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Through x‐ray and infrared examinations of stereoregular polymers and copolymers of vinylaromatic monomers, the occurrence of chain isomorphism phenomena, together with many cases of a new type of isomorphism between monomeric units, have been proved. Isomorphism of macromolecules, allowing true solid solutions, has been observed when melting and crystallizing mixtures of homopolymers (i.e., isotactic polystyrene) and of crystalline copolymers (i.e., styrene‐p‐methylstyrene) having the same identity period and differing very little in lattice constants. Isomorphism among monomeric units occurs in copolymerizing monomers that have a chemical nature and shape slightly different one from the other, e.g., styrene and o‐fluorostyrene. This allows the formation of crystallizable copolymers in the entire composition range. They show physical properties (lattice constants, melting temperatures, etc.) continuously varying between those of the pure homopolymers. The occurrence of isodimorphism phenomena has been observed in crystalline copolymers obtained from monomers the homopolymers of which have different crystalline structures (e.g., styrene‐p‐fluorostyrene), and also in copolymerizing styrene with monomers (e.g., p‐methyl, p‐chlorostyrene) giving only amorphous homopolymers. In the latter case, of course, the copolymers show crystallinity only in a limited composition range.

High molecular weight vinyl fluoride (VF)–vinylidene fluoride (VF2) and vinyl fluoride (VF)–tetrafluoroethylene (TFE) copolymers in a wide range of compositions have been synthesized at low pressure at 25°C. with the aid of a catalytic system consisting of triisobutyl boron activated with molecular oxygen. With the same catalytic system, highly crystalline homopolymers of VF and VF2, melting above the usually reported temperatures, have been obtained. The x‐ray and thermal analyses of crystalline molecular mixtures of PVF and PVF2, as well as of VF–VF2 copolymers, have shown that in both cases the monomeric units of the two types may be present in the same crystallites, giving rise to a phenomenon very similar to the isomorphism in the case of low molecular weight substances; an analogous phenomenon was already described by some of us1 for other polymeric systems. In the actual case we have observed the isomorphism between the VF and VF2 monomeric units in all the range of compositions, both for the homopolymer mixtures and for the copolymers. The VF–TFE copolymer samples, in the range of compositions examined (0–75% in TFE) show a crystal structure essentially similar to that of PVF. Due to the fact that the crystal structure of PVF is not strictly analogous to that of PVFE, in the compositions containing 75–100% TFE units a phenomenon of isodimorphism, or isopolymorphism, probably takes place. Over a wide range of compositions of the monomeric reactant mixture, a marked tendency for formation of alternating (...ABAB...)sequences within the copolymer chains has been observed.

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