Dependence of micro-structure regularity on the degree of molecular asymmetry in polynorbornene derivatives

Abstract

Although the correlation between the regularity of micro-structures and the length of side chains has been widely reported, experiments still require precursors of polynorbornene derivatives due to the complex inter- and intra-chain interplay. This research utilizes hydrogen bonding between amide groups to design molecular models of polynorbornene derivatives (PNbs). The PNbs are abbreviated as Pm-8, where “m” indicates the amount of methylene groups as spacers in the side chain, and “8” refers to the oxyoctyl tail. The aim is to obtain a gradual variation of inter- and intra-chain interplay for further elucidation. Based on an analysis of spatial descriptors, mean square displacement (MSD), and radial distribution function (RDF) by using molecular mechanics (MM) and molecular dynamics (MD) methods, it is found that the asymmetry of side chains plays a more significant role in determining micro-structural regularity than the length. Highly asymmetric side chains are favorable for forming inter-chain hydrogen bonding, promoting interdigitation, and creating a larger scale of regularity in P8-8 and P10-8 molecules. In contrast, the relatively symmetric side chains tend to form intra-chain bonding, resulting in limited regularity within P2-8 and P4-8 molecules. Therefore, unlike regular structures that usually require symmetrical molecules, polynorbornene derivatives with asymmetric side chains have achieved highly ordered orthorhombic lattices in experiments. This study aims to offer an alternative approach to screening effective precursors of long-side chain polymers by means of semi-quantitative geometric parameters, providing a fresh perspective on the subject. 

Graphical Abstract

Chemical structure and molecular geometry of the P8-8 model of polynorbornene derivatives (color code: purple = the main chain, red = O, white = H, gray = C) The P8-8 model of polynorbornene derivatives has an asymmetric micro-structure that promotes interdigitation of side chains, creating a larger scale of regularity in bulk structure. The name of the model is derived from the number of methylene groups as spacers and the oxyoctyl tail in the side chain, both of which are represented by the number 8.