Thermally stable poly(β£-methyl styrene-co-maleimide) (MSMI) and poly(β£methyl styrene-co-4-carboxyphenyl maleimide) (MSCM) substrate polymers were obtained readily by free radical polymerization of comonomers. Introduction of a DR1 chromophore to the maleimide units of MSMI substrate polymer by the Mitsunobu reaction was dependent on the reaction solvent. The degree of substitution of DR1 into the MSMI polymer was bound to be 91.1 mol % and 0.4 mol % by UV spectrometers in the THF and DMF solvent, respectively. DR1 chromophore was, however, substituted in the MSCM polymer at 33.0 mol % by Mitsunobu reaction in the THF solvent. Both substrate and NLO polymer exhibited high thermal stability due to the incorporation of maleimide units in the polymer chain. The glass transition temperature (T g ) and initial decomposition temperature (T i ) of the NLO polymer were in the range of T g Ο 185Β°C and T i Ο 310-345Β°C. The electro-optic coefficient (r 33 ) of NLO polymer was determined with an experimental setup capable of the real-time measurement while varying both the poling field and temperature. The NLO polymer MSMI-THF had a higher r 33 value than MSCM-DR due to an increased degree of substitution of DR1 chromophore. MSMI-THF had a maximum r 33 value of 16 pm/V at 135 MV/m poling field with a 632.8 nm light source.