Absolute rates in anionic copolymerization

A study has been carried out to determine the absolute values of the rate constants in the homogeneous anionic copolymerization of butadiene and styrene, initiated by alkyllithium. The approach used was to measure the rate of formation of disappearance of the styryl anion spectrophotometrically, using the absorption peak at 436 mμ. The solutions of butadienyl and styryl anions (in benzene) were prepared by reacting butyllithium with small amounts of butadiene or styrene, in a preliminary step, until the initiation reaction was complete. Subsequently, each of the anionic solutions was mixed with the opposite monomer, and the cross‐propagation reaction measured spectrophotometrically. It was found that the rate of the reaction of the styryl anion with butadiene was extremely rapid, being virtually completed in less than one minute at room temperature. On the other hand, the rate of reaction of the butadienyl anions with styrene was easily measurable at room temperature, being of the same order as the previously measured homopolymerization rates for butadiene. These measurements enabled a calculation of the following reactivity ratios: rB = 3, rS = .06. This very large difference in cross‐propagation rates can easily account for the “anomalour” behavior previously found by several investigators, wherein butadiene polymerized preferentially in a mixture with styrene, and which had been attributed to special “solvation” effects by the butadiene.