A new chain transfer agent, 5-tert-butylthio-1,3-pentadiene (TBPD or 7,7-dimmethl-6-thia-1,3-octadiene) was used in the free radical polymerization of methyl methacrylate and styrene to produce conjugated diene-end capped macromonomers by a free radical addition-fragmentation mechanism. The chain transfer was found to be degradative. A new kinetic model was proposed to describe the retarded polymerization. The kinetic parameters pertaining to transfer, reinitiation, primary radical termination, and mutual termination of the primary radicals were evaluated at different temperatures permitting precise theoretical prediction of the functionalities. The chain transfer constants, calculated using a modified Mayo’s equation revealed better transfer properties for MMA. The macromonomers were synthesized by high conversion polymerization. Characterizations of the macromonomers revealed that copolymerization predominated over the fragmentation following 1,4-addition, although the former reaction is not detrimental for the chain-end functionalization.