MM361_lec8-9
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Transcript of MM361_lec8-9
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Lec-08
Nework Formation Mechanism
Bifunctional molecules make dimers and trifunctional (f>2) or multifunctional
monomers can actually give you network or interconnected polymers.
If more than two hydroxyl groups are present on one of the monomers, the
chains can be linked together to form a large, strong 3D structure.
Examples: Reaction of phenol with formaldehyde to form ?
Reaction of Phthalic
anhydride with Glycerol to
form glyptal resin.
Chain or Addition Polymerisation
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Free Radical Polymerisation
Unshared electron at carbon atom active site
Free radicals
Initiator molecule (Peroxide)
Homolytic Scission (when peroxide is heated up it converts to free radical)
Addition of the free radical to a monomer
Propagation is a chain reaction
Chain reaction stops when monomers are depleted or chain terminates
Termination by combination or by disproportionation (transfer of proton from one monomer to the other)
Radicals may also react with solvent, initiator, monomer or even other chains (chain transfer)
Free Radical Polymerisation
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Free Radical Polymerisation
Lec-09
Free Radical Polymerisation
Chain Transfer
In addition to propagating and terminating the active site at the chain end
they can engage in reactions with solvent, monomer, initiator, or even other
chains.
This process is called chain transfer, because the process "caps" the growing
chain, usually (but not always) by
saturating it with a proton, but at
the same time generates another
radical that, if it is capable of
adding a monomer, can start a new
chain growing.
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Free Radical Polymerisation:
Chain Transfer to Polymer
Chain transfer can also occur to polymer molecules, resulting in the
formation of a branched molecule. This type of chain transfer becomes
significant when the concentration of polymer is high (i.e., in the later stages
of a batch polymerization), particularly when the polymer has a very
reactive propagating
radical (e.g., polyethylene,
polyvinyl chloride, poly vinyl
Acetate etc.).
Ionic Addition Polymerisation
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Anionic Polymerisation
Monomers can be polymerised anionically if there is an excess of one electron
at the active site i.e. a negative charge.
If the active site is a carbon atom, then this charged species is called a
carbanion. An oxanion is formed during an anionic ring-opening
polymerization.
Initiators:
Grignard reagent (R-X-Mg), organometallic
compounds, alkali metals, metal amides.
Termination does not occur in anionic
Polymerisation because of alkali counterions,
Which have no tendency to combine with
carbanionic active sites.
Such polymers which permanently retain their active centres and continue to
grow until the monomer is available are called living polymers.