Photoinduced Ring-Opening Metathesis Polymerization (PROMP) with Photochemically Generated Schrock-Type Catalysts

摘要

Ring-opening metathesis polymerization (ROMP) is a versatile tool for the preparation of a wide variety of polymers. Since the postulated mechanism by Herisson and Chauvin in 1970 that metal alkylidenes are the active species in this type of reactionand the discovery by Schrock et al. in 1974 that these are generated by an α-H abstraction reaction between two alkyl substituents, a tremendous amount of effort has been directed, successfully, to the design of well-defined one-component ROMP initiators. To date, a broad range of initiators is known with a versatile range of applications. The best studied initiators are those based on early transition metals, in particular molybdenum and tungsten complexes developed by Schrock et al. These compoundsare excellent initiators for a wide variety of monomers, but their main disadvantage is their lack of latency (thermal stability) in the presence of pure monomers. Although there are indications that alkyl complexes of early transition metals can be light-sensitive and even that irradiation can accelerate α-H abstraction reactions, no examples are known, besides our own work on Ru~(II) complexes, using the principle of photoinduced ring-opening metathesis polymerization (PROMP) with well-defined, one-component metal complexes. This is even more surprising when one considers that photoprocesses are known for almost all other polymerization reactions and employed in numerous commercial applications. We report here the first results in the field of photoinduced ROMP (PROMP) reactions using simple alkyl tungsten complexes and light for the in situ generation of active ROMP initiators.