## 3 Results and Discussion
### Choice of Reagents for Mini-Emulsion Polymerization  
The choice of reagents and emulsification parameters play a decisive role in the preparation of particles through miniEP technique. The formation of mini-emulsion systems requires high mechanical mixing in order to reach a steady state, given by a rate equilibrium of droplet fission and fusion. The droplets are then critically stabilized and, during polymerization, the droplets act as \"nanoreactors\" because only droplet nucleation occurs [50]. The general process for miniEP used in this work is displayed in Figure 1. It was not possible to perform DLS of the monomer droplets prior to polymerization, due to multiple scattering effects that result from the high concentration of droplets in the mini-emulsion. It was also not advisable to dilute the mini-emulsion in this state, due to possible displacement of the AUPDS or loss of monomer by diffusion.  
Because the functional active ester group of AUPDS is present in the hydrophilic head group, we chose to perform direct mini-emulsion experiments with a hydrophobic monomer dispersed phase and water as the continuous phase. Thus, when using AUPDS as a surfactant in miniEP the active ester groups should be directed towards the water phase and should be present on the NP surface after polymerization. We chose the monomers MMA and St for particle synthesis because they are  
Figure 1: Schematic representation of the direct mini-emulsion polymerization process with AUPDS surfmer. The reaction mixture is composed of a continuous phase (AUPDS dissolved in water) and a dispersed phase (MMA or St, cross-linker, AIBN, and hexadecane). This mixture is (mini)emulsified using probe ultrasonication, to form critically stable monomer droplets, which are then polymerized. Nucleation occurs predominantly in the monomer droplets and this allows for the production of particles that are a 1:1 copy of the droplets prior to polymerization. Objects shown are not drawn to scale.  
known to form monodisperse NPs in miniEP [50; 51]. Additionally, we investigated if it was possible to produce AUPDS-surface functionalized cross-linked NPs by adding the cross-linkers DVB and EGDMA to St and MMA, respectively. The choice of cross-linker was based on the fact that DVB has similar polymerizable groups to St, the same is valid for EGDMA with MMA [52].  
There are two factors to assure that nucleation only takes place in the monomer droplets: the type of the initiator and the surfactant concentration. In our formulations, the initiator of choice was AIBN, due to the fact that it is only soluble in the dispersed organic monomer phase. This is particularly important to prevent secondary nucleation in the continuous phase (water) for MMA which is partly soluble in water. The AUPDS molecule, due to its amphiphilic character, acts as a surfactant substitute while also acting as a co-monomer. By using low AUPDS concentrations below the critical micelle concentration (CMC), the surfmer does not cover the droplet interface completely [50; 53]. This configuration allows for most of the AUPDS to be positioned on the surface of the particles, thus producing surface-functionalized particles in one step.  
The monomer droplets in mini-emulsion systems are stabilized against the Ostwald ripening effect via the use of co-stabilizers--the so-called (ultra)hydrohobes [53]. Hexadecane (HD) was added as a hydrophobe and it is pertinent to emphasize that the HD is not placed on the interface, and thus its use does not modify the interfacial energy [54].  
In our experiments, we produced NPs with AUPDS as the only surface active agent: PS and PMMA NPs (both non-cross-linked) as well as PS-co-DVB and PMMA-co-EGDMA NPs (both cross-linked). The objective was to systematically test the effect of varying surfmer and cross-linker concentration in order to obtain monodisperse functionalized PMMA and PS NPs with reproducible properties and stability.