Influence of micromixer characteristics on polydispersity index of block copolymers synthesized in continuous flow microreactors

The influence of interdigital multilamination micromixer characteristics on monomer conversions, molecular weights and especially on the polydispersity index of block copolymers synthesized continuously in two microtube reactors is investigated. The micromixers are used to mix, before copolymerization, a polymer solution with different viscosities and the second monomer. Different geometries of micromixer (number of microchannels, characteristic lengths) have been studied. It was found that polydispersity indices of the copolymers follow a linear relationship with the Reynolds number in the micromixer, represented by a form factor. Thus, beside the operating conditions (nature of the first block and comonomer flow rate), the choice of the micromixer geometry and dimension is essential to control the copolymerization in terms of molecular weights and polydispersity indices. This linear correlation allows the prediction of copolymer features. It can also be a new method to optimize existing micromixers or design other geometries so that mixing could be more efficient.     

Pd-catalysed asymmetric mono- and bis-alkoxycarbonylation of vinylarenes

The asymmetric alkoxycarbonylation of vinylarenes catalysed by palladium complexes bearing chiral phosphine ligands has attracted much attention over the last decades. The products of both mono- and bis(alkoxycarbonylation) reactions are important intermediates in the syntheses of pharmaceuticals such as 2-arylpropionic acids, the most important class of non-steroidal anti-inflammatory drugs. In this article, a general overview of the topics will be presented and the recent advances in this field will be particularly detailed. Besides the term alkoxycarbonylation, hydroesterification and hydroalkoxycarbonylation are also used in the literature to describe this reaction. Furthermore, more specific terms such as methoxycarbonylation can be found. In this report, the term alkoxycarbonylation will be used as the general term, and specific terms will be used to unambiguously define which reaction is meant.     

Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation

The role phosphine ligands play in the palladium(ii)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL')(OTf)(2) () [LL' = dcpe (PCy(2)CH(2)CH(2)PCy(2)), dppe, dppm, dppp, cppe (PCy(2)CH(2)CH(2)PPh(2))] are used. Alkyl palladium intermediates of the type [Pd(LL')(CHPhCH(2)Ph)](OTf) ( and ) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the alpha-H of the alkyl ligand of (LL' = dcpe) and linkage isomer ' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (DeltaH( not equal) and DeltaS( not equal)) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD(2)Cl(2). All five of the phosphine systems described are less active than those based on bcope [where bcope is (C(8)H(14))PCH(2)-CH(2)P(C(8)H(14))] and (t)bucope [where (t)bucope is (C(8)H(14))PC(6)H(4)CH(2)P((t)Bu)(2)]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL')(PhCH-CHPh-CPh[double bond, length as m-dash]CHPh)(+) () also being shown to play a role in the alkyne dimerisation step.     

On the origins of sudden adhesion loss at a critical relative humidity: examination of bulk and interfacial contributions

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.     

Evidence for concerted proton-electron transfer in the electrochemical oxidation of phenols with water as proton acceptor. Tri-tert-butylphenol

Proton-coupled electron transfer oxidation of phenols play a prominent role in several natural processes, and one may wonder if their high efficiency is related to the possibility that the electron and proton transfer steps are concerted. The cyclic voltammetric observation of the electrochemical oxidation and reverse reaction has allowed, with the example of 2,4,6-tri-tert-butylphenol in nonbuffered aqueous media, the clear identification of a pathway in which a phenol is directly and reversibly converted into the phenoxyl radical while the generated proton is accepted by a water molecule in a concerted manner. In very basic media, a stepwise mechanism takes place in which the phenol is deprotonated by OH- and the resulting phenoxide ion rapidly oxidized into the phenoxyl radical. As the pH decreases, this pathway progressively shuts down to the advantage of the concerted pathway. The latter assignment is confirmed by the observation of a substantial H/D kinetic isotope effect. At moderately basic pH 10.5, the contributions of the two pathways are about equal and the occurrence of the two competing routes is directly visualized in the cyclic voltammetry response.     

Recovering Invisible Signals by Two‐Field NMR Spectroscopy

Nuclear magnetic resonance (NMR) studies have benefited tremendously from the steady increase in the strength of magnetic fields. Spectacular improvements in both sensitivity and resolution have enabled the investigation of molecular systems of rising complexity. At very high fields, this progress may be jeopardized by line broadening, which is due to chemical exchange or relaxation by chemical shift anisotropy. In this work, we introduce a two‐field NMR spectrometer designed for both excitation and observation of nuclear spins in two distinct magnetic fields in a single experiment. NMR spectra of several small molecules as well as a protein were obtained, with two dimensions acquired at vastly different magnetic fields. Resonances of exchanging groups that are broadened beyond recognition at high field can be sharpened to narrow peaks in the low‐field dimension. Two‐field NMR spectroscopy enables the measurement of chemical shifts at optimal fields and the study of molecular systems that suffer from internal dynamics, and opens new avenues for NMR spectroscopy at very high magnetic fields.     

Clean Donor Oxidation Enhances the H2 Evolution Activity of a Carbon Quantum Dot–Molecular Catalyst Photosystem

Carbon quantum dots (CQDs) are new‐generation light absorbers for photocatalytic H₂ evolution in aqueous solution, but the performance of CQD‐molecular catalyst systems is currently limited by the decomposition of the molecular component. Clean oxidation of the electron donor by donor recycling prevents the formation of destructive radical species and non‐innocent oxidation products. This approach allowed a CQD‐molecular nickel bis(diphosphine) photocatalyst system to reach a benchmark lifetime of more than 5 days and a record turnover number of 1094±61 mol_H2 (mol_Ni)^?1 for a defined synthetic molecular nickel catalyst in purely aqueous solution under AM1.5G solar irradiation.     

General approach for the synthesis of organic-inorganic hybrid nanoparticles mediated by supercritical CO2

We report in this paper novel chemistry that addresses the problem of surfactant solubility in supercritical CO2 for metal nanoparticle synthesis. This new approach for the preparation of organic-functionalized inorganic nanoparticles relies on the reduction of a metal precursor in a CO2-containing insoluble polymer. Reduction of the metal with H2 leads to small nanocrystals stabilized by the polymer with a relatively small polydispersity. The functionalized metal nanoparticles are recovered as a dry powder, free of any organic solvents, which can then be resuspended in an appropriate solvent. This approach limits the number of steps for the preparation of functional nanoparticles which are ready for use. To illustrate this, we report results of the preparation of palladium and silver nanoparticles of 3-5 nm size stabilized with hyperbranched polyamines, functionalized with perfluoroalkyl, perfluorooligoether, non-fluorinated alkyl, polysiloxane, or polyethylene glycol moieties.