21.24: Olefin Metathesis Polymerization: Overview
Recently, the development of olefin metathesis polymerization advanced the field of polymer synthesis. Simply put, the reorganization of substituents on their double bonds between two olefins in the presence of a catalyst is known as the olefin metathesis reaction. The use of metathesis reaction for polymer synthesis is called olefin metathesis polymerization.
Ruthenium-based Grubbs catalyst is the most commonly used catalyst for olefin metathesis polymerization. Grubbs catalyst consists of a carbon-metal double bond, also known as carbene.
The olefin metathesis reaction follows the reversible mechanism as shown in the figure below:
The general mechanism involves the reaction of Grubbs catalyst with an alkene in a [2+2] cycloaddition giving a four-membered metallacyclobutane intermediate which immediately undergoes a ring-opening reaction in a reverse manner to give back the starting material or by breaking different bonds to form a different carbene or a catalyst and a different alkene product.
The new catalyst undergoes [2+2] cycloaddition with a second alkene to form a new metallacyclobutane, giving the metathesis product and a new carbene complex. This new carbene complex is then ready to attack another molecule of starting material, and the cycle is repeated.
There are several ways the olefin metathesis has been implemented for polymer synthesis. However, ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) are the most widely used for polymerization. ROMP is a chain growth, addition polymerization reaction, whereas ADMET is a step-growth, condensation polymerization reaction.