31.5 Olefin Metathesis Polymerization

Olefin Metathesis Polymerization Mechanism and Types

Olefin metathesis polymerization builds complex polymers by rearranging carbon-carbon double bonds using metal catalysts. Instead of adding monomers through radical or ionic pathways, this approach swaps fragments around double bonds, opening up polymer structures that are difficult or impossible to access through other methods.

Two main variants exist: ROMP (Ring-Opening Metathesis Polymerization) and ADMET (Acyclic Diene Metathesis). Both rely on the same core catalytic cycle but start from different monomers and yield different products. The polymers that result still contain double bonds in their backbones, which means you can further modify them after polymerization.

Mechanism of Olefin Metathesis Polymerization

The catalytic cycle centers on a metal-alkylidene complex (a metal bonded to a $\text{CR}_2$ fragment). Here's how it proceeds:

1. The metal-alkylidene reacts with an olefin monomer through a [2+2] cycloaddition, forming a four-membered metallacyclobutane intermediate.
2. The metallacyclobutane breaks apart via a retro [2+2] cycloaddition, but it cleaves in the opposite sense from how it formed. This produces a new olefin (now attached to the growing chain) and regenerates a metal-alkylidene species.
3. The regenerated metal-alkylidene reacts with the next olefin monomer, repeating the cycle.
4. Each cycle extends the polymer chain by one repeat unit while preserving a carbon-carbon double bond in the backbone.

The net result is a redistribution of double-bond partners: the fragments on either side of the original $\text{C=C}$ bonds get shuffled and reconnected into a polymeric chain.

ROMP vs. ADMET Polymerization Methods

Both methods use the same metal-alkylidene catalysts and proceed through the metallacyclobutane intermediate, but they differ in important ways.

ROMP (Ring-Opening Metathesis Polymerization)

• Starts from cyclic olefins such as norbornene, cyclooctene, or cyclopentene.
• The thermodynamic driving force is release of ring strain. Because the cyclic monomer is higher in energy than the open-chain polymer, the equilibrium strongly favors polymerization.
• Each repeat unit retains a double bond in the backbone.
• Capable of producing high-molecular-weight polymers with well-defined structures. With the right catalyst, ROMP can behave as a living polymerization, giving narrow molecular weight distributions.

ADMET (Acyclic Diene Metathesis)

• Starts from linear $\alpha,\omega$-dienes (monomers with a terminal double bond at each end).
• The reaction is a step-growth process: two terminal double bonds undergo metathesis, linking the monomers and releasing ethylene ($\text{CH}_2\text{=CH}_2$) as a small-molecule byproduct.
• Because the reaction is an equilibrium, you must continuously remove ethylene (by vacuum or inert gas purge) to push the equilibrium toward polymer.
• Typically yields lower molecular weights than ROMP, since achieving high conversion in a step-growth process requires very high extents of reaction.

Catalysts and Polymerization Control

Transition metal-alkylidene complexes are essential. The two most important families are:

• Grubbs catalysts (1st and 2nd generation): Ruthenium-based. They are highly active, tolerant of many functional groups, and relatively stable toward air and moisture. The 2nd-generation Grubbs catalyst, which features an N-heterocyclic carbene (NHC) ligand, is more active and longer-lived than the 1st-generation version.
• Schrock catalysts: Molybdenum- or tungsten-based. These are extremely active and can be more stereoselective, but they are much more sensitive to air, moisture, and protic functional groups. They typically require rigorous inert-atmosphere techniques (glovebox or Schlenk line).

Controlling the polymerization:

• Living polymerization is achievable with certain catalyst/monomer combinations (particularly in ROMP). In a living system, initiation is fast relative to propagation, there is no termination, and you get predictable molecular weights with narrow dispersity ($Đ$ close to 1).
• Chain transfer (also called secondary metathesis) can occur when the catalyst reacts with a double bond already in the polymer backbone, scrambling chain lengths. This broadens the molecular weight distribution. Choosing a less active catalyst or running the reaction at lower temperature can minimize it.
• Block copolymers can be prepared by sequential addition of different monomers in a living ROMP system, since the active chain end persists until a new monomer is introduced.

Advantages of Olefin Metathesis Polymerization

Broad functional group tolerance. Especially with Grubbs catalysts, the reaction tolerates esters, ethers, amides, alcohols, and many other groups. This lets you incorporate diverse functionality directly into the polymer without protecting-group strategies.

Post-polymerization modification. The double bonds retained in the polymer backbone are reactive handles. You can:

• Hydrogenate them to give a fully saturated backbone with improved thermal and oxidative stability.
• Epoxidize them to introduce oxirane rings for further crosslinking or ring-opening chemistry.
• Functionalize them via hydroboration or thiol-ene click reactions to attach side groups.

Mild reaction conditions. Many olefin metathesis polymerizations run at or near room temperature. Depending on the catalyst, some reactions can even be performed in air or with undried solvents, which simplifies experimental setup.

Architectural versatility. By controlling monomer feed, catalyst choice, and addition sequence, you can prepare linear homopolymers, block copolymers, graft copolymers, and even cyclic polymers.