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 w as 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 s lightly with increasing temperature (0.42% per 1.0 degrees C). Polymerizati ons 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 monofunctiona l aliphatic initiators: tert-butyl chloride (t-BuCl), 2-chloro-2,4,4-trimet hylpentane (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 u sed as an electron donor. Polymerizations were performed in methylcyclohexa ne/methyl chloride (60:40 v/v) cosolvents at temperatures ranging from -80 to -40 degrees C. Reaction conditions were consistently [DMP] = 2.00 x 10(- 3) M and [IB](0) = 1.0 M. Initiator concentrations were [TIBCl] = [TMPCl] = [t-BuCl] = a[t-Bu-m-DCC] = 2.08 x 10(-2) M. Go-initiator concentrations ra nged from [TiCl4] = 7.20 x 10-3 to 6.79 x 10-2 M. Inspection of the monomer concentration vs time data revealed a number of deviations from first,orde r decay, depending on initiator. It was observed that polymerizations initi ated with t-Bu-m-DCC exhibited an initial regime of rapid monomer consumpti on (RMC), which is accompanied by a significant reaction exotherm. This phe nomenon was attributed to a higher ionization equilibrium constant for the tert-benzylic chloride initiator relative to tert-alkyl chloride PIE chain ends. It was proposed that cations formed from the initiator undergo a numb er 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 i ncreasing 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 comp letely absent with aliphatic initiators; initiation was extremely slow with t-BuCl and sluggish with TMPCl, relative to TIBCl.