wherein R1 is S-Alk-R wherein Alk is an alkylene linkage, preferably a methylene or poly methylene linkage, e.g. a C2 to C6 polymethylene linkage, or a polyalkenylene linkage, e.g. a C3 to C6 alkenylenyl linkage, R is R is -N=C(NR3R4)(NR5R6) or -NR7[(NR3R4)C=NR5], i.e. guanidines or -N=C(R8)(NR9R10), i.e. amidines wherein R3-R7- 10 is H, Alk, Ar or (CH2)nAr wherein Ar is an aryl group and n is an integer of from 1 to 13 or R3 and R4 or R4 and R5 or R5 and R7 or R3 and R7, or R9 and R10, together may be -CH2-CH2- or -CH2-CH2-CH2- and R2 is selected from the group consisting of hydroxyl, lower alkyl and hydroxyl-substituted lower alkyl and R2 is selected from the group consisting of hydroxy and lower alkyl.
