Abstract: A novel process for polymerization or copolymerization of monomers is described, according to which the reaction is carried out in the presence of an anionic initiator and a macroheterocyclic complexing agent of formula ##STR1## in which: EACH R.sub.1 is a hydrogen atom, a hydrocarbon group or an alkoxycarbonyl group, or the two R.sub.1 together can form a group of the general formula: ##STR2## THAT IS TO SAY FORMING A THIRD BRIDGE BETWEEN THE TWO NITROGEN ATOMS IN THE MOLECULE, A is a hydrocarbon group,D is an oxygen or sulfur atom or a hydrocarbon group, with the proviso that at least two of the groups D are oxygen or sulfur atoms and that, if each R.sub.1 is hydrogen, a hydrocarbon group or an alkoxycarbonyl group, one of these two groups D is oxygen or sulfur and the other is oxygen,n and p are integers ranging from 1 to 3, andm is 2 or 3.