Jeremy N. Winter

Order and disorder in Durham polyacetylene

Abstract The Durham route to polyacetylene provides, for the first time, the possibility of controlling the crystalline structure of this fascinating material. This control will enable us to probe the role of the redox doping and relative spatial co-ordinates of the chains in determining electrical conductivity. Before this can be accomplished it is necessary to understand the nature of the order within the variety of materials which can now be produced. Material produced from quiescent films can have a range of structures from largely amorphous (inasmuch as a structure composed of rigid rods can exhibit disorder) to slightly crystalline. By stretching the precursor film during transformation highly oriented, crystalline polyacetylene can be produced. This offers a range of structures and should enable the separation of inter- and intra-chain electronic effects for the first time.

Thermal and photo-oxidative degradation of poly (5′,6′-bis(trifluoromethyl)-bicyclo [2.2.2] octa-5′,7′-diene-2′,3′-diyl)-1,2-ethenediyl, the Durham polyacetylene precursor polymer, and polyacetylene

Abstract The photo-oxidation of the precursor polymer to Durham polyacetylene has been studied by thermal analysis and ultra-violet, infra-red and solid-state nuclear magnetic resonance spectroscopies. It is concluded that the process that is involved in the photolithographic formation of relief images in polyacetylene is initiated by singlet oxygen attack on the polymer film followed by classical auto-oxidation and crosslinking. Polyacetylene was studied as a related model.

Electron spin resonance studies of radicals derived from organic azides

Photochemically or thermally generated t-butoxyl radicals react with primary or secondary alkyl azides R1R2C(H)N3 to afford iminyl radicals R1R2CN·, the e.s.r. spectra of which have been detected. The iminyl radicals undergo self-reaction at close to the diffusion-controlled rate in solution. Iminyl radicals (R2CN·) were also detected during photolysis of the azines R2CN–NCR2(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 [graphic omitted]N· with a rate constant of 4.5 × 102s–1 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 MeN3 or Me3SiN3. 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 σ orbital in the NNN plane. 1-Hydroxy-1-methylethyl radicals react with primary alkyl azides RCH2N3 to form dialkylaminyl radicals, which are thought to have the structure RṄCH2C(OH)Me2. Other α-hydroxyalkyl radicals behave similarly, and a mechanism related to the acid-catalysed decomposition of azides to give imines is proposed.

An improved synthesis of polyacetylene

Ring-opening metathesis polymerization of 3,6-bis(trifluoromethyl)pentacyclo[6.2.0.02,4.03,6.05,7]dec-9-ene gives a soluble polymer which is stable at room temperature and is converted into hexafluoroxylene and polyacetylene on heating.

Homolytic organometallic reactions : XII. Homolytic dealkylation of di-n-butyl-t-butyltin chloride: preferential displacement of the n-butyl radical

Abstract t-Butoxyl radicals react with di-n-butyl-t-butyltin chloride to show the ESR spectrum of the n-butyl rather than the t-butyl radical. Cumyloxyl radicals behave similarly, but trimethylsiloxyl and benzoyloxyl radicals react to give both n-butyl and t-butyl radicals. The unexpected preferential formation of the less stable n-alkyl radical is tentatively ascribed to steric constraints in a 5-coordinate intermediate.

Trialkylsilylaminyl radicals. Part 1. Electron spin resonance studies of the photolysis of silylated hydrazines, hydroxylamines, triazenes, and tetrazenes

Radicals produced by liquid-phase u.v. photolysis of a number of trialkylsilylated hydrazines, hydroxylamines, triazenes, and tetrazenes have been studied using e.s.r. spectroscopy. The bis(trimethylsilyl)-aminyl radical (Me3Si)2N˙, generated by photolysis of (Me3Si)2NN(SiMe3)2, (Me3Si)2NOSiMe3, or (Me3Si)2NNNN(SiMe3)2, is much more reactive than a dialkylaminyl radical. It abstracts hydrogen from aliphatic C–H groups, adds to ethylene, t-butyl isocyanide, and trialkyl phosphites; the bis(triethylsilyl)-aminyl radical behaves similarly. Differences between (R3Si)2N˙ and (R3C)2N˙ are attributed to the σ-donor–π-acceptor substituent effect of the trialkylsilyl ligand. The reactivity of (R3Si)2N˙ is generally similar to that of ButO˙, although the steric congestion at the radical centre in the former results in a preference for attack at less hindered sites. For example, whilst ButO˙ and H2N˙ both react with isobutane to give mainly But˙, (Me3Si)2N˙ gives mainly Bui˙ below room temperature. The trimethylsilyl(methyl)aminyl radical was generated by photolysis of Me3SiN(Me)OSiMe3 or Me3SiN(Me)NNMe and, although its e.s.r. spectrum was not detected, it could be trapped by trialkyl phosphites to give the phosphoranyl radicals Me3SiN(Me)P(OR)3, implying a reactivity greater than that of Me2N˙. Hydrogen abstraction from R3SiN(H)OSiR3 affords R3SiNOSiR3 which rearranges to (R3Si)2NO˙, but RNOSiR3 does not rearrange to R3Si(R)NO˙. The e.s.r. spectra of RNOR, ROSiR3, and R3SiOSiR3 are compared and differences are accounted for in terms of the π-acceptor nature of the R3Si group.

An electron spin resonance study of the photolysis of bis(β-alkoxyalkyl)mercurials

Abstract Bis(β-alkoxyalkyl)mercurials are much more photosensitive than the unsubstituted dialkylmercurials, and when they are irradiated in solution with ultraviolet light, the ESR spectra of the appropriate β-alkoxyalkyl radicals are observed. Spectral data are presented for 14 such radicals and the values of a (H β ) are discussed in terms of possible conformational preferences.

Electron spin resonance studies. Part 63. Selective radical oxidation by titanium(IV) complexes

Evidence is presented that, when complexed with edta in acid solution, titanium(IV) is an effective oxidant for radicals in which tervalent carbon is bonded to a substituent of +M type. A lower limit for k(TiIV–edta +·CHMeOH) is estimated as ca. 108 dm3 mol–1 s–1. When the organic radicals are generated from the TiIII–RO2H couple (R = H or alkyl), a chain reaction operates; but stoicheiometric studies show that the chains are relatively short.

An E.S.R. study of spin-trapping by azodicarboxylates to give hydrazyl radicals

Abstract Addition of photochemically- or thermally-generated radicals (X·) to dialkyl azodicarboxylates gives hydrazyl radicals [RCO 2 (X)N·CO 2 R] which are readily detected by e.s.r. spectroscopy.

Electron spin resonance study of iminyl and triazenyl radicals derived from organic azides

t-Butoxyl radicals react with primary or secondary alkyl azides to produce iminyl radicals, whilst triorganosilyl radicals add to organic azides to give triazenyl radicals.