Concept of product by process

Small molecules are typically specified just by molecular formula and purity. Simple petrochemicals such as methanol and toluene need little more than a boiling point while the specification of analytical chemicals may require a list of measured impurities. Stabilizers may be used and thus require specification in reactive materials such as styrene. It is rare, however, that the process used to manufacture these small molecules is specified as part of the product description. The few exceptions are typically used in food and pharmaceutical applications. 

For polymers, it is quite common to name the process as part of a general description. Terms like emulsion ABS, solution PVC and bulk polystyrene abound in the technical literature. Even when unstated, there may be an implicit understanding of the underlying process. Thus LDPE and LLDPE denote somewhat similar materials made by different processes. 

There are myriad polymerizable materials and polymerization mechanisms. Here we discuss only a few that we hope will illustrate general principles. A first classification of mechanism gives two general types, addition polymerizations (also called chain-growth polymerizations) and step-growth polymerizations. 

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