Chiral Amido-Oxazolinate Zinc Complexes for Asymmetric Alternating Copolymerization of CO 2 and Cyclohexene Oxide Srinivas Abbina and Guodong Du* Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United States * S Supporting Information ABSTRACT: The synthesis and characterization of a series of chiral zinc complexes (L 1a-m )ZnN(SiMe 3 ) 2 (2a-m) with C 1 -symmetric amido-oxazolinate ligands (HL 1a-m = 2-(2′-R 1 NH)phenyl-4-R 2 -oxazoline) have been described. Single-crystal X-ray crystallographic studies confirm that 2a (R 1 = 2,6- dimethylphenyl, R 2 =(S)- i Pr) and 2d (R 1 = 2,6-dimethylphenyl, R 2 =(R)- i Bu) are three-coordinate, mononuclear complexes, and 2k (R 1 = PhCO, R 2 =(S)- i Pr) exists as an amide oxygen-bridged dimer in the solid state with zinc in a distorted tetrahedral geometry. These complexes are viable initiators for alternating copolymerization of carbon dioxide (CO 2 ) and cyclohexene oxide (CHO), yielding poly(cyclohexene carbonate) (PCHC) with good to high carbonate linkage and moderate molecular weights and PDI values, depending on the substituents. The PCHCs produced are typically isotactic, containing up to 72% m-centered tetrads by 2h (R 1 = 2,6-diisopropylphenyl, R 2 =(R)- i Bu), and a rare case of syndiotactic PCHC (57% r-centered tetrads) is obtained with 2j (R 1 = (R)-1-(1-naphthyl)ethyl), R 2 =(R)- i Bu). The asymmetric induction is generally low, with up to 71% SS unit in the main chain of the produced PCHCs. ■ INTRODUCTION Utilization of carbon dioxide (CO 2 ) as a renewable C 1 feedstock has attracted increased interest due to its low cost, nontoxicity, and availability in nature and from many industrial processes. 1 Given its thermodynamic and kinetic stability, one approach is to couple CO 2 with high-energy, ring-strained heterocyclic molecules, leading to formation of alternating copolymers. 2 The most widely studied one is the alternating copolymerization of CO 2 with epoxides, in the presence of catalysts/cocatalysts. 3,4 The resultant aliphatic polycarbonates possess attractive properties, such as biodegradability and low oxygen permeability, and are regarded as promising new generation materials as alternatives to conventional petrochem- ical-derived polymers. Consequently, much effort has been devoted toward the development of efficient catalysts for alternating copolymerization of CO 2 with epoxides. Beginning with Inoue’s discovery of the ZnEt 2 /H 2 O catalyst in 1969, 5 a wide array of catalytic systems of metal complexes with various ligands, such as phenoxides, 6 salen and its derivatives, 3a,7 porphyrins, 8 and others, 9 have been explored to promote the transformation. In particular, zinc-β-diketiminate complexes (Chart 1, A) developed by Coates 10 have been prominent for the copolymerization of CO 2 with epoxides mainly due to their high catalytic activity and precise control over molecular weight and polydispersity. Furthermore, the system is consistent with a living polymerization process. 10,11 A number of strategies have been exploited to improve the thermal and mechanical properties of polycarbonates and to expand their applications. 3 These include copolymerization with epoxide momomers other than the commonly applied cyclohexene oxide (CHO) and propylene oxide (PO), 12 terpolymerization with two different epoxides or other types of monomers, 13 and polymer chain cross-linking. 14 Inspired by the success of chiral catalysts in asymmetric organic synthesis, another strategy is to use them to impart control over the absolute and relative stereochemistry of the resulting polycarbonates. The first examples of asymmetric copolymer- ization of CO 2 and CHO were reported by Nozaki, with a 1:1 mixture of (S)-diphenyl(pyrrolidin-2-yl)methanol and ZnEt 2 as Received: July 24, 2012 Published: October 26, 2012 Chart 1 Article pubs.acs.org/Organometallics © 2012 American Chemical Society 7394 dx.doi.org/10.1021/om3006992 | Organometallics 2012, 31, 7394-7403