Methods are described for preparing graft copolymers from poly(vinyl chloride) or vinyl chloride copolymers comprising reacting these materials with sources of selected metal-centered free radicals in the presence of one or more monomers that can undergo free-radical addition polymerization. The metal-centered free radicals used are capable of abstracting chlorine atoms from the starting polymer to form C-centered radicals that add to the monomer(s) in order to start the growth of branches via a free-radical route. This method is an effective method for producing PVC graft copolymers, and can be used to produce PVC graft copolymers having novel compositions. The methods of the invention are particularly useful for providing highly branched PVC graft copolymers.