TiCl₄ Reaction Order in Living Isobutylene Polymerization at Low [TiCl₄]:[Chain End] Ratios

Abstract

Isobutylene (IB) polymerization kinetics at −80 °C were monitored in real time using mid-infrared ATR-FTIR spectroscopy, with diamond-composite insertion probe and light conduit technology. Monomer concentration as a function of time was obtained by monitoring the absorbance at 887 cm -1 associated with the CH 2 wag of IB. Polymerizations were initiated using 5- tert -butyl-1,3-bis(2-chloro-2-propyl)benzene ( t -Bu- m -DCC) or 2-chloro-2,4,4-trimethylpentane (TMPCl) in conjunction with TiCl 4 co-initiator, in hexane/methyl chloride or methylcyclohexane/methyl chloride (60:40 v/v) cosolvents. Either 2,4-dimethylpyridine (DMP) or 2,6-di- tert -butylpyridine (DTBP) was used as an electron donor (ED). Reaction conditions were [ED] = 2.00 × 10 -3 M, [IB] 0 = 1.0 M, and [TMPCl] (or 2[ t -Bu- m -DCC]) = 2.08 × 10 -2 M. Co-initiator concentrations were designed to be less than or equal to the growing chain end concentration and ranged from [TiCl 4 ] = 7.20 × 10 -3 to 2.28 × 10 -2 M. Steady-state polymerization kinetics were found to be independent of the nature of initiator, slightly faster when conducted with the noncomplexing DTBP rather than DMP and slightly faster when methylcyclohexane rather than hexane was utilized as the hydrocarbon diluent. In all cases, polymerizations exhibited a second-order dependence on the effective TiCl 4 concentration ([TiCl 4 ] eff ). The latter was defined in terms of the nominal amount of TiCl 4 added to the reactor minus the fraction calculated to be unavailable for co-initiation of the polymerization due to formation of a neutral complex with the ED and/or pyridinium salts as a result of proton scavenging. Second-order dependence on the effective TiCl 4 concentration was attributed to the predominance in the propagation reaction of active carbocations associated with dimeric counteranions of the form Ti 2 Cl 9 - .