The process of linking together of monomers, that is, of obtaining macromolecules is called “‘polymerization”. It can be achieved by one of the two processing techniques. (a) Addition Polymerization: In addition or chain polymerization under suitable conditions of temperature and pressure and in the presence of a catalyst called an initiator, the polymer is produced by adding a second monomer to the first, then a third monomer to this dimer, and a fourth to the trimer, and so on until the long polymer chain is terminated. Polyethylene is produced by the addition polymerization of ethylene monomers. This linear polymer can also be converted to a branched polymer by removing a side group and replacing it with a chain. If many such branches are formed, a network structure results.
“Co-polymerization” is the addition polymerization of two or more different monomers Many monomers will not polymerize with themselves. but will copolymerize with other compounds
(b) Condensation Polymerizations:
In this process, two or more reacting compounds may be involved and there is a repetitive elimination of smaller molecules, to form a by-product. For example, in the case of phenol-formaldehyde (bakelite), the compounds are formaldehyde and phenol. Meta cresol acts as a catalyst and the by-product is water. The structure of the ‘Mer’ is more complex. Also, there is a growth perpendicular to the direction of the chain. This is called cross-linking’.
Size of a Polymer:
The polymer chemist can control the average length of the molecules by terminating the reaction. Thus, the molecular weight (the weight of the average molecule, in grams, of 6.02x molecules) or degree of polymerization, D.P., (the number of members in the average molecule) can be controlled. For example, the length of molecules may range from some 700 repeat units In low-density polyethylene to 1,70,000 repeat units in ultrahigh-molecular-weight polyethylene.
These plastics undergo a number of chemical changes on heating and cure to infusible and practically insoluble articles. The chemical change is not reversible Thermosetting plastics do not soften on reheating and can not be reworked. They rather become harder due to the completion of any left-over polymerization reaction. Eventually, at high temperatures, the useful properties of the plastics get destroyed. This is called degradation. The commonest thermosetting plastics are alkyds, epoxies, melamines, polyesters, phenolics, and ureas. for a more related topic click on it Mechanical engineering.