Revising the mechanism of polymer autooxidation

The basic scheme for autooxidation of polymers, originally developed by Bolland, Gee and co-workers for rubbers and lipids, is now widely applied to all types of polymeric materials. According to their scheme, the reaction that makes this process autocatalytic, referred to as the propagation step, is a hydrogen abstraction from the next substrate by the peroxyl radical (ROO˙ + RH → ROOH + R˙). In this study, using advanced quantum-chemical methods, we have shown that this step is actually characterised by largely positive Gibbs free energy (10-65 kJ mol(-1)) for most regular polymers with saturated chains (polypropylene, polyethylene, polyvinyl chloride, polyvinyl acetate, polyurethane, poly(methyl methacrylate) etc.) and even some polymers with unsaturated fragments (polystyrene, polyethylene terephthalate). Neither elevated temperature, nor solvation makes this process thermodynamically favourable. Only when the formed radical centre is conjugated with adjacent double bonds (as in polybutadiene) or captodatively stabilised by two suitable functional groups (such as a carbonyl and a lone pair donor such as oxygen or nitrogen), is the propagation step exoergic. Instead, we show that it is the presence of structural defects, such as terminal or internal double bonds, formed either during polymerisation or in the degradation process itself, that is responsible for the autooxidation of most polyesters and most polyalkenes. Recognition of the real mechanism of autooxidation in polymers is a key to developing strategies for the prevention of their degradation.     

Study the effect of antioxidants on biological activity and on homopolypropylene; mechanical and physical properties

Exposure of polymers to temperature, atmospheric oxygen, or even light could result in some degradation of the polymer properties and features during processing (application), storage and end use. In hydrocarbon polymers, the polymer tend to free radical formation, eventually resulting in chain damage or crosslinking that leads to degradation. Antioxidants are used to terminate these chain reactions by removing radicals. Antioxidants are used in most hydrocarbon polymers including, polypropylene. a good addiction package must be existed to overcome the effect of degradation and save the polymer shape and characteristics. The practical experiment was carried out on a pure polypropylene (intermediate polypropylene resin without additives) and another practical experiment but with adding several types of additives with a certain concentration and study the behavior of polypropylene in all cases with successive extrusions. On other hand Flexible molecular docking on heme oxygenase, an important stress protein that is involved in cellular protection, antioxidant and anti-inflammatory activities, justified the antioxidant activity of the isolated compounds. From the binding energy 3114 and 1680 they could consider to be powerful and available antioxidant.     

Radical transfer reactions in polymers

Free-radical transfer in polymers have been studied by pulse radiolysis and product analysis with the water-soluble polymers poly(vinyl alcohol), poly(acrylic acid), poly(methacrylic acid), polynucleotides and DNA. When OH radicals react with polymers the lifetime of the polymer radical thus created strongly depends on the number of radicals per polymer chain. Moreover, in negatively charged polymers the increased stiffness at high pH results in a remarkable increase of the lifetime of the radicals with respect to recombination. This allows a number of radical transfer reactions to occur (e.g. intramolecular H-transfer, β-fragmentation, depolymerization, reactions with additives).     

Naphthalene diols: a new class of antioxidants intramolecular hydrogen bonding in catechols,naphthalene diols,and their aryloxyl radicals

1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH*), k(ArOH/DOPPH)*, or as antioxidants during inhibited styrene autoxidation, k(ArOH/ROO)*, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k(ArOH/ROO)* = 6.0 x 10(6) M(-)(1) s(-1), of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (deltaBDEs), of 1-6 predict a HAT order of reactivity of 2 < 1 < 3 approximately 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and this effect contributes to the increased HAT activity of 5 and 6 compared to the activities of the catechols. For example, the increased stabilization due to the intramolecular H-bond of 5 radical over 5 parent of 8.6 kcal/mol was about double that of 2 radical over 2 parent of 4.6 kcal/mol. Linear free energy plots of log k(ArOH/DOPPH)* and log k(ArOH/ROO)* versus deltaBDEs for compounds 1-6 along with available literature values for nonsterically hindered monophenols placed the compounds on common scales. The derived Evans-Polanyi constants from the plots for the two reactions, alpha(DOPPH)* = 0.48 > alpha(ROO)* = 0.32, gave the expected order, since the ROO* reaction is more exothermic than the DOPPH* reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log k(ArOH/ROO)* versus deltaBDE plot.     

A DFT study of H-isomerisation in alkoxy-, alkylperoxy- and alkyl radicals: Some implications for radical chain reactions in polymer systems

Intramolecular 1-n H-shift (n = 2, 3… 7) reactions in alkoxy, alkyl and peroxy radicals were studied by density functional theory (DFT) at the B3LYP/6-311+G∗∗ level and compared with respective intermolecular H-transfers. It was found that starting from 1 to 3 H-shift the barrier heights stepwise decrease with increasing n reaching minimum for 1-5 and 1-6 H-shifts. This dependence can be ascribed to the decrease of the strain with increasing transition state (TS) ring size, which is minimal in six- and seven-member rings. The barrier heights of H-shifts in alkyl radicals are systematically larger than those in alkoxy radicals: the respective activation energies (E_a) of 1-5 and 1-6 H-shifts are about 59-67 kJ/mol for alkyl radical and 21-34 kJ/mol for alkoxy radicals. Further increase of the TS ring size in 1-7 H-shifts leads to the increase of the barrier to 44 kJ/mol in the hexyloxy radical and 84 kJ/mol for n-heptyl radical. We have also found that intermolecular H-transfer reactions in all three types of free radicals have smaller barriers than respective intramolecular 1-5 or 1-6 H-shifts by 4-25 kJ/mol. The mentioned difference can be explained in terms of enhanced nonbonding repulsion interaction in the cyclic TS structures compared to respective intermolecular TS. B3LYP/6-311+G∗∗ geometric parameters and imaginary frequencies for 1-n H-shifts TS are consistent with respective calculated barrier heights. Reactivity of some other radicals compared to alkoxy, peroxy and alkyl radicals as well as other factors influencing their reactivity (π-conjugation, steric effect and ring strain in cyclic TS, etc.) are also briefly discussed in relation to free radical reactions in polymer systems.     

Influence of copper-phthalocyanine on the photodegradation of polycarbonate

Dyes and pigments are extensively used in polymer materials to confer colour-changing properties. However, these additives can significantly affect polymer stability against degradation. While the mechanism of stabilization of polymers by some pigments, such as carbon black, has been studied and is well known, the action of chromatic colorants, mainly in the sensitization of the degradation process, remains unclear. Cu-phthalocyanine dye can stabilize polymers against degradation as well as accelerate degradation in other situations. Cu-phthalocyanine incorporated into polycarbonate resulted in an acceleration of the degradation when the material was submitted to photochemical aging. The possible mechanism to explain the photodegradative behavior of PC containing Cu-phthalocyanine is based on the hypothesis that specific interactions among excited states of PC and Cu-phthalocyanine take place and enhance the formation of reactive species in polycarbonate. Excited states of Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in polycarbonate, increasing the formation of free radicals P, which are the starting points for the sequential photo-oxidation reactions that lead to the degradation of the polycarbonate. Electron transfer sensitization is also a possible mechanism: the excited state of Cu-Ph abstracts an electron from PC to form the Cu-Ph radical anion and the PC radical cation. These reactive species in the presence of oxygen can cause oxidation of the aromatic ring.     

Radicals masquerading as electrophiles: a computational study of the intramolecular addition reactions of acyl radicals to imines

Ab initio calculations using 6-311G**, cc-pVDZ, and aug-cc-pVDZ, with (MP2, QCISD, CCSD(T)) and without (UHF) electron correlation, and density functional methods (BHandHLYP and B3LYP) predict that cyclization of the 5-aza-5-hexenoyl and (E)-6-aza-5-hexenoyl radicals proceed to afford the 5-exo products. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVDZ level of theory, energy barriers (deltaE(double dagger)) of 36.1 and 47.0 kJ mol(-1) were calculated for the 5-exo and 6-endo pathways for the cyclization of the 5-aza-5-hexenoyl radical. On the other hand, at the same level of theory, deltaE(double dagger) of 38.9 and 45.4 kJ mol(-1) were obtained for the 5-exo and 6-endo cyclization modes of (E)-6-aza-5-hexenoyl radical, with exothermicities of about 27 and 110 kJ mol(-1) calculated for the exo and endo modes, respectively. Under suitable experimental conditions, the 6-endo cyclization product is likely to dominate. Analysis of the molecular orbitals involved in these ring-closure reactions indicate that both reactions at nitrogen are assisted by dual orbital interactions involving simultaneous SOMO-pi* and LP-pi* overlap in the transitions states. Interestingly, the (Z)-6-aza-5-hexenoyl radical, that cannot benefit from these dual orbital effects is predicted to ring-close exclusively in the 5-exo fashion.     

Radiation induced radical reactions in poly(chloroalkyl methacrylate)s/triallylisocyanurate mixtures

Radical generation after γ-irradiation at 77 K and radical reactions during temperature increase are discussed for the polymers poly(2-chloroethyl methacrylate) and poly(1,2,2,2-tetrachloroethyl methacrylate), mixed with triallylisocyanurate (TAIC). After the radiation interaction radicals are generated in the ester alkyl side groups of the polymer by chlorine abstraction in both polymer/TAIC mixtures. Radical transfer to TAIC occurs during temperature increase to produce allyl radicals. Hence, the high radiation sensitivity of chlorine-containing polymers is combined with the reactivity of allyl radicals of TAIC in such polymer–TAIC mixtures.     

聚甲基丙烯酸甲酯热氧化降解的化学动力学研究

使用质谱、热分析手段研究了PMMA热解反应.结果表明,在氮气中,PMMA-CH=CH2有两个失重阶段,分别对应于主链末端双键引发的断链和主链无规则断链反应,转折点的失重率约为26%.其中,第一阶段的失重速率受扩散过程控制,平均表观活化能E为158.5 kJ/mol, lnA为27.69;第二失重阶段为1.5级化学反应,平均表观活化能E为214.79 kJ/mol, lnA为40.46.在空气中, PMMA也有两个失重阶段,反应机理为1级化学反应,转折点处的失重率约为70%.其中在第一失重阶段平均表观活化能E为130.32 kJ/mol, lnA为24.81,在此阶段中, 过氧化基团的分解反应对PMMA的失重速率有重要影响; 在空气中第二失重阶段平均表观活化能E为 78.25 kJ/mol, lnA为13.97.     

Increased carbon dioxide concentration improves the antioxidative properties of the Malaysian herb kacip fatimah (Labisia pumila Blume)

A randomized complete randomized design (RCBD) 3 by 3 experiment was designed to investigate and distinguish the relationships among production of secondary metabolites (total phenolics, TP; total flavonoids, TF), gluthatione (GSH), oxidized gluthatione (GSSG), soluble carbohydrate and antioxidant activities of the Malaysian medicinal herb Labisia pumila Blume under three levels of CO? enrichment (400, 800 and 1,200 μmol mol?1) for 15 weeks. It was found that the treatment effects were solely contributed by interaction of CO? levels and secondary metabolites distribution in plant parts, GSH, GSHH and antioxidant activities (peroxyl radicals (ROO), superoxide radicals (O?), hydrogen peroxide (H?O?) and hydroxyl radicals (OH). The records of secondary metabolites, glutahione, oxidized gluthathione and antioxidant activities in a descending manner came from the leaf enriched with 1,200 μmol/mol CO? > leaf 800 μmol/mol CO? > leaf 400 μmol/mol CO? > stem 1,200 μmol/mol CO? > stem 800 μmol/mol CO? > stem 400 μmol/mol CO? > root 1,200 μmol/mol CO? > root 800 μmol/mol CO? > root 400 μmol/mol CO?. Correlation analyses revealed strong significant positive coefficients of antioxidant activities with total phenolics, flavonoids, GSH and GSHH indicating that an increase in antioxidative activity of L. pumila under elevated CO? might be up-regulated by the increase in production of total phenolics, total flavonoids, GSH, GSHH and soluble sugar. This study implied that the medicinal potential of herbal plant such as L. pumila can be enhanced under elevated CO?, which had simultaneously improved the antioxidative activity that indicated by the high oxygen radical absorbance activity against ROO, O?, H?O?, and OH radicals.     

Kinetics of polymeric radical oxidation with molecular oxygen in poly(methyl methacrylate) films

In poly(methyl methacrylate) films, the kinetics of the oxidation of polymeric radicals and azobenzenenitrenes with molecular oxygen dissolved in the polymer is studied. The free radicals are produced at 77 K by irradiating the polymer with UV light, fast electrons, or γ rays. The concentration of oxygen is varied from 4.5 × 10¹⁸ to 3.1 × 10¹⁹ cm^?3; the temperature of the reaction, from 90 to 130 K. The reaction is carried out in excess oxygen. The kinetics of radical oxidation is shown to be independent of the type of radiation that stimulates the formation of radicals and coincides with the kinetics of the oxidation of azobenzenenitrenes, which are uniformly dissolved in the polymer. It is concluded that the structure of the polymer in the vicinity of the radicals is virtually the same as the structure of the polymer bulk. The activation energy of the oxygen diffusion coefficient calculated according to the radical oxidation kinetics amounts to ～30 kJ/mol.     

An ESR study of PMMA mechanoradicals and of their interaction with oxygen

Analysis of ESR spectra of mechanoradicals from poly(methyl methacrylate) reveals that after mechanical degradation in vacuo at 77°K, the sample contains two types of primary radicals? CH₂? C(CH₃)(COOCH₃) (I) and CH₂? C(CH₃)(COOCH₃)? CH₂ (II) produced by the breaking of the polymer chain, and secondary radicals ? CH₂? C(CH₃)(COOCH₃)? CH? C(CH₃)? (COOCH₃)? CH₂? (III). With increasing temperature, radical I remains stable while II reacts with methylene hydrogen of the polymer chain giving rise to the secondary radical III, which decays and finally disappears as the temperature rises. After admission of oxygen at 113°K, the polymer radicals react with oxygen with formation of polymer peroxy radicals ROO. and diamagnetic dimers. With increasing temperature the latter dissociate again to the original polymer peroxy radicals which gradually decay, if the temperature is increased further. The present results are compared with earlier ones obtained on poly(ethylene glycol methacrylate) (PGMA).     

The azulene-to-naphthalene rearrangement revisited: a DFT study of intramolecular and radical-promoted mechanisms

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.     

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH(3) radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH(3) radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(3) radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO(2) slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C(2) hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.     

An ab initio and DFT study of some halogen atom transfer reactions from alkyl groups to acyl radical

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that the transition states for the reaction of acetyl radical with several alkyl halides adopt an almost collinear arrangement of attacking and leaving radicals at the halogen atom. Energy barriers (DeltaE(double dagger)) for these halogen transfer reactions of between 89.2 (chlorine transfer from methyl group) and 25.3 kJ mol(-1) (iodine transfer from tert-butyl group) are calculated at the BHandHLYP/DZP level of theory. While the difference in forward and reverse energy barriers for iodine transfer to acetyl radical is predicted to be 15.1 kJ mol(-1) for primary alkyl iodide, these values are calculated to be 6.7 and -4.2 kJ mol(-1) for secondary and tertiary alkyl iodide respectively. These data are in good agreement with available experimental data in that atom transfer radical carbonylation reactions are sluggish with primary alkyl iodides, but proceed smoothly with secondary and tertiary alkyl iodides. These calculations also predict that bromine transfer reactions involving acyl radical are also feasible at moderately high temperature.     

Plasma for Modification of Polymers

The effect of nonpolymer-forming plasma (e.g., plasma of hydrogen, helium, argon, nitrogen) can be viewed as the following two reactions: 1) reaction of active species with polymer, and 2) formation of free radicals in polymer which is mainly due to the UV emitted by the plasma. The incorporation of nitrogen into the polymer surface by N₂ plasma and the surface oxidation by O₂ plasma are typical examples of the first effect. The latter effect generally leads to incorporation of oxygen in the form of carbonyl and hydroxyl and to some degree of cross-linking depending on the type of substrate; however, the degradation of polymer at the surface manifested by weight loss occurs in nearly all cases when polymers are exposed to plasma for a prolonged period of time. The effects of polymer-forming plasma is predominated by the deposition of polymer (plasma polymer); however, with some plasma-susceptible polymer substrates the effect of UV emission from polymer-forming plasma cannot be neglected. The mechanism of polymer formation can be explained by the stepwise reaction of active species and/or of an active specie with a molecule, and the chain addition polymerization of some organic compounds (e.g., vinyl monomers) is not the main route of polymer formation.

Plasma polymers contain appreciable amount of trapped free radicals; however, the concentration is highly dependent on the chemical structure of the monomer. In plasma polymerization, 1) triple bond and/or aromatic structure, 2) double bond and/or cyclic structure, and 3) saturated structure are three major functions which determine the rate of polymer formation and the properties of plasma polymers. The changes of some properties of plasma polymers with time are directly related to the concentration of trapped free radicals in plasma polymers. The amount of trapped free radicals in a plasma polymer is also influenced by the conditions of discharge; however, the UV irradiation from the polymer-forming plasma is not the main cause of these free radicals. Excess amount of free radicals are trapped during the process of polymer formation (rather than forming free radicals in the deposited polymer by UV irradiation). The properties of a plasma polymer is generally different from what one might expect from the chemical structure of the monomer, due to the fragmentation of atoms and/or functions during the polymerization process. This is another important factor to be considered for the modification of polymer surfaces by plasma polymerization.     

Impact of initiator characteristics on the thermal stability of vinylidene chloride copolymers

Two standard vinylidene chloride copolymers, the first containing approximately 9 mass% methyl acrylate and the second containing vinyl chloride at a nominal 15 mass% were prepared by radical suspension techniques using a series of peroxide and azo initiators (all of approximately the same half-life temperature for decomposition). The nature of the initiator could impact the stability of the resulting polymer in two ways. Instability could be introduced either via end-group effects or by attack of residual initiator fragments on the finished polymer during isolation and residual monomer stripping. In this case, the relative thermal stability of the resins produced was assessed by exposing samples to heat and shear in an air environment in a two-roll mill (Brabender Prep-Mill). The rate and extent of degradation was most readily apparent from color development during this treatment. The more thermally stable polymers were produced using initiator radicals that did not attack the polymer during isolation/stripping processes.     

Investigation of the origin of tensile stress-induced rate enhancements in the photochemical degradation of polymers

To investigate the effect of tensile stress on the photochemical degradation efficiencies of polymers, a modified PVC polymer with Cp(CO)3Mo-Mo(CO)3Cp (Cp = eta5-C5H5) units along the backbone was synthesized. The polymer is photochemically reactive because the Mo-Mo bonds are photolyzed with visible light and the resulting radicals are captured with Cl atoms from along the polymer backbone. Of most importance from a mechanistic standpoint, the photochemical degradation reaction occurs in the absence of oxygen, which eliminates the kinetically complicating effect of rate-limiting oxygen diffusion. Tensile stress initially caused the quantum yield of polymer degradation to increase, but, after a certain point, additional stress caused a decrease in the quantum yield. This dependence of quantum efficiency on stress is consistent with a hypothesis in which stress affects the ability of the photochemically generated radicals to recombine. At low to moderate stress, the effect of stress is to increase the separation of the radicals (by recoil), thus decreasing their recombination probability. As the stress increases, however, segments of different chains align, which induces a higher degree of orientation and crystallinity in the polymer, which in turn makes diffusion more difficult. The efficiency of degradation is predicted to decrease accordingly because of decreased radical and/or trap mobility in the ordered polymer. Infrared and X-ray data are presented, showing that the degree of orientation and crystallinity in the polymer does indeed increase with increasing stress.     

Degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers. Effect of internal unsaturation on the stability of Saran copolymers

The thermal degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers containing a constant 9 wt % methyl acrylate and small but varying amounts of phenylacetylene has been examined in the solid phase and in bibenzyl solution. Thermally promoted degradative dehydrochlorination, largely uncomplicated by methyl chloride formation, readily occurs at temperatures approaching 200°C. Incorporation of phenylacetylene into the polymer structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene induces the formation of polyene segments during the polymerization so that all the terpolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing internal unsaturation arises from an increased rate of initiation for the degradation reaction. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. Thus random double bonds have been identified as the principal defect sites responsible for the facile degradation of Saran copolymers. Species which promote the degradation of Saran polymers probably do so by facilitating the introduction of double bonds into the structure. The ratio of hydrogen chloride to stilbene formed for degradation of the terpolymers in bibenzyl solution is ca. 35:1. This is strongly reminiscent of PVDC degradation and suggests that for degradation of either the homopolymer or Saran copolymers the chain-carrying allylic radical pair does not dissociate to any appreciable extent as dehydrochlorination occurs.     

The oxidative degradation of dibenzoazepine derivatives by cerium(IV) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.