## 3 Results  
### Pickering Emulsion Formation and Polymerization  
We have investigated the formation and polymerization of Pickering emulsions from three different monomers: methyl methacrylate (MM), benzyl methacrylate (BM), and tert-butyl acrylate (tBA), with three types of silica NPs bearing three different surface functional groups: nitrile (NP-CN), glycidyl (NP-Gly), thiol (NP-SH). In all cases, we kept the same concentration of NPs and the same water to oil ratio in emulsion preparation, Table S2. The preparation procedure for the NP-CN and NP-SH was reported previously [35]. The synthesis of NP-Gly is given in the experimental methods and SEM images are given in the Figure S1. The obtained emulsions, containing the oil soluble benzoin methyl ether (BME) initiator, were polymerized under exposure to UV lamp for 1 h, as described in the experimental section. Poly(methyl methacrylate) (PMM), poly(benzyl methacrylate) (PBM), and poly( tert-butyl acrylate) (PtBA) microspheres were obtained for each monomer and each particle, except for NP-SH with MM and NP-CN with MM, for which Pickering emulsions could be generated, but no polymer microspheres could be obtained. Qualitatively, from the SEM images, we observe that the microspheres obtained are large and uniform, especially for those obtained by NP-Gly with PMM (Figure 1), PBM (Figure S4), and PtBA (Figure S5), while for the other NPs, the microspheres appear somewhat smaller, their size distribution is broader and deviate somewhat from ideal spherical shape, see Figure S6. In all cases, the microspheres obtained are covered by a self-assembled monolayer of NPs, see Figure 1d, which suggests that the initiation of the polymerization took place in the liquid emulsion droplet. Sometimes smaller microspheres can be identified without NPs on their surface, Figure S7. This can be explained by the polymerization mechanism of Pickering emulsions, which is rather different from the polymerization mechanism of the classic emulsions stabilized by surfactants. On one hand, the mechanism in conventional emulsion polymerization stabilized by surfactants is well-understood, whereas the standard theory notes the existence of three different _loci_ for the nucleation mechanism, micellar nucleation, homogeneous coagulative nucleation, and emulsion droplet nucleation [43; 44]. Although, almost always, the three mechanism are at play in the emulsion polymerization, the dominant mechanism depends on the emulsion type, such that the polymerization _loci_ can shift out of the emulsion droplet toward micelles or the water phase driven by many factors, such as emulsion droplet curvature (emulsion droplet curvature 1/R, R is the radius, increases in the order: emulsions, miniemulsions, microemulsions), by the oil-water interfacial tension, solubility of the monomer in water, the solubility of the polymerization initiator in water or oil, etc. [43] On the other hand, in the case of Pickering emulsions, in the absence of surfactants, the micellar nucleation is excluded, therefore [PERSON] et al. [34] proposed that only two possible mechanisms are present in the initial stage of Pickering emulsion polymerization, namely (i) the homogeneous coagulative nucleation, and (ii) nucleation in the emulsion droplet. In the first case, the oligomers produced in the water phase coagulate to form nuclei that are subsequently swollen by incoming of the monomers from the emulsion droplet reservoir. Emulsion stabilizing nanoparticles attach onto these nuclei and stabilize them, and these ultimately grow into sub-micron particles. An indication that this mechanism is at play is that the obtained polymer particles are only loosely covered with nanoparticles. In contrast, in the second case, when the polymerization is initiated into the emulsion droplet, these transform into solid particles without significant size growth. This mechanism should generate rather large micron size polymer particles.