Control of polymerisation Flashcards
How does RAFT polymerisation work?
Reversible addition-fragmentation transfer
Addition of a special kind of chain transfer agent which controls kinetics: R-S-C(S)-Z where R is radical leaving group and Z controls reactivity of C=S bond.
Mechanism:
Initation by radical source to give active chain
Reversible chain transfer by addition of active chain to CTA. Decomposition via homolytic fission of this complex leading to chain growth. Sequential chain transfer to keep concentration of radical species low.
What are the factors to consider in developing and finetuning control via RAFT polymerisation?
Identity of chain transfer agent and monomer unit. Z group governs reactivity and responsible for intermediate radical stability. End group modification. Identity of R - require good enough leaving group and able to rapidly reinitiate.
For MAMs (good homolytic leaving groups), more reactive RAFT agent good, e.g. S-alkyl and Ph)
For LAMs, (poor homolytic leaving groups), less stabilising Z group favoured to prevent acting as a radical sink.
How does ATRP polymerisation work?
Atom transfer radical polymerisation
Use of transition metal complexes to mediate kinetics. Set up over equilibrium between dormant and active chain, with control achieved via halogen transfer between dormant chain and metal complex.
How does ATRP polymerisation compare with RAFT polymerisation?
Similar scope of reactions available, access through RX chemistry. Residual metal species in ATRP can be problematic. Slightly more limited in functional group tolerance - avoid ligands that may compete.
What factors should be considered in developing and fine-tuning control via ATRP polymerisation?
Choice of ligand and initiator. Ligands with higher denticity tend to increase value of K(ATRP) = kact/kdeact
Initiator: leaving group ability, presence of radical stabilising groups
End group modification
What are the key steps implicated in SET-LRP polymerisations?
Single electron transfer living radical polymerisation.
Activation by halogen complexation to copper complex. SET: Electron transfer between copper complex and RX. Diss: dissociation of radical species to give halide ion. Comp: complexation of halide ion.
Propagation as usual.
Disproportionation reaction of copper complex.
Deactivation by recombination.
What are the benefits and disadvantages of using SET-LRP polymerisation?
Benefits: Wide range of activated monomers tolerated, ultrafast polymerisation, exceptionally high end group fidelity and easy to set up.
Limits: Considerations of what solvent to use, not demonstrated to work well for LAMs, polymerisation of styrene and methacrylamides less well-studied, acid sensitivity
How does ROMP polymerisation work?
Ring opening metathesis polymerisation
Opening of strained cyclic olefins via alkylidene catalyst. Initiation step = complexation to metal, [2+2]-cycloaddition reaction followed by ring opening with metal complex remaining attached. Propagation by continued addition of monomer units. Termination by reaction with alkene.
What are the benefits and downsides to using ROMP?
Advantages: fast, excellent control over chain length, narrow dispersity, access to well-defined block copolymers
Disadvantages: Oxygen sensitive, catalysts can be expensive.
Why is sequence control so important?
Precision allows for recognition of motifs and precise 3D structures found in nature.
What technique has been developed for alternating copolymers?
Use of alternating nucleophilic and electrophilic radicals. Nucleophilic radical attacks electron deficient species, electrophilic radical attacks electron-rich monomer.
What are SMALPS?
Styrene maleic acid lipid particles
How are SMALPs made?
Via formation of alternating polymer of styrene and maleic anhydride followed by hydrolysis to give amphiphile. Amphiphile self-assembles in aqueous solution to form nanodiscs = SMALPs.
What are SMALPS useful for?
Can be used to stabilise and extract membrane proteins to allow the study of them by allowing them to stay surrounded by their native lipid environment.
What techniques have been developed to make multiblock copolymers?
RAFT/SET-LRP/ATRP = Most controlled polymerisation methods can be used to make block copolymers, but order of preparation needs to be considered.
Propagating radical from first block must be a good homolytic leaving group with respect to that of the second block.
