Poly(di-n-propylsilylene) and poly(diethylsilylene-co-di-n-propylsilylene): solid state structure and phase transitions

Abstract

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

Raman intensity and conjugation with participation of ordinary σ‐bonds

“…52,53 In accord with Ref. 5, their Raman spectra, provided that they are excited in the usual way by visible light, should exhibit pre-resonant behavior.…”

“…The vibrational spectra of both polymers along with the band assignments were published previously. 52,53 The assigned Raman spectra of silicon five-membered rings can be found in Refs 54 and 55.…”

Regularities and Peculiarities of Solid Polydialkylsilane Order-Disorder Transitions as Studied by Optical (UV, Raman and IR) Spectroscopy

“…A change in the silicon backbone conformation can be followed most easily by the UV method. In addition, the selection rules operating in the n(SiSi) and n(SiC) regions of the Raman and IR spectra allow determination of the symmetry of the polymer repeating unit and thus the polymer backbone structure [6][7][8][9]13] (see Fig. 2).…”

“…The aim of this paper is to describe characteristic features of polydi-n-alkylsilane transitions based on generalization from the authors' experience [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] and literature data, as well as to demonstrate that the three types of ordering are not necessarily interrelated.…”

Dynamic Mechanical Properties of Extruded Rods of Poly(dimethylsilylene) and Polysilane Copolymers having Methyl and Ethyl Substituents

“…It was found that the copolymers with DM/MP = 7/3-9/1 forms a columnar mesophase at elevated temperatures, as was reported for some other polydialkylsilanes. [9][10][11][12][13][14] We showed that the highly oriented materials of insoluble polysilanes could be prepared by extrusion in the columnar mesophase and that the mechanical properties of oriented PMMMP rods were improved with increasing degree of orientation. [7] It was further shown that the modulus at room temperature decreased with increasing content of MP units, suggesting that the molecular motion of the longer substituents lowers the modulus at room temperature.…”