Weight of Step-Growth Polymer

The molecular weight of a polymer is of prime concern from the practical point of view, for unless a polymer is of sufficiently high inolecular weight ( 10,000) it will not have the desirable strength characteristics. It is important to consider the main factors that affect the polymer molecular weight. 

The data in Table 5.4 reveal that Xn, and hence molecular weight, increases very rapidly with conversion in the high conversion range (p 0.99). An increase in con rsion from 0.990 to 0.999, for example, leads to a 10-fold increase in X thus, it is not practical to try to control polymer molecular weight by adjusting the level of conversion in factory scale step-growth polymerization. An alternative procedure would be to provide, deliberately, an imbalance in the ratio of the two types of functional groups in the feed. 

For example, the use of excess diol in the polymerization of a diol with a diacid [Eq. (5.1)] yields a polyester (VI) with hydroxyl end groups which are incapable of further reaction, since the diacid has been completely reacted  

The use of excess diacid accomplishes the same result the polyester (VII) in this case has only carboxyl groups at both ends when the diol is completely reacted  

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer. The monofunctional monomer limits the polymerization of bifunctional monomers because its reaction with a growing polymer chain produces chain end devoid of a functional group and hence incapable of further reaction. For example, the use of benzoic acid in reaction (5.5) yields a polyamide (VIII) with nonreactive phenyl end groups thereby limiting the polymerization  

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