Reactive melt-processing of poly(styrene-co-maleic anhydride) (SMA) with primary N-alkylamines was carried out in one step in a twin-screw extruder. Ring formation of the imide occurred within the residence time of the extruder, at temperatures exceeding 220-degrees-C and with an excess of primary amines. The increasing chain length of grafted amine onto the SMA backbone depressed the glass transition temperature of poly(styrene-co-N-alkylmaleimides) remarkably.