Polymers class 12 CBSE
THIS IS A PURE CBSE WEBSITE FOR POLYMERS CLASS 12 MADE BY RITIK TATIA
Published on: Mar 4, 2016
Transcripts - Polymers class 12 CBSE
Classifications of polymers
• Polymers are made up of large no of macromolecules.
• The single unit of a polymeric chain is called a monomer.
• This process of formation of polymers from respective monomers is
Classification of polymers
• 1. Natural polymers
These polymers are found in plants and animals. Examples are proteins,
cellulose, starch, resins and rubber.
• 2. Semi-synthetic polymers
Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are
the usual examples of this sub category.
• 3. Synthetic polymers
A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon
6,6) and synthetic rubbers (Buna – S) are examples of man-made polymers
extensively used in daily life as well as in industry.
There are three different types based on the structure of the polymers.
• 1. Linear polymers on Structure
• These polymers consist of long and straight chains. The examples of Polymers are high
density polythene, polyvinyl chloride, etc. These are represented as:
2. Branched chain polymers
• These polymers contain linear chains having some branches, e.g., low density polythene.
These are depicted as follows:
• 3. Cross linked or Network polymers
These are usually formed from bi-functional and tri-functional monomers and contain
strong covalent bonds between various linear polymer chains, e.g. bakelite, melamine,
etc. These polymers are depicted as follows:
• Addition polymers are formed from repeated units of monomer units possessing
double and triple bonds.
• Polymers formed by addition of two different polymers are called co-polymers
• The condensation polymers are formed by repeated condensation reaction
between two different bi-functional or tri-functional monomeric units. In these
polymerisation reactions, the elimination of small molecules such as water, alcohol,
hydrogen chloride, etc. take place.
• The examples are terylene (dacron), nylon 6, 6, nylon 6, etc.
For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine
with adipic acid.
Classification of polymers based on molecular forces
• A large number of polymer applications in different
fields depend on their unique mechanical properties
like tensile strength, elasticity, toughness, etc. These
mechanical properties are governed by intermolecular
forces, e.g., van der Waals forces and hydrogen bonds,
present in the polymer.
• These forces also bind the polymer chains. Under this
category, the polymers are classified into the following
four sub groups on the basis of magnitude of
intermolecular forces present in them.
• These are rubber – like solids with elastic properties. In these elastomeric
polymers, the polymer chains are held together by the weakest intermolecular
forces. These weak binding forces permit the polymer to be stretched.
• A few ‘crosslinks’ are introduced in between the chains, which help the polymer to
retract to its original position after the force is released as in vulcanised rubber. The
examples are buna-S, buna-N, neoprene, etc.
• Fibres are the thread forming solids which possess
high tensile strength and high modulus. These
characteristics can be attributed to the strong
intermolecular forces like hydrogen bonding.
• These strong forces also lead to close packing of
chains and thus impart crystalline nature. The
examples are polyamides (nylon 6, 6), polyesters
• These are the linear or slightly branched long chain
molecules capable of repeatedly softening on heating and
hardening on cooling. These polymers possess
intermolecular forces of attraction intermediate between
elastomers and fibres. Some common thermoplastics are
polythene, polystyrene, polyvinyls, etc.
• These polymers are cross linked or heavily branched
molecules, which on heating undergo extensive
cross linking in moulds and again become infusible.
These cannot be reused. Some common examples
are bakelite, urea-formaldelyde resins, etc.
Classification based on growth polymers
• There are two broad types of polymerisation reactions, i.e., the addition or
chain growth polymerisation and condensation or step growth
Addition Polymerisation or Chain Growth Polymerisation
• In this type of polymerisation, the molecules of the same monomer or
diferent monomers add together on a large scale to form a polymer. The
monomers used are unsaturated compounds, e.g., alkenes, alkadienes and
their derivatives. This mode of polymerisation leading to an increase in
chain length or chain growth can take place through the formation of either
free radicals or ionic species. However, the free radical governed addition or
chain growth polymerisation is the most common mode.
• Free radical mechanism:-
The polymerisation of ethene to polythene consists of heating or exposing to light a
mixture of ethene with a small amount of benzoyl peroxide initiator.
The process starts with the addition of phenyl free radical formed by the peroxide to
the ethene double bond thus generating a new and larger free radical.
This step is called chain initiating step.
As this radical reacts with another molecule of ethene, another bigger sized radical is
The repetition of this sequence with new and bigger radicals carries the reaction
forward and the step is termed as chain propagating step. Ultimately, at some stage
the product radical thus formed reacts with another radical to form the polymerised
Chain terminating step
For termination of the long chain, these free radicals can combine in different ways to form
polythene. One mode of termination of chain is shown as under:
Preparation of important addition polymers
• Teflon is manufactured by heating tetrafluoroethene with a free
radical or persulphate catalyst at high pressures. It is chemically inert
and resistant to attack by corrosive reagents. It is used in making oil
seals and gaskets and also used for non – stick surface coated
• The addition polymerisation of acrylonitrile in presence of a peroxide catalyst leads
to the formation of polyacrylonitrile. Polyacrylonitrile is used as a substitute for
wool in making commercial fibres as orlon or acrilan.
Condensation polymerisation or step growth polymerisation
• This type of polymerisation generally involves a repetitive condensation reaction between
two bi-functional monomers.
• These polycondensation reactions may result in the loss of some simple molecules as water,
alcohol, etc., and lead to the formation of high molecular mass condensation polymers.
• In these reactions, the product of each step is again a bi-functional species and the
sequence of condensation goes on. Since, each step produces a distinct functionalised
species and is independent of each other, this process is also called as step growth
• The formation of terylene or dacron by the interaction of ethylene glycol and terephthalic
acid is an example of this type of polymerisation.
These polymers possessing amide linkages are
important examples of synthetic fibres and are
termed as nylons.
The general method of preparation consists of
the condensation polymerisation of diamines
with dicarboxylic acids and also of amino acids
and their lactams.
Preparation of nylons
• (i) Nylon 6,6: It is prepared by the condensation polymerisation of
hexamethylenediamine with adipic acid under high pressure and at
• Nylon 6, 6 is used in making sheets, bristles for brushes and in textile
• (ii) Nylon 6: It is obtained by heating caprolactum with water at a
• Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.
Preparation of polyesters
• These are the polycondensation products of dicarboxylic
acids and diols.
• Dacron or terylene is the best known example of
polyesters. It is manufactured by heating a mixture of
ethylene glycol and terephthalic acid at 420 to 460 K in the
presence of zinc acetate antimony trioxide catalyst as per
the reaction given earlier.
• Dacron fibre (terylene) is crease resistant and is used in
blending with cotton and wool fibres and also as glass
reinforcing materials in safety helmets, etc.
Preparation of phenol
Also called formaldehyde polymer (Bakelite and related polymers)
• Phenol – formaldehyde polymers are the oldest synthetic
• These are obtained by the condensation reaction of phenol with
formaldehyde in the presence of either an acid or a base catalyst.
• The reaction starts with the initial formation of o-and/or p-
hydroxymethylphenol derivatives, which further react with phenol
to form compounds having rings joined to each other through –
CH2 groups. The initial product could be a linear product – Novolac
used in paints.
• Novolac on heating with formaldehyde undergoes cross linking to
form an infusible solid mass called bakelite. It is used for making
combs, phonograph records, electrical switches and handles of
• Melamine -formaldehyde polymer
Melamine formaldehyde polymer is formed by the
condensation polymerisation of melamine and formaldehyde.
• Copolymerisation is a polymerisation reaction in which a
mixture of more than one monomeric species is allowed to
polymerise and form a copolymer. The copolymer can be
made not only by chain growth polymerisation but by step
growth polymerisation also. It contains multiple units of
each monomer used in the same polymeric chain. For
example, a mixture of 1, 3 – butadiene and styrene can
form a copolymer.
1. Natural rubber
• Rubber is a natural polymer and possesses elastic properties. It is also termed as
elastomer and has a variety of uses. It is manufactured from rubber latex which is a
colloidal dispersion of rubber in water.
• Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-
butadiene) and is also called as cis – 1, 4 – polyisoprene.
• Vulcanisation of rubber: Natural rubber becomes soft at
high temperature (>335 K) and brittle at low temperatures
(On vulcanisation, sulphur forms cross links at the reactive
sites of double bonds and thus the rubber gets stiffened.
The probable structures of vulcanised rubber molecules are
Preparation of synthetic rubbers
• 1. Neoprene
Neoprene or polychloroprene is formed by the free radical
polymerisation of chloroprene.
2. Buna – N
.Buna –N is obtained by the copolymerisation of 1, 3 – butadiene and
acrylonitrile in the presence of a peroxide catalyst.
1. Poly β -hydroxybutyrate – co-β-hydroxy valerate (PHBV)
• It is obtained by the copolymerisation of 3-hydroxybutanoic acid and
3 – hydroxypentanoic acid. PHBV is used in speciality packaging,
orthopaedic devices and in controlled release of drugs. PHBV
undergoes bacterial degradation in the environment.