An additive comprising an active; the interfacial polymerization reaction product of a (poly)isocyanate and a polyamine; a copolymer having maleic anhydride-based repeat units and either or both of olefinic or styrenic-based repeat units; wherein at least a portion of the maleic anhydride-based repeat units are neutralized. A method of preparing an additive comprising providing an oil phase comprising an active, a solvent, a (poly)isocyanate; providing an aqueous phase comprising water and an emulsifier; mixing the oil phase and the aqueous phase to provide an emulsion; providing a polyamine to the emulsion; providing an aqueous dispersion or solution comprising a copolymer having maleic anhydride-based repeat units and either or both of olefinic or styrenic-based repeat units; wherein at least a portion of the maleic anhydride-based repeat units are neutralized; and cofeeding the emulsion and the aqueous dispersion or solution to a spray dryer.

A resin composition including from 30 to 99 wt % of a cellulose-based resin represented by the following formula (1) and from 70 to 1 wt % of a fumaric acid ester polymer containing 30 mol % or more of a fumaric acid diester residue unit represented by the following formula (2); an optical compensation film using the same; and a production method of the optical compensation film. ##STR00001##
(wherein R.sub.1 to R.sub.3 represent a substituent having a carbon number of 1 to 12, and R.sub.4 and R.sub.5 represent an alkyl group having a carbon number of 1 to 12).