Light-scattering studies on polystyrenes in isorefractive poly(methyl methacrylate)–toluene “solvents”

The rms radii of gyration 〈S²〉^1/2 and second virial coefficients Γ₂ of five monodisperse polystyrenes (M × 10^?5 = 1.6, 2.8, 4.2, 6.6) were measured in isorefractive toluene–poly(methyl methacrylate) (M?_v = 4.0 × 10⁴, 1.6 × 10⁵, and 6.3 × 10⁵) “solvents.” For a given PMMA, the concentration at which the θ condition (defined by Γ₂ = 0) was reached was independent of PS molecular weight, but varied inversely with PMMA molecular weight (0.10, 0.056, and 0.023 g/mL, respectively). When this θ condition is reached by adding PMMA to toluene, the radii of gyration are decreased by only about 15%, much less than when it is reached by going to a poor, low-molecular-weight solvent. This reflects the exclusion of PMMA from the PS coils, the internal environment of which is essentially pure toluene.     

Synthesis,purification, and characterization of “perfect” star polymers via “Click” coupling

The copper (I)‐catalyzed azide‐alkyne cycloaddition “click” reaction was successfully applied to prepare well‐defined 3, 6, and 12‐arms polystyrene and polyethylene glycol stars. This study focused particularly on making “perfect” star polymers with an exact number of arms, as well as developing techniques for their purification. Various methods of characterization confirmed the star polymers high purity, and the structural uniformity of the generated star polymers. In particular, matrix‐assisted laser desorption ionization‐time‐of‐flight mass spectrometry revealed the quantitative transformation of the end groups on the linear polymer precursors and confirmed their quantitative coupling to the dendritic cores to yield star polymers with an exact number of arms. In addition to preparing well‐defined polystyrene and poly(ethylene glycol)homopolymer stars, this technique was also successfully applied to amphiphilic, PCL‐b‐PEG star polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Free energy calculations,enhanced by a gaussian ansatz,for the “chemical work” distribution

The evaluation of the free energy is essential in molecular simulation because it is intimately related with the existence of multiphase equilibrium. Recently, it was demonstrated that it is possible to evaluate the Helmholtz free energy using a single statistical ensemble along an entire isotherm by accounting for the “chemical work” of transforming each molecule, from an interacting one, to an ideal gas. In this work, we show that it is possible to perform such a free energy perturbation over a liquid vapor phase transition. Furthermore, we investigate the link between a general free energy perturbation scheme and the novel nonequilibrium theories of Crook's and Jarzinsky. We find that for finite systems away from the thermodynamic limit the second law of thermodynamics will always be an inequality for isothermal free energy perturbations, resulting always to a dissipated work that may tend to zero only in the thermodynamic limit. The work, the heat, and the entropy produced during a thermodynamic free energy perturbation can be viewed in the context of the Crooks and Jarzinsky formalism, revealing that for a given value of the ensemble average of the “irreversible” work, the minimum entropy production corresponded to a Gaussian distribution for the histogram of the work. We propose the evaluation of the free energy difference in any free energy perturbation based scheme on the average irreversible “chemical work” minus the dissipated work that can be calculated from the variance of the distribution of the logarithm of the work histogram, within the Gaussian approximation. As a consequence, using the Gaussian ansatz for the distribution of the “chemical work,” accurate estimates for the chemical potential and the free energy of the system can be performed using much shorter simulations and avoiding the necessity of sampling the computational costly tails of the “chemical work.” For a more general free energy perturbation scheme that the Gaussian ansatz may not be valid, the free energy calculation can be expressed in terms of the moment generating function of the “chemical work” distribution. © 2014 Wiley Periodicals, Inc.     

The “Nickel Effect”

Whereas the reaction of ethylene and triethylaluminum under pressure at 100°C yields trialkylaluminum compounds having long alkyl chains, the presence of small amounts of nickel salts induces the formation of butene. The discovery of this “nickel effect” represents the starting point for the development of the Ziegler catalysts. Comparatively little was formerly known about the nature and the mode of action of the catalytically active nickel species. A basis for the elucidation of the effect was provided by studies on the reduction of nickel compounds by organoaluminum compounds, on the occurrence and existence of nickel hydrides, and on interactions between nickel(0) and Lewis acids as well as organic compounds of main group metals. The most significant result of these studies is the recognition that multicenter bonding systems involving trialkylaluminum compounds and nickel atoms are involved. These react further with complex bonded ethylene in what is probably a concerted manner.     

“Living” radical polymerization of methyl methacrylate with diethyl 2,3-dicyano-2,3-diphenylsuccinate as a thermal iniferter

The bulk polymerization of methyl methacrylate (MMA) initiated with diethyl 2,3-dicyano-2,3-diphenylsuccinate (DCDPS) was studied. This polymerization showed some “living” characteristics; that is, both the yield and the molecular weight of the resulting polymers increased with reaction time, and the resultant polymer can be extended by adding MMA. The molecular weight distribution of PMMA obtained at high conversion is fairly narrow (M_w/M_n = 1.24≈1.34). It was confirmed that DCDPS can serve as a thermal iniferter for MMA polymerization by a “living” radical mechanism. Furthermore, the PMMA obtained can act as a macroinitiator for radical polymerization of styrene (St) to give a block copolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4610–4615, 1999     

“Superpermeability” and “pumping” of atomic hydrogen through palladium membranes

Permeation of atomic as well as molecular hydrogen through palladium membranes has been investigated experimentally in the temperature range from room temperature to 200 °C and at a higher incident flux of hydrogen atoms on palladium surface than in previous studies. The results demonstrate that phenomena of ‘superpermeability’ and ‘pumping’ of atomic gases through metal membranes are of a common nature. A theoretical model based on chemical thermodynamics and diffusion theory adequately describes the quantitative relationships observed in experiments. It was found that permeability of atomic hydrogen depends strongly on the magnitude of surface incident flux and membrane temperature.     

The Correct Meaning of “Coating Efficiency” vs. the “Utilization of Theoretical Efficiency”

The correct meaning of the the term “utilization of the theoretical efficiency” vs. the “coating efficiency” is explained.     

New “localized orbitals” appropriate for post-Hartree—Fock calculations

In this work, we propose a new orbital set based on density matrices of subsystems. Most of the resultant orbitals are localized in the subsystems and resemble the natural orbitals of the subsystems. Correlation between two electrons, each in a distant localized orbital, should be small and the corresponding configurations can be neglected in a calculation. Test calculations are performed on hexatriene and the usefulness of these orbitals is shown. © 1996 John Wiley & Sons, Inc.     

”The criterion was excellence“

The 3rd Euchems Conference is an ideal platform for spotlighting chemistry as the key source of sustainable solutions to the mega‐topics and challenges facing all of us today: energy, food supplies, and resources in a healthy environment. Nachrichten aus der Chemie talked with Francois Diederich and Andreas Hirsch, chairs of the Scientific Committee.