Finite chain approximation for linear and nonlinear polarizabilities of polydiacetylene and polybutatriene

Static longitudinal polarizabilities α_zz and cubic hyperpolarizabilities γ_zzzz are obtained by the FPT-INDO method for finite chain C_4N+2H_2N+4 (N = 1–15) models of polydiacetylene and polybutatriene. For the acetylenic structure the onset of saturation first becomes evident at about C₁₄? C₁₈ for the linear polarizability and C₂₂? C₂₆ for the cubic term. The values per C₄H₂ unit are accurately extrapolated to the infinite chain limit. In the case of the butatrienic structure, no saturation is observed through C₅₄.