Thermolysis Of Polyethylene Hydroperoxides Research Articles

Thermolysis of polyethylene hydroperoxides in the melt, Part 12: Mechanisms and kinetics of thermolysis

The thermolysis of polyethylene hydroperoxides is attributed to the reaction of two hydroperoxide groups. This bimolecular reaction appears as a first-order reaction with the mean values of the hydroperoxide concentrations that can be used for the experimental verification of the kinetics. In low molecular mass liquids and solutions these findings would be irreconcilable. However, in polymer melts, this contradiction is more apparent than real. It is a consequence of the heterogeneous kinetics valid in polymer melts. The bimolecular reaction involves the decomposition of pairs of hydroperoxide groups that are relatively close in the elementary oxidation volumes. By diffusion these hydroperoxide groups can come close enough for reaction. From the chemical point of view the decomposition is a bimolecular reaction. However, from the kinetic point of view it is a first-order reaction of the hydroperoxide pairs. The dependency of the first-order rate on the initial hydroperoxide concentration is explained by the heterogeneous kinetics. The activation energy of the overall process can be related to the sum of the activation energies pertaining to the chemical reaction and to the diffusion process.

Thermolysis of polyethylene hydroperoxides in the melt, Part 11: Relative yields of thermolysis products

The experimental ratios of the main products from polyethylene hydroperoxide thermolysis are examined. Comparison with the corresponding theoretical ratios calculated for different hydroperoxide decomposition reactions allows discriminating between the main hydroperoxide decomposition reactions. The experimental values can usually be explained best by the true bimolecular reaction involving two hydroperoxide groups. Mostly these values are significantly different from the theoretical ratios calculated for the bimolecular reaction with an alcohol group and for the pseudo-monomolecular reaction with a segment of the polymer. The bulk of the results points unequivocally to true bimolecular hydroperoxide decomposition for explaining thermolysis of polyethylene hydroperoxides.

Thermolysis of polyethylene hydroperoxides in the melt, Part 10: Product yields from bimolecular and pseudo-monomolecular hydroperoxide decomposition

The quantitative aspects of some specific decomposition reactions of polyethylene hydroperoxides are re-examined. New data have shown that β-scission of primary alkoxy radicals is negligible in the temperature range of the thermolysis experiments. This is important for the true bimolecular hydroperoxide decomposition for which, in a first approximation, β-scission of primary and secondary alkoxy radicals had been taken into account. The calculation shows that the yields of the main oxidation products such as secondary alcohols, ketones, trans-vinylene groups and aldehydes are not considerably affected by the change. However, the theoretical yields of some minor products such as primary alcohols and of some combination reactions are strongly affected. For the pseudo-monomolecular hydroperoxide decomposition involving a segment of the polymer, the main novelty in comparison with previous work consists in taking into account β-scission of the secondary alkoxy radicals. It allows improving the accuracy of the calculated product yields. Moreover, all the theoretical calculations are on the same level of accuracy and can be used for comparison with the experimental product yields.

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 15: Product yields on bimolecular hydroperoxide decomposition

The product yields are important characteristics of the various hydroperoxide decomposition reactions. The true bimolecular hydroperoxide decomposition is thought to be the dominant reaction in the early stages of polyethylene processing. The reaction probabilities and theoretical yields calculated for the reaction account for the ratio of alcohols to ketones in these early stages. The calculations yield a rate constant for the disproportionation between the alkoxy and peroxy radical that is of the same order as published data for corresponding very low molecular mass radicals. The other reactions of the caged radical pairs are the same as those discussed for the reaction between a hydroperoxide and an alcohol group. They easily account for the experimental data. Pseudo-monomolecular hydroperoxide decomposition involving a hydroperoxide group and a segment of the polymer requires only parameters already envisaged previously. It does not seem to be important for polyethylene processing. However, it has been envisaged for thermolysis of polyethylene hydroperoxides in a press. The product yields calculated in this work for this reaction do not agree with the data from thermolysis of polyethylene hydroperoxides. The calculations allow concluding that pseudo-monomolecular hydroperoxide decomposition cannot be the only reaction on thermolysis of polyethylene hydroperoxides.

Thermolysis of polyethylene hydroperoxides in the melt 5. Mechanisms and formal kinetics of product formation

Many products are formed on thermolysis of polyethylene hydroperoxides. This is especially so for the carbonyl compounds involving ketones, in-the-chain ketones and metyl-ketones, carboxylic acids, aldehydes, esters and γ-lactones. Only γ-lactones can be easily measured as such by IR spectroscopy because they absorb at a wavelength different from that of the bulk of the carbonyl groups. This is also true for the secondary alcohols and the trans-vinylene groups. Formation of all the products observed can be explained by relatively simple mechanisms. Confrontation of the experimental results with formal kinetics based on the assumed mechanisms leads to improved understanding of these mechanisms. It is concluded that hydrogen abstraction by alkoxy radicals is the only significant source for alcohols on thermolysis of polyethylene hydroperoxides. The abstraction of hydroxyl radicals from hydroperoxides, very often envisaged with alkyl radicals, does not contribute to alcohol formation. Trans-vinylene groups seem to result from disproportionation of geminate alkyl radicals. Intramolecular hydrogen abstraction following intramolecular hydroperoxide decomposition might also contribute to double bond formation. Formation of γ-lactones is best explained by monomolecular decomposition of special structures formed on processing in air. The last result from reactions following intramolecular hydrogen abstraction by the alkoxy radical formed on pseudo-monomolecular hydroperoxide decomposition. The reactions proposed are in agreement with formal kinetics as well as with thermochemistry.

Thermolysis of polyethylene hydroperoxides in the melt6. Mechanisms and formal kinetics of product formation in the presence of phenolic antioxidants

The presence of phenols does not affect the nature of thermolysis products. However, the relative amount of the various oxidation products can change significantly. These changes are important from a fundamental point of view. They lead to better understanding of the thermolysis mechanisms. The reduced yield of alcohol groups in the presence of phenols suggests transformation of part of the alkoxy radicals into products other than alcohols. It is attributed to disproportionation between macro-alkoxy and phenoxy radicals yielding a ketone and the original phenol. There is considerable increase of trans-vinylene group formation in the presence of phenols. The explanation is based on another disproportionation reaction involving macro-alkyl and phenoxy radicals to give a double bond and the original phenol molecule. The vinyl groups that can be measured in the presence of phenols are formed on intramolecular hydroperoxide decomposition. The reaction seems to proceed with the tertiary hydroperoxides formed in low-density polyethylene rather than with the normal secondary hydroperoxides. The formation of γ-lactones results from direct decomposition of α,γ-keto-hydroperoxides in 4-position to alcohol groups initially present and also from decomposition of such structures formed on thermolysis. These data allow for an estimation of intramolecular hydrogen abstraction by peroxy radicals in polyethylene. It is found that the last can reach close to 30%.

Thermolysis of polyethylene hydroperoxides in the melt7. Effect of stable nitroxyl radicals

The presence of nitroxyl radicals affects hydroperoxide thermolysis and oxidation product formation. With increasing concentration of a low molecular mass nitroxyl radical the rate of hydroperoxide decomposition at 190 °C decreases at first. Then the rate passes a minimum value and increases again to become at least comparable to that of the control experiment. For any nitroxyl radical concentration the rate of hydroperoxide decomposition remains constant during the entire decomposition. The rate is closer to typical first-order than even in the presence of a phenolic antioxidant. The oxidation products formed in the presence of nitroxyl radicals are mainly alcohols and carbonyl groups, trans-vinylene groups and γ-lactones are formed in smaller amounts. The effect of the nitroxyl radical concentration depends strongly on the oxidation product and the type of rate constant taken into account. The first-order rate constants of alcohol and carbonyl group formation decrease with increasing nitroxyl radical concentration in a way analogous to the rate of hydroperoxide thermolysis. These rates are increasing again for the largest nitroxyl radical concentration in the experiments. The first-order rate of trans-vinylene group formation does not follow the same pattern, it is increasing slightly with the nitroxyl radical concentration. The initial linear rates of product formation show significant differences with the corresponding first-order rates. The initial linear rate of alcohol formation decreases significantly with increasing nitroxyl radical concentration. The corresponding rate of carbonyl formation does not show much variation under the same conditions. However, the initial linear rate of trans-vinylene group formation increases steadily with the nitroxyl radical concentration. The maximum values of the oxidation product concentrations on completion of thermolysis parallel the variation of the initial linear rates with the nitroxyl radical concentration.

Thermolysis of polyethylene hydroperoxides in the melt 8. Mechanisms and formal kinetics in the presence of stable nitroxyl radicals

The presence of nitroxyl radicals affects mainly trans-vinylene and alcohol group formation. The influence of the nitroxyl radicals on the yield of carbonyl and γ-lactone formation is negligible. The effect on hydroperoxide decomposition is best explained by elimination of the chain reaction of induced hydroperoxide decomposition. This is a consequence of the reactions of nitroxyl radicals with alkyl radicals. These reactions involve geminate alkyl radicals as well as single alkyl radicals resulting from prior destruction of a radical pair by reaction with a nitroxyl radical. This is a major aspect of the kinetics of the reactions. The nitroxyl radicals do not disrupt rapidly enough the geminate radicals formed on hydroperoxide thermolysis to yield typical homogeneous, solution-type kinetics. They terminate chains on reaction with geminate and with single radicals. The increase of the rate of formation as well as of the maximum amount of trans-vinylene groups in the presence of nitroxyl radicals can be easily understood. It is caused by the disproportionation of macroalkyl radicals with nitroxyl radicals. It is more difficult to explain the significant decrease of the rate of formation and maximum amount alcohol groups. The only explanation in agreement with mechanisms and kinetics involves unchanged formation of alcohol groups in the presence of nitroxyl radicals. However, the alcohol groups formed by intramolecular hydrogen abstraction by alkoxy radicals are destroyed rapidly after formation of hydroxylamine ethers in the 4-position.

Thermolysis of polyethylene hydroperoxides in the melt3. Experimental kinetics of product formation

Thermolysis of polyethylene hydroperoxides restricted to the initial stages can be readily understood. The same is true for the oxidation products formed on thermolysis. These products are essentially the same as those formed on processing in mixers open to air. They are mainly alcohols and ketones, in-the chain ketones and methyl-ketones. Carboxylic acids and aldehydes are also formed in a small amount. The same is valid for trans-vinylene and γ-lactone groups. The kinetics of oxidation product formation in the initial stages are clearly related to the corresponding kinetics for hydroperoxide decomposition. However, the various products show significant differences in this respect. Thus, thermolysis of hydroperoxides in the initial stages consists of direct hydroperoxide decomposition and free radical induced hydroperoxide decomposition. Formation of carbonyl groups corresponds also to such a process. This means that carbonyl groups result from both direct and induced hydroperoxide decomposition. However, alcohol groups are formed essentially in reactions following direct hydroperoxide decomposition according to the pseudo-monomolecular reaction. Formation of γ-lactones does not proceed according to the same general scheme. Its rate is increasing considerably with the initial oxidation of the samples. Hence, it looks as if it were formed out of products accumulating in the oxidizing polyethylene melt.

Thermolysis of polyethylene hydroperoxides in the melt: 2. Formal kinetics of hydroperoxide decomposition

Thermolysis of polyethylene hydroperoxides in the melt results from different reactions. The first-order rate of hydroperoxide decomposition in the initial stages of thermolysis increases linearly with the initial hydroperoxide concentration. This seems to indicate that in addition to primary hydroperoxide decomposition there is hydroperoxide decomposition induced by the free radicals arising from the primary reaction. The mathematical models deduced from formal kinetics have been compared to these experimental results. It has been found that conventional kinetics based on chain recombination between randomly distributed free radicals could not explain linear increase of the first-order rate with initial hydroperoxide concentration. Conventional kinetics could explain merely linear increase with the square root of this hydroperoxide concentration. The conclusion was the same in the presence and in the absence of phenolic antioxidants. The best explanation of the experimental data involves geminate chain recombination, i.e. termination between two chains originating both from the same initiation reaction yielding a pair of free radicals. In the absence of phenolic antioxidants, geminate chain termination results from the interaction of two alkyl radicals. In the presence of phenols, this geminate chain termination seems to involve essentially an alkyl radical and a phenoxy radical. Besides polymer solutions, polymer melts certainly are the most homogeneous form possible for polymers. Since geminate chain recombination seems to be dominant in polymer melts, it can be assumed that it will be at least as important with polymer states for which diffusion is even more restricted, e.g. polymers in the solid phase.