Organocatalytic Cationic Ring-Opening Polymerization of a Cyclic Hemiacetal Ester

Abstract

We report the bulk cationic ring-opening polymerization of renewably sourced 2-methyl-1,3-dioxan-4-one (MDO) to yield a polyester with hydrolytically and thermally sensitive linkages that facilitate degradation. Neat monomer was successfully polymerized using a variety of protic acids as catalysts. We discovered that, with these catalysts, the cationic polymerization of MDO proceeds via two distinct mechanistic routes, namely, the activated monomer (AM) and active chain-end (ACE) mechanisms. The kinetics of these competing mechanistic avenues were investigated by employing diphenylphosphoric acid (DPP) with or without an alcohol initiator. Without an exogenous initiator, the polymerization propagates via a dioxacarbenium ion that rapidly adds more MDO to produce high-molar-mass poly(2-methyl-1,3-dioxan-4-one) (PMDO). However, we found no clear relationship between [MDO]0/[protic acid]0 and resultant molar mass, suggesting that the ACE mechanism is not well-controlled. This conclusion was further supported...