Polycaprolactone (PCL) grafting on vinyl alcoliol-co-vinyl acetate) (PVA-Ac), was investigated in the melt at high temperature (170 degrees C), below the PVA-Ac melting point, by ring opening polymerization of epsilon-caprolactone initiated by metal alkoxyde sites present in PVA-Ac: no additional initiator was used. The obtained average structures were determined by H-1 NMR. As expected, small grafts, with low average polymerization degree (DP), were obtained, between 4 and 12 h of reaction. These DP are due to exchange reactions between hydroxyl groups and PCL growing chains. The PVA-Ac was shown to be partially substituted by short PCL grafts. The DP linearly increased with the initial Lactone/PVA-Ac ratio, and the substituted alcohol sites rate were limited to 63%. It was shown that the used reactive system is characterized by a quazi-living polymerization mechanism. (c) 2007 Wiley Periodicals, Inc.