o-azo compound and (2) a member selected from the group consisting of an organic unsaturated carboxylic acid and an organic anhydride, the said blending eing at temperatures up to 30 C., (B) reacting the said blend at temperatures up to 250 C., (C) recovering the product from (B) and mixing it with an isocyanate having more than one radical of the formula N=C=X wherein X is a member selected from