Synthesis of Emulsion-templated Porous Polycaprolactone

Emulsion-templated porous materials have gained extensively in applications due to their low density and high porosity. However, by far most this type porous polymers are synthesized using conventional radical polymerization and most of the reactions are thermally initiated. Expanding the polymerization mechanisms available for emulsion-templated polymers synthesis is still highly desired. In this work, Diel-Alder (DA) reaction was used to synthesize emulsion-templated macroporous polycaprolactone (PCL). These macroporous polymers were prepared by DA reaction of polycaprolactone pendant with multiple furan group (PCL-furan) and 1,6-bis(maleimide)hexane (BisM) via a high internal phase emulsion as template. The porous materials have a typical structure of emulsion-templated materials.


Introduction
Emulsion-templated porous materials have gained extensively in applications such as scaffold for tissue engineering, water purification, solid phase supports for catalysis, and membranes for separation processes due to their low density and high porosity [1]. In this method, high (or medium) internal phase emulsion HIPE (or MIPE) is often used as template where the volume fraction of the dispersed phase exceeds 74% [2]. By far most of this type porous polymers are synthesized using conventional radical polymerization and most of the reactions are thermally initiated [1]. However, some important polymers, such as polycaprolactone and poly(lactic acid), can't be synthesized by radical polymerization.
Recently, much of the HIPE-templated polymers research and development has focused on expanding the polymerization mechanisms available for emulsion-templated polymers synthesis. Great efforts have focused upon a variety of polymerization-related issues including thiol-ene and thiol-yne reactions [3], ring-opening metathesis polymerization (ROMP) [4].
During the course of expanding the polymerization mechanisms available for synthesis of emulsion-templated polymers, we considered that the use of Diels-Alder (DA) reaction would be an attractive alternative. DA reaction is one of the most common reactions used in organic chemistry, which involves a straightforward [4 + 2] cycloaddition reaction between an electron-rich diene (i. e. furan and its derivatives) and an electron poor dienophile (i. e. maleic acid and its derivatives) to form a stable cyclohexene adduct [5]. This reaction requires a very low energy to form a cyclohexene ring, can conducted at moderate temperature in gentle condition, and it allows the formation and functionalization of numerous molecules.
Herein, macroporous polycaprolactone was synthesized with DA reaction in a Span 80 stabilized water-in-oil (w/o) HIPE. Polycaprolactone pendant with multiple furan group (PCL-furan) and 1,6-bis(maleimide)hexane (BisM) were synthesized as reactants. And the resulting porous materials have a typical structure of emulsion-templated materials.

2.4Preparation and characterization of porous materials
The oil phase composed of toluene (20 ml), Span80 (2.0 g), PCL-Furan (2.57g, 0.2 mmol, containing furan of 1.9 mmol) and BisM (0.26 g, containing maleimide of 1.9 mmol) was added to a 50 ml beaker. The oil phase was stirred at 600 rpm, 4 ml the water phase containing NaCl of 0.2 mol/L was dropwise added to the oil phase. After the addition of water phase was completed, the mixture was further stirred for 5 min, then put it in an oven for 24 h at room temperature and subsequently freezing-dried.
The phase behavior of the HIPE was evaluated by measuring the delta backscattering of monochromatic light (Ȝ = 880 nm) from the emulsion employing an optical analyzer, Turbiscan Lab Expert (Formulaction, France). As soon as the preparation of the HIPE was

Results and Discussion
As shown in Fig. 1, Turbiscan analysis of this HIPE showed that the backscattering intensity of the emulsion deceased slightly at the beginning of the test, then kept constant during 24 h since it was prepared, which suggested that this system could be emulsion-templating for porous materials. As shown in Fig. 2, DA reaction of PCL-furan and BisM resulted a porous polymers with a well-defined open-cell porous structure, and the average void diameter from the SEM images was 22 ± 4 μm, just like the typical structure of the porous polymers from the radical polymerization of HIPE [2]. This result meant that the DA reaction could be practical reaction approach for preparation of HIPE-templated porous materials.

Summary
Emulsion-templated macroporous PCL was prepared via DA reaction between PCL-furan and 1,6-bis(maleimide)hexane in a high internal phase emulsion template. The porous materials have a typical structure of emulsion-templated materials, and the average void diameter of the obtained polyHIPEs sample was 22 ± 4 μm.