Olefin metathesis polymerization is a powerful tool for making complex polymers. It works by rearranging carbon-carbon double bonds using metal catalysts, allowing for the creation of diverse polymer structures with useful properties.

This method comes in two main flavors: ROMP and ADMET. Both use similar catalysts but differ in their starting materials and products. The resulting polymers can be further modified, making them versatile building blocks for various applications.

Olefin Metathesis Polymerization Mechanism and Types

Mechanism of olefin metathesis polymerization

• Involves rearrangement of carbon-carbon double bonds catalyzed by metal-alkylidene complex
• Proceeds through metallacyclobutane intermediate formed by reaction of metal-alkylidene with olefin monomer
• Metallacyclobutane undergoes retro [2+2] cycloaddition yielding new metal-alkylidene and olefin
• Newly formed metal-alkylidene reacts with another olefin monomer continuing polymerization cycle
• Results in redistribution of carbon-carbon double bonds and formation of polymeric product

ROMP vs ADMET polymerization methods

• ROMP (Ring-Opening Metathesis Polymerization)
  • Polymerizes cyclic olefins (norbornene, cyclopentene)
  • Driven by release of ring strain
  • Produces polymers with double bond in backbone of each repeat unit
  • Synthesizes high molecular weight polymers with well-defined structures
• ADMET (Acyclic Diene Metathesis)
  • Polymerizes linear $\alpha,\omega$-dienes
  • Produces polymers with double bond at end of each repeat unit
  • Requires continuous removal of ethylene gas to drive equilibrium towards polymerization
  • Typically yields lower molecular weight polymers compared to ROMP
• Both ROMP and ADMET use same metal-alkylidene catalysts and proceed through metallacyclobutane intermediate

Advantages of olefin metathesis polymerization

• Compatible with wide range of functional groups (esters, ethers, amides)
  • Allows incorporation of diverse functionalities into polymer structure
• Resulting polymers contain carbon-carbon double bonds for post-polymerization modification
  • Double bonds can be hydrogenated to improve thermal and oxidative stability
  • Double bonds can be functionalized via epoxidation, hydroboration, thiol-ene addition
• Synthesizes various polymer architectures
  • Linear polymers, block copolymers, graft copolymers prepared by controlling monomer feed and catalyst selection
• Conducted under mild conditions
  • Many reactions carried out at room temperature or slightly elevated temperatures
  • Tolerant to air and moisture depending on specific catalyst used

Catalysts and Polymerization Control

• Transition metal catalysts are essential for olefin metathesis polymerization
  • Grubbs catalysts: ruthenium-based, highly active and tolerant to functional groups
  • Schrock catalysts: molybdenum or tungsten-based, highly active but sensitive to air and moisture
• Living polymerization can be achieved with certain catalyst systems
  • Allows for precise control over molecular weight and polymer architecture
• Chain transfer can occur, affecting molecular weight distribution
  • Can be controlled through choice of catalyst and reaction conditions