Polymers DOI: 10.1002/ange.201310523 From Syndiotactic Homopolymers to Chemically Tunable Alternating Copolymers: Highly Active Yttrium Complexes for Stereoselective Ring-Opening Polymerization of b-Malolactonates** CØdric G. Jaffredo, Yulia Chapurina, SophieM. Guillaume,* and Jean-FranÅois Carpentier* Abstract: Alternating copolymers constitute an attractive class of materials. It was shown previously that highly alternated poly(b-hydroxyalkanoate)s (PHAs) can be prepared by ring- opening polymerization (ROP) of mixtures of two different enantiomerically pure 4-alkyl-b-propiolactones. However, the approach could not be extended to PHAs with chemically tunable functional groups, which is highly desirable to access original advanced materials. Reported herein is the first highly syndioselective and controlled ROP of racemic allyl and benzyl b-malolactonates (MLA R ;R = allyl, benzyl) using an yttrium complex supported by a tetradentate dichloro-substi- tuted bis(phenolate) ligand. This highly active catalyst allows the nearly perfect alternating copolymerization of MLA Allyl and MLA Benzyl . Hydrogenolysis of the benzyloxycarbonyl or functionalization of the allyl pendant groups opens a route towards a new class of functional alternating copolymers. Nature taught us how vital the sequence is for macro- molecules such as DNA and proteins. Controlling the architecture and composition of synthetic macromolecules is also, of course, of paramount importance so as to obtain polymers with optimized properties for applications. Among the broad variety of synthetic materials, regularly alternating copolymers are of special interest, as optimal positioning of functional substituents can induce improved performance over that of copolymers having lower periodicity (block, gradient, random). [1] Major advances have been achieved in this field over the last two decades thanks to the use of coordination polymerization with well-defined metal com- plexes. [1] Poly(hydroxyalkanoate)s (PHAs) are a class of aliphatic polyesters having a three-carbon-atom backbone structure, and differ by their substituent on the 4-position. These polymers, which are naturally occurring in different micro- organisms in their isotactic form, represent a highly valuable targets as substitutes for nonpetrochemical-based plastics and for use in biomedical applications. [2] PHAs can also be synthetically prepared by ring-opening polymerization (ROP) of b-lactones. [3] However, the stability of the latter monomers makes this process challenging, and even more so if stereoselectivity is simultaneously addressed. [3] Yttrium complexes with tetradentate bis(phenolate) ligands are highly active catalysts for the living ROP of simple b-lactones bearing alkyl substituents at C4; the prototypical example is b-butyrolactone (BL). [4] Additionally, some of these catalysts, when applied to racemic mixtures of these chiral monomers, can achieve high syndioselectivity (P r , probability of racemic linkage, up to 95%) by a chain-end control mechanism. [4] By taking advantage of this unique syndioselectivity of yttrium catalysts, Coates, Thomas, and co-workers devised a smart, new strategy for making original alternating PHAs by performing ROP on 1:1 mixtures of two different enantio- merically pure 4-substituted-b-propiolactones of opposite absolute configuration. [5] Yet, the demonstration is limited to monomers bearing nonfunctional substituents [(fluoro)alk- yls] or substituents that do not allow easy chemical trans- formations (ethers). Alkyl b-malolactonates (MLA R ) are monomers that, upon ROP, lead to poly(b-malic acid) derivatives (PMLA R ), which are PHAs of high interest because of the potentially accessible pendant functional carboxylate moieties. Although a variety of catalysts/initiators have been disclosed for the ROP of derivatives such as MLA Bz (Bz = benzyl), none of them has shown stereoselectivity towards racemic monomers and thus afford only atactic [6] or slightly isotactic-enriched PMLA Bz . [7] Herein we report on highly active and syndioselective yttrium catalysts for the ROP of rac-MLA R derivatives (R = Bz, All = allyl). Unexpectedly, in light of previous studies on ROP of lactide and more simple b-lactones such as BL, [4] high stereoselectivity has been achieved with an o,p-dichloro- substituted bis(phenolate) ligand and not with ligands bearing bulky aryl/alkyl substituents. First, homopolymerization experiments were performed using yttrium isopropoxide initiators generated in situ by using 1 equivalent of iPrOH and the compounds 1a–d (Scheme 1), which were selected based on previous work on the ROP of rac-lactide (LA) and rac-BL. [4] All four catalytic systems were highly active for the controlled ROP of rac- MLA R at 20 8C in toluene, thus allowing full conversion of 100 equivalents of MLA R within 2–120 minutes (Table 1). These activities under mild reaction conditions with turnover frequencies (TOF) of at least 3000 h 1 using the 1a/iPrOH system (Table 1, entries 1 and 5) are quite remarkable. Although in line with those observed in the ROP of BL with similar systems, [4] these TOF values are the highest ones [*] C. G. Jaffredo, Dr. Y. Chapurina, Dr. S. M. Guillaume, Prof. Dr. J.-F. Carpentier Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, UniversitØ de Rennes 1 35042 Rennes (France) E-mail: sophie.guillaume@univ-rennes1.fr jean-francois.carpentier@univ-rennes1.fr [**] This research was financially supported in part by the Region Bretagne (Ph.D. grant to C.G.J.). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201310523. A ngewandte Chemi e 2725 Angew. Chem. 2014, 126, 2725 –2729  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim