In this work, ring opening insertion polymerization (ROIP) of epsilon-caprolactone (epsilon-CL) using a series of hydrogen phosphonates (H-phosphonates) as initiators was investigated. The ROIP occurred by a coordination-insertion mechanism containing two steps. First, the carbonyl carbon was attacked by the phosphorus atom of the H-phosphonate tautomerization (a phosphine-like structure) and the acyl-oxygen bond was broken. An intermediate was formed by the coordination of the former carbonyl carbon and acyl-oxygen of epsilon-CL to phosphorus atom. Then the phosphorus-alkoxide of H-phosphonate was cleavaged to form acyl-alkoxide bond. Poly(epsilon-caprolactone) (PCL)-inserted H-phospbonates (PCL-HPs), which was not only the product of the occurred ROIP but also the initiator for the next ROIP, were produced. After 60 min of microwave irradiation (510 W), PCL with a number-average molar mass of 7800 g/mol and monomer conversion over 92% was obtained. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Cbem 47: 6214-6222, 2009