TiCl4 reaction order in living isobutylene polymerization at low [TiCl4]:[chain end] ratios

Isobutylene (IB) polymerization kinetics at -80 degrees C were monitored in real time using midinfrared ATR-FTIR spectroscopy, with diamond-composite insertion probe and light conduit technology. Monomer concentration as a fu nction of time was obtained by monitoring the absorbance at 887 cm(-1) asso ciated with the = CH2 wag of IB. Polymerizations were initiated using 5-ter t-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC) or 2-chloro-2,4,4-tr imethylpentane (TMPC1) in conjunction with TiCl4 co-initiator, in hexane/me thyl chloride or methylcyclohexane/methyl chloride (60:40 v/v) cosolvents. Either 2,4-dimethylpyridine (DMP) or 2,6-di-tert-butylpyridine (DTBP) was u sed as an electron donor (ED). Reaction conditions were [ED] = 2.00 x 10(-3 ) M, [IB](0) = 1.0 M, and [TMPC1] (or 2[t-Bu-m-DCC]) = 2.08 x 10(-2) M. Go- initiator concentrations were designed to be less than or equal to the grow ing chain end concentration and ranged from [TiCl4] = 7.20 x 10(-3) to 2.28 x 10(-2) M. Steady-state polymerization kinetics were found to be independ ent of the nature of initiator, slightly faster when conducted with the non complexing DTBP rather than DMP and slightly faster when methylcyclohexane rather than hexane was utilized as the hydrocarbon diluent. In all cases, p olymerizations exhibited a second-order dependence on the effective TiCl4 c oncentration ([TiCl4](eff)). The latter was defined in terms of the nominal amount of TiCl4 added to the reactor minus the fraction calculated to be u navailable for co-initiation of the polymerization due to formation of a ne utral complex with the ED and/or pyridinium salts as a result of proton sca venging. Second-order dependence on the effective TiCl4 concentration was a ttributed to the predominance in the propagation reaction of active carboca tions associated with dimeric counteranions of the form Ti2Cl9-.