An improved process for free radical polymerization is produced making it possible to control the growth steps of a polymerization to produce homopolymers and copolymers, including block and graft copolymers. The process uses a long half-life radical initiators and compounds which have the structure of formula I wherein X is a group having at least one carbon atom and is such that the free radical X. is capable of polymerizing the unsaturated monomer by free radical polymerization, and the radical functionality resides on the or one of the carbon atoms, R1, R2, R5 and R6 represent the same or different straight chain or branched substituted or unsubstituted alkyl groups of a chain length sufficient to provide steric hindrance and weakening of the O—X bond, and R3 and R4 represent the same or different, straight chain or branched, substituted alkyl groups or R3CNCR4 may be part of a cyclic structure which may have fused with it another saturated or aromatic ring, the cyclic structure or aromatic ring being optionally substituted.