The effect of substituents on the conformational rigidity of the six-membered dihydrocycle constituting the repeating unit of heterocyclic polymers was studied by the semiempirical AM1 method and by conformational analysis. The polymer rigidity can be controlled over a wide range by choosing appropriate substituents in the dihydrocycle. An original procedure was proposed for calculating the Kuhn segment of the copolymer taking into account the Boltzmann factor.