Mechanistic, kinetic, and energetic study of titanium tetrachloride-coinitiated quasiliving cationic polymerization of isobutylene and styrene

Quinn Aubry Thomas


The fundamental, mechanistic study of the cationic polymerization of isobutylene and styrene has been the focus of study in many research groups and continues to be an important endeavor in our research laboratory. Recently developed real-time in situ ATR-FTIR monitoring of quasiliving cationic polymerization systems has afforded the continuous measurement of monomer concentration with the necessary speed and sensitivity. The ability to monitor closely the different stages of polymerization has resulted in the observation of details of the initiation and propagation mechanisms that are difficult or impossible to observe with traditional gravimetric analysis. The objectives of this work were to unambiguously assign the order of propagation with respect to Lewis acid and to understand the fundamental differences between styrene and 113 propagation through the determination of relative kinetic rate constants and energies of activation. Due to disagreements in the literature it was necessary to conduct a detailed kinetic investigation to determine the kinetic order of the propagation rate with respect to the Lewis acid for IB polymerizations over a range of temperatures. Polymerizations were initiated using 2-chloro-2,4,4-trimethylpentane (TMPCl) and co-initiated with TiCl 4 . Polymerizations were carved out using Lewis base (LB) di-tert -butylpyridine (DtBP) at [DtBP] = 2 or 8 × 10-3 M, [IB] = 2.0 or 1.0 M, [TMPCl] = 0.054 or 0.025 M and [TiCl4 ] = 0.01-0.05 M in methylcyclohexane/methyl chloride (MCHex/MeCl) 60/40 or 63/37 (v/v) cosolvent system. Coinitiator concentrations were designed to be ≤ the growing chain end concentration and ranged from [TiCl4 ] = 1.0 to 5.0 × 10-2 M. In all cases, polymerizations exhibited a second order dependence on the effective TiCl4 concentration ([TiCl4 ]eff ) after appropriate temperature corrections were made. [TiCl4 ]eff was defined in terms of the nominal amount of TiCl4 added to the reactor minus the fraction calculated to be unavailable for coinitiation of the polymerization due to formation of pyridinium salts as a result of proton scavenging. Second-order dependence on the effective TiCl4 concentration was attributed to the predominance in the propagation reaction of chain end ionization by dimeric Ti2 Cl8 , and thus counteranions of the form Ti 2 Cl9Θ . Polymerization of IB and styrene using tert -benzylic initiator 5-tert -butyl-1,3-bis (2-chloro-2-propyl)benzene (bDCC), was the focus of the remaining studies herein reported. (Abstract shortened by UMI.)