Increasing the Mooney viscosity in a stepwise manner in the production of high-molecular-weight polybutadiene having more than 95 wt.% cis-1,4 units and less than 1 wt% 1,2-vinyl units, comprises: (i) polymerizing at least one monomer comprising butadiene and/or isoprene, in the presence of at least one inert, organic solvent and in the presence of at least one catalyst based on neodymium carboxylate at temperatures of -20[deg] C to 150[deg] C; (ii) terminating the polymerization by adding protic compounds; and (iii) adding sulfur chlorides to the polymerizate. Increasing the Mooney viscosity in a stepwise manner in the production of high-molecular-weight polybutadiene having more than 95 wt.% cis-1,4 units and less than 1 wt% 1,2-vinyl units, comprises: (i) polymerizing at least one monomer comprising butadiene and/or isoprene, in the presence of at least one inert, organic solvent and in the presence of at least one catalyst based on neodymium carboxylate at temperatures of -20[deg] C to 150[deg] C; (ii) terminating the polymerization by adding protic compounds; and (iii) adding sulfur chlorides to the polymerizate. The sulfur chlorides are treated with a carboxylic acid, fatty acid, and/or fatty acid ester before adding to the polymerizate. Independent claims are also included for: (1) a high molecular weight neodymium-catalyzed polybutadiene obtained by the above mentioned method; and (2) rubber mixtures comprising a polybutadiene.
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