The syn dimer of cyclobutadiene (tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum chemical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously after ionization or after subsequent warming or illumination of the samples. Based on these findings, we propose a mechanistic scheme which involves a partitioning of the incipient radical cation of TOD between two electronic states. These two states engage in (near) activation-less decay to the more stable valence isomers, cyclooctatetraene (COT*+) and a bis-cyclobutenylium radical cation BCB*+. The latter product undergoes further rearrangement, first to tetracyclo[4.2.0.0(2,4).0(3,5]oct-7-ene (TCO*+) and eventually to bicyclo[4.2.0]octa-2,4,7-triene (BOT*+) which can also be generated photochemically from BCB*+ or TCO*+. The surprising departure of syn-TOD*+ from the least-motion reaction path leading to BOT*+ can be traced to strong vibronic interactions (second-order Jahn-Teller effects) which prevail in both possible ground states of syn-TOD*+. Such effects seem to be more important in determining the intramolecular reactivity of radical cations than orbital or state symmetry rules.
After suitable drying, the subject monomers in the form of neat liquids undergo radiation-induced poly merization with no apparent side reactions and high conversions to precipitatable polymers of low molec ular weight. A high frequency of chain (proton) transfer to monomer is indicated by the fact that the kinetic chain lengths are estimated to be several hundred times larger than the range of DP n values (12-4). Structural characterization of the limonene oxide polymer by 1 H and 13 C NMR spectroscopy provides conclusive evidence that the polymerization proceeds by the opening of the epoxide ring to yield a 1,2 -trans polyether. Similar NMR studies on the polymers formed from the α-pinene and ß-pinene oxides show that the opening of the epoxide ring for these monomers is generally accompanied by the concomitant ring opening of the cyclobutane ring structure to yield a gem-dimethyl group in the main chain.The radiation-induced cationic polymerization of vinyl and unsatu rated monomers in the liquid state has been studied for over 25 years, and the essential features of this type of polymerization appear to be well established (1,2)· In contrast to cationic polymer ization by catalysts where the propagating species is usually described as a solvated ion pair, the distinctive characteristic of cationic polymerization induced by high energy radiation is that propagation occurs by free ions with very large rate constants, the range of kp values for observable polymerization being from 10 M"" 1 s" 1 to 10 8 M" 1 s" 1 . Since the concentration of free ions is typically about 10" 10 M for dose rates obtainable from kilocurie Co gamma-radiation sources, the rates of polymerization are very sensi tive to traces of impurities, including water (3), that can function as efficient terminating agents. Consequently, much attention has been paid to the development of stringent experimental techniques for the rigorous drying of monomers (1H>), since otherwise this type of polymerization may go unrecognized.
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