Real-time monitoring of carbocationic polymerization of isobutylene via ATR-FTIR spectroscopy: The t-Bu-m-DCC/DMP/BCl3 system

The kinetics of isobutylene (IB) polymerization were monitored using real-t ime, in-situ ATR-FTIR spectroscopic analysis. Polymerization was initiated using the 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC)/2,4-d imethylpyridine (DMP)/boron trichloride (BCl3) system in pure methyl chlori de (MeCl) solvent, over a range of [BCl3], [IB], and temperatures. Profiles of [IB] as a function of time were obtained by monitoring the absorbance a t 887 and/or 1656 cm(-1), associated with the =CH2 wag and C=C stretch, res pectively, of IB. First-order kinetic plots were constructed to obtain the apparent rate constant, k(app), for polymerization under specific condition s. Over the range of [BCl3] studied, the kinetic order of the reaction with respect to BCl3 was found to be close to unity but was depressed below uni ty at high [BCl3] due to depression of the overall solvent polarity by BCl3 . A similar decrease in polarity of the reaction medium resulted in a decre ase in the k(app) as the initial IB concentration, [IB](0), was increased. Polymerizations conducted in the range -80 to -30 degrees C revealed a rela tively low and positive apparent energy of activation, E-a, of 1.7 kcal/mol for the rate of polymerization, which was attributed to the polar 100% MeC l solvent system; the k(app) of the polymerization increased by 130% as tem perature was increased over this range. Over the range of [BCl3], [IB], and reaction temperatures studied, this system produced well-defined, low mole cular weight PIE exhibiting extremely narrow molecular weight distribution (PDI=1.05) and possessing tert-chloride chain ends exclusively, with no evi dence of direct or moisture initiation or chain termination or transfer occ urring during the experimental lifetime.