AbstractThe molar optical rotation at 589 nm, the UV‐, CD‐and ORD‐spectra of atactic and highly isotactic random copolymers of 4‐methyl‐1‐penten‐3‐one (4MP3O) and (S)‐methyl‐1‐hexen‐3‐one ((S)‐4MH3O) were measured in different solvents at different temperatures. The ellipticity observed in the isotactic copolymers between 240 and 320 nm is much larger than that calculated for a mixture of the two homopolymers, showing that in the isotactic (but not in the atactic) copolymers chiral conformations, giving contributions of negative sign to the CD, prevail also in the monomeric units arising from the non‐chiral monomer. The decrease of the ellipticity with increasing temperature for the copolymers is not linear; the average value between 20°C and 60°C is larger for the copolymers than for the homopolymers. The results indicate that in the isotactic copolymers main‐chain helical conformations of one handedness are favoured over those of the opposite screw sense, and that achiral units incorporated in helical sections are forced to assume chiral conformations similar to the conformations that chiral units assume in the same helical section.