Examples of suitable modifiers are maleic acid, methyl maleate, itaconic acid, tetrahydrophthalic acid, anhydrides and imides of these, fumaric acid, the mono- and diesters of these acids, for example with C1- and C2-C8 alkanols, the mono- or diamides of these acids, such as N-phenylmaleimide and maleohydrazide.