rs, etc., or (3) A and B are hydrogen, D is a carboxylic acid ester group and E is a methylene carboxylic acid ester group, e. g., itaconic acid esters, or (4) A and D are carboxylic acid ester groups as above, E is a methylene carboxylic acid ester group and B is hydrogen, e. g., aconitic acid esters.