Variable-temperature carbon-13 and silicon-29 CPMAS NMR studies of poly(di-n-hexylsilane)

Abstract

Abstract : Poly(di n-hexylsilane) has been studied in the solid state using variable temperature 13C and 29Si CPMAS NMR spectroscopy. It has been found that the silane backbone exists in different conformations above and below the thermochromic temperature transition of ca. 307 K. The low temperature form is characterized by a silicon resonance at ca. -20.8ppm which is assigned to the all trans conformation, consistent with other spectroscopic and theoretical calculations. As the temperature was increased above the lowest measured value of 240 K, a second narrow peak at ca. -24.1ppm was observed upfield of the all trans peak in the 29Si NMR spectra. The relative area of the upfield peak increased gradually from 280 to 304 K until about 25% of the intensity was in the upfield resonance. At 310 K a sharp transition occurs, and the remaining 75% of the low field peak completely disappears. The aliphatic side chains at low temperatures are tightly restricted with the C-1 methylenes present in two conformations at temperatures below 300 K. These two C-1 conformations are present in equal populations and are probably rigid and alternating across and/or along the silane backbone. A single C-1 resonance is observed at higher temperatures. At temperatures at which two silicon resonances are observed, the C-2 and C-3 methylene carbons are each simultaneously observed as broad and narrow resonances. The broad component is dominant at low temperatures which corresponds to the downfield all trans silicon resonance while the narrow component observed at higher temperatures corresponds to the high field silicon resonance. The C-4 through C-6 resonances are observed to be relatively narrow but their motions are not isotropic.