Poly(di-n-propylsilylene) and poly(diethylsilylene-co-di-n-propylsilylene) have been investigated by Raman, IR, and UV spectroscopy, TGA, DSC, wide angle X-ray diffraction, and solid state l8C and MSi NMR. No IR-Raman coincidences were found in the Si-Si or Si-C stretching regions for (n-Pr2Si)", demonstrating that the main chain has a planar all-trans configuration, T" consistent with the UV absorption at 355 nm. From the Si-C stretching bands, all of the n-propyl groups appear to have the same conformation.(n-Pr2Si)n(Et2Si)m also has the T. configuration at room temperature, and a tetragonal crystal lattice. Both polymers undergo a first-order transition to a mesomorphic phase, above 222 °C for (n-Pr2Si)" and above 113 °C for the copolymer. ylsilylene) [ ( Me2Si) "] 3 poly (diethylsilylene) [(Et^SDJ,3-5 and poly(di-n-propylsilylene) [(n-Pr2Si)"]4 all have been shown to have a planar, all-trans zigzag (T") structure for the main polymer chain. Poly(di-n-butylsilylene)6-7 and poly(di-n-pentylsilylene)7•8 however, adopt 7/3 helical structures under ambient conditions. The all-trans structure returns with six-carbon side chains; the wellstudied poly(di-n-hexylsilylene) has the T" structure up to its transition temperature at 42 °C and a mesomorphic structure above that.9-11 Poly(di-n-heptylsilylene) also takes the T" structure at ambient temperatures.10'12Here we report a study of (n-Pr2Si)" and the related polysilane copolymer poly(diethylsilylene-co-di-n-propylsilylene), (n-Pr2Si) "(Et2Si) m. Our results confirm the T" conformation for poly(di-n-propylsilylene), reported from earlier electron and X-ray diffraction studies,4 and show that at room temperature the copolymer also has a planar zigzag conformation of the silicon chain. These results also allow a closer description of the structures, especially at elevated temperatures where both polymers have a firstorder phase transition.
II. Results and DiscussionA. Poly(di-n-propylsilylene) at Room Temperature. The ultraviolet spectrum of (n-Pr2Si)" was previously reported to consist of three peaks at 312, 324, and 360 nm, but the earlier authors considered that impurities
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