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product and the Ni/Ti ratio. In the same spirit, ..... 9401 as a robust biocatalyst with a high enantioselectivity for benzylic (R) secondary alcohols. Shvo's catalyst ...
Chem Soc Rev REVIEW ARTICLE Tandem catalysis: a new approach to polymers Cite this: Chem. Soc. Rev., 2013, 42, 9392

Carine Robert and Christophe M. Thomas* The creation of polymers by tandem catalysis represents an exciting frontier in materials science. Tandem catalysis is one of the strategies used by Nature for building macromolecules. Living organisms generally synthesize macromolecules by in vivo enzyme-catalyzed chain growth polymerization reactions using activated monomers that have been formed within cells during complex metabolic

Received 2nd August 2013

processes. However, these biological processes rely on highly complex biocatalysts, thus limiting their

DOI: 10.1039/c3cs60287g

industrial applications. In order to obtain polymers by tandem catalysis, homogeneous and enzyme catalysts have played a leading role in the last two decades. In the following feature article, we will

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describe selected published efforts to achieve these research goals.

Introduction Biocatalysts are an invaluable source of inspiration for chemists. From simple materials, these natural catalysts produce a large variety of structurally complicated compounds. Such a high degree of complexity is obtained with the help of efficient biocatalytic reactions involving multienzymatic systems which can accelerate several reactions in a sequential manner.1 By contrast, in classical synthetic chemistry, individual transformations are conducted as stepwise processes punctuated by the purification and isolation of intermediates at each stage of the sequence.2 Chimie ParisTech, UMR CNRS 7223, 11 rue Pierre et Marie Curie, 75005 Paris, France. E-mail: [email protected]

Carine Robert obtained her PhD degree in 2012 under the supervision of Prof. C. Thomas. She then moved to the University of Tokyo (Japan) and worked for one year as a post-doctoral fellow with Prof. K. Nozaki, supported by the Japan Society for the Promotion of Science. In 2013, she was appointed as an assistant professor at Chimie ParisTech (Paris, France) in the research group of Prof. Christophe Carine Robert Thomas where her research interests are directed towards the conception of new catalysts for the polymerization of cyclic polar monomers.

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In the past years, a challenge for chemists has been to design systems that mimic multienzymatic activity. In particular, a large number of investigations have been directed towards designing tandem reactions.3,4 Tandem catalysis involves the sequential or concurrent action of two or more catalytic cycles in a single reactor to yield a product with minimum workup, or change in conditions. More complex combinations may be envisioned. In particular, additional reagents may be required for a specific catalyst cycle to perform its function.5 These new catalytic schemes that take advantage of unstable intermediates have several advantages over multistep syntheses, including time- and cost-savings, atom economy, waste reduction and energy consumption. In addition, circumventing the need to store and transport harmful

Christophe Thomas obtained his PhD degree in 2002 working under ¨ss-Fink the supervision of Prof. Su (Switzerland). He then joined Prof. Coates’ group at Cornell University (USA) as a postdoctoral fellow supported by the Swiss National Science Foundation (2002–2003). After spending one year in Prof. Ward’s laboratories, he was appointed as an assistant professor at the University of Rennes (France) in 2004. Christophe Thomas In 2008, he was promoted to professor at Chimie ParisTech (Paris, France). His research interests comprise the study of fundamental processes in organometallic chemistry with an emphasis on polymerization catalysis and the control of stereochemistry.

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chemicals by generating them in situ increases safety and provides environmental protection. Also the synthesis of complex products that are otherwise difficult to obtain comes within reach. However, it has been shown that different parameters must be taken into account for a successful tandem reaction. In particular, when multiple chemistries are performed in a single system, selectivity is a key issue. In addition one of the most important conditions is catalyst compatibility: best control is achieved when there is no interference between the catalytic species. Furthermore, the reactivity of the active sites must be well matched so that the product of one catalytic cycle does not hamper the overall tandem sequence. Therefore tandem catalysis is becoming increasingly important for synthesizing small molecules.6–8 These methods are recognized as powerful tools in several areas of organic chemistry, including natural product synthesis, drug discovery, and process chemistry.9–11 Tandem catalysis is also one of the strategies used by Nature for building macromolecules.12 Indeed, through billions of years of evolution, Nature selected efficient ways for synthesizing macromolecules by in vivo enzyme-catalysed chain growth polymerization reactions using activated monomers that have been formed within cells.13 Such a high degree of perfection is obtained with the help of efficient biocatalytic processes. For instance, aliphatic polyesters, such as polyhydroxyalkanoates, are synthesized under mild conditions from renewable feedstocks, such as sugars or fatty acids. However, these biological processes rely on highly complex biocatalysts, such as PHA synthases,14 thus limiting their industrial applications. Inspired by these biocatalysts, several research groups anticipated that metal complexes would be of interest in order to achieve effective tandem preparation of polymers with new structures and functions, in particular for the synthesis of polyolefins.15 The early discoveries in these tandem processes have been extensively and remarkably covered in two comprehensive reviews by Bazan and will not be duplicated herein.15,16 The present review is concerned with recent selected papers that have described methodologies and strategies that allow the tandem synthesis of polymers, such as polyolefins, polydienes, polyesters, polycarbonates (and the corresponding copolymers). Herein, we have focused our attention on those systems that incorporate at least one synthetic catalytic species and can involve homogeneous or enzymatic reaction sites.

production of linear low-density polyethylene by addition of ethylene to a mixture of two molecular catalysts. In this tandem catalysis, one complex dimerizes or oligomerizes ethylene to a-olefins while the second site incorporates these a-olefins into a growing polyethylene chain. A variety of classical catalyst combinations are available for this purpose. Better control over the polymerization process, and therefore product properties, is attained by the use of homogeneous single-site catalysts. The best-behaved tandem processes take advantage of well-defined catalysts that require stoichiometric quantities of activators. One such system employs a nickel-based complex 1 and a cationic titanium derivative 2 (Scheme 1). The nickel sites are responsible for converting ethylene to 1-butene or mixtures of 1-butene with 1-hexene. These olefins are then copolymerized with ethylene at the titanium sites to produce LLDPE with Mw up to 330 000 g mol1. Notably, it was shown that it is possible to obtain a linear correlation between the branching content in the polymer product and the Ni/Ti ratio. In the same spirit, Miller reported that a zirconium-based polymerization catalyst and a chromiumbased oligomerization catalyst in combination with methylaluminoxane (MAO) constituted a tandem catalyst system for converting ethylene alone to linear low density polyethylene with short and long branches (total branches: 14.1–43.5 branches per 1000 carbon atoms, Mn up to 1810 g mol1, Mw up to 114 900 g mol1).17 By adjusting the polymerization conditions (i.e., catalysts/MAO ratio, ethylene pressure, prepolymerization time), the branching characteristics of polyethylene could be varied in a controlled manner. A major breakthrough has very recently been achieved by Marks and Delferro who synthesized and fully characterized a heterobimetallic complex, consisting of a constrained-geometry titanium olefin polymerization centre covalently linked to a chromium bis(thioether)amine ethylene trimerization centre.18 The resulting catalyst selectively produces linear low-density polyethylene with molecular weights as high as 460 000 g mol1 and exclusively n-butyl branches in conversion-insensitive densities of B18 branches/1000 carbon atoms, which are 17 and 3 times, respectively, those achieved by tandem mononuclear Ti and Cr catalysts under identical reaction conditions. Interestingly, these results suggest that proximity of the catalytic centres alters the propagation and chain-transfer characteristics of the heterobimetallic catalyst.

Synthesis of linear low-density polyethylene using molecular catalysts

In order to synthesize functionalized polyolefins, Crivello demonstrated that a wide variety of mono-, di- and multifunctional allyl ethers can be polymerized using a cobalt-catalyzed tandem isomerization and cationic polymerization (Scheme 2).19 Employing dicobalt octacarbonyl in combination with organosilanes, the polymerization of these monomers took place rapidly to give high molecular weight polymers (Mn = 760–24 000 g mol1, Mw/Mn = 1.1–1.9). However, monomers containing hydroxyl, amide, imide, urethane, or sulfide functional groups failed to undergo polymerization. Sita described the synthesis of new classes of microphaseseparated polyolefin of well-defined structure with cationic

Due to the accessibility of the monomer and the attractive properties of linear low-density polyethylene (LLDPE), the development of new and efficient initiators for this tandem polymerization process is a major scientific goal.15 As a result, a significant amount of recent research has focused on the discovery and development of new catalytic systems for this process.16 Tandem catalysis for branched polyethylene synthesis has progressed from using ill-defined heterogeneous mixtures to taking advantage of well-defined homogeneous organometallic catalysts. Notably, interesting results have been obtained for the

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Synthesis of (co)polymers using molecular catalysts

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Scheme 1

Tandem synthesis of LLDPE.

Scheme 2

Synthesis of functionalized polyolefins.

amidinate-based zirconium complexes.20 The use of amidinatebased zirconium precatalysts 3, which upon activation by the borate cocatalyst, [PhNMe2H][B(C6F5)4], can allow the stereospecific living polymerization of 1-hexene, and also the living cyclopolymerization of 1,5-hexadiene to produce high molecular weight poly(methylene-1,3-cyclopentane) (PMCP) materials possessing extremely narrow polydispersities (Mw/Mn o 1.1). Therefore, using a tandem polymerization/cyclopolymerization strategy, these Ziegler–Natta catalysts were shown to produce new poly(1-hexene)-b-PMCP and poly(1-hexene)-b-PMCP-bpoly(1-hexene) di- and triblock copolymers of narrow polydispersity (Mn = 20 000–31 000 g mol1, Mw/Mn = 1.1) (Scheme 3).

Scheme 3

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High-field 13C NMR spectra of these block copolymers revealed that both block materials were highly isotactic. Notably, these classes of polyolefin block copolymers can microphase-separate into ordered morphologies with feature sizes on the nanometerlength scale. Several research groups developed tandem metathesis– hydrogenation processes using Grubbs-type ruthenium carbene complexes as catalysts. In 1997, McLain et al. reported the synthesis of an ethylene–methylacrylate copolymer by the ring-opening metathesis polymerization (ROMP) of an ester-functionalized cyclooctene using Cl2(PCy3)2RuQCHCHQCPh2, followed by the hydrogenation of the resulting polymer.21 Under hydrogen

Synthesis of microphase-separated polyolefins.

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Review Article pressure, the metathesis catalyst residue was supposed to be converted to RuHCl(PCy3)2. Then Wagener developed an alternative procedure involving sequential homogeneous metathesis– heterogeneous hydrogenation: addition of silica gel to a completed homogeneous metathesis polymerization catalyzed by Cl2(PCy3)2RuQCHPh allowed the formation of a highly effective heterogeneous catalytic system that was able to hydrogenate the unsaturated polymer, even at low pressure (Mn = 1500 g mol1, Mw/Mn = 1.9).22 Also, the osmium complex OsHCl(CO)(PiPr3)2 was reported to catalyze the ROMP of norbornene and 2,5norbornadiene to give cis-syndiotactic poly(norbornene) and poly(norbornadiene), respectively. In both cases the resulting polymers have a high cis content, up to 95%. In addition, this osmium derivative was used as a catalyst for tandem ringopening metathesis polymerization and hydrogenation of norbornene and 2,5-norbornadiene (Mn = 11 000–310 000 g mol1, Mw/Mn = 1.1–2.8).23 The stereoregularity of the cyclopentane and cyclopentene ring sequences in poly(norbornene) and poly(norbornadiene) of the hydrogenated derivatives was also found to be syndiotactic. In 2007 Fogg et al. demonstrated that it was possible to manipulate both the active site and the ancillary ligands to transform a highly active ROMP catalyst 4 into an efficient ligand-stabilized hydrogenation catalyst (Scheme 4).24 The resulting assisted-tandem catalyst enabled them to convert sterically demanding monomers and a long-chain polymer into saturated, chain-length precise polymers (Mn = 13 000–104 000 g mol1, Mw/Mn o 1.1). Grubbs and coworkers reported the ability of the single difunctional complex 5 to mediate three mechanistically different reactions (Scheme 5).25 This ruthenium-based complex was conveniently prepared from Cl2(PCy3)2RuQCHPh and allyl 2-bromo-2-methylpropionate and incorporates both a ROMP and an atom-transfer radical polymerization (ATRP) initiator. The authors initially confirmed that 5 initiated both ROMP and ATRP independently. The ROMP of 1,5-cyclooctadiene (COD) in solution or bulk afforded poly(butadiene) (PBD), in yields ranging from 85 to 95% and with Mw/Mn nearly two. Interestingly, these results were similar to those obtained when Cl2(PCy3)2RuQCHPh was used as the ROMP initiator. Also, addition of methyl methacrylate (MMA) to a solution of 5 in toluene afforded poly(methyl methacrylate) (PMMA) in 75% yield. In addition, a linear relationship between monomer

Scheme 4

Chem Soc Rev conversion and polymer molecular weight was observed and nearly monodispersed polymers (Mw/Mn o 1.2) were obtained. These observations suggested that complex 5 effectively controlled the polymerization. Complex 5 was then employed in the one-pot copolymerization of COD and MMA under a variety of conditions. Analysis of the resulting polymers by 1H NMR spectroscopy indicated the presence of both PBD and PMMA in the expected ratios. Gel permeation chromatography confirmed that PBD–PMMA diblock copolymers were formed as monomodal polymer distributions were observed (Mw/Mn 1.5–1.6) with experimental molecular weights in agreement with their theoretical values. Kinetic studies suggested that a single ruthenium alkylidene complex successfully mediated two mechanistically distinct polymerizations, simultaneously. Finally, at the conclusion of a COD–MMA copolymerization using complex 5, the residual ruthenium species can be transformed into a catalyst capable of hydrogenating the unsaturation in the polymer backbone (formed during the ROMP of the cyclic olefin). Choi reported extremely fast tandem ring-opening/ringclosing metathesis polymerization of a monomer containing two unreactive functional groups, cyclohexenyl and alkynyl, by using a third generation Grubbs catalyst 6 (Scheme 6).26,27 The authors demonstrated that this tandem system produced polymers with controlled molecular weights and narrow polydispersities at low temperature (Mn = 8000–41 000 g mol1, Mw/Mn = 1.2–1.8). To explain this extremely fast polymerization, Choi et al. proposed an ‘‘alkyne-first’’ mechanism (Scheme 7). In this mechanism, the catalyst would react with the terminal alkyne, after which the tandem ring-opening/ring-closing metathesis reaction involving the adjacent cyclohexene would occur, generating the propagating carbene complex 7. This is in accordance with previous studies that suggested that a Grubbs catalyst supported by an N-heterocyclic carbene ligand reacts more favorably with alkynes than with alkenes.28 Also, the terminal alkyne is kinetically more accessible than the 3-substituted cyclohexene potentially obtained via a ‘‘cyclohexene-first’’ mechanism. This type of controlled polymerization allowed the preparation of block copolymers using other conventional living metathesis polymerizations. The diene on the backbone of the polymer was then post-functionalized by sequential Diels–Alder and aza-Diels–Alder reactions, which led to selective functionalization depending on the stereochemistry of the diene.

Tandem synthesis using a ROMP catalyst.

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Scheme 5

Polymer synthesis using ROMP and ATRP.

Scheme 6

Tandem synthesis of a polydiene.

Scheme 7

‘‘Alkyne-first’’ mechanism involved with complex 6.

Inspired by the enzymatic ping-pong mechanism, Nozaki et al. designed a conceptually new approach, which employed allylboration and hydroformylation in an iterative and alternating manner (Scheme 8).29 Therefore, the authors successfully synthesized alternating and regioregular vinyl monomer–vinyl alcohol copolymers possessing multiple hydroxyl groups in a periodical manner, which have rarely been prepared by other synthetic methods (Mn = 800– 2400 g mol1, Mw/Mn B 1.5). Synthesis of sequence-regulated functionalized vinyl polymers is one of the most challenging goals in polymer science30 and this study provides opportunities to synthesize new functional polymeric materials by a polymerization methodology in which the main chain is propagated by alternating repetition of two mechanistically distinct transformations. In 2010 Coates and coworkers reported a highly efficient method for the synthesis of poly(3-hydroxybutyrate) by the carbonylative

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polymerization of propylene oxide (Scheme 9).31 Poly(3-hydroxybutyrate) (PHB) is a naturally occurring biodegradable and biocompatible polyester. Current methods to synthesize PHB include bacterial fermentation, ring-opening polymerization of b-butyrolactone (BBL), and direct copolymerization of propylene oxide (PO) and carbon monoxide (CO).32 Although fermentation produces high molecular weight PHB, the process is energy-intensive and necessitates polymer separation from the bacterial culture. The direct copolymerization of CO and PO is atom-economical but suffers from low monomer conversion. The living ring-opening polymerization of BBL yields a high molecular weight polyester, although it requires the purification of a toxic lactone. In contrast, the use of compatible epoxide carbonylation and lactone polymerization catalysts allowed Coates et al. to carry out a one-pot reaction that eliminates the need to

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Scheme 8

Ping-pong polymerization.

Scheme 9

Tandem synthesis of polyesters from epoxides.

isolate and purify the toxic b-butyrolactone intermediate (Mn = 2000–52 000 g mol1, Mw/Mn = 1.1–1.5). One of the best combinations was achieved by using complex 8 [(ClTPP)Al(THF)2]+[Co(CO)4] (ClTPP = meso-tetra(4-chlorophenyl)porphyrinato) with 9: this catalytic system showed high activities for both the carbonylation and polymerization stages of the reaction, respectively. This multicatalytic approach maintains the atom economy of the CO and PO copolymerization and provides the high-molecular weight polymer achieved by BBL polymerization. Darensbourg reported a tandem strategy combining two living polymerization techniques, salenCo(III)X-catalyzed CO2–styrene

Scheme 10

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oxide (SO) copolymerization and ring-opening polymerization of lactide with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), for the synthesis of poly(styrene carbonate-b-lactide) copolymers (Scheme 10).33 The authors used water as the chain transfer and/or chain terminator reagent, which is added at the end of the salenCo(III)X-catalyzed SO–CO2 copolymerization to in situ generate hydroxyl groups at the end of the polymer chains. The resulting polycarbonates with hydroxyl end groups can thus be directly employed as macroinitiators to subsequently initiate ROP of lactide and synthesize the expected diblock copolymers (Mn = 7900–20 000 g mol1, Mw/Mn B 1.1). Thanks to the living polymerization nature of both steps in this tandem strategy, the

Tandem synthesis of polycarbonates.

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Scheme 11

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Tandem synthesis of polyesters from epoxides and dicarboxylic acids.

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Review Article This selective and quantitative hydrogenation of the chlorine terminals in PMMA-Cl was achieved via direct transformation of the polymerization catalyst. Therefore this tandem process is practically versatile compared with previously reported systems employing hazardous tin reagents for hydrogenation or expensive and exotic silyl enolates for end-capping. Sawamoto also reported the synthesis of sequence-regulated copolymers via tandem catalysis consisting of metal alkoxidesmediated transesterification of methacrylates with alcohols and ruthenium-catalyzed living radical polymerization of their comonomers (Mn = 10 000–18 000 g mol1, Mw/Mn B 1.3).37 This tandem catalysis allowed the direct regulation of the multimonomer sequence that was difficult via one-pot process. Random-gradient copolymers were generally obtained with the highly selective transesterification of primary methacrylates in the presence of tertiary counterparts, and gradient-block copolymers were synthesized via iterative tandem catalysis due to the high controllability of ruthenium-catalyzed living radical polymerization. The tandem catalysis coupled with polymerization and transesterification can be extended to random or block copolymerization, which are achieved by sequentially and variably starting either polymerization or transesterification, respectively. Guan et al. developed the first example of tandem catalytic coordination/living radical polymerization (LRP) for efficient synthesis of nanoparticles for bioconjugation (Mn = 53 000– 478 000 g mol1, Mw/Mn = 1.5).38 Using a chain walking palladium-a-diimine catalyst, macroinitiators bearing multiple radical initiation sites were prepared and used subsequently in a Cu(I)-mediated ATRP for synthesizing dendritic polymer nanoparticles. Addition of an N-acryloyloxysuccinamide (NAS) monomer at the end of the ATRP afforded N-hydroxysuccinamideactivated polymer nanoparticles as convenient scaffolds for bioconjugation. This methodology is based on the fact that the addition of an acrylate monomer will generate a more stable secondary C–Br bond which is significantly less reactive for further polymerization. This allows the functionalization of each methacrylate chain end with an acrylate unit (i.e., NAS).39 Final conjugation with both small dye molecules and protein (e.g., ovalbumin) yielded nanoparticle conjugates with relatively high dye or protein per particle ratio (Mn = 3600 g mol1, Mw/Mn = 1.2). In 2008 Mantovani and Haddleton described a one-pot process consisting of a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) and a living radical polymerization.40 A CuBr–iminopyridine catalytic system was found to catalyze both processes. Interestingly, the relative rates of CuAAC and LRP can be tuned by appropriate changes of different parameters such as the concentration of the copper system, the reaction temperature and the nature of the solvent employed. In particular, the rate of the CuAAC process appeared to be relatively sensitive to the nature of the solvent used, which may be partly explained in terms of the different ability of the various solvents to coordinate the copper centre. In addition, the implementation of this concept for the synthesis of glycopolymers yielded well-defined materials in a simplified manner. Thus, mannose-functional polymers were prepared by

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Chem Soc Rev CuAAC–LRP from propargyl methacrylate and 2 0 -azidoethyla-mannopyranoside. In that case, DMSO was used as solvent as it solubilizes the mannose azide starting material, propargyl methacrylate, and the polymer product. 1H NMR showed that the two reactions occurred simultaneously. Increasing the catalyst concentration increased the rate of both LRP and CuAAC. Size exclusion chromatography analysis of the resulting glycopolymers showed Mw/Mn B 1.1, indicating controlled polymerization. In 2011 Hillmyer described the formation of nanostructured vinyl thermosets by a reaction induced phase separation mechanism using several vinyl monomers starting from a new, reactive poly(lactide)-b-poly(cyclooctene-s-5-norbornene2-methylene methacrylate)-b-poly(lactide) block polymer prepared by combining polymerization mechanisms (Mn = 55 000 g mol1, Mw/Mn = 1.4). Firstly the synthesis of a multiply functional hydroxyl telechelic poly(cyclooctene-s-5-norbornene-2-methylene methacrylate) was achieved by ring opening metathesis (co)polymerization of cis-cyclooctene and 5-norbornene-2-methylene methacrylate using the second generation Grubbs catalyst in combination with a symmetric chain transfer agent bearing hydroxyl functionality.41 Secondly, the resulting hydroxyltelechelic polymer was used as a macroinitiator for the ring opening transesterification polymerization of rac-lactide to form reactive poly(lactide)-b-poly(cyclooctene-s-5-norbornene2-methylene methacrylate)-b-poly(lactide) triblock polymers. Finally, the triblocks were cross-linked by free radical copolymerization with several vinyl monomers including styrene, divinylbenzene, methyl methacrylate, and ethylene glycol dimethacrylate to afford nanostructured vinyl thermosets. Synthesis of copolymers using enzyme–radical initiator combinations Howdle demonstrated that a simultaneous one-pot combination of enzymatic and chemical polymerization systems in supercritical carbon dioxide (scCO2) leads to controlled synthesis of block copolymers (Mn = 23 000–41 000 g mol1, Mw/Mn = 1.2–2.1).42 This was made possible by combining the enzymatic ring-opening polymerization (ROP) of e-caprolactone with the atom transfer radical polymerization of methyl methacrylate (Scheme 12). The authors demonstrated that both enzyme- and metal-based ATRP catalysts act concurrently in scCO2. Although e-CL is being consumed by the enzymatic ROP, use of e-CL as a cosolvent allowed the ATRP-catalyzed growth of the PMMA block to proceed with good control. Finally, the data indicated that the two catalytic systems are robust under these conditions and can tolerate each other. Synthesis of copolymers using molecular catalyst–enzyme combinations In 2005 Meijer provided proof of principle of iterative tandem catalysis.43 In this polymerization strategy, the concurrent action of two fundamentally different catalysts is required to achieve chain growth. For instance, combining ruthenium-catalyzed racemization with lipase-catalyzed ring-opening enabled the efficient oligomerization of (S)-6-methyl-e-caprolactone ((S)-6-MeCL).

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Scheme 12

Tandem synthesis of a copolymer.

Scheme 13

Tandem synthesis of polyesters.

The authors showed that the lipase-catalysed ring-opening of o-substituted lactones, such as 6-MeCL, results in a ring-opening product bearing a secondary alcohol. Since lipases generally only accept the (R) enantiomer of a secondary alcohol as the nucleophile, propagation halts after the ring-opening of an (S)-6-MeCL molecule. In situ racemization of the terminal secondary alcohol of the propagating polymer chain can provide reactive chain ends, yielding enantio-enriched oligomers of 6-MeCL. Also, Meijer

Scheme 14

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reported the use of dynamic kinetic resolution (DKR) to secondary diols and diesters to afford chiral polyesters (Mn = 3300– 15 400 g mol1) (Scheme 13).44 In order to obtain chiral polyesters of high enantiomeric excess value and good molecular weight, the authors employed Shvo’s catalyst 13 and Novozym 435. Novozym 435, Candida antarctica lipase B immobilized on a resin, was selected as the acylation catalyst since it is well studied in DKR processes and has proven itself

Tandem synthesis of polyesters.

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Review Article as a robust biocatalyst with a high enantioselectivity for benzylic (R) secondary alcohols. Shvo’s catalyst 13 was chosen as the racemization catalyst because it was found to show excellent compatibility with Novozym 435. The optimal polymerization conditions of 1,1 0 -(1,3-phenylene) diethanol and diisopropyl adipate required Shvo’s catalyst and Novozym 435 in the presence of 2,4-dimethyl-3-pentanol as the hydrogen donor (Scheme 13). With these conditions, chiral polyesters were obtained with molecular weights up to 15 000 g mol1 and an enantiomeric excess value up to 99%. Aliphatic secondary diols also resulted in enantio-enriched polymers but at most an enantiomeric excess of 46% was obtained with molecular weights in the range of 3300–3700 g mol1. This low enantiomeric excess originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliphatic diols. The reaction times are still relatively long which results from the slow racemization activity of the Shvo’s catalyst under the conditions employed. Nevertheless, iterative tandem catalysis is an effective way to prepare chiral polymers from a variety of optically inactive monomers. Heise and coworkers demonstrated that DKR can be combined with enzymatic polymerization for the synthesis of chiral polyesters from racemic secondary diols.45 The applied catalyst system consisted of the Noyori-type ruthenium catalyst 14 and Novozym 435. Since this catalyst combination tolerates a wide range of acyl donors, it was also suitable for difunctional acyl donors for the formation of polycondensates. Based on the high enantioselectivity of Candida antarctica lipase B in the esterification of secondary benzylic alcohols, the racemic a,a0 -dimethyl-1,4-benzenedimethanol was chosen as the diol component and dimethyl adipate as the acyl donor (Scheme 14). The reaction was allowed to proceed over four days, during which time the molecular weight was seen to gradually increase to 3000–4000 g mol1 according to size exclusion chromatography. Moreover, the optical rotation of the reaction mixture increased from 0.61 to 1281 during the process, which further confirms the enrichment of the (R,R) enantiomer in the reaction product.

Conclusion We have described selected important advances in achieving the tandem synthesis of (co)polymers. These studies demonstrate that tandem reactions should yield novel materials for a wide range of applications. However, despite recent significant advances in tandem catalysts, some major points remain to be addressed and improved. Although research efforts have been dedicated to tandem initiating systems and processes, the number of active, productive and selective initiators remains limited. This might be improved most likely by designing new tandem systems, more reactive and more robust toward large amounts of monomer. In addition, it can be anticipated that supported heterogeneous catalysts would be highly valuable for tandem reactions and would represent an attractive approach for the design of cleaner processes. This should allow the synthesis of new types of improved innovative (co)polymers

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Chem Soc Rev with original properties and would clearly increase the number of applications for polymers.

Acknowledgements ´gion ˆIle-de-France are thanked for financial CNRS, ENSCP and Re support. CMT is grateful to the Institut Universitaire de France.

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