A synthetic route is provided to prepare poly(.omega.-hydroxycarboxylic acid) polymers via the ring-opening polymerization of .omega.-hydroxycarboxylic acid cyclic oligomers. The cyclic oligomers can be prepared directly from biorenewable sources, such as 3-hydroxypropionic acid. The method can be used to prepare high molecular weight polymers from the cyclic oligomers. Good molecular weight control can be obtained for both solution state and melt polymerizations.