Polymerization reactions are generally divided into two broad classes: step growth or polycondensation reactions (examples would include the synthesis of polyamides and polyesters), and chain growth processes such as those used to prepare polyethylene or polystyrene. These processes are illustrated schematically in Figure 1.
The statistical nature of step and chain growth polymerization processes ensures that the products of such reactions must be heterogeneous. Conventional polymeric materials thus consist of mixtures of chains, often characterized by relatively broad distributions of chain length or composition. In many materials applications, this kind of molecular heterogeneity is advantageous since it suppresses crystallization and helps to preserve desirable properties such as optical clarity or elasticity. On the other hand, synthetic developments that afford improved control of macromolecular architecture have had profound impact on materials science and technology. As examples, one can cite the discovery of Ziegler-Natta polymerization, now used to prepare billions of pounds per year of crystalline polyolefins, or the development of living anionic polymerization of olefins, which led directly to block copolymers and the commercially important thermoplastic elastomers.
The advent of recombinant DNA methods has provided a basis for developing polymeric materials characterized by essentially absolute uniformity of chain length, sequence, and stereochemistry. This article outlines the principles governing the cloning and expression of artificial genes, and examines the potential role of artificial proteins in polymer materials science.