Aqueous Polymerization of Styrene Promoted by Water-Soluble Robust Ruthenium Hydride Complexes

Aqueous polymerization of styrene in biphasic media (styrene/water) has been achieved by water-soluble robust mononuclear hydride complexes [(η⁶-C₆Me₆)Ru^II(bpy)H]_n(X) {[1]_n(X), where X = SO₄ (n = 2) or CF₃SO₃ (n = 1), bpy = 2,2‘-bipyridine}. The hydride complex [1]₂(SO₄) was synthesized from the reaction of an aqua complex [(η⁶-C₆Me₆)Ru^II(bpy)(H₂O)](SO₄) {2(SO₄)} with a water-soluble hydrogen donor HCOONa in H₂O in a pH range of 4−12 at 70−100 °C. The structures of [1]₂(SO₄) and 1(CF₃SO₃) were determined by X-ray analysis, ¹H and ²H NMR, IR, and electrospray ionization mass spectrometry (ESI-MS). X-ray analysis has revealed that complex 1(CF₃SO₃) adopts a distorted octahedral geometry with the Ru atom coordinated by one η⁶-C₆Me₆ ligand, one bidentate bpy ligand, and one terminal hydrido ligand that occupies a bond position. Complex [1]₂(SO₄) reacts with excess amounts of styrene in biphasic media to provide polystyrene in a 62% isolated yield for 8 h. The polydispersity (M_w/M_n) of the obtained polystyrene is 1.8. The isolated yield of polystyrene shows a maximum around pH 8. The pH-dependence is similar to the pH-dependent formation of 1. Growing polymer intermediates [(C₆Me₆)Ru(bpy){(C₂H₃)(C₆H₅)}_nH]⁺ (n = 1−4) were directly observed by ESI-MS.