme 42) and deprotonation pathway B via the deprotonation of the αcarbon, leading to an active species with a cumulated double bond (Scheme 42). The fact that the use of a more basic phosphine led to a slightly broadened Đ of 1.38, as a result of the enhanced deprotonation pathway, was argued to support the proposed competition of the two initiation mechanisms. 4.1.5. Divinyl Acrylic Monomers. The chemoselective polymerization of divinyl acrylic monomers, such as vinyl methacrylate (VMA), allyl methacrylate (AMA), and 4vinylbenzyl methacrylate (VBMA) (Table 6), where one vinyl group is selectively polymerized while the other vinyl group remains unreacted for further postpolymerization, provides a convenient approach to functional acrylic polymers. However, completely chemoselective polymerization of such monomers still has been a demanding task in late-stage radical polymerization,388−394 as well as in anionic polymerization395,396 and group transfer polymerization397,398 when conducted at −20 °C or above. Recent advances have enabled complete chemoselectivity through employing coordination−addition polymerization by chiral cationic zirconocenium catalysts.399,400 As the LPP proceeds through a bimolecular, activated monomer propagation mechanism, it is possible that only the vinyl group

Scheme 41. Proposed Propagation “Catalysis” Cycle for the LPP of Polar Vinyl Monomers Based on the Results of Kinetic and Mechanistic Studiesa

a

r.d.s. = rate-determining step.

well for extended Michael-type monomer 4-VP with CC CCCN functionality, as revealed recently by Rieger et al.358 Thus, the polymerization of 4-VP by a CLA, AlEt3/PMe3, proceeded in a controlled manner, producing P(4-VP) with a medium MW of Mn = 84.0 kg/mol and a narrow Đ of 1.21 (TOF = 92 h−1, I* = 25%). Switching to an LP with a bulky LB or LA, namely, AlEt3/PCy3 or Al(iBu)3/PMe3, decreased the TOF to 17 and 20 h−1, respectively. ESI-MS analyses of the

Scheme 42. Possible Mechanisms of the Initiation Process Involved in the LP-Mediated Polymerization of DEVP

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anionic polymerization by nBuLi at −78 °C or radical polymerization by AIBN at 60 °C for these divinyl acrylic monomers produced an insolubly cross-linked polymer even at low conversions. The postfunctionalization by the “thiol−ene” click reaction between the pendant double bond of PVBMA and PhCH2SH provided a convenient approach to functional acrylic polymers. Very recently, the same group utilized the Dip-iPr-NHO/Al(C6F5)3 system for the random copolymerization of MMA with VMA to produce PMMA with pendent double bonds that can be converted into a series of PMMA-gPLA brush copolymers by the thiol−ene click reaction, followed by DBU-promoted organocatalytic ROP of raclactide.355 The chemoselective polymerization of bifunctional AMA with the NHC/B(C6F5)3 LPP systems was also reported by Chen et al.362 For example, the chemoselective polymerization by IiPr(Me)/B(C6F5)3 in neat AMA achieved 60% conversion in 4 h, producing un-cross-linked, soluble, syndiotactic PAMA (Mn = 46.4 kg/mol, Đ = 1.62, rr = 83%) with complete retention of the allyl moiety located on the side chain. Recently, Chen et al. also described the synthesis of multi-vinylfunctionalized γ-butyrolactones, including γ-vinyl-γ-methyl-αmethylene-γ-butyrolactone (γVMMBL) and γ-allyl-γ-methyl-αmethylene-γ-butyrolactone (γAMMBL) (Table 6) starting from biorenewable feedstocks, which were effectively and chemoselectively polymerized by LPs consisting of E(C6F5)3 (E = Al, B) LA and NHC LB [ItBu, IiPr(Me)] to afford functional poly(MMBL)s.402 At RT, near-quantitative monomer conversion was achieved in ∼1 h (monomer/LA/LB = 200/2/1, yield 96−98%), and the LPP proceeded exclusively via the enchainment of the conjugated α-methylene double bond. The resulting vinyl-functionalized polymers had unimodal but relatively broad molecular weight distributions (Đ = 1.99− 2.96), which were soluble in common organic solvents and stable at RT. In particular, the Al-based LP produced a polymer (Mn = 116−122 kg/mol) with approximately 5.5 times higher MW than that produced by the B-based LP (Mn = 20.3−22.4 kg/mol). The resulting PγVMMBL and PγAMMBL exhibited Tg values of 197 and 154 °C, respectively, which are lower than that of the related PγMMBL (227 °C), showing the plasticizing effect of the vinyl group substitution. Noteworthy here is that

conjugated with the carbonyl moiety, which is activated through coordination of the carbonyl group to the LA, will be polymerized, while leaving the nonconjugated vinyl group intact, thus potentially achieving complete chemoselective polymerization (Scheme 43). Scheme 43. Proposed Bimolecular, Activated Monomer Propagation Mechanism for the Chemoselective Polymerization of Divinyl Acrylic Monomers by LPs

In 2014, Lu and co-workers reported chemoselective polymerization of dissymmetric divinyl acrylic monomers by Al(C6F5)3-based LPs under mild conditions, affording soluble polymers with high MW and narrow Đ values.401 Utilizing B(C6F5)3 as the LA and Dip-NHO as the LB for VBMA (Table 6) polymerization did not show any activity at RT up to 24 h. Switching to Al(C6F5)3-based LP systems, the polymerization of VBMN exhibited moderate to high activity depending on the structure of the LB employed [TOF (h−1): Dip-iPr-NHO (200) < Dip-NHO (600) < PPh3 (12000)] with I* = 35−48%, producing PVBMA with Mn = 42.0−58.0 kg/mol and Đ = 1.22−1.39. The polymerization proceeded exclusively through the enchainment of the methacrylic CC bond and leaving the nonconjugated vinyl group unreacted. The chemoselective polymerizations of the other two divinyl acrylic monomers, AMA and VMA (Table 6), were also accomplished with high activity by Al(C6F5)3-based LPs (TOF = 800−19200 h−1), producing vinyl-functionalized polymers with similar Đ values (1.21−1.37) but higher MWs (Mn = 140−640 kg/mol) compared to those in VBMA polymerization. In contrast,

Scheme 44. Possible Chain Initiations and Propagations Involved in the ROP of Cyclic Esters by LPs for Polymer Chain Formation

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Table 7. Summary of LP-Mediated ROP

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Table 7. continued

PγVMMBL can undergo thermally induced curing upon heating at higher than 240 °C to form cross-linked materials. Very recently, Xu et al. also achieved the chemoselective polymerization of AMA, VMA, and VBMA utilizing LPs comprised of homoleptic RE aryl oxide complexes RE(OAr)3 (RE = Sc, Y, Sm, La; Ar = 2,6-tBu2C6H3) and phosphines PR3

(R = Ph, Cy, Et, Me), affording soluble polymers bearing pendant CC double bonds with Mn up to 153 kg/mol and Đ = 1.23−2.00.403 The catalytic activity of polymerizations was found to be dependent on the ionic radii of RE ions (Sc < Y < Sm < La) and electronic/steric profiles of the LBs (PPh3 < PCy3 < PMe3 ≈ PEt3). 10590


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Scheme 45. Structures of NHC-Tethered Titanium(IV), Yttrium(III), Magnesium(II), and Zinc(II) Bifunctional Complexes Employed for the ROP of Lactide

1.47). In their subsequent work,407 a set of saturated NHCtethered magnesium(II) and zinc(II) complexes (20−23; Scheme 45) were also shown to be effective for the ROP of rac-lactide to yield polymers with medium MWs (Mn = 16.0− 27.5 kg/mol, Đ = 1.32−1.57). The change of the steric profile of the N-substituent has a significant effect on the polymerization rate in the case of magnesium complexes (20, 45 min, conv = 26.9%; 22, 45 min, conv = 97.7%; rac-lactide/Mg = 100/1), whereas zinc complexes exhibited similar activity (21, 17 h, conv = 91.9%; 23, 16 h, conv = 78.3%). The differences in the rates of polymerization were rationalized by the proposed different initiating processes: the polymerization was initiated by the tethered NHC group in the case of the magnesium complexes, while the initiating group was speculated to be the alkoxide group for the zinc complexes, considering the weaker Mg−carbene C bond than that of the zinc analogue. An LPP system consisting of a cationic zinc complex supported by the diaminophenolate (NNO) ligand (24; Scheme 46) and a tertiary amine [e.g., 1,2,2,6,6-pentamethylpi-

4.2. Ring-Opening Polymerization

As early as in the 1990s, Dubois and Jérôme found that the addition of LBs such as picoline or phosphines enhanced the activity of Sn(Oct)2- or Al(OiPr)3-catalyzed coordination ROP of lactide, with the polymer MW and dispersity being affected in various degrees.404,405 In both catalyst cases, it was considered that the LB was coordinated to the metal, which polarized the metal−alkoxide bond and thereby facilitated monomer insertion. Thus, the added organic base would simply act here as a ligand that modulates the reactivity of the metal− alkoxide species. In recent years, the investigation of ROP by LPs with metal or organic LAs in combination with LBs has not only brought about the enhanced ROP activity, but also led to improved selectivity in the ROP. Regarding the mechanism of the LP-mediated ROP, taking LPP of cyclic esters as the example, it proceeds through coordination of the carbonyl moiety of the monomer to the LA, which enhances the electrophilic character of the carbonyl carbon, thus facilitating nucleophilic ring-opening of the monomer by the LB itself (Scheme 44A), an LB-activated alcohol (initiator or OHterminated growing polymer chain) via H-bonding (Scheme 44B), or the LB-activated monomer via proton abstraction (Scheme 44C). As the mechanism is widely proposed in the LB-mediated organopolymerization, the details of the corresponding mechanisms will depend on some key properties of the LB (Lewis basicity, nucleophilicity, steric hindrance, etc.), the acidity of the monomer, and use of an alcohol initiator. This section will review recent progress made in the LP-mediated ROP of heterocyclic monomers, including lactide, lactones, cyclic carbonates, NCAs, and ring-opening copolymerization (ROC) of CO2 (carbonyl sulfide or cyclic anhydrides) with epoxides, with key results being summarized in Table 7. The ROP by LPs based on metal complex LAs supported by ancillary ligand will be covered first, and then the simple LP systems on the basis of the main-group LAs without an ancillary ligand will be described. Examples of mono- or binary dual catalytic systems, in which the activation of both initiator and monomer is via H-bonding interaction, such as thiourea−amino derivatives or TBD, are not covered in this review. 4.2.1. Cyclic Esters. In 2006, Arnold and co-workers employed NHC-tethered titanium(IV) alkoxide (18) and yttrium(III) amide (19) complexes that acted as intramolecular LPs for the ROP of rac-lactide (Scheme 45).406 Titanium complex 18 produced PLA initially with a narrow dispersity at RT (rac-lactide/Ti = 100/1, 2 min, yield 85%, TOF = 2550 h−1, Mn = 2.3 kg/mol, Đ = 1.17), but after which the dispersity became broader with increased reaction time, due to competitive transesterification processes. The resultant polymer was attached with an imidazolium group, suggesting that the ring-opening initiation is effected by nucleophilic attack of the NHC group. The more Lewis acidic yttrium complex 19 mediated a more rapid polymerization (rac-lactide/Y = 10000/ 1, 15 min, TOF = 34000 h−1, yield 85%, Mn = 66 kg/mol, Đ =

Scheme 46. LP System Consisting of a Cationic Zinc Complex (LA) and PMP (LB) for the ROP of rac-Lactide

peridine (PMP)] was utilized to promote efficient and controlled ROP of rac-lactide by Guillaume, Bourissou, Carpentier, and co-workers.408 In the presence of neopentyl alcohol (neo-PentOH) as the initiator, near-quantitative conversion of the monomer was achieved in 3 h to produce atactic PLA with Mn up to 14.5 kg/mol and Đ values ranging between 1.2 and 1.4 (rac-lactide/neo-PentOH/24/PMP = 100/ 1/0.5/0.5, TOF ≈ 33 h−1). No polymerization was observed when the polymerization was conducted in the presence of neoPentOH with only 24 or PMP, and the authors postulated that 24/amine LA/LB LP cooperatively activates the monomer and initiating/propagating alcohol. In situ formation of zinc alkoxides that initiated the polymerization was ruled out as no reaction occurred in an equivalent mixture of 24/PMP/neoPentOH. The polymerization activity increased with an increase in the basicity of the amine. Increasing the amount of alcohol with respect to the catalytic amounts of 24/amine LP allows the simultaneous growth of several polymer chains per metal center without affecting either the control or the rate of polymerization, comparable to the “immortal” ROP processes by organometallic catalysts. The 24/amine LP system was also efficient for the ROP of TMC (TOF ≤ 3600 h−1, TMC/24/ 10591


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Scheme 47. NHC−Zn(C6F5)2 and NHC−Metal Chloride Adducts Employed for the ROP of Cyclic Esters

BnOH/NEt3 = 1000/1/5/5, bulk, Mn < 20 kg/mol, Đ ≈ 1.7), the activity of which was much higher than that of 24/BnOH without the amine (TOF ≤ 100 h−1, TMC/24/BnOH = 500/ 1/5, bulk).409 Variation of the nature of the anion (e.g., {H2N[B(C6F5)3]2}− and [EtB(C6F5)3]−) did not significantly affect the performance of these catalyst systems, while the change of the amine followed the same trend observed in the ROP of rac-lactide. A series of NHC−Zn(C6F5)2 neutral LP adducts (25; Scheme 47) were also investigated for the ROP of β-BL, raclactide, and TMC.410 In the presence of such monomers, NHC−Zn(C6F5)2 adducts can be dissociated to generate free NHC and Zn(C6F5)2, which promote the ROP of β-BL, but the activity was low (TOF = 2.7−4.0 h−1, β-BL/Zn = 100, 90 °C, toluene), and the Mn values of the resultant polymers (Mn = 0.8−1.7 kg/mol, Đ = 1.17−1.22) were much lower than the theoretical values. The existence of crotonate/H chain ends implied that the NHC acted as a base, abstracting the α-H of Zn(C6F5)2-activated β-BL, which led to the alkyl cleavage to trigger the polymerization. The NHC−Zn(C6F5)2 adducts were also found to ring-open polymerize rac-lactide (TOF = 5 h−1, Mn = 3.8−9.2 kg/mol, Đ = 1.96−2.13) and TMC (TOF = 50 h−1, Mn = 14.1−23.9 kg/mol, Đ = 1.39−1.93) at RT (monomer/Zn = 100), and afford polymers with ill-controlled MW. Unlike β-BL polymerization, the obtained PTMC contained MeO−/HOCH2CH2CH2O−carbonate chain ends deriving from the methanolysis of an imidazolium chain end, which suggested that the polymerization was initiated by the nucleophilic attack of the NHC. In this context, Buchmeiser, Naumann, and co-workers also employed isolated NHC/metal halide LP adducts (26−29; Scheme 47) as the latent precatalysts for the ROP of ε-CL,411 one of which (26) has already been shown to act as a latent catalyst in the successful synthesis of polyurethanes with high catalytic activity compared to other systems on account of the cooperative polymerization mechanism where the free carbene and SnCl2 formed by thermal dissociation serve as a base for deprotonation of polyol and an LA for activation of the isocyanate, respectively.412 The polymerization activity strongly depended on the nature of the LA and dissociation ability of the NHC−LA adducts. No polymer was generated at RT, with the exception of 27, thanks to the weak coordination of the NHC to the Mg center, which resulted in the formation of a small amount of the free NHC to initiate the polymerization. At relatively high temperature (70 °C, ε-CL/BnOH/LP = 280/2/1, neat, 5 h), all of these adducts were active, where weakly bound 27 polymerized faster than 28, which in turn was more efficient than more strongly associated 26. Further raising the temperature to 90 and 130 °C, which facilitated the dissociation of the adduct, accelerated the polymerization for all the NHC−LA adducts (TOF ≈ 560 h−1, 15 min). In the case of 29-mediated polymerization, a high temperature of 130 °C was necessary to achieve quantitative conversion, whereas the polymerization at 70 °C was only marginally active (TOF = 2.5 h−1). The Mn values of the

obtained polymers were in the range of 6.0−16.5 kg/mol (Đ = 1.64−1.93). In 2015, Dove, Naumann, and co-workers described the combination of simple metal halides [MgX2 (X = Cl, Br, I), YCl3, AlCl3, etc.] with NHCs [s-IMes (1,3-dimesitylimidazolin2-ylidene), IMes, IiPr] and organic bases (DBU, DMAP) as the LPs for the ROP of cyclic esters,413 especially macrolactone ωpentadecalactone (PDL), a renewable monomer which has attracted increasing interest,414−422 thanks to the properties of its corresponding polymer (PPDL) being similar to those of linear low-density polyethylene.415 This study showed that isolated NHC−MgCl2 adduct 27 and the mixture of s-IMes and MgCl2 displayed an identical catalytic effect for the polymerization of PDL. In contrast, the use of YCl3 as the LA led to conversion about half as high as observed with MgCl2, while AlCl3 effected an extremely low conversion, and the addition of FeCl3, ZnCl2, BPh3, and Bi(OTf)3 did not induce any polymerization. For different magnesium halide salts, MgI2 exhibited the highest polymerization activity, which was explained by its cationic character resulting from dissociation of one iodide ligand. Other LBs, such as IMes, IiPr, DBU, and DMAP, were also active in the presence of MgX2. The order of activity was observed to be MgX2 > YCl3 ≫ AlCl3 and MgI2 > MgBr2 > MgCl2 in every case regardless of the nature of the nucleophile, indicating that the LA−monomer activation/ interaction is the decisive factor in this catalytic system. The combination of MgI2 and DBU was found to be the most active catalytic system for PDL polymerization (TOF = 308 h−1, Mn = 70.7 kg/mol, Đ = 1.80, DBU/MgI2/BnOH/PDL = 1/5/1/200, 110 °C, 30 min, toluene). As the mechanism widely proposed in organopolymerization, such LP-mediated polymerization proceeded through nucleophilic ring-opening of LA-activated PDL by either activated alcohol (initiator or OH-terminated growing polymer chain, Scheme 44B) or by the NHC itself (Scheme 44A). Interestingly, the MgI2/DMAP LP system showed monomer selectivity, where the polymerization gradually loses its activity when the nature of the monomer shifts more distant from the original PDL (2 h, conversion of εCL 95%, conversion of δ-VL 38%, conversion of rac-lactide 5%, conversion of β-BL 0%). In the case of copolymerization of εCL and δ-VL (1/1), only MgI2 and YCl3 coupled with DMAP resulted in isolated polymer (MgI2, 2 h, 52% conversion; YCl3, 10 min, 58% conversion). In the same year, Waymouth and Chang found that addition of an appropriate amount of LiCl (1 M) to IiPr(Me)-mediated ROP of δ-VL rendered a more controlled polymerization (TOF = 532.5 h−1) with higher I* (108.0−130.6%), predictable Mn (≤5.46 kg/mol), and narrow dispersities (Đ = 1.08−1.26), in sharp contrast to the polymerization by IiPr(Me) alone (TOF = 540.0 h−1, Mn = 19.6 kg/mol, Đ = 1.35, I* = 36.7%).423 The presence of LiCl also influenced the topology of the resultant PVL, in which only linear chains were produced at high LiCl concentrations, while cyclic PVLs were isolated in the absence of LiCl. Naumann et al. employed NHOs (6, 30−32; Scheme 48) as the LB, in combination with simple metal halides (MgI2, MgCl2, YCl3, 10592


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10.9−28.1 kg/mol, Đ = 1.72−2.10, 100 °C, THF, pressure tube). Fully randomized copolymer with 50% PDL content was realized by NHO 6/MgI2, as a result of transesterification. Moving to δ-VL/PDL copolymerization (110 °C, toluene), a relatively sluggish conversion of δ-VL ( MgCl2 > LiCl, and the saturated NHO backbones and the introduction of substituents on the exocyclic olefinic carbon can disfavor the adduct formation. When the lactone coordinates to a metal halide (MgCl2), the most likely binding mode is via the carbonyl oxygen. It was also demonstrated that in a metal-free setting an initiating alcohol cannot be deprotonated by NHO, while in the presence of MgCl2 the same process is exothermic with a low barrier. In 2013, the controlled ROP of L-lactide and ε-CL by LPs was established by Amgoune and Bourissou et al. through combining Zn(C6F5)2 as the LA with an organic base (an amine and a phosphine).134 It was found that weakly interacting Zn(C6F5)2/PMP LP was active toward L-lactide polymerization in 2-methyltetrahydrofuran (Me-THF)427 at 65 °C (TOF = 5.7 h−1, L-lactide/LA/LB = 30/1/1), affording PLA with a relatively narrow Đ of 1.3, albeit a low I* of only 17%. This LPP system showed a strong solvent dependence: the polymerization proceeded at a significantly lower rate in toluene or THF than that in Me-THF, and the formed PLA was contaminated by poly(THF) in THF, while no polymerization occurred in CH2Cl2. No polymer was formed when PMP or Zn(C6F5)2 was used alone at 65 °C after 12 h, indicating the cooperative activation of the LP is crucial to the polymerization. Moving to more nucleophilic LBs led to more efficient polymerization, with the TOF value following the order of PPh3 (7.0 h−1) < DMAP (13.7 h−1) < PnBu3 (14.9 h−1) (Mn ≤ 50.6 kg/mol, Đ = 1.1−1.6). More interesting, the resulting PLA obtained by Zn(C6F5)2/PMP has the cyclic architecture, as corroborated by the absence of end groups at whatever the MW of the PLA. Additionally, the MW of PLA had a linear increase with monomer conversion, while the Đ value slightly increased from 1.1 to 1.5 over time. On the basis of these results, the authors hypothesized that the key backbiting cyclization step occurs at

Scheme 48. Structures of NHOs Employed in This LPP Study and Proposed Equilibrium between Dissociated NHO/Metal Halide Pairs (Active for Polymerization) and NHO−Metal Halide Adducts (Inactive for Polymerization)

ZnI2, AlCl3) as the LA to achieve controlled homo- and copolymerization of ε-CL and δ-VL at RT.424 In the polymerization of ε-CL, the polymerization activity was determined by the LA: MgI2 (TOF ≈ 36 h−1) > YCl3 > ZnI2 ≈ AlCl3 > MgCl2, while in the δ-VL polymerization, ZnI2 (TOF ≈ 36 h−1) has gained reactivity in relation to YCl3 and MgI2. This LPP system was also successful for copolymerization of equimolar ε-CL and δ-VL (NHO/BnOH/metal halide/δ-VL/ ε-CL = 1/2/5/100/100), where the most rapid monomer consumption was observed for MgI2 and the slowest for AlCl3. Additionally, the metal halide is also the decisive factor for the copolymerization selectivity: MgI2, MgCl2, ZnI2, and AlCl3 favored the incorporation of δ-VL, whereas YCl3 showed a preference for ε-CL. In all cases, relatively narrow dispersity values were observed (Đ = 1.05−1.37) for the obtained polymers, with a linear correlation of Mn and conversion, despite the moderate I* value of ∼50%. In contrast, NHO 6 alone promoted the uncontrolled polymerization, and NHOs 30−32 were inactive toward ROP of ε-CL or δ-VL. These results demonstrated that suitable metal halides not only activate the monomer to facilitate the ring-opening process, but also suppress side reactions (transesterification and enolization286) arising from a reduction of the free, active NHO in solution as a result of competitive formation of inactive NHO− metal halide adduct (Scheme 48). It is also conceivable that the activation of the main chain ester group by the metal halide is weaker than that of the lactone, accordingly leading to a higher selectivity of ring-opening over transesterification. More recently, Naumann et al. successfully extended the above NHO (6, 31−32)/metal halide (MgI2, MgCl2, YCl3, ZnI2, LiCl) LP systems to homopolymerization of PDL, copolymerization of PDL with five-, six-, and seven-membered lactones (γ-BL, δ-VL, ε-CL), and copolymerization of γ-BL with δ-VL and ε-CL.425 With a NHO/BnOH/LA/PDL ratio of 1/2/5/100 at 110 °C, all three NHOs paired with metal halides were able to mediate PDL homopolymerization (conversion = 85−97%, 4 h, MgI2 > MgCl2 > YCl3 > ZnI2), producing PPDL with Mn = 10−40 kg/mol and relatively broad dispersity of 1.46−1.79, due to the existence of transesterification at high temperature. Various copolymers were obtained by employing this LP system where the monomer feed ratio was fixed to 1/1. Compared to PDL homopolymerization, copolymerizations of PDL with ε-CL proceeded more slowly (conversion = 54−97%, 4 h) and produced copolymers with broader dispersity (Mn = 10593


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Scheme 49. Two Possible Mechanistic Pathways Proposed for the LP-Mediated ROP of Lactide Leading to the Formation of Cyclic PLA

CL) and l-poly(PDL-b-LA)] by sequentially polymerizing PDL and ε-CL or L-lactide and subsequent nucleophilic substitution by alcohol after full monomer conversion. It has been shown that bulky and electron-deficient Ph2CHOH was the most suitable alcohol that exclusively attacked the acylazolium species rather than the ester group in the polymer chain, thereby leading to the formation of the linear polymer without random chain scission. LPs based on group 13 LAs are the early system explored for the ROP. In 2012, Chen et al. initially employed Al-based LPs consisting of highly acidic Al(C6F5)3 LA and PtBu3 LB for the ROP ε-CL, albeit low activity (TOF = 23.2 h−1, 0.125 mol % catalyst), producing PCL with Mn = 74.0 kg/mol and Đ = 2.76.147 Recently, Nakayama and co-workers described the controlled ROP of L-lactide by employing Al(C6F5)3·THF/ PMes3 or PPh3 in the presence of BnOH to produce PLA with relatively low dispersity (Mn = 7.2−25 kg/mol, Đ = 1.11−1.46, TOF < 4.0 h−1, L-lactide/LA/LB/BnOH = 100/1/1/1, 100 °C, 24 h).270 The ROP of ε-CL by this LP system was faster (TOF < 170 h−1) but less controlled (Mn = 4.1−10 kg/mol, Đ = 1.41−1.67). Dagorne et al. utilized adducts of NHC (IMes, IDipp, ItBu)−group 13 metal alkyl (MR3; M = Al, Ga, In; R = Me, Et, C6F5) for the ROP of rac-lactide.430 A controlled ROP was achieved when adducts of IMes−AlR3 (R = Me, TOF = 100 h−1; R = Et, TOF = 24.7 h−1) were employed as catalysts at RT (rac-lactide/LP adduct = 100, Mn = 13−15 kg/mol, Đ = 1.31−1.40, I* = 90.3−107.1%), whereas the more sterically bulky adducts IDipp−AlR3 (R = Me, Et) required heating (90 °C) for polymerization to occur, and mediated much less active and less controlled ROP (TOF = 3.9−4.5 h−1, Mn = 5.1−8.2 kg/mol, Đ = 1.32, I* = 52.8−76.5%). Interestingly, the IMes− AlMe3 adduct selectively produced imidazolium-ended linear PLA, while the IDipp−AlMe3 adduct led to the formation of cyclic PLA. Heating a linear PLA obtained by IMes−AlMe3 after complete conversion can quantitatively convert into the cyclic topology, ascribing to the thermally induced intramolecular cyclization. The IMes−Al(C6F5)3 adduct with the more acidic LA exhibited a much lower activity (TOF = 0.63 h−1) presumably due to the stronger Al−NHC interaction, which thereby disfavored the dissociation of the adduct. No polymerization activity was observed at RT in the case of ItBu− AlMe3, and a poor control of the ROP process when IMes− GaMe3 and IMes−InMe3 were utilized (TOF = 1.8−10.0 h−1, Đ = 2.22−2.54).

the very end of the polymerization and/or during workup. As a consequence, it provided a new entry to sequential polymerization of ε-CL and L-lactide, affording PCL-b-PLA cyclic diblock copolymer, which was not possible in the NHCmediated ring-closing polymerization of lactide previously reported by Waymouth and Hedrick as it promotes reinitiation rather than chain extension.27 Two possible polymerization mechanisms were proposed (Scheme 49). The basic pathway was proposed to proceed via deprotonation of the activated LA by a stronger base but a poorer nucleophile (such as PMP) to generate a zinc enolate that initiates the polymerization (Scheme 49A). The other pathway was proposed to proceed via zwitterionic species where the Lewis acidic Zn(C6F5)2 activates the monomer through the coordination, while the amine or phosphine LB (DMAP, P n Bu 3 , and PPh 3 ) cooperatively acts as a nucleophile to generate a zinc alkoxide (Scheme 49B). The above Zn(C6F5)2-based LPP mechanism was further exploited by Li et al. for the synthesis of cyclic polymers.428 Thus, combination of Zn(C6F5)2 with organic bases, including DMAP, IMes, DBU, and 7-methyl-1,5,7-triazabicyclo[4.4.0]decane-5-ene (MTBD), afforded LPs with different degrees of LA−LB interactions, thus modulating the ROP of L-lactide, and a direct relationship between polymerization activity and the degree of interaction of the LP can be observed. For example, the most interacting Zn(C6F5)2/DMAP pair exhibited the lowest activity (TOF = 46.3 h−1, L-lactide/LA/LB = 50/1/1, toluene, 80 °C), while the less interacting Zn(C6F5)2/IMes showed moderate activity (TOF = 92.6 h−1), and the least interacting Zn(C6F5)2/DBU and Zn(C6F5)2/MTBD exhibited the highest activity (TOF = 277.8 h−1). Moreover, the interaction between LA and LB was weakened at high temperature or in solvents with a high dielectric constant, resulting in further enhanced polymerization activity. The analysis of the unquenched polymerization mixture produced by Zn(C6F5)2/MTBD LP revealed that the polymerization proceeded via zwitterionic species through the bifunctional activation of the LP. Very recently, Li et al. also showed that Zn(C6F5)2/DBU LP promoted the controlled ROP of PDL, achieving a TOF up to 198 h−1.429 In line with the ROP of Llactide by the same LP system, the resulting PPDL has a cyclic topology. Cyclic block copolyesters c-poly(PDL-b-CL) and cpoly(PDL-b-LA) were obtained by sequential addition of PDL and ε-CL or L-lactide. The authors also developed an effective strategy to produce linear diblock copolymers [l-poly(PDL-b10594


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Scheme 50. Proposed Mechanism for the Living ROP of δ-VL or ε-CL by NHO/Al(C6F5)3 LPs

Figure 9. SC-XRD structures of the zwitterionic, tetrahedral intermediate derived from the reaction of NHO (8) + Al(C6F5)3 + ε-CL (left column), which models intermediate 34, and the corresponding ring-opened product (right column), which models the precursor of intermediate 35. Reprinted from ref 435. Copyright 2017 American Chemical Society.

1.02).435 Owing to the high nucleophilicity of NHO, NHO/ Al(C6F5)3 LPs exhibited a much higher activity (ε-CL/LA/LB = 400/2/1; 8, TOF = 22.2 h−1; 33, TOF = 114.2 h−1) than the other Al(C6F5)3-based LPs (LB = DBU, DMAP, PMes3, ItBu). Interestingly, relative to the 8/Al(C6F5)3 system (Đ = 1.27− 1.77, I* = 56−97% (δ-VL), I* = 49−59% (ε-CL)), 33/ Al(C6F5)3 not only promoted the polymerization with a significantly higher I* (δ-VL, I* = 78−127%; ε-CL, I* = 70− 106%) and enhanced activity which was 4.8 and 6 times higher in the ROP of δ-VL and ε-CL, respectively, but also produced polyesters with much lower Đ values (1.06−1.15). The living characteristics of this LPP system, cleanly confirmed by the linear growth of Mn values with an increase of conversions and monomer-to-initiator ratio as well as the chain extension experiments, enabled the synthesis of well-defined di- and triblock copolymers regardless of the monomer addition order (PCL-b-PVL, PCL-b-PVL-b-PCL) with narrow dispersity (Đ < 1.15). The kinetic study revealed that 33/Al(C6F5)3-mediated ROP is zeroth-order with respect to both monomer and Al(C6F5)3 concentrations but has a first-order dependence on the concentration of NHO 33. Coupled with structurally characterized key intermediates (34 and 35; Scheme 50, Figure 9), the authors proposed that the polymerization is initiated by the nucleophilic attack of the Al(C6F5)3-activated monomer by NHO 33 to form the zwitterionic, tetrahedral intermediates, followed by its ring-opening and proton transfer to generate

Hillmyer, Tolman, and co-workers reported the stereoselective ROP of rac-lactide using a simple LP system prepared in situ from InCl3, BnOH, and NEt3, affording PLA with good MW control (Mn = 11.5−159 kg/mol, Đ = 1.06−1.12) and high heterotacticity (Pr ≤ 0.97, where Pr is the probability of racemic placement of monomer stereoisomers),431,432 despite the absence of an ancillary ligand that is typically required in the metal-based coordination polymerization systems.433,434 All three components are necessary for the polymerization to occur (TOF ≤ 40 h−1). The polymerization rate was affected by the lactide stereoisomer [kobsd(rac-lactide) ≈ kobsd(meso-lactide) > kobsd(L-lactide)], which can be utilized as a protocol for purifying D-lactide spiked with small amounts (ca. 5%) of Llactide and meso-lactide. Kinetic studies showed the process to be first-order in monomer and InCl3 concentrations and zeroth-order with respect to both BnOH and NEt 3 concentrations. The authors inferred that ROP occurs via a coordination−insertion mechanism that employs an [InCl(3−n)(OBn)n]m as the propagating species (with NEt3H+ as the counterion). The role of the amine is to act as a base to generate the dinuclear dianionic complex and is not an integral part of the catalytically active species. The living ROP of δ-VL or ε-CL was developed by Zhang and Chen et al. in 2017 by using Al(C6F5)3/NHO (8, 33; Scheme 50), affording polyesters with high Mn (up to 855 kg/ mol, Đ as low as 1.02) and narrow dispersity (Đ as low as 10595


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Scheme 51. Proposed Pathway for Ring Contraction of rac-Lactide by B(C6F5)3-Based FLPs

Scheme 52. Epimerization of meso-Lactide into rac-Lactide Catalyzed by the B/N LP and Subsequent Kinetic Resolution Polymerization of the Resulting rac-Lactide into Isotactic PLA and Optically Resolved D-Lactide

Scheme 53. Proposed Mechanism for the Controlled ROP of NCAs by LPs

lactide, often regarded as a “waste” side product that needs to be removed during the production of L-lactide, into rac-lactide via rapid epimerization catalyzed by B(C6F5)3/DABCO LP (Scheme 52).437 The use of the relatively nonpolar solvent toluene was essential for this rapid and quantitative epimerization, because it enabled the precipitation of the resulting rac-lactide from solution once formed due to solubility differences, which constantly disrupts the thermodynamic equilibrium to ensure the completion of epimerization. Even with a low catalyst loading of 0.01 mol %, 95.4% meso-to-raclactide conversion was accomplished in just 5 min (TOF = 114480 h−1). The authors also developed a highly enantioselective bifunctional chiral organic catalyst (Scheme 52) which combines three essential elements (β-isocupreidine core, thiourea functionality, and binaphthylamine framework) into a single molecule that can asymmetrically activate both the monomer (by the chiral thiourea) and the propagating hydroxyl species (by the chiral amine). Under optimized conditions employed for kinetic resolution polymerization of rac-lactide, this bifunctional chiral organic catalyst preferentially polymerized L-lactide and kinetically resolved D-lactide with a high stereoselectivity factor of 53 and an ee of 91% at 50.6% monomer conversion. The epimerization and kinetic resolution polymerization can be coupled into a one-pot process,

zwitterionic enolaluminate active species which attack the incoming monomer activated by Al(C6F5)3, thus entering into the chain propagation cycle (Scheme 50). The ring-opening process of the zwitterionic, tetrahedral intermediates was considered as the rate-determining step. The initial attempt of the ROP of δ-VL and rac-lactide by B(C6F5)3/phosphine (PtBu3, PCy3) or amine (2,6-lutidine, N,N-dimethylaniline, TMP) FLPs was made by Stephan and co-workers in 2011.436 However, ring-opening of δ-VL and ring contraction of rac-LA were observed without any formation of polyesters. Mechanistically, the ring-opening of the lactone resulted from the activation of the C−O bond by B(C6F5)3, prompting nucleophilic attack by the amine or phosphine LB to generate the zwitterionic species. Switching to rac-lactide, the ring-contracted product was proposed to be initiated by deprotonation of B(C6F5)3-activated rac-lactide by the organic base, presumably due to the steric demand of the methyl groups of rac-lactide that precluded the nucleophilic attack of the sterically encumbered organic base. The resulting carbanion undergoes nucleophilic attack of the carboanion to the carbonyl C, affording the ring-contracted product (Scheme 51). On the other hand, using the B(C6F5)3/1,4-diazabicyclo[2.2.2]octane (DABCO) LP (which forms a stable CLA), in 2015, Zhu and Chen discovered a strategy for quantitatively converting meso10596


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Scheme 54. Copolymerization of CO2 with Epoxides by Different LPs and Initiation Pathways Involved in the Copolymerization

controlled polymerizations (TOF = 3.1−5.6 h−1, Mn = 6.4− 11.6 kg/mol, Đ = 1.24−1.29). Most recently, Yang et al. also explored a simple, efficient LP system by substituting Fmes2BF with zinc acetate [Zn(OAc)2] as the LA.439 Thus, the Zn(OAc)2/aniline (or its derivatives) system also mediated the controlled ROP of Glu-NCA under mild conditions in a relatively short reaction time (TOF ≤ 30 h−1, Mn ≤ 44 kg/mol, Đ = 1.10−1.30, Glu-NCA/LA/LB = 50/ 1/1). 4.2.3. Copolymerization of CO2 (COS and Anhydrides) with Epoxides. LP catalysis was also successfully employed for alternating copolymerization of CO2 [also carbonyl sulfide (COS) and cyclic anhydrides] with epoxides recently, which was traditionally catalyzed by transition-metal complexes that usually required time-consuming multistep synthesis, providing a straightforward route to polycarbonates (PCs) [poly(monothiocarbonate)s or polyesters]. In 2016, Gnanou, Feng, and co-workers established the controlled copolymerization of CO2 with cyclohexene oxide (CHO) by utilizing AliBu3 as the LA and a lithium halide (LiCl, LiBr) or alkoxide (LiOBn) as the LB initiator (Scheme 54),440 which selectively produced the alternating copolymer with a high carbonate content of 95− 99% in 84−92% yield after 10 h (TOF ≤ 2 h−1, Mn = 2.1−5.8 kg/mol, Đ = 1.10−1.30, CHO/LB/AliBu3 = 20/1/0.4, 60−80 °C), without the formation of the cyclic carbonate product. The copolymerization by the AliBu3/LiCl (LiBr) pair was proposed to be initiated by the nucleophilic attack of Cl− (Br−) to AliBu3activated CHO, resulting in ring-opening of CHO to form the growing alkoxide species, which then reacted with Li+-activated CO2 to yield the lithium-bound carbonate for the following alternating chain propagation (Scheme 54, pathway A). On the other hand, when LiOBn was utilized as the initiator, the polymerization was proposed to be started by the reaction between OBn− and Li+-activated CO2 (Scheme 54, pathway B). Because of the existence of an equilibrium between free AliBu3 and LiOBn and their ate complex [LiBnO−AliBu3], switching from a more dissociating medium (THF) to relatively nonpolar toluene led to sluggish copolymerization. In addition, α,ωbifunctional PC or PSt-b-PC (PSt = polystyrene) and PI-b-PC (PI = polyisoprene) block copolymers were also synthesized by using the triethylene glycol precursor or by one-pot sequential addition with styrene or isoprene initiated first (Scheme 54). Recently, the homopolymerization of propylene oxide or oxetane and copolymerization of propylene oxide and oxetane

effectively transforming meso-lactide directly into isotactic PLLA and D-LA. 4.2.2. N-Carboxyanhydrides. The scope of the LPmediated ROP has extended beyond cyclic esters (e.g., lactones and lactides). Recently, Yang et al. reported the utilization of the borane/amine LPs for the controlled ROP of N-carboxy anhydrides (NCAs) to synthesize well-defined polypeptides (Scheme 53), opening up a new opportunity for the application of LP-mediated polymerizations.438 Control runs revealed that the ROP of γ-benzyl-L-glutamate (Glu-NCA) by the LB aniline alone was completed in 1 h (Glu-NCA/LB = 50/1, RT), affording the corresponding polypeptide with a relatively broad dispersity (Đ = 1.63), as expected for a normal amine mechanism, whereas the bulky borane LA, bis[2,4,6-tris(trifluoromethyl)phenyl]boron fluoride (Fmes2BF), was incapable of initiating the polymerization. However, when Fmes2BF and aniline were combined, the ROP became controlled, producing the polypeptide with a measured Mn (7.5−12.0 kg/ mol) close to the calculated value and a narrow Đ value (1.28), although the polymerization time was extended to 8 h to achieve a 99% monomer conversion (TOF = 6.3 h−1, GluNCA/LA/LB = 50/1/1). Different from the typical mechanism of LPP where LA activates the monomer through the coordination, it was proposed that, in this case, the bulky borane LA captures the terminal amino group of the propagating chain to form a new LP intermediate (Scheme 53), which is essential for restraining side reactions. Because the nucleophilicity of the aniline initiator and the terminal amine of the propagation chain end are weakened by the interaction with the LA, a reduced rate of the polymerization, but a more controlled polymerization process was observed, as compared to the aniline-mediated ROP. Additionally, the controlled ROP of ε-benzyl-L-lysine N-carboxyanhydride (Lys-NCA) was also achieved by the Fmes2BF/aniline LP (TOF = 9.9 h−1, Mn = 11.9 kg/mol, Đ = 1.22). Under the same conditions, bis[2,4,6tris(trifluoromethyl)phenyl]boron hydride (Fmes2BH)/aniline pair exhibited a low activity for the ROP of Glu-NCA (TOF = 2.5 h−1), producing the polymer with a broad dispersity (Đ = 1.53), while the B(C6F5)3/aniline pair was totally inactive, highlighting the importance of choosing a sterically and electronically balanced LA, relative to the pairing LB, for achieving an efficient and controlled LPP. Switching to other primary amines, such as p-methoxyaniline, p-bromoaniline, and benzylamine, in combination with Fmes2BF also led to 10597


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for the alternating copolymerization of COS with CHO at 80 °C, with the alternating selectivity over 99% (TOF ≤ 102 h−1, M n ≤ 56.5 kg/mol, Đ = 1.20−1.50). However, the copolymerization of COS with phenyl glycidyl ether by BEt3/ DTMeAB (or DBU) led to relatively lower alternating selectivity (88%) or high cyclic thiocarbonate (90%), and these LP catalytic systems were inert toward copolymerization of COS with electron-deficient styrene oxide (SO). The same group also achieved one-pot synthesis of COS/PO-based diblock or triblock copolymers by using the BEt3/guanidine (or quaternary onium salts) LP and polyethylene glycol (PEG) as the macromolecular chain transfer agent (CTA).450 Thus, the addition of the CTA to COS/PO copolymerization enhanced the polymerization activity (TOF of PO up to 240 h−1), higher than that of the copolymerization without CTA, while the perfect alternating degree and regioregularity were maintained for the poly(monothiocarbonate) block. Very recently, Wang, Li et al. described the employment of zinc alkyl (or aryl)/amine LPs for alternating ring-opening copolymerization of various cyclic anhydrides [phthalic anhydride (PA), cyclohexanedicarboxylic anhydride (CHA), succinic anhydride (SA)] with epoxides [PO, CHO, epichlorohydrin (ECH), SO] to semiaromatic and aliphatic polyesters (Scheme 56).451 In the case of CHO/PA copolymerization, the

have also been realized by using onium salts in combination with aluminum alkyls.441,442 Shortly after, the same group reported the simple, metal-free synthesis of colorless PC via controlled copolymerization of CO2 with epoxides by utilizing organic BEt3 as the LA in combination with onium halides or onium alkoxides derived from alcohol deprotonation by organic phosphazene superbases (tBu-P4, tBu-P2) (Scheme 54).443 In the case of propylene oxide (PO) copolymerization (PO/BEt3/initiator = 50−1000/2/1, 60 °C, 10 h), the NBu4Cl/BEt3-based system stood out with respect to the other systems studied, exhibiting a relatively higher activity with a TOF of 50 h−1 and also producing the polymers (Mn ≤ 50.0 kg/mol, Đ = 1.10−1.20) with a higher carbonate content (>90%) and better product selectivity (92− 99% vs cyclic carbonate). Moreover, good regioselectivity can be achieved by this catalytic system, showing the presence of ∼82% head/tail linkage. When copolymerizing CO2 with CHO, perfectly alternating PCs (carbonate content >99%) were obtained with the product selectivity over 94% (Mn ≤ 76.4 kg/ mol, Đ = 1.10−2.10). The highest TOF of 600 h−1 was achieved by the PPNCl/BEt3 pair at a low catalyst loading of 4000/2/1 (80 °C, 6 h), which was much higher than that of the AliBu3-based catalytic system. The copolymerization was proposed to proceed with the LA BEt3 working as the activator for the epoxide, while the organic anion of the onium salt acts as the initiator for the copolymerization and the organic cation activates CO2 to form the cation-bound carbonate, similar to the AliBu3-based catalytic system. Alternating and regioselective copolymerization of COS with epoxides was also achieved by the BEt3-based LPs (Scheme 55),

Scheme 56. Alternating Ring-Opening Copolymerization of Anhydrides with Epoxides Catalyzed by Zinc Alkyl (or Aryl)/Amine LPs

Scheme 55. Copolymerization of COS with Epoxides by LPs

as revealed by Zhang, Darensbourg, and co-workers in 2017.444 This copolymerization strategy provided a new metal-free approach to convert COS, an environmentally harmful gas derived from the burning of fossil fuels, coal gas, and many chemical processes,444 into colorless, transparent, high-performance poly(monothiocarbonate).445−449 In this study, LBs, including DBU, TBD, and quaternary onium salts [e.g., dodecyltrimethylammonium bromide (DTMeAB), NBu4Cl, PBu4Cl, PPh4Br, and PPNCl], when combined with BEt3, can catalyze this copolymerization, while other LPs such as BPh3/ PPh3 (DBU or TBD) and B(C6F5)3/PPh3 (DBU or TBD) were ineffective. In the case of PO/COS copolymerization, an alternating copolymer product (84% to >99%, vs cyclic thiocarbonate) with high chemoselectivity (with an undetectable ether linkage) and high regioselectivity (>99% tail-to-head linkage) was accomplished at RT (PO/BEt3 = 250−1000, TOF ≤ 119 h−1, Mn ≤ 92.5 kg/mol, Đ = 1.20−1.80). In most instances, oxygen−sulfur exchange reactions between COS and epoxides, which would generate random thiocarbonate and carbonate units, were effectively suppressed, except those polymerizations conducted at high reaction temperatures or high BEt3 loadings. This LP catalytic system was also effective

cooperation of DMAP with Zn(C6F5)2 [Zn(C6H5)2 or ZnEt2] yielded alternating copolymers with an ester linkage higher than 99% (TOF ≤ 102 h−1, Mn ≤ 56.5 kg/mol, Đ = 1.20−1.50, 110 °C), while replacing DMAP with DBU or MTBD maintained the perfect alternating degree, but led to a suppressed catalytic efficiency. The copolymerization of PA with PO by Zn(C6F5)2/ DMAP had a reactivity similar to that of the CHO copolymerization, affording the alternating copolymer with high regioselectivity (TOF = 101 h−1, Mn = 7.4 kg/mol, Đ = 1.25). An enhanced polymerization rate was achieved in the copolymerization of PA with ECH (TOF = 210 h−1) or SO (TOF = 166 h−1), presumably due to the existence of the electron-withdrawing group in the monomer, but producing copolymers with relatively low MWs. For copolymerization of epoxides with cyclic anhydrides by the LPs, deprotonation of αH of the anhydrides with organic bases was observed, resulting in a decrease of activity in the copolymerization of CHO with SA (TOF = 21 h−1) or CHA (TOF = 56 h−1), which can be suppressed to some extent by using nonpolar solvents such as 10598


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general, relatively nonpolar and noncoordinating solvents are employed as the medium in LPP, while strong donating solvents such as DMF are purposely avoided because such solvents preferentially coordinate with the LA, thus poisoning the LA and leading to the suppression of monomer activation. Toluene (dielectric constant ε = 2.38) is the most widely used solvent in LPP, but in the cases when the dissolvability of the resultant polymer (e.g., PγMMBL) in toluene is poor, polar but noncoordinating solvents, such as xylene, bromobenzene, chlorobenzene, dichloro(or fluoro)benzene, and CH2Cl2 (ε = 8.93), are required to achieve a homogeneous polymerization. It is worth mentioning that extra caution should be exercised when CH2Cl2 is used in LPP due to rapid decomposition of some commonly used but rather sensitive LAs and LBs as well as the active species in this solvent. For example, the NHCs with less steric hindrance (e.g., IMe and IiPr) are unstable in this solvent,362 and direct contact of CH2Cl2 with Al(C6F5)3 must be avoided as they react rapidly to form [ClAl(C6F5)2]2.117 However, Al(C6F5)3/monomer adducts (isolated or generated in situ) are stable in CH2Cl2. The observed high activity in the LPP of polar monomers by a CLA, a stable adduct in the solid state, was attributed to an equilibrium between the CLA and the “frustrated” free LA + LB form in the presence of a suitable solvent or monomer for effective chain initiation.374 Therefore, the dissociating ability of the CLA in a solvent or monomer significantly impacts the LPP behavior, especially the polymerization activity and I* value. The decreased activity with an increase in the LA−LB adduct strength was directly observed due to the difficulty of releasing the free LB and LA for initiating the polymerization.374 In this context, solvents with a relatively high dielectric constant can bring about an enhanced chain initiation rate.428 Moreover, CLA-mediated polymerizations are usually carried out using a monomer activation procedure to achieve high activity by premixing the monomer and the LA in a solvent first, followed by the addition of the LB to initiate the polymerization.16,147,346 The reverse addition sequence by direct mixing of the LA and the LB first generally generates a stable CLA, thus leading to the sluggish polymerization. In the cases where the formation of the stable CLA can still occur even in the monomer activation procedure, LPPs usually exhibit low to moderate I* values ( Al(ORF)3 (RF = C(CF3)3). Angew. Chem., Int. Ed. 2008, 47, 7659−7663. (68) Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A. On a Quantitative Scale for Lewis Acidity and Recent Progress in Polynitrogen Chemistry. J. Fluorine Chem. 2000, 101, 151−153. (69) Mallouk, T. E.; Rosenthal, G. L.; Muller, G.; Brusasco, R.; Bartlett, N. Fluoride-Ion Affinities of GeF 4 and BF 3 from Thermodynamic and Structural Data for (SF3)2GeF6, ClO2GeF5, and ClO2BF4. Inorg. Chem. 1984, 23, 3167−3173. (70) Haartz, J. C.; Mcdaniel, D. H. Fluoride-Ion Affinity of Some Lewis-Acids. J. Am. Chem. Soc. 1973, 95, 8562−8565. (71) Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85, 3533−3539. (72) Drago, R. S.; Wayland, B. B. A Double-Scale Equation for Correlating Enthalpies of Lewis Acid-Base Interactions. J. Am. Chem. Soc. 1965, 87, 3571−3577. (73) Parr, R. G.; Szentpaly, L. v.; Liu, S. B. Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922−1924. (74) Maynard, A. T.; Huang, M.; Rice, W. G.; Covell, D. G. Reactivity of the HIV-1 Nucleocapsid Protein p7 Zinc Finger Domains 10605


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Polymerization of Polar Monomers Mediated by Main-Group Lewis

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