Carbonylative polymerization of oxetanes initiated by acetyl cobalt complexes

Four acetyl cobalt complexes, [AcCo(CO)(3)P(p-tolyl)(3)] (1; p-tolyl = 4-Me-C6H4), [AcCo(CO)(3)P(OPh)(3)] (2), [AcCo(CO)(3)P(NMe2)(3)] (3), and [AcCo(CO)(2)(dppp)] (4; dppp=1,3-bis(diphenylphosphanyl)propane), were synthesized, characterized, and examined as catalysts for the unprecedented carbonylative polymerization of oxetanes. Copolymers containing ester (4-hydroxyalkanoate) and/or ether units were obtained with complexes 1 and 2, but not with complexes 3 and 4 either in the presence or absence of additional phosphorus ligands. The ester unit/ether unit ratio varied in the range 21:79-63:37, and the highest ester/ether ratio of 63:37 was achieved by using complex 1 in the presence of a further 5 equivalents of P(OPh)(3). Although direct carbonylative polymerization is possible, preformation and ring opening of the gamma-lactone is also suggested as an alternative pathway.