Tracer techniques for the determination of monomer reactivity ratios. IV. Monomer reactivity ratios for styrene–isoprene copolymerization

The ionization chamber‐vibrating reed electrometer assay technique has been improved in precision (±0.25% average deviation) and used to determine monomer reactivity ratios for the styrene—isoprene copolymerization. Values of r1 (styrene) = 0.50 (100°) and 0.54 (80°) and r2 = 1.92 (100°, 80°) have been obtained for the radical initiated copolymerization. For γ‐irradiation initiation at 22°, r1 = 0.66 and r2 = 1.88. For butyllithium initiation, r1 = 0.03 and r2 = 5.9. The difference in the radical and γ‐radiation initiation values is considered in terms of possible partial ionic character of the radiation initiation process and possible temperature dependence of the reactivity ratios (r1) over the 22–100°C. range. The latter indicates a difference in activation energy of 0.8 kcal./mole for the radical to styrene and radical to isoprene propagation steps.