Polymerization of racemic alpha-methyl-beta-propiolactone (MPL) was carried out using Zn-, Sn-, La-, Y-. and Al-based organometallic catalysts. It was found that Zn(C2H5)(2)/H2O (1/0.6), ((C4H9)-C-n)(2)Sn- (OCH3)(2), Al((OC3H7)-C-i)(3), and (C2H5)(2)AlOC2H5 produced atactic poly(alpha-methyl-beta-propiolactone) (PMPL), whereas ((C4H9)-C-n)(3)SnOCH3 and Y((OC3H7)-C-i)(3) formed PMPL of low isotacticity with isotactic (i) diad fractions between 0.53 and 0.56. Since the Sn-based catalysts were syndioregulating for beta-methyl-beta-propiolactone (BL) polymerizations, placement of the lactone methyl substituent at the alpha- instead of the beta-position alters steric interactions between the incoming monomer and the Sn catalyst-chain end so that syndioaddition is no longer preferred. Isobutylaluminoxane (IBAO) produced a PMPL sample that can be separated by acetone into two fractions : an acetone-insoluble (AI) PMPL fraction of high isotacticity (i-diad fraction 0.85) and high molecular weight (M(n) = 662 000 g/mol) and an acetone-soluble (AS) PMPL fraction of low isotacticity (i-diad fraction 0.57) and low molecular weight (M(n) = 6740 g/mol). Chain stereosequence analysis at the triad level was performed by carbon (C-13) nuclear magnetic resonance at 75.4 MHz for PMPL products from IBAO catalysis. Excellent agreement was found between the experimental results and those calculated by the enantiomorphic-site model, suggesting that the configuration of the adding monomer was primarily controlled by the configuration of the catalytic sites. The AI fraction was highly crystalline based on DSC analysis (peak melting temperature 123 degrees C, enthalpy of fusion 16.3 cal/g). Methylaluminoxane (MAO) gave distinctly different results than IBAO-catalyzed MPL polymerization. Specifically, the number or activity of catalytic sites that lead to highly isotactic PMPL was less for MAO than for IBAO. For the polymerization of both BL and MPL, IBAO catalysts appear to function similarly so that at least two types of catalytic sites are present which produce a highly isotactic fraction with high molecular weight and a low isotacticity fraction with low molecular weight.