For example, once polyesters form, they cannot readily be purified, and, for purely practical reasons, it is often difficult to attempt to
increase the molecular weight of a polyester once it has
been removed from the reaction vessel. It is therefore
necessary to aim at conditions most likely to yield products
of the required molecular weight with minimum
risk of decomposition or side reactions that could complicate
the structure of the products or induce the formation
of constituents that might hinder the course of
polymerization.