ε-Caprolactone: Activated monomer polymerization; controversy over the mechanism of polymerization catalyzed by phosphorus acids (diarylhydrogen phosphates). Do acids also act as initiators?

Abstract

Whenever Activated Monomer Mechanism (AMM) is involved in cationic ring-opening polymerization of cyclic esters, giving biodegradable polymers, catalysts and initiators are used. In some studies of AMM polymerization of ε-caprolactone (CL), without external initiator added, it was proposed that protonic catalysts are initiating themselves. We have shown by DOSY and other analytical methods for derivatives of phosphoric acid (claimed to act also as initiators) that these catalysts are not initiating polymerization and are separate from macromolecules. Traces of water are initiators. Understanding the role of catalysts in the bulk polymerization of CL allowed preparation of poly(ε-caprolactone) with Mn up to 140,000, for the first time over 105 in AMM.