We report the experimental evaluation of the intensity-dependent index of refraction of a new class of third order nonlinear materials, side chain polymers containing 4-dialkylamino-4'-nitro-stilbene and 4-dialkylamino-4'-nitro-diphenylbutadiene as side groups. The measurements, based on nonlinear grating coupling into planar waveguides with 30 ps pulses at 1.064-mu-m, showed large electronic nonresonant n2 Kerr coefficients.