The homopolymerization of 2,3-dimethyl-1,3-butadiene and the copolymerization of 1,3-butadiene/2,3-dimethyl-1,3-butadiene using the catalyst system AlEt2Cl-Co(acac)2. A structural investigation of the products

The homopolymerization of 2,3-dimethyl-1,3-butadiene with the catalyst system AlEt₂Cl-Co(acac)₂ affords polymers consisting of about 81% cis-1,4-units and 19% 1,2-units. The copolymerization of 1,3-butadiene/2,3-dimethyl-1,3-butadiene using the same system is of the ideal type and the composition of the copolymers is practically identical with that of the monomer mixture. The structure of the copolymers depends on their composition. The butadiene units, which are more than 95% cis-1,4 in the homopolymer, become partially 1,2 in the copolymers; their amount increases with increasing dimethylbutadiene content. Conversely, the fraction of dimethylbutadiene units with 1,2-structure decreases in the copolymers with increasing butadiene content. In the copolymers containing more than 50% of butadiene, the dimethylbutadiene units are practically all cis-1,4. The analysis of sequence distribution shows that the 1,2-butadiene units in the copolymers are not randomly distributed but are mostly adjacent to dimethylbutadiene units. The results obtained with dimethylbutadiene are compared with those reported for the homopolymerization of isoprene and the copolymerization of isoprene/butadiene using the same catalyst system.