Structure and reactivity in cationic polymerization of butadiene derivatives. III. 2‐phenylbutadiene

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Abstract
Cationic polymerization of 2‐phenylbutadiene (2‐PBD) has been investigated. Polymerization were performed by SnCl4·TCA, WCl6, and BF3·OEt2 as catalysts in methylene chloride. 2‐PBD polymerized easily and gave low molecular weight polymers. The polymerization proceeded to give a polymer having 1,4‐structure without 1,2‐ or 3,4‐structure. The double bonds of the polymer were partially consumed, probably owing to cyclization and chain‐transfer reactions. 2‐PBD was 0.66 times as reactive as styrene and 1.2 times as reactive as isoprene in the copolymerization at −78°C by SnCl4·TCA in methylene chloride. Reactivities of ring‐substituted 2‐PBD obeyed the Hammett relation with ρ+ = −2.04. The 13C chemical shift of ring‐substituted 2‐PBD was measured. Chemical shift values for C1 and C3 were correlated with Hammett σ, but those for C2 and C4 were almost unaffected by the substituents. On the basis of experimental results, the transition state of the cationic polymerization of 2‐PBD was depicted as a benzylic cation rather than a phenylallylic one.