Poly{3-[(S)-2-methylbutoxy]phenyl isocyanate} was shown to be locally helical, but a rather flexible wormlike chain of persistence length 3 nm. Its optical activity changed remarkably on temperature-solvent and molecular weight, vanishing at a specific temperature T-c irrespective of molecular weight, exhibiting a cooperative transition of helical conformations. Such optical activity data were well explained by a theory of Lifson et al. on the helix reversal model, taking the conformations of the terminal residues explicitly. This polymer is locally helical and rodlike because of its conjugating main-chain amide bonds, which are conjugating also with side-chain phenyl rings. However, the hitter conjugation weakens the main-chain stiffness, with almost no contribution to the global stiffness from helix reversal points.