Model mono-, di-, and tri-ω-functionalized three-arm star polybutadienes. Association behavior in dilute solution by dynamic light scattering and viscometry

The dilute solution properties of three-arm star polybutadienes with one, two, and three dimethylamino or sulfobetaine end groups were studied in tetrahydrofuran (THF) and cyclohexane by dynamic light scattering and viscometry. The polymers were prepared by anionic polymerization techniques using [3-(dimethylamino)propyl]lithium or sec-BuLi as initiator and methyltrichlorosilane as linking agent. The dimethylamino groups were converted to highly polar sulfozwitterion groups by reaction with cyclopropanesultone. In the case of the amine-capped polybutadienes, the hydrodynamic behavior in cyclohexane and THF was almost the same, meaning the absence of any appreciable association in cyclohexane. For the zwitterionic polymers, the hydrodynamic, R_H, and viscometric radii, R_v, in cyclohexane are larger than those for the corresponding amine-capped samples, showing the existence of aggregation. The difference decreases as the number of functional groups increases, which is in agreement with previous static methods results. The micelles formed by the monofunctional species behave hydrodynamically as star polymers with functionality equal to 2N_w, where N_w is the weight-average degree of association. The R_v/R_H ratio is lower than unity (0.51-0.89), meaning that during the flow in the viscometer there is some disassociation of the aggregates.