# A View on the Synthesis and Characterization of Porous Microspheres Containing Pyrrolidone Units
## 3 Results and Discussion
Figure 2: Relative distributions of the microsphere volume as a function of diameter: (**a**) poly(GMA-_co_-EGDMA)_1 copolymer; (**b**) poly(NVP-\(\omega\)-EGDMA)_1 copolymer. found in the copolymer synthesized in the presence of toluene/dodecane mixture (2.80%). The application of a more polar diluent (decan-1-ol) instead of dodecane resulted in an increase in the amount of nitrogen and consequently in the pyrrolidone unit content in the poly(NVP-_co_-EGDMA)_2 and poly(NVP-_co_-EGDMA)_3 materials. The diluent composition also had a great impact on porous structure formation. Nitrogen sorption analysis was employed to determine the key parameters of the porous structure for all copolymers studied. The isotherms of nitrogen adsorption/desorption on selected copolymers are presented in Figure 4.  
The copolymer with the most developed specific surface (456 m\({}^{2}\)/g) area was obtained when toluene was applied as the pore-forming diluent (Table 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Copolymer**} & **Specific Surface Area** & **Pore Volume** & **Pore Diameter** & **N** \\  & \(S_{BET}\) **(m\({}^{2}\)/g)** & \(V\) **(cm\({}^{3}\)/g)** & \(D_{BJH}\) **(nm)** & **\%** \\ \hline poly(NVP-_co_-EGDMA)\_1 & 456 & 0.675 & 31 & 3.01 \\ poly(NVP-_co_-EGDMA)\_2 & 88 & 0.624 & 31 & 3.45 \\ poly(NVP-_co_-EGDMA)\_3 & 76 & 0.313 & 50 & 3.60 \\ poly(NVP-_co_-EGDMA)\_4 & 267 & 0.566 & 37 & 2.80 \\ poly(NVP-_co_-EGDMA)\_5 & 190 & 0.275 & 40 & 2.97 \\ poly(NVP-_co_-EGDMA)\_6 & 101 & 0.215 & 37 & 3.80 \\ poly(NVP-_co_-EGDMA)\_7 & 81 & 0.284 & 38 & 4.40 \\ poly(NVP-_co_-EGDMA)\_8 & 59 & 0.319 & 47 & 6.40 \\ poly(GMA-_co_-EGDMA)\_1 & 109 & 0.483 & 39 & - \\ poly(GMA-_co_-EGDMA)\_2 & 72 & 0.280 & 52 & - \\ poly(GMA-_co_-EGDMA)\_1+p & 101 & 0.378 & 38 & 1.51 \\ poly(GMA-_co_-EGDMA)\_2+p & 68 & 0.210 & 51 & 2.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The basic parameters of porous structure and nitrogen weight fraction of the investigated copolymers. Figure 4: The isotherms of nitrogen adsorption (\(\bullet\)) and desorption (\(\bigcirc\)) on selected copolymers determined at 77 K.  
This behavior could be clarified on the basis of the toluene solubility parameter, which was equal to 18.2 (MPa)\({}^{0.5}\). This value indicated the good solvation power of toluene. In an initially homogenous polymerization system containing a good solvent, phase separation (indispensable for the formation of porous morphology) occurred at a greater conversion of monomers to polymer. During this late phase separation, a huge number of individual nuclei were created. Toluene competed with the monomer for solvation within the nuclei, resulting in a relatively low local concentration of monomers inside the nuclei. Attractive forces between individual nuclei were weak, thus limiting coalescence. Consequently, small globules were formed. They were interconnected and created a polymeric network with a high surface area. The pore size distribution had the maximum in the region of micro- to mesopores. The incorporation of a poorer solvent (dodecane) led to earlier phase separation because of the limited solubility of the polymer phase in the polymerization mixture. The newly created phase favorably swelled with the monomers. The monomer concentration within the nuclei was relatively high. It led to coalescence and a subsequent increase in their size. As a result, the formed globules and the free voids between them were larger. Consequently, larger pores and a less developed specific surface area were noticed for the poly(NVP-_co_-EGDMA)_4 copolymer. A further increment in the dodecane concentration in the organic mixture led to further diminution of the specific surface area, as can be seen for the poly(NVP-_co_-EGDMA)_5 copolymer. Evidently, the solvent controlled copolymer porosity via polymer chain solvation in the early stages of polymerization. The porosity of the synthesized copolymers was also strongly dependent on the molar ratio of the comonomers.