End-Functionalized Block Copolymers of Styrene and Isoprene: Synthesis and Association Behavior in Dilute Solution

The dilute solution properties of di-and triblock copolymers of isoprene and styrene having a highly polar sulfozwitterion group on one or both ends of the chain were studied in the nonpolar solvent carbon tetrachloride (a good solvent for polystyrene and polyisoprene) by low-angle laser light scattering. The copolymers were prepared by anionic polymerization using high-vacuum techniques with [3-(dimethylamino) propyl]lithium as initiator. The dimethylamino end group of the chain was transformed to a zwitterion by reaction with cyclopropanesultone. The monofunctional zwitterion-capped copolymers were found to associate in carbon tetrachloride, whereas the difunctional versions formed gels even below the coil overlap concentration, c^*. The aggregation number was found to decrease with increasing base molecular weight due to the excluded volume repulsions of the copolymeric tails. Comparison with the corresponding ω-zwitterionic polyisoprene homopolymers in cyclohexane and in carbon tetrachloride shows that for the same base molecular weight the materials with the homopolymeric tails aggregate more strongly. Aggregation numbers for the copolymers are of the same order of magnitude as the degree of association measured for a zwitterion-capped polystyrene. The presence of the phenyl rings in the copolymer chain seems to reduce the extent of aggregation. The effect of position and number of the functional groups on the strength of the association is also discussed.