## 3 Classical Mechanism of Pore Formation in Macroporous Polymers
### Formation of Pores in a Tube during Polymerization  
The external conditions of polymerization into a tube are different from those shown in the suspension process. First, a single phase consisting of an organic mixture is present in the tube. The interfacial tension between the aqueous and the organic phase, characteristic of the process of suspension, is absent. Moreover, in contrast to the drops that stay in constant motion in the aqueous phase as a result of agitation, the content of the tube mold remains static during polymerization. The basic mechanism of polymerization in the presence of porogenic solvent described above, including precipitation of the nuclei and shrinkage, is general and independent of the polymerization technique. However, solid nuclei or their clusters have higher density than the polymerization mixture. Thus, in the absence of mixing and if the rate of polymerization is slow, solid nuclei can settle and accumulate in the bottom of the mold (tube). Nuclei (not stirred) and clusters sediment in the bottom of the tube and form a little organized porous structure with large pore volume in the early stages of polymerization. In addition, they present a high specific surface area since the nuclei retain their individuality. Then, as the nuclei come into contact with one another, some growing polymer chains connect the nuclei to each other and the structure is fixed. The polymerization process continues both in solution (above the rod in formation), and within the large pores of the swollen material (formed in the early stages), which leads to the formation of some new nuclei. Thus, the large pores formed initially disappear and the individuality of the polymer nuclei is also lost due to the polymerization reaction that occurs inside the pores and by the capture of the dissolved polymer chains, resulting in a decrease of the surface areas. In contrast to the \"shaped rods\", the growing nuclei within the droplets dispersed into the aqueous phase in the suspension polymerization do not lose their individuality. Drops spin by agitation and the centrifugal force allows the nuclei to move into the drop. The nuclei are able to maintain their individuality for a maximum period of time, to grow separately and to package within the particle (drop). As a result, the gaps between the globules forming a single particle are smaller. As mentioned above, the \"dynamics of the systems\" seems to be the cause of the different pore size distributions between the rods and particles. The experimental conditions used to obtain porous characteristics (pore size, pore volume and specific surface area) intended for a given polymeric material will depend upon the application field. By varying the reaction parameters in the preparation of these polymers, porous properties can be modified by several orders of magnitude. The parameters that are modified more frequently to achieve this kind of materials are the following: the composition of the porogenic mixture, the reaction temperature, the cross-linking monomer concentration, and the concentration of the radical initiator [48]. The effect of temperature is usually studied since the polymerization temperature can be a variable to consider, adjusting the pore size distribution of the molded rods without requiring a change in the composition of the polymeric mixture. [PERSON] [49] and [PERSON] and [PERSON] [48, 50] concluded that the higher the temperature, the smaller the pores. The polymerization temperature-porous structure relation is a consequence of the increasing decomposition rate of the initiator (and the overall polymerization rate) on increasing the temperature. From these reports, it is known that the higher the reaction temperature, the greater the number of free-radicals generated per unit of time and the greater the number of nuclei and microspheres formed (the amount of monomers in the system is the same for each polymerization).