Ring-opening metathesis polymerization of 3,6-bis(trifluoromethyl)pentacyclo[6.2.0.O*~~.O~~~.O~~~]dec-9-ene gives a soluble polymer which is stable at room temperature and is converted into hexafluoroxylene and polyacetylene on heating.Conjugated polymers usually display insulator or poor semiconductor behaviour, but often they can be reversibly modified by electron-transfer reactions to provide materials which display conductivities characteristic of n-or p-type semi-conductors or metals depending on the kind and extent of modification.' Polyacetylene is the paradigm for this field of research and the overwhelming majority of studies have been carried out on a material produced by direct polymerization of acetylene using a technique first described by Shirakawa.2 This material, 'Shirakawa polyacetylene ,' is usually obtained in the form of an open fibrillar mat with a density of ca. 0.4 g cm-3 and variable amounts of catalyst residuel.3 which may have important effects on the observed properties of the material.4 During the last few years we have developed an alternative effective two-stage synthesis of polyacetylene via a soluble precursor polymer (Scheme 1) .5--7 This approach allows the problems of regulating the polymer morphology and purity to be tackled. It is now clear that the material R2 L Scheme 1. A two-stage route to polyacetylene, see refs. 5-7. R Scheme 2. Summary of the photochemistry of the tricyclo[4.2.2.02.']deca-3,7,9-triene system, see refs. 13-16.

Photochemically or thermally generated t-butoxyl radicals react with primary or secondary alkyl azides R1R2C( H) N3 to afford iminyl radicals R 'R2C=N*, the e.s.r. spectra of which have been detected. The iminyl radicals undergo selfreaction at close to the diffusion-controlled rate in solution. lminyl radicals (R,C=N*) were also detected during photolysis of the azines R,C=N-N=CR, (R = H or Me), but photolysis of cyclobutanone azine yielded only the spectrum of the 3-cyanopropyl radical, formed by ring-opening of the strained cyclic iminyl radical. In contrast, the 4-cyanobutyl radical undergoes irreversible cyclisation to give the iminyl radical dH,(CH,),C=N* with a rate constant of 4.5 x 1 O2 s-at 259 K. Triorganosilyl radicals add to a variety of organic azides to form adducts which are either 1,3-or 3.3-triazenyl radicals. 15N-Labelling studies establish that the central nitrogen of the original azide gives rise to the largest hyperfine splitting in the triazenyl adducts of MeN, or Me,SiN,.The available evidence is considered most consistent with formation of a 1,3-triazenyl radical in which the unpaired electron is mainly associated with the central nitrogen and in a o orbital in the NNN plane. 1 -Hydroxy-I -methylethyl radicals react with primary alkyl azides RCH,N, to form dialkylaminyl radicals, which are thought to have the structure RACH,C(OH) Me,. Other a-hydroxyalkyl radicals behave similarly, and a mechanism related to the acid-catalysed decomposition of azides to give imines is proposed.

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The radical (Me,Si),N-, generated by U.V. photolysis of (Me,Si),NN(SiMe,), or of (Me,Si),NOSiMe,, readily abstracts hydrogen from hydrocarbons, adds to ethylene, to phosphorus(II1) compounds, and to t-butyl isocyanide, and is hence much more reactive than the dimethylaminyl radical.