Water-soluble polycarbonate having pendent carboxyl groups on the main-chain carbons is reported for the first time. This paper describes synthesis and enzyme-catalyzed ring-opening polymerization of a novel carbonate monomer, 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (1). Various commercially available lipases were screened for their ability to polymerize 1 in bulk at 80 degrees C. Monomer conversion and molecular weight of the polymer were significantly influenced by the source of the enzyme. For example, of the seven Lipases screened, lipase AK (from Pseudomonas fluorescens) gave the highest monomer conversion (97%) and molecular weight (M-n = 6100). In reactions carried out under identical experimental conditions, no polymerization was observed when thermally deactivated lipase AK was used. Debenzylation of 2 by catalytic hydrogenation led to the corresponding linear polycarbonate, 3, with pendent carboxyl groups. Presence of pendent carboxyl groups is expected to enhance the biodegradability of the polycarbonate and facilitate a variety of potential biomedical applications, e.g., as polymeric drug carriers in time-controlled drug delivery systems.