Alternating Copolymerization of Propylene Oxide with Cyclic Anhydrides Catalyzed by Zinc Glutarate (ZnGA)/CaH2

Abstract

Ring-opening alternating copolymerization (ROAC) represents a promising methodology for polyester synthesis, which faces the critical challenge of developing efficient catalytic systems to expand substrate applicability and enhance organocatalytic polymerization performance. In this investigation, a zinc glutarate (ZnGA)/CaH₂ complex was implemented as an innovative Lewis acid-base pair catalyst for the ROAC of propylene oxide (PO) with phthalic anhydride (PA) and succinic anhydride (SA). Systematic examinations were conducted regarding the influences of reaction modalities, Lewis base classifications and proportions, and reactant stoichiometries on catalytic efficiency and selectivity. The successful synthesis of fully alternating poly(PO-alt-PA) and poly(PO-alt-SA) architectures was unequivocally verified through ¹H NMR and ¹³C NMR analyses, demonstrating a 47.5% reduction in polyether content relative to conventional ZnGA-monocatalyzed systems. The ZnGA/CaH₂ catalytic system retains competitive advantages under optimized conditions (75°C, achieving 100% yield, TOF = 430 h^–1, and > 99% ester linkage selectivity). Mechanistic insights into chain initiation and propagation processes suggest that within the ZnGA/CaH₂ coordination complex system, the copolymerization preferentially generates highly active pre-coordinated zinc alkoxide (–Zn–OR) intermediates containing ionic reactive species, thereby facilitating PA incorporation and augmenting catalytic selectivity.