The ring-opening polymerization of a monomer containing a free carboxylic acid group is reported for the first time. The monomer, 5-methyl-5-carboxyl-1,3-dioxan-2-one (MCC), was copolymerized with trimethylene carbonate (TMC) in an enzymatic ring-opening polymerization conducted in bulk at 80 degreesC. The low-melting TMC comonomer also solubilized the high-melting MCC monomer, allowing for solvent-free polymerizations. Six commercially available lipases were screened, and Candida antarctica lipase-B (Novozym-435) and Pseudomonas cepacia lipase were selected to catalyze the copolymerization because of their higher monomer conversions. Higher molecular weight polymers (weight-average molecular weight = 7800-9200) were prepared when Novozym-435 was used, with less MCC incorporated into the copolymer than used in the monomer feed. However, Pseudomonas cepacia lipase showed good agreement between the molar feed ratios and the molar composition, but the molecular weights (weight-average molecular weight = 3600-4800) were lower than those obtained when Novozym-435 was used. C-13 NMR spectral data were used for microstructural analysis, which suggested the formation of random, linear, and pendant carboxylic acid groups containing polycarbonates with hydroxyl groups at both chain ends.