radical, depending whether n is equal to 2, 3, 4 or 5 respectively, and A is the residue of a polyol of structure A(OH)n and A is a member selected from the group consisting of polymethylene glycols, polyoxyalkylene glycols, polyethertriols, polyethertetraols, glycerol, trimethylolalkanes and xylitol and of mono(arylsulphonylurethanes) of formula (I) ##STR37## wherein Ar is an aromatic group;R is