Statistical co-crystallization of different constitutional repeating units which may either belong to the same copolymer chains (copolymer isomorphism) or originate from different homopolymer chains (homopolymer isomorphism). Isomorphism is a general term: in the strict sense, the crystal structure is essentially the same throughout the range of compositions; in isodimorphism or isopolymorphism, there are two or more crystal structures, respectively, depending on composition.