A new form of controlled growth free radical polymerization leading to narrow polydispersity polymers and/or block copolymers is described. The process is based on the polymerization of monomers in the presence of macromonomers of general structure CH2=C(Z)CH2(A)n [(A)n= radical leaving group, Z = activating group] and displays many of the characteristics of living polymerizations. The process is most suited to methacrylic monomers but with the appropriate choice of reaction conditions (high temperatures and/or low conversions) it can also be applied to acrylic and styrenic monomers. The macromonomers are conveniently prepared by catalytic chain transfer to alkyl cobalt(III) complexes or by addition-fragmentation chain transfer. The factors which determine the efficiency of cobalt complexes for molecular weight reduction in free radical emulsion and solution polymerization of methyl methacrylate are also discussed.