Polymer Chemistry (DSE - I, Old Syllabus Odisha) (Sem V Kerala University) (Core XVII, New Education Policy)

Notes on Polymer Chemistry for BSc Honours

Introduction:

The word polymer comes from the greek words 'poly' meaning many and 'meros' meaning parts. Thus a polymer is a large molecule made up of smaller units called monomers that are chemically linked together.

n CH₂=CH₂ ----------> -(CH₂-CH₂)-_n 

    These polymers can found in nature or can be synthetically prepared or man-made. Some examples of natural polymers are DNA, Proteins, polysaccharides, cellulose, polypeptide and natural rubber. Similarly some examples of man made polymers include polythene, PVC, Polyamides, Nylon etc.

Difference between polymers and macromolecules:

We have already defined polymers which necessarily contain repeating monomer units. For example: Polythene contains the repeating unit of ethene monomer unit.

    Macromolecules have a bigger molecular structure and higher molecular weight but have no repeating units. For example: fat molecules contain glycerol and different fatty acids.

 Degree of polymerization (DP):

The number of monomer unit or repeating unit in a polymer molecule is called the degree of polymerization. The molecular weight of a polymer is equal to the product of molecular weight of monomer unit and degree of polymerisation.

    The DP is an important characteristics  of a polymer that affects its physical properties. A higher DP indicates better mechanical property  such as strength and durability.

Classification of polymers:

1.Based on origin:

a.Natural polymers:

The polymers obtained from natural sources are called natural polymers or biological polymers. Examples include: Rubber, DNA, Proteins, polysacharides, cellulose, polypeptide etc.

b.Synthetic polymers:

The polymers which are prepared in laboratory or industry from low molecular weight molecules are called synthetic molecules. Examples include: Nylon, PVC, polyethylene etc.

2.Based on composition:

a.Homopolymers:

A polymer which is formed by the same repeating units or monomer is called a polymer. Example: PVC (ployvinylchloride) is formed by the repetition of single monomer unit called vinyl chloride. Other examples are polystyrene, polypropylene etc.

b.Copolymers: 

A polymer which is formed by the repetition of two different monomeric units is called a copolymer. 

    A copolymer can further be classified as random or alternating or block or graft copolymers.

Random Copolymers: Where two different monomer units are arranged randomly in a polymer chain without any particular sequence. Like A-B-B-A-A-A

Alternating copolymers: Where the two different copolymers are aranged in alternating  manner in a polymer chain. A-B-A-B-A-B

Block copolymer: Where two different monomer units make a group and make the polymer chain like: A-A-A-B-B-B-A-A-A-

Branch Copolymer: Where side chains grow from the main polymer chain or from a common point of the polymer. Branched copolymers can further be classified as Star copolymer, Comb copolymer and Brush copolymer which can be represented in the following way:

3.Based on Mechanism:

a.Addition polymers: 

These polymers are formed by the repeated addition of monomer units without the elimination of any byproduct such as HOH or ROH etc. Examples include PVC, PE, PA etc.

n CH₂=CH₂ ----------> -(CH₂-CH₂)-_n 

Here ethylene molecules repeat to form polyethylene (PE).

In these polymers the molecular weight of the polymer is an integral multiple of the molecular weight of the monomer unit.

b.Condensation polymers:

These polymers are formed by the condensation of monomer units with the elimination of simple molecule such as HOH or ROH etc. Examples include PET, Nylon 6,6 etc.