The genetics in polymers: Crystallization as a fingerprint of the molecular microstructure

Hidden details of the molecular structure of polymers can be revealed from the retro-analysis of the crystallization behavior. The way a polymer crystallizes and the selection of a specific polymorphic form upon crystallization contain all the information about the structure of macromolecules. The intriguing challenge is how to extract this information from the formed crystals. The key is the construction of a model and the availability of model systems of polymer crystallization based on the exploitation of the phenomena of polymorphism and isomorphism and on the understanding of how such phenomena are affected by the presence of microstructural defects. The explanatory case study of isotactic polypropylene (iPP) prepared with Ziegler-Natta catalysts is here described. iPP shows an intricate polymorphic behavior, which may be successfully exploited to build models of the crystallization behavior and reveal hidden details of the complex molecular structure dictated by the heterogenous multi-site Ziegler-Natta catalysts. A model of the crystallization behavior of iPP has been defined through deep analysis of the crystallization of iPP and its copolymers synthesized with soluble organometallic catalysts that produce a simple molecular structure with perfectly random distribution of defects. In these model systems the crystallization of the α and γ forms depend only on the average length of the regular propene sequences that, in turn, depends on the concentration of defects. A precise correlation between the amount of crystallized γ form and the average length of regular isotactic propene sequences is the basic of this model. This correlation can be used in more complex samples where the concentration of defects is not known or when the distribution of defects is not random as in Ziegler-Natta iPPs, as calibration curve to evaluate the length of regular propene sequences from the crystallization behavior by simply measuring the maximum amount of crystallized γ form. The retro-analysis of the crystallization behavior of iPP allows to disclose hidden aspects of the molecular structure of the extremely complex Ziegler-Natta polypropylene and provides evidence of the blocky structure of chains of iPP (and polyolefins in general) synthesized with Ziegler-Natta catalysts. This retro-analysis exploits the presence of polymorphism and is possible only for polymers that can crystallize in different polymorphic forms that exhibit thermodynamic stability and crystallization kinetics which depend on the molecular structure and, in particular, on the presence of defects.