High molecular weight poly(1-hydrido-1,3,3,5,5-pentamethyltrisiloxane) (II) with a highly regular microstructure has been prepared by dilithium diphenylsilanediolale-catalyzed ring-opening polymerization of 1,3,3,5,5-pentamethylcyclotrisiloxane. The microstructure of II was determined by Si-29 NMR. Chemical modification of II has been achieved. Quantitative hydrosilylation of II with benzophenone can be catalyzed by activated dihydridocarbonyltris(triphenylphosphine)ruthenium. Karstedt-catalyzed hydrosilylation of II with pentafluorostyrene has been achieved. The high molecular weight modified copolymers also have regular microstructures.