Alternating Selective Copolymerization of Lactide and (epsilon)-Caprolactone with Dizinc Complexes of Modular, Binucleating Bis(pyrazolyl)alkanes

Poly(lactic acid) is the most successful biodegradable synthetic polymer. Yet the physical properties of poly(lactic acid) need to be improved for it to replace petrochemical polymers or for it to serve emerging applications. Sequence-selective copolymerization of lactide with lactones can provide more versatile polyesters, but ideal alternating selectivity remains elusive. We report binucleated dizinc catalysts that show exceptional alternating selectivity in this reaction. Metalation of binucleating bis(pyrazolyl)alkanes (PDRH) gives neutral complexes PDRZn2X3 (X = Cl, Br, I) with two dis-tinct zinc sites. Anion metathesis gives cationic complexes [PDRZn2X2]+ with a C2-symmetric and (mu)-phenolate structure. The neutral iodide complexes PDRZn2I3 polymerize lactide with moderate dispersity (Ð = 1.181.38) in the presence of propylene oxide. Much lower activity was obtained with monozinc analogs, implicating a cooperative polymerization. End group analysis and active site interrogation are consistent with coordination insertion polymerization by a dialkoxide complex, generated by epoxide ring-opening. Based on model dizinc alkoxides, in situ NMR analysis, and DFT modeling, we provide a plausible active site structure and mechanistic rationale for cooperativity. We obtained copolymers of lactide with (epsilon)-caprolactone, by copolymerization or by modification of commercial poly(lactic acid), that show the highest degree of alternation obtained with a main group catalyst. Kinetic analysis indicates caprolactone incorporation by transesterification with unusual alternating selectivity. We also obtained copolymers with (gamma)-butyrolactone, (delta)-valerolactone and glycolide by this method. This report also represents a rare use of bimetallic catalysts to obtain new selectivity in lactide polymerization.