Stereoselective polymerisation

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

Distinguish between stereospecific and stereoselective polymerisations. Give examples of each. 

As in stereoselectivity, the degree of stereoelection is reduced as the chiral centre is positioned further away from the C = C bond only racemic a-olefins with a chiral carbon atom in the a- or -position to the double bond have been stereoselectively polymerised. Note that, in general, stereoelectivity is lower than stereoselectivity. 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

When D, L-alanine A-carboxylic acid anhydride was subjected to polymerisation with triethylaluminium as the catalyst, the polymer formed, polyalanine, was found [168,174] to consist of two fractions a water-insoluble fraction which was rich in isotactic sequences and a water-soluble atactic fraction. This indicated that some stereoselective polymerisation had occurred [75]. 

Complex 18 polymerises propene to crystalline isotactic polypropene after activation with methylaluminoxane. X-ray crystallography shows that 18 has a C2 symmetry axis and that both coordination sites are equally shielded. The related complexes, bis l-(5-cholesten-3a-yl)indenyl zirconium dichloride and diraethylsilyl-his(2-methyl-4-r-butyl-cyclopentadienyl) zirconium dichloride, can also be activated by methylaluminoxane for the stereoselective polymerisation of propene. i . ... 

The lipase-catalysed ROP of the four-membered P-BL was first reported by Nobes and co-workers [74], Poly(3-hydroxybutyrate), with a weight average MW ranging from 256 to 1,045 Da were prepared after several weeks of polymerisation using approximately equal weights of P-BL and lipase. An enantioselective polymerisation of four-membered lactones was demonstrated. Racemic MPL was stereoselectively polymerised by PsCL to generate an optically active (S)-enriched polyester with... 

Metalloporphyrins as catalysts of chain transfer in radical polymerisation and stereoselective oxidation. L. Karmilova, G. V. Ponomarev, B. R. Smirnov and I. M. Bel yovskii, Russ. Chem. Rev. (Engl. Transl), 1984, 53,132 (44). 

In this chapter we will discuss a few topics in the area of alkene polymerisations catalysed by homogeneous complexes of early and late transition metals (ETM, LTM). One of the main research themes for the ETM catalysts has been the polymerisation of propene, while industries have also paid a lot of attention to metallocenes giving LLDPE (linear low-density polyethylene, for thinner plastic bags). In less than a decade a completely new family of catalysts has been developed which enables one to synthesise regioselective and stereoselective polymers of a wide variety of monomers. These new catalysts are starting to find application on industrial scale, but as yet only reports on commercialisation of (branched) polyethylene and polystyrene have appeared. 

Recently many subtle effects of the ligand structure, concentrations of alkene, and conditions on the polymerisation have been reported to have significant effects on molecular weight, regioselectivity, branching, stereoselectivity or enantioselectivity, incorporation of other monomers, chain transfer, etc. Often these subtle effects can be understood from the mechanism, or they contribute to the understanding of the detailed processes going on. 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

An analysis of polymer end groups provided insight into the mechanism of stereo-control in such catalysts. The first polymerisation step, where propene inserts into a Zr-Me bond, is in fact not stereoselective, while the insertion into a Zr-iso-butyl bond proceeds with high enantioselectivity. Ligand stereo-control operates therefore by an indirect mechanism the ligand determines the conformation of the polymery] chain, and this in turn influences the preferred orientation of the incoming alkene [127], as illustrated in structure 89 for a syndiospecific case. 

We refer the readers to a useful body of books and reviews in the bibliography which will prove helpful to investigators determining the mechanism of radical reactions. The early two-volume compendium edited by Kochi has much valuable information, even though 30 years old, and most modern texts on radicals provide excellent guidance to radical synthesis and mechanism. We shall not discuss stereochemistry explicitly which now forms an important part of the mechanisms of radical reactions except to note that excellent stereoselectivities can be obtained in radical reactions with a clear understanding of the mechanisms involved. Many concepts in radical polymerisations are equally applicable to small molecule reactions and we refer the reader to an excellent account on the subject by Moad and Solomon. 

From the scientific point of view, however, polymerisation of higher a-olefins is of considerable interest, since the relationship between the structure of the catalyst and that of the resulting polymers may be explained by considering the influence of the size of the alkyl group in the monomer. Moreover, polymerisation of chiral a-olefins allows study of stereoselection or stereoelection phenomena during polymerisation. 

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