Stereospecific chain polymerization

The versatility of polymerization resides not only in the different types of polymerization reactions and types of reactants that can be polymerized, but also in variations allowed by step-growth synthesis, copolymerization, and stereospecific polymerization. Chain polymerization is the most important kind of copolymerization process and is considered separately in Chapter 7, while Chapter 9 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of polymerization conditions, including the more recent metallocene-based Ziegler-Natta systems. Synthetic approaches to starburst and hyperbranched polymers which promise to open up new applications in the future are considered in an earlier chapter dealing with step-growth polymerization. 

A chain polymerization can be radicalar, anionic, cationic, or stereospecific according to the type of initiator used. The reaction may occur in bulk, in suspension, in emulsion, as in solution. It is necessary to choose adequate temperatures for a good control of the processes. 

Tsuruta T, Kawakami Y. Anionic ring-opening polymerization stereospecificity for epoxides, episulfldes and lactones. In Eastmond GC, Ledwith A, Russo S, Sigwalt P, editors. Comprehensive Polymer Science. Chain polymerization. Volume 3. Oxford Pergamon Press 1989, Part I. p 489. 

Fig. 8. 1,6-Anhydro ring opening polymerization Stereospecific activated chain...

IX. Stereospecific Chain Polymerization and Copolymerization by Coordination Catalysts... 

IX. STEREOSPECIFIC CHAIN POLYMERIZATION AND COPOLYMERIZATION BY COORDINATION CATALYSTS... 

As could be seen in this short overview, the PVC field exhibits considerable complexity. The most important factor that controls chain branching, stereospecifity, and crystallinity is the polymerization temperature. 

The most efficient enantioface discriminating agents seem to be transition metal complexes covalently bound to the growing chain end, which are also able to achieve a very high regio-selectivity in the attack to the double bond. Unfortunately, the type of monomers which are polymerized stereospecifically with this type of catalysts are mainly unsaturated hydrocarbons. Propylene (14) and butadiene (46) can be polymerized by the above catalysts both to isotactic and syndiotactic polymers. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

The structure of the chain, i.e., whether it is a helix or a random coil, might influence not only the rate but also the stereospecificity of the growing polymer. For example, it is plausible to expect that in normal vinyl polymerization helix formation might favor specific placement, say isotactic, while either placement would be approximately equally probable in a growing random coil. Formation of a helix requires interaction between polymer segments, and this intramolecular interaction is enhanced by bad solvents particularly those which precipitate the polymer. 

The polymerase is stereospecific. It accepts only the D-(-)-stereoisomer which is generally formed by the NADPH-linked reductase. With respect to chain length of the activated fatty acids the specificity of the polymerase varies in different organisms. It links not only C4 3-acyl moieties but also C5 compounds when forming the polyester molecule [26]. It also polymerizes 3-hy-droxy-, 4-hydroxy-, and 5-hydroxyalkanoates from C3 to C5 monomers, but not C6 or higher (e.g., in R. eutropha) [27-31]. In pseudomonads, in contrast, it links C6 to C14 3-hydroxyalkanoyl-CoA [32]. 

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