Oligomerisation & Polymerisation of Alkenes Flashcards
What is the Dewar-Chatt-Ducanson model?
- It is a widely used model in organometallic chemistry that explains the bonding interaction between transition metals and π-acceptor ligands
- It involves a σ-donation from ligand→metal
- And π-backdonation from filled metal→ligand
What is an Oligomer?
An oligomer is a molecule made up of a small number of repeating monomer units (typically 10-20), which is larger than a simple molecule but not as large as a polymer
Explain what happens in the 3 broad polymerisation process steps: Initiation, Propagation & Termination?
- Initation - Process to start the polymerisation
- Propagation - Process of polyymer chain growth to larger size
- Termination - Process(es) by which chain growth is stopped
What is Chain Transfer?
- Processes to control molecular weight of the polymer
- One chain is terminated and a new chain is initiated
- Reduces molecular weight without sacrificing yield
Alkene Oligomerisation is thermodynamically allowed as C=C bond strength is less than double the C-C bond stregth BUT are kinetically slow in the absence of a catalyst
How do transition metal complexes affect this?
- Transition metal complexes can cause dimerisation, oligomerisation or polymerisation catalysts
- This is dependant on the relative probability of chain termination compared that of chain propagation
In the absence of hydrogen, alkyl chain growth is…
…usually terminated by β-elimination of hydrogen
If AlR₃ is present during the polymerisation process then…
…transfer of the polymer chain from the transition metal to aluminium can occur
What drives the rate of termination
- You get 70kcal from the reformation of M-H bond + 50kcal from alkene reformation if the new C-H bond (100kcal) formed in the polymerisation process was broken
How can we control the selectivity of the Nickel catalyst so either oligomers or polymers can be favoured?
Through the choice of ligands
Oligomer formation vs Polymer formation is favoured when…
- Oligomer formation is favoured if kt is similar to kp
- Polymer formation is favoured if kp is larger than kt
- E.g. with a d⁰ complex, e.g. [Cp₂MR]⁺, where M = Ti, Zr or Hf, β-elimination of hydrogen is not a easy process thus kp>kt
Describe why the structure of this pre-catalyst is benefical?
(Note it is used in SHOP - Shell Higher Olefin Process)
- Benefit of the carboxylate is it makes the nickel more electron deficient, making the phosphine a bit more electron rich
- PPh₂ is a PPh₃ derivative
- The cycloctodiene is a 2e- donor which acts as a chelating ligand to the metal (2e- donor on 1 side and 1e- donor at the other)
- The 1e- donor is a weak bond, so upon heating will break via hydride elimination
What happens once the catalyst has been activated in the SHOP process
- Charge on Ni(II) meaning it has increased lewis acidity - helping it coordinate to this alkene
- Ni goes from a 14e- to 16e- species (d⁸ - square planar - coordinately saturated)
What happens SHOP cycle once alkene coordination has occured?
- Hydride migration/cis-ligand migration
- Dropped from 16e- to 14e- due to the generation of a free coordination site
What happens in the SHOP cycle once hydride migration has occured?
- Another Alkene coordination follwed by a chain migration
- Do the the addition then removal of a free coordination site
- This works because the rate of the propagation is a little faster than the β-hyride elimination - allowing the chain to grow to the desired length
How does the SHOP cycle terminate?
Through β-hyride elimination
What is the Schulz-Flory distribution
- The relative ratios of polymers of different length after a polymerisation process based on their relative probabilities of occurence
- Results in this distribution of C12-C-18
- In the SHOP cycle this results in a mixture because kp and kt are so close in rate
What happens during the Seperation process at the end of the SHOP cycle?
- Catalysts remain in the polar solvent; non-polar alkenes are separated in a second liquid phase and “skimmed off” from the reaction mixture
What can we do with the unwanted chain lengths after the seperation process at the end of the SHOP cycle?
Unwanted chain lengths isomerised to internal olefins and undergo metathesis to give useful olefins (£)
Polymer tacticity refers to the relative positions of groups pendan to the polymer backbone chain
What is an Atactic polymer?
- An atactic polymer is a type of polymer in which the sterochemistry (spatial arrangement) of the pendant group along the polymer backbone is random
- Baically random sterochemistry
Polymer tacticity refers to the relative positions of groups pendant to the polymer backbone chain
What are isotactic and syndiotactic polymers
- Isotactic polymer is where all substituent groups are aligned on the same side of the polymer chain
- Syndiotactic polymers is where the substituent groups alternate sides along the polymer backbone is a regular pattern
How does metallocenes interact with the Zr metal centre in this example?
- Zr(iv) has a +4 oxidation state so doesn’t experience stabilisation of the olefine adduct by back-bonding
Identify the intermediates of this alkene polymerisation reaction
What does MAO stand for in this reaction?
- [MeAlO]ₙ
- Acts as an anti-crown - lots of empty pz orbitals meaning it can act as an anion-receptor and strip the chloride away from the metal complex
What is the product of this reaction?
- Protonolysis of the M-alkyl bond
- PhNMe₃ can coordinate M and suppress activity
Chain growth proceeds through Cossee-Ariman mechanism (based on migratory insertion)
What is the product?
- Reaction with alkene is very reactive due to no-backbonding
- Resulting in ligand migration being very quick
- Goes to the most substituted terminus because if there is no-backbonding there is a positive charge at this substitued carbon
- AND Cp are sterically bulky so adding further away from that
α-agostic interactions can be important in stabilising the transition state for alkene insertion
Show how it occurs
- The σ C-H bond is very close to the electron deficient metal centre - semi-coordinated
What is hydrogenolysis in terms of alkene polymerisation?
- Hydrogenolysis: to give a saturated polymer
- Small amount of H₂ are added to the polymerisation reactor
What are some advantages of Metallocene catalysts?
- Lower molecular weight distribution, i.e. sharply defined polymer length
- Single site for reaction
- High activity comparable to heterogenous case
- Variation in ligand, leading to tailor-made polymers
What are some disadvantages of Metallocene catalysts?
- Major problem with tthese catalysts is their oxophilic nature which can make them highly air moisture sensitive
- Oxophilic nature also limits the functional groups which can be present in the monomer
- Heterogenous catalysts form isotactic polypropylene with propene
- For simple homogenous catalysts there is no control of the tacticity
- Alkenes like propene are prochiral (non-chiral molecule which can beome chiral)
- Two carbon, symmetric metallocenes induce the production of isotactic polypropylene through enantiomorphic control, where bridge between aromtic groups resticts the rotation of rings
- How does this work
- When switching from left to right, its always on the same stero face of the alkene that has to react
- This catalyst can be used to control the production of syndiotactic polymerisation
- How does it work
- In this case we build a mirror plane into this
- Which means the alkene has to swap fromt the R to C face
- Which gives the up and down arrangement
- However, this can often makes mistakes
