2. What are Polymers?
A polymer is a large molecule or a macromolecule which essentially is a
combination of many subunits. The term polymer in Greek means ‘many
parts’. Polymers may be naturally found in plants and animals (natural
polymers) or may be man-made (synthetic polymers).
Polymers are all created by the process of polymerization wherein their
constituent elements, called monomers, are reacted together to form
polymer chains, i.e., 3-dimensional networks forming the polymer bonds.
4. TYPES OF POLYMERS
HOMOPOLYMERS:
Homopolymer is made from only one type of monomer
unit. In the repeating structural unit of a homopolymer,
only one type of monomer unit is present.
For example,Polythene, Teflon, PAN and nylon-6.
COPOLYMERS:
copolymer is made from two (or more) types of
monomer units. In the repeating structural unit
of a copolymer, two (or more) types of monomer
unit are present.
For example,Buna-S, Buna-N, polyester,
Bakelite
5. Linear Polymers:
The structure of polymers containing long and straight
chains falls into this category. PVC, i.e. poly-vinyl
chloride,
is largely used for making pipes and electric cables is
an example of a linear polymer.
Branched-chain Polymers:
When linear chains of a polymer form branches, then
such polymers are categorized as branched chain
polymers. For example, Low-density polythene.
Cross-linked Polymers:
They are composed of bifunctional and trifunctional
monomers. They have a stronger covalent bond in
comparison to other linear polymers. Bakelite and
melamine are examples in this category.
7. ADDITION POLYMERISATION
A chain polymerization or addition polymerization is a reaction that yields a polymer product
which is the exact multiple of monomers. Thus the mechanism is called addition polymerization.
Addition polymerization is initiated by initiators and carried out in three different types of
mechanism.
1.Free radical mechanism
2.Ionic mechanism
3.co-ordination mechanism
Common examples of addition polymerization
are polyethylene, polyvinyl chloride (PVC),
acrylics, polystyrene, polytetrafluoroethylene,
and polyoxymethylene (acetal).
8. FREE RADICAL CHAIN POLYMERISATION
1.INITIATION: Initiators are unstable compounds and undergo homolytic fission to
produce free radicals which react with π electrons of the monomer tomproduce
monomer free radical.
Bynzoyl peroxide, hydrogen peroxide are good initiators for free radical chain
polymerization.
2.PROPAGATION: The monomer free radical reacts with a number of monomers
rapidly resulting in the chain growth with free radical site at the end of the chain
producing a living polymer. Bt adding fresh monomers to the living polymer with free
radical site, again chain growth starts. Hence it is called living polymer.
3.TERMINATION: Termination(to stop chain growth) of the growing polymer chain is
carried by ‘coupling’ and ‘disproportionation’, resulting in the dead polymer. In
coupling, the collision of two growing chains causes the union of the two chains at
their free radical site producing a dead polymer.
9.
10. CO-ORDINATION POLYMERISATION (or) ZEIGLER-NATTA POLYMERISATION
Zeigler and Natta discovered that in the presence of a combination of transition
metal halides like TiCl4, ZnBr3 etc with an organometallic compounds like triethyl
aluminium or trimethyl aluminium, stereospecific polymerisations can be carried out.
Combination of metal halides and organometallic compounds are called Zeigler
Natta catalysts.
CATIONIC CHAIN POLYMERISATION:
The heterolytic fission of the initiator results in cationic and anionic chain
polymerization.
ANIONIC CHAIN POLYMERISATION:
An anion produced by organo alkali compounds like ethyl sodium, methyl
potassium, butyl lithium etc initiates the chain polymerization.
11. STEP POLYMERISATION (or)
CONDENSATIONPOLYMERISATION
Step polymerization takes place by condensation reactions of functional groups of the
monomers with the elimination of byproducts like H2o, Hcl etc.. Hence known as
condensation polymerization. The following are the characteristics of concensation
polymerisaition:
The monomers contain functional groups like –OH, -COOH, NH2, RCOOR, halides etc.
The functionality of the monomer must be two or more than two.
The polymer is built up by a slow step wise condensation reaction of the functional
groups of the monomer.
The reaction are not exothermic.
The molecular weight of the polymer is not the sum of the molecular weights of the
monomers.
The polymers produced are living polymers containing functional groups at the end of
the chain.
