Transformation of Step-Growth Polymerization into Living Chain

Review pubs.acs.org/CR

Transformation of Step-Growth Polymerization into Living ChainGrowth Polymerization Tsutomu Yokozawa* and Yoshihiro Ohta Department of Material and Life Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan ization is that the functional groups of the monomer and polymer show the same reactivity,1−3 step-growth polymerization could be transformed into chain-growth polymerization if the polymer end group formed by reaction of the monomer becomes more reactive than the monomer itself. 4−14 Furthermore, the end group of the condensation polymer is a stable functional group, not a reactive species such as a radical or an ion, and therefore, chain-growth condensation polymerization (CGCP) should show living polymerization behavior. Indeed, perfectly monodisperse biopolymers such as polypeptides,15,16 DNA,17 and RNA18 are produced via a CGCP mechanism. Artificial CGCP of an X−AB−Y monomer has been achieved by activation of the polymer end group in the following two ways: (1) the reactivity of the polymer end group CONTENTS Y is enhanced when a change of substituent effect on Y is induced by B−A bond formation between the monomer and 1. Introduction 1950 polymer (Scheme 1a), and (2) the polymer end group Y is 2. CGCP Based on the Substituent Effect 1950 activated by catalyst M transferred intramolecularly after the 2.1. Basic Principle 1950 monomer has reacted with the polymer end, −B−M−Y 2.2. Monomers Based on the Resonance Effect 1951 (Scheme 1b). In this review, we will focus on developments 2.3. Monomers Based on the Inductive Effect 1951 in CGCP based on these two approaches since 2013. We have 2.4. Architecture 1952 previously reviewed earlier work in these areas, as well as other 3. CGCP by Catalyst Transfer 1952 19−21 approaches to CGCP. 3.1. Background 1952 3.2. Monomers 3.2.1. Grignard Monomers 3.2.2. Boronic Acid Ester Monomers 3.2.3. Other Monomers 3.3. Mechanism of Catalyst-Transfer Condensation Polymerization 3.4. Functionalization of Polymer Ends 3.5. Architecture 3.5.1. All Conjugated Polymers 3.5.2. Conjugated Polymers and Conventional Polymers 4. Miscellaneous Chain-Growth Polymerization 5. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments References

1953 1953 1954 1956

2. CGCP BASED ON THE SUBSTITUENT EFFECT 2.1. Basic Principle

CGCP based on the substituent effect proceeds in the following way (Scheme 2). The proton of the monomer H−AB−Y, which has both nucleophilic and electrophilic sites, is first abstracted with base to generate an anion, −A. The strong electrondonating substituent effect of the −A anion deactivates the electrophilic site B−Y, which prevents the anion monomer −A− B−Y from reacting with another −A−B−Y (self-condensation). Scheme 1. Chain-Growth Condensation Polymerization in Two Manners

1956 1957 1958 1958 1960 1961 1963 1963 1963 1963 1963 1964 1964

1. INTRODUCTION Conventional condensation polymerization proceeds in a stepgrowth polymerization manner, and it was thought that “living” condensation polymerization was essentially impossible, because living polymerization is a chain-growth polymerization. However, since the basic principle of step-growth polymer© 2015 American Chemical Society

Special Issue: Frontiers in Macromolecular and Supramolecular Science Received: July 6, 2015 Published: November 10, 2015 1950

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Scheme 2. Basic Principle of CGCP Based on the Substituent Effect

Scheme 3. CGCP of 1 (R = OOB, R′ = C8H17) and Deprotection26

When an electrophile B′−Y as an initiator reacts with the anion monomer to form a B′−A bond, the electron-donating ability of the B′−A bond becomes weaker than that of the −A anion, resulting in increased reactivity of the terminal electrophilic site B−Y. The next monomer attacks this B−Y site, and growth continues in a chain-polymerization manner, in which the monomer selectively reacts with the polymer end group. A change of substituent effect at the nucleophilic site can be transmitted to the electrophilic site by means of both a resonance effect and an inductive effect. We will separately review monomers utilizing these two effects. monomer 2′ generated with LiHMDS did not undergo selfcondensation. However, polymerization did not proceed even in the presence of an initiator ester, and only a 1:1 adduct of 2 and the initiator was formed. Accordingly, this chemistry was applied to stepwise synthesis of oligo(o-benzamide) by using Nalkylisatoic anhydride 3 (Scheme 4).37,38 Furthermore, ladder polymer synthesis was attempted by means of polymerization of 4, bearing both o-aminobenzoic acid ester and isatonic anhydride moieties, with LiHMDS, but a linear polymer was obtained, as well as the cyclized products (Scheme 5).39

2.2. Monomers Based on the Resonance Effect

Typical monomers are para-substituted phenylene monomers, CGCP of which affords polyamides,22−26 polyesters,27,28 and polyethers29−32 with narrow polydispersity. Furthermore, 2,6substituted naphthalene33 and diphenylene bearing an electronwithdrawing group at the para-position34−36 also undergo CGCP to afford polyamide and polyether, respectively (Table 1). Monomer 1 underwent CGCP with lithium 1,1,1,3,3,3hexamethyldisilazide (LiHMDS) and LiCl in THF at −10 °C even if an alkyl group was present at the 3-position of the monomer. When R and R′ were 4-(octyloxy)benzyl (OOB) and octyl groups, respectively, polyamide obtained by CGCP was converted into poly(3-octylbenzamide) by treatment with sulfuric acid (Scheme 3). This polyamide was soluble in acetone, N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, methanol, and trifluoacetic acid (TFA), in contrast to poly(p-benzamide), which is insoluble in most organic solvents.26 o-Aminobenzoic acid ester 2 was expected to undergo CGCP based on the resonance effect. Indeed, the amide anion

2.3. Monomers Based on the Inductive Effect

meta-Substituted phenylene monomers undergo CGCP, leading to polyamides under optimized conditions,40−44 although the inductive effect is less efficiently transmitted, compared to the resonance effect (Table 2). Monomer 5 with a methoxyethoxymethyl (MEM)-protected hydroxyl group and OOB-protected amino group afforded the corresponding polyamide, although the para counterpart was hardly polymerized. The protecting groups on the oxygen and amide nitrogen of the polyamide were simultaneously removed with TFA to afford well-defined poly(amidophenol), which was converted to poly(benzoxazole) by heating.44 AB2-type monomer 6 also underwent CGCP to

Table 1. Monomers Based on the Resonance Effect

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Scheme 4. Stepwise Synthesis of Oligo(o-benzamide)38

(Sn(Oct)2) to yield a block copolymer with low polydispersity. Furthermore, the hydroxyl group could be converted to an atom transfer radical polymerization (ATRP) initiator unit and a chain transfer agent unit for reversible addition−fragmentation chain transfer (RAFT) polymerization, and block copolymers of hyperbranched polyamide and a vinyl polymer were obtained by ATRP and RAFT polymerization (Scheme 6).48 Recently, Prehn and Boyes conducted CGCP from an initiator on the surface of a silicon wafer or from silica to obtain poly(N-octyl-p-polybenzamide) brushes (Scheme 7).49 The film thickness increased to 12 nm with increasing reaction time. However, further extension was difficult because of blocking of the reactive polymer end groups. In the course of our study of CGCP, we found that poly(Nalkyl-p-benzamide)s50,51 and poly(N-alkyl-2,6naphthalenecarboxamide)s52−54 adopt a helical conformation. Recently, we synthesized poly(p-benzamide) having isopropylsubstituted tris(ethylene glycol) (TEG) as a chiral Nsubstituent by means of CGCP. In comparison with the previous helical polyamide with a methyl-substituted chiral TEG,50 this bulkier isopropyl counterpart increased the abundance of the helical structure. This is probably due to the increased difference of stability between the right- and lefthanded helical structures of the polymer (Scheme 8).55

Scheme 5. Attempt To Synthesize a Ladder Polyamide from 439

give a hyperbranched polyamide, the molecular weight of which can be controlled up to 40000 while retaining low polydispersity (Mw/Mn < 1.2).45−47 2.4. Architecture

Aromatic polyamides, polyesters, and polyethers obtained by means of CGCP thus far described strictly contain the initiator unit and end group in a polymer chain, like living polymerization, and architectures such as block copolymers, star polymers, and graft copolymers have been synthesized. Since we have previously reviewed these architectures,19,21 only recent examples are described here. Several block copolymers of hyperbranched polyamide and conventional polymer were synthesized from hyperbranched macroinitiators. The hydroxyl group in the initiator unit of hyperbranched polyamide initiated ring-opening polymerization of ε-caprolactone in the presence of tin(II) 2-ethylhexanoate

3. CGCP BY CATALYST TRANSFER 3.1. Background

We have investigated Ni-catalyzed Kumada−Tamao coupling polymerization of bromothiophene Grignard monomer 7, the regioregular polymerization of which had been developed by McCullough,56−59 during the course of development of CGCP based on the substituent effect. We anticipated that the Ni(0)

Table 2. Monomers Based on the Inductive Effect

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Scheme 6. Block Copolymer of Hyperbranched Polyamide and a Conventional Polymer48

Scheme 7. Aromatic Polyamide Brushes from a Silicon Surface49

implying that the anticipated chain-growth polymerization based on a change of the substituent effect was not involved in this polymerization. We conducted several studies on the polymerization mechanism, and proposed that the Ni(0) catalyst is inserted into the intramolecular C−Br bond after transmetalation and reductive elimination, resulting in CGCP (Scheme 9b).64

Scheme 8. Poly(p-benzamide) Having an IsopropylSubstituted Chiral Tris(ethylene glycol) Side Chaina

3.2. Monomers

3.2.1. Grignard Monomers. Many Grignard monomers undergo Ni-catalyzed CGCP (Table 3). Donor monomers were first developed: thiophenes,19,65−67 selenophenes,68,69 pyrroles,70 phenylenes,71,72 fluorenes,73 cyclopentadithiophenes,74 dithienosilole,75 and even nonconjugated bithienylmethylene.76 Recently, acceptor monomers such as pyridines,77 benzotriazoles,78 and thiazoles79,80 and diaryl monomers81 have been reported. On the other hand, Willot and Koeckelberghs reported that Grignard thienothiophene monomer 8 was not polymerized with Ni catalysts, whereas chain-growth polymerization of the zinc counterpart 9 proceeded with a Pd catalyst, albeit in an uncontrolled manner (Scheme 10).82 Monomer 8 was first reacted with the Ni(II) catalyst to form a dimer of bromothienothiophene and to generate Ni(0). However, they formed a strong π complex, as determined by 31P NMR spectroscopy, and even the intramolecular C−Br bond of the dimer did not undergo oxidative addition. The Pd catalysts tend to form weaker π complexes, and polymerization proceeded. They proposed a chain-growth polymerization mechanism including termination on the basis of the findings that the molecular weight was slightly increased with increasing feed ratio of monomer to the Pd catalyst, and that the molecular

a

Reprinted with permission from ref 55. Copyright 2015 John Wiley & Sons, Inc.

catalyst would insert selectively into the terminal C−Br bond of the polymer chain, rather than the C−Br bond of monomer 7, because the strong electron-donating chloromagnesio moiety of 7 deactivates the C−Br bond for oxidative addition (Scheme 9a). Indeed, the polymerization of 7 with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) at room temperature proceeded in a chain polymerization manner.60,61 McCullough also reported Ni-catalyzed chain-growth polymerization of bromothiophene zinc monomer62 and 763 at the same time. However, the molecular weight was not controlled by addition of active aryl halides bearing an electron-withdrawing group, 1953

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Scheme 9. CGCP of 7 with Ni Catalyst: (a) Anticipated Mechanism Based on the Substituent Effect, (b) Real Mechanism Based on Intramolecular Transfer of the Catalyst64

Table 3. Grignard Monomers for Ni-Catalyzed CGCP

formed in the initial stage of the reaction of 10 with Ni(II) catalyst would trap Ni(0). The polymerization of 11 also

Scheme 10. Polymerization of Thienothiophene Monomers 8 and 982

Scheme 11. Unfavorable Monomers for Ni-Catalyzed CGCP83,84

afforded a low-molecular-weight polymer, the matrix-assisted laser desorption/ionization (MALDI) mass spectrum of which showed a terminal group formed by reaction with THF (the reaction solvent). Koeckelberghs reported that dibromophenanthrene was not quantitatively converted into a Grignard monomer with i-PrMgCl·LiCl (Scheme 11).84 3.2.2. Boronic Acid Ester Monomers. Pd-catalyzed Suzuki−Miyaura coupling polymerization of AB-type boronic acid ester monomers also proceeds via the chain-growth polymerization mechanism. Polythiophene,85−87 polyphenylene,88 polyfluorene,86,89 poly(phenanthrene),84 and poly(fluorene-alt-benzothiadiazole)90 were synthesized in a con-

weight was independent of the amount of bromobenzene used as a transfer agent. We attempted CGCP of two kinds of phenylenevinylene Grignard monomers, 10 and 11, with a Ni catalyst for synthesis of well-defined poly(phenylenevinylene) (PPV) (Scheme 11).83 However, 10 was not polymerized, probably because the diene 1954

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Table 4. Boronic Acid Ester Monomers for Pd-Catalyzed CGCP

Scheme 12. Proposed Mechanism of Intermolecular Transfer of Pd(0) Catalyst to Phenylenevinylene Boronic Acid Ester Monomer91

Table 5. Zinc- and Tin-Containing Monomers for Ni- or Pd-Catalyzed CGCP

THF. This surprising result indicates that a small amount of water promotes intramolecular transfer of the Pd catalyst in Suzuki−Miyaura CGCP.87 Most Suzuki−Miyaura CGCP uses tBu3P(Ar)PdX as an initiator, but Geng reported that Nheterocyclic carbene (NHC)-ligated Pd complex Pd(IPr)(OAc)2 was effective for CGCP of fluorene and thiophene boronic acid ester monomers.86 In this polymerization, the Pd(II) complex is first reduced to Pd(0) by the reaction of 2 equiv of monomers and reductive elimination, which is slower than the initiation with t-Bu3P(Ar)PdX, so the molecular weight distribution is somewhat broad (Mw/Mn ≈ 1.6). We also attempted to synthesize two phenylenevinylene boronic acid ester monomers, the boronic acid ester counter-

trolled manner (Table 4). The polydispersity of these polymers is greater (Mw/Mn = around 1.3) than that of polymers obtained by Ni-catalyzed CGCP. However, different polymer end groups are easily introduced into the polymer by initiation with Ar− Pd−X complexes, and termination can be done with many kinds of boronic acids or esters (see section 3.4). We investigated CGCP of triolborate monomer 12, which can be polymerized without addition of base in dry solvent; side reactions, deboronation and dehalogenation, are caused by water and base in the reaction mixture. Contrary to our expectation, however, polythiophene with a narrow molecular weight distribution and controlled polymer end groups was obtained in THF/water, as compared to the reaction in dry 1955

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Scheme 13. Mizoroki−Heck Polymerization of 13 with tBu3P(tolyl)PdBr and Dicyclohexylmethylamine (Cy2NMe)101

parts of 10 and 11 in Scheme 11, with the t-Bu3P(Ar)PdX initiator. High-molecular-weight PPV was obtained, in contrast to the result from Ni-catalyzed Kumada−Tamao coupling polymerization, but the polymerization did not proceed via the CGCP mechanism, affording PPV with a broad molecular weight distribution.83 We then investigated why CGCP of monomers containing a carbon−carbon double bond (CC) did not proceed by examining model reactions of dibromostilbene derivatives with aryl boronic acid esters in the presence and absence of additives containing CC. We found that Pd(0) on the CC bond of the substrate readily undergoes intermolecular transfer to CC of another substrate or additive, and that this intermolecular transfer of the catalyst can be suppressed by introduction of alkoxy groups at the orthoposition(s) to CC.91 Therefore, the failure of Pd-catalyzed Suzuki−Miyaura CGCP of phenylenevinylene boronic acid ester is probably due to intermolecular transfer of the Pd(0) to CC of another monomer (Scheme 12). 3.2.3. Other Monomers. Zinc- and tin-containing monomers undergo CGCP to afford polythiophenes,62,92,93 polyfluorenes,94 poly(dithienosilole),95 poly(naphthalenediimidealt-dithiophene),96−98 poly(perylenediimide-alt-dithiophene),99 and poly(phenyleneethynylene)100 (Table 5). Ni-catalyzed polymerization of both zinc-containing thiophene monomers,62,92 dithienosilole monomer,95 and naphthalenediimide−dithiophene monomer96,97 and Pd-catalyzed polymerization of tin-containing thiophene monomer93 and phenyleneethynylene monomer 1 0 0 proceeded in a living polymerization manner. Polymerization of fluorene monomer with Pd(CH3CN)2Cl2/t-Bu3P proceeded very rapidly at room temperature even with a small amount of the catalyst (turnover numbers (TONs) were above 100000, and turnover frequencies (TOFs) were up to 280 s−1), indicating chain-growth polymerization based on intramolecular transfer of the catalyst. However, the polymerization involved chain termination and did not show living polymerization nature.94 Similar behavior was observed in polymerization of naphthalenediimide− dithiophene monomer98 and the perylenediimide−dithiophene monomer99 with the same Pd catalyst. We attempted catalyst-transfer Mizoroki−Heck polymerization of p-iodostyrene 13 with a t-Bu3P(tolyl)PdBr initiator for the synthesis of well-defined PPV,101 because t-Bu3PPd0 has a high affinity for a π-conjugated backbone, as shown in Pdcatalyzed CGCP, and enables exceptionally mild Mizoroki− Heck coupling reaction even at room temperature.102 However, PPV with a hydrogen atom at one end and an iodine atom at the other was formed until the middle stage, and the polymer end groups were converted into tolyl and hydrogen in the final stage (Scheme 13). The molecular weight did not increase until about 90% monomer conversion and then sharply increased after that. The molecular weight distribution was broad. These results indicated that the mechanism is a conventional step-growth polymerization. The occurrence of step-growth polymerization is presumably due to the low coordination ability of H−Pd−I(tBu3P), formed in the catalytic cycle of the Mizoroki−Heck coupling reaction, to π-electrons of the PPV backbone; reductive elimination of HI from this Pd species with base would take place after diffusion into the reaction mixture. Sanji and Iyoda reported transition-metal-free chain-growth polymerization of conjugated monomers containing pentafluorophenyl and trimethylsilyl groups with fluoride ion (Scheme 14).103,104 Tetrabutylammonium fluoride (TBAF) was the best fluoride ion source, and the molecular weight was

controlled by the feed ratio of monomer to TBAF. A pentacoordinated fluorosilicate is involved as a key intermediate Scheme 14. Transition-Metal-Free CGCP for the Synthesis of π-Conjugated Polymers103,104

and selectively reacted with the pentafluorophenyl group at the para-position. They proposed that the occurrence of chaingrowth polymerization is attributed to intramolecular transfer of fluoride anion due to anion−π interaction or to a change of the substituent effect. Block copolymers were also synthesized by utilizing the living polymerization nature. 3.3. Mechanism of Catalyst-Transfer Condensation Polymerization

Kiriy105,106 and Koeckelberghs107 independently revealed that Kumada−Tamao catalyst-transfer polymerization of Grignard thiophene monomer 7 with Ni(dppp)Cl2 involves not only unidirectional growth but also bidirectional growth, and the tailto-tail thiophene diad, which is formed by the reaction of 2 equiv of 7 with Ni(dppp)Cl2 in the initiation step, is present both at the terminal and inside the backbone of polythiophene (Scheme 15). Koeckelberghs pointed out that successive block copolymerization of A and B with Ni(dppp)Cl2 in one pot yields not only AB diblock but also BAB triblock copolymers. For the exclusive synthesis of AB diblock copolymers, external Ar−Ni−Br initiators should be used108 (see section 3.4). Koeckelberghs reported Pd-catalyzed CGCP of zinc-containing thiophene monomer through the substituent effect, not via the catalyst-transfer mechanism.109 The polymerization with a Pd initiator having a Ruphos phosphine ligand involves dissociation of Pd0[Ruphos] from the propagating end into the solution. However, the electron-donating chlorozincio moiety deactivates the C−Br bond of the monomer for 1956

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Scheme 15. Mechanism of Kumada−Tamao Catalyst-Transfer Condensation Polymerization of 7 with Ni(dppp)Cl2107

oxidative addition from Pd0[Ruphos], which instead undergoes oxidative reinsertion into the C−Br bond at the end of a polymer chain (Scheme 16). This mechanism was proposed on the basis of the fact that the molecular weight was decreased in proportion to the amount of 2-bromo-3-hexylthiophene, which

primary Ni(II) complex, the ligand of which is replaced with dppp. Kiriy has developed a convenient method for the synthesis of the Ni initiator118 by reacting o-tolylmagnesium chloride or (3-hexylthiophene-2-yl)magnesium chloride with Ni(dppe)Cl2 or Ni(dppp)Cl2 at 0 °C. Monotransmetalation is attributed to steric hindrance of the ortho-substituents to reaction of the second Grignard reagent with the monotransmetalated Ni complex (Scheme 17). In the case of Suzuki−Miyaura CGCP, t-Bu3P(Ph)PdBr is an effective Pd initiator, and the phenyl group was introduced on a

Scheme 16. Pd-Catalyzed CGCP through the Substituent Effect109

Scheme 17. Synthesis of Ni Initiators for Kumada−Tamao CGCP114,116,118

has a nondeactivated C−Br bond. If the catalyst remains complexed with the propagating polymer chain, the amount of 2-bromo-3-hexylthiophene should have no effect. 3.4. Functionalization of Polymer Ends

In Kumada−Tamao catalyst-transfer condensation polymerization with Ni(dppp)Cl2 or Ni(dppe)Cl2 (dppe = 1,2bis(diphenylphosphino)ethane), functional groups can be introduced on one or both ends by using Grignard reagents. Allyl, ethynyl, and vinyl Grignard reagents afford monofunctional products, whereas aryl and alkyl Grignard reagents yield difunctional products.110−112 An ArNi(dppp)X (X = Br or Cl) complex initiates Kumada− Tamao CGCP from the Ar group, and the polymer chain propagates unidirectionally. This protocol enables us to introduce different functional group on both ends by using this Ni initiator and quenching Grignard reagent.113 For the synthesis of ArNi(dppp)X, ArX is first reacted with Ni(PPh3)4114,115 or Et2Ni(2,2′-bipyridine)116,117 to generate the

chain end of polyfluorene,89,119 poly(p-phenylene),88 and polythiophene.85 A synthetic method for this Pd complex was established by Hartwig120 before the finding of Suzuki−Miyaura CGCP. Accordingly, many kinds of Pd initiators ligated by tBu3P have been synthesized and used for Suzuki−Miyaura CGCP.121−124 Recently, a dimeric Pd(II) complex with P(oTol)3 (14) was reported to initiate Suzuki−Miyaura CGCP to 1957

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Table 6. Pd Initiators for Suzuki−Miyaura CGCP

Table 7. All Conjugated Block Copolymers Prepared by Successive Metal-Catalyzed CGCP

3.5. Architecture

yield well-defined polyfluorene with a triarylamino group at one end125 (Table 6). Hu used Pd2(dba)3 (dba = dibenzalacetone)/t-Bu3P with ArX (X = I or Br) for in situ generation of ArPd(t-Bu3P)X, which is an efficient initiator for Suzuki−Miyaura CGCP.126,127 ArX with various substituents (Cl, Br, F, NO2, CN, COPh, CO2Et, OMe, HOCH2) at the para-position can be used for in situ generation of Pd initiators, and well-defined polyfluorenes with corresponding functional groups at one end were obtained. The in situ-generated ArPd(t-Bu3P)X complexes generally afforded a polymer with a narrower molecular weight distribution, compared to the polymer obtained with the isolated Pd catalyst. An additional amount of t-Bu3P in the in situ-generated Pd initiator system (2 equiv of t-Bu3P to Pd(0) from Pd2(dba)3 was used) helps to stabilize Pd(0) species via coordination to form more stable Pd(t-Bu3P)n (n ≥ 2) complexes.

Many architectures, including block copolymers, graft copolymers, and star polymers, have been synthesized since the discovery of CGCP for π-conjugated polymers.113,128−133 Herein, we will focus on recent developments, and this section is subdivided into architectures from all conjugated polymers and architectures from conjugated polymers and conventional polymers. 3.5.1. All Conjugated Polymers. Block copolymers reported recently are summarized in Table 7. Sugiyasu and Takeuchi synthesized block copolythiophene 15 of poly(3hexylthiophene) (P3HT) and polythiophene bearing a bulky terphenyl side chain, which works as a sheath for the polythiophene backbone, by means of Ni-catalyzed Kumada− Tamao CGCP. The block copolymer underwent a microphase separation, comprising an ensemble of stacked and isolated polythiophenes.65 1958

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Table 8. All Conjugated Random Copolymers

Poly(3-hexylselenophene) block,134,135 gradient,135 and random copolymers135,136 with P3HT were synthesized by Kumada−Tamao CGCP. Seferos similarly prepared poly(3hexyselenophene)-b-poly(3-(2-ethylhexyl)thiophene) and poly(3-(2-ethylhexyl)selenophene)-b-poly(3-(2-ethylhexyl)thiophene), and investigated the thin film morphology of a series of selenophene−thiophene diblock copolymers 16.69 Block copolymers with identical side chains underwent phase separation, appearing ordered on the 1−2 nm scale, but relatively disordered on the 1−2 μm scale. However, in block copolymers with different side chains on each block, the 1−2 nm scale order was maintained, but a more ordered lamellar structure was observed on the 1−2 μm scale. A block copolymer of poly(2,5-bis(hexyloxy)-1,4-phenylene) (PPP) and P3HT was synthesized by Kumada−Tamao CGCP,137,138 and its thin film morphology was investigated.139 Both PPP and P3HT show crystallinity, and crystallization and microphase separation co-occurred during the thin film formation process due to the incompatibility and crystallization of the two segments. Dai prepared block copolymer 17 bearing a 2-ethylhexyl side chain in the polythiophene block, which decreases the crystallinity, and found that 17 was able to cocrystallize into a single uniform domain comprising PPP and poly(3-(2-ethylhexyl)thiophene) main chains with mutually interdigitated side chains spaced between. 140 Dai also synthesized PPP-b-P3HT/cadmium sulfide hydroxide (CdS− OH) nanohybrids, which can be used as bicontinuous nanochannels for efficient charge separation and charge transport.141 Koeckelberghs found that Kumada−Tamao coupling polymerization of cyclopentadithiophene (CPDT) monomer proceeded in a chain-growth polymerization manner with Ni(dppp)Cl2, and synthesized block copolymer 18 with P3HT.74 In the block copolymerization, thiophene monomer was polymerized first, and then more donor CPDT monomer was added to the reaction mixture. The reversed order block copolymerization did not afford 18, similarly to the case of block copolymerization of PPP and P3HT.137 Block copolymer 19 was independently synthesized by the Dubois and Kiriy groups. Dubois polymerized dithienosilole Grignard monomer and then thiophene Grignard monomer in the presence of Ni(dppp)Cl2,75 whereas Kiriy adopted the

reversed order copolymerization with Ni(dppe)Cl2 and used chlorozincio dithienosilole monomer instead of the corresponding Grignard monomer.95 The latter method afforded block copolymer 19 with a narrower molecular weight distribution. Although Kumada−Tamao CGCP of thienopyrazine monomer was less controllable, Koeckelberghs conducted successive polymerization of thiophene monomer and then thienopyrazine monomer with Ni(dppp)Cl2 to obtain block copolymer 20.142 Koeckelberghs and his colleagues found that polymerization of zinc-containing monomers with Pd0[Ruphos] involved the CGCP mechanism (based on substituent effects), not the catalyst-transfer mechanism, as noted in section 3.3, which means that the polymerization order would not affect the controlled synthesis of block copolymers; therefore, they synthesized six kinds of polythiophene−polyfluorene−polyselenophene triblock copolymers 21 in all possible orders from an initiator by using three zinc-containing monomers.143 Furthermore, they introduced hydrogen donor and acceptor functionalities as the end groups of different poly(alkylthiophene)s, and obtained the supramolecular hydrogenbonded diblock copolymer 22.144 Seferos found that nickel(II) diimine catalyst was effective for CGCP of electron-deficient benzotriazole Grignard monomer78 and tried to synthesize a block copolymer with electron-rich P3HT.145 Block copolymer 23 was successfully obtained by chain extension from either electron-deficient poly(benzotriazole) or electron-rich P3HT, in contrast to the case of Ni(dppp)Cl2-catalyzed Kumada−Tamao block copolymerization137 as mentioned in connection with the synthesis of block copolymer 18.74 Metal-catalyzed CGCP enables us to synthesize not only block copolymers but also random copolymers (Table 8). Koeckelberghs conducted copolymerization of 3-alkyl-2-bromo5-(chlorozincio)thiophene (normal monomer) and 3-alkyl-5bromo-2-(chlorozincio)thiophene (reversed monomer) in the presence of Pd[Ruphos] catalyst with varying feed ratio of the two monomers, obtaining a series of chiral poly(alkylthiophene)s 24 with different regioregularities.146 It is interesting that the highest crystallinity under kinetic conditions was not obtained for fully regioregular poly(alkylthiophene); instead, a small amount of regio-irregularity increased the crystallinity. 1959

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Dai synthesized random copolymers 25 of P3HT and poly((hexyloxy)thiophene) by means of Ni(dppp)Cl2-catalyzed CGCP of Grignard monomers.147 The band gap energy of 25 was decreased with increasing composition ratio of poly((hexyloxy)thiophene), because the highest occupied molecular orbital (HOMO) value of poly((hexyloxy)thiophene) (−4.56 eV) is higher than that of P3HT (−5.27 eV), in spite of the small difference of the lowest unoccupied molecular orbital (LUMO) level between the two polymers. On the other hand, random copolymer 26 of P3HT and poly((hexylthio)thiophene), similarly synthesized by Seferos, showed a decreased HOMO value; the copolymers showed an 11−18% increase in the open-circuit voltage (VOC) relative to that of a P3HT/PC71BM device.148 This increase was independent of the copolymer composition over the 50:50 to 85:15 range. Koeckelberghs approached band gap control of poly(alkylthiophene) by incorporation of phenylene units.149 Since chain-growth random copolymerization of thiophene Grignard monomer and phenylene Grignard monomer with a Ni catalyst is not possible because of difficult transfer of the catalyst from the more donating thiophene monomer to the less donating phenylene monomer,137 thiophene Grignard monomer was copolymerized with a Grignard thiophene−bromophenylene biaryl monomer to obtain random copolymer 27. The band gap increased linearly with increasing phenylene content. Seferos synthesized donor−acceptor random copolymers 28 consisting of poly(dithienosilole) and poly(benzotriazole) by means of Kumada−Tamao CGCP with a nickel diimine catalyst.150 Furthermore, benzotriazole monomer was partially polymerized, followed by addition of dithienosilole monomer to afford poly(benzotriazole)-b-[poly(dithienosilole)-ran-poly(benzotriazole)]. Before this work, Kiriy had conducted copolymerization of the anion radical of naphthalenediimide− dithiophene as an acceptor monomer and Zn-containing fluorene as a donor monomer with a t-Bu3P-ligated Pd catalyst. The obtained copolymer 29 was not a random copolymer but a sharp gradient or blocklike copolymer.151 In this copolymerization, fluorene monomer was polymerized much faster, and the resulting polyfluorene was able to initiate polymerization of the second monomer, presumably acting as a macroinitiator. As for star polymers, triarm P3HT152 and tetraarm polyfluorene153 were synthesized by Kumada−Tamao CGCP with an external trifunctional Ni initiator and by Suzuki− Miyaura CGCP with a tetrafunctional Pd initiator generated in situ, respectively (Scheme 18). Higashihara applied formation of the Ni initiator to synthesize P3HT-grafted donor−acceptor alternating conjugated polymers (Scheme 19).154 3.5.2. Conjugated Polymers and Conventional Polymers. Many kinds of block copolymers of polythiophene and conventional polymers have been synthesized by using endfunctionalized polythiophenes as a macroinitiator or a macroterminator.113,131 Chao recently synthesized a series of P3HT-bpolyisoprene (PI) diblock copolymers and PI−P3HT−PI triblock copolymers by means of the reaction of the aldehyde end group of P3HT with living anionic PI (Scheme 20).155 Diblock copolymers with a PI fraction of less than 40 wt % exhibited parallel straight fibers longer than several micrometers accompanied by concurrent enhanced crystallinity of P3HT, whereas triblock copolymers showed highly curved interdomain boundaries with significantly depressed crystallinity of P3HT. Koeckelberghs introduced a phenol group on the end of polythiophene by use of a hydroxy-protected phenol nickel

Scheme 18. Multifunctional Initiators for Star Conjugated Polymers152,153

Scheme 19. P3HT-Grafted Donor−Acceptor Conjugated Polymers154

initiator for Kumada−Tamao CGCP, and formed a supramolecular graft copolymer of poly(4-vinylpyridine) and polythiophene through hydrogen bonding (Scheme 21).156 Incidentally, polythiophenes containing phenol, thiol, pyridine, or phosphonic end groups were used to prepare hybrid 1960

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Scheme 20. Diblock and Triblock Copolymers of P3HT and Polyisoprene155

Scheme 21. Supramolecular Graft Copolymer156

4. MISCELLANEOUS CHAIN-GROWTH POLYMERIZATION We attempted to control condensation polymerization of AA and BB monomers by use of a solid-phase-support-bound Scheme 23. Attempt To Control the Polymer Ends of the Condensation Polymer from AA and BB Monomers by Use of a Solid-Phase-Support-Bound Reagent160

materials with inorganic nanoparticles such as Pt, Fe2O3, CdSe, ZnS, and so on.157,158 Synthesis of these block copolymers and graft copolymers requires end functionalization of polythiophenes. However, Wu reported one-pot block copolymerization of thiophene Grignard monomer and conventional vinyl monomers such as styrene, tert-butyl acrylate, and alkoxyallene with a dppp-ligated π-allylnickel complex.159 Vinyl monomer was first polymerized with the Ni complex, and the growing chain end of these vinyl polymers was able to initiate the polymerization of thiophene Grignard monomer to afford block copolymers (Scheme 22). Furthermore, the π-allylnickel complex can be used as an Scheme 22. One-Pot Synthesis of the Block Copolymer of Polystyrene and P3HT159

reagent, although this polymerization has not yet been transformed to chain-growth polymerization. We thought that selective synthesis of linear polymers by controlling the polymer end groups would be a way to achieve CGCP of AA and BB monomers, because cyclization takes place during all stages of polymerization. Accordingly, condensation polymerization of diacid chloride and diol was carried out in the presence of oxime

external initiator for the thiophene Grignard monomer, and successive chain extension of alkoxyallene and thiophene Grignard monomer was conducted to yield P3HT-b-poly(alkoxyallene)-b-P3HT triblock copolymer. 1961

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Scheme 24. Chain-Growth Supramolecular Polymerization of Porphyrin 30a

a

Reprinted with permission from ref 161. Copyright 2014 Nature Publishing Group.

mode and was eventually transformed to the H-aggregate (faceto-face stacking) mode over several days. However, when an aliquot of H-aggregate 30 solution was added to a freshly prepared sample of J-aggregate 30, the J-aggregate nanoparticles were converted to a fibrous H-aggregate within a few hours (Scheme 24). The length of the obtained H-aggregate was proportional to the ratio of the total concentration of the added J-aggregate to the initial concentration of the H-aggregate, and the polydispersity was 1.1 for all stages. Aida reported chain-growth supramolecular polymerization of monomer M with initiator I, the process of which is the same as that of typical living polymerization through formation of covalent bonds.162 Compound M adopts a cagelike closed conformation because of intramolecular hydrogen bonding of the side chains. However, compound I, an N-methylated derivative of M, converted the intramolecular hydrogen bonding of M to intermolecular bonding with I to afford 1:1 complex I−M. Since I−M has a free CO bond, like I, another M was complexed with this terminal. Growth continued with conversion of intramolecular to intermolecular hydrogen bonding of M (Scheme 25). The degree of polymerization was increased in proportion to [M]0/[I]0, and the polydispersity was in the range of 1.2−1.3. Furthermore, this chain-growth polymerization proceeded stereoselectively and enabled optical resolution of a racemic monomer.

resin or oxime silica gel, followed by cleavage of the formed polyester from the solid-phase support with aniline.160 We expected that the reaction between polymers attached to the monofunctional reagent on the solid-phase support would be suppressed because of the bulkiness of the support. The molecular weight would be controlled by the feed ratio of the monomer to this solid-phase-support-bound reagent, and the molecular weight distribution could be made narrower by controlling monomer addition to the reaction mixture containing the support reagent. However, the polyester obtained after cleavage from the solid-phase support with aniline contained not only polyester with anilide at one end, but also polyester with anilides at both ends (Scheme 23). The latter polyester was presumably formed by the reaction of the oxime moieties in the solid-phase support with the acid chloride moiety of polyester in an intramolecular manner. A supramolecular polymer is generally formed in a stepgrowth polymerization manner, and there is an equilibrium between polymerization and depolymerization, depending on the monomer concentration. Therefore, controlled synthesis of a supramolecular polymer is a challenge in supramolecular chemistry. However, transformation of step-growth polymerization to living chain-growth polymerization has recently been achieved to obtain a supramolecular polymer with controlled molecular weight and low polydispersity. The first example was reported by Takeuchi and Sugiyasu.161 The porphyrin molecule 30 overlapped in the J-aggregate (slipped face-to-face stacking) 1962

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Scheme 25. Chain-Growth Supramolecular Polymerization of Corannulene Monomer M with Initiator Ia

a

Reprinted with permission from ref 162. Copyright 2015 American Association for the Advancement of Science.

Biographies

5. CONCLUSION We have reviewed recent developments in the area of CGCP utilizing the substituent effect and CGCP via catalyst transfer, as well as chain-growth supramolecular polymerization. Transformation of step-growth polymerization into living chaingrowth polymerization has great potential for production of new materials, such as block and graft copolymers of condensation polymers and conventional polymers. We have focused on polycondensation and supramolecular polymerization in this review; other step-growth polymerizations, such as polyaddition and addition−elimination polymerization, have not yet been investigated for transformation into chain-growth polymerization, except for some work on cyclic addition polymerization of triacetylene monomers.21,163 The methods used to transform the mechanism in each polymerization were found almost by chance, as described in this review. However, further studies of this mechanistic transformation may lead to a breakthrough in the development of more systematic strategies.

Tsutomu Yokozawa was born in Chiba in 1957. He received his B.S. (1981), M.S. (1983), and Ph.D. (1987) in organic chemistry from the Tokyo Institute of Technology under the direction of Professor Nobuo Ishikawa and Professor Takeshi Nakai. In 1985, he had already started an academic career in the Research Laboratory of Resources Utilization, Tokyo Institute of Technology, as a Research Associate, and was promoted to Assistant Professor in 1988. He joined the Department of Applied Chemistry, Kanagawa University, as a Lecturer in 1991, and was promoted to Associate Professor in 1993. From 1997 to 1998, he worked as a visiting scientist at the University of Illinois at Urbana-Champaign with Professor J. S. Moore. He was promoted to Full Professor in 1999. He was also a researcher for Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), from 2001 to 2005 and a guest

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +81-45-4815661, ext. 3846. Notes

The authors declare no competing financial interest. 1963

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professor at Wuppertal University in 2010. He received the Award of the Society of Polymer Science, Japan, in 2007. He has been Asian Editor (chemistry and synthesis) of the journal Polymer since 2014. His research interest covers controlled synthesis of polymers and supramolecular chemistry of polymers, as well as synthetic organic chemistry.

Yoshihiro Ohta was born in Kanagawa, Japan, in 1983. He received his B.S. (2006), M.S. (2008), and Ph.D. (2011) in applied chemistry from Kanagawa University under the direction of Professor Tsutomu Yokozawa. After working as a Research Assistant Professor with Professor Atsushi Takahara at Kyushu University (2011), he joined the faculty of Kanagawa University, where he is presently an Assistant Professor. His research interests include synthesis and characterization of hyperbranched polymers, functional block copolymers, and graft copolymers with well-defined structures.

ACKNOWLEDGMENTS Our study was supported by a Grant-in-Aid (No. 24550141) for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013− 2018 (MEXT is the Ministry of Education, Culture, Sports, Science and Technology). We also thank Dr. Eisuke Baba for creating the Table of Contents and Title Page graphics. REFERENCES (1) Carothers, W. H. Polymers and Polyfunctionality. Trans. Faraday Soc. 1936, 32, 39−53. (2) Flory, P. J. Molecular-Size Distribution in Linear Condensation Polymers. J. Am. Chem. Soc. 1936, 58, 1877−1885. (3) Flory, P. J. Fundamental Principles of Condensation Polymerization. Chem. Rev. 1946, 39, 137−197. (4) Lenz, R. W.; Handlovits, C. E.; Smith, H. A. Phenylene Sulfide Polymers. III. The Synthesis of Linear Poly(Phenylene Sulfide). J. Polym. Sci. 1962, 58, 351−367. (5) Newton, A. B.; Rose, J. B. Relative Reactivities of the Functional Groups Involved in Synthesis of Poly(Phenylene Ether Sulfones) from Halogenated Derivatives of Diphenyl Sulfone. Polymer 1972, 13, 465− 474. (6) Lovering, J. R.; Ridd, J. H.; Parker, D. G.; Rose, J. B. Polymerization and Related Reactions Involving Nucleophilic Aromatic Substitution. Part 2. The Rates of Reaction of Substituted 4-Halobenzophenones with the Salts of Substituted Hydroquinones. J. Chem. Soc., Perkin Trans. 2 1988, 1735−1738. (7) Hibbert, D. B.; Sandall, J. P. B.; Lovering, J. R.; Ridd, J. H.; Yousaf, T. I. Polymerization and Related Reactions Involving Nucleophilic Aromatic Substitution. Part 3. Mathematical Models of the Polycondensation Reactions of Halobenzophenones. J. Chem. Soc., Perkin Trans. 2 1988, 1739−1742. 1964

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DOI: 10.1021/acs.chemrev.5b00393 Chem. Rev. 2016, 116, 1950−1968