Importance of the singlet–triplet transition of ethylene in its photopolymerization in the presence of oxygen

The rate of the photopolymerization of ethylene initiated by a mercury are lamp was investigated under a pressure of 400 kg/cm² at 30 ± 5°C by varying the wavelength of incident light. Ethylene was found to polymerize by ultraviolet light below about 3500 Å. The yield decreased gradually as the wavelength became longer, and no polymer was obtained at wavelength greater than 3900 Å. The addition of a small amounr (<100 ppm) of oxygen promoted the polymerization. Ultraviolet absorption spectat measured for the mixture of ethylene and a small amount of oxygen displayed several absorption peaks in the region 2700–3500 Å. The absorption began at about 3680 Å and became stronger with the concentration of oxygen. The average separation between the peaks was about 1000 cm^?1. The spectra were also measured for oxygen containing a small amount of ethylene. Similar absorption peaks with those described above were observed. On the basis of these results, it was pointed out that an excited triplet ethylene is formed under the irradiation of light due to a perturbing effect of oxygen contained in ethylene monomer and the reaction between the excited ethylene and oxygen is important in initiating the polymerization.