SOLID-STATE CROSS POLARIZATION MAGIC ANGLE SPINNING C-13 NMR INVESTIGATION OF ALKOXY-SUBSTITUTED POLY(PARA-PHENYLENE VINYLENE) HOMOPOLYMERS AND COPOLYMERS

Abstract

The comparison of solid state and solution C-13 NMR spectra of the model compound trans, trans-2,5-dimethyl-1,4-bis(2-(3,4,5-trimethoxyphenyl)ethenyl) benzene showed that essentially the same conformation exists in both states. The molecule is almost planar exhibiting an ortho-methyl-syn form. For the cis, cis-isomer a non-planar form is proposed in which the three phenyl rings are perpendicular to the plane containing the double bonds. The information from the model compounds was used to analyse the cross polarization/magic angle spinning (CP-MAS) C-13 NMR spectra of a series of 2,5-di-alkoxy-substituted homo- and copolymers of poly(para-phenylene vinylene) (PPV). For the all-trans polymers we propose the ortho-alkoxy-anti conformation and for the cis-containing polymers we propose the conformation in which the phenyl rings are perpendicular to the olefinic plane. The first atom of the alkoxy chain is always in the phenyl plane. Estimates of the degree of polymerization gave 9 < n < 18 for these compounds synthesized via Wittig reactions. The value agrees well with results of other methods. Measurements of relaxation times (T1-rho(H), T2(H), T(CH), T2(C)) were performed on a selection of the compounds. The T1-rho(H) values vary linearly with the length of alkoxy chain. The molecular dynamics of alkoxy-substituted PPVs is dominated by a segmental mobility of these chains, with minimal interference from other molecular motions. Methyl rotation is more rapid in long and unbranched chains than in short, or branched ones. A rationalization of specific densities observed in these polymers can be given in terms of the observed molecular mobility.