Design, Behavior, and Recycling of Silica-Supported CuBr−Bipyridine ATRP Catalysts

Abstract

A new strategy for immobilizing CuBr/bipyridine (bpy) complexes on silica surfaces is described. An immobilizable, organosilane-containing bpy ligand (SdMBpyTMS) is synthesized and complexed with CuBr followed by addition to four different silica supports, mesoporous SBA-15 with 50 and 100 Å pores, controlled pore glass (CPG) with 240 Å pores, and nonporous Cab-O-Sil EH5. The resulting solids are characterized by a battery of techniques including thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), FT-Raman spectroscopy, 13 C and 29 Si magic angle spinning (MAS) and cross-polarization magic angle spinning (CP-MAS) spectroscopy, low-temperature nitrogen physisorption, and elemental analysis. Characterization of these solids revealed that a mixture of covalently immobilized mono- and bis-copper coordinated complexes, uncoordinated ligand, and in some cases physisorbed copper exists on the silica surface. The resulting solids are utilized for ATRP of methyl methacrylate (MMA). Whereas catalysts supported on mesoporous SBA-15 are ineffective at controlling the polymerization, CuBr/SdMBpy complexes immobilized on CPG(240) and Cab-O-Sil are effective at mediating the controlled polymerization of methyl methacrylate. Polymerizations with these catalysts achieved >70% conversion, narrow molecular weight distributions (1.29 < PDI < 1.52), and low (undetectable) residual copper content in the final polymer. Application of the “immobilized/soluble hybrid” methodology with these silica supported catalysts did not result in well-controlled polymerizations due to strong partitioning of the soluble catalyst onto the silica support under the conditions employed here, perhaps elucidating a limitation of this methodology with silica-supported catalysts. A new methodology for catalyst regeneration is described utilizing a simple treatment of the used catalyst with AIBN. The AIBN regenerated catalysts can be recycled with moderate conversions and narrow molecular weight distributions comparable to the first catalyst use.