A polymer is a large macromolecule of high to very high molecular weight which consists of many repeating units called monomers which are covalent bond to one another. In many cases, monomers are linked together in a row, like links in a chain. However branching and cross-linking between chains does occur. Monomers can even be bonded together in a two- or three-dimensional polymer network.
Polymers are very abundant on the surface of earth; either in nature or artificially prepared. We come across polymers each and every minute we live. Artificially produced polymers include plastics, elastomers, fibers, and materials of intermediate characteristics. When you get up in the morning and brush your teeth, the toothbrush you have is made of plastic-that is a polymer. Its bristles are fibers-again a polymer.
We use plastic bags, cards, paper, wood, clothing, paints, and endless items that have the basic chemical structure of a polymer. Human and animal bodies are mainly made up of protein. Proteins are the building blocks of life. They are polymers too, with α-amino acids as repeating units linked together with peptide bonds.
A polymer can be classified into a homopolymer, which consists of only one type of monomer, or a copolymer (sometimes called a heteropolymer), which consists of two or more types of monomers.
To create polymers from monomers (single units), a chemical reaction called polymerization (linking of monomers) occurs.
There are two types of polymerizations, that of addition polymerization and of condensation polymerization.
The new broad terms for addition and condensation polymerizations are step-growth polymerization and chain-growth polymerization. This is the case because nowadays polymerization is also carried out by ring-opening or by complex mechanisms other than addition or condensation.