Kinetic investigations of the controlled/living polymerization of isobutylene via in situ real-time ATR-FTIR spectroscopy

Andrew Bradley Donnalley

Abstract

The objective of this research was to apply recent advances in analytical instrumentation, specifically real-time in situ monitoring via ATR-FTIR spectroscopy, to the most challenging aspects of isobutylene (IB) polymerization. Initial investigations of IB polymerization kinetics were monitored in real time using mid-infrared FTIR-ATR 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. Inspection of the first-order plot generated from the FTIR data revealed a number of deviations from linearity, which were attributed primarily to a transient rise and subsequent fall in reactor temperature caused by the initial large exotherm of polymerization. These small differences in instantaneous rate were not detected by the gravimetric method due to insufficient accuracy and density of data points. Refinements in reactor design and data analysis facilitated a detailed kinetic investigation to determine the kinetic order of the propagation rate with respect to the Lewis acid, for IB polymerizations at -80°C. Polymerization kinetics were found to be independent of the nature of the initiator, slightly faster when conducted with the non-complexing DTBP, and slightly faster when methylcyclohexane was utilized as the hydrocarbon diluent. In all cases, polymerizations exhibited a second order dependence on the effective TiC1 4 concentration ([TiC14 ]eff ). Second-order dependence on the effective TiC14 concentration was attributed to the predominance in the propagation reaction of active carbocations associated with dimeric counteranions of the form [Special characters omitted.] . Additionally, it was noted that first-order analysis of polymerizations initiated with the aliphatic initiator TMPCl, passed through the origin, whereas polymerizations initiated by the aromatic t -Bu-m -DCC all exhibited positive finite y-intercepts. The origin of the positive y-intercepts observed for t-Bu-m-DCC initiated IB polymerizations was probed by performing a series of experiments using the aromatic difunctional initiator t -Bu-m -DCC and a series of monofunctional aliphatic initiators. 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. RMC behavior was completely absent with aliphatic initiators; initiation was extremely slow with t -BuCl and sluggish with TMPCl, relative to TIBCl. (Abstract shortened by UMI.)