Ring-opening polymerization of cyclic monomers by complexes derived from biocompatible metals. Production of poly(lactide), poly(trimethylene carbonate), and their copolymers
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Tridentate Schiff base calcium derivatives have been prepared and shown to be very effective catalysts for ring-opening polymerization of both lactides and trimethylene carbonate to produce high molecular weight polymers with narrow polydispersity. Optimization of the calcium catalyst was achieved changing the imine backbone, initiator, and substituent on the phenolate ring of the calcium complex. Turnover frequencies were obtained up to 1124 h-1 for a melt polymerization of L-lactide carried out at 110 C. Solution studies in CDCl3 demonstrated the polymerization reaction is first order in [monomer] and [catalyst], with k = 19.9 M-1 h-1 for lactide polymerization and k = 500.0 M-1 h-1 for trimethylene carbonate polymerization at ambient temperature. The activation parameters for the ring-opening polymerization of L-lactide using catalyst 1a were determined to be H = 73.5 3.8 kJ/mol and S = -42.5 12.6 J/(mol K), and catalyst 1a was shown to produce heterotactic polylactide from rac-lactide with a Pr value of 0.73. It was also found that the ring-opening polymerization of trimethylene carbonate is a lower energy process than that of lactide polymerization as revealed by its activation parameters, which were determined to be H = 37.9 3.1 kJ/mol and S = -135.1 11.4 J/(mol K). Furthermore, this class of catalysts afforded from biocompatible metals is very effective at producing diblock copolymer from trimethylene carbonate and lactides. Kinetic studies of the copolymerization of trimethylene carbonate and L-lactide are presented as a function of the feed ratio of the two cyclic monomers. 2008 American Chemical Society.