The curing retardation and mechanism of high temperature vulcanizing silicone rubber filled with superconductive carbon blacks

The curing retardation and mechanism of high‐temperature vulcanizing silicone rubber (HTV SR) filled with superconductive carbon black (CB) BP2000 have been studied experimentally and theoretically. The results show that both rubber matrix and CBs have influences on the peroxide curing of rubber/CBs composites. The retardation does not appear as prominent in nature rubber (NR)/BP2000 composites as in HTV SR/BP2000 composites. Quantum chemistry calculations reveal that the curing retardation of HTV SR/BP2000 composite should not be attributed to the curing reaction dynamics of HTV SR molecules. Fourier transform infrared (FTIR) and X‐ray photoelectron spectroscopy (XPS) analyses of CBs show that the effect of radical scavenging of phenol‐OH groups existing on BP2000 surface is the main reason for the retardation in the peroxide curing reaction. The effect is found to be more effective in HTV SR/BP2000 composite and thus retards its curing. The curing retardation does not appear in silicone rubber (SR)/BP2000 composites vulcanized by condensation reaction, and the resulting vulcanizates have excellent physical properties. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.     

Two different reaction mechanisms of cinnamate side groups attached to the various polymer backbones

Cinnamate polymers are well known photoreactive polymers due to [2+2] cycloaddition reaction of cinnamate side group. In this work, we have found that the cinnamate side groups could be also reacted by thermal energy, and this reaction is presumed to attribute to the radical reaction of carbon double bond in the cinnamate groups. Contrary to the photocycloaddition reaction of the cinnamate side groups, the thermal reaction of cinnamate side group was closely related to the flexibility of polymer backbone. The difference of the mechanism between the photocycloaddition reaction and thermal crosslinking reaction was confirmed by ¹H NMR and ¹³C NMR analysis of the model compound.     

Deep Inside Cucurbiturils: Physical Properties and Volumes of their Inner Cavity Determine the Hydrophobic Driving Force for Host–Guest Complexation

Cucurbit[n]urils (CBn) bind guest molecules through a combination of electrostatic interactions with the carbonyl rims and hydrophobic interactions with the inner cavity. Investigations with solvatochromic probes in CB7 reveal that the polarity of the cavity resembles that of alcohols (e.g., n-octanol), while its polarizability (P=0.12) and apparent refractive index (n_D=1.10±0.12) are extremely low, close to the gas phase. The calculated molecular quadrupole moments of CBs are extremely large (Θ_zz=−120 to −340 Buckingham). A survey of reported binding constants of neutral guests and hydrophobic residues that form 1 : 1 inclusion complexes with CB6, reveals a preferential inclusion of C3–C5 residues in its cavity. The largest guests which show non-negligible binding contain 7 heavy atoms (excluding hydrogen). For CB7, the strongest binding is observed for guests with adamantyl (10 heavy atoms) and ferrocenyl groups (11 heavy atoms), while the largest guests known to be complexed are carborane and the adduct of two pyridine derivatives (12 heavy atoms). The evaluation of different volumes shows that the most meaningful cavity, namely that responsible for binding of hydrophobic residues, is confined by the planes through the oxygen carbonyls. The volume of this inner cavity follows the formula V/ų=68+62(n−5)+12.5(n−5)², affording representative cavity volumes of 68 ų for CB5, 142 ų for CB6, 242 ų for CB7, and 367 ų for CB8. The volume of the 2 bond dipole regions is comparably smaller, amounting, for example, to 2×35 ų for CB6. The analysis of packing coefficients for representative sets of known guests with clearly defined hydrophobic binding motifs reveals average values of 47 % for CB5, 58 % for CB6, 52 % for CB7, and 53 % for CB8, which are well in line with the preferred packing (“55 % solution”, see S. Mecozzi, J. Rebek, Chem. Eur. J. 1998 , 4, 1016–1022) in related supramolecular host–guest assemblies. The driving force for binding of hydrophobic guests and residues by CBs is interpreted in terms of the unimportance of dispersion interactions (owing to the low polarizability of their cavity) and the dominance of classical and nonclassical hydrophobic effects related to the removal of very-high-energy water molecules (2 for CB5, 4 for CB6, 8 for CB7, and 12 for CB8) from the cavity.     

Chemistry in Restricted Spaces: Select Photodimerizations in Cages,Cavities, and Capsules

Carrying out chemical transformations under environmentally sustainable conditions has become one of the important current goals of chemistry. In this context, conducting reactions under solvent-free conditions (crystals, zeolites, clay, etc.) and in water has attracted considerable attention. Since most molecules either do not crystallize and or do not dissolve in water, the two approaches are complimentary. To make molecules solubilize in water one needs to employ water-soluble hosts such as micelles, cavitands, and capsules. To achieve selectivity, one should provide a confined environment within which the motions of reactant molecules are restricted to that in free solution. The confined space in which the reaction takes place independent of the host environment could be defined in terms of the “reaction cavity” originally presented by Cohen and Schmidt. In this mini-review, examples of photodimerization of olefins carried out in cavitands such as cucurbiturils, cyclodextrins, calixarenes, and octa acid are presented. Results are discussed in terms of the reaction cavity concept, which is applicable to reactions in both solids and water.     

Photochemical and photophysical reactions of poly(propylene imine) dendrimers tethering cinnamamide groups

Here we report on photochemical and photophysical properties of poly(propylene imine) dendrimers tethering cinnamamide groups at the peripheral positions. Photoexcitation of the dendrimer solutions with 313 nm brought about monotonous decrease of absorption band of trans-cinnamamide around 270 nm as a result of trans-to-cis photoisomerization and [2+2] photocycloaddition. The first-generation dendrimer showed preferential formation of cis-isomer, whereas photocycloaddition was more favorable for the third- and fifth-generation dendrimers. Interestingly, the third- and fifth-generation dendrimers could encapsulate phosphorescent donors into the dendrimer nanocavities. When the dendrimers capturing the donors were excited at 365 nm, photocycloaddition proceeded efficiently through triplet-triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin’s formula, we found that this triplet-triplet energy transfer is quenched within a radius of ∼0.5 nm. Such triplet-triplet energy transfer within dendrimer nanocavities would provide promising strategy to design and fabricate novel molecular devices by utilizing the dendrimers.     

An Approach to the Synthesis of the Manzamine Alkaloids Via the Vinylogous Amide PHotocycloAddition/Retro-Mannich Fragmentation/Mannich Closure Cascade (pharM)

The application of the vinylogous amide [2+2] photocycloaddition/retro-Mannich fragmentation/ M annich closure cascade ( pharM ) to the synthesis of the pentacyclic ring system of the anti-leukemic marine alkaloid manzamine A is presented. Two approaches to the synthesis of the requisite pentacycle are described: (a) the transannular photocycloaddition of an 18-membered vinylogous amide; and (b) photocycloaddition of an acyclic vinylogous amide, followed by macrolactamization of the derived pharM closure product to generate the pentacyclic ring system.     

Preparation and electrochemical characterization of platinum and ruthenium catalysts deposited on fluorinated carbon supports

The effect of fluorination on carbon blacks (CBs) was investigated by analyzing the CB surface functional groups. Also, fluorinated CB supported platinum–ruthenium electrocatalysts were studied. The surface characteristics of the fluorinated CB-supported catalysts were determined by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS), and their crystallinity was evaluated using X-ray diffraction (XRD). The electrochemical properties of the fluorinated CB-supported platinum and ruthenium (Pt/Ru CBs) catalysts were analyzed by cyclic voltammetry (CV). From the FT-IR results, the introduction of a fluorine atom onto CBs was confirmed by the existence of a carbon–fluorine (C–F) characteristic peak. The XPS result also showed that the fluorine content increased with increasing heat treatment temperature. The XRD analysis revealed that the catalysts are composed of mostly Pt and Ru. This result agreed with the fact that the electrochemical activity of the Pt/Ru CB catalysts was enhanced due to the small size and a high loading of catalysts by surface modification of the CBs.     

Fluorophore-Functionalized and Top-Covered Resorcin[4]arene Cavitands

This review describes our recent developments in the field of resorcin[4]arene cavitands. We present the syntheses of cavitands bearing pyrene, anthracene, and BODIPY dyes as fluorophores on the cavitand walls. These systems have been used to examine the conformational switching process between the closed vase and the open kite forms that are characteristic for this class of cavitands. In a second research direction, we prepared top-covered resorcin[4]arene-based, switchable container molecules, and investigated their binding and switching properties.     

Effect of nanosized carbon black on the morphology,transport, and mechanical properties of rubbery epoxy and silicone composites

Three different types of nanosized carbon black (CB), Printex XE2 (CBP), Vulcan XC72, and Printex 140 U (CBU), were dispersed by mechanical mixing in rubbery epoxy (RE) and silicone to produce composites. It was found that the maximum possible loading of CB in the polymers depended on the surface area of CB. For a given loading, all three CBs produced similar improvements in the thermal conductivity of the resulting composites, but their effects on the electrical conductivity varied and ranged from insulating composites with CBU to conducting composites with CBP. CBP produced a greater improvement in the electrical conductivity than the thermal conductivity of the polymers compared to the other CBs. This was attributed to the high structure of CBP, which led to the formation of a concatenated structure within the matrix. The CB/silicone composites had a similar thermal conductivity to that of the CB/RE composites, but only the CBP/silicone composite produced at 8 wt % loading was electrically conducting. The compression and hardness properties of RE were also significantly improved with the addition of CB. However, in the case of silicone, only CBP had a considerable effect on the compression properties. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Self-healing supramolecular hydrogels fabricated by cucurbit[8]uril-enhanced π-π interaction

Novel self-healing supramolecular hydrogels have successfully been fabricated through reversible cucurbit[8]uril (CB[8])-enhanced π-π interaction. Naphthaline groups in the side chains of copolymers and CB[8] molecules are employed as cross-linkers to form 1:2 ternary complex by host-guest interaction. Furthermore, the dipole-dipole interaction between the polar carbonyl groups of CB[8] and quaternary ammonium cation also contributes to the formation of self-healing property. It is found that the molar ratio of CB[8] to naphthaline units has a great influence on its self-healing property. This work represents a facile approach for fabricating cucurbituril-based self-healing supramolecular hydrogels, which can be potentially applied in several fields.     

阳离子交换树脂催化合成丙烯酸环己酯

以丙烯酸和环己烯为原料,磺酸基苯乙烯系强酸性阳离子交换树脂为催化剂合成丙烯酸环己酯,考察了反应温度、催化剂用量、烯酸摩尔比、反应时间对酯化反应的影响.结果表明,合成丙烯酸环己酯的最佳条件为:反应温度85℃,催化剂质量分数5%,n(酸)∶n(烯)=2∶1,反应时间5h.在最佳工艺条件下,环己烯转化率为87.2%,丙烯酸环...     

Photodegradation of EPDM/MWCNT nanocomposites: Effect of singlet oxygen

The photo-oxidation of ethylene propylene diene monomer (EPDM)/multiwall carbon nanotubes (MWCNT) nanocomposites has been studied under accelerated UV-irradiation (λ ≥ 290 nm) in the presence of singlet oxygen. The rate of photodegradation of EPDM/MWCNT is found to be higher when compared to the pristine polymer in presence of singlet oxygen. The enhancement of the rate of degradation of composite has been described through [2 + 2] photocycloaddition reaction between singlet oxygen and double bonds on composite which is followed by cleavage. The rate of degradation and the formation of new functional groups on the composite has been monitored by FTIR spectroscopy and found to be increased with irradiation time. The changes in surface morphology have been studied by scanning electron microscope. Differential scanning calorimetry measurements revealed an increase in the glass transition temperature of photodegraded EPDM and EPDM/MWCNT nanocomposites. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

不同取代度纤维素苯甲酸酯负载铂催化苯加氢

以N,N-二甲基乙酰胺/氯化锂(DMAc/LiCl)为溶剂溶解纤维素,吡啶(Py)为催化剂,苯甲酰氯为酯化剂在均相条件下进行酯化反应,通过控制苯甲酰氯的量得到了不同取代度(DS)的纤维素苯甲酸酯(CB),溶胀测试结果表明不同取代度的CB在苯、环己烯和环己烷中的溶胀度不同。将微波还原得到的铂纳米簇与CB杂化后得到Pt/CB杂化膜,并将其用于苯催化加氢反应中。利用FT-IR、XPS、XRD、DSC和TEM对不同取代度的CB和杂化膜的结构与性能进行了表征。研究发现膜的溶胀度直接影响苯的转化率和环己烯的选择性,活性评价结果显示,苯的转化率随着取代度的增大而增大,最高为7.29%;环己烯的选择性随取代度的增大而减小,最高为0.43%。     

Ultrastable Host–Guest Complexes and Their Applications

This review describes monovalent synthetic receptor–ligand (or host–guest) pairs with extremely high binding affinity, comparable to that of the biotin–avidin pair, and their applications. Cucurbit[7]uril (CB[7]), a member of the host family cucurbit[n]uril (CB[n], n=5–8, 10), forms ultrastable host–guest complexes with ferrocene-, adamantane- or bicyclo[2.2.2]octane-based molecules having ammonium groups properly positioned to interact with the carbonyl oxygens at the portals of CB[7]. The extremely high affinity is achieved by a large enthalpic gain arising from the near perfect size/shape complementarity between the rigid CB cavity and the rigid core of the guest molecules, with the critical assistance of the positive entropy change due to the extensive dehydration of the host and guest. The high stability of the complexes allowed us and others to explore several biological applications such as immobilization of biomolecules on a solid surface, protein isolation, triggering intracellular events, and regulating enzymatic activities. These complexes with their exceptional affinity, chemical robustness, simple preparation, biocompatibility, and easy handling may replace the biotin–(strept)avidin system in diverse areas of research, including affinity chromatography, high throughput biochemical assays, imaging, and sensor technologies.     

Topochemically photoreacted fluorescent dimers of 2,3-dicyanopyrazines

Novel fluorescent materials derived from 2,3-dicyanopyrazine were synthesized and subjected to photodimerization reaction. Styryl substituents were attached by Wittig reaction, and the [2+2] photocycloaddition of the 2,3-dicyanopyrazine, either in solution or as a thin film, was studied with irradiation under a high-pressure Hg lamp. The resulting compounds were characterized by ¹H NMR, FT-IR and elemental analysis. Spectral changes of UV-visible absorption intensity and fluorescent intensity were examined at specific exposure intervals. While the cyclobutane ring of dimers induced a discrete π-conjugation with aryl substituents to show a hypsochromic shift of absorption and emission spectra, the fluorescence intensity was increased and the specific lowest unoccupied molecular orbital (LUMO) levels were formed compared to monomers.     

Transition metal-carboryne complexes: synthesis, bonding, and reactivity

The construction and transformation of metal-carbon (M-C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M-C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal-carbon (M-C(cage)) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M-C(cage) bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal-carboryne complexes. Our work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M-C(cage) bonds, we prepared a series of group 4 and group 10 transition metal-carboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M-C(cage) bonds with electrophiles. Structural and theoretical studies on metal-carboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M-C σ and M-C π bonds, similar to that observed in metal-benzyne complexes. The nickel-carboryne complex (η(2)-C(2)B(10)H(10))Ni(PPh(3))(2) can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes. The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium-carboryne species, generated in situ from Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2), reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations. Studies of metal-carboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal-benzyne complexes. Other transition metal-carborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.     

The Stabilization Effect of Tris(hydroxymethyl)aminomethane and Oxidation of Carbon Black on the Resistivity-Temperature Character of Carbon Black-Filled Polyethylene

Carbon black (CB)-filled high density polyethylene (HDPE) composites, incorporating with or without tris(hydroxymethyl)aminomethane (Tris), were prepared by melt blending. The effects of CB oxidation and Tris incorporation on the resistivity-temperature characters were studied. The negative temperature coefficient effect (NTC) effect and the isothermal variation of resistance with time were reduced by incorporating Tris, and oxidation of CB enhanced the effects of Tris addition. The reason lies in the increase of polarity resulting from CB oxidation and the interaction between Tris and CBs.     

PTC effect in HDPE filled with carbon blacks modified by Ni and Au metallic particles

Several types of positive temperature coefficient (PTC) composites are prepared by using Ni and Au modified carbon black. The major aim of this work was the lowering of the room temperature resistivity of the composites by enhancement of electrical conduction of the CB particles by incorporating metallic particles. Investigations showed that the metal particles fill the cavities and surface defects of the CB and thus surface free area reduces after modification. Metallic particles also change the nature of the CB particles after modification. Several types of PTC composites were prepared by using modified and unmodified CB by several loading level of filler. Measuring electrical properties of the PTC samples showed that the Ni modified CB reduce room temperature resistivity to lower than that of PTC composites prepared by unmodified CB. PTC composites prepared by Au modified CBs showed very different properties compared to PTC composites prepared by Ni modified CB. Their resistivities were quite higher than the others showing a poor compatibility between the matrix and Au modified CB. The effect of thermal annealing also investigated on the electrical properties of the prepared composites. Room temperature resistivities reduced for most of the samples while PTC intensities increased after annealing. Theoretical bases are employed to discuss the room temperature resistivity and PTC behavior of the composites before and after annealing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Applications of Cucurbit[n]urils (n=7 or 8) in Pharmaceutical Sciences and Complexation of Biomolecules

Cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]) are two key members of the cucurbit[n]uril (CB[n]) family of macrocycles. Because of the good water solubility of CB[7] and the unique ternary binding properties of CB[8], these two macrocycles have attracted increasing attentions in recent years. In particular, many promising reports of exciting applications regarding CB[7] and CB[8] have emerged in the pharmaceutical sciences and complexations of biomolecules, which has become one of the most important areas of potential applications of CB[n]s. This review summarizes the applications of macrocyclic CB[7], CB[8] and their derivatives as supramolecular platforms that have been developed in recent years within the field of pharmaceutical sciences and biomolecular sciences, and discusses the current challenges and future prospects of this area.     

The encouraging improvement of polyamide nanofiltration membrane by cucurbituril-based host–guest chemistry

The considerable performance enhancement of small molecule-sieving nanofiltration membrane has been achieved by the functional combination between host–guest chemistry and interfacial polymerization (IP) for the first time in this work. First, the water-insolubility of cucurbit[6]uril (CB6) was ameliorated by constructing host–guest complex (CB6-PIP) with piperazine. Second, the incorporation of water-soluble CB6-PIP in the selective layer via IP leads to the generation of not only the enlarged conventional polyamide network tunnels but also rotaxane tunnels. Such enrichment of solvent transport tunnels contributes to an amazing pure water permeability of 15.5–25.4 Lm⁻²bar⁻¹h⁻¹, three times higher than that of traditional polyamide membranes, with a high R/MgSO₄ of 99.5–92.5%, perfect SO₄²⁻/Cl⁻ selectivity due to the electronegative contribution of CB6, as well as untapped potential in organic solvent nanofiltration. This work not only provides a fire-new strategy to design new type of NF materials but also promotes the application of CBs in many other fields.