Concerted Reactions of Charge Transfer and Covalent Bond Formation in Ionized Alkylbenzene−Isobutene Clusters. Copolymerization of Styrene−Isobutene and α-Methylstyrene−Isobutene Clusters

Abstract

Mixed aromatic−isobutene clusters (Am.In, where A = toluene, p-xylene, mesitylene, 1,2,4-trimethylbenzene, styrene, and α-methylstyrene) have been ionized by resonant two-photon ionization in the vicinity of the aromatic's transition, and by two-photon ionization using 248 nm and 193 nm photons. Intracluster reactions leading to the formation of isobutene dimer cation C8H16•+ are observed following the two photon ionization of the binary clusters only when the aromatic precursor has an ionization potential greater than or equal to that of the isobutene dimer. The observation of this process in the gas phase and within clusters suggests that a similar initiation mechanism may take place in solution following the photoionization of an appropriate aromatic initiator. Evidence has been presented that points to the successive covalent additions of isobutene molecules on styrene and α-methylstyrene radical cations within the binary clusters. The intracluster reactions appear to yield two intermediate isomers. One isomer is consistent with an acyclic 1,4-radical cation which can initiate further polymerization via cationic or radical propagation depending on the nature of the available monomers in the cluster. The second isomer has probably a cyclic structure in which the ionic and the radical sites are interacting, and this gives rise to a stable product which manifests itself in the appearance of an enhanced ion intensity for the styrene−isobutene or the α-methylstyrene−isobutene radical cation. The effect of water on the cluster copolymerization has been examined. Intracluster proton transfer reactions within the (α-methylstyrene)(isobutene)(water)n species with n ≥ 3 producing protonated water clusters have been observed. The results are consistent with the distonic structure of the α-methylstyrene−isobutene radical cation which can initiate either cationic or radical propagation. These cluster studies present good model systems to examine the early stages of copolymerization and the reactivity ratios of different monomers.