Polysiloxane diblock copolymers containing a pure polysiloxane backbone were prepared by the functionalization of poly(dimethylsiloxane)-b-poly(methylvinylsiloxane) copolymers. The copolymers were obtained by the sequential anionic copolymerization of either 1, 3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane or 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane with hexamethylcyclotrisiloxane. The two vinyl monomers showed large differences in the propagation rates, but both could be used for the formation of polysiloxane block copolymers. Differences in the polymerization sequences were investigated and revealed that better control was obtained if the slower propagating monomer was polymerized first. The method permitted the synthesis of block copolymers with molecular weight distributions around 1.4 and lower and high block purities. The vinyl groups of the block copolymers were quantitatively and selectively functionalized by hydrosilation or epoxidation reactions.