Polymers are complex and giant molecules and are different from low molecular weight compounds like, say, common salt. To contrast the difference, the molecular weight of common salt is only 58.5, while that of a polymer can be as high as several hundred thousands. These big molecules or "macro molecules"
are made up of much smaller molecules.
- The small molecules, which combines to form a big molecule, could have one or more chemical compounds.
- To make this more feasible, let us imagine a set of rings of the same size and material.
- When these rings are interlinked, the chain formed could be considered as a representation of a polymers of molecules from same compound. Alternatively, the individual rings could be made of varied sizes and materials, and one which are joined to represent a polymer made of molecules from different compounds.
- So, Polymer can be defined as a complex molecule formed by interlinking of many smaller molecules.
- This interlinking of many units has given the polymer its name, 'poly' meaning many and 'mer' meaning part, in Greek.
For example, a gaseous compound, Butadiene, with a molecular weight of 54 combines nearly 4000 times and gives a polymer, known as polybutadiene (a synthetic rubber) with about 200,000 molecular weight.
Butadiene + butadiene + butadiene + ------------ + butadiene → Polybutadiene
One can thus see how a substance with as small a molecular weight as 54 grows to become a giant molecule of 54 x 4000 ≅ 200,000 molecular weight.
It is essentially the 'giantness' of the size of the polymer molecule that makes its behavior different from that of a smaller compound or a monomer.
Polymers, we know, are made up of many small molecules, which are combined to form a single long or large molecule. The individual units of molecules from which a polymer is created or evolved are better known as monomers and the process by which these monomers are linked with one another to form a big polymer molecule is called as 'Polymerization'
- The monomers combining to form a polymer can be of the same type or of different types altogether.
- When the monomer molecules just add up to form the polymer, the process is called as "Addition polymerization".
- The monomer species in this case retains its structural identity when it gets converted into a polymer.
- For example, the molecules of ethylene monomer can add to form polyethylene, in which the structural identity of ethylene is retained.
- When, however, molecules do not just add on, but also undergo some reaction in forming the polymer, the process is called as 'condensation polymerization'. Here, the two molecules, condense to form a polymer. The condensation takes place between two reactive functional groups, like carboxyl group (of an acid) and the hydroxyl group (of an alcohol).
While forming the polymer, however, water molecules also get eliminated. We can, therefore, see that while in addition polymerization, the molecular weight of the polymer is roughly equal to the sum of all monomers combining, in condensation polymerization, the molecular weight is less than the sum of the combining monomers.
Depending upon their origin, polymers can be grouped as natural or synthetic polymers. Natural polymers are those polymers which are isolated from natural resources. Examples:
Cotton, silk, wool and rubber.
These polymers are mostly isolated from plant and animal resources. Though they are processed to get the end product, since the basic material comes from a natural source, these polymers are termed as natural polymers.
Other polymers like cellophane, cellulose rayon, leather, etc
. These are chemically modified versions of natural polymers.
The polymers synthesized from lower mass molecular compounds are better known as synthetic polymers. Synthetic polymers, again can be organic or inorganic polymers. They can also be either thermosetting or thermoplastic polymers.Example:
Polyethylene, PVC, nylon, terylene, etc.PVC or polyvinyl chloride, has a formula of -[CH2 - CH(Cl) -]n-
Some of the important polymers are listed below
||-[-O-(R)-O-C(=O) - (R) - C(=O)-]n-
||-[-NH-(R)x - NH - C(=O) - (R)y-C(=O)-]n-
|Polyvinyl alcohol (PVA)
Polymer is a generic name given to a huge number of materials having high molecular mass. These substances exist in innumerable forms and varieties as these molecules contains a wide range and variety of atoms in their molecules.
Polymers have varied chemical structures, different physical properties like the mechanical and thermal characteristics, etc., and can be classified as mentioned below. Depending upon their origin, polymers are classified as - Natural and syntheic polymers
1. Organic and inorganic polymers
A polymer which has a backbone chain made up of carbon atoms is termed as an organic polymer.
The atoms or molecules attached to the side chains of the main carbon atoms backbone are, however, usually those of hydrogen, oxygen, nitrogen, etc. The majority of synthetic polymer is organic and they are very extensively studied. In fact, the variety of organic polymers are so huge that when we refer these as 'polymers'
, we usually mean the organic polymers.The molecules of inorganic polymers whereas contain no carbon atom in their long chain. Some examples of inorganic polymers are glass and silicone rubber.
2. Thermoplastic and Thermo setting polymers
- Some polymers soften on heating and can be converted into any shape that they can retain on cooling.
- The process of heating, re-shaping and retaining the same even on cooling could be repeated several times over and such polymers, that usually turns soft after heating and then stiffen after cooling down are better known as thermoplastics.
- Polyethylene, PVC, nylon and sealing wax are the examples of thermoplastic polymers.
- Some polymers, on heating invariable undergo chemical changes and then convert themselves into an in fusible mass.
- They are like the egg white, which on heating solidifies and, once set, cannot be reshaped.
- The polymers that become in fusible and insoluble mass on heating, are called 'thermosetting' polymers.