3,4-di-(2′-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with 2,4-toluenediisocyanate, 1,6-hexamethylenediisocyanate and 3,3′-dimethoxy-4,4′-biphenylenediisocyanate to yield novel Y-type polyurethanes (4-6) containing 3,4-dioxybenzylidenemalononitrile groups as nonlinear optical (NLO) chromophores, which are parts of the polymer backbones. The resulting polyurethanes 4-6 were soluble in common organic solvents such as acetone and DMSO. Polymers 4-6 exhibited a thermal stability up to 260°C from thermogravimetric analysis (TGA) with differential scanning calorimetry (DSC) with T g values in the range of 96–152°C. The SHG coefficients (d33) of poled polymer films were around 6.8 × 10-9 esu. These poled polymers exhibited a high thermal stability of dipole alignment even at 20°C higher than T g , and no SHG decay was observed below 175°C due to the partial main chain character of the polymer structure, which is acceptable for NLO device applications.