Styrene oxide (SO) was polymerized with a diethylzinc/alpha-pinene oxide (ZnEt2/alpha-PiO) catalyst system under various conditions. Polystyrene oxide (PSO) thus obtained had a regular head-to-tail and isotactic structure. The number-average molecular weight reached 4.07 x 10(4), and the molecular weight distribution was 5.7 (M-w/M-n). The glass-transition temperature of PSO was about 47 to 50 degrees C, depending on the molecular weight. The molar ratio of ZnEt2 to alpha-PiO, 2 : 1, led to a high molecular weight of PSO in an 89.2% yield within 72 h.