David E. Bergbreiter

Self-Assembled, Sub-Micrometer Diameter Semipermeable Capsules.

Semipermeable nanoscopic hollow spheres or cages with diameters in the sub-micrometer to the 100 nm range are accessible by the self-assembly of polymers onto sub-micrometer sized particles (Möhwald et al.) or by the formation of shell-cross-linked polymeric micelles (Wooley et al.) followed by chemical degradation of the underlying particle or micelle core (see scheme).

Covalent layer-by-layer assembly—an effective, forgiving way to construct functional robust ultrathin films and nanocomposites

Layer-by-layer (LbL) assembly is a versatile way to construct thin film nanocomposites and to modify surfaces. This technology is broadly useful because it is a simple and forgiving synthetic method. Most commonly, this approach to fabrication of an interface involves multilayer ionic assembly of polyelectrolytes. While ionic grafts are easy to prepare, ionic assemblies cannot be used in all applications. This highlight focuses on an alternative approach that uses covalent bonds to form multilayer grafts. Selected examples of this chemistry showing the scope of this methodology in the formation of ultrathin film nanocomposites and its potential are discussed below.

Superhydrophobic Surfaces Formed Using Layer-by-Layer Self-Assembly with Aminated Multiwall Carbon Nanotubes

A convenient and simple route to functionalized multiwall carbon nanotubes (MWNTs) using the reaction of the amine (NH) groups of polyethyleneimine (PEI) with MWNTs in N,N-dimethylformamide (DMF) at 50 degrees C is described. The product functionalized MWNTs (MWNT-NH-PEI) contain 6-8% by weight PEI based on elemental analysis, thermal gravimetric analysis, and titration. The products form stable emulsions in water below pH 9 and can be derivatized to form alkylated MWNTs that are dispersible in organic media. Such MWNT-NH-PEI nanoparticles can also be used in covalent or ionic layer-by-layer assembly to form nanocomposite thin films on functionalized polyethylene (PE) films and powders. Such nanocomposite films were analyzed by contact angle analysis, atomic force microscopy (AFM), and confocal Raman microscopy. These analyses show that these superhydrophilic surfaces have micro/nanoroughness with a roughly uniform distribution of MWNT nanoparticles. Superhydrophobic PE films can be formed either from ionic layer-by-layer self-assembly of MWNT-NH-PEIs and poly(acrylic acid) or from covalent layer-by-layer self-assembly of MWNT-NH-PEIs and Gantrez if the final graft is acrylated with a mixed anhydride prepared from ethyl chloroformate and octadecanoic acid. The resulting octadecylated surface produced by five covalent layer-by-layer deposition steps has a water contact angle of 165 degrees and a sliding angle of less than 5 degrees. The corresponding surface produced by five ionic layer-by-layer deposition steps has a water contact angle of 155 degrees but exhibits water pinning. The ionically assembled nanocomposite graft is labile under acidic conditions. The covalently assembled graft is more chemically robust.

WATER-SOLUBLE POLYMER-BOUND, RECOVERABLE PALLADIUM(0)-PHOSPHINE CATALYSTS

Abstract The synthesis of a water-soluble polymer-bound Pd(0)-phosphine catalyst is described. This soluble polymeric catalyst is soluble in aqueous or mixed aqueous/organic media and has high activity in nucleophilic allylic substitution and in sp-sp 2 coupling reactions of aryl iodides with terminal alkynes. The catalyst can be recycled efficiently by solvent or thermal precipitation methods.

The use of soluble polymers to effect homogeneous catalyst separation and reuse

Abstract The use of soluble polymeric ligands for homogeneous catalysts separation is reviewed with emphasis on work from the authors laboratory. Examples discussed include polyethylene-bound catalysts, poly(alkene oxide)-bound catalysts, poly( N -isopropylacrylamide)-bound catalysts, fluorous polymer-bound catalysts and amphoteric polymer-bound catalysts. The utility of these systems is also discussed in the broader context of polymer-supported catalysis and the advantages and limitations of soluble polymeric ligands are discussed in this context.

Polyisobutylene-anchored N-heterocyclic carbene ligands.

The synthesis of polyisobutylene (PIB)-supported N-heterocyclic carbenes (NHCs) that are useful as ligands for recoverable/recyclable organometallic complexes is described. Both PIB-bound carbenes analogous to SIMes and IMes as well as carbene precursors bound to PIB via 1,2,3-triazoles by alkyne-azide couplings are described. Both Ag(I) and Ru(II) complexes of these carbenes are shown to be phase selectively soluble in heptane. Hoveyda-Grubbs second-generation catalysts containing a PIB-supported NHC have also been used to catalyze ring-closing metathesis.

Catalytic cyclopropanation with transition metal salts of soluble polyethylene carboxylates

Abstract Syntheses of Rh(II) carboxylate salts of terminally funcfionalized polyethylene carboxylic acids arc described. These polymer-bound transition metals are homogeneous catalysts for cyclopropanation of alkenes at 100°C and have activities and selectivities like transition metal acetates but are quantitatively recoverable at 25°C.

Comparison of Covalently and Noncovalently Functionalized Carbon Nanotubes in Epoxy

Carbon nanotubes typically require the use of a dispersing or stabilizing agent to prevent significant aggregation during incorporation into a polymer matrix. These additives must be strongly associated, either covalently or physically, to achieve their purpose. In this study, multi-walled carbon nanotubes (MWNTs) were dispersed into an epoxy matrix using polyethylenimine (PEI) as a dispersant that was either covalently attached to the nanotubes or physically mixed to result in only noncovalent interaction. Epoxy composites containing covalently modified MWNTs exhibited greater storage modulus and reduced electrical conductivity.

Microwave promoted Heck reactions using an oligo(ethylene glycol)-bound SCS palladacycle under thermomorphic conditions

Palladium catalyzed Heck couplings utilizing an air-stable, water-soluble oligo(ethylene glycol)-bound SCS palladacycle catalyst and microwave irradiation lead to formation of several cinnamic acid derivatives with reaction times of less than 1 hour. Such couplings of various aryl halides with alkene acceptors occur in an air atmosphere with aqueous and organic solvents are described. Recycling of the catalyst was accomplished using a 10% aqueous DMA–heptane thermomorphic system that was advantageously homogeneous during these microwave promoted reactions and biphasic during the catalyst recovery step.

Thermomorphic Polyethylene-Supported Olefin Metathesis Catalysts

The preparation of polyethylene-oligomer (PE(olig))-supported N-heterocyclic carbene ligands (NHCs) and their Ru complexes is described. These complexes are structurally analogous to their low molecular weight counterparts and can serve as thermomorphic, recoverable/recyclable ring-closing metathesis (RCM) catalysts. Because of the insolubility of PE(olig)-supported species at 25 °C, such complexes can perform homogeneous RCM reactions at 65 °C and, upon cooling, precipitate as solids. This allows for their quantitative separation from solutions of products.