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-time, 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-dimethylpyridine (DMP)/boron trichloride (BCl 3 ) system in pure methyl chloride (MeCl) solvent, over a range of [BCl 3 ], [IB], and temperatures. Profiles of [IB] as a function of time were obtained by monitoring the absorbance at 887 and/or 1656 cm -1, associated with the CH 2 wag and C C stretch, respectively, of IB. First-order kinetic plots were constructed to obtain the apparent rate constant, k app, for polymerization under specific conditions. Over the range of [BCl 3 ] studied, the kinetic order of the reaction with respect to BCl 3 was found to be close to unity but was depressed below unity at high [BCl 3 ] due to depression of the overall solvent polarity by BCl 3 . A similar decrease in polarity of the reaction medium resulted in a decrease in the k app as the initial IB concentration, [IB] 0, was increased. Polymerizations conducted in the range −80 to −30 °C revealed a relatively 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% MeCl solvent system; the k app of the polymerization increased by 130% as temperature was increased over this range. Over the range of [BCl 3 ], [IB], and reaction temperatures studied, this system produced well-defined, low molecular weight PIB exhibiting extremely narrow molecular weight distribution (PDI = 1.05) and possessing tert -chloride chain ends exclusively, with no evidence of direct or moisture initiation or chain termination or transfer occurring during the experimental lifetime.