The kinetics of the thermal polymerization of acetylene

Abstract

The thermal reactions of acetylene in Pyrex tubes at 352 to 472°C have been studied by analysis of the amounts of acetylene and the simpler products at definite time intervals; the mean composition of the polymer was also found from pressure measurements com­bined with the analytical results. Increase in surface area caused a marked increase in the rate of the reaction. By comparing the results obtained in normal and packed tubes, the kinetics of both the homogeneous and the surface reactions have been elucidated. The former is almost entirely a polymerization reaction, and is second order with a velocity constant k = 3.72 x 10 13 e -50 200/ RT . Nitric oxide (0.3%) completely inhibits this reaction, which is a radical process with a chain length of about 100. The polymer is initially C 4 H 4 , and the mean molecular weight increases slowly as the reaction proceeds. A radical chain mechanism with a high rate of chain transfer is proposed. The surface reaction is first order with respect to the acetylene concentration in the gas phase, with an activation energy of 42.7 kcal, and is also completely inhibited by nitric oxide (25%), the chain length being about 5. The mean initial composition of the polymer from this reaction is C 4 H 2.8 ; hydrogen and ethylene are also important products. A radical mechanism is proposed for the formation of polymer and hydrogen; the ethylene is considered to come from direct hydrogenation of acetylene. The kinetics of the reaction between nitric oxide and acetylene, which is largely heterogeneous, have also been studied.