Full Quantification of the Light-Mediated Gilch Polymerization

Alkoxy-substituted poly(p-phenylenevinylenes) (PPVs) continue to be major workhorse materials in optoelectronics, ranging from thin film electronics to bioimaging. An attractive synthetic route toward PPVs is the Gilch polymerization. Nevertheless, obtaining control over molecular weight and chain constitution is challenging due to the free-radical nature of this reaction. In this work we quantitatively show with in situ UV-irradiation NMR spectroscopy how control over the Gilch polymerization can be enhanced by irradiation with UV-light. The potential of this method has been demonstrated but never interpreted within a quantitative framework, resulting in a lack of mechanistic and kinetic insight. We account for this not only by in situ analyzing and modeling the photochemical Gilch polymerization but also by characterizing the photolysis of the starting material. The latter shows that the solvent THE likely acts as radical transfer agent in the Gilch pathway and similar precursor-based biradical routes. We perform two photopolymerization runs: (i) under continuous UV-irradiation and (ii) by applying a short UV pulse while monitoring the chemical response of the mixture. Since existing models of the Gilch polymerization are inadequate for describing the recorded time-concentration profiles, we develop a new model that couples thermal and photoinduced polymerization. Numerical curve fitting quantifies the rate constants associated with both pathways. We demonstrate that (i) a photoactivated p-quinodimethane species Q* reacts in the (re)initiation and propagation steps, (ii) photopropagation is significantly faster than photoinitiation, and (iii) the photochemical reactions are considerably faster than their thermal analogues, which allows for decoupling the thermal and photochemical pathways at temperatures low enough to suppress the former.