Chemical and Biomolecular Engineering Faculty Research


Effect of Reaction Conditions on the Kinetics of Living Isobutylene Polymerization at High Initiator/ticl4 Ratios

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The kinetics of isobutylene (IB) polymerization initiated by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 in methylcyclohexane (MeCHx)/methyl chloride (MeCl) or hexane (Hx)/MeCl 60/40 v/v at −80 °C were investigated in the range of [TMPCl]0/[TiCl4]0 > 1 using a real-time fiber-optic mid-IR monitoring technique. The rate of IB consumption was monitored by following the disappearance of the 1655 cm-1 IR band, characteristic of the C C stretching vibration. Both the area and the height of the 1655 cm-1 band were found to be directly proportional to IB concentration under the conditions investigated. From polymerization rate data the overall rate constant kp‘ = kpKeq was calculated, where kp is the rate constant of propagation and Keq = k1/k-1 is the equilibrium constant for the dormant/active equilibrium characteristic of living IB polymerization. The investigation revealed that kp‘ was very sensitive to solvent quality. kp‘ ≈ 0.5−1.7 L2 mol-2 s-1 was found using MeCHx, while kp‘≈ 1.7−3.4 L2 mol-2 s-1 was obtained using Hx from different providers and/or with and without further purification. GC analysis and real-time IR monitoring revealed the presence of carbonyl contaminants in the solvents, which were found to form a complex with TiCl4. Regardless of solvent quality, the reaction order in TiCl4 was found to be invariably 1 under the conditions investigated. Monomer concentration and temperature were found to have a profound effect on the polymerization; kp‘ increased exponentially with an increase of [IB]0 or a decrease in temperature. The apparent activation enthalpy of propagation was found to be ΔHa = −26.4 kJ/mol.

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