Chemical and Biomolecular Engineering Faculty Research


Living Carbocationic Polymerization Xiv: Living Polymerization of Isobutylene with Ester.Ticl4 Complexes

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The living polymerization of isobutylene (IB) has been induced by cumyl acetate·TiCl4 and cumyl propionate·TiCl4 complexes in CH2Cl2/n-C6H14 and CH3Cl/n-C6 H14 mixtures under conventional laboratory conditions at −40°C. The living nature of the polymerization was demonstrated by linear M̄n versus Wp (g PIB formed) plots starting at the origin. DPn obeys [IB]/[cumyl acetate·TiCl4] and molecular weight distributions (MWD) are very narrow, M̄w/M̄n = 1.05–1.13. PIBs having M̄n >40000 have been prepared. The effects of the nature of the solvent, solvent composition, nature of the ester, ester/TiCl4 ratio and concentrations on rates, M̄ns and MWDs have been investigated. The living polymerization is faster by the cumyl acetate-TiCl4 than by the cumyl propionate·TiCl4 complex. The rate of living polymerizations is negligible at CuAc/TiCl4 = 1 and increases significantly in the presence of stoichiometric excess of TiCl4 over the ester. Undesirable initiation due to protogenic impurities can be suppressed by employing relatively nonpolar (hexane-rich) media, by increasing the ester/TiCl4, ratio, or by the addition of a non-initiating ester (ethyl acetate); however, the rates decrease as a consequence of these measures. Polymerizations induced by mixtures of cumyl acetate·TiCl4/cumyl propionate·TiCl4 complexes yield monomodal narrow MWDs indicating rapid ester exchange at the growing site which in turn suggests ionic active species. The kinetic findings are explained by a mechanistic scheme that rests on two propositions: Propagation involves activated (most probably ionized) polymer-ester·Lewis acid complexes, and free (uncomplexed) Lewis acid causes irreversible destruction of living propagation.

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Macromolecular Symposia





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