the other a- (CH2),- group which is uninterrupted or interrupted by-O-,-S-, or by-NR7-, and/or is unsubstituted or substituted by Ci-Cea) kyt ; R7 is C1-C5alkyl ; is from 1 to 4; m is from 1 to 7; n is from 1 to 4; when n = 2, Ri, R5 or R6 is a bivalent alkyl group; or Ri and R2 together with the 2 nitrogen atoms linking them form a- (CH2) m- ring ; when n = 3, Ri, R5 or R6 is a trivalent alkyl g