A new efficient dianionic initiator dipotassium diphenylsilanediolate (Ph2Si(OK)(2)) was synthesized by reacting diphenylsilanediol with potassium in the mixture of THF and benzene. The anionic ring-opening polymerization of octamethylcyclotetrasiloxane (D-4) or tetramethyltetravinylcyclotetrasiloxane (D-4(Vi)) initiated by Ph2Si(OK)(2) was systematically investigated. A number of factors including the nature of initiators, promoters, and monomers, the molar ratio of promoter to initiator (C-p/C-i ratio) and monomer to initiator (C-p/C-i ratio) affect the polymerization remarkably. Using N-methyl-2-pyrrolidinone as the promoter, a series of well-defined difunctional H-Si-terminated polydimethylsiloxanes (PDMSs) were prepared with high monomer conversion (> 90%) and relatively low polydispersity (< 1.6) at 30 degrees C. By hydrosilation of the synthesized PDMS with ally end-capped poly(ethylene oxide) (PEO), amphiphilic PEO-b-PDMS-b-PEO triblock copolymers with low polydispersities and high block purities were obtained. (C) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 153-159, 2007