2,5-Diketopiperazines (DKPs) are the smallest cyclic dipeptides found in nature with various attractive properties. In this study, we have demonstrated the successful modification of proline-based DKPs using anionic ring-opening polymerization (AROP) as a direct approach. Four different proline-based DKPs with various side chains and increasing steric hindrance were used as initiating species for the polymerization of 1,2-epoxybutane or ethoxyethyl glycidyl ether in the presence of t-BuP4 phosphazene base. The addition of a Lewis acid, tri-isobutyl aluminum, to the reaction mixture strongly decreased the occurrence of side reactions. Impact of the DKP side-chain functionalities on molar mass control and dispersity was successfully evidenced. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1008-1016