Controlled Radical Polymerization Catalyzed by Ruthenium Complexes: Variations on Ru-Cp~#

摘要

A series of isoelectronic ruthenium-based complexes of the general formula [RuX(Cp~#)L_2] (Cp~# = cyclopentadienyl or cyclopentadienyl derivatives) were synthesized, and their relative catalytic activities were determined by monitoring the atom transfer radical polymerization of methyl methacrylate, n-butyl acrylate, and styrene. [RuCl(Cp~*)(PPh_3)_2] and [RuCl(Ind)(PPh_3)_2] were found to be highly efficient catalysts for ATRP, producing polymers with narrow molecular weight distributions (M_w/M_n < 1.2). The following order of increasing efficiency was determined : [RuCl(Cp)(PPh_3)_2] [RuCl(Ind)(PPh_3)_2] < [RuCl(Cp~*)(PPh_3)_2]. In sharp contrast, ruthena-carboranes were inefficient in ATRP, demonstrating therefore the prominent role of the Cp~# ligand. The effect of the phosphine ligands was also investigated, and additional studies indicated that the release of a phosphine ligand occurred prior to the activation of the carbon-halogen bond of both the initiator and polymer growing chain end by the unsaturated ruthenium center.