The alternating copolymerization of propylene oxide and carbon monoxide was investigated with cobalt complexes. The NaCo(CO)(4)/amine catalyst system selectively yielded oligo(3-hydroxybutyrate)s bearing a polymerizable crotonate end group, whereas the use of Co-2(CO)(8) as a cobalt source resulted in a smaller concentration of the crotonate end group and a high degree of polymerization. The high selectivity for the oligoesters with the crotonate end group with the NaCo(CO)(4)/amine system was attributed to its more basic reaction condition. (C) 2004 Wiley Periodicals, Inc.