Backbone Chemical Composition and Monomer SequenceEffects on Phenylene Polymer Persistence Lengths

摘要

Despite a vast body of the literature devoted to the use of phenylene polymers in the fabrication of graphene nanoribbons, the study of the physical properties of these precursors still poses open questions whose answers will certainly contribute to the design of more efficient/precise synthesis protocols. Particularly, persistence length measurements combined with size exclusion chromatography techniques assign both semiflexible to semirigid structures depending on the molecular weight of the precursor (Narita. et al.Nat. Chem. 2014, 6, 126−132 [] []). Peculiarly, these results suggest an apparent structural change upon increasing the length of the polymers. To address this puzzle, we use single-chain models to study the stiffness of polyphenylene precursors in a theta-like solvent as a function of chain composition and monomer sequence. Steric effects are isolated by considering random walk chains with segment length distributions and the position of monomers determined by the nature of the arene substitution along the backbone. Moreover, two homopolymer limiting cases are defined,that is, meta and para sequences, by associating two types of monomersto each possible substitution pattern. We consider, within these twolimiting cases, chains with different compositions and monomer sequences.We compute persistence lengths, mean square end-to-end distances,and gyration and hydrodynamic radii. We find that distinct valuesof the persistence length for apparently the same chain chemistryare the result of different mixing ratios and the arrangement alongthe chain of the two positional isomers of the same monomer. Finally,we discuss the relation between two-dimensional density of the numberof crossings and the length of polyphenylene segments as they wouldoccur upon strong chain adsorption onto a substrate.