The nonlinear optical properties of polyaminoborane, -(NH2-BH2)(N)-, and polyiminoborane, -(NH-BH)(N)-, oligomers are studied by using ab initio schemes, taking explicitly into account the dynamic electron correlation effects. We report the evolution with chain length of the geometries, charges, dipole moments, polarizabilities, and first hyperpolarizabilities of both systems. The first hyperpolarizabilities are rationalized in terms of the delocalization/asymmetry interplay.