Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials

Abstract

The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH₂ sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morphology is enabled by the precise halogen placement in the chain and appears to be driven by the selection of the nucleus stem length in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows models for generating new materials based on nanostructures at the lamellar and sublamellar level not feasible in classical branched polyethylenes