Highly (S)-enriched substituted poly(is an element of-caprolactone)s were synthesized from 4-methyl-is an element of-caprolactone (4-MeCL) and 4-ethyl-is an element of-caprolactone (4-EtCL) by lipase Novozym-435 (from Candida antarctica) catalyzed ring-opening polymerizations. The polymerizations were performed in bulk, thus eliminating the need for solvents in the polymerization process. Poly(4-EtCL) and poly(4-MeCL) having >95% enantiomeric purity (ee(p)) have been prepared. Number-average molecular weights of the poly(4-EtCL) and poly(4-MeCL) were 4400 and 5400, respectively. The effect of reaction temperature on enzyme enantioselectivity, polymer molecular weight, and monomer conversion was also investigated at 45 and 60 degreesC. Contrary to many literature reports and conventional wisdom, the enantioselectivity of the lipase was greater at 60 degreesC, the higher reaction temperature. The solventless polymerization process appeared to be diffusion-controlled in which the monomer conversion and polymer molecular weight increased at higher reaction temperature.