Abstract: The invention relates to a process for the polymerization or copolymerization of at least one monomer which is polymerizable by the radical route in the presence of a stable free radical from the nitroxide family. The stable free radical is &bgr;-substituted and provides excellent control of polydispersity while ensuring a better rate of polymerization or copolymerization, if they are compared with the stable free radicals used in the prior art.

Abstract: This process is characterized in that at least one of the said monomers is polymerized or copolymerized in bulk, solution, emulsion or suspension at a temperature which may be as low as 0.degree. C., in the presence of an initiator system comprising at least one radical-generator compound other than an arenesulphonyl chloride; and at least one catalyst consisting of a metal complex represented by the following formula (I): Ma.sub.a (L).sub.n in which: M represents Ni, Pd or Pt; A represents a halogen or a pseudohalide; the groups L are ligands of the metal M, which are chosen independently from those containing at least one from among N, P, As, Sb, and Bi, it being possible for at least two of these ligands to be connected together by one or more divalent radicals; a is an integer from 1 to 5; n is an integer from 1 to 4; with 4.ltoreq.a+n.ltoreq.6, a+n representing the coordination number of the complex.

Abstract: The present invention relates to the stabilization of a polymer by a stable free radical and in particular to the use of a stable free radical for inhibiting the degradation of a polymer by a mechanism which does not involve oxygen. This use can be carried out when the polymer is in the presence or in the absence of oxygen. The degradation which can be inhibited can be a depolymerization or a crosslinking.