Block copolymers with zwitterionic groups at specific sites: synthesis and aggregation behavior in dilute solutions

Well-defined linear diblock and triblock copolymers of styrene and isoprene, nearly symmetric in composition, with one or two sulfobetaine associogenic groups at the ends of the polymer chain or the junction points between blocks, were synthesized by anionic polymerization high-vacuum techniques. The synthetic strategy used the combined initiation of polymerization with 3-dimethylaminopropyllithium, living end-capping with 1-(4-dimethylaminophenyl)-1-phenylethylene, and postpolymerization reaction with cyclopropanesultone. The association behavior of these macromolecules in dilute solutions in the nonpolar solvent CCl₄, good for both blocks, was studied by a number of methods, including static and dynamic light scattering and viscometry. The number and position of the associogenic groups dramatically influenced the association behavior of these model block copolymers. The end-functionalized samples formed larger aggregates than the junction-point-functionalized ones. The difference was attributed to stronger excluded volume effects when the zwitterion group was located within the chain than when the group was at the chain end(s).