Syndiotactic 1,2‐polybutadiene with Co‐CS2 catalyst system. III.1H‐ and 13C‐NMR study of highly syndiotactic 1,2‐polybutadiene

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Abstract
The 1H and 13C‐NMR spectra of highly crystalline syndiotactic 1,2‐polybutadiene (s‐PB) are discussed in order to clarify the mechanism of butadiene polymerization with cobalt compound–organoaluminum–CS2 catalysts. Cis opening of the double bonds in the syndiotactic polymerization is affirmed by the study of the copolymer from perdeuteriobutadiene and cis,cis‐1,4‐dideuteriobutadiene. S‐PB (mp 210°C) has 99.7% 1,2 units, 0.3% isolated cis‐1,4 units, and 99.6% syndiotacticity. Polymer ends (2‐methyl‐3‐butenyl group and conjugated diene structure) are also determined. The differences in free energy of activation between 1,2 and cis‐1,4 propagation and between syndiotactic and isotactic propagation are 14.0 and 9.6 kcal/mol, respectively, for Co(acac)3‐AlEt3‐AlEt2Cl‐CS2, and 6.7 and 5.7 kcal/mol, respectively, for the aluminum‐free Co(C4H6)(C8H13)CS2 system. The conformation of s‐PB in o‐dichlorobenzene at 150°C is described by the sequence (tt)1.6(gg)(tt).