The polymerization of propylene oxide to obtain a high-molecular-weight polymer with an atactic structure required for the application as artificial blood vessels was investigated using combinations of organozinc and organotin compounds as catalyst. The composition of the most active catalyst, resulting from the reaction of diphenyltin sulfide with bis(3-dimethyl-aminopropyl)zinc, was found to be R(C6H5)2Sn(SZn)2R with R = (CH2)3N(CH3)2. Using this catalyst, an anionic coordination polymerization was observed with neither stereoselectivity nor living type or cationic features. At low catalyst concentration (0,03 mol-% Zn) a high-molecular-weight poly(propylene oxide) (PPOX) was obtained in 80-90% yield ([bar M ]w = 500000; 40% isotactic). Lowering of the catalyst concentration and increasing the polymerization temperature changed the kinetics and the stereochemistry of the polymerization leading to polymers of lower molecular weight and to a decrease in the isotactic PPOX fraction to 20%, probably due to an association of the catalytic species.