Our research program focuses on two areas: polymer chemistry and surface science. We study fundamental and practical problems in nanotechnology, biotechnology, and asymmetric catalysis of small organic molecules through the controlled synthesis of new materials. These materials are new polymer architectures, monolayers, or vesicles for catalysts that introduce new reactivities or selectivities.
Effecient site-isolation for otherwise impossible cascade (Pot-in-pot reactions):
Simple site-isolation of >99.5% of Grubbs catalyst within polydimethylsiloxane (PDMS) thimbles allowed new cascade reactions with m-Chloroperoxybenzoic acid (MCPBA). The catalysts and MCPBA did not diffuse through PDMS, but small organic reagents did and reacted on both sides of the thimbles.
Polymer Chemistry:
We actively pursue the synthesis of comb polymers for applications as new photonic materials. Comb polymers are a new architecture with a polymeric backbone and regularly and densely spaced arms. Because the arms are also polymers, comb polymers have ultrahigh molecular weights. Our syntheses of comb polymers are diverse and use the Grubbs’ first and second generation catalysts, atom transfer radical polymerizations, and ring opening polymerizations of lactide. We synthesized ultrahigh molecular weight polymers with degrees of polymerization along the backbone up to 2,000 and total molecular weights from one million to over sixty million grams per mole. Most notably, we synthesized comb block copolymers that assembled in the solid state into photonic band gap materials. These polymers assembled into ordered morphologies with large domain sizes that made the material appear colored.
Surface Chemistry:
Organic monolayers are critically important in science and technology as they allow the surface chemistry of a surface to be controlled using organic reactions. Monolayers on Au and SiO2 have found so many applications in diverse fields that any listing will be incomplete. The field of monolayers on silicon (as opposed to silicon dioxide) is far less well developed, but holds terrific promise as silicon is a critically important electronic material. We developed some of the first mild methods to assemble, pattern, and functionalize organic monolayers on hydrogen-terminated Si(111) using TEMPO. These monolayers are stable for months upon exposure to air and for weeks upon immersion in refluxing methylene chloride. We developed two methods to pattern these monolayers by transforming surface carboxylic acids into anhydrides and reacting them with amines, and we reacted olefin-terminated monolayers with the Grubbs’ catalyst and an olefin in solution to functionalize the surface by cross metathesis. We used polydimethylsiloxane (PDMS) to pattern these monolayers on the nm- to micrometer size scales. Our method to pattern them on the nm-size scale is called “Patterning by Etching at the Nanoscale” or PENs for short. Our goal in this area is to integrate the selectivity of organic monolayers with the electrical properties of silicon to fabricate new sensors that are more stable than those on gold or glass.
Small Molecule Catalysis:
We recently began a new research program in our group to add new properties to homogeneous catalysts. For instance, in a recent paper in JACS, we occluded the Grubbs’ catalysts into mm-sized slabs PDMS. These slabs were added to MeOH/H2O mixtures where the catalysts did not diffuse from the PDMS, but small molecules diffused into the slabs and reacted. We introduced functional group selective reactions by controlling the diffusion of reagents into PDMS. PDMS is a hydrophobic polymer so polar reagents such as salts diffuse very slowly into its interior, but noncharged molecules diffuse into it rapidly. By controlling the charge on molecules, we can control whether they react with occluded Grubbs’ catalysts. In addition, we discovered that some reagents react by olefin isomerization rather than olefin metathesis with occluded Grubbs’ catalysts. Finally, we reacted the Schrock catalyst with olefins in ionic liquids and developed a new method to separate small molecules from ionic liquids.
