L6 - 8: Ring-Opening Metathesis

Question 1

ROMP: Polymerisation type, basis for mechanism

Answer 1

• Chain-growth polymerisation process
• Mechanism is based on alkene metathesis (C=C double bond exchange process) -> unsaturation in the monomer is conserved in the monomer

Question 2

Why is ROMP so useful?

Answer 2

• Ease of functionalisation
• Inherent properties of polymer (particularly rigidity)
• Aids forming very complex polymer products
• Industrial purification; after cracking, less stringent purification required to get the starting material

Question 3

How are ROMP reactions controlled?

Answer 3

• All steps are reversible
• As such, the reaction is equilibrium controlled (depending on thermodynamics)
• The driving force is the release of ring strain, balanced by entropy (entropy decreases from monomer to polymer, and enthalpy is negative)
• Monomers with higher ring strain have an equilibrium constant closer to a high conversion

Question 4

Types of chain transfer possible in ROMP:

Answer 4

• Intermolecular chain transfer (part of polymer reacts with a different metal, resulting in scrambling of the polymer) -> metal and R chain swap OR metal:metal, R:R forming dead chain)
• Intramolecular chain transfer (backbiting where a double bond on the polymer reacts with its own metal catalyst) -> constricts polymer chain, excising a cyclic species (requires large monomer, won’t be able to form anything smaller than 6 membered due to ring strain)
• Demonstrate?…

Question 5

Conditions for chain transfer in ROMP?

Answer 5

• Typically occur at later stage when concentration of monomer is outweighed by existing chains (statistically more likely to react with each other)

Question 6

What types of monomers are used for ROMP?

Answer 6

• 4 membered, 5, 7, 8, cyclooctatetraene etc
• Lots of work in bicyclic monomers (e.g. nonbornene) which has huge ring strain, very reactive

Question 7

Possible structures for ROMP of norbornene:

Answer 7

• Cis isotactic (S:R, S:R)
• Cis syndiotactic (S:R, R:S)
• Trans syndiotactic (S:R, S:R)
• Cis syndiotactic (S:R, R:S)

Question 8

Advantages of Tantalum based ROMP catalysts (Compared to Ti); further features

Answer 8

• Easily prepared from a commercially available material (TaCl5)
• Higher activity than Ti complexes
• At high conversions, secondary metathesis becomes competitive with propagation
• Use of more electron rich thiolate ligands reduces electrophilicity of metal centre

Question 9

Deleterious effect of high activity:

Answer 9

• After a point, high activity can lead to side reactions and loss of control of the reaction

Question 10

Potential basis for ROMP catalysts: A few basic comparisons on basis of reactivity

Answer 10

• Titanium (many early examples) -> quite reactive
• Molybdenum -> ‘’ ‘’
• Ruthenium -> lower reactivity; better functional group tolerance (monomers with oxygen etc)
• Tantalum
• Tungsten
• Many transition metals have been relatively successful

Question 11

How can ROMP be used to prepare conducting polymers?

Answer 11

• Several approaches used
• CF3 substituted norbornadiene -> very soluble monomer, disubstituted benzene can be removed as a byproduct -> conductive polymer
• Atom economy approach
• One step approach using cyclooctatetraene