Emulsion templated poly(thiol-enes): Selective oxidation improves mechanical properties

Highlights

• Porous polymer network with interconnected cellular architecture prepared via high internal phase emulsion precursors.

• Tris [2-(3-mercaptopropionyloxy) ethyl] isocyanurate and 1,6-hexanediol diacrylate used to apply a thiol-ene click chemistry.

• Selective oxidation leads to improved mechanical properties and offers possibility of tuning thermomechanical properties.

Abstract

Porous thiol-ene polymer networks were prepared with different sulfur contents from high internal phase emulsions (HIPEs). A rigid thiol, tris [2-(3-mercaptopropionyloxy) ethyl] isocyanurate (TEMPIC), together with 1,6-hexanediol diacrylate (HDDA), was employed for this study. A modification of the polymer network was performed via the oxidation of the thioethers present in the polymer backbone to sulfoxides and sulfones. Infrared spectroscopy measurements confirmed that the selective oxidation of thioether groups was possible. Furthermore, the changes in thermomechanical properties were determined by dynamic mechanical analysis and thermogravimetric analysis. A significant increase in glass transition temperature from 4.9 to 41.5 °C for the resulting sulfone could be observed for networks with the highest sulfur content. Changes in mechanical properties were determined via tensile and compression testing and showed considerable improvement of tensile and compressive strength after the oxidation. The behavior of the materials upon hydrolytic stress was investigated and led to the highest mass loss for sulfone materials.

Introduction

The emulsion templating approach is a well-established pathway to macroporous polymers with high pore volumes and pore diameters greater than 500 nm. [1] When the continuous phase of a high internal phase emulsion is polymerized, after the removal of the droplet phase, resulting polymers (polyHIPEs) exhibit an interconnected pore architecture. PolyHIPEs are used for adsorption, absorption, catalysis, column supports, membranes, and a wide range of biomedical applications. [[2], [3], [4]] Depending on the intended use, different polymerization mechanisms for the synthesis of polyHIPEs with varying properties and functionalities are available. Radical chain growth mechanism is most commonly used. [5] Other established mechanisms include RAFT [6], ATRP [7], ROMP [8], and step-growth mechanisms for the synthesis of polyurethanes or poly(thiol-enes) via click chemistry. [9,10]

Thiol-ene click chemistry as a route to polyHIPEs has been on the rise in recent years. In general, research regarding thiol-ene polyHIPEs is frequently aimed at biomedical applications because the resulting materials are typically less brittle than purely chain-growth (meth)acrylate-based materials. [4] Thiol-ene polyHIPEs also contain a significant amount of degradable ester linkages and can be functionalized in several ways. Functionalization of remaining free thiol-groups was demonstrated in some cases. [11,12] The group of Cameron demonstrated the post-polymerization functionalization of free thiol groups within thiol-ene polyHIPEs to change hydrophilicity/hydrophobicity or attach chemical markers such as fluorescein O-acrylate. [12] They also illustrated the direct incorporation of other functionalizable pentafluorophyl acrylate into thiol-ene polyHIPEs. [13] Since it is difficult to predict the amount of free thiol groups remaining in the network after polymerization, this second approach should allow for higher and more controlled incorporation of functional groups.

Another way of targeting material properties of thiol-ene polyHIPEs is the choice of monomers and the amount of thiol used in the system. Depending on the amount of thiol and the alkene monomer used, the thiol can act as a chain-transfer agent in a mixed chain- and step-growth mechanism or in a pure step-growth mechanism resulting in different material properties. [14] The most frequently used thiols for polyHIPEs are pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and trimethylolpropane tris(3-mercaptopropionate) (TMPTMP), typically leading to more hydrophobic materials. Monomers used with these two thiols include acrylates, vinyl esters, alkynes and allyls. [15,16] Only recently, a hydrophilic thiol, ethoxylated trimethylolpropane tris(3-mercaptopropionate) (ETTMP), together with PEG-(meth)acrylates, was incorporated to create hydrogel polyHIPEs through an oil-in-water HIPE. [17]

A way of modifying the material properties of thiol-ene polymers is the oxidation of resulting thioethers to sulfoxides or sulfones. Sulfoxides and sulfones exhibit desirable thermomechanical and chemical properties, challenging to attain in a direct synthetic way. The synthesis of poly(aryl ether sulfones) which involved monomer functionalization and a strict protocol for successful synthesis was reported. [18] Aliphatic sulfoxides or sulfones have not been reported for polyHIPEs yet but offer potential since the T_g of thiol-ene polymers is usually low and might pose a problem for intended applications. Oxidation of sulfur in thiol-ene polyHIPEs overall promises improved material properties and a potential change in hydrophilicity and degradability which might be relevant for potential applications.

Another aspect that has not been mentioned in previous thiol-ene polyHIPE applications as tissue engineering scaffolds is the potential of the material to react to oxidative stress, a phenomenon induced by (auto)inflammatory reactions within living organisms. [19] Thiol or sulfur(II) containing molecules most commonly function as metal binders or redox buffers within biological systems. [20] Thus considering potential oxidative effects on the polymer network when creating thiol-ene polyHIPEs for biological applications should be considered as they could not only lead to different material properties but also be utilized for oxidation responsive drug release or, in an undesired way, lead to depolymerization and the release of toxic low molecular weight species.

Approaches to synthesize polysulfoxides and sulfones include direct synthesis, e.g. the route chosen by Cameron and Sherrington, [18] and a post-processing route where the polysulfides are transformed to sulfoxides and sulfones. Podgórski et al. described the oxidation of polysulfide networks to high-performance polysulfone networks with hydrogen peroxide. [21] Recently a similar approach for the selective oxidation of polysulfide latexes to sulfoxides or sulfones by the group of Landfester also included hydrogen peroxide and an even milder oxidant tert-butyl hydroperoxide solution (t-BuOOH), to exclusively yield sulfoxides. [22] The terms sulfide or thioether are used regarding these types of materials. In this work, the term polythioether or thioether is used.

The aim of this study was to improve the toughness and thermomechanical properties of polythioether polyHIPEs via an oxidative post-polymerization processing approach. This can be achieved through the oxidation of the polythioethers to polysulfoxides and -sulfones. Furthermore, polyHIPEs with different thiol contents were investigated to study how the amount of sulfur(II) present in the polymer backbone can affect the overall material properties before and after oxidation. In that matter, a facile procedure suitable for hydrophilic and hydrophobic polythioether polyHIPEs employing t-BuOOH or hydrogen peroxide was developed for a newly developed thiol-ene polyHIPE to achieve either porous sulfoxide or sulfone networks. [22] The synthesis of polyHIPEs and the respective oxidation procedure is shown in Fig. 1.