Real-Time Monitoring of Carbocationic Polymerization of Isobutylene via ATR-FTIR Spectroscopy: The t-Bu-m-DCC/DMP/BCl3 System

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Polymers and High Performance Materials


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 (BCl3) system in pure methyl chloride (MeCl) solvent, over a range of [BCl3], [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 =CH2 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 [BCl3] studied, the kinetic order of the reaction with respect to BCl3 was found to be close to unity but was depressed below unity 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 decrease 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 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 [BCl3], [IB], and reaction temperatures studied, this system produced well-defined, low molecular weight PIE 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.

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