Macromolecular isomorphism

Bunn and Peiser (3) first recognized macromolecular isomorphism in synthetic materials in the case of the ethylene/vinyl alcohol copolymers and in polyvinylalcohol itself. Successively, they suggested this possibility also for natural rubber. Subsequently, many other examples of macromolecular isomorphism were described. We shall see in the following that they refer mainly to stereoregular vinyl polymers and copolymers, fluorinated polymers and copolymers, copolyamides, and polyesters. In this review we shall refer only to synthetic materials, excluding therefore such important examples of isomorphism as those occurring in polypeptides and polynucleotides. 

As well known, the crystalline structure of polymers is generally characterized by a comparatively high proportion of defects compared with the case of low molecular weight substances these defects may be due either to chemical faults or may be simply attributed to the mechanism of crystallization. As a consequence, this means that most polymers can contain a small proportion of extraneous units in the crystal state. However, we will not consider as real cases of macromolecular isomorphism those having a concentration of either component below 5%. 

An interesting phenomenon of macromolecular isomorphism was detected in copolymers of a-butene with a-M-hexene, a-M-octene, a-n-decene, a-w-dodecene, and a- -octadecene (<5). In these copolymers isomorphous replacement occurs between the butene units and the methylene sequences of the side groups in both the known modifications I and II of poly-a-butene. 

In addition to the general steric requirements reported in the introductory section for macromolecular isomorphism, if chains differ in chemical structure, they must also show some degree of compatibility to intimate mixing and not too much different crystallization kinetics. The first condition is strictly similar to the one that applies to liquid mixtures. As a well known example, liquids without reciprocal affinity in general cannot form a unique phase. Attempts to obtain mixed crystals from polyethylene and polyvinyl or polyvinylidene fluoride has been unsuccessful hitherto, in spite of the similarity in shape and size of their chains. In view of the above somewhat strict requirements, it is not surprising that relatively few examples of this type of isomorphism have been reported. 

Although this phenomenon may be included in the general class of structural disorder in crystalline polymers, we think convenient to mention it here, because of its close relation to macromolecular isomorphism in the usually accepted meaning. 

A trivial case of macromolecular isomorphism involves the mixing of species differing only in an isotope, for example, as isotactic polypropylene and isotactic polydeuteropropylene (Natta et ai, 1958). More interesting examples can be realized by melting together such polymers as poly(vinyl fluoride) and poly(vinylidene fluoride) (Natta et a/., 1971) or poly(isopropyl vinyl ether) and poly(sec-butyl vinyl ether) (Allegra and Bassi, 1969) that form isomorphic pairs at all relative compositions. 

Several vinyl copolymers show macromolecular isomorphism when the side groups are sterically ordered. If, asi it always happens in the examples known hitherto, the order is of the isotactic type, we have called them isotactic (copolymers). 

For a discussion of macromolecular isomorphism, see Refs 2,90,91, and some of the papers quoted therein. 

G. Allegra and I. W. Bassi. Isomorphism in synthetic macromolecular systems ,... 

Allegra, G. and Bassi, J. W. Isomorphism in Synthetic Macromolecular Systems. 

Isomorphism in macromolecular systems may be defined as the statistical substitution, within a single crystalline phase, between monomer units differing either in chemical structure or in conformation or in configuration. The distribution of the different monomer units needs not to be totally random. As we shall see, there are examples of isomorphous systems consisting of a mixture of different homopolymer chains. In these cases the randomness is confined to the macromolecules... 

The most important conditions to be fulfilled in order to have isomorphism in a macromolecular system are ... 

From the preceding sections it is clear that isomorphism of monomeric units in synthetic copolymeric systems is a quite general phenomenon. We wish to recall here that the requirements for the isomorphous substitution in the macromolecular field are similar to those holding for solid solutions in ionic or metallic crystals however, the degree of... 

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