Screw-sense-selective polymerization of the chiral isocyanide monomers derived from phenylalanine with NiCl2 as a catalyst in methanol to yield helical-conjugated polyisocyanide was investigated with respect to the thermal stability of its helical conformation. Poly(1-tert-butoxycarbonyl-2-phenylethyl isocyanide) (poly1c) took a stable helical conformer independent of the polymerization temperature. In poly(1-ethoxycarbonyl-2-phenylethyl isocyanide) (poly2c), which had slightly smaller side groups, the helical conformation was thermally destabilized. The specific rotation and circular dichroism of poly2c prepared at temperatures greater than 40 degreesC were considerably depressed in comparison with the values for poly2c prepared at or below room temperature. Additionally, poly2c prepared at low temperatures exhibited reversible temperature-dependent specific rotation and circular dichroism, whereas poly1c showed few changes. It is suggested that polyisocyanide derived from phenylalanine takes various helical conformers (i.e., from tightly to loosely coiled helices), the thermal stability of which depends on the size of the side group.