Physico-chemical aspects of polyethylene processing in an open mixer. Part 14: Product yield on reaction of hydroperoxide with alcohol groups

The views from solution-type chemistry cannot be transposed to polymer melts. The bimolecular reaction between hydroperoxide and alcohol groups is a typical example in this respect. It is the dominant hydroperoxide decomposition reaction in the advanced stages of polyethylene processing in the high temperature range (170-200 °C). In part, the alkoxy/alkoxy radical pair resulting from the decomposition reacts directly in the cage by disproportionation and combination. In part, it is transformed into a caged alkoxy/alkyl radical pair following hydrogen abstraction by one of the alkoxy radicals. The alkoxy/alkyl radical pair reacts by combination or disproportionation in the cage or is transformed into an alkyl/alkyl radical pair on hydrogen abstraction by the remaining alkoxy radical. Combination or disproportionation of the alkyl/alkyl radical pair is considerably faster than oxygen addition and/or cage disruption. The bimolecular reactions of the three caged radical pairs remove the free radicals from the polymer melt. There is no chain reaction initiated by those radicals. This explains previous findings concerning the absence of initiation by the hydroperoxides formed and decomposed in polyethylene melts. The relative quantities of oxidation products calculated from the model are compatible with the experimental results. The comparison yields quantitative values for the probabilities of the different cage reactions. The rate constants for the reactions between the alkoxy/alkoxy and alkoxy/alkyl radical pairs can be deduced from the probabilities, taking intramolecular hydrogen abstraction by the alkoxy radical as a reference. They are comparable to the corresponding rate constants for low molecular mass compounds in solution. The knowledge of these values allows calculation of the oxidation product yields at any temperature.