Poly[2-(carboxymethylthio)ethyl]methylsiloxane-block-polydimethylsiloxane, 1, and poly{[2-(carboxymethylthio)ethyl]methylsiloxane-co-dimethylsiloxane}-block-polydimethylsiloxane, 2, were synthesized by ene-thiol addition of mercaptoacetic acid to respective precopolymers 3 and 4, containing vinyl groups bound to silicon. The precopolymers 3 and 4 were obtained by sequential anionic copolymerization of hexamethylcyclotrisiloxane, D-3, with 2,4,6-trimethyl-2,4,6-trivinyl-cyclotrisiloxane, 5, and D-3 with 2,4,4,6,6,-pentamethyl-2-vinylcyclotrisiloxane, 6, respectively. The method permits to synthesise the block copolymers with a narrow molecular weight distribution and a high block purity. The ene-thiol addition was performed on free radical route, using AIBN as initiator. The reaction proceeds selectively, leading to the transformation of vinyl groups in the precopolymers into the beta-adducts.