GAS-PHASE REACTIONS BETWEEN TI(+) AND ISOBUTYLENE - CATIONIC POLYMERIZATION AND OBSERVATION OF C4H9+(C4H8)(N) BY HIGH-PRESSURE MASS-SPECTROMETRY

The gas-phase reactions between Ti+ and isobutylene in a variable pressure laser vaporization source have been investigated. At low pressure, the dominant reaction path is the elimination of Ha followed by sequential additions of isobutylene molecules onto TiC4H6+. At higher pressure (0.3 Torr), the eliminative reactions are quenched and a new condensation channel is opened which corresponds to the generation of a polymeric ion of the form C4H9+(C4H8)(n). This channel proceeds via the generation of C4H9+ from a pressure-dependent chain transfer reaction of the Ti+ containing ions to isobutylene monomers. The reaction rates among the series C4H9+(C4H8)(n) vary significantly with n. The first two steps are very fast and the steps generating the ions with n = 3-5 are relatively slow. The present study suggests that it is possible to alter the mechanism of gas-phase cationic polymerization and perhaps control the composition of the final products by controlling the total pressure in the reaction source.