THERMOCHROMIC BEHAVIOR OF POLYSILANES CONCERNING THE DEPENDENCE ON THE SEQUENCE LENGTH

Poly(dihexylsilane) and poly(dibutylsilane) having a wide range of mol ecular weights were prepared by the photodegradation of the correspond ing high-molecular-weight polymers. These polymers showed characterist ic thermochromism. Thus, the absorption maximum observed for both poly mers at around room temperature disappeared and generated a new band a t longer wavelengths at low temperature. The transition temperatures ( T-c) (at which the intensities of these two absorption bands become eq ual) of these polymers were apparently proportional to the inverse of the number-average molecular weight ((M) over bar(n)(-1)) until the po lysilanes reach a certain value of molecular weight. Beyond that value , the transition temperature was no longer dependent on the molecular weight. The thermochromic transition temperature also depended on the size of the alkyl substituents. These results have been satisfactorily explained by a model of the thermochromism based on a cooperative tra nsition between the ordered trans and gauche states for poly(dialkylsi lane)s.