Abstract
The effect of ligand of Lewis acid on the cationic polymerization of isobutylene (IB) using TiCl4/TiBr4-mixed co-initiator is reported. The polymerization proceeds in a living fashion; however, the polymerization rate decreases with decreasing TiCl4/TiBr4 ratio. The active co-initiator in this polymerization system is the mixed dimer, Ti2ClxBr(8-x), not the dimer of TiCl4 or TiBr4. From the molecular weight distribution vs conversion plots, the rate constant of deactivation, k-i, and ionization, , the apparent equilibrium constant of ionization, , and the thermodynamic parameters are calculated and discussed. By the stepwise replacement of Cl to Br the Lewis acidity decreases, which results in a decreased ionization rate constant while the deactivation rate constant remains essentially unchanged. Therefore, the decreasing overall rates of polymerization with decreasing TiCl4/TiBr4 ratio are exclusively due to the lower equilibrium constant of ionization, which results in lower ion pair concentration.
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