Process for producing polybutadine rubber with enhanced mechanical strength

A process for producing a polybutadiene rubber with enhanced mechanical strength, comprising the two successive steps of:

• (A) polymerizing a first polymerization mixture comprising 1,3-butadiene and a polymerization medium in the presence of a cis-1,4-polymerization catalyst which comprises:
• (1) a cobalt compound selected from the group consisting of cobalt complexes with beta-diketones of the formula (II):
• ##STR3## wherein R.sup.1 and R.sup.2 represent an aliphatic hydrocarbon radical having 1 to 3 carbon atoms, respectively, and R.sup.3 and R.sup.4 represent at least one member selected from the group consisting of a hydrogen atom and an aliphatic hydrocarbon radical having 1 to 3 carbon atoms, respectively;
• a cobalt complex with .beta.-keto acid esters of formula (III):
• ##STR4## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are as defined above;
• cobalt salts of an organic carboxylic acid having 6 or more carbon atoms;
• and a cobalt halide complex of the formula (IV):
• CoXnYm (IV)
• wherein X represents a halogen atom, n represents an integer of 2 or 3, Y represents an organic ligand capable of forming a complex in coordination with the cobalt halide and m represents an integer of from 1 to 4, and
• (2) a dialkyl aluminum halide of the formula (I):
• AlR.sub.2 X (I)
• wherein R represents an alkyl radical having 2 to 8 carbon atoms and X represents a halogen atom, to convert at least a portion of the amount of said 1,3-butadiene to a cis-1,4-polybutadiene;
• and
• (B) polymerizing a second polymerization mixture comprising the resultant cis-1,4-polybutadiene from Step (A), unreacted 1,3-butadiene and a polymerization medium in the presence of a 1,2-polymerization catalyst which comprises:
• 1. said cobalt compound;
• 2. said dialkyl aluminum halide;
• 3. carbon disulfide;
• and
• 4. an electron donor organic compound selected
• from the group consisting of an aliphatic polyether, an aliphatic carboxylic ester, an aliphatic ketone, an aliphatic acetal, an aliphatic N,N-dialkyl amide, an aliphatic dialkyl sulfoxide and a trialkyl phosphate, to convert said cis-1,4-polybutadiene and said 1,3-butadiene to a polybutadiene rubber, whereby the resultant polybutadiene rubber comprises 70% to 95% by weight of a boiling n-hexane-soluble fraction thereof having an intrinsic viscosity of from 1 to 5 determined in toluene at a temperature of 30.degree. C. and containing 95% or more of a cis-1,4-structure, and 5% to 30% by weight of a boiling n-hexane-insoluble fraction having a reduced viscosity of from 0.5 to 5, determined in tetrahydronaphthalene at a temperature of 135.degree. C., and a melting point of 180.degree. C. or more, and containing 85% or more of a 1,2-structure.