Spectroscopic measurements on spontaneously polymerizing styrene, 2. The estimation of the reactivity of the two Diels-Alder-isomers towards polymer radicals

Spectroscopic measurements at 335 nm on polymerizing styrene have been carried out with spontaneously polymerizing as well as dibenzoyl peroxide (BPO)-initiated bulk systems. The evidence for the formation of two Diels-Alder-intermediates, 1a and 1b , out of which only 1a seems to be capable to produce initiating radicals by reaction with the monomer, can also be derived from the development of the absorption, characteristic for the intermediates, during the BPO-initiated polymerization. From the dependence of the overall rate constants of consumption of 1a , k_2,aM] , and 1b , k_2,bM] , on the rate of polymerization the reactivity of 1a and 1b towards polymer radicals can be estimated. Whereas 1b , the less reactive intermediate, seems to be mainly consumed by polymer radicals through copolymerization, chain transfer being unimportant, (the rate constant of consumption of 1b by polymer radicals, k_3,b, relative to the constant of chain propagation k_p ranges from 2,1 at 60°C to 3,2 at 80°C) the corresponding ratio for 1a , k_3,a/k_p, which goes from 113 at 80°C to 143 at 60°C, is supposed to represent primarily the constant of chain transfer of 1a in styrene polymerization. Under the condition of spontaneous polymerization the reaction with polymer radicals contributes 16% (for 1a ) and 5–8,5%, depending on temperature (for 1b ), to the overall rate of consumption of the intermediates. The stationary concentration of 1a , the isomer which accordingly is solely responsible for initiation as well as chain transfer in spontaneous styrene polymerization, is calculated to range from 5,2·10^?6 mol l^?1 at 60°C to 8,5·10^?6 mol l^?1 at 80°C.