ESR study of the photo-induced decomposition of polypropylene in the presence of N, N, N′, N′-tetramethyl- p-phenylenediamine

Abstract

The mechanism of uv ( λ > 325 nm) photodegradation of polypropylene (PP) containing N, N, N′, N′-tetramethyl- p-phenylenediamine (T 4MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T 4MPD cation radical (T 4MPD +·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T 4MPD +·. One component was a sharp quartet which was assigned to a methyl radical, ·CH 3·. The other component, a singlet, was attributed to a trapped electron, e t −. By comparison of the ESR spectrum of deuterated T 4MPD with that of the normal compound it was found that 60 ∼ 70% of the methyl radicals arose from the added T 4MPD due to β-scission, which also formed the N, N, N′-trimethyl- p-phenylenediamine radical, T 3MPD·. The T 3MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T 3MPD −, which releases the electron to reproduce the T 3MPD· radical at elevated temperatures. This production of the radical T 3MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ⋍ 168 K to 185 K. The remaining fraction, 30 ∼ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T 4MPD ∗ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ∼CH 2CH(CH 3)CH 2ĊO from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ∼CH 2CH(CH 2·)CH 2CHO.