Genetic Engineering of Polymeric Materials
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Engineering of Polymeric Materials
David A. Tirrell, Maurille J. Fournier, and Thomas L. Mason Introduction 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 g r o w t h processes such as those used to prépare polyethylene or polystyrène. Thèse processes are illustrated schematically in Figure 1. T h e statistical n a t u r e of step a n d chain g r o w t h polymerization processes ensures that the products of such reactions must be heterogeneous. Conventional polymeric materials thus consist of m i x t u r e s 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 préserve désirable properties such as optical clarity or e l a s t i c i t y . O n t h e o t h e r h a n d , s y n t h e t i c d e v e l o p m e n t s that afford improved control of macromolecular architecture hâve had profound impact on materials science and technology. As examples, one can cite the discovery of Ziegler-Natta polymerization, 1 now used to prépare billions of pounds per y e a r of c r y s t a l l i n e polyolefins, or the development of living anionic polymerization of olefins, 2 which led directly to block copolymers and the commercially important thermoplastic elastomers. T h e a d v e n t of r e c o m b i n a n t DNA methods has provided a basis for developing polymeric materials characterized by essentially absolute uniformity of chain length, séquence, and stereoc h e m i s t r y . This article o u t l i n e s the principles governing the cloning and expression of artificial gènes, and ex-
MRS BULLETIN/JULY1991
amines the potential rôle of artificial proteins in polymer materials science. Principles Figure 2 outlines the essential steps in applying recombinant DNA methods to the préparation of new proteinlike polymers. The primary amino acid séquence of the polymer of interest is first encoded into a complementary séquence of DNA, which is constructed via solid-phase organic synthesis and enzymatic ligation. 3 Insertion of the synthetic gène into an appropriate expression vector is then followed by transformation of a host organism in w h i c h p r o t e i n e x p r e s s i o n is a n t i c i pated. In bacterial hosts, which remain the most widely used, f e r m e n t a t i o n and product isolation complète the process. Synthesis of materials in multigram quantifies by this route is quite
Genetic Instability Bacterial cells and E. coli in particular p o s s e s s m u l t i p l e p a t h w a y s by which genetic information can be rearranged and in some instances deleted. 4 Because foreign DNA imposes a metabolic b u r d e n on t h e cell, g e n e r a l l y w i t h o u t conferring any compétitive g r o w t h advantage, loss of such DNA from d i v i d i n g cell p o p u l a t i o n s is always a conçern. This probl
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