## 3 Classical Mechanism of Pore Formation in Macroporous Polymers
### Formation of Pores in a Tube during Polymerization
Synthetic polymers like poly(ethylene glycol) (PEG) could cause lateral chain aggregation leading to the formation of large pores in the preparation of monoliths [6]. [PERSON] _et al._[51] demonstrated that the pore dimensions of monolithic materials were improved by varying the molecular weight of linear PEG dissolved in the porogen mixture. They synthesized monoliths from acrylic or methacrylic monomers with a variety of terminal groups with and without ethylene glycol links of differing lengths in the side chains, in combination with triethylene glycol dimethacrylate (TEGDMA) and trimethyll-propane trimethacrylate (TRIM) as cross-linkers. [PERSON] _et al._[36] prepared macroporous polymer rods from _N_-acryloyl-tris(hydroxymethyl) aminomethane (NAT) and _N,N'_-methylenebisacrylamide (BIS) using different porogenic mixtures and demonstrated that the rod of higher porosity and pore size was formed when dimethylsulfoxide (DMSO) and a 1:1 combination of tetradecanol and PEG 6,000 were used as coporogens. Then, other macroporous polymeric rod systems were prepared by those authors [37, 38] using NAT and GMA as mono vinyl monomers and BIS or TRIM as cross-linking agents. The reactions were performed in the presence of a ternary porogenic diluent composed of DMSO, tetradecanol and PEG 6,000. The results showed that a lower polymerization temperature and an increase in the amount of nonsolvating diluents (tetradecanol and PEG 6,000) in the reaction mixture led to products with higher porosity. On the other hand, porogens such as supercritical carbon dioxide [5], were used to yield structurally controlled porous cross-linked poly(methacrylate) monoliths. [PERSON] _et al._[5] demonstrated that it is possible to \"fine-tune\" the porous morphology in cross-linked poly(TRIM) monoliths by changing the CO\({}_{2}\) density. In addition, in another research [55] they described the synthesis of porous cross-linked poly(methacrylate) monoliths from radical polymerization using 1,1,1,2-tetrafluoroethane (R134a) as the porogenic solvent. The solvent separation with this porogen was simple and no washing or drying steps were required due to its low boiling point. Whereas variables such as temperature and porogenic system affect the polymer porosity without changes in the rod composition, the concentration of cross-linking agent affects the porous properties and composition of polymer networks. A decrease in the percentage of the cross-linking agent in the reaction mixture to yield monoliths leads to a shift in the pore size distribution curve toward larger pore sizes [36]. This behavior could be due to the fact that an increase in the cross-linking agent concentration could lead to the formation of more cross-linked nuclei in the early stage of the polymerization reaction allowing an earlier separation phase. The higher cross-linking density of nuclei limits their swelling, and then both monomer diffusion into the nuclei and the real coalescence of nuclei formed in the later stage of the reaction do not occur. Therefore, microglobules have smaller sizes and consequently the voids between them are smaller.