Synthetic materials like nylon ropes, tyres, electrical switches, non sticky coating teflon, toys, foot wear, plastic buckets, polythene bags, paints, naturally occurring starch, cellulose, leather, wool, silk, cotton ... all are made of polymers. In Greek 'poly' means many, 'mer' means part. A polymer is a large molecule (with high molecular mass) which is formed by the combination of large number of small basic units (monomers). The process of formation of the polymer is called polymerization. There is several ways of classification of polymers. Some of the classifications are:
 

¤  Classification based on Source: The polymers which are formed by the nature (plants and animals) are called 'natural polymer's (eg. starch, cellulose). The polymers which are man made are called "Synthetic polymers" (eg: Terylene, Bakelite). If the polymers are derived from naturally occurring materials, they are called "semi synthetic polymers" (eg: cellulose nitrate, cellulose rayon).
 

¤  Classification based on structure: The polymers which contain long and straight chains are called "linear polymers" (eg: P.V.C., polythene) where as polymers with branched chains are "branched chain polymers" (eg: low density polythene, glycogen). Bakelite, melamine are cross linked polymers which contain bi or tri functional monomers.

¤ Classification based on heat treatment:
Thermoplastic polymers: The polymers which are soften on heating. Generally they have linear structure and are formed by addition as well as condensation polymerization. eg: P.V.C, Nylon, polythene.
Thermosetting polymers: The polymers which becomes hard on heating. They have cross linked structure and are formed by condensation polymerization. eg: polysiloxane, Bakelite.
 

¤  Classification based on mechanism:
When condensation takes place between 2 different bi functional or tri functional monomer molecules, "condensation polymer" is formed.

When addition takes place between the same molecules having double or triple bonds, "homo polymers" are formed. If it takes place between two different molecules having double or triple bonds, "co polymers" are formed. Co polymers are classified into Random, Graft, alternating and Block co polymers.

¤  Classification based on molecular forces: Molecular forces like hydrogen bonds, Vander Waals forces between the molecules are responsible for toughness, elasticity, tensile strength of polymers. Following 4 types of sub groups come under this category.
(i)  Elastomers: The polymer that can be stretched on applying force (due to cross links) and returns to its normal shape and size on removal of the force.
   eg: Buna - N, Buna - S, Neoprene.
(ii)  Fibers: The polymer that forms thread like solids due to strong hydrogen bonding.
    eg: Nylon 6, 6, Terylene.
      Thermoplastic polymers and thermosetting polymers also comes under this category.
 

Natural rubber: Joseph Priestley named the solid material as rubber because it is used to erase pencil marks. It is prepared from colloidal milky suspension (called latex) obtained from rubber tree. Latex contains about 35% rubber. Rubber can be coagulated by the addition of HCOOH or CH₃COOH to latex. Further it is refined by break down and compounding.
         Natural rubber is considered as linear polymer of 2 methyl 1, 3 buta diene (isoprene). It is also called as '' cis - 1, 4 polyisoprene". It is hydrocarbon having molecular formula C5H8. The isoprene Units are joined by head - to - tail by 1, 4 links.

The transform of ubber is called "gutta percha". The  of rubber is between 1,30,000 to 3,40,000. It an be stretched like a spring as it has coiled structure due to weak Vander Waals forces.

Natural rubber (cis - 1 , 4 polyisoprene)
Synthetic rubbers are either homo polymers of 1, 3 - buta diene derivatives or co - polymers of 1, 3 - buta diene or its derivatives. eg : Buna - S, Neoprene.
 

Vulcanization of rubber: The word "vulcanization" was coined by Thomas Hamcock. As natural rubber becomes soft at high temperature, brittle at low temperature, has low tensile strength, high water abosbing capacity, soluble in non-polar solvents. So natural rubber is heated with sulphur to improve physical properties.
* "The process of heating natural rubber up to 373 K to 415 K with sulphur to improvise physical properties like elasticity, tensile strength etc is called "vulcanization".

It can be accelerated by the catalysts zinc stearate or zinc oxide. Ebonite (non-elastic) is formed on addition of 40 - 45% sulphur to natural rubber.
            Vulcanization takes place at allylic groups ( –CH₂ group present next to the double
bond) and forms cross links at these centres.

Vulcanized rubber                 Cross linking in Vulcanization
 

Bio - Polymers: Carbohydrates, proteins, nucleic acids are essential for living systems, one type of carbohydrates, poly saccharides are polymers of D - glucose. Starch, cellulose, glycogen (animal starch), Dextrin are polymers of D - glucose. There are formed due to different modes of linkage between D - glucose units. Proteins are high molecular weight complex bio polymers of amino acids present in all living cells.

They are formed through peptide linkages between amino acids.
Bio - degradable polymers: As polythene like polymers cause many environmental problems, new biodegradable synthetic polymers are designed and developed.
PHBV: Poly β - hydroxy butyrate - Co - β - hydroxy valerate (PHBV) is a copolymer of 3 hydroxy butanoic acid and 3 - hydroxy pentanoic acid. It is formed due to condensation of these 2 monomers

  The excess amount of 3 - hydroxy butanoic acid makes PHBV tougher while excess amount of 3 - hydroxy pentanoic acid makes it more flexible. PHBV is used to make capsules, used in orthopedic devices, and in speciality packaging.
Polyglycolic acid and poly lactic acid: Poly glycolic acid poly lactic acid are formed due to polymerization of the monomers glycolic acid and lactic acid.
Nylon - 2 Nylon - 6:  It is a biodegradable alternating poly amide copolymer, formed due to condensation of glycine and amino caproic acid
                   n H₂N − CH₂ − COOH + n H₂N − (CH₂)₅ − COOH
                                                         ↓
                   [ − HN − CH₂ − CO − NH − (CH₂)₅ − CO − ] _n + 2n H₂O
                                                   Nylon-2  Nylon-6
Molecular weights of polymers: The properties of a polymer changes as the number of monomer Units per polymer molecule varies from sample to sample so several types of molecular weights are used. They are  (Z - average molecular weight),  (viscosity average molecular weight),  (number average molecular weight) and  (Weight average molecular weight).

Number average molecular weight can be determined chemically by and group analysis method or physically by colligative properties.
                    
The ratio of  and  is known as "poly dispersity index" (PDI). PDI for natural
polymers is equal to unity. Where as for synthetic polymers PDI > 1.
                                                 

Summary of some important polymers