The study of ring-opening metathesis polymerisations of strained bicyclic monomers mediated by well-defined ruthenium complexes

Abstract

When 7-alkoxynorbomadiene monomers are subjected to Ring-Opening Metathesis Polymerisations (ROMP) mediated by RuСІ(_2)(РСуз)(_2)(=СНРh) (A), regeneration of the initiator is observed. As the steric bulk of the 7-alkoxy functionality is decreased the extent of regeneration observed decreases accordingly (7-tert-butoxy: 27 %， 7- isopropoxy; 18 %, 7-ethoxy: 8 %, 7-methoxy: 3 %). The initial monomer to initiator ratio ([M]o/[I]o) was also found to effect the extent of regeneration. The ROMP of bicyclic olefin monomers containing oxygen functionality in positions other than the bridgehead carbon results in no regeneration of the initiator. Similarly, subtle variations to the chemistry of the ligands of initiator A result in no regeneration being observed, possibly due to the electronic effects of the ligands on the metal centre. Two propagating alkylidene species are apparent in Ή NMR spectra during the ROMP of monomer 1 mediated by initiator A, their identity is revealed by the addition of either free phosphine or a phosphine scavenger (CuCl) to the system once the polymerisation reaches completion. The resonance appearing as a triplet at 19.36 ppm is assigned to the alkylidene protons of bisphosphine species, whereas a broad resonance at -17.5 ppm is assigned to a monophosphine species in which oxygen contained within the propagating polymer backbone chelates to the ruthenium centre. The latter species is stable in solution and active for ROMP. Comparison are drawn between this species and КиСІ2(РСуз)(=СН-0-0- /-РгСбН4) (F), a stable ruthenium complex containing internal oxygen chelation, and NMR experiments reveal that they are similar in structure. The ligand exchange and ROMP behaviour of initiator F is also studied. A new methodology which permits the recovery of ruthenium alkylidene complexes from ROMP systems mediated by initiator A is described. The regenerated complexes are recovered in good yields and successfully employed in subsequent olefin metathesis reactions. This novel approach of induced regeneration is of significant interest due to the simple addition of terminating agents to ROMP reactions resulting in expensive non-recyclable ruthenium initiators being converted into cost effective recyclable catalysts.