A new polymer for nonlinear optics (NLO) based on maleimide/methylvinylisocyanate precursor polymers is introduced. This sidechain copolymer is cured after spin-coating and before poling, which, by a thermally activated fragmentation reaction, leads to increased glass transition temperature, increased density of NLO chromophores, and reduced solubility. NLO materials will play an important role in optical telecommunications, but the problem of the chromophores relaxing back into the isotropic state must first be solved.