A prevalent but erroneous notion is that useful polymer, such as those given in Table 29-1, can be, and are, made by slap-dash procedures applied to impure starting materials. This is far from the truth; actually, the monomers used in most large-scale polymerizations are among the purest known organic substances. Furthermore, to obtain uniform commercially useful products, extraordinary care must be used in controlling the polymerization reactions. The reasons are simple - namely, formation of a high-molecular-weight polymer requires a reaction the proceeds in very high yields, and purification of the product by distillation, crystallization, and so on, is difficult, if not impossible. Even a minute contribution of any side reaction that stops polymer chains from growing further will seriously affect the yield of high polymer.