A new catalyst system for the alternating copolymerization of propylene oxide and carbon monoxide to form aliphatic polyesters is developed. The reaction of propylene oxide and carbon monoxide using CO2(CO)(8)/A(6,7-dihydro-5,8-dimethyldibenzo[bj]-1,10-phenanthroline)/PhCH2Br as a catalyst, afforded atactic PHB in high molecular weight (M-n = 19400, M-w/M-n = 1.63) and 55% yield at 80 degreesC under 900 psi of carbon monoxide in benzene.