Initiation Effects in the Living Cationic Polymerization of Isobutylene

Isobutylene (IB) polymerization kinetics 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 CH2 wag of IB. The measured intensity of this band was found to decrease slightly with increasing temperature (0.42% per 1.0 °C). Polymerizations were initiated using the aromatic difunctional initiator 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC) and a series of monofunctional aliphatic initiators: tert-butyl chloride (t-BuCl), 2-chloro-2,4,4-trimethylpentane (TMPCl), and 2-chloro-2,4,4,6,6-pentamethylheptane (TIBCl). The co-initiatior was TiCl4, and the Lewis base 2,4-dimethylpyridine (DMP) was used as an electron donor. Polymerizations were performed in methylcyclohexane/methyl chloride (60:40 v/v) cosolvents at temperatures ranging from −80 to −40 °C. Reaction conditions were consistently [DMP] = 2.00 × 10-3 M and [IB]0 = 1.0 M. Initiator concentrations were [TIBCl] = [TMPCl] = [t-BuCl] = 2[t-Bu-m-DCC] = 2.08 × 10-2 M. Co-initiator concentrations ranged from [TiCl4] = 7.20 × 10-3 to 6.79 × 10-2 M. Inspection of the monomer concentration vs time data revealed a number of deviations from first-order decay, depending on initiator. It was observed that polymerizations initiated with t-Bu-m-DCC exhibited an initial regime of rapid monomer consumption (RMC), which is accompanied by a significant reaction exotherm. This phenomenon was attributed to a higher ionization equilibrium constant for the tert-benzylic chloride initiator relative to tert-alkyl chloride PIB chain ends. It was proposed that cations formed from the initiator undergo a number of propagation steps prior to the first ion-pair collapse and that this number yields an estimate of the number of IB units consumed per successful ionization during propagation. This number was observed to decrease with increasing temperature. It was also proposed that the RMC event may offer a means for the experimental determination of the absolute rate constant for IB propagation in TiCl4-co-initiated polymerizations. RMC behavior was completely absent with aliphatic initiators; initiation was extremely slow with t-BuCl and sluggish with TMPCl, relative to TIBCl.