## 3 Classical Mechanism of Pore Formation in Macroporous Polymers
### Formation of Pores in a Tube during Polymerization
Increasing the number of nuclei and microspheres necessarily decreases their size so that smaller voids or pores between them appear in the final copolymer. In contrast, if the polymerization temperature is low, the rate of polymerization is slow and the transfer of a substantial part of the monomers from the solution in the nuclei can occur, resulting in the growth of the nuclei in a larger size. The pore size distribution is shifted toward greater pores. On the other hand, as reported by [PERSON] and [PERSON] [48], the addition of a good solvent to the monolith polymerizing system results in a phase separation that occurs in the \"later stages\" of polymerization (where cross-linking dominates the phase separation process). Contrarily, the addition of a poorer solvent to the system causes an earlier phase separation in the polymer. The new phase swells with the monomers (if they are liquids) because these are thermodynamically better solvents for the polymer than the porogenic solvent. Therefore, the local concentration of monomers in the swollen gel nuclei is higher than that in the surrounding solution and the polymerization reaction proceeds mainly in these swollen nuclei rather than in the solution. Those nuclei formed are likely to be adsorbed by the large preglobules developed earlier by coalescence of many nuclei, increasing the size of the preglobules. Thus, the globules formed in the system are larger and the voids between them (pores) are larger as well. If the solvent quality improves, the good solvent competes with monomers in the solvation of nuclei, the local monomer concentration is lower and the globules are smaller. As a result, porous polymers formed in more solvating solvents have smaller pores. If the monomers are solids and were dissolved in the medium, the effect depends on the solvating power of both monomers and polymer chains in formation [37]. In the case of an increase in the amount of nonsolvating diluent, a decrease is observed in the solvating power of both monomers and polymer chains in formation. Therefore, the nuclei of polymers in formation could segregate and capture preferably monomers from the local solution, and consequently increase their size. This process possibly takes place since the monomers probably tend to pass from a less polar medium to stay within the more polar swollen nuclei, which leads to a higher local concentration of monomers in the swollen nuclei than that in the surrounding solution. The polymerization reaction would proceed mainly in the swollen nuclei rather than in the solution. At the same time, the precipitated microglbules can attract the polymer chains in formation and coalesce with them, which would lead to a further increase in their size and therefore in the size of the pores formed between them. Contrarily, if the solvating power of the solvent increases, the polarity within the nuclei is similar to that in the solution, the local monomer concentration is lower since the monomers are not forced to adsorb preferably into the nuclei and the polymerization occurs with form of nuclei that remain individualized. A large number of nuclei compete for the remaining monomers, leading a higher number of small globules that aggregate with small pores. As concluded previously [6], the thermodynamic conditions that induce phase separation during polymerization depend on the composition of the polymerization mixture containing monomers and the porogen. The porogenic solvent controls the porous properties of the monolith through the solvation of the polymer chains in the reaction medium during the early stages of polymerization. In view of this, the solvating power of the diluent has a critical effect on the porous structure of monoliths. The addition of a good or a bad solvent produces changes in the phase separation and consequently in the structural heterogeneity [49]. However, it is important to emphasize that high-molecular-weight linear polymers were used to create macropores as inert diluents [6, 36, 37, 49, 51, 52, 53, 54].