COMPARATIVE POLYMERIZATION IN THE GAS-PHASE AND IN CLUSTERS .2. ELECTRON-IMPACT AND MULTIPHOTON-INDUCED REACTIONS IN ISOBUTENE AND BENZENE/ISOBUTENE CLUSTERS

Electron Impact (EI) ionization of isobutene clusters leads to the production of predominantly the radical cation series (C4H8)(n)(.+) with a small fraction of the carbocation series C4H9(C4H8)(n)(+) The dimer radical cation exhibits unusual abundance under a wide range of experimental conditions which is consistent with a covalent bond formation in agreement with the gas phase results. The observed trimer stability at low ionization energy suggests the formation of a cyclic structure. Several closed and open shell ions have been observed within the clusters and have been attributed to dissociation products from the dimer and trimer. Mixed benzene/isobutene clusters (BmIn) have been ionized by EI, resonant two-photon ionization in the vicinity of the benzene's A(lg) --> B-2u, 6(0)(1) transition (similar to 259 nm), and two-photon ionization using 248- and 193-nm photons. In all cases, the resulting ion distributions are dominated by the series I2+Bm, thus reflecting the stability of the covalent bonded isobutene dimer cation. The study of clusters indicates that the formation of the dimer cation is more favorable than that of the carbocation under the cold beam conditions. This is consistent with the gas phase behavior where the condensation channel increases at low temperatures. Metastable peaks associated with the tert-butyl cation series C4H9(C4H8)(n)(+), n > 2, have been observed in the time-of-flight (TOF) experiments indicating a significant change in the rate of the addition reactions past n = 2. This is consistent with the gas phase measurements which showed that the second polymerization step leading td the formation of C3H9(C4H8)(2)(+) exhibits an unusually large entropy change. Correspondingly, the cluster results indicate that the third and higher polymerization steps are delayed after ionization. The relation between the gas phase and cluster studies and the implications for the bulk polymerization are discussed.