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INTRODUCTION:�

The term polymer is derived from two greek words poly means many and mers means parts.
Thus Polymers are composed of large number of repeating units or molecules called monomers. thus Polymer is made up of thousands of monomers joined together to form large molecule called macromolecule. The unique characteristic of a polymer is that a molecule is either a long chain or a network of repeating units which are covalently bonded together.
Polymers are molecular materials and are generally noncrystalline at ordinary temperatures. The common polymers are made from compounds of carbon. Some polymers are made from inorganic chemicals such as silicates and silicones. The naturally obtained polymers includes proteins, cellulose, starch etc. they are commonly found in leather, wool, cotton, silk, rubber etc. there are also synthetic polymerssuch as polyethylene, polystyrene, nylon, terylene, Dacron etc

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POLYMERIZATION MECHANISM:

Polymerization is the process of linking of monomers together to form Polymer. Polymerization process start with breaking of double bonds (C=C) of monomers which requires large energy to start reaction.

Monomers link together to form polymers as follows

Monomer (Ethylene C₂H₄) Polymer ( Polyethylene)

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The Polymerization mechanisms of monomers are achieved by two ways

Addition Polymerization
Condensation Polymerization

Addition Polymerization:

It is also known as growth polymerization

The addition of same kind of monomers are known as homopolymerization

A+A+A+A+A+A+A = -A-A-A-A-A-A-A

The addition of different kinds of monomers is known as co-polymerization

A+B+A+B+A+B = A-B-A-B-A-B

In case of addition polymerization, the covalent bonds of monomers are broken and monomers are combined together by single bonds

The conversion of propylene into polypropylene is one of the example of addition polymerization.

H H H H H H

2( C = C) = C - C - C – C –

H H H H H H

Propylene Polypropylene

Similarly in co-polymerization two or more polymers are joined to form single chain

Styrene (A) and Butadiene (B) molecules are joined to produce Synthetic rubber

Styrene (A) + Butadiene (B) = A-B-A-B

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Condensation Polymerization:�

It is stepwise process. Here molecules are brought together to interact and hence produce polymer and byproduct instead of breaking carbon double bonds. Byproduct is water, hydrogen, chloride etc.
The simple example of condensation polymerization is to produce Bakelite from phenol and formaldehyde

+ = + H₂O

Phenol formeldehyde bakelite

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Comparison between Homopolymers and Co-Polymers�

Homopolymers	Copolymers
These polymers have same type of monomer units.	This is also known as heteropolymer as they are composed of two different kinds of monomers.
They can be classified as linear, branched, cross linked and network homopolymers.	They can be classified as alternating copolymer, block copolymer, graft copolymer and random copolymers.
Linear homopolymers have linear long polymeric chain of same types of monomer units. For example; HDPE	In an alternating copolymer, the two monomers are arranged in an alternative way and can be represented as ABABABABAB.
Branched homopolymers have short or long branches bonded on parent polymeric chain such as LDPE.	In random copolymer the monomers are arranged in any order such as AABAAABBBBAB.
Cross linked and network homopolymers have braches on parent chain which are interconnected with each other to form cross linked and network polymer.	In block copolymers, two blocks of homopolymers are joined together.It can be represented as AAAAAAABBBBBBB.
Short term stiffness	Long term stiffness
Short term impact strength	Long term impact strength
Example: Polyethylene, Poly(vinyl chloride), Polypropylene, Polystyrene	Example: Nylon-6,6, polyester, protein, Buna-S

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DEGREE OF POLYMERIZATION:�

Degree of polymerization is the number of monomer units in the polymer.

It is defined as the ratio of molecular weight of a polymer and molecular weight of the repeat or monomeric unit.

Molecular weight of the polymer

i.e DP =-------------------------------------------

Molecular weight of the monomeric unit

M_n

DP =--------

M_o

Where M_n and M_o are average molecular weight of the polymer and molecular weight of the repeated unit respectively.

The average molecular weight of the polymer is given by

M_n = Σ n_iM_i

where M_i is molecular weight of its fraction and n_i is the number of fraction of molecules.

Degree of polymerization will be useful for understanding the different types of polymerization mechanisms.

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Defects in the Polymers:�

The point defect concept in polymers is different than in metals and ceramics. In polymers point defects are observed in crystalline regions. Chain ends are considered as a defects because they are chemically dissimilar. Also vacancies at chain ends are also defect.

Figure below represents point defects in polymer materials.

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�Defects in the Polymers:�

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HOMOPOLYMER AND COPOLYMER

On the basis of origin, polymers can be classified as natural and synthetic polymers. On the basis of chemical nature, they can be classified as organic and inorganic polymers. Organic polymers have carbon atoms in the polymer backbone of chain. For example; polypropylene, poly (vinyl chloride) are organic polymers because the polymeric backbone is composed of carbon atoms. Inorganic polymers do not have carbon atoms in their polymeric chain such as Silicone rubbers.

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On the basis of monomer composition, polymers can be classified as homo-polymers and co-polymers.

Homopolymers can be defined as the polymers which have one type of constituent monomer units in polymeric chain. They can be represented as;

�

-A-A-A-A-A-A-A-A-A-A-A-A-

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Copolymers are composed of two or more different types of monomer units and generally represented as;

�

-A-B-A-B-A-B-A-B-A-B-

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Homopolymers can be linear, branched or cross-linked which depends on the linkage between monomer units.

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Homopolymer Examples�Polyethylene: It is one of the most common examples of Homopolymer which is composed of ethylene monomer units. �

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Poly (vinyl chloride): It is commonly known as PVC. It is a thermoplastic which can be mold in different shapes. It is supplied in powder form which shows resistance against oxidation and degradation. It is a chain growth polymer of vinyl chloride (CH2=CHCl).

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Mechanical Properties of Polymers

Specific gravity of Polymers lies between 0.09 to 0.3 which is less than metals ( 0.3 to 12)
Thermal conductivity of polymers is very low.
Thermal expansion of polymers is five times higher than metals.
Modulus of rigidity of polymers is low.
Most of Polymer plastics becomes brittle and yellow, when exposed to sunlight for longer duration.
They are good electrical insulators.
The stress-strain relation relationship in a polymers is not linear like that of metals.
Polymers can be deform elastically and plastically

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Deformations:�

The change in shape and size of the material when external forces are applied is called deformation.

There are two types of deformations

1)Elastic deformation and 2) Plastic deformation

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Elastic Deformation�

The reversible strain produces in the material is called elastic deformation.
Elastic deformation produces temporary change in shape of material.
After removal of load or force the specimen comes to it’s original dimensions.
Specimen is elastic up to the yield point.
Due to elastic deformation ductility of material do not changes.
Due to elastic deformation there is no change in strength and hardness of material.
Examples of elastic materials are springs, metal bar etc.

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Plastic Deformation�

The sufficient stress produces permanent deformation is called plastic deformation.
Plastic deformation produces permanent change in shape of material.
After removal of load or force the specimen does not comes to it’s original dimensions.
Specimen becomes plastic beyond yield point.
Due to elastic deformation ductility of material do not changes
Due to elastic deformation there is increase in strength and hardness of material.
Examples of plastic materials are polythene bag, stainless steel, aluminium alloy etc

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Reinforced Polymers:�

Fibre-reinforced plastic (FRP) (also called fiber-reinforced polymer, or fiber-reinforced plastic) is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass (in fibreglass), carbon
(in carbon fiber reinforced polymer), Other fibres such as paper, wood, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.
FRPs are commonly used in the aerospace, automotive, marine, and construction industries.

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Applications of Polymers

In view of their light color, light weight, easy shapability and low cost, plastic materials occupies many of the household articles ( chairs, buckets, pens, toys etc.).
Phenolics ( Bakelite), a kind of synthetic polymer has been widely used as insulating components in generators, transformers and switch gears in heavy electrical engineering.
Bakelites are also used in printing circuit boards in electronics and telecommunication industries.
Phenolic resigns are used as varnish in all electrical windings in all types of motors.
Teflon, a good insulator, finds application both in high and low temperatures.

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Applications of Polymers

6) Many of the fibrous cheap materials such as wood, silk, cotton are used as insulating materials.

7) Due to improved mechanical strength and thermal resistance, rubbers are used as insulators in cables.

8) Plastics and polyesters are used in chemical engineering, building construction, marine, road transportation, dairy and agriculture industries.

9) In medical field polymers like Dacron, Teflon, and Polyurethane are used for replacement of heart valves, blood vessels.

10) Play ground equipments, various balls, golf clubs,

Swimming pools and protective helmets are produced from polymers.

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