Tacticity Effect on the Conformational Properties of Polypropylene and Poly(ethylene-propylene) Copolymers

Tacticity is one of the most important features of stereoregular polymers, with significant impact on morphology and on a variety of properties such as conformational, thermal, rheological, mechanical, etc. In this work we focus on tacticity effects on the conformations and more specifically on the unperturbed dimensions of single polypropylene (PP) homopolymer and both block and random poly(ethylene-propylene) copolymer chains. The equilibration of all chains is achieved by applying the single-chain Monte Carlo algorithm of Tzounis et al. [Macromolecules 2017, 50, 4575-4587]. In agreement with past studies, we find that tacticity has a significant effect on the stiffness of PP homopolymer. The characteristic ratio exhibits a nonmonotonic dependence on the fraction of meso dyads along the PP chains, which results from two competing mechanisms, revealed by analysis of the torsional states of the PP chain backbones. A simple theoretical model is developed to describe the dependence of the stiffness of poly(ethylene-block-propylene) copolymer on its propylene content and on the tacticity of its PP block. Finally, the effect of both tacticity and propylene content on the stiffness of poly(ethylene-random-propylene) chains, used to model ethylene-propylene monomer (EPM) materials, is found to be in very good qualitative agreement with available rotational isomeric state (RIS) model predictions.