Define polymer in brief

A polymer is a large molecule (macromolecule) formed by joining many small repeating units called monomers by chemical bonds. Its relative molecular mass is usually very high, in thousands to millions. Examples: polyethylene from ethylene, PVC from vinyl chloride, and natural polymers like rubber, cellulose and protein.

• Polymerization is the chemical process of joining monomers into the polymer, by addition (e.g. ethylene to polyethylene) or condensation (e.g. phenol and formaldehyde to Bakelite, with water released).
• Polymers can be natural (rubber, wood, cellulose) or synthetic (plastics, synthetic fibres, resins).
• Based on structure they may be linear, branched or cross-linked, which decides whether they are thermoplastic or thermosetting.

 n [CH2=CH2]  --heat,pressure,catalyst-->  [-CH2-CH2-]n
   ethylene                                  polyethylene