Schrock’s Catalyst | Olefin Metathesis
Olefin Metathesis, Grubbs Reaction - Organic chemistry
Secondary metathesis reactions (controlled by catalyst choice and reaction conditions) also affect the product distribution. Recoordination of an alkene on the growing polymer chain with the catalyst can lead to cyclic oligomers through a ring-closing metathesis reaction (“backbiting”). Chain transfer (cross metathesis) between a growing polymer unit and an adjacent polymer alkene also leads to broadened molecular weights. Chain transfer can also be used to improve processability of the resulting polymer – addition of an acyclic olefin (chain-transfer agent) can limit chain molecular weights and introduce terminal functional groups.
Olefin Metathesis Grubbs Reaction
Careful balance of catalyst, monomer, and other factors can offer excellent control of the polymer structure. In terms of homogeneous catalysts, most tungsten and molybdenum catalysts (Schrock catalysts) have rapid initiation rates and can produce “living” polymerizations with excellent control of polydispersity and chain tacticity, but the low functional group tolerance limits the monomers available. Ruthenium metathesis catalysts (Grubbs catalysts) tend to have slower initiation rates, often leading to higher polydispersities, but their air stability and greater tolerance for functional groups makes them “user friendly” and enables use of a wide range of functional monomers and additives.
Kinetically controlled E-selective catalytic olefin metathesis
Employing the standard cross-metathesis assay previously described, the chelated-adamantyl catalyst provides the heterocoupled product in 87% Z-selectivity at 64% reaction conversion. Interestingly, allylbenzene also undergoes self-metathesis during the above cross-metathesis assay, providing the homocoupled product in >95% Z-selectivity. Also noteworthy is the fact that the best experimental conditions involve performing the reaction in a 1:1 mixture of THF:H2O, thus, dry solvents are not a requirement. However, it is noted that strict exclusion of oxygen is required.