Correlation of thermal degradation mechanisms: Polyacetylene and vinyl and vinylidene polymers

The thermal degradation of poly(vinyl bromide) (PVB), poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(vinyl fluoride) (PVF), poly(vinylidene chloride) (PVC₂), and poly(vinylidene fluoride) (PVF₂) has been studied by direct pyrolysis–mass spectrometry (DP-MS) and flash pyrolysis–gas chromatography–mass spectrometry techniques. Vinyl and vinylidene polymers exhibit two competitive thermal degradation processes: (1) HX elimination with formation of polyene sequences which undergo further moleculaar rearrangements, and (2) main-chain cleavage with formation of halogenated or oxigenated compounds. The overall thermal degradation process depends on the prevailing decomposition reaction in each polymer; therefore, different behaviors are observed. The thermal degradation of polyacetylene (PA) has also been studied and found important for the elucidation of the thermal decomposition mechanism of the title polymers.     

Copolymers of vinyl fluoride

Vinyl fluoride was polymerized by photochemical initiation in a continuous-flow cylindrical reactor at room temperature and at pressures of up to 30 atm. Copolymers with vinyl acetate were prepared in order to improve the solubility and processability of poly(vinyl fluoride) (PVF). The copolymers were hydrolyzed to the corresponding vinyl alcohol copolymers and yielded hydrophilic films that are strong and flexible only when swollen by water. It was found that on hydrolysis the T_g, T_m, and heat of fusion as well as degree of crystallinity increased. It was suggested that PVF and the copolymers with vinyl alcohol are isomorphous.     

Toward a Complete Understanding of the Vinyl Fluoride Spectrum in the Atmospheric Region

A deep and comprehensive investigation of the vinyl fluoride (CH₂CHF) spectrum in the atmospheric window around 8.7 μm is presented. At first, the ro‐vibrational patterns are modelled to an effective Hamiltonian, which also takes into account the coupling of the C? F stretching vibration, ν₇, with the neighbouring vibrational combination ν₉+ν₁₂. The obtained Hamiltonian gives very accurate simulations and predictions of the ro‐vibrational quantum energies. Then, in the main part of the work, an experimental and theoretical study of vinyl fluoride self‐broadening collisions is carried out for the first time. The broadening coefficients obtained experimentally are compared with those calculated by a semiclassical theory, demonstrating a significant contribution of collisional coupling effects between lines connecting pairs of degenerate (or nearly degenerate) rotational levels. Finally, the experimentally retrieved integrated absorption coefficients are used to calculate the absorption cross‐section of the ν₇ normal mode, from which dipole transition moments are derived. The obtained results provide a deep insight into the spectral behaviour of vinyl fluoride, in a spectral region of primary relevance for atmospheric and environmental determinations. Indeed, the data presented constitute an accurate model for the remote sensing of vinyl fluoride—a molecule of proved industrial importance which can lead to hazardous effects in the atmosphere and affects human′s health.     

IR laser-induced decomposition of poly(vinyl chloride-co-vinyl acetate): Control of products by irradiation conditions

IR laser-induced, ablative decomposition of poly(vinyl chloride-co-vinyl acetate) was examined under different irradiation conditions and its volatile and solid products were characterized by mass spectroscopy, infrared spectroscopy, Raman spectroscopy and UV spectroscopy and EDX-measurements. The laser decomposition of the copolymer, compared with that of poly(vinyl acetate) and poly(vinyl chloride), is revealed to be a more efficient process leading to solid films with the proportion of Cl- and CH₃C(O)O-groups controlled by irradiation conditions.     

Controlling Factors for the Conversion of Trifluoroacetate Sols into Thin Metal Fluoride Coatings

Metal fluoride films were prepared from trifluoroacetate sols by a spin-coating method. Effects of preparation conditions on the deposition of fluoride films have been examined in order to control the thickness and the refractive index of the films. It was suggested that the fluoride was formed by the thermal decomposition of the trifluoroacetate sols generating gaseous phases such as (CF₃CO)₂O, CO and CO₂. This reaction mechanism seemed to largely effect the deposition of the fluoride films in terms of the formation of pores. Characteristics of the fluoride films could be changed by varying the heat treatment. The refractive index of the MgF₂ films decreased with increasing heating temperature, which might result from the increased porosity of the films. Drying the film before the heat treatments was effective in the control of the refractive index.     

Reactions of tetrachlorodiborane(4) with fluoroolefins

Tetrachlorodiborane(4) reacts with trifluoroethylene to give dichloro-2,2- difluorovinylborane and both isomers of dichloro-2-chloro-2-fluorovinylborane. These compounds can be converted to the corresponding difluoro (halovinyl)boranes by treatment with SbF₃. Reaction of B₂Cl₄ with vinyl fluoride gives 1,1,2-tris(dichloroboryl)ethane. A facile halogen exchange with B₂Cl₄, was observed for a number of fluoroolefins, includin vinyl fluoride, 1,1-difluoroethylene, 2-fluoropropene, and 3,3,3-trifluoropropene. No reaction was observed with 2,3,3,3-tetrafluoropropene.     

Fluorocyclopropanes. IV. Copolymers of perfluorocyclopropene

Perfluorocyclopropene undergoes free-radical copolymerization with ethylene, isobutylene, cis- and trans-2-butene, vinyl acetate, methyl vinyl ether, vinyl chloride, styrene, acrylonitrile, tetrafluoroethylene, vinyl fluoride, and vinylidene fluoride. The copolymerization proceeds most readily with electron-rich olefins such as methyl vinyl ether (to yield a 1:1 copolymer), but conditions were found to give copolymers with electron-deficient olefins such as tetrafluoroethylene and vinylidene fluoride. Copolymers with methyl vinyl ether, tetrafluoroethylene, vinyl fluoride, and vinylidene fluoride were examined in detail. Evidence is presented that the perfluorocycloproply ring is incorporated intact into the copolymer and can be subsequently isomerized to a perfluoropropenyl unit by heating at 200–300°C.     

Mechanism for the gas‐phase hydrogen fluoride‐mediated decomposition of peroxyacetyl nitrate (PAN) studied by DFT method

Density functional theory has been used to study the mechanism of the decomposition of peroxyacetyl nitrate (CH₃C(O)OONO₂) in hydrogen fluoride clusters containing one to three hydrogen fluoride molecules at the B3LYP/6‐311++G(d,p) and B3LYP/6‐311+G(3df,3pd) levels. The calculations clarify some of the uncertainties in the mechanism of PAN decomposition in the gas phase. The energy barrier decreases from 30.5 kcal mol^?1 (single hydrogen fluoride) to essentially 18.5 kcal mol^?1 when catalyzed by three hydrogen fluoride molecules. As the size of the hydrogen fluoride cluster is increased, PAN shows increasing ionization along the O? N bond, consistent with the proposed predissociation in which the electrophilicity of the nitrogen atom is enhanced. This reaction is found to proceed through an attack of a fluorine to the PAN nitrogen in concert with a proton transfer to a PAN oxygen. On the basis of our calculations, an alternative reaction mechanism for the decomposition of PAN is proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Many‐body energy decomposition analysis of cooperativity in hydrogen fluoride clusters

This article studies the cooperativity present in hydrogen fluoride clusters, (FH)_n, by means of a many‐body decomposition of the binding energy. With the aim of quantifying how the results depend on the calculation level, the partition was performed from dimer to hexamer at the RHF, MP2, and density functional (B3LYP) levels, and for the heptamer and octamer at the RHF and B3LYP levels, using a 6‐31++G(d, p) basis set in all cases. We obtain that, for a proper representation of the cooperative effects in hydrogen fluoride, at least the inclusion of the three‐body terms is fundamental. The contributions are found to be underestimated at the RHF level and overestimated at the B3LYP level, with respect to the MP2 results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

CO2 laser-induced interaction between sulfur hexafluoride and carbon disulfide

Interaction between carbon disulfide and sulfur hexafluoride is excited by cw CO₂ laser radiation and has been investigated for different ratios over a pressure range from 3.1 to 34.6 kPa. The reaction yields sulfur tetrafluoride, sulfur, carbon, thiocarbonyl fluoride, tetrafluoromethane and hexafluoroethane, the ratio of these latter products is dependent on the partial pressure of sulfur hexafluoride in the initial CS₂-SF₆ mixture. Interaction is considered to include both the SF₆-sensitized decomposition of carbon disulfide and reaction between sulfur hexafluoride and carbon disulfide.     

Tetrabutylammonium fluoride-induced dehydrobromination of vinyl bromides to terminal acetylenes

Tetrabutylammonium fluoride (TBAF) was found to be a mild and efficient base for the dehydrobromination of vinyl bromides. Treatment of various 2-bromo-1-alkenes with TBAF·3H₂O in DMF yielded terminal acetylenes in high yields without undue regard to the presence of water.     

Reaktion von Lanthanfluorid mit Halogenalkanen

Reactions of Lanthanum Fluoride with Halogenoalkanes Reactions are reported on lanthanum fluoride as a fluorinating reagent and as a catalyst for the fluorination and dismutation of halogenoalkanes. Carbon tetrachloride, monofluorotrichlormethane, and chloroform are partially fluorinated by lanthanum fluoride. In the presence of HF, lanthanum fluoride acts as catalyst the fluorine transfer to CCl₄, CHCl₃ and C₂F₃Cl₃. Monofluorotrichloromethane, difluorodichloromethane, and trifluorotrichloroethane dismutate on lanthanum fluoride. Results on the thermal decomposition of the above-mentioned fluorochloroalkanes are communicated.     

Fluorothiocarbonyl compounds. VI. Free-radical polymerization of thiocarbonyl fluoride

Thiocarbonyl fluoride, CF₂?S, and thiocarbonyl chlorofluoride, CFCl?S, undergo addition polymerization in free radical-initiated systems. In addition, both compounds copolymerize with various unsaturated compounds, including typical vinyl and vinylidene monomers. The chlorofluoride, because of its rapid polymerization rate, copolymerizes best with very active monomers, of which 2,3-dichloro-1,3-butadiene is an example. Thiocarbonyl fluoride polymerizes best at low temperatures. The trialkylborane—oxygen redox couple has been adapted to free-radical polymerizations and copolymerizations from ?60 to ?120°C. With such initiation CF₂?S has been copolymerized with terminal and internal olefins, vinyl compounds, allyl compounds, and acrylic esters. Copolymerization with propylene is unusual, in that it proceeds in a manner that strongly favors a product composed of two molecules of CF₂?S for each propylene. In other cases, product compositions are more responsive to the ratio of monomers charged.     

Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500-2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late observations. We thus include a laser schlieren study (1717-2332 K, 75 < P < 482 torr in 10 and 4% vinyl fluoride in Kr) of this dissociation. This latter work also includes a set of experiments using shock-tube time-of-flight mass spectrometry (4% vinyl fluoride in neon, 1500-1980 K, 500 < P < 1300 torr). These time-of-flight experiments confirm the theoretical expectation that the only reaction in vinyl fluoride is HF elimination. The dissociation experiments are augmented by laser schlieren measurements of vibrational relaxation (1-20% C(2)H(3)F in Kr, 415-1975 K, 5 < P < 50 torr, and 2 and 5% C(2)H(4)F(2) in Kr, 700-1350 K, 6 < P < 22 torr). These experiments exhibit very rapid relaxation, and incubation delays should be negligible in dissociation. An RRKM model of dissociation in 1,1-difluoroethane based on a G3B3 calculation of barrier and other properties fits the experiments but requires a very large DeltaE(down) of 1600 cm(-1), similar to that found in a previous examination of 1,1,1-trifluoroethane. Dissociation of vinyl fluoride is complicated by the presence of two parallel HF eliminations, both three-center and four-center. Structure calculations find nearly equal barriers for these, and TST calculations show almost identical k(infinity). An RRKM fit to the observed falloff again requires an unusually large DeltaE(down) and the experiments actually support a slightly reduced barrier. These large energy-transfer parameters now seem routine in these large fluorinated species. It is perhaps a surprising result for which there is as yet no explanation.     

Studies on the kinetics of decomposition of diacyl peroxides: VI. CIDNP in the products of thermolysis of lauroyl peroxide and 3,5,5-trimethylhexanoyl peroxide

The ¹H NMR spectra recorded during the thermolysis of lauroyl peroxide and of 3,5,5-trimethylhexanoyl peroxide in ODCB containing 2-iodopropane show CIDNP multiplet effect. It was observed that the relative concentrations of 1-iodoundecane and 2-iodopropane, both polarized and normal species, changed with time. The relaxation time T₁ of the α-protons in 1-iodoundecane and vinyl protons of CH₂ in 1-undecene, enhancement factor of the protons of polarized species and the rate constant of decomposition for lauroyl peroxide have been determined. The mechanism of the decomposition of the entitled peroxides is discussed in terms of the radical pair theory.     

The CW-CO2 laser induced reaction of 1,1-difluoroethane with chlorine

The CW–CO₂ laser induced reaction of CF₂HCH₃ (1) with Cl₂ yields vinylidene fluoride (2) as the main product; other products are CH₃CF₂Cl (3), CF₂ClCH₂Cl (4), CF₂ = CHCl (5), CF₂ = CCl₂ (6) and CF₂ Cl₂ (7). The yield dependence of 2 on the CF₂HCH₃/Cl₂ ratio, the laser irradiation time, the laser power and the pressure of the gaseous reactants have been investigated. Furthermore, the TEA–CO₂ laser induced reaction of 1 with Cl₂. the CW–CO₂ laser induced reaction of 2 and 2 with Cl₂ have also been studied in order to gain more mechanistic insights for this complicated reaction system. Apparently, CF₂ClCH₃ but not CF₂HCH₂Cl. is the main precursor to 2. Interestingly, it has been found that the relatively strong double bond of CF₂ = CH₂ can be broken by laser irradiation. The possibility of applying this laser methodology to the production of vinylidene fluoride has been discussed.     

Effect of Calcination Temperature on La-Modified Al2O3 Catalysts for Vapor Phase Hydrofluorination of Acetylene to Vinyl Fluoride

A La-modified Al₂O₃ catalyst was prepared with deposition-precipitation method. The effect of calcination temperature on the reactivity for vapor phase hydrofluorination of acetylene to vinyl fluoride. The catalysts calcined at different temperatures were characterized using NH₃-TPD, pyridine-FTIR, X-ray diffraction, and Raman techniques. It was found that the calcination process could not only change the structure of these catalysts but also modify the amount of surface acidity on the catalysts. The catalyst calcined at 400 oC exhibited the highest conversion of acetylene (94.6%) and highest selectivity to vinyl fluoride (83.4%) and lower coke deposition selectivity (0.72%). The highest activity was related to the largest amount of surface acidity on the catalyst, and the coke deposition was also related to the total amount of surface acidic sites.     

Pressure and laser intensity effects on kinetic rates in binary reactive gas mixtures

The irradiation of binary reactive gaseous mixture by a CW CO₂ laser at changing pressure of absorber or transparent reactant have been performed at over-all pressures of several hundreds torrs with a maximum laser intensity of 680 W.cm⁻². A radial thermodiffusion effect leading both to a transient depletion of absorber concentration in the irradiation zone and a decrease of the rate of decomposition have been shown. The maximum yield of decomposition which appears at a given intensity and pressure of absorber when the pressure of reactant is changed is general for the binary gas mixtures tested ; it may be understood in terms of laser energy deposited in the mixture and distributed between the absorber and reactant partners. A rate decomposition dependent on the square of incident laser intensity and proceeding from this distribution is also exhibited.     

Potential for rotation about C(sp2)-O and C(sp2)-S bonds: electron diffraction results for CH2CH-OCH3 and CH2

Electron diffraction of gaseous methyl vinyl ether and methyl vinyl sulphide show that both compounds exist as a mixture of two conformers, one of these is the syn form with a planar heavy atom skeleton. For methyl vinyl ether the second conformer may also have a planar skeleton (anti form), but a gauche form with a torsional angle close to 180° cannot be ruled out. For the sulphide a gauche form is found. Structural parameters are given in Table 2. Ab initio calculations gave energy minima for syn and anti forms for CH₂CH-OH and for syn and gauche forms for CH₂CH-SH.