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 epsilon-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(epsilon-caprolactone) with M-n up to 140,000, for the first time over 10(5) in AMM. (C) 2019 Elsevier Inc. All rights reserved.