SCIENCE CHINA Application of Named Reactions in Polymer Synthesis

SCIENCE CHINA Chemistry

• REVIEWS • November 2015 Vol.58 No.11: 1695–1709 · SPECIAL TOPIC · Progress in Synthetic Polymer Chemistry doi: 10.1007/s11426-015-5447-1

Application of named reactions in polymer synthesis

Xue Jiang1, Chun Feng1*, Guolin Lu1,2 & Xiaoyu Huang1*

1Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Chinese Academy of Sciences; Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China 2State Key Laboratory of Molecular Engineering of Polymers; Department of Macromolecular Science, Fudan University, Shanghai 200433, China

Received April 17, 2015; accepted April 22, 2015; published online June 30, 2015

In recent years, with the rapid development of polymer science, the application of classical named reactions has transferred from small-molecule compounds to polymers. The versatility of named reactions in terms of monomer selection, solvent environment, reaction temperature, and post-modification permits the synthesis of sophisticated macromolecular structures under conditions where other reaction processes will not operate. In this review, we divided the named reactions employed in polymer-chain synthesis into three types: transition metal-catalyzed cross-coupling reactions, metal-free cross-coupling reactions, and multi-components reactions. Thus, we focused our discussion on the progress in the utilization of these named reactions in polymer synthesis.

named reaction, polymer synthesis, cross-coupling reaction, multicomponent reaction

1 Introduction actions has transferred from the synthesis of small molecule compounds to polymers. Over the past few years, polymeric materials have been Organic chemistry is an amazing subject because of its im- very important to our lives; however, compared with bio- pressive power to construct complex and diverse molecular logical processes such as DNA replication or ribosomal architectures. Over its approximately 200-year history, protein synthesis, synthetic polymerization methods remain chemists have devoted themselves to the discovery and in- very primitive [2]. Polymer chemists have had a long-term vention of new chemical reactions, thereby promoting the interest in developing new synthetic methods to control the development of synthetic methodology. As indispensable shape of synthetic macromolecules. A number of living tools for chemical synthesis, many reactions were defined polymerization methods have been identified since the in- by the names of the inventors in commemoration of these troduction of anionic polymerization by Szwarc in 1956 [3]. discoveries [1]. The so-called “named reactions”, especially Group transfer polymerization (GTP) [4], living anionic and the carbon–carbon bond-formation reactions, have played cationic ring opening polymerization [5], living carbo- an enormously significant role in shaping chemical synthe- cationic polymerization [6], ring-opening metathesis sis because they supply key steps in the construction of so- polymerization (ROMP) [7], and controlled/“living” radical phisticated building blocks. Although new synthetic meth- polymerization (CRP) [8] techniques are currently known ods are developed every day, many of these classical reac- for the precise synthesis of macromolecular architectures tions are being constantly re-examined in order to explore with defined molecular weights and end-groups. Although new applications. As a result, the application of named re- named reactions play an important role in the syntheses of monomers [9,10] and many named reactions such as the *Corresponding authors (email: [email protected]; [email protected]) Glaser coupling reaction [11,12] and Williamson reaction

[13] have been applied for post-polymerization modifica- groups of Kumada [26] and Corriu [27] in 1972. Since the tions, only a few of them have been introduced to construct discovery of the chain-growth mechanism in 2004 [22–25], the main chain. Kumada polymerization has been considered to be a well- In this review, we divided those named reactions em- suited tool for the preparation of well-defined polymers ployed in polymer-chain synthesis into three types: transi- with low polydispersities, controllable molecular weights, tion metal-catalyzed cross-coupling reactions, metal-free and chain-end functionality. A plethora of electron-rich cross-coupling reactions, and multi-component reactions. monomers based on thiophene, fluorene, selenophene, pyr- We focus much of our discussion on recent synthetic de- role, benzene, and carbazole have been successfully pol- velopments in these reactions, with an emphasis on their ymerized using appropriate nickel catalysts, and the proper- utilization for polymers. ties of the obtained materials matched those expected for a well-controlled polymerization process [28]. To further expand the scope of controlled polycondensation 2 Transition metal (TM)-catalyzed cross- towards other electron-deficient building blocks, Pammer et coupling reactions al. [29] recently presented the synthesis of head-to-tail regioregular poly(4-alkylthiazole) compounds via Kumada- Research in -conjugated polymers (CPs) has rapidly de- coupling polycondensation of 2,5-dihalogenated-4-alkyl- veloped, driven by many potential applications in organic thiazoles (Scheme 1). Subsequent analysis showed that the electronics such as nonlinear optics, organic photovoltaic number average molecular weights (Mn) of poly(4-nonyl- cells, light-emitting diodes, and field effect transistors thiazole) (C9-PTz) were 1.9–2.4 kDa with a Polydispersity [14–18]. The established transition metal (TM)-catalyzed Index (PDI) of about 1.1–1.3 and the Mn of poly(4-tridecyl- cross-coupling reactions including Kumada, Suzuki, Stille, thiazole) (C13-PTz) were 2.9–3.0 kDa (PDI1.1–1.2). In both Negishi and Heck reaction, are powerful and indispensable cases, the low polydispersity showed the superiority of this tools to obtain the desired extended -conjugated systems kind of polymerization. In addition, C9-PTz and its regioi- such as polythiophenes, polyfluorenes, polyphenylenes, somer PBTz, which are similar to P3HT, exhibited electro- poly(arylethynylene)s and so on [19–21]. chemical and optical band gaps, while their frontier orbits are CPs are commonly prepared via Pd-catalyzed cross- stabilized by 0.3–0.5 eV relative to those of polythiophene. coupling reactions, which are usually followed by conventional step-growth polycondensation mechanisms. However, 2.2 Suzuki-Miyaura polymerization the synthesis of such materials via conventional step-growth polycondensation face the challenging issues of a low de- Since 1979, when Suzuki and Miyaura [30] first reported a gree of control over the molecular weight, polydispersity, stereoselective synthesis of arylated (E)-alkenes by the re- end-group functionality, and regioregularity. To improve action of 1-alkenylboranes with aryl halides in the presence this, new synthetic methods are required to access a variety of a palladium catalyst, Suzuki-Miyaura coupling between of well-defined materials. A major breakthrough occurred organoboron compounds and organic halides has found when McCullough [22,23] and Yokozawa [24,25] simulta- wide applications in the synthesis of numerous organic neously reported nickel-catalyzed Kumada polymerization compounds. This useful reaction was not introduced to via a chain-growth mechanism for synthesizing poly(3- polymer synthesis until Schlüter and co-workers [31] syn- hexylthiophene). In chain-growth polymerization, an initia- thesized a poly(p-phenylene) bearing ﬂexible side chains in tor reacts with an AB-type monomer to begin the polymeri- 1989. This new polymerization method based on Suzuki- zation, and subsequent monomer additions occur at the Miyaura cross-coupling, which is also referred as Suzuki chain end. If the termination pathways are minimal or ab- polycondensation (SPC), has found widespread applications sent, this method produces polymers with precise molecular for the synthesis of polymers with directly coupled aromatic weights and narrow molecular weight distributions. Moreo- units [32]. In comparison with the AB approach, the AA/BB ver, the copolymer microstructure can also be controlled by approach of SPC is more popular, most likely because the the relative reactivity of the monomers and their order of monomers are easier to synthesize. For example, a high addition. The initial discovery of Kumada polymerization molecular weight copolymer comprised of alternating sparked a flurry of activity in this field, and other Pd/Ni- catalyzed polymerization reactions with chain-growth mechanism were also reported.

2.1 Kumada polymerization

Cross-coupling of organomagnesium (Grignard) reagents with aryl or alkenyl halides in the presence of a nickel- Scheme 1 Synthesis of C9-PTz and C13-PTz via Kumada polymerization phosphine catalyst was independently reported by the [29]. Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1697 benzothiadiazole (BTZ) and cyclopentadithiophene (CDT) 2.3 Stille polymerization units was synthesized by Müllen et al. (Mn=50 kDa, Scheme 2) [33]. This -conjugated polymer can self-assemble into Stille coupling between stannanes and aryl halides is anoth- lamellar stacks with hole mobilities as high as 1.4 cm2 V−1 s−1. er palladium-catalyzed reaction. Early work concerning the reaction between organostannanes and electrophilic partners Further optimization produced BTZ-CDT copolymers with even higher crystallinity, and the hole mobilities reached an was published by Eaborn et al. [38] and Kosugi et al. [39] in 2 −1 −1 1976 and 1977, respectively. A year later, in 1978, Stille et exceptionally high value of up to 3.3 cm V s [34]. Although SPC became an extremely powerful tool for al. [40] improved the reaction conditions, and the reaction was soon known as the Stille coupling reaction. Along with the synthesis of an array of conjugated polymers, controlled Suzuki-Miyaura coupling, the Stille reaction also offers the Pd(0)-catalyzed cross-coupling polymerization for the syn- opportunity to design polymers with functional moieties thesis of conjugated polymers with controlled lengths and manifesting many highly desirable properties [41]. For ex- low polydispersities remained elusive. In 2005, Hu et al. [35] ample, Jin et al. [42] synthesized two donor-acceptor (D-A) established that Pd(0)/t-Bu P is a powerful catalyst system 3 medium band-gap polymers (Scheme 4) via microwave- for efficient preferential oxidative addition, which was es- assisted Stille polymerization, which were utilized for bulk sential to expand SPC from step-growth into chain-growth. heterojunction polymer solar cells. The weight-average mo- Then, Yokozawa [36] reported some initial studies on the lecular weights (M ) of the obtained P1 and P2 polymers mechanism of this polymerization system, which showed w were 36 kDa with a PDI of 1.2 and 32 kDa with a PDI of that AB-type monomers were polymerized with PhPd- 1.3, respectively. (t-Bu3P)Br as the initiator. Recently, Hu et al. [37] de- In the case of Stille polymerization, it is understood that scribed the controlled Pd(0)/t-Bu3P-catalyzed Suzuki cross- the reaction mainly occurs via step-growth polycondensa- coupling polymerizations of AB-type monomers via the tion of AA/BB approach [41]. To obtain controlled poly- chain-growth mechanism with ArPd(t-Bu3P)X (X=I, Br, Cl) mers, the chain growth mechanism also needs to be intro- or Pd2(dba)3/t-Bu3P/ArX as the initiator (Scheme 3). By duced to this polymerization. Recently, a commercially comparison, combinations of p-BrC6H4I, p-HOCH2C6H4Br available palladium N-heterocyclic carbene (NHC) complex and p-PhCOC6H4Br with Pd2(dba)3/t-Bu3P were identified (i.e., Pd-PEPPSI-IPr) was used to induce Stille catalyst as highly robust initiation systems, producing polymers in a transfer polycondensation of an AB-type monomer, i.e., fair yield with low PDIs of 1.13–1.20. In addition, an addi- SnHTBr (Scheme 5) [43]. The resultant high molecular tional amount of t-Bu3P in the initiation system helped low- weight poly(3-hexylthiophene) (P3HT) was regioregular, er the PDIs, likely by stabilizing the Pd(0) species in the and the polymer length could be modulated (Mn=7–73 kDa, initiation system via coordination to form more stable PDI=1.10–1.53) by varying the catalyst concentration. Pd(t-Bu3P)n (n2) complexes. As a result, chain-growth polycondensation of AB-type monomers has become a pop- 2.4 Negishi polymerization ular alternative for the preparation of well-defined -conjugated polymers. Inspired by the discovery of the Kumada cross-coupling

Scheme 2 Benzothiadiazole-cyclopentadithiophene (BTZ-CDT) copolymers with exceptionally high hole mobilities prepared by Suzuki polycondensation [33].

Scheme 3 Pd(0)/t-Bu3P-catalyzed Suzuki cross-coupling polymerizations with ArPd(t-Bu3P)X or Pd2(dba)3/t-Bu3P/ArX as initiator [37]. 1698 Jiang, et al. Sci China Chem November (2015) Vol.58 No.11

Scheme 4 Microwave-assisted Stille polymerization [42].

Scheme 5 Synthesis of P3HT using a Pd-NHC catalyst [43]. reaction, Pd- or Ni-catalyzed cross-coupling reactions be- 2.5 Mizoroki-Heck polymerization tween organozincs and aryl, alkenyl, or alkynyl halides, was Mizoroki-Heck cross-coupling is one of the most powerful reported by Negishi and co-workers [44]. With the devel- tools for the creation of alkenyl-aryl bonds, and was dis- opment of new efficient Pd catalysts for cross-coupling re- covered independently by Mizoroki and Heck in the early actions of small molecules, Negishi polymerization, which 1970s [46,47]. It is recognized as a powerful and widely uses non-toxic and strongly nucleophilic zinc-organic-based used tool for the syntheses of elaborated molecules that monomers, is a promising technique for industrial-scale present interesting physical, biological, or pharmaceutical production. Kiriy et al. [45] recently reported that a properties [48–51]. Mizoroki-Heck based polymerizations, Pd/t-Bu3P-catalyzed Negishi chain-growth polycondensa- however, have generally found fewer application than the tion of AB-type monomers could be used to increase the aforementioned methods, which could be because of the molecular weight of polyfluorene up to 120 kDa, with un- difficulty in preparing the precursors. Recently, Itsuno et al. precedented high catalyst turnover numbers (TON>200000) [52] reported the synthesis of novel polymeric chiral cata- −1 and turnover frequencies (TOF>280 s ), in contrast, the lysts using Mizoroki-Heck polymerization (Scheme 7). related step-growth polycondensation of functionally related Since the reaction is relatively tolerant of many functional AA/BB-type monomers proceeded with TON and TOF groups, polymers containing cinchonidinium moieties in the values that were two orders of magnitude lower (Scheme 6). main chain were successfully afforded via AA/BB approach Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1699

with base occurs after diffusion into the reaction mixture.

2.6 Sonogashira-Hagihara polymerization

In 1975, Sonogashira and Hagihara [55] reported that sym- metrically substituted alkynes could be prepared under mild conditions in the presence of a catalytic amount of Pd(PPh3)Cl2 and copper(I) iodide. Recently, the so-called Sonogashira-Hagihara cross-coupling reaction has received much attention as it is the most common approach for preparing poly(arylethynylene)s. Sanda et al. [56] synthesized novel optically active poly(phenyleneethynylene)s bearing azobenzene moieties in the main chains with m,m′- and p,p′-linkages by Sonogashira-Hagihara coupling polymerization of the corresponding monomers (Scheme 9). The two Scheme 6 Negishi chain-growth polycondensation of AB-type monomers and Negishi step-growth polycondensation of AA/BB-type monomers [45]. corresponding polymers (i.e., poly(1-2m) and poly(1-2p)), which had Mn values of 10700 and 9400, were obtained in 70% and 86% yields, respectively. Through trans-cis isom- using the cinchona alkaloid- derived dimer and diiodide. erization of the azobenzene moieties, poly(1-2m) could Furthermore, the obtained chiral polymers showed excellent achieve a reversible conformational change between folded catalytic activities towards asymmetric benzylation reac- and unfolded structures upon UV- and visible-light irradia- tions to yield the corresponding phenylalanine derivatives tion. In contrast, poly(1-2p) showed very little conforma- with high yields and levels of enantioselectivity. tional transformation or azobenzene isomerization because Fu and co-workers [53] reported an exceptionally mild of the stiff main chain, which was more strongly stabilized Mizoroki-Heck coupling reaction in the presence of by hydrogen bonding and extended conjugation through the Cy2NMe (Cy=cyclohexyl) and Pd/t-Bu3P in 1994. Yokoza- p,p′-linked azobenzene units. wa and co-workers [54] recently investigated Mizoroki- Heck coupling polymerization of 1,4-bis[(2-ethylhexyl)- oxy]-2-iodo-5-vinylbenzene with this Pd initiator to synthe- 3 Metal-free cross-coupling reactions size poly(phenylenevinylene) (PPV) (Scheme 8). The polymerization proceeded even at room temperature when The aforementioned transition metal-catalyzed coupling 5.5 equivalents of Cy2NMe was used as a base; however, reactions have enabled the design and synthesis of a variety the molecular weight distribution of the obtained PPV was of -conjugated polymers. Additionally, metal-free named broad. Matrix-assisted laser desorption ionization-time of reactions which can form carbon–carbon bonds or carbon- flight (MALDI-TOF) mass spectroscopy showed that PPV heteroatom bonds, such as Michael reaction, Horner- with H/I end-groups formed until the middle stage, after Wadsworth-Emmons reaction, Diels-Alder reaction, Baylis- which the end-groups were converted into tolyl and H. The Hillman reaction, Bergman reaction etc., also serve im- Mn did not increase until about 90% monomer conversion portant roles in polymer synthesis, enabling the develop- and then increased sharply after that, indicating convention- ment of novel polymers from diverse monomer feedstocks. al step-growth polymerization. The occurrence of step- growth polymerization, rather than catalyst-transfer chain- 3.1 Michael addition reaction growth polymerization is presumably due to the low coordination ability of H-Pd(II)-I(t-Bu3P) to p-electrons of PPV Michael addition is a facile method to form carbon–carbon backbone; reductive elimination of H–I from this Pd species bonds from nucleophiles and activated olefins or alkynes

Scheme 7 Synthesis of cinchona alkaloid-derived chiral polymers by Mizoroki-Heck polymerization [52]. 1700 Jiang, et al. Sci China Chem November (2015) Vol.58 No.11

butanol (ABOL) to cystamine bisacrylamide (CBA) and bisacryloylpiperazine (BAP), two different functionalized PAAs were synthesized with Mw of 5.9 and 5.5 kDa (PDI= 1.13), respectively. Water-soluble P(CBA-ABOL) self- assembles into cationic nanocomplexes with oppositely charged proteins, such as human serum albumin (HSA), at Scheme 8 Synthesis of PPV via Mizoroki-Heck polymerization [54]. the physiological pH value of 7.4, then rapidly disaggre- gates in a reductive (intracellular) environment after uptake. Because of the cleavage of repetitive disulfide linkages, the [57]. Benefitting from the features of mild reaction condi- bioreducible PAAs had excellent efficiencies for intracellu- tions, high conversions, and high functional group tolerance, lar protein delivery and should be applicable for oral protein the Michael addition reaction has recently gained attention delivery. as a polymer synthesis tool for tailored macromolecular Another alternative synthetic path to achieve high mo- architectures [58]. Like the transition metal-catalyzed poly- lecular weight polymers is to use an AB-type monomer as a merizations, Michael polymerization also lends itself to single precursor. Zhang et al. [61] reported an efficient both AA/BB-type and AB-type polymerization. strategy to prepare a series of azobenzene(azo)-containing Poly(amido amine)s (PAAs) are a family of synthetic main-chain liquid crystalline polymers (LCPs) via facile polymers obtained by stepwise polyaddition of primary and Michael addition polymerization (Scheme 11). Three azo secondary amines to bisacrylamides [59]. Because they are polymers with different lengths of flexible spacers were degradable in aqueous media, many PAAs are remarkably synthesized with the characters of rather good thermal sta- biocompatible. Engbersen et al. [60] developed an effective bilities, relatively low glass transition temperatures, a broad intracellular protein delivery system based on linear PAAs range of liquid crystalline mesophases, and reversible pho- (Scheme 10). Through Michael polyaddition of 4-amino-1- toresponsive behaviors. Furthermore, the presence of

Scheme 9 Sonogashira-Hagihara coupling polymerization of D-hydroxyphenylglycine-derived diiodophenylene monomer 1 with 3,3′- and 4,4′- diethynylazobenzene monomers 2m and 2p [56].

Scheme 10 Synthesis of P(CBA-ABOL) and P(BAP-ABOL) [60]. Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1701 secondary amino groups in the backbones not only made 9,9-dialkyl-fluorene-2,7-dicarboxaldehydes and benzobi- them highly reactive precursors for various new functional soxazole monomers (Scheme 13). This method not only linear and cross-linked azo LCPs, but also led to the for- produced polymers with all trans double bonds, but prevents mation of supramolecular hydrogen-bonded LCPs; these cross-linking reactions, incomplete double-bond formation, compounds have very appealing properties including flexi- and other undesirable structural defects. The resulting ble preparation, facile reconstruction and recyclability, and poly(fluorene vinylene-co-benzobisoxazole)s exhibited pro- good mechanical and photomechanical properties. mising brightness in guest-host OLEDs with the advantages of good solubility, good thermal stability, and high electron 3.2 Horner-Wadsworth-Emmons reaction affinity.

The Horner-Wadsworth-Emmons (HWE) reaction is a 3.3 Diels-Alder reaction chemical reaction with widespread use in organic synthesis for the production of E-alkenes using stabilized phospho- The Diels-Alder (DA) reaction is a well-known reaction nate carbanions and aldehydes (or ketones) [1]. Polymers involving [4+2] cycloaddition of a dienophile and diene [1]. generated using the Horner-Wadsworth-Emmons condensa- Taking advantage of the fact that the DA and the opposite tion reaction have become a reality in recent years through retro-Diels-Alder (rDA) reactions can be controlled by the effort of chemists. Poly(thienylenevinylene) (PTV) has temperature, the DA reaction, especially with the furan/ proven to be promising for applications in photovoltaic de- maleimide couple, has been applied for the production of vices because of its unique properties of low oxidation po- novel polymer materials bearing recyclable, mendable, and tential despite a small band-gap [62]. To improve the syn- thermo-responsive features [65]. Shibata et al. reported DA thesis of well-defined PTV, a new synthetic strategy was polymerization of difurfurylidene diglycerol (DFDG) and performed by Hadziioannou et al. (Scheme 12) [63]. 4,4′-bismaleimidodiphenylmethane (BMI), to afford a Poly[3,4-dioctyl(2,5-thienylenevinylene)] (DO-PTV) was bio-based linear polyimide (DFDG-BMI) (Scheme 14) [66]. designed and successfully synthesized from AB-type After reprecipitation, the Mw of the obtained DFDG-BMI monomers using HWE condensation. In this method, excess reached 7900. In addition, the corresponding monomers monofunctional end-capper, i.e., 2-thiophenecarbaldehyde, could be regenerated through retro-DA depolymerization of was employed to control the degree of polymerization and DFDG-BMI in N,N-dimethyl-formamide at around 100– ensure complete conversion of the reactive phosphonate 120 °C. In addition to the AA/BB-type monomers, self- group. This type of condensation reaction generally gener- polymerizable AB-type monomers were also prepared. ates DO-PTV with well-controlled lengths, although the Swager et al. [67] described the synthesis of 1,4-epoxy- polydispersities were rather high; however, purification via 5,12-dihexyloxy-1,4-dihydrotetracene, and its polymeriza- column chromatography minimized this drawback. tion was carried out in the melt phase at high pressure in Jeffries et al. [64] also reported a synthetic route to new solution (Scheme 15). Additionally, a novel poly(iptycene) benzobisoxazole copolymers via HWE polycondensation of ladder polymer was successfully prepared via the dehydration

Scheme 11 Synthetic route of main-chain LCPs [MP-m (m=2, 6, 10)] via Michael addition polymerization, and supramolecular hydrogen bonding interac- tion of these main-chain LCPs [61].

Scheme 12 Synthesis of DO-PTV via Horner-Wadsworth-Emmons polycondensation [63]. 1702 Jiang, et al. Sci China Chem November (2015) Vol.58 No.11

Scheme 13 Synthesis of poly(fluorene vinylene-co-benzobisoxazole)s via Horner-Wadsworth-Emmons polycondensation [64].

Scheme 14 Synthesis of DFDG and DFDG-BMI [66]. of the above DA polyaddition polymer, using pyridinium synthetic steps from inexpensive, commercially available p-toluenesulfonate and acetic anhydride. reagents. The reported strategy provided a welcome addi- Taking advantage of the aza-Diels-Alder reaction, which tion to the polymer chemist’s toolkit by providing ready is known as the Povarov reaction, Gorodetsky et al. [68] access to a diverse library of polyquinoline-type materials. have demonstrated a previously unknown synthetic route to 4,6-linked polyquinolines (Scheme 16). The AB-type 3.4 Baylis-Hillman reaction monomer was designed to incorporate the requisite alkyne The Baylis-Hillman reaction is a carbon–carbon single and aldimine functionalities within a single substrate as well bond-forming reaction between the -position of an acti- as an alkyl chain for enhanced solubility. Thus, polyquino- vated alkene and an aldehyde, or more generally a carbon lines with a unique architecture were obtained in only two electrophile, in the presence of a suitable nucleophilic catalyst such as a tertiary amine [1]. This reaction has attracted interest in organic synthesis because it is an atom-econom- ical coupling that occurs under mild conditions without heavy metals and can be carried out with stereochemical control to generate a polyfunctional scaffold for further transformations. Unfortunately, this highly efficient reaction has been overlooked for a long time in polymer synthesis. Based on Baylis-Hillman polymerization, Klok et al. [69] reported a novel strategy for the synthesis of side-chain functional polyesters via an AA/BB approach from bifunctional acrylates and bifunctional dialdehydes (Scheme 17). The degree of the polymerization reached 25 with DABCO as the catalyst. In addition, the side-chain hydroxyl and vi- Scheme 15 The Diels-Alder polymerization of AB-type monomers, and nyl groups could be further modified to generate a diverse dehydration of the precursor polymer [67]. range of bifunctional polyesters. Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1703

Scheme 16 Synthesis of bifunctional monomer and polyquinoline [68].

Scheme 17 Synthetic strategy for side-chain functional polyesters via Baylis-Hillman polymerization [69].

3.5 Bergman reaction 4 Multi-component reactions The Bergman reaction, which involves intramolecular cyclization of enediyne compounds, was first reported by Multi-component reactions (MCRs) are a type of modular Bergman et al. in 1972 [70]. Due to the cytotoxicity of the and highly efficient reactions in which three or more dif- biradical intermediate, it is widely used in pharmaceutical ferent starting materials react to generate a final complex research and organic synthesis [71]. Since 1994, when Tour prduct in a one-pot procedure. Since the pioneering work in et al. [72] described a radical polymerization route to obtain 1850 by Strecker, who first discovered the synthesis of poly(p-phenylene)s and polynaphthalenes, diradicals from -amino-nitriles through a three-component reaction, Bergman cyclization have been regarded as very promising MCRs increasingly fascinated chemists because they com- and attractive monomers for preparing conjugated aromatic bine various reactants in a one-pot and one-step process to polymers and carbon-rich materials without transition-metal generate single products under mild conditions [79,80]. The catalysts (Scheme 18) [73]. strikingly efficient MCRs were only recently introduced to Hu’s group [74,75] successfully synthesized rod-coil polymer chemistry. While some MCRs such as the Biginelli brush conjugated polymers via Bergman cyclization poly- reaction, Kabachnik-Fields reaction, and Hantzsch reaction merization. As shown in Scheme 19, enediyne monomers were found to collaborate well with controlled radical with either polycaprolactone or Frechet-type dendrimers polymerization to facilely generate a series of well-defined were initially prepared and then subjected in bulk to a polymers with different side groups [81–83], MCRs espe- reﬂuxing diphenyl ether or dimethyl-o-phthalate bath un- cially the Passerini reaction and Ugi reaction, are frequently der vacuum for thermal Bergman cyclization. Two of the used and are a very powerful approach to directly generate obtained polymers are readily soluble in common organic diverse functional polymers [84]. solvents. The higher molecular weight fractions could be isolated through repeated centrifugation or dialysis, alt- 4.1 Passerini-3CR reaction hough the size distributions of the obtained brush polymers were wide in all cases. Chiral conjugated polymers with The Passerini-3CR reaction, which involves the reaction of rigid backbones were also synthesized using a similar pro- an aldehyde with a carboxylic acid and an isocyanide to cess [76,77]. Furthermore, the same group reported for the generate an -acyloxycarboxamide in one step, was first first example of on-surface formation of one-dimensional described by Passerini in 1921 (Scheme 21) [1,85]. Given polyphenylene chains through Bergman cyclization on the mild reaction conditions, atom-economy, and tolerance Cu(110) by combining high-resolution UHV-STM imaging to many functional groups, the Passerini reaction has been and DFT calculations (Scheme 20) [78]. widely used in organic synthesis and drug development. In contrast, for polymer synthesis, the reaction remained unex- plored until 2011, when Meier’s group [86] first showed that diverse monomers prepared by Passerini-3CR could be polymerized using acyclic diene metathesis (ADMET) polymerization. Furthermore, they demonstrated a novel Passerini polymerization method that involves the use of a dicarboxylic acid and dialdehyde in combination with Scheme 18 Bergman cyclization and related polymerization [72,73]. structurally diverse isocyanides to directly form high 1704 Jiang, et al. Sci China Chem November (2015) Vol.58 No.11

Scheme 19 Synthesis of brush conjugated polymers via Bergman cyclization [74,75].

Scheme 20 Mechanism of Bergman cyclization and radical polymerization of DNHD [78].

Scheme 21 Synthetic strategy for Passerini polymerization via the AA/BB/C approach [86–88] (color online). molecular weight polyesters (Scheme 21(a)) [86]. The re- very powerful synthetic strategy to produce functionalized sulting polymers with a more regular repeated unit structure polymers from two bifunctional building blocks. Li et al. had molecular weights of up to 56 kDa. Therefore, Passerini [87] then followed this strategy using diisocyanides instead multi-component polymerization (MCP) was found to be a of dialdehydes to prepare sequence-regulated poly(ester- Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1705

Scheme 22 Synthetic strategy for photodegradable polymers based on Passerini MCP [89]. amide)s (Scheme 21(b)); they found that the molecular (P4HB)-type polyesters with controlled degradation behav- weight was directly dependent on the amount of mono- iors and non-acidic degradation products. group component. The easily controlled polymer molecular weight together with the functional group tolerance offers many possibilities for the construction of a variety of new 4.2 Ugi four-component reaction polymer architectures using this synthetic strategy. Very The Ugi four-component reaction (Ugi-4CR), which was recently, the same research group extended this synthetic discovered in 1959, is another famous MCR that is an ex- concept of multifunctionalization to stimulus-responsive tension of the Passerini-3CR with an additional component polymers and realized the first example of multi-function- —a primary amine [1]. The reaction without catalyst is also alization of photodegradable polymers (Scheme 22) [89]. an atom-economic and environmentally friendly reaction as In order to further expand the scope of applications for only water is expelled as a byproduct and the chemical Passerini MCP, Li et al. [88] achieved polyamides with new yields are generally high. Meier’s group [91] first studied combinations by employing diisocyanides, dialdehyde, and Ugi-4CR to synthesize highly substituted ,-dienes amide various carboxylic acids (Scheme 21(c)). Compared to tra- monomers for ADMET polymerization, affording a series ditional amide-bond syntheses, these polyamides could pro- of highly diverse substituted polyamides. Like Passerini- vide a platform for subsequent modification since the car- 3CR, Ugi-4CR can produce polymers via polycondensation boxylic acids can be appropriately selected to introduce functional groups. or polyaddition processes by using bifunctional components. The above-mentioned polymerizations can be described It is necessary to use AA/BB/C/D combinations to realize as an AA/BB/C approach. Taking advantage of the aspect the polymerization process. In this way, six different poly- that the composition of the products is strictly governed by amides were synthesized, under very mild reaction condi- the reaction mechanism, a new AB/C approach can also be tions and without the use of any catalyst, by varying the utilized to regulate the monomer distribution, in other words combination of available bifunctional monomers (Scheme to control the monomer sequence (Scheme 23). Li et al. [90] 25) [92]. The obtained finely tuned macromolecular struc- synthesized a linear functional polyester through Passerini tures mainly reached molecular weights greater than 10 kDa MCP of (E)-4-oxobut-2-enoic acid (AB monomer) and dif- and showed only glass transitions. Moreover, the facile in- ferent isocyanides (C monomer) (Scheme 24). During the troduction of functional groups enables post-polymerization polymerization, isomerization of the double bonds occurs, modifications. generating two different repeating units in the polymer Ugi five-component condensation (Ugi-5CC) is a modi- backbone. Further hydrogenation of these precursor poly- fied Ugi-4CR reaction that offers a novel strategy for the mers would yield functional poly(4-hydroxybutyrate) incorporation of CO2 into polymers. For this purpose, 1,12- diaminododecane and 1,6-diisocyanohexane, together with

isobutyraldehyde, methanol, and CO2, were used by Meier’s group [93] to directly synthesize polyamides by Meier’s group (Scheme 26). By varying the reaction conditions, the corresponding polyamides bearing methyl carbamate side-

chains showed remarkable Mn values of 20.2 kDa with polydispersity of 1.71. Encouraged by the formation of hydan- toin by the methyl carbamate derivatives [94], the polymer was further converted into polyhydantoin in a very straight- Scheme 23 Two different approaches to Passerini MCP based on AA/BB/C-type (a) and AB/C-type monomers (b). forward fashion. 1706 Jiang, et al. Sci China Chem November (2015) Vol.58 No.11

Scheme 24 Synthesis of functional P4HB by Passerini MCP and subsequent hydrogenation [90].

Scheme 25 General representation of all six possible monomer combinations that are polymerizable via Ugi-4CR (using two bifunctional and two monofunctional monomers) [92].

5 Conclusions

The synthesis of polymers using classical named reactions is a new but promising field of work. With further study of the mechanism, polymers with controlled molecular weights could be achieved after optimization of the reaction conditions. Considering that the versatility of the named reactions in terms of monomer selection, solvent environment, reaction temperature, and post-modification permits the synthesis of unique sophisticated macromolecular structures, the established named reactions that exhibit high efficiency and selectivity may provide numerous ways to develop polymeric tools. Despite many significant discoveries and developments, this methodology is by no means problem-free. In all of these reactions, the synthesis of bifunctional AA- or AB-type monomers seems to be the main challenge. In addition, transition-metal catalysts are relatively expensive

and difficult to remove completely; the heavy metal impuri- Scheme 26 Ugi-5CC of 1,12-diaminododecane, isobutyraldehyde, tert- ties in the resulting polymers may affect their electronic butyl isocyanide, carbon dioxide, and an alcohol leading to substituted dicarbamates [93]. properties. Compared with the living polymerization methods, Jiang, et al. Sci China Chem November (2015) Vol.58 No.11 1707 this methodology still often gives only low molecular tion for the synthesis of well-defined condensation polymers and weight polymers with high polydispersities, which may be a -conjugated polymers. Chem Rev, 2009, 109: 5595–5619 21 Xu S, Kim EH, Wei A, Negishi EI. Pd- and Ni-catalyzed cross- problem for applications that require high molecular coupling reactions in the synthesis of organic electronic materials. Sci weights. 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