Effect of Reaction Conditions on the Kinetics of Living Isobutylene Polymerization at High Initiator/TiCl4 Ratios

The kinetics of isobutylene (IB) polymerization initiated by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl 4 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 /[TiCl 4 ] 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 k p ‘ = k p K eq was calculated, where k p is the rate constant of propagation and K eq = k 1 / k - 1 is the equilibrium constant for the dormant/active equilibrium characteristic of living IB polymerization. The investigation revealed that k p ‘ was very sensitive to solvent quality. k p ‘ ≈ 0.5−1.7 L 2 mol -2 s -1 was found using MeCHx, while k p ‘≈ 1.7−3.4 L 2 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 TiCl 4 . Regardless of solvent quality, the reaction order in TiCl 4 was found to be invariably 1 under the conditions investigated. Monomer concentration and temperature were found to have a profound effect on the polymerization; k p ‘ increased exponentially with an increase of [IB] 0 or a decrease in temperature. The apparent activation enthalpy of propagation was found to be Δ H a = −26.4 kJ/mol.