The poly(n-hexyl isocyanate), poly(p-tolyl isocyanate), and copoly(n-hexyl-p-tolyl isocyanate) which contains 16 mole percent of p-tolyl isocyanate content have been prepared by low temperature anionic polymerization in dimethyl formamide. Narrow-distribution fractions of each polymer species were obtained by carefully controlled successive fractional precipitation method. Number average molecular weight(Mn) and osmotic second virial coefficients were determined by osmometry in toluene at 37℃. The intrinsic viscosity behaviors of each series of polymer fractions have been examined over the molecular weight range of 2.9*10⁴ to 7.3*10⁵ g/mole in toluene, chloroform, and tetrahydrofuran at 25℃, as well as under theta conditions. Thetatemperatures of 16.4℃ for poly(n-hexyl isocyanate), 14.8℃ for poly(p-tolyl isocyanate), and 13.2℃ for the copolymer of n-hexyl and p-tolyl isocyanate were obtained from the phase separation studies in toluene. The unperturbed molecular dimensions in dilute toluene solution have been determined by a number of extrapolation methods for data at 25℃. The value of short-range interaction parameter, A=(2>₀/Mn)^1/2, of copolymer was found as 1.200*10^-8cm·mol^1/2/g^1/2. This was comparable with the value obtained for poly(p-tolyl isocyanate), 1.114*10^-8, but was found to be far below than that of poly(n-hexyl isocyanate), 3.24*10^-8. The molecular shape of copolymer in toluene at theta condition, therefore, was estimated as a interrupted helix structure with short blocks of n-hexyl isocyanate units.