Rigidification of Poly(p-phenylene)s through ortho-Phenyl Substitution.

Anna Baun,Zijie Qiu,George Fytas,Akimitsu Narita,Zuyuan Wang,Klaus Müllen,Svenja Morsbach

doi:10.1021/acs.macromol.0c00810

Abstract

A sterically π-congested ortho-phenylated poly(p-phenylene) (PPP) has been synthesized with unprecedentedly high molecular weights up to 29 kDa after fractionation, as confirmed by gel permeation chromatography coupled with a multiangle laser light scattering detector. The chain translation diffusion coefficient obtained from dynamic light scattering experiments displayed strong scaling (∼Lw–0.8) to the chain contour length, indicating a rodlike shape with remarkably high rigidity of this novel PPP. These results provide an interesting insight into the relationship between the structure and the chain stiffness of PPP-based polymers and challenge the validity of the existing diffusion models in polymer physics.

Highlights

The chain stiffness of polymers is characterized by the persistence length, which is a measure of the chain rigidity and allows for a clear distinction between flexible and rigid-rod polymers.[1]
Unsubstituted PPPs exhibit exceptional mechanical strength, stiffness, and high thermal stability due to the rigid aromatic backbone of the paraconnected phenylene units.[9,10]. Their applications are hampered by their vanishing solubility, which induces structural defects and leads to low molecular weights during their preparations, typically through a Ni-mediated polycondensation of aromatic dihalides[11] or an oxidative coupling of benzene in the presence of CuCl2 and AlCl3.12,13 To circumvent these drawbacks, several protocols have been employed: (i) chemical transformation of solubilized precursor polymers to yield unsubstituted PPPs14 and (ii) substitution of PPPs to enhance their solubility.[15−17] Especially, substituted PPPs bearing alkyl and/or additional functional groups have been studied as shape-persistent polymers
We observed precipitation of the oligomers during the polymerization, which indicated that the tert-butyl groups did not provide sufficient solubility

Summary

INTRODUCTION

The chain stiffness of polymers is characterized by the persistence length, which is a measure of the chain rigidity and allows for a clear distinction between flexible and rigid-rod polymers.[1]. A similar persistence length (lp = 8 nm) was observed for poly[6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydrofluorene] C, despite its extended planarized segment, where rotation about interring bonds of the backbone was hindered.[22] In contrast, PPP-based polymer D with further extended planarized segments and bulkier alkylphenyl groups demonstrated a larger persistence length of 25 nm. While π-conjugation across these interring bonds is largely suppressed due to the pronounced torsion, PPP 1 showed a remarkable bathochromic shift upon extension of the conjugation. This was attributed to a through-space conjugation among the π-orbitals of the phenyl substituents. The obtained orthophenylated PPPs were very short, with only up to 15 repeating

RESULTS AND DISCUSSION

CONCLUSION

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