The laser-flash-initiated polymerization as a tool of evaluating (individual) kinetic constants of free-radical polymerization, 2. The direct determination of the rate of constant of chain propagation

When a polymerizable system is subjected to periodic light flashes, which induce the formation of primary radicals, a pseudostationary state is established which is characterized by a periodic profile of the (polymer) radical concentration. Within such a period of length t₀ the radical concentration will decay according to a second-order rate law. At the end of this period the radicals, which have escaped termination up to this moment, have propagated up to a chain length L₀ = t₀·k_p·c_M, k_p representing the propagation rate constant and c_M the monomer amount concentration. When the next flash arrives these radicals are opposed to a strongly increased overall concentration of radicals which leads to an enhanced probability for their termination. As a consequence the formation of dead polymer molecules with a chain length close to L₀ is favoured. The chain-length distribution of polystyrene prepared under such pseudostationary conditions, which was evaluated by gel permeation chromatography, in fact exhibits such a peak. The analysis of the theoretical distribution curves, derived in this communication, reveals that it is easily possible to correlate this peak to L₀, independently of the mode of termination (disproportionation or combination). Thus, a method of evaluating k_p is derived without any reference to the termination rate constant k_t and largely independent of all features which usually cause problems in the evaluation of k_p and k_t (such as primary radical termination etc.). The experimental results agree fairly well with the data reported in literature, especially with those obtained from the number of particles and the rate of polymerization in emulsion systems.