Free-radical degradation of solid poly(methyl methacrylate) initiated by photoreduction of the chloride complexes of Fe 3+. Kinetics and mechanism

Abstract

The kinetics have been studied quantitatively and a mechanism proposed for the free-radical degradation of solid PMMA initiated by photoreduction of the chloride complexes of Fe 3+. The polymer was exposed in an inert atmosphere at 77 and 293 K to light of various spectral compositions with γ > 300 nm, not absorbed by the polymer. The rate of the build-up of the double isolated and conjugated bonds, formed as a result of thermal transformations of the PMMA macroradicals at 293 K, has been studied. It is shown that the isolated double bonds are generated at a rate equal to the initiation rate W, and the conjugated double bonds are generated at the rate of 0.3 W i. A mechanism is proposed closely describing the observed regularities. It has been found that the degradation of PMMA irradiated at 77 K results from the thermal decomposition of macroradicals on heating the samples and is dependent upon the spectral composition of the light. The probability of degradation is 0.16 per photoreduced Fe 3+ ion for light γ < 370 nm and decreases to only 0.009 for light with γ > 390 nm. It is concluded that macrochain breaking under these conditions is due to the thermal decomposition of the macroradicals ∼CH 2C(CH 3)CH 2∼ . At 293 K the photoinitiated PMMA degradation is a free-radical, but not a chain, process independent of the intensity and spectral composition of the light (in the wavelength range 313–390 nm), molecular mass of the polymer and film thickness. Degradation in an inert atmosphere is characterised by a probability factor per photoreduced Fe 3+ ion (α) which increases with the degree of conversion of the initiators. The rate of degradation in an atmosphere of HCl is directly proportional to the initiation rate W i. It is concluded that, at 293 K in an enert atmosphere, the rupture of macromolecules is due to the thermal decomposition of both the primary macroradicals ∼C(CH 3)(COOCH 3)ĊH-C(CH 3)(COOCH 3)∼ and the radicals ∼CH 2-Ċ(CH 3)CH 2∼ formed by addition of low-molecular radicals to the radical reaction products in this system, i.e. the isolated middle double bonds.