The chemical composition and bonding structure of B–C–N–H thin films deposited by reactive magnetron sputtering

The chemical composition and bonding structures of B–C–N–H films fabricated by medium frequency magnetron sputtering, with N₂+CH₄+Ar gas mixture sputtering the boron target, were investigated. XPS and FTIR spectrometric analyses show that the increase of CH₄ flow rate during deposition causes an increase of the C content in the films. The increase in the CH₄ flow rate promotes an increase in the B–C, C–N single and C?N double bonds which are the components of the hybridized B–C–N bonding structure. From the results of Raman spectroscopy analysis, it is seen that the intensity of the D band of the films' Raman spectrum decreases with increasing CH₄ flow rate, indicating a decrease of the sp²‐phase content or the sp² C cluster size. The decreases of I_D/I_G also reflect the formation of more boron‐ or nitrogen‐ bound sp³‐coordinated carbons in the films. Copyright © 2009 John Wiley & Sons, Ltd.     

Solvent‐Mediated Transformation from Achiral to Chiral Nickel(II) Metal–Organic Frameworks and Reassembly in Solution

Reactions of 5‐nitroisophthalic acid (NO₂‐H₂ip), 1,4‐bis(imidazol‐1′‐yl)butane (bimb), and Ni(NO₃)₂ ? 6 H₂O gave rise to four metal–organic frameworks (MOFs), [Ni₂(NO₂‐ip)₂(bimb)_1.5]_n ( 1 ), [Ni₄(NO₂‐ip)₃(bimb)₂(OH)₂(H₂O)]_n ? (CH₃CH₂OH)_{0.5 n} ( 2 ), [Ni(NO₂‐ip)(bimb)_1.5(H₂O)]_n ? (H₂O)_n ? (CH₃CH₂OH)_{0.5 n} ( 3 ), and [Ni(NO₂‐ip) (bimb)(μ‐H₂O)]_n ? (H₂O)_n ( 4 ). The metal/ligand ratio, pH value, and solvent exerted a subtle but crucial influence on the formation of complexes 1 – 4 , which possess different visual color and crystal structures. Complex 1 exhibits a twofold interpenetrating 3D pillared bilayer framework composed of binuclear and mononuclear Ni^II units, whereas complex 2 is a 3D chiral network that consists of asymmetric tetranuclear Ni^II units. Complexes 3 and 4 are 3D layer‐pillared frameworks that consist of mononuclear Ni^II ions and a 3D six‐connected network of μ‐water‐bridged dinuclear Ni^II units, respectively. Interestingly, achiral 4 can be transformed into chiral 2 by using a solvent‐mediated single‐crystal‐to‐single‐crystal process without any chiral auxiliary. Magnetic analyses of 2 and 4 show the occurrence of antiferromagnetic interactions. Complex 3 is difficult to obtain directly as a single solid phase, but it can be homogeneously formed by solvent‐mediated transformations from 1 , 2 , and 4 .     

Formation of alkane‐phosphonic acid self‐assembled monolayers on alumina: an in situ SPR study

The formation process of n‐alkane phosphonic acid CH₃ (CH₂)_n?1 PO(OH)₂ (n = 10,12,14,18) self‐assembled monolayers (SAMs), deposited from ethanol solutions on aluminum oxide, has been monitored in situ using surface plasmon resonance (SPR) spectroscopy. In addition, the two‐solvent approach is used to obtain both film thickness and refractive index of the fully formed adsorbed layers. A densely packed adsorbed layer is formed only for the longest molecules with n = 18. The chain length and solution concentration dependence of formation kinetics are studied, and the existence of two distinct kinetic steps is revealed. Fittings of the experimental results with various kinetic models are performed. Our analysis suggests that during the first kinetic step, a transition from a lying‐down to a standing‐up phase takes place, and the growth of this standing‐up phase is governed by second‐order kinetics. The second slow kinetic step is described by a modified first‐order Langmuir law. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechanism for the gas‐phase reaction between OH and 3‐methylfuran: A theoretical study

The mechanism for the OH + 3‐methylfuran reaction has been studied via ab initio calculations to investigate various reaction pathways on the doublet potential energy surface. Optimizations of the reactants, products, intermediates, and transition structures are conducted using the MP2 level of theory with the 6‐311G(d,p) basis set. The single‐point electronic energy of each optimized geometry is refined with G3MP2 and G3MP2B3 calculations. The theoretical study suggests that the OH + 3‐methylfuran reaction is dominated by the formation of HC(O)CH?C(CH₃)CHOH (P7) and CH(OH)CH?C(CH₃)C(O)H (P9), formed from two low‐lying adducts, IM1 and IM2. The direct hydrogen abstraction pathways and the S_N2 reaction may play a minor or negligible role in the overall reaction of OH with 3‐methylfuran. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single‐Molecule‐Magnet Nodes within a Metal–Organic Framework

Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three‐dimensional array. Herein, we show that isonicotinic acid N‐oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy₂ single‐molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single‐crystal to single‐crystal transformation between the phases Dy₂(INO)₄(NO₃)₂?2 solvent (solvent=DMF (Dy₂‐DMF), CH₃CN (Dy₂‐CH₃CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy₂‐DMF and 76 cm^?1 for Dy₂‐CH₃CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy₂ nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.     

Study of the Efficiency of UV and Visible‐Light Photocatalytic Oxidation of Methanol on Mesoporous RuO2–TiO2 Nanocomposites

Mesoporous RuO₂–TiO₂ nanocomposites at different RuO₂ concentrations (0–10 wt %) are prepared through a simple one‐step sol–gel reaction of tetrabutyl orthotitanate with ruthenium(III) acetylacetonate in the presence of an F127 triblock copolymer as structure‐directing agent. The thus‐formed RuO₂–TiO₂ network gels are calcined at 450 °C for 4 h leading to mesoporous RuO₂–TiO₂ nanocomposites. The photocatalytic CH₃OH oxidation to HCHO is chosen as the test reaction to examine the photocatalytic activity of the mesoporous RuO₂–TiO₂ nanocomposites under UV and visible light. The photooxidation of CH₃OH is substantially affected by the loading amount and the degree of dispersion of RuO₂ particles onto the TiO₂, which indicates the exclusive effect of the RuO₂ nanoparticles on this photocatalytic reaction under visible light. The measured photonic efficiency ξ=0.53 % of 0.5 wt % RuO₂–TiO₂ nanocomposite for CH₃OH oxidation is maximal and the further increase of RuO₂ loading up to 10 wt % gradually decreases this value. The cause of the visible‐light photocatalytic behavior is the incorporation of small amounts of Ru⁴⁺ into the anatase lattice. On the other hand, under UV light, undoped TiO₂ shows a very good photonic efficiency, which is more than three times that for commercial photocatalyst, P‐25 (Evonik–Degussa); however, addition of RuO₂ suppresses the photonic efficiency of TiO₂. The proposed reaction mechanism based on the observed behavior of RuO₂–TiO₂ photocatalysts under UV and visible light is explored.     

The hydroxyl radical reaction rate constant and products of 3,5‐dimethyl‐1‐hexyn‐3‐ol

A bimolecular rate constant,k_DHO, of (29 ± 9) × 10^?12 cm³ molecule^?1 s^?1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with 3,5‐dimethyl‐1‐hexyn‐3‐ol (DHO, HC?CC(OH)(CH₃)CH₂CH(CH₃)₂) at (297 ± 3) K and 1 atm total pressure. To more clearly define DHO's indoor environment degradation mechanism, the products of the DHO + OH reaction were also investigated. The positively identified DHO/OH reaction products were acetone ((CH₃)₂C?O), 3‐butyne‐2‐one (3B2O, HC?CC(?O)(CH₃)), 2‐methyl‐propanal (2MP, H(O?)CCH(CH₃)₂), 4‐methyl‐2‐pentanone (MIBK, CH₃C(?O)CH₂CH(CH₃)₂), ethanedial (GLY, HC(?O)C(?O)H), 2‐oxopropanal (MGLY, CH₃C(?O)C(?O)H), and 2,3‐butanedione (23BD, CH₃C(?O)C(?O)CH₃). The yields of 3B2O and MIBK from the DHO/OH reaction were (8.4 ± 0.3) and (26 ± 2)%, respectively. The use of derivatizing agents O‐(2,3,4,5,6‐pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O‐bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible DHO/OH reaction mechanisms based on previously published volatile organic compound/OH gas‐phase reaction mechanisms. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 534–544, 2004     

Palladium–cadmium sulfide nanopowder at oil–water interface as an effective catalyst for Suzuki–Miyaura reactions

A simple and effective strategy is described for the synthesis of Pd–CdS nanopowder by the reduction of an organopalladium(II) complex, [PdCl₂(cod)] (cod = cis ,cis ‐1,5‐cyclooctadiene), in the presence of CdS quantum dots (QDs) at a toluene–water interface. We investigated the impact of addition of CdS QDs on catalytic activity of Pd nanoparticles (NPs). The Pd–CdS nanopowder functions as an efficient catalyst for Suzuki–Miyaura reactions for the formation of carbon–carbon bonds. There is a high electron density on Pd NPs and due to their high electron affinity they behave as an electron scavenger from CdS increasing the rate of oxidative addition, which is the rate‐determining step of the catalytic cycle, and, just as we expect, the C─C coupling reaction with the Pd–CdS nanopowder is faster and occurs in less time than that with Pd nanocatalysts. Compared to classical reactions, this method consistently has the advantages of short reaction times, high yields in a green solvent, reusability of the catalyst without considerable loss of catalytic activity and low cost, and is a facile method for the preparation of the catalyst.     

Generalized Nonaqueous Sol–Gel Synthesis of Different Transition‐Metal Niobate Nanocrystals and Analysis of the Growth Mechanism

A general nonaqueous route for the synthesis of phase‐pure transition‐metal niobate (InNbO₄, MnNb₂O₆, and YNbO₄) nanocrystals was developed based on the one‐pot solvothermal reaction of niobium chloride and the corresponding transition‐metal acetylacetonates in benzyl alcohol at 200 °C. All samples were carefully characterized by XRD, TEM, HRTEM, and energy‐dispersive X‐ray (EDX) analysis. The crystallization mechanism of these niobate nanocrystals points to a two‐step pathway. First, metal hydroxide crystals and amorphous niobium oxide are formed. Second, metal niobate nanocrystals are generated from the intermediates by a dissolution–recrystallization mechanism. The reaction mechanisms, that is, the processes responsible for the oxygen supply for oxide formation, were found to be rather complex and involve niobium‐mediated ether elimination as the main pathway, accompanied by solvolysis of the acetylacetonate ligands and benzylation reactions.     

Synthesis and catalytic activity of binuclear Mn(III,III)–BINOL complexes for epoxidation of olefins

The novel binuclear complexes [Mn₂^{(III, III)}(BINOL)₃L₂]2H₂O, where, L = 2, 2′‐bipyridine (Bpy) or 1,10‐phenanthroline (Phen) and BINOL = 1, 1′‐bi‐2‐naphthol were synthesized and characterized by elemental analyses, magnetic susceptibility and various spectral methods. The catalytic activity of these complexes was studied for the epoxidation reaction of unfunctionalized olefins like styrene, 1‐hexene, 1‐octene and 1‐decene. The products thus obtained were analyzed by GC. The epoxidation reactions were carried out, in the presence of catalyst with different oxidants, to study the effect of the nature of the oxidant on the reactions. The different oxidants used were the peroxide oxygen donor (e.g. TBHP and H₂O₂), mono oxygen donor (e.g. PhIO) and dioxygen donor (e.g. molecular O₂). TBHP was found to be the best oxidant for the epoxidation reaction. To study the effect of the solvent on the epoxidation, the reactions were carried out in different media, such as a polar media (e.g. with CH₃OH as solvent), non‐polar media (e.g. with CH₂Cl₂ and C₆H₆ as solvents) and coordinating solvent (e.g. CH₃CN). The maximum epoxide formation was observed in CH₂Cl₂ medium. The epoxidation reactions with optically active BINOL catalysts under optimum established conditions were carried out to examine the enantioselectivity of the catalysts. The complexes were, however, found not to be enantioselective. Copyright © 2006 John Wiley & Sons, Ltd.     

Nickel‐Catalyzed Mizoroki–Heck‐ versus Michael‐Type Addition of Organoboronic Acids to α,β‐Unsaturated Alkenes through Fine‐Tuning of Ligands

Various arylboronic acids reacted with activated alkenes in the presence of [Ni(dppe)Br₂], ZnCl₂, and H₂O in CH₃CN at 80 °C to give the corresponding Mizoroki–Heck‐type addition products in good to excellent yields. Furthermore, 1 equivalent of the hydrogenation product of the activated alkene was also produced. By tuning the ligands of the nickel complexes and the reaction conditions, Michael‐type addition was achieved in a very selective manner. Thus, various p‐ and o‐substituted arylboronic acids or alkenylboronic acid reacted smoothly with activated alkenes in CH₃CN at 80 °C for 12 h catalyzed by Ni(acac)₂, P(o‐anisyl)₃, and K₂CO₃ to give the corresponding Michael‐type addition products in excellent yields. However, for m‐substituted arylboronic acids, the yields of Michael‐type addition products are very low. The cause of this unusual meta‐substitution effect is not clear. By altering the solvent or phosphine ligand, the product yields for m‐substituted arylboronic acids were greatly improved. In contrast to previous results in the literature, the present catalytic reactions required water for Mizoroki–Heck‐type products and dry reaction conditions for Michael‐type addition products. Possible mechanistic pathways for both addition reactions are proposed.     

Stereoselective Solid‐State Synthesis of Substituted Cyclobutanes Assisted by Pseudorotaxane‐like MOFs

Regioselective photodimerization of trans‐4‐styrylpyridine (4‐spy) derivatives is performed using pseudorotaxane‐like Zn‐based metal organic frameworks MOFs as templates. The formation of rctt‐HT (head‐to‐tail) dimers is achieved by confining pairs of coordinated 4‐spy derivative ligands within hexagonal windows and then irradiating them with UV light. It is also possible to achieve a photodimerization reaction where two different substituted 4‐spy ligands are included in such a MOF material. The ether bond formation is employed to protect the sensitive ‐OH group of HO‐spy and the methyl group of CH₃O‐spy is subsequently removed after the formation of cyclobutane derivative in the CH₃O‐spy‐based MOF. Introducing substituents at the 2‐ or 3‐position of the phenyl group of 4‐spy does not significantly affect the rate of the dimerization process except in the case of the strongly electron‐withdrawing nitro group where the rate is significantly decreased. These results are in striking contrast to the mixtures of photoproducts and low yields obtained by untemplated photodimerization in organic solvents.     

Theoretical study on the gas‐phase reaction mechanism between rhodium monoxide and methane for methanol production

The gas‐phase reaction mechanism between methane and rhodium monoxide for the formation of methanol, syngas, formaldehyde, water, and methyl radical have been studied in detail on the doublet and quartet state potential energy surfaces at the CCSD(T)/6‐311+G(2d, 2p), SDD//B3LYP/6‐311+G(2d, 2p), SDD level. Over the 300–1100 K temperature range, the branching ratio for the Rh(⁴F) + CH₃OH channel is 97.5–100%, whereas the branching ratio for the D‐CH₂ORh + H₂ channel is 0.0–2.5%, and the branching ratio for the D‐CH₂ORh + H₂ channel is so small to be ruled out. The minimum energy reaction pathway for the main product methanol formation involving two spin inversions prefers to both start and terminate on the ground quartet state, where the ground doublet intermediate CH₃RhOH is energetically preferred, and its formation rate constant over the 300–1100 K temperature range is fitted by k_CH3RhOH = 7.03 × 10⁶ exp(?69.484/RT) dm³ mol^?1 s^?1. On the other hand, the main products shall be Rh + CH₃OH in the reactions of RhO + CH₄, CH₂ORh + H₂, Rh + CO +2H₂, and RhCH₂ + H₂O, whereas the main products shall be CH₂ORh + H₂ in the reaction of Rh + CH₃OH. Meanwhile, the doublet intermediates H₂RhOCH₂ and CH₃RhOH are predicted to be energetically favored in the reactions of Rh + CH₃OH and CH₂ORh + H₂ and in the reaction of RhCH₂ + H₂O, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Theoretical study on the reaction mechanisms and stereoselectivities of DABCO‐catalyzed Rauhut–Currier/cyclization reaction of methyl acrylate with 2‐benzoyl‐3‐phenyl‐acrylonitrile

With the aid of density functional theory (DFT) calculations, we have investigated the mechanisms and stereoselectivities of the tandem cross Rauhut–Currier/cyclization reaction of methyl acrylate R₁ with (E)‐2‐benzoyl‐3‐phenyl‐acrylonitrile R₂ catalyzed by a tertiary amine DABCO. The results of the DFT calculations indicate that the favorable mechanism (mechanism A) includes three steps: the first step is the nucleophilic attack of DABCO on R₁ to form intermediates Int1 and Int1‐1, the second step is the reaction of Int1 and Int1‐1 with R₂ to generate intermediate Int2(SS,RR,SR&RS), and the last step is an intramolecular S_N2 process to give the final product P(SS,RR,SR&RS) and release catalyst DABCO. The S_N2 substitution is computed to be the rate‐determining step, whereas the second step is the stereoselectivity‐determining step. The present study may be helpful for understanding the reaction mechanism of similar tandem reactions.     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

A Theoretical Study of the Reductive Elimination of CH3EH3 from cis‐[Pt(CH3)(EH3)(PH3)2] (E = Si,Ge) in the Presence of Acetylene

The reaction mechanism of the elimination of CH₃EH₃ from the platinum complexes cis‐[Pt(CH₃) · (EH₃)(PH₃)₂] (E = Si, Ge) in the presence of acetylene has been studied using gradient‐corrected DFT calculations at the B3LYP level. The reaction proceeds in two steps. The first step is the formation of the acetylene complex [Pt(CH₃)(HCCH)(EH₃)(PH₃)] which occurs in a associative/dissociate pathway via the five‐coordinated intermediate [Pt(CH₃)(HCCH)(EH₃)(PH₃)₂]. The rate‐determining step is the elimination of CH₃EH₃ via a four‐coordinated transition state. The alternative mechanism via direct dissociation from the five‐coordinated intermediates has higher activation barriers. The calculated activation energies of the model reactions are in good agreement with experimental results. The silyl complex has a lower barrier for the elimination reaction than the germyl complex. The calculated transition states show that the reason for the lower barrier is the strength of the nascending C–Si bond, which is higher than the C–Ge bond. The results are in agreement with the postulated mechanism of Ozawa et al. (Organometallics, 1998 , 17, 1018).     

Cycloadditions of Aryl‐Substituted 1,2,4‐Triazines with 2‐Cyclopropylidene‐1,3‐dimethylimidazolidine – Zwitterions as Discrete Intermediates

Cycloadditions of 2‐cyclopropylidene‐1,3‐dimethylimidazolidine ( 1 ), a strong, electron‐rich C‐nucleophile, with a variety of aryl‐substituted 1,2,4‐triazines occur at temperatures between ?100 and +100°, depending on the substitution pattern. At low temperatures, zwitterions, formed by nucleophilic attack of 1 on the triazines, could be detected spectroscopically and, in some cases, isolated. Two types of zwitterions were found: 1) those where the new bond was linked to C(5) of the triazine and which were formed in a reversible dead‐end equilibrium, and 2) those where the new bond was linked either to C(3) or C(6). The latter exhibited the same regiochemistry as the final cycloadducts, and might be intermediates of a two‐step Diels–Alder reaction. Energies and structural characteristics for stationary points in the reaction of monosubstituted triazines with 1 in the gas phase and in CH₂Cl₂ solution were calculated at the Becke3LYP/6‐311+G(d,p)//Becke3LYP/6‐31G(d) level of theory. Different reaction mechanisms are discussed on the basis of steric, electronic, and solvent effects.     

Synthesis of 4‐substituted styrene compounds via palladium catalyzed Suzuki–Miyaura reaction using bidentate Schiff base ligands

Air‐stable symmetric Schiff base have been synthesized and proved to be efficient ligands for Suzuki–Miyaura reaction between aryl bromides and arylboronic acids using PdCl₂(CH₃CN)₂ as palladium source under aerobic conditions. The coupling reaction proceeded smoothly using N,N‐bis(anthracen‐9‐ylmethylene)benzene‐1,2‐diamine (L₇) as ligand to provide 4‐substituted styrene compounds in good yields. Copyright © 2009 John Wiley & Sons, Ltd.     

Formation pathways of DMSO2 in the addition channel of the OH‐initiated DMS oxidation: A theoretical study

The production of dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO₂) in the dimethyl sulfide (DMS) degradation scheme initiated by the hydroxyl (OH) radical has been shown to be very sensitive to nitrogen oxides (NO_x) levels. In the present work we have explored the potential energy surfaces corresponding to several reaction pathways which yield DMSO₂ from the CH₃S(O)(OH)CH₃ adduct [including the formation of CH₃S(O)(OH)CH₃ from the reaction of DMSO with OH] and the reaction channels that yield DMSO or/and DMSO₂ from the CH₃S(O₂)(OH)CH₃ adduct are also studied. The formation of the CH₃S(O₂)(OH)CH₃ adduct from CH₃S(OH)CH₃ (DMS‐OH) and O₂ was analyzed in our previous work. All these pathways due to the presence of NO_x (NO and NO₂) and also due to the reactions with O₂, OH and HO₂ are compared with the objective of inferring their kinetic relevance in the laboratory experiments that measure DMSO₂ (and DMSO) formation yields. In particular, our theoretical results clearly show the existence of NO_x‐dependent pathways leading to the formation of DMSO₂, which could explain some of these experimental results in comparison with experimental measurements carried out in NO_x‐free conditions. Our results indicate that the relative importance of the addition channel in the DMS oxidation process can be dependent on the NO_x content of chamber experiments and of atmospheric conditions. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009     

Temperature‐dependent self‐assembly and rheological behavior of a thermoreversible pmma–PnBA–PMMA triblock copolymer gel

Self‐assembly and mechanical properties of triblock copolymers in a mid‐block selective solvent are of interest in many applications. Herein, we report physical assembly of an ABA triblock copolymer, [PMMA–Pn BA–PMMA] in two different mid‐block selective solvents, n‐butanol and 2‐ethyl‐1‐hexanol. Gel formation resulting from end‐block associations and the corresponding changes in mechanical properties have been investigated over a temperature range of ?80 °C to 60 °C, from near the solvent melting points to above the gelation temperature. Shear‐rheometry, thermal analysis, and small‐angle neutron scattering data reveal formation and transition of structure in these systems from a liquid state to a gel state to a percolated cluster network with decrease in temperature. The aggregated PMMA end‐blocks display a glass transition temperature. Our results provide new understanding into the structural changes of a self‐assembled triblock copolymer gel over a large length scale and wide temperature range. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 877–887