Need for Stoichiometric Control

2-6 MOLECULAR WEIGHT CONTROL IN LINEAR POLYMERIZATION 2-6a Need for Stoichiometric Control 

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer, a monomer with only one functional group. Acetic acid or lauric acid, for example, are often used to achieve molecular weight stabilization of polyamides. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction. Thus, the use of benzoic acid in the polyamide synthesis yields a polyamide (XI) with phenyl end groups that are unreactive toward polymerization. 

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