Hexabutyl guanidinium acetate (HBG . OAc) was synthesized and successfully used as a catalyst for the ring-opening polymerization (ROP) of lactides. The experimental results indicated that the guanidinium salt HBG . OAc showed satisfactory catalytic behavior. Polymerization in bulk (120 degreesC, 18 h) produced polylactides with moderate molecular weights (number-average molecular weight = 2.0 X 10(4)) and very narrow molecular weight distributions (polydispersity index = 1.07-1.12). A kinetic study of polymerization in bulk with HBG . OAc as an initiator revealed that the polymerization possessed typical characteristics of living polymerization. A ROP mechanism by HBG . OAc was proposed on the basis of the additive effect of the polymerization and the H-1 NMR characterization of the microstructure of the product polymers. (C) 2004 Wiley Periodicals, Inc.