Typical of such reagents are polyaldehydes such as glutaraldehyde and polyisocyanates such as 2,4-tolylenediisocyanate, 1,6-diisocyanatohexane and activated polyfluoro compounds such as 1,5-difluoro-2,4-dinitrobenzene and P,P'-difluoro-m,m'-dinitrodiphenylsulfone.