The terminal radicals of the compounds of formula B1 are according to the definitions of R2 and R3 and if X represents --COO-- or --CONR5 --, the acyl radicals of acrylic or methacrylic acid or the monoacyl radicals of maleic, fumaric or itaconic acid, or if monoalkyl esters of these acids with straight or branched chain alkanols of 1 to 10 carbon atoms, such as methanol, ethanol, butanol, diisobu