Polymers
2.0 Polymers and polymerization
2.0 Polymers and polymerization
Macromolecules, both natural and man-made, owe their great size to the fact that they are polymers that is, each one is made up of a great many simpler units — identical to each other or at least chemically similar - joined together in a regular way. They are formed by a process called polymerization, the joining together of many small molecules to form very large molecules. The simple compounds from which polymers are made are called monomers.
Polymers are formed in two general ways:
1. In chain-reaction polymerization, there is a series of reactions each of which consumes a reactive particle and produces another, similar particle; each individual reaction thus depends upon the previous one. The reactive particles can be free radicals, cations, or anions.
A typical example is the polymerization of ethylene in which the chain-carrying particles are free radicals, each of which adds to a monomer molecule to form a new, bigger free radical. $$Rad. + C{H_2} = C{H_2} \to RadC{H_2} = C{H_2} - \mathop \to \limits^{C{H_2} = C{H_2}} RadC{H_2}C{H_2}C{H_2}C{H_2} - \mathop \to \limits^{C{H_2} = C{H_2}} ...$$
2. In step reaction polymerization, there is a series of reactions each of which is essentially independent of the preceding one; a polymer is formed simply because the monomer happens to undergo reaction at more than one functional group.
A diol, for example, reacts with a dicarboxylic acid to form an ester; but each moiety of the simple ester still contains a group that can react to generate another ester linkage and hence a larger molecule, which itself can react further, and so on.
