Tungsten Oxo and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes for the Visible-Light-Induced Ring-Opening Metathesis Polymerization of Dicyclopentadiene

Abstract

A system for the visible-light-controlled ring-opening metathesis polymerization (ROMP) of endo,endo-dicyclopentadiene (DCPD) was developed by combining thermally latent group 6 olefin metathesis initiators, whose vacant binding site is occupied by an N-heterocyclic carbene (NHC), with a photoredox catalyst. Similar ruthenium-based systems have previously been utilized in numerous photolithographic applications. Here, we were able to adapt the approach to group 6 metathesis catalysts for the first time. Extensive screening of 31 different olefin metathesis initiators and five different photoredox catalysts revealed that both tungsten oxo and tungsten imido alkylidene NHC complexes together with 2,4,6-triphenylpyrylium tetrafluoroborate (TPPT) as a photocatalyst are most suitable for the desired application. We propose that, upon irradiation of a mixture of the corresponding olefin metathesis catalyst and TPPT with visible light (440 nm), the excited pyrylium compound oxidizes the NHC ligand and a metathesis-active Schrock-type catalyst is formed. This system can be applied to two-dimensional (2D) photolithography in the ROMP of DCPD. Photomasks were employed to produce cross-linked polymers with great spatial resolution within minutes by applying visible light as the sole stimulus giving access to polymers those compared to many other systems do not require thermal or UV post-curing. Notably, mixtures of the two catalysts and DCPD were latent for several days, and catalyst loadings as low as 100 ppm were sufficient for the desired applications proving the robustness and high activity of the developed system.