Combining a ligand photogenerator and a Ru precatalyst: a photoinduced approach to cross-linked ROMP polymer films

Although metathesis photoinduced catalysis is now well established, there is little development in thin film preparation using photochemically activated ring-opening metathesis polymerization (ROMP). Herein, a N-heterocyclic carbene (NHC) photogenerator (1,3-bis(mesityl)imidazolium tetraphenylborate) is combined with an inactive metathesis catalyst ([RuCl₂(p-cymene)]₂) to generate under UV irradiation an active catalyst (p-cymene)RuCl₂ (NHC), that is capable of producing in a single step cross-linked copolymer films by ROMP of norbornene with dicyclopentadiene. The study shows that the photoinitiated catalytic system can be optimized by increasing the yield of photogenerated NHC through a sensitizer (2-isopropylthioxanthone), and by choosing [RuI₂(p-cymene)]₂ as precatalyst to provide a long-term photolatency. The cross-linked polymer structure is investigated by a range of techniques including gel content measurement, FT-IR and solid-state ¹³C NMR spectroscopy, TGA and DSC, which reveal a cross-linking mechanism proceeding through both metathesis and olefin coupling.