The problem of synthesizing macromolecules of accurately predetermined structure is an intriguing one; not only does such a synthesis provide a challenge to our chemical ingenuity but it also offers the possibility of providing a new range of materials with potentially valuable properties. To achieve a synthesis of this kind, the random features associated with conventional polymerization and copolymerization must be eliminated. Apart from gross structure defects such as chain branching and wrong-way addition, randomness in addition polymerization arises because the nature of the reaction between the propagating chain and the monomer is not uniquely defined. Thus, in homopolym-erization, addition may lead to either iso or syndiotactic placement, while in copolymerization further ambiguity is introduced by the possibility of reaction between a given propagating chain and any monomer species present; similar remarks also apply to conventional condensation copolymerization.