Monomer-starvation

Under the conditions of a high monomer concentration, kinetic trapping of the two different coordination sites can reasonably be expected to be competitive with site epimerization and, therefore, conversely, the dynamic process of site epimerization would be expected to become more dominant under conditions of monomer starvation. 

Storey et al. [140] point out that in living polymerizations of monomers like isobutylene that are co-initiated by TiCLt temperatures as low as —80°C, the livingness is limited not by chain transfer to monomer but rather by a unimolecular termination process. Unimolecular terminations often involve p-proton expulsions to produce polymers with terminal unsaturation. They claim, however, that this does not happen here. Rather the normal ferf-chloride chain ends of polyisobutylene formed by this type of polymerization gradually become depleted. They propose, therefore, that an isomerization mechanism takes place instead in the presence of an active Lewis acid, tmder monomer starvation conditions. It can be illustrated as follows [140] ... 

For both benchmark catalysts C2-I-I and C2-I-IH4, isotacticity (% mmmni) decreases substantially, e.g. from 87 to 54.7% by decreasing the monomer concentration from 11 to 0.4 mol/L in the case of C2-I-I. Under catalyst starvation conditions, that is [M] 0, the resulting oligomers were fully atacticP ... 

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