Controlled catalysis: The combination of magnetically and gravimetrically recoverable catalysts such as the nanoparticle‐bound base (see picture; blue) and the resin‐bound acid (green), respectively, allow the application of non‐complementary catalysts to multistep, one‐pot reaction cascades (e.g. A→B→C). The catalysts are recovered after reaction and reused in subsequent, unrelated reactions.

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, 13C and 29Si 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.

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A systematic study of the effect of the synthesis method and catalyst structure on the atom transfer radical polymerization (ATRP) performance of copper(I) bromide/pyridylmethanimine complexes supported on silica is described. Four different synthetic routes, including multistep-grafting (M1), two-step-grafting (M2), one-pot (M3), and preassembled-complex (M4) methods, have been evaluated on three different silica supports (mesoporous SBA15 with 48-and 100-Å pores and nonporous Cab-O-Sil EH5). The resulting solids have been used for ATRP of methyl methacrylate. The catalysts allow for moderate to poor control of the polymerization, with polydispersity indices (PDIs) ranging from 1.46 to greater than 2. The materials made with the preassembled-complex (M4) and one-pot (M3) approaches are generally more effective than those prepared with a grafting method (M1 and M2) on porous silica, whereas all the methods provide similarly performing catalysts on the nonporous support. Nonporous Cab-O-Sil EH5 is the most effective support because of its small particle size, lack of porosity, and relative compatibility in the reaction media. All the catalysts leach copper into solutions in small amounts. In addition, the catalysts can be effectively recycled, with improved controlled character in recycle runs (PDI ϳ 1.2). Control experiments have shown that this improved performance of the used catalysts is likely due to the presence of a soluble Cu(II) complex in the materials that effectively deactivates the growing polymer chain, leading to narrow PDIs and controlled molecular weights.

Metal complex catalysts that are covalently tethered to oxide supports have been utilized for many years as hybrid molecular/ heterogeneous catalysts. In this work, recent results from our laboratory in the area of silica-tethered metal complex polymerization catalysts are reviewed. The critical parameters for catalyst design when catalyst recovery and recycle are important are discussed in the context of silica-immobilized CuBr complexes for atom transfer radical polymerization (ATRP) as well as silica-tethered Zn-bdiiminate (BDI) complexes for lactide or epoxide/CO 2 polymerization. Additionally, a new strategy for the preparation of siteisolated metal complex catalysts on a silica surface is illustrated through the design of tethered constrained geometry catalysts (CGCs) for ethylene polymerization. The novel synthetic protocol is demonstrated to result in catalysts that appear to be more welldefined and that have improved catalytic properties. Major challenges and future directions in the field of oxide-tethered metal complex catalysis are discussed.