Mechanistically designed dual-site catalysts for the alternating ROMP of norbornene and cyclooctene

Mechanistic work on the olefin metathesis reaction by well-characterized ruthenium carbene complexes led to the rational design and synthesis of a modified, unsymmetrical "first-generation" catalyst, which, in contrast to either first- or second-generation systems with symmetrical ligands, converts a mixture of two cycloolefin monomers to a largely alternating copolymer. The mechanistic concept of a homogeneous catalyst that switches between more than one state at each turnover is general. The structures of the complexes, determined by mass spectrometry, NMR, and X-ray crystallography, reveal some unexpected features, which explain sequence errors in the copolymer.