Cationic polymerization in which a cyclic monomer yields a monomeric unit that is either acyclic or contains fewer rings than the cyclic monomer.
Examples: Monomers used in cationic ring-opening polymerization are cyclic ethers, lactones and lactams.
Source:
PAC, 2022, 94, 1093. 'Terminology for chain polymerization (IUPAC Recommendations 2021)' on page 1130 (https://doi.org/10.1515/pac-2020-1211)