## 1 Introduction  
Porous polymer membranes play a key role in various application fields, such as separation analysis [1, 2, 3], sensors [4, 5], energy storage and conversion, etc. [6, 7, 8, 9]. An ideal porous polymer membrane should possess a highly interconnected open-cellular structure and high toughness. The highly interconnected open-cellular structure is beneficial to achieving high mass transfer flux with low resistance, while high toughness helps to maintain structure integrality when subjected to external force. The preparation method of porous polymer materials usually includes foaming, phase separation, pore-forming agent, etc. [10, 11, 12]. However, these methods lack the regulation of the pore structure of materials.  
By contrast, polymerized high internal phase emulsions (polyHIPEs) provide a facile method to build open-cellular structures with high porosity and good pore connectivity.  
Typical polyHIPEs are monoliths with a high glass transition temperature (T\({}_{\rm g}\)), which are produced by free radical crosslinking copolymerization of monomers such as styrene (St) and divinylbenzene (DVB). Although having a high modulus and strength, typical polyHIPEs are extremely brittle, thus easily leading to fragmentation during the preparation of membranes. The mechanical properties of typical polyHIPEs can only be measured by a compression test instead of a tensile test [13]. Therefore, improving the toughness of polyHIPEs is critical in order to expand their application fields [13; 14]. In order to address the above problems, [PERSON] et al. [13; 15; 16; 17] prepared polyHIPE membranes with improved toughness by ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). However, ROMP of DCPD requires strict control of reaction conditions, such as temperature, pressure, and solvent selection, to avoid the occurrence of side reactions and the instability of the product structure [15; 16].  
Free radical polymerization is a more convenient process due to the wide range of applied monomers and mild reaction conditions. But to prepare polyHIPE membranes with high open-cellular extent and high toughness via free radical polymerization is still challenging. By copolymerizing acrylate soft monomers, the T\({}_{\rm g}\) of the polymer can be significantly reduced, thereby improving the toughness of polyHIPEs [18]. However, due to the high polarity of the acrylate monomer, Ostwald ripening is overwhelming when its dosage is large, which is detrimental to the formation of a stable emulsion [19]. Furthermore, copolymerizing too many soft monomers will lead to the collapse and closure of pores, affecting the formation of an open-cellular structure [20]. To date, the concerned research is limited, and there is a lack of discussion on the intrinsic structure and mechanical properties of the materials.  
Previous studies have shown that controlled/\"living\" free radical polymerization (CLRP) realizes the controllability of the polymerization process by changing the kinetic characteristics, thereby obtaining a more uniform crosslinking structure and further improving the mechanical properties of the material [21; 22; 23; 24; 25; 26]. This provides a new direction for the preparation of polyHIPE membranes with high open-cellular extent and high toughness. In this paper, polyHIPE membranes composed of monomers, including styrene (St), butyl acrylate (BA), and divinylbenzene (DVB), composite emulsifiers including sorbitol monooleate (Span 80) and sodium dodecyl benzene sulfonate (DDBSS), are firstly prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization, one of the controlled/\"living\" free radical polymerizations. The effects of RAFT polymerization and monomer composition on the intrinsic structure and mechanical properties of polyHIPE membranes are systematically investigated in order to achieve high open-cellular extent and high toughness.