Ring strain is one of the most important factors affecting the polymerizability of a cyclic monomer. In this work, the ring strains in gamma-butyrolactone, delta-valerolactone and epsilon-caprolactone were inferred on the basis of infrared and Raman spectroscopy data. The thermodynamic factors influencing the ability to polymerize of these three lactones and other cyclic diesters in the glycolide series were studied using molecular modelling methods. The ring strain was observed to increase with increasing ring size for the three lactones studied increasing the thermodynamic scope for these monomers to polymerize. For the glycolide series, ring strain and polymerizability decreased with increasing substitution on the alpha-carbon.