Stereoselective Copolymerization of Ring-Strained Pentacyclic Anhydride with Cyclohexene Oxide: Cocatalyst-to-Catalyst Ratio Dependence of Polymerization Mechanism

The stereospecific synthesis of well-defined polyesters with high glass-transition temperature (T g) is considerably challenging via ring-opening copolymerization (ROCOP) of bulky cyclic anhydride and epoxides under a binary metal complex/cocatalyst system. Meanwhile, a deeper understanding of the influence of the cocatalyst-to-metal complex ratio on copolymerization remains poor. An iron(III) aminotriphenolate complex (APFe)-mediated ROCOP of cyclohexene oxide with a pentacyclic anhydride derived from maleic anhydride and anthracene was reported in the presence of bis(triphenylphosphine)iminium chloride (PPNCl), selectively yielding cis-polyester with a T- g value of 228.4 C-degrees. The copolymerization rate and mechanism were highly dependent on the [PPNCl]/[APFe] molar ratio, where the copolymerization rate demonstrated a "V" shape and the polymerization mechanism switched from a cationic pathway to a coordination anionic approach. These variations are consistent with a similar trend of Lewis acidity in binary catalysts despite the turning point appearing at variant PPNCl-to-APFe ratios. Furthermore, the role of anhydride was explored as a chain transfer agent, which generated key active species during the copolymerization progress in the absence of a nucleophilic cocatalyst. This work presents a new viewpoint to understand the influence of cocatalyst-to-catalyst on the copolymerization behavior of the metal complex-based binary catalytic system.