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Title: chapter #15: polymer
Description: these notes are related to polymers which lies under the subject chemistry.
Description: these notes are related to polymers which lies under the subject chemistry.
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Chemistry Notes for class 12 Chapter 15
Polymers
The word polymer has a Greek origin
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Polymer is defined as a
chemical substance of a high molecular mass formed by the combination of a large number of
simple molecules, called monomers
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g
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is known as
polymerisation
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A macromolecule mayor may not contain monomer units, e
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, chlorophyll (C55H72O5N4Mg) is
a macromolecule but not a polymer since there are no monomer units present so we can
conclude that all polymers are macromolecules while all macromolecules may not be polymers
in nature
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i
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, in plants or animals
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(ii) Synthetic polymers The polymers which are prepared in the laboratory are known as
synthetic polymers or man-made polymers, e
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, polythene, synthetic rubber, PVC, nylon-66,
teflon, orlon etc
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ncerthelp
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g
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Classification of Polymers Based on Structure
(i) Linear polymers These are the polymers in which the monomer units are linked to one
another to form long linear chains
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The close
packing results in high densities, tensile strength and high melting and boiling points
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g
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(ii) Branched chain polymers In such polymers, the monomer units are linked to form long
chains with some branched chains of different lengths with source
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Thus, they have low densities, low tensile
strength as well as low melting and boiling points
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(iii) Cross-linked polymers or network polymers In such polymers, the monomer units are
linked together to form three dimensional network
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Examples of cross linked polymers are bakelite, glyptal, melamine-formaldehyde
polymer etc
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Addition
polymers have the same empirical formula as their monomers
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Copolymers The polymers which are obtained by the polymerisation of two or more
monomers are called copolymers
(ii) Condensation polymers The polymers which are formed by the combination of monomers
with the elimination of small molecules such as water, alcohol, hydrogen chloride etc
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g
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Classification of Polymers Based on Molecular Forces
1
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g
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The
weak binding forces permit the polymers to be stretched
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2
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These are widely used for making clothes, nets, ropes, gauzes, etc
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The fibres are crystalline in nature and have sharp melting
points
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3
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These forces are intermediate of the forces present in the
elastomers and in the fibres
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Thus, they can be cast into different shapes by using suitable
moulds, e
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, polyethene and polystyrene
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These are added to I plastics to make
them soft rubber like
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Thermosetting plastics These are normally semifluid substances with low molecular
masses
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As a result, they also become three dimensional in nature
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Types of Polymerisation
1
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For this polymerisation monomers used are unsaturated compounds like alkenes; alkadienes
and their derivatives
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chain growth
polymerisation occurs by the following mechanisms:
Free radical addition polymerisation
Cationic polymerisation
Anionic polymerisation
(i) Free radical addition polymerisation The monomers used are generally monosubstituted
alkenes
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Mechanism The reaction involves the following steps
Step I Chain initiation step In this step, peroxide undergoes homolytic fission, e
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, benzoyl
peroxide on heating produces phenyl initiator free radical
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Step III Chain termination step There are three ways of chain termination: Coupling
reaction, disproportionation reaction, chain transfer reaction
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) and protonic acids such as H2SO4, HF, etc
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It involves the following steps:
Step I
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Step Growth Polymerisation
Condensation polymerisation which occurs in a stepwise manner with elimination of some
smaller molecules like H2O, NH3, HCI, ROH, etc
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g
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,
undergo step Growth Polymerisation
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Thus, the polymer sample contains chain of varying lengths and hence, its molecular
mass is always expressed as an average molecular mass
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It is determined by light scattering and ultracentrifugation method
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In
other words, such polymers are more homogeneous
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Polyolefins
These are obtained by the addition polymerisation of ethylene and its derivatives
1
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(i) Low density polythene (LDP)
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It is tough, flexible, transparent, chemically inert as well as poor conductor pf electricity
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It is used in the insulation of electricity carrying wires and manufacture of queeze bottles, toyes
and flexible pipes
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It is also chemically inert and more tougher and
harder
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2
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It is thermoplastic
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Polyvinylchloride (PVC)
It is used for making rain coats, toys, electrical insulation
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4
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It is chemically inert and resistant to attack by corrosive reagent
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6
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Polyamides
The polymers which contain an amide linkage in chain are known as pOlyamide, e
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, nylon-6,
6
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Nylon-66
It is obtained by the condensation of adipic acid and hexamethylenediamine with the
elimination of water molecule
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The polyamides are identified by numbers
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As in the above case, the carbon atoms are 6 in each case,
therefore the product is described as _nylon-66
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It shows good resistance to
abrasion
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It is used in bristles for brushes and in
textile
2
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Resins
1
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ncerthelp
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The reaction involves the formation of methylene
bridge at ortho, para or both ortho and para positions
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Uses
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Bakelite is used for making combs, photograph records, electrical switches etc
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2
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It has assumed great importance these days particularly in making
crockery
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3
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Natural Rubber
Natural
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In the polymer chain of natural rubber, the residual double bonds are located between C2 and
C3 of the isoprene unit
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In the natural rubber, there is no polar substituent
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The cis-configuration gives the polymeric chain of natural rubber a coiled
structure
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When the force is
removed, the chain returns back to its original coiled shape
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It can be used only in the temperature range 10°C-50°C
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It has higb water
absorption capacity
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As such, it cannot
be used very extensively for commercial puposes
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This process of introducing -S-S- crosslnks in the structure of natural rubber by
heating with sulphur at 11O°C is called vulcanlsation of rubber
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Zinc oxide accelerates the rate of
vulcanisation
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These crosslinks tend to limit the motion of chains relative to each other
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Neoprene
Polymer formed by polymerisation of chloroprene is neoprene or synthetic rubber
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6
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It is formed as follows
Properties and Uses
It is insulator in nature and is used for making conveyor belts and printing rollers
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g
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1
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The polymer is known by several commercial names such as lucite, acrylite, plexiglass and
perspex
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It is used, in the manufacture of unbreakable lights, protective coatings, dentures, and in
making windows for aircrafts
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Glyptal
It is a polyester having crosslinks
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It is obtained by condensation of
ethylene glycol and phthalic acid or glycerol and phthalic acid
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It
is, therefore, used in the manufacture of paints and lacquers
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Terylene (Dacron)
It is a condensation product of ethylene glycol and terephthalic acid
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Properties and uses
Terylene is highly resistant to the action of chemical and biological agents
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It can also be blended with wool or cotton to obtain fabrics of desired
composition
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It is also used for preparing magnetic recording
tapes, conveyer belts, aprons for industrial workers etc
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e
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g
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Nature has provided us a variety of polymers which can be produced by the biological systems
in plants and animals
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g
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In the biological system, these polymers decompose or hydrolyse in the Presence of
different enzymes
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Aliphatic polyesters are the common examples of biodegradable Polymers
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2
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Some More Impotant Polymers
1
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2
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3
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4
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These are the copolymer
of epichlorohydrin and bisphenol-A
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Thikol is another variety of synthetic rubber which is a copolymer of ethylene chloride
and sodium tetrasulphide (Na2S4)
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Dynells a copolymer of vinyl chloride and acrylonitrile and is used for making human
hair wigs
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Silk Is a thread like natural polymer which is obtained from cocoons of sllk worms
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8
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Title: chapter #15: polymer
Description: these notes are related to polymers which lies under the subject chemistry.
Description: these notes are related to polymers which lies under the subject chemistry.