Asymmetric, regio- and stereo-selective alternating copolymerization of CO2 and propylene oxide catalyzed by chiral chromium Salan complexes

Chiral chromium complexes of tetradentate N,N'-disubstituted bis(amino-phenoxide) (designated as Salan, a saturated version of Schiff-base Salen ligand) in conjunction with an ionic quaternary ammonium salt can efficiently catalyze the copolymerization of CO2 with racemic propylene oxide (rac-PO) at mild conditions to selectively afford completely alternating poly(propylene carbonate) (PPC) with similar to 95% head-to-tail linkages and moderate enantioselectivity. These new catalyst systems predominantly exceed the previously much-studied SalenCr(III) systems in catalytic activity, polymer enantioselectivity, and stereochemistry control. The chiral diamine backbone, sterically hindered substitute groups on the aromatic rings, and the presence of sp(3)-hydridized amino donors and its N,N'-disubstituted groups in chiral SalanCr(III) complexes all play significant roles in controlling polymer stereochemistry and enantioselectivity. Furthermore, a relationship between polycarbonate enantioselectivity and its head-to-tail linkages in relation to regioselective ring-opening of the epoxide was also discussed on the basis of stereochemical studies of PPCs derived from the copolymerization of CO2 with chiral PO at various conditions. (C) 2008 Wiley Periodicals, lnc.