eferred examples of N-hydroxy imides of formula (I) that may be used in the present invention are those for which: n=0 and R1???R2???R3???R4???H (N-hydroxysuccinimide) (compound 1 below)n=0, R3???R4???H and R1 and R2 together form a bond (N-hydroxymaleimide) (compound 2 below)n=0, R3 is a phenyl group, R4???H and R1 and R2 together form a bond (N-hydroxy-2-phenylmaleimide) (compound 3 below)n=0, R