Provided is a method comprising reacting the anionic living end of a polymer with a compound selected from the group consisting of tin halides and silicon halides to produced a polymer with a homolytically cleavable group containing a Sn—C bond or a Si—C bond. An engineered plastic may then be produced by adding the polymer containing a homolytically cleavable group containing a Sn—C bond or a Si—C bond to a solvent comprising at least one vinyl monomer; (b) optionally adding at least one additional inert solvent; (c) optionally adding additives selected from the group consisting of extender oils, modifiers, and antioxidants; and (d) initiating polymerization of the at least one vinyl monomer by the use of an initiator and/or heat, wherein a free radical is produced from the polymer containing a homolytically cleavable group containing a Sn—C bond or a Si—C bond.