Polymerization of Isobutylene Initiated By Laser Vaporization of Metal Targets
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Polymerization of Isobutylene Initiated By Laser Vaporization of Metal Targets Walter Vann, George M. Daly and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth University Richmond, VA 23284-2006
ABSTRACT
A novel technique involving the initiation of polymerization using laser vaporization of metal targets is being explored. In the experiment, metal ions such as Ti+, Zr+, Ag+ are generated in the gas phase by pulsed laser vaporization/ionization of a metal target using the second harmonic of a Nd:YAG laser (532 nm). The ions are pulled toward a monomer liquid (isobutylene which can only be polymerized by cationic mechanisms) by applying appropriate electric fields across the reaction chamber. Under different experimental conditions (temperature, pressure of a carrier gas, laser energy, electric field strength, reaction time) different polymeric products (viscous liquids or solids) were obtained with average molecular weights ranging from 200 to greater than 106. Full characterization of these materials is currently in progress. This technique could lead to a powerful approach for the synthesis of a wide variety of polymeric materials with unique properties such as stability, strength and photo and electrical conductivity. INTRODUCTION
The fundamental mechanism that governs ionic polymerization is an important question in a number of scientific disciplines as well as practical applications. Information regarding the exact nature of initiation by free ions remains largely unavailable. Under normal circumstances, ionic propagation in solution may not be due to free ions but to ion pairs. The propagation rate constants for free ions are greater than those for an ion pair. Furthermore, cationic polymerization often stops before complete monomer consumption since the ion pair eventually recombines and terminates the process [1]. Our approach to a fundamental understanding of cationic polymerization involves two parallel investigations: (1) polymerization in gas phase clusters, and (2) polymerization of liquid monomers using gas phase ions. Intracluster polymerization reactions possess a distinct form of chemistry that has not been previously explored. In the gas phase at low pressures, eliminative polymerization may predominate. However, in the same systems at high pressure, the ionic intermediates may be stabilized and addition without elimination may occur. In clusters, our first results show evidence for both stabilization by fast evaporation (boiling off) of solvent molecules and for eliminative polymerization. These studies involved cationic polymerization in isoprene [2] and vinyl chloride [3] clusters following electron impact ionization. The results showed, for the first time, that sequential . lymerization reactions, with 2-10 condensation steps, can occur on a time scale of a few microseconds following the ionization of the neutral clusters. This occurs despite the fact that condensation reactions are exothermic, and the energy released could dissociate the cluster. The competition between the
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