[Show abstract][Hide abstract] ABSTRACT: Studies of the hydrogen evolution reaction (HER) were carried out on base-inhibited vapor phase polymerized (BI-VPP) poly(3,4-ethylenedioxythiophene) (PEDOT)-poly(ethylene glycol) (PEG) thin films synthesized on several substrates, including gold, glassy carbon, and titanium, in an acidic electrolyte. The proton reduction overpotential became smaller and the current–potential (i-E) scans moved toward the thermodynamic potential with time. However, they did not move below the proton reduction potential of the substrate. Collectively, the results suggested to us that the polymer film was porous and that the porosity increased with time when the electrode was immersed in solution, with the hydrogen evolution catalysis taking place on the conducting substrate beneath the polymer rather than on our BI-VPP PEDOT-PEG thin films. Comparison of the HER on gold substrates with different impurities was also studied and traces of Pt and Pd at the ppm level significantly improved the HER activity of Au.
[Show abstract][Hide abstract] ABSTRACT: Using stopped flow and other spectroscopic techniques, the thermodynamic parameters of the coupling reaction between 1,3-dimesitylimidazolylidene and phenyl isothiocyanate were determined (H(o) = -96.1 kJ mol(-1) and ΔS(o) = -39.6 J mol(-1) K(-1)). On the basis of these data which indicated that the reaction was reversible (K(eq) = 5.94 × 10(14) M(-1) at 25 °C; k(f) = 252 M(-1) s(-1); k(r) = 4.24 × 10(-13) s(-1)), the adduct formed from the two aforementioned coupling partners was used as a latent catalyst to facilitate the [2 + 2 + 2] cyclotrimerization of phenyl isocyanate and the polymerization of DL-lactide.
The Journal of Organic Chemistry 01/2011; 76(1):301-4. DOI:10.1021/jo101850g · 4.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using the copper-catalyzed 1,3-dipolar “click” cycloaddition reaction, poly(sulfone)s containing pendant azide moieties were functionalized with various quantities of sodium 3-(prop-2-ynyloxy)propane-1-sulfonate and crosslinked with 1,7-octadiyne. The degrees of sulfonation and crosslinking were systematically varied by changing the ratios of the aforementioned reagents. The polymers were cast into membranes, acidified, and then tested for proton conductivity, methanol permeability, and membrane-electrode assembly (MEA) performance. The membranes showed a reduction in methanol permeability with increasing concentration of crosslinker and exhibited performance on par with direct methanol fuel cells containing Nafion-based membranes.
[Show abstract][Hide abstract] ABSTRACT: Conjugated polymers (CPs) have become essential to a diverse array of fields, ranging from biosensing to photovoltaics to optoelectronic materials. Given their widespread use, there is a growing need for variants whose functions may be modulated via exogenous stimuli. Alteration of CP structure could effect the desired changes and may be actuated in response to (1) chemical, (2) electrochemical, (3) photochemical, or (4) thermal impetuses. In this Perspective, we present a review of CPs reported to respond to chemical, electrochemical, and photochemical stimuli along with our own efforts using thermal stimuli. A particular emphasis has been directed toward structurally dynamic CPs that exhibit changes in their electronic properties, which could enable access to new classes of smart and other functional materials. During the course of our discussion, we consider the advantages and disadvantages inherent to each of the aforementioned methods, along with the potential applications of the polymers derived therefrom. We conclude this Perspective with a brief overview of the accomplishments in this nascent field, reflecting upon the unconquered challenges and contemplating the opportunities present for future discoveries involving structurally dynamic CPs.
[Show abstract][Hide abstract] ABSTRACT: A study was conducted to demonstrate the formation of structurally dynamic materials based on bis(N-heterocyclic carbene)s (NHC) and bis(isothiocyanate). It was demonstrated that all the challenges involved in the construction and application of self-healing materials were overcome by the chemistry of NHCs substrates that formed reversible adducts with a variety of reagents, including other NHCs, organic electrophiles, and transition metals. The study also explored the feasibility of preparing structurally reversible polymers based on NHCs chemistry. Bis(NHC) was selected as a suitable monomer for the investigations, as its reactive carhene moieties were connected through an extended π-system and featured flexible branched alkyl N-subslituents to enhance solubility in organic sohents.
[Show abstract][Hide abstract] ABSTRACT: Sulfonated poly(ether ether ketone) (sPEEK) membranes were blended with various functionalized imidazoles. The effect of the pKa of the added heterocycle on the dry proton conductivities of the blended membranes was evaluated over the temperature range of 40-150 degrees C. These membranes showed nonmonotonous conductivities with respect to temperature, as well as a clear correlation of peak temperature conductivity to the pKa of the heterocycle. We use a theoretical model based on the reaction equilibria between sPEEK's sulfonic acid groups and the basicity of the added heterocycles in order to better understand the mechanistic origins of the observed temperature-conductivity profiles.
The Journal of Physical Chemistry B 08/2009; 113(30):10063-7. DOI:10.1021/jp904192t · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A click by any other name: Coupling bis(N-heterocyclic carbene)s with bis(azide)s afforded a novel class of conjugated polytriazenes. These polymers were rendered electrically conductive upon doping, and fluorene-containing variants exhibited luminescence. This adaptation of N-heterocyclic carbene (NHC)/azide coupling chemistry to polymer synthesis reveals the potential of NHCs as building blocks for accessing polymers having useful electronic properties.
[Show abstract][Hide abstract] ABSTRACT: The syntheses of two new dithioester-based reversible addition- fragmentation chain transfer (RAFT) agents containing imidazolium Z groups and their potential in controlling free radical polymerizations of styrene are discussed. Starting with commercial available 1,3-dimesitylimidazolylidene, these agents were synthesized in two or three high yielding steps that required no chromatography, distillation, or other elaborate purification procedures. The products were isolated using straightforward filtration techniques. These agents were found to be effective for preparing well-defined polystyrenes at relatively low chain transfer agents (CTA):initiator ratios. The synthesis approach can be significant in facilitating access of RAFT agents that are highly functionalized or exhibit tunable activities through judicious N-substitution, electronic modulation of N-heterocyclic carbene component, or variation of the counteranion.
[Show abstract][Hide abstract] ABSTRACT: This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.
The Journal of Physical Chemistry B 01/2007; 110(51):25568-72. DOI:10.1021/jp065990a · 3.30 Impact Factor