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)/TiCl4 in methylcyclohexane (MeCHx)/methyl chl oride (MeCl) or hexane (Hx)/MeCl 60/40 v/v at -80 degrees C were investigat ed 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 fo llowing 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 co nstant 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 dorm ant/active equilibrium characteristic of living IB polymerization. The inve stigation revealed that k(p)' was very sensitive to solvent quality. k(p)' approximate to 0.5-1.7 L-2 mol(-2) s(-1) was found using MeCHx, while k(p)' approximate to 1.7-3.4 L-2 mol(-2) s(-1) was obtained using Hx from differ ent 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 u nder the conditions investigated. Monomer concentration and temperature wer e found to have a profound effect on the polymerization; k(p)' increased ex ponentially with an increase of [IB](0) or a decrease in temperature. The a pparent activation enthalpy of propagation was found to be Delta H-a = -26. 4 kJ/mol.