The polydimethyisiloxane with a very narrow molecular weight distribution and free of macrocyclics was synthesized by polymerization of hexamethyleyclotrisloxane (D3) in toluene with benzyltrimethylammonium bis (o-phenylenedioxy) phenylsiliconate as a catalyst and dlmethyl sulfoxide (DMSO) as a promoter．Oxygen must be excluded，otherwise it WOuld lower the reactivity of the catalyst．As oxygen is removed from the reaction system，the rate of the polymerization is obviously increased and D3 can be con-verted to polymer approximately to 100 percent．The polydlmethylsiloxane thus obtain-ed is a living polymer．Characteristics of the nonequilibrium polymerization of D3 in the absence of oxygen were investigated．