Ring-opening polymerization (ROP) of e-caprolactone and L-lactide (LA) was studied using stannous(II) 2-ethylhexanoate (Sn(Oct)2) with N,N-dimethylformamide-dimethyl acetal (DMF-DMA). DMF-DMA showed a tenfold improvement in catalytic activity over that of Sn(Oct)2 under the same conditions. It also enhanced the capability to control molecular weight in the synthesis of small molecular weight polymers of polycaprolactone and polylactide (PLA). The high molecular weight polymerization demonstrated a strong capability to control molecular weight for the polymerization of LA: a molecular weight of PLA exceeding 400,000 was obtained at very low catalytic loadings. The individual polymerization rates of other tin reagents with DMF-DMA also clearly increased. Applying this methodology could drastically reduce the time and cost required for the fabrication of these products to increase the competitive advantage of manufacturers. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012