OXIDATION-KINETICS OF POLY(4-METHYLPENTENE)

The thermal oxidation (in air between 70 and 150 degrees C) of unstabi lised samples of poly(4-methylpentene) (PMP) were studied by infrared spectrometry and by gravimetry. The behaviour was compared to PP expos ed in the same conditions. The thermal oxidation was found to exhibit two well differentiated kinetic regimes both for PP as for PMP. The fi rst regime predominates at temperature higher than 90 degrees C. It is characterized by an activation energy of the pseudo induction period of about 100 kJ mol(-1). The induction period of mass gain is about 1, 8 time shorter than the induction period of carbonyl gain. These chara cteristics, and the fact that induction periods are of the same order for both polymers, are consistent with a ''close loop mechanistic'' sc heme with unimolecular POOH decomposition. The advantages of this hypo thesis compared to other proposed hypothesis (bimolecular POOH decompo sition, heterogeneous oxidation...) are discussed. The main theoretica l difficulty is that of explaining why the unimolecular process predom inates, whereas POOH are obviously hydrogen bonded, even at very low c onversions of the oxidation process. This point remains to be explaine d. The second kinetic regime predominates at temperature lower than 90 degrees C. It is characterized by a considerably lower activation ene rgy. We have examined many hypothesis to explain the existence of a tr ansition at that temperature (where neither PMP nor PP display a physi cal transition). A first hypothesis is that unimolecular POOH decompos ition is replaced by a bimolecular one, which can explain a decrease o f the apparent activation energy of about 50% The bimolecular initiati on creates pairs of radicals (POO. + PO). It can be imagined that a pa rt of them desactivate mutually by termination after rearrangement of PO by P scission. In this case, the transition at 90 degrees C could b e also a result of the temperature induced change in molecular mobilit y. Above 90 degrees C, the radicals would diffuse almont freely out of the cage. In contrast, below 90 degrees C, termination would become c ompetitive with diffusion. A supplementary research is needed to fully elucidate the cause of this transition at 90 degrees C. The eventuali ty of the existence of spatial heterogeneity of the oxidation process has been discussed. It is well known that oxidation is heterogenous at the morphological scale, but there is no evidence of heterogeneity at smaller scale, where the chemical kinetic would not be applicable.