Poly(methyl methacrylate)-block-polysulfide-block-poly(methyl methacrylate) copolymers were synthesized for the first time through a new method involving the free radical polymerization of MMA in the presence of a thiocol oligomer as a chain transfer agent, followed by chemical oxidation of the remaining SH end-groups. The chain transfer constant of the SH end-groups of the thiocol was estimated from the rate of consumption of the thiol groups versus the rate of consumption of the monomer (C-T=0.67). The triblock copolymers synthesized were characterized by SEC and H-1 NMR measurements.