The ring-opening polymerization (ROP) of styrene oxide (SO) was carried out using 3-phenyl-1-propanol (PPA) as the initiator and a phosphazene base, 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phospho oranylidenamino]-2 Lambda(5),4 Lambda(5)-catenadi(phosphazene) (t-Bu-P(4)), as the catalyst at room temperature. The polymerization proceeded in a living manner, which was confirmed by the kinetic and chain extension experiments, to produce the poly(styrene oxide) (PSO) with a controlled molecular weight (S200-21 800 g mol(-1)) and narrow molecular weight distribution (<1.14). The (1)H NMR and MALDI-TOF MS measurements of the obtained PSO dearly indicated the presence of the PPA residue at the chain end. In addition, the t-Bu-P(4)-catalyzed ROP of SO with functional initiators, such as 4-vinylbenzyl alcohol, 5-hexen-1-ol, 6-azide-1-hexanol, and 3-hydroxymethyl-3-methyloxetane, successfully afforded the corresponding end-functionalized PSO with precise molecular control. The t-Bu-P(4)-catalyzed ROP of SO proceeded throup the beta- and alpha-scissions as the main and minor ring-opening manners on the basis of the microstructure of the PSOs analyzed by the (13)C NMR measurement, which was clarified in the model reactions corresponding to the initiation and propagation. For the thermal analysis of PSO, the glass transition temperature and 5% weight loss temperature were found to be 34 and 310 degrees C, respectively.