Olefin Metathesis Polymerization: Acyclic Diene Metathesis (ADMET)

ADMET uses open-chain alkenes with two double bonds at the chain ends, such as 1,8-nonadiene. Two equivalents of these terminal dienes react in the presence of Grubbs catalyst to form a dimer and release ethylene gas. The ethylene escapes during the reaction, driving product formation forward and enabling polymer chain growth.

ADMET involves formation of a metallacyclobutane intermediate through [2+2] cycloaddition of one double bond from a terminal diene with Grubbs catalyst. This cyclobutane intermediate immediately undergoes ring-opening to generate a new catalyst species. The new catalyst then reacts with another diene molecule, and this cycle continues to build the polymer chain.

ADMET is a reversible process, so ethylene gas must be removed from the reaction mixture to drive polymerization to completion. As ethylene escapes during the reaction, it favors product formation by shifting equilibrium toward polymer growth. This removal is essential for achieving high molecular weight polymers efficiently.

ADMET is highly efficient for polymerization and can produce polymers with molecular weights as high as 80,000 amu. This high efficiency makes ADMET a valuable method for synthesizing polymers with controlled and substantial molecular weights compared to some traditional polymerization approaches.

ADMET allows high control over functional groups, unlike traditional radical chain growth polymerization. The formed unsaturated polymer can be further manipulated through hydrogenation or other functionalizations. This versatility enables synthesis of modern polymers, including completely linear polyethylene, with precise structural design.

ADMET uses acyclic terminal dienes as substrates and produces linear unsaturated polymers with ethylene as a byproduct. Unlike ring-opening metathesis polymerization, which uses cyclic monomers, ADMET employs open-chain alkenes. Both methods utilize Grubbs catalyst and metathesis mechanisms but differ in monomer structure and polymer architecture.

ADMET produces unsaturated polymers containing residual double bonds that enable further chemical manipulation. These polymers can undergo hydrogenation to saturate the double bonds or be subjected to other functionalizations. This post-synthesis versatility allows chemists to tailor polymer properties for specific applications.