Alternating Selective Copolymerization of Lactide and ε-Caprolactone with Dizinc Complexes of Modular, Binucleating...

Poly(lactic acid) is the most successful biodegradable synthetic polymer. Yet, its physical properties need to be improved to replace petrochemical polymers or 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 (PD^RH) gives neutral complexes PD^RZn₂X₃ (X^- = Cl^-, Br^-, I^-) with two distinct zinc sites. Anion metathesis gives μ-phenolate cationic complexes [PD^RZn₂X₂]⁺. The neutral iodide complexes PD^RZn₂I₃ polymerize lactide with moderate dispersity (Đ = 1.18-1.38). Monozinc analogs give much lower activity, implicating a cooperative polymerization. End-group analysis and active site interrogation are consistent with coordination insertion polymerization by a dialkoxide complex. 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 and ε-caprolactone showing the highest degree of alternation obtained with a main group catalyst. Mechanistic analysis implicates transesterification with unusual alternating selectivity. We also obtained copolymers with γ-butyrolactone, δ-valerolactone, and glycolide. This report also represents a rare use of bimetallic catalysts to obtain new selectivity in lactide polymerization.