The Mercury Photosensitized Polymerization of Cyclopropane

An investigation has been made of the polymerization of cyclopropane with mercury (³P₁) atoms in a static system over the pressure range from 1–400 mm at 30°C. The only important product of the reaction is a liquid polymer with the formula (C₃H₆)_n, where n is probably about 6. Small amounts of other products, of low molecular weight, are present at initial pressures below 10 mm. Data are presented which show that propylene is formed only at low initial pressures, suggesting its origin from the collision of trimethylene biradicals with the wall. The over‐all rate of consumption of cyclopropane becomes independent of initial pressures above 40 mm. The quantum yield of cyclopropane disappearance in the complete quenching region is 0.136. A mechanism is proposed for the polymerization involving the initial formation of an energy‐rich trimethylene biradical by the reaction $$\mathrm{C}+\mathrm{Hg}{(}^3{P}_1\mathrm{)\to }{\mathrm{R}}_1^{*}+\mathrm{Hg}{(}^1{S}_0\mathrm{).}$$ The activated radical can then either reform cyclopropane upon collision or form a higher biradical: $${\mathrm{R}}_1^{*}+\mathrm{C}\text{→2}\mathrm{C},$$ $${\mathrm{R}}_1^{*}+\mathrm{C}\to {\mathrm{R}}_2^{*},\mathrm{etc}\mathrm{.,}$$ where C = cyclopropane, R₁ = ·(CH₂)₃·, and R₂ = ·(CH₂)₆·. Chain termination will occur when the polymer biradical can no longer furnish sufficient energy to break the C–C bond in cyclopropane. The radical will then stabilize itself by a proton shift. On the basis of the above mechanism, combined with the analytical data, reaction (2) must be at least 43 times faster than reaction (3). Evidence is also presented for the fact that cyclopropane does not react with the H atoms formed by the collision of Hg(³P₁) atoms with hydrogen.