Macromolecules 1998, 31, 7985-7987
7985
Polymeric Organosilicon Systems. 28. Preparation and Properties of Novel σ-π Conjugated Polymers with Alternating Disilanylene and 2,5-Diethynylenesilole Units in the Backbone
Scheme 1
Joji Ohshita, Nobuhisa Mimura, Hidekazu Arase, Mitsunori Nodono, Atsutaka Kunai,* Kenji Komaguchi, and Masaru Shiotani
Scheme 2
Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Mitsuo Ishikawa* Department of Chemical Technology, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurashima, Kurashiki, Okayama 712-8505, Japan Received January 27, 1998 Revised Manuscript Received September 9, 1998
Introduction. Organosilicon polymers composed of an alternating arrangement of a organosilicon unit and π-electron system represent a new class of conjugated polymers.1 They are potentially useful as functional materials, such as organic semiconductors1a and photoresists.2 In addition, this type of polymer can be used as thermal precursors to silicon carbide.1d,3 Recently, silole ring systems have been extensively studied as the new π-electron system with unusual optical properties. The remarkably red-shifted UV absorptions and high electron-transporting properties of the compounds containing the silole ring would result from their low-lying lowest unoccupied molecular orbital (LUMO) due to the σ*-π* interaction between the silole silicon atom and the π-orbitals.4 Although many papers concerning the synthesis of polymers and oligomers containing the silole ring have been published to date,5,6 only a few refer to polymers bearing 2,5-linked silole rings in the backbone.6 In the course of studies concerning the synthesis of silole-containing conjugated polymers,7,8 we have prepared polymers having repeating organosilicon-2,5-diethynylenesilole units in the backbone, by dehydrobromination of 2,5-dibromosilole and diethynyldi- and monosilanes in the presence of a CuI/Pd(PPh3)4 catalyst.9 Results and Discussion. The starting monomer 2,5-dibromo-1,1,3,4-tetraphenylsilole (1) was prepared as reported by Tamao et al.6b Tamao et al. have also reported the synthesis of 2,5-bis(phenylethynyl)-1,1,3,4tetraphenylsilole from the reaction of 1 and phenylacetylene in the presence of a CuI/Pd(PPh3)4 catalyst in a mixed solvent of THF and triethylamine.6b First, we examined a model reaction to confirm that CuI/Pd(PPh3)4-catalyzed dehydrobromination is applicable to the synthesis of 2,5-bis(silylethynyl)silole. Thus, when 1 was treated with 2 equiv of (trimethylsilyl)acetylene under similar conditions, 2,5-bis(trimethylsilylethynyl)1,1,3,4-tetraphenylsilole (2) was obtained in 94% yield, as the sole volatile product (Scheme 1). Similar treatment of dibromosilole 1 with diethynyldiand monosilanes (3a-c), followed by reprecipitation of the organic products from methanol-chloroform, afforded polymers 4a-c in 38-63% yields, as shown in Scheme 2 and Table 1. The rather low yields for the polymers are ascribed to the formation of methanolsoluble oligomers which were separated by reprecipi-
Table 1. Preparation of Polymers 4a-c polymer
yield (%)
mp (°C)
Mw (Mw/Mn)a
4a 4b 4c
38 45 63
88-91 78-84 84-88
8 000 (1.6) 12 000 (1.2) 13 900 (2.4)
a
Determined by GPC, relative to polystyrene standards. Table 2. Properties of Polymers 4a-c, 5, and 6 TGAb
CVe
UVa
weight conductivity polymer (λmax/nm) Td5 c loss (%)d Epa (V)f Epc (V)g (S cm-1)h 4a 4b 4c 5 6
412 415 408 300 310
354 401 434 309 474
52 59 67 76 79
0.58 0.44 0.62 0.52 0.52 0.42 not observed 0.76 0.52
3.7 × 10-3 1.9 × 10-3 2.1 × 10-4
