In this letter, we report high electro‐optic methylene di(phenylene isocyanate) (MDI)/DR19 side‐chain polymer film polymerization by vapor deposition polymerization. For samples deposited at substrate temperatures from 10 to 30 °C, the electro‐optic (EO) coefficient, r33, was measured to be 5 pm/V after poling. A lifetime of about one week was obtained. The highest EO effect observed were films deposited at −40 °C and polymerized after poling. The EO coefficient of these samples is about 24 pm/V while the lifetime is only about 30 min. The effect of substrate temperature, the ratio of monomers, and the poling temperature on the nonlinearity of the films are studied. © 1996 American Institute of Physics.