Science of Synthesis

Liu, J.; Johnson, J. A., Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (2016) 2, 99.

Ring-opening metathesis polymerization (ROMP) is a chain-growth polymerization that utilizes cyclic alkenes with greater than ~5 kcal · mol−1 strain energy, such as cyclobutene, cyclopentene, Z-cyclooctene, and norbornene, as monomers (‌Scheme 1‌).[‌1‌] As is typical for metathesis reactions, ROMP is thermodynamically controlled; the release of ring strain in the monomer provides the driving force for polymerization. There are many examples of ROMP initiators, and our focus here is on discrete organometallic complexes with at least one N-heterocyclic carbene (NHC) ligand. The vast majority of these are ruthenium-based complexes (tungsten- and molybdenum-based NHC complexes are discussed in Section ‌2.1.5.3‌). Developments in the field of N-heterocyclic carbene–ruthenium ROMP initiators for alkene metathesis have led to (1) improved functional-group, moisture, and oxygen tolerance; (2) improved control of polymer microstructure (e.g., Z vs. E alkene configuration and/or tacticity); (3) reduced secondary metathesis reactions, i.e. reaction of the growing polymer chain with internal polymer alkenes in either an intermolecular or intramolecular fashion (“backbiting”); and (4) improved rates of initiation (ki) and rates of propagation (kp).[‌2‌] Due to these developments, ROMP has become a powerful technique for the synthesis of a wide range of functional, well-defined polymers for a variety of applications.

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References