ESR studies on the conversion of mechanoradicals of poly(methyl methacrylate) and its self-degradation

Mechanisms of the conversion of poly(methyl methacrylate) (PMMA) radicals produced by ball-milling have been investigated by means of ESR. The spectrum observed after a short milling time (0·1 h) at 77 K in vacuum was resolved into the three components, a triplet, a nonet and a doublet. The nonet, the triplet and the doublet are ascribed to the three kinds of radical:

respectively. On the other hand, it was found that the spectrum after milling for more than 18 h at 77 K in a vacuum consisted of the nonet and the doublet, which had been converted from the triplet. The conversion of the triplet to the doublet during long milling suggests inter- and/or intra-hydrogen abstraction by the radical responsible for the triplet.The doublet observed after milling for longer than 18 h did not change its line-shape up to 263 K but on further warming to 273 K for 0·1 h the spectrum changed to give a triplet component. This reproduction of the triplet was interpretated in terms of β-bond scission to the unpaired electron of the radical (C), due to the thermal treatment. The bond scission was confirmed by the observed decrease in the molecular weight after the heat treatment. The radical (B) formed by the β-scission abstracts the hydrogen in the PMMA parent molecule to produce a radical of the species (C). This cyclic reaction leads to a self-degradation.