Disclosed is a process for the preparation of compounds of formula (I) wherein R1 is C1-C4 alkoxy; R2 is hydrogen or a group of formula —CH2—PO3H2 or is a group of formula R1-O—(CH2—CH2O)n—CH2—CH2—; R3 is an alkylene, optionally substituted with one or more —NR2-groups; R4 and R5 are both a group of formula —CH2—PO3H2 or one of them is a group of formula —CH2—PO3H2 and the other is a group of formula R1-O—(CH2—CH2—O)-CH2-CH2-; n is an integer between 4 and 50; which comprises: a) reacting a compound of formula R1-O—(CH2—CH2—O)n—CH2—CH2—OH with SOCl2 or a similar reagent to give a compound of formula R1-O—(CH2—CH2—O)n—CH2—CH2—Cl; b) reacting the compound obtained in a) with an amine of formula NH2—R3′-NH2 wherein R3′ is an alkylene optionally substituted with one or more —NH-groups; c) reacting the compound obtained in b) with formaldehyde and phosphorous acid.