Radical sensitization of acetylene pyrolysis

Abstract

The kinetics of acetylene polymerization initiated by neopentane (Np) or acetone (Ac) decompositions has been investigated in a static reactor dynamically coupled to a modulated beam mass spectrometer between 850–950 K. Overall rates follow the expression: R = −d[C_2H_2]/dt = k_s[X]^(1/2)[C_2H_2] + k_u[C_2H_2]_2 (I), where X represents Np or Ac and k_s, k_u the rate constants of the sensitized and unsensitized reactions, respectively. The rate law of the sensitized reaction clearly suggests a chain polymerization mechanism with k_s = k_p(k_i/k_t)^(1/2) (i, t, and p stand for initiation, termination, and propagation, respectively). Remarkably, the derived values of k_p are nearly independent of the sensitizer, although Ac acts as a source of methyl radicals whereas Np also produces hydrogen atoms, and fall in the expected range for the addition of vinylic radicals to acetylene. It is shown that a chain transfer process involving the fast [1,5] intramolecular hydrogen atom shift in 4-methyl-buta-1,3-dien-1-yl radicals (CH_3-CH=CH-CH=ĊH) followed by further addition to C_2H_2 and aromatization, transforms methyl radicals into hydrogen atoms and is able to account for the presence of toluene among the products of the sensitized reactions. Based on current thermochemical data for the but-1-en-3-yn-2-yl radical (CH_2=Ċ-C≡CH) and present rates of propagation it is argued that if the unsensitized polymerization of acetylene also proceeded by a vinyl radical chain, then even the most favorable self-initiation reaction: 2C_2H_2 = C_4H_3 + H (a), would be far too slow. Finally, present results also show that acetone at impurity levels (⩽ 0.1%) can not provide fast enough spurious initiation rates in chain mechanisms for the “unsensitized” acetylene pyrolysis at pressures above 10 torr.