Timothy E. Patten

Polymers with Very Low Polydispersities from Atom Transfer Radical Polymerization

A radical polymerization process that yields well-defined polymers normally obtained only through anionic polymerizations is reported. Atom transfer radical polymerizations of styrene were conducted with several solubilizing ligands for the copper(I) halides: 4,4′-di-tert-butyl, 4,4′-di-n-heptyl, and 4,4′-di-(5-nonyl)-2,2′-dipyridyl. The resulting polymerizations have all of the characteristics of a living polymerization and displayed linear semilogarithmic kinetic plots, a linear correlation between the number-average molecular weight and the monomer conversion, and low polydispersities (ratio of the weight-average to number-average molecular weights of 1.04 to 1.05). Similar results were obtained for the polymerization of acrylates.

Atom Transfer Radical Polymerization and the Synthesis of Polymeric Materials

The development of new polymeric materials is based on the availability of methods, principally living polymerizations, that allow well-defined polymers to be prepared. Living polymerizations are chain-growth polymerizations that proceed in the absence of irreversible chain transfer and chain termination. Provided that initiation is complete and exchange between species of various reactivities is fast, one can adjust the final average molecular weight of the polymer by varying the initial monomer-to-initiator ratio (DPn = D[M]/[I]0) while maintaining a narrow molecular weight distribution (1.0 < Mw/Mn < 1.5). [8,9] Also, one has control over the chemistry and structure of the initiator and active end group, so polymers can be end-functionalized and block copolymerized with other monomers. Thus, using only a few monomers and a living polymerization, one can create many new materials with vastly differing properties simply by varying the topology of the polymer (i.e., comb, star, dendritic, etc.), the composition of the polymer (i.e., random, periodic, graft, etc.), or the functional groups at various sites on the polymer (i.e., end, center, side, etc.) (Fig. 1). Examples of such materials prepared by atom transfer radical polymerization (ATRP) are shown later in this review. Much of the academic and industrial research on materials development has focused on coordination, cationic, anionic, and ring-opening polymerizations due to the availability of controlled/living polymerizations of these types. Free-radical polymerizations accounted for approximately half of the total production of polymers in the United States in 1995. Despite its tremendous utility, a significant drawback to free-radical polymerization is the lack of macromolecular structure control due to near diffusion-controlled radical coupling and disproportionation. Therefore, the development of controlled/living radical polymerization methods has been a long-standing goal in polymer chemistry. The last five years have seen the realization of this goal and the rapid growth in the development and understanding of new controlled radical polymerizations. In this discussion, we give a brief overview of recent developments in controlled radical polymerizations and describe in more depth the progress that has been made in the development of ATRP.

Synthesis and aqueous solution properties of polyelectrolyte-grafted silica particles prepared by surface-initiated atom transfer radical polymerization

A range of polyelectrolyte-grafted silica particles have been prepared by grafting suitable initiators onto near-monodisperse, 304-nm-diameter silica particles using siloxane chemistry, followed by surface-initiated atom transfer radical polymerization (ATRP) of four ionic vinyl monomers, namely sodium 4-styrenesulfonate (SStNa), sodium 4-vinylbenzoate (NaVBA), 2-(dimethylamino)ethyl methacrylate (DAM), and 2-(diethylamino)ethyl methacrylate (DEA) in protic media. The resulting polyelectrolyte-grafted silica particles were characterized using dynamic light scattering (DLS), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), helium pycnometry, and diffuse reflectance infrared Fourier transfer spectroscopy (DRIFTS). The TGA results indicated that the polyelectrolyte contents of the silica particles could be varied from 0.6% to 6.0% in weight. SEM studies revealed several surface morphologies for the grafted polyelectrolytes and XPS analysis of the particle surface also provided good evidence for surface grafting. Combined aqueous electrophoresis and DLS studies confirmed that these polyelectrolyte-grafted silica particles had pH-dependent colloid stabilities, as expected. Cationic polyelectrolyte-grafted silica particles were colloidally stable at low or neutral pH, but became aggregated at high pH. Conversely, anionic polyelectrolyte-coated silica particles became unstable at low pH. It was found that the rate of surface-initiated ATRP was substantially slower than the analogous solution polymerization. Finally, there was some evidence to suggest that, at least in some cases, a significant fraction of polymer chains became detached from the silica particles during polymerization.

Observation and analysis of a slow termination process in the atom transfer radical polymerization of styrene

Abstract Under conditions in which the rate of polymerization is slow, we have observed the slow elimination of HBr from the polymer endgroups in the ATRP of styrene. Experimental evidence indicates that this process is likely due to the solvent effect on the stability of 1-PEBr at 110 °C. A second elimination reaction was observed in a hydrocarbon solvent. The major contribution to the second elimination process comes from the reaction of the Cu(II) species, formed after atom transfer, with the growing polymeric radical which presumably occurs via a one electron oxidation process. The bimolecular rate constant for the reaction of the growing polymeric radical with the Cu(II) species in an atom transfer process is approximately 10 3 to 10 4 times greater than for the same in a termination process. Thus, the chemoselectivity for atom transfer is very high, and the effect of this termination reaction is minimal under conditions in which the concentration of monomer is high and the concentration of Cu(II) species is at the minimum necessary to ensure good molecular weight control. These data also suggest that effect of this reaction is negligible for styrene polymerizations yielding low molecular weight polymer and that it should result in an upper molecular weight limit to styrene ATRP.

Improving Hematite’s Solar Water Splitting Efficiency by Incorporating Rare-Earth Upconversion Nanomaterials

Confounded by global energy needs, much research has been devoted to convert solar energy to various usable forms, such as chemical energy in the form of hydrogen via water splitting. Most photoelectrodes, such as hematite, utilize UV and visible radiation, whereas ∼40% infrared (IR) energy remains unconverted. This work represents our initial attempt to utilize IR radiation, that is, adding rare-earth materials to existing photoelectrodes. A simple substrate composed of hematite film and rare-earth nanocrystals (RENs) was prepared and characterized. Spectroscopy evidence indicates that the RENs in the composite absorb IR radiation (980 nm) and emit at 550 and 670 nm. The emitted photons are absorbed by surrounding hematite films, leading to improvement of water splitting efficiency as measured by photocurrent enhancement. This initial work demonstrates the feasibility and concept of using RENs for utilizing more solar radiation, thus improving the efficiency of existing solar materials and devices.

Fabrication and Characterization of Rare-Earth-Doped Nanostructures on Surfaces

This article presents a simple and practical means to produce rare-earth-based nanostructures, as well as a combined characterization of structure and optical properties in situ. A nanosphere lithography strategy combined with surface chemistry enables the production of arrays of β-NaYF(4):Yb,Er nanorings inlaid in an octadecyltrichlorosilane matrix. These arrays of nanorings are produced over the entire support, such as a 1 cm(2) glass coverslip. The dimension of nanorings can be varied by changing the deposition conditions. A home-constructed, multifunctional microscope integrating atomic force microscopy, near-field scanning optical microscopy, and far-field optical microscopy and spectroscopy is utilized to characterize the nanostructures. This in situ and combined characterization is important for rare-earth-containing nanomaterials in order to correlate local structure with upconversion photoluminescence. Knowledge gained from the investigation should facilitate materials design and optimization, for instance, in the context of photovoltaic devices and biofluorescent probes.

pH Responsive Polymer Cushions for Probing Membrane Environment Interactions

A robust and straightforward method for the preparation of lipid membranes upon dynamically responsive polymer cushions is reported. Structural characterization demonstrates that complete, well-packed membranes with tunable mobility can be constructed on the polymeric cushion. With this system, membrane conformational changes induced by cellular cytoskeleton interactions can be modeled. The membrane can be tailored to screen the cushion from changes in pH or allow rapid response to the pH environment by incorporation of protein ion channels. This elementary system offers a means to replicate the conformational changes that occur with the cellular cytoskeleton and has great potential for fundamental biophysical studies of membrane properties and membrane-protein interactions decoupled from the underlying solid support.

Biomolecule detection via target mediated nanoparticle aggregation and dielectrophoretic impedance measurement

A new biosensing system is described that is based on the aggregation of nanoparticles by a target biological molecule and dielectrophoretic impedance measurement of these aggregates. The aggregation process was verified within a microchannel via fluorescence microscopy, demonstrating that this process can be used in a real time sensor application. Positive dielectrophoresis is employed to capture the nanoparticle aggregates at the edge of thin film electrodes, where their presence is detected either by optical imaging via fluorescence microscopy or by measuring the change in electrical impedance between adjacent electrodes. The electrical detection mechanism demonstrates the potential for this method as a micro total analysis system (microTAS).

Polymer Brushes under High Load

Polymer coatings are frequently used to provide repulsive forces between surfaces in solution. After 25 years of design and study, a quantitative model to explain and predict repulsion under strong compression is still lacking. Here, we combine experiments, simulations, and theory to study polymer coatings under high loads and demonstrate a validated model for the repulsive forces, proposing that this universal behavior can be predicted from the polymer solution properties.

Confined polymer systems: synergies between simulations and neutron scattering experiments

Molecular simulations and neutron reflectivity are both extremely valuable tools for determining the structure of soft matter at interfaces and under confinement. The high resolution structural information provided by these techniques allows us to obtain a thorough understanding at the molecular level. Here we present examples of polymer thin films and show the advantages these two techniques offer and how we can combine the approaches in order to provide a complete structural and thermodynamic picture.