Cationic coordinated polymerizations steric effects

The titanium trichloride-diethylaluminum chloride catalyst converted butadiene to the cis-, trans,-trans-cyclododecatriene. Professor Wilke and co-workers found that the particular structure is influenced by coordination during cyclization between the transition metal and the growing diene molecules. Analysis of the influence of the ionicity of the catalyst shows effects on the oxidation and reduction of the alkyls and on the steric control in the polymerization. The lower valence of titanium is oxidized by one butadiene molecule to produce only a cis-butadienyl-titanium. Then the cationic chain propagation adds two trans-butadienyl units until the stereochemistry of the cis, trans, trans structure facilitates coupling on the dialkyl of the titanium and regeneration of the reduced state of titanium (Equation 14). 

Substituent Effects So reactive are the coordinatively unsaturated metallocenium cations that one would expect that introduction of substituents into the cyclopenta-dienyl rings would modulate catalytic activity. (11) If substituent effects could be understood, it should be possible to modify the catalyst system to produce polymers having predetermined properties. We suspect that the needed detailed understanding will be hard to achieve for two reasons. First, the polymerization involves a multi-step mechanism and substituents may affect different steps differently. Second, steric and electronic effects of substituents may act in concert or in opposition and they must therefore be disentangled. Table 1 shows the effect of structural changes... 

Reaction of a-diimines with vinyl acetate (VAc) unfortunately does not result in polymerization reactions. The low coordination ability to a cationic Pd complex for this functional olefin is due to an inaeased sterical hindrance. The, compared to ethene, higha n(C=C) frontier orbital (HOMO) energy should promote a n-coordination to the metal. Unfortunately, effective a-coordination via the oxygen atom is also possible and is assumed to be a severe restriction to the incorporation. Furthermore, the insertion rate of VAc into metal-alkyl bonds is reduced compared to ethene and can occur either in a... 

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