Enzymatic ring-opening copolymerization of 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MBC) with trimethylene carbonate (TMC) was investigated. A route to aliphatic polycarbonates decorated with pendent carboxylic acid groups is demonstrated. Lipase from Pseudomonas fluorescens (AK) was selected to perform the copolymerization at various monomers feed ratios. Copolymers with different composition were prepared by varying the monomer feed ratio from 10% to 80% MBC. H-1, C-13 and H-1-C-13 HETCOR NMR spectra were used to analyze the microstructure of the copolymers. The]H NMR spectra indicated lower incorporation of TMC in the copolymer than was expected from the monomer feed ratio. Analysis of the C-13 spectra did not indicate an ordered structure but instead suggested the formation of random polymers. This was further confirmed by the thermal data obtained for representative samples. The thermal properties at different feed ratios of poly[MBC-co-TMC] copolymers were investigated by differential scanning calorimetry (DSC). No melting temperature (T-m) for either homopolymers or copolymers was observed. A plot of 1/T-g(K) vs the weight composition of MBC in the copolymers was constructed and was consistent with copolymers that tend toward random distribution; this was confirmed from the NMR data.