7 - Constitutional Regularity in Linear Condensation Polymers

Abstract

Constitutional isomerism in chemical compounds arises from differences in the nature or sequence of bonding (e.g. Me2O is a constitutional isomer of EtOH). Linear polymer chains give rise to a constitutional isomerism if they contain ‘nonsymmetric’ monomeric units such that arrangements with different mutual orientations can be distinguished (a sufficient condition for this is that the ‘nonsymmetric’ units have no two-fold rotation axes unless they are parallel to the polymer backbone). This type of structural isomerism has been known to exist in vinyl polymers for almost 50 years1 and has been well studied; the terms ‘head-to-tail’ and ‘head-to-head/tail-to-tail’ are associated with it. For vinyl polymers it has been learned that constitutional isomerism is generally not controllable, since vinyl polymerization is almost always characterized by a high regioselectivity,2–4 which forces nearly perfect head-to-tail arrangements of monomeric units. It is possible to obtain regular head-to-head/tail-to-tail alignment by special synthetic routes (see, for instance, work by Vogl and collaborators5, 6). The properties of head-to-head/tail-to-tail vinyl polymers were observed to be remarkably similar to those of the usual head-to-tail versions, but for a vinylidene polymer stark differences in properties (such as melting point and glass transition temperature) were observed,6 thus the question of the potential influence of this type of isomerism on properties cannot be answered definitely.