## 2 Results and Discussion  
### Synthesis of Porous P(S-DVB) Microparticles  
As illustrated in Figure 1, the synthesis process of monodisperse porous polymer microparticles was based on monodisperse linear polystyrene(PS) seed particles, undergoing porogen swelling, monomer and crosslinker swelling, polymerization and extraction. Monodisperse PS seed particles prepared by dispersion polymerization have an average diameter of 2.7 um, and the coefficient of deviation (CV) is only about 1.9% (Figure 2). The monodispersity was inherited when porous polymer microparticles were prepared. Dibutyl phthalate (DBP) is a seed swelling activator and porogen is a classical plasticizer. Toluene as a porogen is an excellent solvent of polystyrene. These two organic solvents can be diffused into PS seed particles from their emulsion. After being swollen with porogen, monomer, crosslinker and initiator, polymerization occurs in the seed particles to form an interpenetrating polymer network. Linear PS and porogens stay in the crosslinked polymer network and are then extracted by tetrahydrofuran (THF) to generate the porous structure. The preparation conditions of porous polymer microparticles are summarized in Table 1.  
Figure 1: Schematic illustration of the preparation of porous polymer microparticles. Figure 2: SEM images of 2.7 μm PS seed microspheres synthesized by dispersion polymerization (**a**) and the histogram of the distribution of PS seed microspheres (**b**). The porosity of porous polymer microparticles is greatly influenced by porogens. Figure 3 shows the morphologies of porous polymer microparticles prepared by using different ratios of toluene to DBP as porogens. When only DBP was applied, no pores were observed on the surface of the microparticles (Figure 3a). When the toluene ratio was increased, different sizes of pores with uniform distribution appeared on the surface of the microparticles (Figure 3b-d). When only toluene was used, a few pores were observed (Figure 3e). The BET (the abbreviation of Brunauer, Emmett and Teller) (Figure 3f) result showed that the largest surface area was obtained when the toluene to DBP ratio was 3:1. The swelling temperature can also influence the morphology of porous P(S-DVB) microparticles. As shown in Figure 4, porous P(S-DVB) microparticles have the roughest surface when 25 \({}^{\circ}\)C of swelling temperature is applied. A smoother surface and smaller pore size were generated by either decreasing or increasing the swelling temperature. Temperature generally has a great impact on the swelling kinetics. PS seeds might not be well swelled by porogens and monomers under low temperature, leading to dense porous structures. On the contrary, high temperature would accelerate prepolymerization of monomers during swelling, leading to a smoother surface and smaller pore size. Using different crosslinking agents for synthesizing porous P(S-DVB) microparticles presents different morphologies. Figure 5 shows the SEM and TEM images of porous particles that were synthesized by using different crosslinking agents. We can see that the porous microparticles have larger pore size when ethylene dimethacrylate (EDMA) is used as a crosslink, while the microparticles \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Toluene** & **DBP** & **Swelling Temperature** & **DVB** & **EDMA** \\  & **(mL)** & **(mL)** & **(\({}^{\circ}\)C)** & **(mL)** & **(mL)** \\ \hline P(S-DVB)-1 & 0 & 2.4 & 35 & 2.0 & 0 \\ P(S-DVB)-2 & 0.6 & 1.8 & 35 & 2.0 & 0 \\ P(S-DVB)-3 & 1.2 & 1.2 & 35 & 2.0 & 0 \\ P(S-DVB)-4 & 1.8 & 0.6 & 35 & 2.0 & 0 \\ P(S-DVB)-5 & 2.4 & 0 & 35 & 2.0 & 0 \\ P(S-DVB)-6 & 1.2 & 1.2 & 15 & 2.0 & 0 \\ P(S-DVB)-7 & 1.2 & 1.2 & 25 & 2.0 & 0 \\ P(S-DVB)-8 & 1.2 & 1.2 & 45 & 2.0 & 0 \\ P(S-EDMA) & 1.2 & 1.2 & 35 & 0 & 2.0 \\ \hline \hline \end{tabular}  
* Dibutyl phthalate(DBP); divinylbenzene(DVB); ethylene dimethacrylate(EDMA).  
\end{table} 
Table 1: Preparation conditions of porous polymer microparticles.