Stereospecific polymerizations stereoselectivity mechanism

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... 

The examination of models for stereospecific Ziegler-Natta polymerizations indicate the importance of the nonbonded interactions for the stereoselectivity mechanisms. 

A necessary (but not sufficient) prerequisite for models of catalysts for the stereospecific polymerization of 1-olefins polymerization, is the stereoselectivity of each monomer insertion step. The possible origin of stereoselectivity in this class of systems was investigated through simple molecular mechanics calculations [11, 14, 24, 32, 52, 78-80, 82-86]. 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

The mechanism of stereospecific polymerization of 1,3-dienes is also categorized as an insertion polymerization and simplified representations of the stereoselectivity for cis (Eq. (2.88)) and irons (Eq. (2.89)) enchainments are shown below (Zohuri et al., 2012). 

Needless to say, vinyl polymerization is one of the most important methods for polymer synthesis. A variety of carbon-carbon (C-C) main chain polymers have been prepared by the vinyl polymerization of monomers with diverse substituents, via radical, cationic, anionic, or coordination mechanism. Furthermore, with the technological achievement such as living and stereoselective (or stereospecific) polymerizations, fine-tuning of the polymer structure with respect to molecular weight and tacticity has been realized in a number of examples. In particular, polymers obtained with vinyl polymerization (vinyl polymer) as represented by polyethylene, polypropylene, polystyrene, and poly(methyl methacrylate) have contributed to the progress of modern society in various aspects as useful synthetic materials. 

Abstract Metallocene complexes that serve as stereoselective olefin polymerization catalysts are described. The polymerization of propylene, styrene, methyl methacrylate, 1,3-dienes, non-conjugated dienes and cycloolefins is discussed. The stereochemistry of monomer insertion is governed by the chiral steric environment of catalysts derived from a ligand structure (catalytic-site control) or a chiral center in the polymer chain (chain-end control). The mechanism of formation of isotactic and syndiotactic polymers in each monomer and catalyst is explained. Non-metallocene catalysts for stereospecific polymerization are also mentioned. 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

It can be concluded that studies of a subtle elements of propagation, particularly in the coordinated processes, are still needed. Perhaps the advanced quantum mechanical calculations methods would be very helpful for this purpose. The p-substituted p-lactones are still waiting for the stereospecific initiator allowing the stereoelective and/or stereoselective polymerizations to be carried out. Finally, initiators of the stereocontrolled polymerization operating in bulk, high-temperature processes are also wanted. 

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