Catalysts stereospecific chain polymerization

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

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

In principle, an asymmetric polymerization might be obtained either by asymmetric induction brought about by an asymmetric terminal group initiating the stereospecific chain growth, or by the steric control, done by the asymmetric catalyst, of the electrophilic or nucleophilic attack of the growing chain on the monomer, or by both factors. 

In our studies an attempt was made to obtain detailed information on the mechanism of stereospecific diene polymerization initiated by several catalyst systems. To this end the structure and some chemical reactions of compounds used as models of the system "catalyst-growing polymer chain were studied in detail. 

The different behavior of the catalysts apparently arises from the nature of the transition metal of the catalyst. It seems reasonable to treat the mechanism of stereospecific olefin polymerization in terms of coordination ionic catalysts, regarding the valence state, coordination number, and nature of ligands of the transition metal as a matter of primary importance. In such an approach the polymerization mechanism is based on the character of metal—carbon bond by which a growing polymer chain is linked to the transition metal. 

The stereospecific living polymerization of a trimethylsiloxane derivative of cyclohexanediol has been achieved using bis((rj3-allyl) trifluoroacetylacetonato-nickel(II)( as a catalyst. Although a pure para-linked polycyclohexadiene precursor is obtained, pyrolysis of the difunctional cyclohexadiene is accompanied by chain scission and formation of more or less structurally well-defined low molecular weight products. 

Finally a type of addition polymerization calling for the employment of stereospecific catalysts has recently aroused much interest in the laboratory as well as in industry. With the aid of such catalysts, an addition polymerization yields certain poljmers in which the main chain as well as tire chemical substituents are dtuated in a highly ordered spatial pattern. Polymers of physical and chemical characteristics are thus obtained which contrast markedly from those formed in normal polymerizations. 

Polymerization Ziegler-Natta, supported metal oxides such as PhiUps, Unipol and metallocene Stereospecific chain formation required weak interaction/complexation with any initiators/catalysts metal coordination complexes required... 

The ternary MoOCl -w-Bu Sn-EtOH catalyst induces living polymerization of not only 1-choro-l-alkynes but also PhA with bulky ortho substituents [42,43]. The presence of bulky ortho substituents (e.g., CF, SiMeg) is essential to achieve excellent living polymerization, which is probably because such substituents are able to sterically preclude chain transfer and termination. Stereospecific living polymerization of er -butylacetylene is possible with MoOCl -w-Bu Sn-EtOH, which gives a polymer with a narrow MWD [44]. The NMR spectrum of the formed polymer has shown that the cis content of main-chain double bond reaches 97% for poly(fert-butylacetylene) prepared at -30 °C. 

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. 

A large part of the stereospecific behavior of polymerization catalysts presented in this review can be rationalized in the framework of a stereoselectivity mechanism involving a chiral orientation of the growing chain. The discovery... 

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

---