Stereospecific copolymerization

The 1970s saw the introduction of higher activity catalysts based on magnesium chloride-supported titanium that improved the control of the physical properties of the polyethylene—molecular weights, stereospecificity, and the degree of copolymerization. 

A somewhat different situation arises in the copolymerization of a racemic monomer with an optically active monomer of similar structure in the presence of a conventional stereospecific (or stereoselective) catalyst (299, 321). Examples concern the copolymerization of racemic 3,7-dimethyl-1-octene with (5)-3-methyl-l-pentene and of racemic jec-butyl vinyl ether with various optically active vinyl ethers. In all cases there was preferential copolymerization of one of the two enantiomers of the racemic monomer with the second monomer and simultaneous formation of an optically active homopolymer containing predominantly the noncopolymerized antipode, according to Scheme 20. The two products are easily separated, due to their different solubilities. 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

Another important use of BC13 is as a Friedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cydophosphazenes to polymers (81,82) polymerization of olefins such as ethylene (75,83—88) graft polymerization of vinyl chloride and isobutylene (89) stereospecific polymerization of propylene (90) copolymerization of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerization of norbomene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

A template polymer complex, which incorporates N-benzyl-D-valine with almost 100 % stereospecificity, has been synthesized by copolymerization of A-P2-[Co (R,R )-A, /V -bis(4-(vinylbenzyloxy)salicylidene]-l,2-diaminocyclohexane -(A-benzyl-D-valine)], styrene, and divinylbenzene, followed by dissociation of the coordinated amino acid 115). 

Hence, cation radical Diels-Alder copolymerization leads to the polymer of a lower molecular weight and lower polydispersity index than does cation radical polymerization-homocyclobutanation. Nevertheless, the copolymerization occurs under very mild conditions and is regio- and stereospecific (Bauld, Aplin, et al. 1998). This reaction appears to occur by a step-growth mechanism rather than by the more efficient cation radical chain mechanism proposed for the poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation radical Diels-Alder polymerization. ... 

Using Al(i-C4H9)3/TiCl4 catalyst for copolymerization of styrene with substituted styrenes, the reactivity ratios showed that cationic copolymerization occurred at Al/Ti< 1 and that stereospecific coordinated anionic copolymerization took place at Al/Ti > 2.5. 

Natta et al. [108] have studied the copolymerization of styrene with substituted styrenes on ZN catalysts. They came to the conclusion that monomer coordination to the electron-poorest part of the catalytic complex is one of the basic characteristics of this stereospecific copolymerization. The relation between log rf1 and SCp is typically cationic [104], This is a direct proof of Natta s ideas. This also means that the vinyl group of the monomer is coordinated to the catalytic complex by the P carbon and by its nearest neighbours. 

During recent years an increasing evidence has accumulated that discernible types of centers exist in Z—N catalysts, particularly in their heterogeneous versions. The centers may differ in their kp values, monomer coordination abilities, stereospecificities and reactivities in copolymerization. This concept can explain — at least qualitatively — wide MWD of polyolefins, composition heterogeneity of copolymers and specific responses of the catalyst performance to electron-donor additives. The origin of the differently behaving centers should be seen in a diversity of chemical processes... 

An excessively high temperature may diminish the chance of detecting some structural characteristics that can be obtained only from the study of the dimers, trimers, tetramers, etc. of the polymer. If only the monomer and small molecules are generated during pyrolysis, information on properties such as stereospecificity or copolymeric structure is lost. For this reason, Tgq values around 600° C may be more appropriate for certain studies using analytical pyrolysis. 

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. 

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