21.26: Olefin Metathesis Polymerization: Acyclic Diene Metathesis (ADMET)
Acyclic diene metathesis polymerization or ADMET polymerization involves cross-metathesis of terminal dienes, such as 1,8-nonadiene, to give linear unsaturated polymer and ethylene. As ADMET is a reversible process, the formed ethylene gas must be removed from the reaction mixture to complete the polymerization process.
Similar to cross-metathesis, ADMET also involves the formation of metallacyclobutane intermediate by [2+2] cycloaddition of one of the double bonds of a terminal diene with Grubbs catalyst. The cyclobutane intermediate immediately undergoes ring-opening to form a new catalyst which reacts with another diene, and the cycle continues to form the polymer chain. The advantage of the ADMET is that the formed unsaturated polymer can be further manipulated either by hydrogenation or by other functionalizations.
As the ADMET allows a high degree of control for different functional groups than the traditional radical polymerization process, it has been efficiently used to synthesize a variety of modern polymers. For example, the synthesis of completely linear polyethylene is possible by an ADMET polymerization reaction.