Provided is a method of converting an anionic living end to a protected free radical "living" end in a well-controlled manner, which enables copolymerization by plural mechanisms. The method is particularly useful for copolymerization of anionically polymerizable monomers and free radical polymerizable monomers, and therefore useful for production of PB-g-PS, HIPS, ABS, and MBS etc. Also provided is a reversibly coupled polymer, which, when decoupled, generates "living" free radical for further copolymer chain reaction.