Modeling Termination Kinetics of Non-Stationary Free-Radical Polymerizations

Abstract

Pulsed‐laser induced polymerization is modeled via an approach presented in a previous paper.[1] An equation for the time dependence of free‐radical concentration is derived. It is shown that the termination rate coefficient may vary significantly as a function of time after applying the laser pulse despite of the fact that the change in monomer concentration during one experiment is negligible. For the limiting case of t ≫ c–1 (kpM)–1, where c is a dimensionless chain‐transfer constant, kp the propagation rate coefficient and M the monomer concentration, an analytical expression for kt is derived. It is also shown that time‐resolved single pulse‐laser polymerization (SP–PLP) experiments can yield the parameters that allow the modeling of kt in quasi‐stationary polymerization. The influence of inhibitors is also considered. The conditions are analyzed under which M (t) curves recorded at different extents of laser‐induced photo‐initiator decomposition intersect. It is shown that such type of behavior is associated with a chain‐length dependence of kt.

Critically Evaluated Termination Rate Coefficients for Free‐Radical Polymerization, 1. The Current Situation

“…the work of refs. [31,36,37,46] Even if benchmark k t values can be identified for low conversion, bulk MMA and styrene systems, as proposed, it is appropriate to ask what the use of such values would be. Obviously they would be applicable for only a narrow range of polymerization conditions, but that is still of technical utility.…”

Chain-length dependence of free-radical termination rate deduced from laser single-pulse experiments

“…Even more important is the observation that upon variation of the DMPA concentration or of laser pulse intensity, the SP-PLP traces intersect. As has been shown by PREDICIm simulation and, more recently, also by solving a reduced kinetic scheme analytically, [39] such crossings can only occur in situations where k t is chain-length dependent and, at the same time, the primary free-radical species differ in their tendency to react with the monomer. This finding opens the novel route for determining chain-length dependent k t that has been used in the present paper.…”

Chain-Length Dependence of Termination Rate Coefficients in Acrylate and Methacrylate Homopolymerizations Investigated via the SP−PLP Technique

“…11 DMPA (2,2-dimethoxy-2-phenylacetophenone), which has been used in previous studies as the photoinitiator, is nonideal in that it decomposes into two fragments one of which does not add to monomer but primarily reacts with free radicals and thus acts as an inhibitor. 12 This has been demonstrated via PREDICI simulations 10,13 and by ESI analysis. 14 If monomer conversion per pulse is too small, as with methacrylates, where it may be less than 0.01%, a sequence of laser pulses is applied to the monomerphotoinitiator mixture, and the resulting signals are coadded.…”