Synthesis of macrocyclic poly(ethylene oxide)s containing a protected thiol group: a strategy for decorating gold surfaces with ring polymers

Abstract

We introduce an approach to synthesize macrocyclic poly(ethylene oxide)s containing a pendant protected thiol group (pSH-CPEO), which is demonstrated to be able to attach to gold surfaces without prior deprotection. Our strategy is based on a bimolecular approach by which a di-alkyne molecule derived from thiol-protected 3-mercapto-1,2-propanediol and a series of PEO bis(azides) of M_n = 2, 6, and 11 kg mol⁻¹ are coupled via copper-catalyzed azide–alkyne cycloaddition. The cyclization reaction was verified by size exclusion chromatography and matrix-assisted laser desorption ionization time of flight mass spectrometry. In addition, we used fast scanning calorimetry to evaluate the glass transition temperature (T_g) of the synthesized pSH-CPEOs. Thanks to the extremely rapid cooling power of this technique, PEO crystallization can be circumvented and, therefore, its fully amorphous state can be investigated. The results confirmed higher T_g values for macrocycles compared to their chemically equivalent linear precursors. This result highlights the importance of the chain end in affecting the T_g of polymers. Finally, to demonstrate the ability of pSH-CPEO to covalently attach to gold surfaces, pSH-CPEO samples were allowed to react with gold-coated glass slides and the surface properties were compared with those of the samples obtained by the reaction of linear α-thiol, ω-methoxy PEO (with and without protection of the thiol group) and gold-coated glass slides. X-ray photoelectron spectroscopy data confirmed the formation of Au–S linkages as well as the removal of the thiol protector group through the quantitative analysis of the chemical composition at the surface. The contact angle data of pSH-CPEO/gold exhibited increased hydrophilicity compared to bare gold and topological effects at the interface. A reaction mechanism between the 2,4-dinitrobenzene-protected thiol group and the gold surface is also proposed.