Synthesis of stereoregular polymers through ring-opening metathesis polymerization

Area of Science:

• Polymer Chemistry
• Organic Synthesis
• Catalysis

Background:

• Norbornenes and norbornadienes are inexpensive monomers for ring-opening metathesis polymerization (ROMP).
• Controlling polymer tacticity (stereochemical arrangement) is crucial for desired material properties, especially after hydrogenation.
• Previously, achieving polymers with a single, defined tacticity was challenging.

Purpose of the Study:

• To explore advanced catalytic methods for stereoselective synthesis of polynorbornenes via ROMP.
• To understand and utilize mechanisms like enantiomorphic site control and stereogenic metal control for precise polymer architecture.
• To enable the production of hydrogenated polymers with specific, single tacticity structures.

Main Methods:

• Utilized well-defined ROMP catalysts, including molybdenum imido alkylidene complexes with chiral ligands.
• Employed molybdenum and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators.
• Investigated monomer addition pathways to control stereochemistry during polymerization.

Main Results:

• Achieved cis,isotactic-poly(2,3-dicarbomethoxynorbornadiene) using chiral molybdenum catalysts via enantiomorphic site control.
• Produced cis,syndiotactic polynorbornenes and substituted norbornadienes using MAP initiators via stereogenic metal control.
• Demonstrated that stereogenic metal control allows alternating incorporation of enantiomers from racemic monomers.

Conclusions:

• Precise control over polymer tacticity (cis,isotactic and cis,syndiotactic) is now achievable through specific ROMP catalyst design.
• While all four basic polymer structures may not be accessible, the ability to form pure tactic polymers is significant for applications.
• Further mechanistic understanding of ROMP polymerization by well-defined initiators promises more elaborate stereoregular polymers and copolymers.