Noncovalent Hydrogen Bonds Tune the Mechanical Properties of Phosphoester Polyethylene Mimics.

Hisaschi T Tee,Kaloian Koynov,Tobias Reichel,Frederik R Wurm

doi:10.1021/acsomega.9b01040

Hisaschi T Tee, Kaloian Koynov + Show 2 more

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https://doi.org/10.1021/acsomega.9b01040

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Abstract

Polyethylene mimics of semicrystalline polyphosphoesters (PPEs) with an adjustable amount of noncovalent cross-links were synthesized. Acyclic diene metathesis copolymerization of a phosphoric acid triester (M1) with a novel phosphoric acid diester monomer (M2) was achieved. PPEs with different co-monomer ratios and 0, 20, 40, and 100% of phosphodiester content were synthesized. The phosphodiester groups result in supramolecular interactions between the polymer chains, with the P–OH functionality as an H-bond donor and the P=O group as an H-bond acceptor. A library of unsaturated and saturated PPEs was prepared and analyzed in detail by NMR spectroscopy, size exclusion chromatography, differential scanning calorimetry, thermogravimetry, rheology, and stress–strain measurements. The introduction of the supramolecular cross-links into the aliphatic and hydrophobic PPEs showed a significant impact on the material properties: increased glass-transition and melting temperatures were observed and an increase in the storage modulus of the polymers was achieved. This specific combination of a flexible aliphatic backbone and a supramolecular H-bonding interaction between the chains was maximized in the homopolymer of the phosphodiester monomer, which featured additional properties, such as shape-memory properties, and polymer samples could be healed after cutting. The P–OH groups also showed a strong adhesion toward metal surfaces, which was used together with the shape-memory function in a model device that responds to a temperature stimulus with shape change. This systematic variation of phosphodiesters/phosphotriesters in polyethylene mimics further underlines the versatility of the phosphorus chemistry to build up complex macromolecular architectures.

Highlights

Inspired by nature, supramolecular chemistry uses hydrogen bonding,[1−4] metal−ligand interactions,[5] or donor−acceptor π−π stacking[6] to assemble small molecules or polymers into materials with an extensive range of properties.[7]
Complex structures are generated by hydrogen bonding such as proteins, which fold into specific three-dimensional structures to enable their function, or DNA with hydrogen bonding between the nucleic acids, which plays a crucial role in the double-helical structure.[8]
The strength of a single hydrogen bond (H-bond) is ca. 10−40 kJ mol−1 relatively weak; multiple H-bonds result in high cohesive energies, which can act as physical cross-links in polymer networks

Summary

Introduction

Supramolecular chemistry uses hydrogen bonding,[1−4] metal−ligand interactions,[5] or donor−acceptor π−π stacking[6] to assemble small molecules or polymers into materials with an extensive range of properties.[7]. 10−40 kJ mol−1 relatively weak; multiple H-bonds result in high cohesive energies, which can act as physical cross-links in polymer networks. By choice of the H-bonds, the material properties can be designed in advance due to weak H-bonds having a fast bond exchange, which results in stimuli responsiveness, whereas strong H-bonds having a retarded bond exchange leading to solidlike properties. Several technological concepts such as self-healing,[9] shape-memory processes,[10] and dynamic energy dissipation[11] have been achieved by the incorporation of H-bonds into polymers.

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