General Pore Formation Mechanism of Organic Monoliths

1 General Pore Formation Mechanism of Organic Monoliths 

Due to the fact that thermally initiated free radical copolymerization is by far the most routinely employed method for fabrication of organic monolithic stationary phases, the pore formation mechanism is discussed for this particular kind of polymerization. 

Other modes of copolymerization, like photochemically or chemically initiated free radical polymerization, ROMP, or polycondensation reactions, in the presence of inert diluents are, however, supposed to be comparable with respect to the formation of support porosity. 

The polymerization mixture for the preparation of rigid, macroporous monolithic materials in an unstirred mold generally contains a monovinyl compound (monomer), a divinyl compound (crosslinker), an inert diluent (porogen), as well as an initiator. The mechanism of pore formation of such a mixture has been postulated by Seidl et al. [101], Guyot and Bartholin [102], and Kun and Kunin [103] and can be summarized as in the following text. 

The thermal initiator, present in the polymerization mixture, decomposes at a certain temperature accompanied by disposal of radicals that initiate the polymerization reaction of monomer as well as cross-linking molecules in solution. After becoming insoluble in the employed polymerization mixture (strongly dependent on the nature of porogenic solvent and on the degree of cross-linking), the polymer nuclei precipitate. 

---