12/09/2014

1

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

12/09/2014

2

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

12/09/2014

3

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.

12/09/2014

4

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

12/09/2014

5

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.