Strategies for the Synthesis of Well-Defined Star Polymers by Anionic Polymerization with Chlorosilane Coupling and Preservation of the Star Architecture during Catalytic Hydrogenation

Abstract

The synthesis of isoprene-based star polymers by anionic polymerization, chlorosilane coupling, and catalytic hydrogenation was studied in detail, with the goal of obtaining and preserving a well-defined star architecture. The coupling reaction between polyisoprenyllithium (PI-Li) and chlorosilanes, which can be intractably slow in apolar media, was dramatically accelerated by the concurrent addition of tetrahydrofuran (THF) with the coupling agent. In the presence of THF, PI-Li produces a yellow color allowing the living end concentration to be tracked visually, enabling near-stoichiometric coupling, as demonstrated by the synthesis of 6-arm star polymers and star block copolymers. Moreover, glassblowing techniques are not required. Star polymers coupled with chlorosilane terminating agents lost arms during catalytic hydrogenation. Several common hydrogenation catalysts were evaluated. The degree of degradation depended on the identity of the catalyst and the temperature of the reaction—more active catalysts and higher temperatures led to more scission—but not the reaction time. Degradation closely tracked the hydrogenation reaction indicating that scission is a catalytically activated process. This phenomenon is attributed to a decreasing affinity of the polymer for the catalyst with increasing saturation. Degradation of the star architecture depended strongly on the steric environment at the core. It was shown that the steric strain can be reduced by tuning the topology of the coupling agent to limit the number of arms per Si atom. The steric constraints at the core were further relaxed by adding a short run of butadiene units to the arms just prior to coupling, which nearly eliminated degradation during hydrogenation of a 6-arm star.