Molecular weight and molecular weight distribution of polymers prepared from butyllithiums

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Abstract
The calculated kinetic molecular weight Mk was compared with experimentally determined viscosity molecular weight Mv of polybutadiene, polyisoprene, and polystyrene initiated with n‐BuLi, sec‐BuLi, and tert‐BuLi. The discrepancy between Mk and Mv depends on the ratio of rates of initiation Ri and propagation Rp, which in turn depends on initiator structure and monomer. In the alkyllithium‐initiated polymerization system, Mk based on the [RLi] consumed rather than charged, is almost equal to the actual molecular weight. The ratio of Ri and Rp also affects the molecular weight distribution. The order of monodispersity for polydienes is sec‐BuLi ≅ tert‐BuLi > n‐BuLi. For styrene, it is sec‐BuLi ≫ n‐BuLi ≫ tert‐BuLi. The fractionation data are in perfect agreement with the predictions of the kinetic data. The ideal conditions for the preparation of monodispersed polymer by means of anionic polymerization are also discussed.

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