## 1 Introduction
Whereas standard surfactants (e.g., SDS or CTAB) have disadvantages, such as surfactant migration or desorption from the particles post polymerization, using polymerizable surfactants--often referred to as surfmers (surface active monomers)--leads to covalent attachment of the surfactant molecules at the particle surface [23; 24; 25]. Surfmers are amphiphilic compounds consisting of a polymerizable group (usually hydrophobic), and a hydrophilic head group (neutral or charged), connected by a spacer (usually an alkyl chain with at least six methylene groups) [22]. Surfmers in which the head group is a functional group are referred to as functional surfmers [22; 23; 24; 26; 27]. An overview on surfmer structures and their applications has been recently presented by [PERSON] and [PERSON] (2014) [22]. During polymerization, the surfmers are directly incorporated into the polymeric backbone of the particles--while predominantly present on the particle surface. Hence, the use of functional surfmers allows for a controlled display of the functional head groups with a covalent attachment to the surface of the particles [23; 24; 27; 28]. In this manner, the use of functional surfmers enables the production of surface functionalized particles through a one-pot synthesis, and the obtained particles may be further used toward specific applications [22]. As aforementioned, a wide range of surfmer molecules has been developed so far and polymeric particles have been developed using surfmers bearing cationic [29; 30; 31] or anionic head groups [32; 33; 34], as well as non-ionic [35; 36; 37]. The positive charge on cationic surfmers is commonly derived from quaternary ammonium groups [38; 39; 40; 31] and may include functional moieties, for instance in fluorinated surfrmers [29; 41]. A prominent cationic surfmer example is the \(p\)-(11-acrylamido)undecanoylyphenyl dimethylsulfonium methyl sulfate (AUPDS), a water soluble surfmer that presents an active ester functionality that acts as an anchor group. AUPDS was synthesized by [PERSON] et al., who used it in the preparation of polymer NPs, but only through the EP technique [42]. The active ester group offers optimal reactivity under mild reaction conditions with primary amines, a chemical function widely present in proteins. Consequently, the configured particles with this customized functional surface are promising candidates for multifunctional platforms suitable for biomedical applications, as reported previously [43; 44]. Surfactants bearing sulfonium head groups have also been reported [45] and surfmers bearing active ester groups have been described for the preparation of functionalized PS particles, but via emulsion polymerization and not miniEP [46; 47]. Using surfmer molecules to obtain functional particles systems that are both monodisperse and reproducible is a challenging task and, for a better understanding of the mechanistic approach of miniEP using surfmers and of the colloidal properties of the obtained particles, a systematic study of different formulations is necessary. The aim of this work is to investigate the role of AUPDS as a surfactant and co-monomer in a thermally-initiated miniEP system using either styrene (St) or methyl methacrylate (MMA) as co-monomers. Cross-linked particles and the effect of varying cross-linker concentrations in these surfmer-functionalized particle systems were also investigated. The obtained particle properties, such as particle size, distribution, and surface charge were considered and the different formulations were compared.