Fate of Trapped Acrylate Radicals. Chain Oxidation

2 Fate of Trapped Acrylate Radicals. Chain Oxidation 

As we have seen, the trapped radicals have an appreciable stability at room temperature and under vacuum. After five months, more than 50 % of the radicals were still present in the sample 

Heating of the samples caused a partial decay of the radical concentration which was accompanied by additional polymerization After 1 h at 80 C, about 65 o of the radicals had survived and after an additional heat treatment of 1 h at 120 °C still about 15% of the radicals could be detected 

The deuteration experiment shows that hydrogen abstraction does occur but the long lifetime of the radicals indicates that transfer of the radical sites by subsequent abstraction of tertiary H-atoms (which is a zero-enthalpy process) must be rather restricted. Otherwise the radicals would terminate each other rapidly. The transport of radical sites at room temperature is obviously limited to distances shorter than the average distance between trapped radicals (10-14 nm at a radical concentration of (3 1.5) 10 M). 

Oxygen reacts rapidly with trapped radicals, but no peroxy radicals could be detected. Instead, the ESR signal gradually decayed to zero in about 2 h. At intermediate stages the spectrum was unchanged, only its amplitude was reduced. 

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