We synthesized two optically active helical polysilanes, poly [(S)-3,7-dimethyloetyl-3-phenylpropylsilane] (PS1) and poly [(R)-3,7-dimethyloetyl-3-phenylpropylsilane] (PS2), hearing a flexible and rodlike silicon main chain and enantiopure alkyl side chains with (S)- and (R)-chiral centers, respectively, at the gamma-positions. PS1 and PS2 underwent a thermodriven helix-helix transition at 10 degreesC in isooctane. Circular dichroism (CD) and UV studies demonstrated the transition characteristics, such as the transition temperature, population of right- and left-handed helical motifs, global shape, and screw pitch. At -80 degreesC, the dissymmetry ratio suggested that a preferential right-handed or left-handed screw sense was present in the polymer chains of PS1 and PS2, respectively. However, above the transition temperature, the appearance of a bisignate cotton band in the CD spectra suggested that both right-handed screw-sense, tight helical segments and left-handed screw-sense, loose helical segments coexisted in the same chains of PS1 and PS2. (C) 2004 Wiley Periodicals, Inc.