Abstract: A process of preparing a bimodal or multimodal homopolymer of a conjugated diene or a bimodal or multimodal copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound consists in polymerizing the monomer(s) in a reaction medium at a temperature of between 20.degree. C. and 200.degree. C. in the presence of a catalyst system formed of the reaction product of:(a) an organic compound of a metal of group 3A of the periodic classification of elements of the Mendeleev Table having one of the following formulas;M.sup.1 M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4M.sup.2 (M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4).sub.2M.sup.3 R.sup.1 R.sup.2 R.sup.3M.sup.1 OM.sup.3 R.sup.1 R.sup.2with (b) at least one electron-donor compound containing at least one heteroatom,and adding to the reaction medium during the course of the polymerization reaction a compound of a transition metal of groups 1b to 7B and 8 or a magnesium compound of the general formula Mg(A).sub.2.

Abstract: A process of preparing a bimodal or multimodal homopolymer of a conjugated diene or a bimodal or multimodal copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound consists in polymerizing the monomer(s) in a reaction medium at a temperature of between 20.degree. C. and 200.degree. C. in the presence of a catalytic system comprising an organolithium initiator, a barium, strontium or calcium compound, and an organometallic compound of a metal of group 2B or 3A of the periodic classification of elements of the Mendeleev Table and adding hydrogen as a modifying agent to the reaction medium during the course of the polymerization reaction.

Abstract: A process of preparing a bimodal or multimodal homopolymer of a conjugated diene or a bimodal or multimodal copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound consists in polymerizing the monomer(s) in a reaction medium at a temperature of between 20.degree. C. and 200.degree. C. in the presence of a catalyst system comprising an organolithium initiator, a barium, strontium or calcium compound, and an organometallic compound of a metal of group 2B or 3A of the periodic classification of elements of the Mendeleev Table, and adding to the reaction medium during the course of the polymerization reaction a compound of a transition metal of groups 1B to 7B and 8 or a magnesium compound of the general formula Mg(A).sub.2.

Abstract: A process of preparing a bimodal or multimodal homopolymer of a conjugated diene or a bimodal or multimodal copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound consists in polymerizing the monomer(s) in a reaction medium at a temperature of between 20.degree. C. and 200.degree. C. in the presence of a catalyst system formed of the reaction product of:(a) an organic compound of a metal of group 3A of the periodic classification of elements of the Mendeleev Table having one of the following formulas:M.sup.1 M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4M.sup.2 (M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4).sub.2M.sup.3 R.sup.1 R.sup.2 R.sup.3M.sup.1 OM.sup.3 R.sup.1 R.sup.2with (b) at least one electron-donor compound containing at least one heteroatom,and adding hydrogen as a modifying agent to the reaction medium during the course of the polymerization reaction.