Association behavior of living anionic lipophobic head‐groups in hydrocarbon mileau

Abstract

AbstractThe commercial applications of anionic polymerization have become wide‐spread with regard to the preparation of linear and star shaped homopolymers and block copolymers (Diene‐35TM, KratonTM, VectorTM, SolpreneTM, K ResinTM and ShellVisTM). Furthermore, randomized styrene/butadiene linear copolymers (SolpreneTM) and low molecular weight, 10K or less, polybutadienes (LithenesTM) of various microstructures are commercially available. Recently Liquid KratonsTM with OH functional units in place on one or both chain ends have become items of commerce. The lithium based systems yield materials of uniform composition and molecular weights with virtually monodisperse molecular weight distributions. Both molecular weight and composition are tunable via the simple expedient of manipulating the monomer/initiator ratio. Although the synthetic aspects of these anionic systems have been well‐mastered and exploited over the years the mechanistic features have remained controversial; issues which revolve around active center aggregation behavior and the reactivity (or its absence) of these self‐assembled anionic lipophobic head‐groups. Thus a study of these systems was undertaken using modern scattering techniques and data analysis procedures.Small angle neutron scattering (SANS) in combination with dynamic and static light scattering, DLS and SLS, has been used to evaluate the association behavior of the styryl‐ and dienyllithium head‐groups in benzene and cyclohexane solutions. Both types of lipophobic active centers were found to aggregate as dimers which in turn can self‐assemble to yield large‐scale wormlike micelles (prolate ellipsoids). These systems thus emulate the behavior of diblock copolymers and surfactants in their capacity to form flexible cylindrical micelles. It was also found for all head‐groups that the combination of long chain lengths and high polymer concentrations favored the presence of the dimer structure. This commonality of aggregation state is in consonance with results given in 1964.