Pulsed NMR study of molecular motion in the uncured diglycidyl ether of bisphenol-A

The diglycidyl ether of bisphenol-A, an uncured epoxy resin, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1p, and T₂ have been measured over the temperature range from ?160 to 200°C. The resin was studied in its monomeric form and in two mixtures containing higher oligomers. The relaxation times are interpreted in terms of the molecular motion in the resins. The motion responsible for relaxation in the solid monomer form is thought to be methyl group reorientation at low temperatures and general molecular motion at high temperatures. The motions are characterized by activation energies of 5 kcal/mole and 33 kcal/mole, respectively. The solid mixtures exhibit similar effects to the monomer, but an additional relaxation mechanism is observed which is attributed to segmental motion. This motion is characterized by an activation energy of 12–15 kcal/mole. The self-diffusion coefficient was measured in the liquid monomer, and the activation energy for self-diffusion is found to be 11 kcal/mole.     

Kinetics of the I2-catalyzed isomerization of allyl chloride to cis- and trans-1-chloro-1-propene. The stabilization energy of the chloroallyl radical

The I₂-catalyzed isomerization of allyl chloride to cis- and trans- l-chloro-l-propene was measured in a static system in the temperature range 225–329°C. Propylene was found as a side product, mainly at the lower temperatures. The rate constant for an abstraction of a hydrogen atom from allyl chloride by an iodine atom was found to obey the equation log [k,/M^?1 sec^?1] = (10.5 ± 0.2) ?; (18.3 ± 10.4)/θ, where θ is 2.303RT in kcal/mole. Using this activation energy together with 1 ± 1 kcal/mole for the activation energy for the reaction of HI with alkyl radicals gives DH⁰ (CH₂CHCHCl? H) = 88.6 ± 1.1 kcal/mole, and 7.4 ± 1.5 kcal/mole as the stabilization energy (SE) of the chloroallyl radical. Using the results of Abell and Adolf on allyl fluoride and allyl bromide, we conclude DH⁰ (CH₂CHCHF? H) = 88.6 ± 1.1 and DH⁰ (CH₂CHCHBr? H) = 89.4 ± 1.1 kcal/ mole; the SE of the corresponding radicals are 7.4 ± 2.2 and 7.8 ± 1.5 kcal/mole. The bond dissociation energies of the C? H bonds in the allyl halides are similar to that of propene, while the SE values are about 2 kcal/mole less than in the allyl radical, resulting perhaps more from the stabilization of alkyl radicals by α-halogen atoms than from differences in the unsaturated systems.     

Relation of the decay of free radicals in irradiated polyethylene to the molecular motion of the polymer and the configurations of the free radicals

Decay reactions of the free radicals produced in irradiated polyethylene (high-density and low-density materials) were examined in connection with the molecular motion of the matrix polymer. Three temperature regions, in which the free radicals decay very rapidly, at around 120, 200, and 250°K, were designated T_A, T_L, and T_B, respectively. The decay of the free radicals at these temperatures had activation energies in high-density polyethylene of 0.4 kcal/mole for T_A, 9.4 kcal/mole for T_L, and 18.4 kcal/mole for T_B. In low-density polyethylene these quantities were 0.7 kcal/mole for T_A, 23.1 kcal/mole for T_L, and 24.8 kcal/mole for T_B. Comparison of time constants for the decay reactions and for molecular motion of the matrix polymer indicate that the decay in T_A and T_B is closely related to molecular motion in the amorphous regions of the polymer. The decay of the free radicals at T_L in high-density polyethylene is due to molecular motion associated with local mode relaxation at lamellar surfaces, while that of low-density polyethylene is due to local mode relaxation in the completely amorphous region. Steric configurations of the free radicals which decay in the respective temperature regions were also investigated.     

Proton NMR study of molecular motion in polydimethylsiloxane

Proton spin–lattice relaxation times T₁ were measured for two samples of polydimethylsiloxane (PDMS), one with weight-average molecular weight M_w = 77,400 and the other with M_w = 609,000. Two T₁ minima and a T₁ discontinuity were observed for each compound. The high-temperature T₁ minima were attributed to a stretching and flexing motion of the PDMS chain. Quantitative comparison of the relaxation data with a theoretical model developed for this motion allowed the activation energy, 2.3 kcal/mole, and the maximum angular displacement of the methyl group symmetry axis to be determined. The latter was found to be 31°, independent of sample molecular weight. The low-temperature minima were ascribed to methyl reorientation with an activation energy of 1.6 kcal/mole. The T₁ discontinuities were attributed to melting and allowed the degree of crystallinity to be estimated.     

The Electrocyclic Cyclopropyl-Allyl Rearrangement. Preliminary communication

MINDO-2 SCF calculations indicate that ring-opening of cyclopropyl radical (I) to allyl radical (II) is more favourable via a disrotatory reaction path, the calculated activation energy being ～30 kcal/mole. (For conrotatory opening the activation energy was found to be ～44 kcal/mole.) The two critical motions of the nuclei during the transformation are found to be strongly decoupled, i.e. rupture of the CH₂βCH₂ bond precedes rotation of the CH₂ groups. As predicted by qualitative theories both ring-opening modes are unfavourable since they involve a change in electronic ground-state symmetry between I and II. The preferred ring-opening mode is discussed qualitatively in terms of Evans' principle.     

Energy disposal in the three-centered elimination of df from 1,1,2-trifluoroethane-1-d1

The chemical activation data for three- and four-centered hydrogen fluoride elimination from CH₂FCDF₂ have been analyzed to assign the energy released to the olefin fragment in the three-centered process and to estimate the threshold energies for elimination channels. Based upon the cis–trans isomerization rates of CHF = CHF, 78% of the total available energy was released to the olefin fragment for the αα channel. The analysis suggests the existence of an appreciable barrier (～10 kcal/mole) for the reverse reaction, addition of the CH₂FCF carbene to DF. The threshold energies for αα, αβ, and βα elimination from 1,1,2-trifluoroethane-1-d₁ were assigned as 71, 68, and 68 kcal/mole, respectively. Analysis of the chemical activation data for 1,1,2,2,-tetrafluoroethane, without distinguishing between the three- and four-centered elimination channels, suggests a threshold energy of ～75 kcal/mole.     

Isothermal degradation of poly-2,2′-(m-phenylene)-5,5′-bibenzimidazole

The isothermal degradation of poly-2,2′-(m-phenylene)-5,5′-bibenzimidazole in vacuo has been studied. Measurement of the increase in pressure with time, coupled with infrared analysis, was used to determine the distribution of the degradation products. Processes A and B with different second-order rate laws were determined to be significant in the temperature range of 550–700°C. Process A leads to the formation of equimolar quantities of hydrogen and ammonia and has an activation energy of 68 kcal/mole. Process B leads to the production of HCN, NH₃, and H₂ in the ratio of 1:1:2.5 and has an activation energy of 77 kcal/mole. The activation energies and the rate laws are consistent with a mechanism in which the initial degradation step is the bimolecular reaction of two aromatic rings.     

Radiation copolymerization of vinylene carbonate with isobutyl vinyl ether

The radiation-induced copolymerization of vinylene carbonate (M₁) with isobutyl vinyl ether (M₂) has been investigated over the temperature range of 40–80°C. The monomer reactivity ratios r₁ and r₂ were determined to be 0.118 and 0.148, respectively, and an activation energy of 7.6 kcal/mole (31.8 kJ/mole) was calculated for the copolymerization process.     

Isothermal and nonisothermal creep of lightly crosslinked cis-1,4 polybutadiene

A nonisothermal creep experiment has been analyzed to ascertain its suitability for determining the temperature dependence of low activation energy viscoelastic processes in elastomers far above T_g. The nonisothermal method was employed to determine the activation energy for creep near 35°C in a lightly crosslinked cis-1,4 polybutadiene elastomer at small strains within the linear viscoelastic region, and at various large deformations up to rupture. The observed activation energy was essentially independent of the level of strain, and the value of ΔH_a (～11 kcal/mole) determined via the nonisothermal creep method was in good agreement with the result (～12 kcal/mole) obtained via time-temperature superposition of isothermal linear viscoelastic creep data. The nonisothermal data allowed for an estimate of the volume of the “flow unit” associated with the controlling viscoelastic creep mechanism, attributed here to slippage of entanglements within the lightly crosslinked network.     

Studies in ring-opening polymerization. II. Cyclohexane spiro-5-1,3,2-dioxathiolan-4-one 2-oxide

The effect of spiro cyclohexane substitution on the polymerizability of the 1,3,2-dioxathiolan-4-one-2-oxide ring in various solvents has been examined. The steric hindrance of the cyclohexane ring inhibits the bimolecular chain propagation reaction which involves direct attack by a terminal hydroxyl group on the ring and which has been shown to occur in simpler dioxathiolan systems. The conjoined cyclohexane ring does not, however, markedly affect the “thermal” polymerization which occurs in nonhydroxylic solvents and in which chain propagation is thought to involve a reactive α-lactone intermediate. The rate-determining step in the sequence of reaction leading to polymer formation is a ring-scission process in which sulfur dioxide is evolved and the α-lactone intermediate formed. The values of the activation energy (25–30 kcal/mole) and frequency factor (10¹¹–10¹³sec^?1) associated with this reaction are, therefore, those which govern the the overall polymerization, since the subsequent steps are sensibly instantaneous. In the presence of adventitious traces of water the resultant polymer, poly(1-hydroxycyclohexanecarboxylic acid) has one carboxyl and one hydroxyl endgroup per chain. Polymers having M?_n ～ 15,000 are readily obtained; these are amorphous materials, in contrast to the analogous poly-β-ester and dialkyl-substituted poly-α-esters which are crystalline. At temperatures in excess of 120°C a competitive first-order fragmentation reaction leading to the formation of cyclohexanone, carbon monoxide, and sulfur dioxide was observed. Kinetic studies demonstrated that this reaction, which is characterized by an activation energy of ～40 kcal/mole is unimportant, in the sense that it does not interfere with polymer formation at temperatures below 100°C.     

Further studies on the dissociation of HBr behind shock waves

Previously reported shock tube studies of the dissociation of HBr in the temperature range of 2100–4200°K have been extended to lower temperatures (1450–2300°K) in pure HBr. The course of reaction was followed by monitoring the radiative recombination emission in the visible spectrum from Br atoms. The results imply that, in the lower range of temperatures, the activation energy of dissociation, E in the expression AT^?2e^?E/RT, can be approximated by the HBr bond energy (88 kcal/mole). It was also found that, in this temperature range, the rate of HBr dissociation is sensitive to the Br₂ dissociation rate and the HBr + Br exchange rate. When these rates were adjusted to bring computed reaction profiles into agreement with experimental ones, it was found that the higher-temperature data could also be fitted reasonably well with an HBr dissociation activation energy of 88 kcal/mole, contrary to the conclusions of our previous work, which favored an activation energy of 50 kcal/mole. The “best value” for k₁^Ar, the rate coefficient for HBr dissociation in the presence of Ar as chaperone, appears to be 10^{21.78 ± 0.3} T^?2 10^?88/θ cc/mole sec, where θ = 2.3 RT/1000; that for k₁^HBr, is 10^22.66T^?210^?88/θ. A detailed review is given of the rate coefficients for the other pertinent reactions in the H₂–Br₂ system, viz., Br₂ dissociation and reactions of HBr with H and Br.     

Thermal degradation and oxidation of polystyrene studied by factor-jump thermogravimetry

Factor-jump thermogravimetry has been used to study the activation energy of polystyrene degrading in a vacuum, in N₂ flowing at 4 mm/s and in $\text{N}{}_2\text{O}{}_2$ mixtures. The results show the activation energy to be 44·9 ± 0·2 kcal/mole (188 ± 0·8 kJ/mole) for degradation above 350°C in vacuum or in flowing N₂. This agrees well with work reported in 1949 by Jellinek⁷ but with few results reported subsequently.The apparent activation energy for polystyrene losing weight above 280°C in an atmosphere of abundant O₂ is 21·5 ± 0·2 kcal/mole (90·2 ± 0·8 kJ/mole). In all cases where O₂ was deliberately introduced (partial pressures >4 mm Hg), the sample degraded to a black tar and the activation energy was ≤30 kcal/mole, depending on the amount of oxygen present and on the thermal history of the sample.     

The dissociation of 3-chloro-1-butene and the resonance energy of the chloro-allyl radical

Methane is a primary product of pyrolysis of 3-chloro-l-butene at temperatures in the range 776–835°K, and from its rate of formation values have been obtained for the limiting high-pressure rate constant of the reaction These may be represented by the expression log [(k₁)_∞/sec^?1] = (16.7 ± 0.3) ? (71.5 ± 1.5)/θ, where θ = 2.303RT kcal/mole. Assuming a zero activation energy for the reverse reaction and that over the experimental temperature range the rates at which a methyl radical adds on to chlorobutene are comparable to those at which it abstracts hydrogen, the activation energy for the dissociation reaction leads to a value of 83.2 ± 1.9 ckal/mole for D(H? CHClCH:CH₂) at 298°K. Taking D(H? CHClCH₂CH ₃) = 95.2 ± 1.0 kcal/mole a value of 12.0 ± 2.1 kcal/mole is obtained for the resonance energy of the chloroallyl radical. This value in conjunction with resonance energies obtained in earlier work indicates that substitution of a hydrogen atom on the carbon atom adjacent to the double bond in the allyl radical leads to no significant variation in the allylic resonance energy.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-chloroethyl and ethylenechloronium cations, two possible intermediates in the electrophilic addition of chlorine to ethylene, have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set.These geometries were then used to compute more accurate wave functions using Dunning's double-zeta basis set. The bridged chloronium ion was found to be more stable by 9.35 kcal/mole, the opposite order of stability from the C₂H₄F⁺ ions. Interconversion of the two C₂H₄Cl⁺ cations was computed to have a barrier of 6.25 kcal/mole.The activation energy for this chlorination reaction, using the ethylenechloronium cation and a chlorine anion at infinite separation as the model for the activated complex, was computed to be 128.7 kcal/mole, showing that this is not a feasible gas phase reaction.     

The effect of pressure on the volume and the dielectric relaxation of linear polyethylene

The effects of pressure on the α (ca. 70°C, 1 kHz) and γ (ca. ?100°C, 1 kHz) relaxations of linear polyethylene were studied dielectrically between 0 and 4 kbar. Equation of state (PVT) data were also determined in the range of interest of these relaxations. The sample was rendered dielectrically active through oxidation (0.8 C?0 per 1000 CH₂). The α process (which occurs in the crystalline fraction) could be studied over a much wider temperature range than heretofore possible due to the effect of pressure in increasing the melting point. Examination of relaxation strength from 50 to 150°C showed that there must be two crystalline relaxation processes: the well-known α relaxation plus a competing one. The α relaxation is believed to be due to a chain twist–rotation–translation mechanism that results in rotation–translation of an entire chain in the crystal. The relaxation strength of the α process decreases and therefore indicates the presence of a second (faster and not directly observed) process that increases in intensity with increasing temperature. It is postulated that the second process is due to motion of defects that become more numerous through thermal injection at higher temperatures. Analysis of the relaxation data along with the PVT data allowed the constant volume activation energy for the α relaxation to be determined. It was found to be 19.4 ± 0.5 kcal/mole. The constant volume activation energy is important in modeling calculations of the crystal motions and is significantly smaller than the atmospheric constant pressure activation energy of 24.9 kcal/mole. The effect of pressure on the activation parameters and shape of the γ process was also determined. There has been controversy over whether the γ process occurs only in the amorphous fraction or in both the amorphous and crystalline phases. Since the two phases have quite different compressibilities, increasing the pressure should change the shape of the loss curves (versus frequency and temperature) if the process occurs in both phases. The shape (but not location) of the loss curves was found to be remarkably independent of pressure. This finding strengthens the view that the γ process is entirely amorphous in origin.     

Catalytic action of metallic salts in autoxidation and polymerization. IV. Polymerization of methyl methacrylate by cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide or dioxane hydroperoxide

Polymerization of methyl methacrylate was carried out by four initiating systems, namely, cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide (t-Bu HPO) or dioxane hydroperoxide (DOX HPO). Dioxane hydroperoxide systems were much more effective for the polymerization of methyl methacrylate than tert-butyl hydroperoxide systems, and cobaltous acetylacetonate was more effective than cobaltic acetylacetonate in both hydroperoxides. The initiating activity order and activation energy for the polymerization were as follows: Co(acac)₂–DOX HPO (E_a-9.3 kcal/mole) > Co (acac)₃–DOX HPO (E_a = 12.4 kcal/mole) > Co(acac)₂–t-Bu HPO (E_a = 15.1 kcal/mole) > Co(acac)₃–t-Bu HPO (E_a-18.5 kcal/mole). The effects of conversion and hydroperoxide concentration on the degree of polymerization were also examined. The kinetic data on the decomposition of hydroperoxides catalyzed by cobalt salts gave a little information for the interpretation of polymerization process.     

Polymerization of styrene with ZrCl4 and ZrCl3 in combination with Al(C2H5)3

The polymerization of styrene with two catalyst systems consisting of Al(C₂H₅)₃ in combination with ZrCl₄ or ZrCl₃ has been studied. The rate of polymerization with catalyst concentration was first-order with ZrCl₄ system and second-order with ZrCl₃ system, but at higher catalyst concentrations in both cases, the rate progressively decreases and finally attains a low value. The rate of polymerization is, however, proportional to the square of the monomer concentration in both the cases. The overall energy of activation was 10.9 kcal./mole and 6.45 kcal./mole in these systems. The polymers obtained with ZrCl₄ were of lower molecular weights as compared to those obtained with ZrCl₃. The polymers in both the cases had amorphous character.     

Trimethylenemethane: Activation energy for ring-closure of the diradical

The energy of activation for ring-closure of ground state triplet trimethylenemethane (I) to methylenecyclopropane has been measured by following the rate of dissappearance of the electron spin resonance spectrum over the temperature range –155° to –140°, in a series of frozen solid matrices. The experiments described make use of 3-methylenecyclobutanone and methylenecyclopropane as precursors to trimethylene-methane.Kinetic results obtained starting from methylenecyclopropane were most satisfactory and lead to an energy of activation for ring-closure of 7 kcal/mole. This value is significantly smaller than the aprpox. 20 kcal/mole barrier estimated on the basis of theoretical models. Truncation of the barrier by a tunnelling mechanism is made unlikely by the finding that trimethylenemethane-d₆(I-d₆) undergoes ring-closure with the same 7 kcal/mole energy of activation as the parent I.     

A kinetic study of the thermal elimination of hydrogen fluoride from 1,2-difluoroethane. Determination of the bond dissociation energies D(CH2F?CH2F) and D(CH2F?H).

The kinetics of the thermal elimination of HF from 1,2-difluoroethane have been studied in a static system over the temperature range 734–820°K. The reaction was shown to be first order and homogeneous, with a rate constant of where θ = 2.303RT in kcal/mole. The A-factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has been compared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2-difluoroethane by the RRKM theory and the bond dissociation energy, D(CH₂F? CH₂F) = 88 ± 2 kcal/mole, derived. It follows from thermochemistry that ΔH_f⁰(CH₂F) = -7.8 and D(CH₂F? H) = 101 ± 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.     

Silane pyrolysis

We show that silane pyrolysis is initiated by decomposition on the amorphous silicon surface, with an activation energy E_a of 56 kcal/mole. The observed surface decomposition rate is only weakly dependent on silane pressure. Much faster delayed decomposition rates, approximately independent of surface area and proportional to pressure, are shown to be initiated by surface reactons. A model for surface decomposition is given. Also a model for gas reactions is suggested based on H atom or SiH₃ release by surface decomposition, causing chain reactions that process the gas to higher silanes that decompose rapidly. This model can explain the previous observations that the initial disilane formation rate and the delayed decomposition rate were independent of the surface area to volume ratio A/V, which had misled previous investigators to suggest homogeneous initiation processes.