21.19: Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)

Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring strain drives the reaction forward.

The characteristic feature of the ROMP is that the unsaturation present in the reactants is conserved in the products. The reactivity of cycloalkenes towards ROMP decreases with an increase in the ring size.

Poly(phenylene vinylene) (PPV), an important polymer having alternating phenyl and vinyl groups used for electro-optical applications, is prepared by ROMP reaction. Acetyl-substituted bicyclooctadiene undergoes ROMP in the presence of Grubbs catalyst and gives processable polymer which upon heating loses two moles of acetic acid and aromatizes to form poly(phenylene vinylene) polymer.

Figure 1. ROMP synthesis of poly(phenylene vinylene).

Recall that ring-opening metathesis polymerization, or ROMP, utilizes the olefin metathesis reaction to synthesize polymers.

ROMP involves strained cycloalkenes as substrates, such as cyclopentene.

The release of the ring strain upon open-chain product formation acts as a driving force for the reaction.

Initially, cyclopentene reacts with Grubbs catalyst, a metal carbene, and forms a metallacyclobutane intermediate.

Subsequently, the intermediate undergoes a ring-opening reaction to give a new substituted carbene.

This new carbene further reacts with another molecule of cyclopentene via a ring-opening reaction and extends the alkyl chain.

Repetition of these steps leads to the formation of an unsaturated polymer product.

Generally, the reactivity of cycloalkenes towards ROMP increases with an increase in the ring strain or a decrease in the ring size.

ROMP reactions are useful in synthesizing fully conjugated and highly unsaturated materials by retaining the unsaturation of monomers in the polymeric product.