Process for preparing bimodal and multimodal polymers of conjugated dienes

Claim 1

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, consisting in polymerizing the monomer(s) in a reaction medium at a temperature of between 20° C. and 200° 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.4 M.sup.2 (M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4).sub.2 M.sup.3 R.sup.1 R.sup.2 R.sup.3 M.sup.1 OM.sup.3 R.sup.1 R.sup.2 in which M 1 represents an alkali metal, M 2 represents an alkaline earth metal, M 3 represents a metal of group 3A, R 1 , R 2 , R 3 represent an alkyl or aralkyl radical and R 4 represents either an alkyl or aralkyl radical or a radical XB in which X represents an oxygen, sulfur or nitrogen atom and B represents an alkyl or aralkyl radical or a radical M 3 (R 5 R 6 ) in which R 5 , R 6 represent an alkyl or aralkyl radical, with (b) at least one electron-donor compound containing at least one heteroatom selected from the group consisting of aprotic polar compounds, protic polar compounds and compounds formed of the reaction products of protic polar compounds with an alkali metal or with an alkaline earth metal, characterized by adding to the reaction medium during the course of the polymerization reaction, as a modifying agent which is not a polymerization initiator, a compound of a metal of groups 1B to 7B and 8 of the periodic classification of elements of the Mendeleev Table or a magnesium compound of the general formula Mg(A) 2 in which A represents an alkyl radical having from 1 to 10 carbon atoms or an alcoholate, phenate, beta-diketonate or carboxylate radical; said modifying agent being added in such amount that the molar ratio of the modifying agent to the organic compound of the metal of group 3A is between 0.01 and 20.