Anionic and Cationic Ring-Opening Polymerization of 2,2-Divinyl-4,4,6,6-tetramethylcyclotrisiloxane. Synthesis of Regular Poly(2,2-bis (perfluoroalkyl-1'H,1'H,2'H,2'H-dimethylsilylethyl)-4,4,6,6 -tetramethyltrisiloxanes) by Chemical Modification of Regular Poly(2,2-divinyl-4,4,6,6-tetramethyltrisiloxane)/