A copolymer [P(MMA-co-TBPM)] was prepared by the radical polymerization of methyl methacrylate (MMA) and 2,2,6,6-tetramethyl-4-benzyloxyl-piperidinyl methacrylate (TBPM) with azobisisobutyronitrile as an initiator. TBPM was a new monomer containing an activated ester. Both the copolymer and TBPM were characterized with NMR, IR, and gel permeation chromatography in detail. It was confirmed that P(MAU-co-TBPM) could initiate the graft polymerization of styrene by the cleavage of the activated ester of the TBPM segment. This process was controllable, and the molecular weight of the graft chain of polystyrene increased with the increment of conversion.