Thermal Rearrangement Reaction Research Articles

Molecular design of free volume structure in thermally rearranged polybenzoxazole membranes for gas separation studied by positron annihilation

AbstractIn this work, 2,2‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (APAF) was used as the fixed diamine monomer, and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA), 3,3′,4,4′‐biphenyltetracarboxylic diandhydride (BPDA) and pyromellitic dianhydride (PMDA) were selected as four different dianhydride monomers, respectively, to prepare the thermally rearranged polybenzoxazole (TR‐PBO) gas separation membranes with different structures via a thermal rearrangement reaction at 450°C. The microstructure of TR‐PBO membranes was investigated using X‐ray diffraction and positron annihilation spectroscopy. All the TR‐PBO membranes exhibit bimodal free volume structure, containing ultramicropores (<0.7 nm) and micropores (<2 nm) with size of 3.6–4.6 and 7.6–8.4 Å, respectively. As the steric hindrance of the bridging groups in the dianhydride monomers increases, the radius and number of pores in the membranes increases, leading to an increase in fractional free volume (FFV). The pure gas permeation test shows a positive correlation between gas permeability and FFV in the TR‐PBO membranes, indicating that an increase in FFV is favorable for the diffusion of gas molecules. Particularly, the ultramicropores play a decisive role on the gas selectivity. All the TR‐PBO membranes exhibit excellent gas permeaselectivity. Among them, the H2/CH4 separation performance of APAF‐6FDA and APAF‐ODPA membranes approaches the 2008 Robeson upper bound, which is due to the high number of both micropores and ultramicropores. Our results demonstrate that the free volume structure of polymer membranes can be designed by selecting monomers with different structures and utilizing thermal rearrangement reactions, allowing for the preparation of gas separation membranes with high permeability and selectivity.

Preparation of fluorinated poly(benzoxazole-co-imide) with low dielectric constants based on the thermal rearrangement reaction of o-hydroxy polyimides

This paper proposes a new strategy to prepare PIs with excellent overall performance by combining various modifications such as triphenyl pyridine structure, long-chain anhydride, trifluoromethyl, and thermal rearrangement reaction.

Gas separation performance and physical aging of tubular thin-film composite carbon molecular sieve membranes based on a polyimide of intrinsic microporosity precursor

We present a study on the fabrication of tubular thin-film composite CMS membranes based on an intrinsically microporous polyimide of intrinsic microporosity (PIM-PI), PIM-6FDA-OH. Besides the inherent structural similarity between the PIM-PI and CMS membranes (i.e. microporosity with pores <20 Å), the unique feature of the chosen precursor is its ability to undergo a thermal rearrangement (TR) reaction which constitutes an additional mechanism of microporosity evolution in addition to the pyrolysis process. By using Raman spectroscopy and in-situ thermal spectroscopic ellipsometry we tracked the structural TR- and pyrolysis-related evolution in CMS films as thin as 100 nm. Our study revealed a pronounced acceleration of the microstructure collapse (densification) due to physical aging that occurred in ultra-thin films. These, and our previous findings, suggest that excessive reductions in selective layer thickness in microporous amorphous materials, such as PIMs or CMS, may not be beneficial to obtaining highly efficient membranes. Instead, we have shown that excellent and stable separation properties could be achieved by PIM-PI-derived CMS membranes with thicker, ∼3 μm, selective layers (e.g. CO2, H2 permeances of >200 GPU, with CO2/CH4, CO2/N2, and O2/N2 selectivities of 43.0, 41.0, and 7.5, respectively) even after 3 months of aging.

Thermally rearranged covalent organic framework with flame-retardancy as a high safety Li-ion solid electrolyte

Solid polymer electrolytes have demonstrated high promise to solve the safety problems caused by conventional liquid electrolytes in lithium ion batteries. However, the inherent flammability of most polymer electrolyte materials remains unresolved, hence hindering their further industrial application. Addressing this challenge, we designed and constructed a thermal-responsive imide-linked covalent organic framework (COF) bearing ortho-positioned hydroxy groups as precursors, which can conduct a thermal rearrangement to transform into a highly crystalline and robust benzoxazole-linked COF upon heating. Benefiting from the release of carbon dioxide through thermal rearrangement reaction, this COF platform exhibited excellent flame retardant properties. By contrast, classic COFs (e.g., boronate ester, imine, olefin, imide linked) were all flammable. Moreover, incorporating polyethylene glycol and Li salt into the COF channels can produce solid polymer electrolytes with outstanding flame retardancy, high ionic conductivity (6.42 ​× ​10−4 ​S ​cm−1) and a high lithium-ion transference number of 0.95. This thermal rearrangement strategy not only opens a new route for the fabrication of ultrastable COFs, but also provides promising perspectives to designing flame-retardant materials for energy-related applications.

Synthesis and characteristics of HAB‐6FDA thermally rearranged polyimide nanocomposite membranes

AbstractThe thermal rearrangement of polyimides of ortho‐positioned functional group membranes improves the gas permselectivity properties of the polyimide precursor. For this experiment, HAB‐6FDA polyimide was synthesized from 3,3 dihydroxy‐4,4‐diamino‐biphenyl (HAB) and 2,2‐bis‐(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) by chemical imidization. A sample was modified from a pure polymer to silica nanoparticle composition. Furthermore, a modification was carried out by thermal rearrangement reaction at temperatures of 350, 400, and 450°C. The thermal property of these membrane films was characterized by differential scanning calorimetry (DSC), FTIR, opacity experiment, and free volume analysis. Permeability decreases with an increase in the kinetic diameter of gasses, which is normal behavior for glassy polymers. The composition of silica nanoparticles slightly changes the permeability in the polyimide. The combined effect of silica nanoparticles and thermal rearrangement of the HAB‐6FDA membrane has shown an excellent performance. The thermal rearrangement with nanocomposite shows a significant impact on a larger effect on permeation for lighter gases, that is, H2, CO2, and O2, compared with N2 and CH4. Particularly for H2/CH4 gas pair, it lies over Robeson's 2008 upper bound limit, which fits the composition in the novel class for the gas separation membranes.

Thermally rearranged (TR) HAB-6FDA nanocomposite membranes for hydrogen separation

Thermally rearranged (TR) polymers exhibited a good balance of high permeability and high selectivity. For this purpose HAB-6FDA polyimide was synthesized from 3,3 dihydroxy-4,4-diamino-biphenyl (HAB) and 2,2-bis-(3,4-dicarboxyphenyl) hexafluoro propane dianhydride (6FDA) by chemical imidization. Initially, the sample was modified from pure polymer to silica nanofiller doped polymer membrane. Further the modification was done by thermal rearrangement reaction at 350 °C temperature. This modification causes a mass loss in polymer structure and therefore enhances the fractional free volume (FFV). The gases used for the permeation test were H2, CO2, N2 and CH4. Selectivity was calculated for H2/CO2, H2/N2 and H2/CH4 gas pairs and plotted in the Robeson's 2008 upper bound and compared with reported data. The transport properties of these gases have been compared with the unmodified membrane. Permeability of all the gases has increased to that of unmodified polymer membrane. Thermally rearranged polymer nanocomposite exhibits higher gas permeability than that of silica doped and pure polymer. Also the selectivity for H2/CO2 and H2/N2 gas pairs exceeds towards Robeson's upper bound limit. It crosses this limit dramatically for H2/CH4 gas pair. Polymer nanocomposite can be utilized to obtain high purity hydrogen gas for refinery and petrochemical applications.

Synthesis of poly condensed oxygen-containing helical heterocycles via rearrangement of benzofused-2,2′-diphenoquinone derivatives

The thermal rearrangement reaction of helical-shaped quinone (benzofused-2,2′-diphenoquinone derivative) afforded three types of poly-condensed oxygen-containing heterocyclic compounds. One type of them was spiro compound which is consisting of cyclopentadiene and γ-butyrolactone. The other two compounds were helically condensed compounds, one of them containing as cyclopentadiene and pyranone ring and the other consists of fused furan and oxepin ring. These products were could be selectively synthesized by the choosing modification of reaction conditions. Moreover, the mechanistic aspects of these reactions are also discussed.

Preparation and properties of polybenzoxazole-based gas separation membranes: A comparative study between thermal rearrangement (TR) of poly(hydroxyimide) and thermal cyclodehydration of poly(hydroxyamide)

This comparative study focuses on the cyclodehydration reaction of poly(hydroxyamide)s (PHAs) and the thermal rearrangement (TR) reaction of aromatic poly(hydroxyimide)s (APIs) to produce polybenzoxazoles (PBOs) for application as gas separation membranes. Systematic comparisons between the two routes showed that PHA cyclodehydration can produce fully converted PBO membranes at a temperature more than 100 °C lower than the API-TR route. Gas separation performance of PHA–PBOs is comparable to API-TR polymers, and PHA–PBOs are considerably more mechanically robust. More importantly, PBO films can be produced equally well in both air and inert atmospheres via the PHA cyclodehydration route. The PHA–PBOs formed in air have statistically the same physical and transport properties as those formed in N2. The present study provides fundamental information regarding the PHA-to-PBO cyclodehydration process.

Effect of polymer structure on gas transport properties of selected aromatic polyimides, polyamides and TR polymers

Thermally rearranged (TR) polymers are formed through a thermally induced solid-state reaction of polyimides or polyamides that contain nucleophilic reactive groups ortho-positioned to their diamine. Naturally, the transport properties of TR polymers are intimately related to the chemical structure and reactivity of their precursors. Herein, we report characterization and transport properties for three poly(hydroxyimide) precursors prepared via thermal imidization in solution and for their corresponding TR polymers. Structural modifications to the polymer backbone can be used to control thermal rearrangement reaction kinetics. In regards to TR polymer formation, samples prepared from diamines with biphenyl functionality reacted more efficiently than those prepared from diamines with hexafluoroisopropylidene-linked aromatic units. However, hexafluoroisopropylidene functional units provided the highest combinations of permeability and selectivity for separations involving H2, N2, O2, CH4, and CO2. Differences in permeability between samples correlated well with changes in free volume, and 3 poly(hydroxyimide)s showed unusually high selectivities for their given free volume. The effect of synthesis route was also investigated for a specific TR polymer derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Poly(hydroxyimide) precursors prepared via thermal imidization in solution and thermal imidization in the solid-state showed nearly identical permeabilities and selectivities regardless of synthesis route. However, after thermal rearrangement, the TR polymers prepared from polyimides synthesized via solid-state imidization have higher gas permeabilities than their solution-imidized analogs. In addition to light gas permeabilities, plasticization effects were investigated with CO2 hysteresis loops for all samples, and pure-gas olefin/paraffin permeabilities were determined for a TR polymer derived from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and 6FDA. With the exception of HAB–6FDA polyimides, pure-gas CO2 feed pressures up to approximately 50bar do not reveal a plasticization pressure point, but conditioning effects are observed for most samples. APAF–6FDA TR polymers have pure-gas permeabilities and selectivities beyond the propylene/propane upper bound.

ChemInform Abstract: Rearrangement Reactions of 1,1‐Divinyl‐2‐phenylcyclopropanes.

Abstract1,1‐Divinyl‐2‐phenylcyclopropanes are a class of substrates showing a rich array of both thermal and photochemical rearrangement reactions.

ChemInform Abstract: Synthesis of trans‐Cycloalkenes via Enantioselective Cyclopropanation and Skeletal Rearrangement.

An efficient one-pot two-step procedure for asymmetric synthesis of piperidine-fused trans-cycloalkenes is reported. The method comprises the initial enantioselective installation of another cyclopropane ring onto methylenecyclopropanes and the subsequent thermal skeletal rearrangement in which the installed and inherent cyclopropane rings are both opened. A concerted mechanism is proposed for the latter thermal rearrangement reaction together with a closed transition state model.

Synthesis of trans-cycloalkenes via enantioselective cyclopropanation and skeletal rearrangement.

An efficient one-pot two-step procedure for asymmetric synthesis of piperidine-fused trans-cycloalkenes is reported. The method comprises the initial enantioselective installation of another cyclopropane ring onto methylenecyclopropanes and the subsequent thermal skeletal rearrangement in which the installed and inherent cyclopropane rings are both opened. A concerted mechanism is proposed for the latter thermal rearrangement reaction together with a closed transition state model.

Synthesis of 2-(2-Fluorophenyl)-2-methylamino-Cyclohexanone as a New Ketamine Derivative

The main goal of this article is to the present research on the development of ketamine derivatives. The target molecule was a fluoroderivative of ketamine, for which a multistep synthesis has been reported. This novel ketamine derivative, 2-(2-fluorophenyl)-2-methylamino-cyclohexanone, has been called fluoroketamine by our research group. The starting fluorobenzonitrile was reacted with the appropriate Grignard reagent followed by the bromination reaction to obtain α-bromocyclopentyl-(2-fluorophenyl)-ketone. The reaction of the obtained ketone with methylamine at −40 °C then resulted in the formation of α-hydroxycyclopentyl-(2-flourophenyl)-N-methylamine. Finally, the five-memberd ring cyclopentanol was expanded to the cyclohexylketone by a thermal rearrangement reaction. The HCl salt of the target molecule, which is soluble in water, was obtained by the acidification of the free fluoroketamine with HCl. Preliminary animal tests on mice have shown that the resulting fluoroketamine has some advantages over ketamine in terms of the effective dose and the recovery time.

ChemInform Abstract: Rearrangement of Aryl Geranyl Ethers.

AbstractThe thermal rearrangement reactions of aryl geranyl ethers are studied.

ChemInform Abstract: Regioselective Addition of Cyclic Nitrones to Feist′s Acid Dimethyl Ester.

AbstractCyclic nitrones regioselectively add to Feist′s acid dimethyl ester (II) yielding diastereoisomeric mixtures of isoxazolidines which are susceptible to thermal rearrangement reactions.

Rearrangement of Aryl Geranyl Ethers

This article describes the thermal rearrangement reactions of aryl geranyl ethers. These reactions depend on the structure of the aryl moiety of the substrate and the reaction conditions used. The naphthyl ethers underwent a [1,3]-alkyl shift, followed by acid-catalyzed intramolecular cyclization. The microwave-assisted rearrangement of isoquinolinyl ether showed a pattern of an abnormal Claisen rearrangement. The multi step rearrangement of the quinolyl ether afforded a spiro compound. These new reactions were used to synthesize novel heterocyclic compounds.

A prospective study on the application of thermally rearranged acetate-containing polyimide membranes in dehydration of biofuels via pervaporation

Dehydration of biofuels (bioalcohols) derived from fermented products via distillation process is always energy intensive and often limited by the formation of azeotropic mixtures between alcohols and water. We have explored the feasibility of thermally rearranged polybenzoxazole (PBO) membranes in dehydration of biofuels via pervaporation process. The polyimide based precursor was synthesized by polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3′-dihydroxybenzidine diamine (HAB) and cast into dense membranes. The resultant polyimide membranes were then subjected to thermal rearrangement reaction to initiate the solid-state chemical reconstruction into PBO. The effects of heating temperature and duration on pervaporation performance were systematically investigated. In general, the thermally rearranged PBO membranes showed an impressive enhancement in permeation flux with reasonable separation performance as compared to its polyimide precursor. In addition, the pervaporation performance was found to be greatly affected by the thermal rearrangement temperature as compared to the dwell duration. Furthermore, the stability of the thermally rearranged PBO membrane has been proven by continuously monitoring the pervaporation performance at 80°C for 250h. The long-term data revealed that there was no significant changes in separation performance that signifies the feasibility of the thermally rearranged PBO membrane in the separation and purification of biofuels via pervaporation process in biorefineries.

Theoretical study of the thermal rearrangement of chloromethylsilanes, and its mechanism

The thermal rearrangement reactions of chloromethylsilane, (chloromethyl)dimethylsilane, and (chloromethyl)vinylsilane have been studied by use of the density functional theory method at the B3LYP/6-311G(d, p) level. The structures of the reactants, transition states, and the products were determined and fully optimized. The geometries of the different stationary points and the harmonic vibrational frequencies were calculated at the same level. The results showed that thermal rearrangement of the chloromethylsilanes occurred via one pathway. The chlorine atom migrated from the carbon atom to the silicon atom, and the hydrogen atom migrated simultaneously from the silicon atom to the carbon atom through a double-three-membered-ring transition state, forming methylchlorosilane, trimethylchlorosilane, and vinylmethylchlorosilane. The energy barriers of the three rearrangements calculated at the B3LYP/6-311G(d, p) level were 217.4, 201.6, and 208.7 kJ mol−1, respectively. The effects of alkyl substituents on silicon atom are discussed. Changes of thermodynamic functions, equilibrium constant, and reaction rate constant were calculated in accordance with Eyring transition-state theory over the temperature range 400–1,500 K.

Role of Gas-phase Reactions and Thermal Gradient Control in Carbon Nanotube Synthesis

ABSTRACTWe investigate the role of precursor thermal rearrangement and surface catalytic reactions in the synthesis of vertically aligned carbon nanotubes (VA-CNTs) by acetylene-based, chemical vapor deposition (CVD) and demonstrate a millimeter-long growth of single-walled CNT (SWNT) without water assistance. A substrate heater was used to create an ascending temperature gradient from gas injection to catalyst substrate. Whereas temperature of catalyst substrates primarily determines their catalytic activity, it is a thermal condition of a gaseous mixture in the CVD chamber that also influence growth yield and structural features of as-grown CNTs. Employing Egloff’s characterization, [1] we discuss the importance of various gas thermal zones in producing high-quality nanotubes with augmented growth efficiency. We continue to report production of millimeter-long, VA-SWNT having a mean diameter of 1.7 ± 0.7 nm, catalyzed by iron on an alumina support. Important finding is that a million of aspect ratio of SWNT arrays can be produced, without water assistance, via combined action of an ascending temperature gradient toward catalyst substrate and low partial pressures of acetylene carbon feedstock. Our results do not only emphasize the role of precursor thermal rearrangement in CNT synthesis, but also offer a practical route to the modulation of such complex phenomena for an ultrahigh-yield growth of narrow VA-SWNT.

Refractive Index Changes in Polymers Bearing Pendant Active Ester Groups by Thermal Rearrangement Reaction

Abstract Linear and branched polymers bearing S-(2-benzoxazolyl) thioester groups were selectively synthesized by free radical and photoinitiated radical polymerization, respectively, of S-(2-benzoxazolyl)-4-vinylthiobenzoate. Upon heating, the pendant S-(2-benzoxazolyl) thioester groups were converted to 3-acylbenzoxazoline-2-thione groups, accompanied with a change of refractive index (Δn) of about +0.01 in both cases. The structural conversion was confirmed by UV–vis and IR spectroscopy, as well as comparison with authentic polymers bearing 3-acylbenzoxazoline-2-thione groups (similarly synthesized from 3-(4-vinylbenzoyl)benzoxazoline-2-thione). Application of such thermally induced rearrangement reactions may provide a new approach for obtaining materials whose refractive index can be changed by heat (e.g., generated by a laser beam).