Activation Energy Of Cyclization Research Articles

Copolymers of Acrylonitrile and Acrylic Acid: Effect of Composition and Distribution of Chain Units on the Thermal Behavior of Copolymers

The copolymers of acrylonitrile and acrylic acid are synthesized for the first time by trithiocarbonate-mediated reversible addition-fragmentation chain transfer polymerization in DMSO at a content of comonomers in the reaction mixture of 40%. Various regimes of acrylic acid introduction into the reaction are tested. This makes it possible to synthesize copolymers with close average composition and different unit distribution in a chain. It is shown that, with an increase in the fraction of acrylic acid in the copolymer, the contribution of the ionic mechanism of cyclization grows, which is favorable for widening the temperature interval of cyclization and decrease in the intensity of heat flow. Reduction in the rate of introduction of acrylic acid into copolymerization and, as a consequence, its more uniform distribution in the chain lead to analogous effects and cause a decrease in the activation energy of cyclization by both the ionic and radical mechanism. Thus, the use of reversible addition-fragmentation chain transfer polymerization with the dosed introduction of the comonomer allows one to tune the properties of the copolymers during their heat treatment.

High molecular weight of poly(acrylonitrile-co-3-aminocarbonyl-3-butenoic acid methyl ester) used as carbon fiber precursor: preparation and stabilization

A bifunctional 3-aminocarbonyl-3-butenoic acid methyl ester (ABM) was used to prepare high molecular weight poly(acrylonitrile-co-3-aminocarbonyl-3-butenoic acid methyl ester) [P(AN-co-ABM)] by suspension polymerization in aqueous medium. The influence of ABM monomer feed ratios on the structure and stabilization of P(AN-co-ABM) was characterized by element analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Based on reactivity ratio calculation, the comonomer ABM possesses greater reactivity than acrylonitrile. The molecular weight of P(AN-co-ABM) in this work is 10 times larger, as compared to the one prepared by solution polymerization; it is the key to producing high-performance carbon fiber. The FTIR, XRD and DSC results illustrate that the stabilization of P(AN-co-ABM) is significantly improved as compared with PAN homopolymer and P(AN-AA-MA), for instance, lower initiation temperature, smaller cyclization activation energy (Ea), broadened exothermic peak and larger stabilization extent, due to the ionic cyclization initiated by ABM, which makes the resultant P(AN-co-ABM) a potential precursor for carbon fiber.

Structures and cyclization behaviors of gel-spun cellulose/polyacrylonitrile composite fibers

Polyacrylonitrile (PAN)/cellulose composite fibers have been produced by dry-jet gel spinning through their co-solution. The rheological properties of PAN/cellulose/dimethylacetamide/LiCl solutions containing different cellulose contents from 0 to 10 wt% were characterized, and 5 wt% PAN/cellulose composite solution shows the best solution homogeneity. During gel spinning, the cellulose forms elongated particles inside the gelation bath, and the particle diameters depend on the as-spun draw ratio. It was found that the glass transition of PAN fibers shifts to higher temperatures along with the increase of cellulose content, and the glass transition activation energy of PAN chains becomes higher when cellulose particles become smaller. Regardless the changes of cellulose amount (2–10 wt%) and particle diameter (7.1–1.4 μm), the cyclization activation energy of PAN/cellulose composite fibers is 13–17% lower than that of neat PAN fibers. Our experiments suggest that the addition of cellulose in PAN fibers has no direct effect on the cyclization reaction of PAN chains. Instead, the released by-products during the pyrolysis of cellulose at high temperature degradation affect the cyclization reaction of PAN chains.

Preparation, Stabilization and Carbonization of a Novel Polyacrylonitrile-Based Carbon Fiber Precursor.

The quality of polyacrylonitrile (PAN) precursor has a great influence on the properties of the resultant carbon fibers. In this paper, a novel comonomer containing the sulfonic group, 2-acrtlamido-2-methylpropane acid (AMPS), was introduced to prepare P(AN-co-AMPS) copolymers using itaconic acid (IA) as the control. The nanofibers of PAN, P(AN-co-IA), and P(AN-co-AMPS) were prepared using the electrospinning method. The effect of AMPS comonomer on the carbon nanofibers was studied using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Raman spectrum. The structural evolutions of PAN-based nanofibers were quantitatively tracked by FTIR and XRD during the thermal oxidative stabilization (TOS) process. The results suggested that P(AN-co-AMPS) nanofibers had the lower heat release rate (ΔH/ΔT = 26.9 J g−1 °C−1), the less activation energy of cyclization (Ea1 = 26.6 kcal/mol and Ea2 = 27.5 kcal/mol), and the higher extent of stabilization (Es and SI) during TOS process, which demonstrated that the AMPS comonomer improved the efficiency of the TOS process. The P(AN-co-AMPS) nanofibers had the better thermal stable structures. Moreover, the carbon nanofibers derived from P(AN-co-AMPS) precursor nanofibers had the better graphite-like structures (XG = 46.889). Therefore, the AMPS is a promising candidate comonomer to produce high performance carbon fibers.

Incorporation of Alkali Lignin in Polyacrylonitrile: Phase Separation, Coagulation, and Cyclization Kinetics.

In this study, we systematically investigated the phase separation behaviors of polyacrylonitrile (PAN)/alkali lignin (AL)/dimethyl sulfoxide (DMSO) systems and found that the addition of AL causes phase separation and the systems form sea (PAN)/island (AL) types of structures. Interestingly, the AL-rich domains are very stable even after a long time of storage up to 15 days. Additionally, how the phase separation affected the solution rheology, the coagulation process and PAN cyclization were explored. The addition of AL in PAN/DMSO solutions changes the solution viscosity and gelation behaviors. Also, the existence of AL-rich domains accelerates the coagulation rate of the PAN solution in water. Because AL degrades at a lower temperature than PAN, it reduces the PAN cyclization temperature but leads to a higher cyclization activation energy, which could be caused by their different initiation mechanisms. These results would be useful to understand how the addition of AL affects the PAN solution structures, solution rheology, solution coagulation behaviors, and PAN stabilization reactions.

Investigation of structural conversion and size effect from stretched bundle of electrospun polyacrylonitrile copolymer nanofibers during oxidative stabilization

The ultimate goal of this research is to develop continuous carbon nanofibers with superior mechanical strength. The hypothesis is that aligned and stretched polyacrylonitrile (PAN) copolymer nanofibers may be an innovative type of precursor, because the nanofibers possess high degrees of morphological and structural perfections; and due to size effect (i.e., the nanofibers are ~20 times thinner than the conventional microfibers), the formation of structural heterogeneities (e.g., sheath/core structure) can be effectively prevented/mitigated. In this study, a bundle consisting of aligned PAN copolymer nanofibers was first prepared by electrospinning (with nanofibers collected by a flowing water system) followed by 3 times stretching; the oxidative stabilization of precursor nanofiber bundle was then investigated. Compared to conventional microfibers, electrospun nanofibers had smoother surface, higher density, and higher degrees of crystallinity and macromolecular orientation. As a result, the activation energy for cyclization of nitrile groups was higher; nevertheless, the diffusion of oxygen molecules was easier and the distribution of oxygen was more uniform particularly when the stabilization temperature was 240°C or higher. Such a situation led to the reduction of activation energy for oxidative dehydrogenation, facilitated the structural conversion into aromatic and supra-molecular structures, and also prevented the formation of sheath/core structure.

Differential scanning calorimetry (DSC) as a tool for probing the reactivity of polyynes relevant to hexadehydro-Diels-Alder (HDDA) cascades.

The differential scanning calorimetry (DSC) behavior of a number of alkyne-rich compounds is described. The DSC trace for each compound exhibits an exothermic event at a characteristic onset temperature. For the tri- and tetraynes whose [4 + 2] HDDA reactivity in solution has been determined, these onset temperatures show a strong correlation with the cyclization activation energy. The studies reported here exemplify how the data available through this operationally simple analytical technique can give valuable insights into the thermal behavior of small molecules.

Chemical Modification for PAN Fibers during Heat-treatment Process

Chemical modification for Polyacrylonitrile (PAN) fibers during heat-treatment process were systematically studied by DSC, FT-IR, EA, XPS, etal. Comparing with original PAN fibers, chemical reactions, structures and elemental compositions of fibers modified with potassium permanganate (KMnO4) solutions were totally changed at a certain extent. KMnO4 had reduced the activation energy of cyclization, decreased the area and widened the peak of exothermic curve, decreased the velocity of cyclization reaction, increased the oxygen content about 67%, hence increased C-O-C and C=O groups and the core/shell ratio.

Thermo-chemical reactions and structural evolution of acrylamide-modified polyacrylonitrile

Thermal properties of acrylonitrile (AN)-acrylamide (AM) copolymers for carbon fibers were studied by DSC and in situ FTIR techniques in nitrogen (N2) and air flows. The cyclization mechanism and stabilization behavior of polyacrylonitrile (PAN) were discussed. In N2 flow, it was found that AM had the ability to initiate and accelerate cyclization process, which was confirmed by the fact that the initiation of nitriles shifted to a lower temperature. Compared to AN homopolymer, the initiation temperature of cyclization was ahead 32 K by introducing 3.59 mol% AM into the copolymer. The exothermic reaction was relaxed due to the presence of two separated exothermic peaks. Accompanied by DSC, in situ FTIR and calculation of activation energy, the two peaks were proved to be caused by ionic cyclization and free radical cyclization, respectively, and the corresponding cyclization mechanism was proposed. With increasing in AM content, the ionic cyclization tends to be dominant and the total heat liberated first increases and then decreases. For AN homopolymer, the activation energy of cyclization is 179 kJ/mol. For AN-AM copolymer (containing 3.59 mol% AM), the activation energy of ionic cyclization is 96 kJ/mol and that of free radical cyclization is 338 kJ/mol. In air flow, similar cyclization routes occur and the difference is the contribution of oxidation. The oxygen in environment has no remarkable effect on cyclization of AN homopolymer but retards the cyclization of AN-AM copolymers. For AN-AM copolymer with 3.59 mol% AM, the cyclization temperature is postponed 10°C in air.

Activation Energy and Thermodynamic Parameters of Cyclization in Purified Natural Rubber Latex Using a Trimethyl Silyl Triflate

The property of natural rubber (NR) is more interesting, except for some properties which need to improve. The cyclization reaction is an interesting reaction for modified NR to thermoplastic elastomer. The kinetic cyclization of deproteinized natural rubber (DPNR) or purified NR latex effectively performed in latex phase by using trimethylsilyl-trifluoromethane sulphonate or trimethyl silyl triflate (TMSOTF) is studied at various temperatures and times. The cyclized products are confirmed by NMR spectroscopies including one and 2D NMR. The kinetic of cyclization is investigated. It is found that the degree of cyclization in DPNR is a function of cyclization conditions. The rate constant (k) is 200 (s-1) at 100°C based on 95% of regression and the activation energy of cyclization in DPNR latex is 147.66kJ/mol. Some thermodynamic parameters: enthalpy, entropy, and free energy activation of 144.6kJ/mole, -269.11J/mole, and 246.9kJ/mole, respectively, are obtained for the cyclization of DPNR. The kinetic and thermodynamic parameters of cyclization obtained from this study indicate that an increase in the process temperature would increase the rate of cyclization formation.

Effect of adding sulphur to the dope on properties of modified polyacrylonitrile fibers

Ammonium-modified polyacrylonitrile (PAN) dope was prepared by free-radical solution copolymerization of acrylonitrile (AN) and ammonium itaconate (AIA), adding different weight percent amount of sulphur into quantitative dope and then the dope was spun into precursor fibers. Effects of sulphur on the characteristics of the copolymers and precursor fibers even the oxidated fibers or the resultant carbon fibers were studied. It has been found that the adding sulphur could obviously increase the hydrophilicity of the copolymer. But in the spinning processes, with the concentration of the adding sulphur increasing, the apparent minus jet stretch of as-spun fibers varied from −21% to −66%, the crystallinty increased from 21.2% to 40.5%, the void volume, fineness, tenacity and elongation at-break of the ammonium-modified precursors all decreased, which could result in the increase of the comprehensive properties of the resultant precursors. It was testified that the precursors developed from the copolymer with the higher concentration of adding sulphur have higher degree of orientation. The resultant carbon fibers developed from precursors of the adding sulphur show an improvement in tensile strength of about 30%. The activation energy of cyclization can be reduced from the original 150 kJ mol−1 to about 110 kJ mol−1. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Calculated Effect of Substitutions on the Regioselectivity of Cyclization of α-Sulfenyl-, α-Sulfinyl-, and α-Sulfonyl-(5R)-5-hexenyl Radicals

Calculations of the activation energy of cyclization of alpha-sulfenyl-, alpha-sulfinyl-, and alpha-sulfonyl-5-hexenyl radicals and their respective 5-methyl-5-hexenyl counterparts at the G3MP2B3 level agree quite well with experimental data. The alpha-sulfinyl-5-hexenyl radical exhibits unexpected regioselectivity (93.99:6.01) via the 5-exo mode, whereas the alpha-sulfenyl- and alpha-sulfonyl-5-hexenyl radicals show increasing branching ratios of the 6-endo product. In contrast, the cyclization of the alpha-sulfur-based 5-methyl-substituted counterparts yields essentially the 6-endo products in all cases; in particular, the alpha-SO2-5-CH3-5-hexenyl radical gives high regioselectivity (98.85:1.15) via the 6-endo mode. Several other 5-substituted moieties, including the electron-withdrawing (CN and NO2) or electron-donating substituents (NH2), also proceed preferentially to 6-endo closure. The alpha-sulfonyl-5-amine-5-hexenyl radical is calculated to proceed to exclusively the 6-endo product, a demonstration of the high synthetic value of this reactant.

Temperature-Dependent Tail−Tail Dynamics of Pyrene-Labeled Poly(dimethylsiloxane) Oligomers Dissolved in Ethyl Acetate

We have determined the tail-tail dynamics and energetics of linear poly(dimethylsiloxane) (PDMS) polymers (M(n) = 3100; M(w)/M(n) = 1.07) that have been terminally labeled with pyrene (Py-PDMS-Py) in ethyl acetate between 255 and 323 K. The upper critical solution temperature (theta(u)) for the PDMS/ethyl acetate system is 278 K. The Py-PDMS-Py photophysics follow the Birks model at all of the temperatures studied. However, there is evidence for an increase in the local composition of PDMS surrounding the Py- residues in Py-PDMS-Py below the theta temperature. The recovered activation energy for Py-PDMS-Py tail-tail cyclization is the same as the activation energy for diffusion in ethyl acetate. The activation energy for cyclization is about fourfold larger in comparison to the barrier for PDMS internal backbone rotations. The estimated internal activation barrier for Py-PDMS-Py in ethyl acetate is in line with the internal barrier rotation for PDMS backbone rotation. The intramolecular excimer binding energy for Py-PDMS-Py in ethyl acetate is similar to the intermolecular pyrene excimer. In the ground state, the mean distance between the Py-PDMS-Py tail segments does not change over the temperature range studied. A comparison of these temperature-dependent Py-PDMS-Py/ethyl acetate results with previous results on Py-PDMS-Py in pure supercritical CO2 and CO2-dilated liquids shows that the addition of CO2 to the liquid phase offers the way to tune to Py-PDMS-Py tail-tail dynamics over the greatest range.

Critical distance model for the energy of activation of the Bergman cyclization of enediynes

AbstractThe reaction profile of the Bergman cyclization of enediynes is written as a cubic polynomial expansion about the critical distance. Using the “simple sewing” approximation, a relationship is derived that expresses the energy of activation as a function of the critical distance, with the effect of other geometric and electronic factors accounted for in the expansion coefficients. A training set of 10 representative enediynes were selected and input parameters were calculated by density functional theory at the B3LYP/6‐311G* level of theory. All calculations were corrected for zero‐point energy. All singlet biradical calculations were also checked for wave function stability. Calibration curves were then constructed that correlate the activation energy of cyclization of the molecules in the test set with the critical distance parameter. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Thermal degradation behaviour of two methacrylate polymers with side chain amide groups

2-Oxo-2-phenylamino)ethylene methacrylate (PAMA) and (2-oxo-2-benzylamino)ethylene methacrylate (BAMA) were prepared by reaction of sodium methacrylate with N-phenyl chloroacetamide and N-benzyl chloroacetamide, respectively, which were obtained from reaction of chloroacetyl chloride with aniline or benzylamine. The thermal degradations of poly(PAMA) and poly(BAMA) prepared by a free-radical procedure were investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 °C under vacuum, followed by product collection, and using IR spectra of partially degraded polymer. The products volatile at degradation temperature but not at ambient temperature were collected on the cooled upper part of the degradation tube [cold ring fraction, CRF P for poly(PAMA), CRF B for poly(BAMA)]. Product analysis studies showed that only N-benzyl hydroxyacetamide formed as major product in thermal degradation of poly(BAMA) and the monomer did not, whereas in the case of poly(PAMA) N-phenyl hydroxyacetamide and the monomer both formed as major products. For poly(BAMA), the activation energy of cyclization is calculated as 47.4 kJ mol −1; for poly(PAMA) the activation energy of the depolymerization is calculated as 58 kJ mol −1. The mechanism of thermal degradation including formation of the major products is discussed.

Modeling the deamidation of asparagine residues via succinimide intermediates

Density functional theory (B3LYP/6-31G*) has been used to study the cyclization, deamidation and hydrolysis reactions of a model peptide. Single point energy calculations with the polarized continuum model drastically lower the activation energy for cyclization in a basic medium. Confirmation of the experimental results that cyclization is slower than deamidation in acidic media and the opposite is true in basic media has enabled us to propose mechanisms for both processes.

Direct Measurements of the Kinetics of 3-exo Radical Cyclizations Using Radical Reporter Groups

The E and Z isomers of the 4-(2,2-diphenylcyclopropyl)-3-butenyl radical (1), produced by laser flash photolysis of the corresponding PTOC esters in THF, cyclized to the (2,2-diphenylcyclopropyl)(cyclopropyl)methyl radical (2) that rapidly opened to 1,1-diphenyl-4-cyclopropyl-3-butenyl radicals (3). Radicals 3 were monitored by UV spectroscopy, but the observed rate constants were for the initial, relatively slow cyclizations of radicals 1 to radical 2. The Arrhenius functions determined in the temperature range of 20−58 °C were log(k/s-1) = (11.46 ± 0.38) − (9.10 ± 0.54)/θ for (E)-1 and log(k/s-1) = (12.34 ± 0.32) − (10.10 ± 0.45)/θ for (Z)-1 where θ = 2.3RT in kcal/mol and errors are at 2σ. Radical (Z)-1 cyclizes somewhat faster than radical (E)-1 as a result of a more favorable entropy of activation and despite the fact that the activation energy for cyclization of (Z)-1 through the requisite syn-transition state is greater than that for cyclization of (E)-1 through an anti-transition state.

Cyclopolymerization of ether dimers of α-(hydroxymethyl)acrylic acid and its alkyl esters: substituent effect on cyclization efficiency and microstructures

Free-radical cyclopolymerization of six ether dimers of α-(hydroxymethyl)acrylic acid and its alkyl esters was kinetically investigated to evaluate the factors that control the cyclopolymerizability. A strong effect of ester substituent on cyclopolymerizability was observed and cyclization efficiency at 80°C increased in the following order: methyl, ethyl, isobornyl, t-butyl and adamantyl esters. That is, cyclization efficiency is found to be affected by the number of carbons linked to the central ester carbon (which defines the ‘effective’ bulkiness) rather than by the ‘apparent’ bulkiness, which encompasses the total mass or size of the whole substituent. Kinetic considerations of the temperature dependence of cyclization efficiency indicated that a strong contribution of the steric factor for bulky substituents (which inhibits intermolecular monomer addition) resulted in high cyclization efficiency despite a disadvantage in activation energy for cyclization. A linear relationship between the difference of activation energies ( E c − E i) and the logarithm of the ratio of collision frequency factors ( ln( A c A i ) ) observed for various esters led to recognition of an iso-efficiency point at which all esters show the same cyclization efficiency ( k c k i = 2.0 mol l −1 at −45°C). This phenomenon is caused by the fact that both parameters change in proportion to the bulkiness of the ester substituent. The ether dimer of α-(hydroxymethyl)acrylic acid showed unexpectedly high cyclization efficiency (intermediate between secondary and tertiary esters) despite having the lowest substituent bulkiness. The kinetic parameters obtained showed some deviation from the linearity observed for the ester derivatives. The large A c A i obtained was attributed to the capability of hydrogen-bond formation between acid groups, which causes a favourable conformation for cyclization. Microstructures of the cyclopolymers were investigated by observation of carbonyl carbon peaks in the 13C nuclear magnetic resonance spectra. The peaks observed were assigned to four microstructures based on the combination of trans/cis configuration of the tetrahydropyran ring and racemic/meso configuration between the rings. Both the trans and racemic ratios increased with the bulkiness of the ester substituent. The values of E cis − E trans and A trans A cis for the t-butyl ester cyclopolymer indicated that trans formation is slightly favourable in both activation energy and steric factor, a result similar to that reported for the racemic/meso ratio in radical polymerization of some alkyl methacrylates. On the other hand, racemic formation between rings was suggested to be sterically unfavourable by the A racemic A meso ratio = 0.55 . The glass transition temperatures for the cyclopolymers were found to be much higher than for the corresponding acrylates or methacrylates, and the thermal stabilities of some of the derivatives, especially the adamantyl compound, were also high.

Study on the thermal behaviour of copolymers of acrylonitrile with halogenated carboxylic acids

AbstractThe thermal stability of AN copolymers was studied by DSC technique in order to get information on the influence of comonomers containing halogenated carboxylic acid on the stabilization of PAN. It is shown that the activation energy of cyclization is reduced in the case of the copolymer with bromoacrylic acid (BrAA, 86.2 kJ/mol) compared with chloroacrylic acid (ClAA, 158 kJ/mol), acrylic acid (AA, 128 kJ/mol) and itaconic acid (IA, 102 kJ/mol). Copolymer AN/BrAA permits the stabilization at higher temperature than AN/IA and AN/AA, helping to shorten the stabilization time. It is proved that comonomer ClAA is not superior to BrAA in the effectiveness of thermal stabilization. The mechanism of the cyclization reaction of AN copolymer containing halogenated carboxylic acid is discussed.

Preparation of high-performance carbon fibres from PAN fibres modified with cobaltous chloride

Preparations of high-performance carbon fibres have been tested and the results are reported in this article. Polyacrylonitrile (PAN) fibres modified with cobaltous chloride have increased the crystal size, the crystallinity and the orientation, and also improved the tensile strength by about 15–40% and the modulus by about 10–20% of the resulting carbon fibres, which were carbonized at 1300 °C. The oxygen content, theAI value and the density for the modified PAN fibres are smaller than those for unmodified PAN fibres. Because the formation of the ladder polymer in the modified PAN fibres is slow and gradual during the stabilization process, the resulting carbon fibres have a better modulus than carbon fibres developed from unmodified PAN fibres. The activation energy of cyclization,E a, is increased to 180.0 from 156.6 kJ mol−1 after the modification process. The modified PAN fibres do not reduce the time required for stabilization. The carbon fibres developed from the modified PAN fibres have a greater stacking size,L c, than those developed from unmodified PAN fibres. A higher stacking size, and therefore a higher number of crystalline carbon basal planes, is one of the reasons for the improved modulus of the carbon fibres.