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The synthesis of poly(2,5-dialkoxy-p-phenylene)s (PPP) by palladium-catalyzed polycondensation of 2,5-dialkoxy-4-bromophenylboronic acids is described. Number-average degrees of polymerization of ca. 30 have been reached at best. The characterization of the polymers by NMR, IR, UV-VIS and fluorescence spectra, data on the bulk structure obtained by X-ray methods, melting behavior and thermal stability are reported. PPP's with long alkoxy side chains exhibit a layered phase structure, those with short or branched alkoxy side chains, cylindrical packing. Poly(2,5-dibutoxy-p-phenylene) was prepared with chain lengths 7 5 P, I 30. The polymer with P, of 30 shows thermal transitions in the bulk phase at 182°C and 227 "C. It forms an anisotropic melt above 227 "C which only starts decomposing if heated above 300 "C. The polymers of shorter chain length show isotropic melts, with the melting point strongly depending on chain length.

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