New substituted polyacetylenes with phenyleneethynylene side groups [-(C6H4-CC)n-SiiPr3; n=1, 2]:: Synthesis, characterization, spectroscopic, and photoelectric properties

The selective, terminal triple bond polymerization of HC=C-C6H4-C=C-(SiPr3)-Pr-i (1) and HC=C-C6H4-C=C-C6H4-C=C-(SiPr3)-Pr-i (2) yields soluble, high-MW polyacetylenes P1 and P2 with phenyleneethynylene-type pendant chains. The [Rh(cod)(OCH3)](2)- and MoCl5-based catalysts show 100% polymerization selectivity to terminal triple bonds and give high-cis (NMR: Rh, 94% cis; Mo, 70% cis) polymers having a medium extent of pi-conjugation. WOCl4-based catalysts reacting in benzene marginally also insert internal triple bonds and give low-cis polymers (<10% cis) having a high extent of conjugation. Addition of 1,4-dioxane to the WOCl4-based catalyst considerably reduces or even eliminates the internal triple bond insertion and increases the polymer molecular weight, cis-unit content (approximate to 35% cis) and extent of pi-conjugation. The pi-pi* band of P1 pendant groups is closely correlated with the polymer microstructure: the cis-unit content in P1 can be reliably ascertained from the A(292)/A(268) UV-absorbance ratio. Raman bands of the main-chain v(C=C) mode exhibit both frequency and intensity dispersion with extent of conjugation. On the other hand, only intensity dispersion is conclusively observed for the v(Ph-C-chain) band at 1341 cm(-1) that, upon correction to the dispersion effect, is in a good correlation with the cis-unit content obtained from NMR spectra. Xerographic and transient photoconductivity measurements have shown that the photogeneration in these polymers proceeds through the side group excited states, and the charge carrier's (holes) transport is controlled by hopping between the main chain conjugated sequences.