Dinuclear salen compounds of zirconium and hafnium are efficient initiators for the solvent-free ring-opening polymerization of cyclic ester monomers and lactides. There is a correlation between the theoretical and experimental number-average molecular weights (M-n's) in these polymerizations. Polymerization of beta-butyrolactone gives poly(3-hydroxybutyrate) with a good M-n and molecular weight distribution.