A kinetic analysis of retardation of polymerization and depolymerization reactions has been carried out. Using the derived expressions the validity of the current methods of determining rates of chain initiation from retardation data has been examined. Correlation of the theoretical expressions with experimental results is discussed.
On exposure to ultra-violet light of wave-length 2537 Å, polymethylmethacrylate degrades rapidly to monomer at temperatures above 130° C
in vacuo
. The general features of this reaction are described, and the behaviour of polymethylmethacrylate compared with that of other vinyl polymers under similar conditions. The diffusion of monomer through polymethylmethacrylate is a comparatively slow process. The effect of this on the experimental results is described, and the limited conditions under which true rates of degradation can be measured are outlined. The overall quantum efficiency of the photo-degradation reaction is greater than 10
2
, and the intensity exponent about 0·5 for most samples. Number-average molecular weights of samples of degraded polymers have been measured, the results being similar to those obtained in an earlier study of the thermal-degradation. The results show that the mechanism of degradation is a true depolymerization process, involving initiation, a depropagation reaction, and (under most circumstances) mutual termination of reaction chains.
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