Uniform-size polystyrenes (PSs) having a diol end-group 2 were synthesized through the reaction of a living PS with a chlorosilane derivative having a trimethylsilyl-protected diol function, i.e. 3-chlorodimethylpropoxy-1,2-bis(trimethylsiloxy)propane 1, and the subsequent deprotection by precipitation into an acidic methanol solution. The polyaddition reaction of 2 with 4,4’-diphenylmethanediisocyanate followed by the chain extension with 1,4-butanediol produced polyurethane-PS graft copolymers 3 with a controlled graft segment length and graft content.