Selective Formation of Diblock Copolymers Using Radical Trap-Assisted Atom Transfer Radical Coupling

Polystyrene (PSt) radicals and poly(methyl acrylate) (PMA) radicals, derived from their monobrominated precursors prepared by atom transfer radical polymerization (ATRP), were formed in the presence of the radical trap 2-methyl-2-nitrosopropane (MNP), selectively forming PSt-PMA diblock copolymers with an alkoxyamine at the junction between the block segments. This radical trap-assisted, atom transfer radical coupling (RTA-ATRC) was performed in a single pot at low temperature (35 °C), while analogous traditional ATRC reactions at this temperature, which lacked the radical trap, resulted in no observed coupling and the PStBr and PMABr precursors were simply recovered. Selective formation of the diblock under RTA-ATRC conditions is consistent with the PStBr and PMABr having substantially different KATRP values, with PSt radicals initially being formed and trapped by the MNP and the PMA radicals being trapped by the in situ-formed nitroxide end-capped PSt. The midchain alkoxyamine functionality was confirmed by thermolysis of the diblock copolymer, resulting in recovery of the PSt segment and degradation of the PMA block at the relatively high temperatures (125 °C) required for thermal cleavage. A PSt-PMA diblock formed by chain extenstion ATRP using PStBr as the macroinitiator (thus lacking the alkoxyamine between the PSt-PMA segements) was inert to thermolysis. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3619–3626

A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production

We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production

We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H-2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid-solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H-2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H-2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U. S., by using less than 0.7% of the U. S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

Selective Formation of Diblock Copolymers Using Radical Trap-Assisted Atom Transfer Radical Coupling

Polystyrene (PSt) radicals and poly(methyl acrylate) (PMA) radicals, derived from their monobrominated precursors prepared by atom transfer radical polymerization (ATRP), were formed in the presence of the radical trap 2-methyl-2-nitrosopropane (MNP), selectively forming PSt-PMA diblock copolymers with an alkoxyamine at the junction between the block segments. This radical trap-assisted, atom transfer radical coupling (RTA-ATRC) was performed in a single pot at low temperature (35 degrees C), while analogous traditional ATRC reactions at this temperature, which lacked the radical trap, resulted in no observed coupling and the PStBr and PMABr precursors were simply recovered. Selective formation of the diblock under RTA-ATRC conditions is consistent with the PStBr and PMABr having substantially different K-ATRP values, with PSt radicals initially being formed and trapped by the MNP and the PMA radicals being trapped by the in situ-formed nitroxide end-capped PSt. The midchain alkoxyamine functionality was confirmed by thermolysis of the diblock copolymer, resulting in recovery of the PSt segment and degradation of the PMA block at the relatively high temperatures (125 degrees C) required for thermal cleavage. A PSt-PMA diblock formed by chain extenstion ATRP using PStBr as the macroinitiator (thus lacking the alkoxyamine between the PSt-PMA segements) was inert to thermolysis. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3619-3626

A Computational Analysis of the Collision-Induced Dissociation Mechanisms of the Dipeptide Glycine-Serine Under Mass Spectrometry

Collision-induced dissociation (CID) of peptides using tandem mass spectrometry (MS) has been used to determine the identity of peptides and other large biological molecules. Mass spectrometry (MS) is a useful tool for determining the identity of molecules based on their interaction with electromagnetic fields. If coupled with another method like infrared (IR) vibrational spectroscopy, MS can provide structural information, but in its own right, MS can only provide the mass-to-charge (m/z) ratio of the fragments produced, which may not be enough information to determine the mechanism of the collision-induced dissociation (CID) of the molecule. In this case, theoretical calculations provide a useful companion for MS data and yield clues about the energetics of the dissociation. In this study, negative ion electrospray tandem MS was used to study the CID of the deprotonated dipeptide glycine-serine (Gly-Ser). Though negative ion MS is not as popular a choice as positive ion MS, studies by Bowie et al. show that it yields unique clues about molecular structure which complement positive ion spectroscopy, such as characteristic fragmentations like the loss of formaldehyde from the serine residue.2 The increase in the collision energy in the mass spectrometer alters the flexibility of the dipeptide backbone, enabling isomerizations (reactions not resulting in a fragment loss) and dissociations to take place. The mechanism of the CID of Gly-Ser was studied using two computational methods, B3LYP/6-311+G* and M06-2X/6-311++G**. The main pathway for molecular dissociation was analyzed in 5 conformers in an attempt to verify the initial mechanism proposed by Dr. James Swan after examination of the MS data. The results suggest that the loss of formaldehyde from serine, which Bowie et al. indicates is a characteristic of the presence of serine in a protein residue, is an endothermic reaction that is made possible by the conversion of the translational energy of the ion into internal energy as the ion collides with the inert collision gas. It has also been determined that the M06-2X functional¿s improved description of medium and long-range correlation makes it more effective than the B3LYP functional at finding elusive transition states. M06-2X also more accurately predicts the energy of those transition states than does B3LYP. A second CID mechanism, which passes through intermediates with the same m/z ratio as the main pathway for molecular dissociation, but different structures, including a diketopiperazine intermediate, was also studied. This pathway for molecular dissociation was analyzed with 3 conformers and the M06-2X functional, due to its previously determined effectiveness. The results suggest that the latter pathway, which meets the same intermediate masses as the first mechanism, is lower in overall energy and therefore a more likely pathway of dissociation than the first mechanism.