Conformational Transitions in Poly{<i>n</i>-hexyl-[(<i>S</i>)-3-methylpentyl]silylene} in Dilute Solution: Temperature and Molecular Weight Dependence Detected by Circular Dichroism

Abstract

Ten narrow-distribution samples of poly{n-hexyl-[(S)-3-methylpentyl]silylene} (PH3MPS) ranging in weight-average molecular weight from 3.1 × 103 to 8.7 × 105 in isooctane, n-hexane, and methylcyclohexane were studied by circular dichroism and ultraviolet absorption over a wide temperature range (from −75 to 85 °C). To follow the conformational transition, Kuhn's dissymmetry factor gabs as a measure of helicity was determined as a function of molecular weight and temperature from the ratio of circular dichroism to absorbency. The molecular weight dependence of gabs was analyzed by a statistical mechanical theory based on a conformational picture of an alternating sequence of right-handed and left-handed helices intervened by helix reversals. The free energy of the helix reversal was much smaller than those for polyisocyanates investigated previously [poly((R)-1-deuterio-n-hexyl isocyanate) and poly((R)-2-deuterio-n-hexyl isocyanate)] at the same temperatures although its solutions showed large circular dichroism. Therefore, it was shown that the correlation lengths of the helix for this polymer were much shorter than the polyisocyanates at the same temperature. Thus, the helix reversal is the dominant molecular process in the conformational properties of PH3MPS.