Suspension and Bulk Polymerization Techniques

The preparation of polymers through free radical polymerization can be carried out in several ways, both in laboratory and industrial scales. Among the techniques, bulk, suspension, emulsion and solution polymerization methods are of the most practical importance. 

In the case of bulk polymerization, the initiator is dissolved in the monomer and the viscosity of the system increases with progressing polymerization from liquid, through the state of gel ( gel-effect ) to solid polymer. This polymerization technique has many disadvantages, among others the transfer of exothermic reaction heat from the system is very complicated. The reaction heat reaches values as high as 85 kj/ mol, and because polymers are poor heat conductors, this may cause the temperature of the system to reach the boiling point of monomer and consequently the polymer is foamed by vapours of monomer. 

These troubles can be eliminated using suspension polymerization method. Here, the monomer with dissolved initiator is dispersed in 

Micron-size particles can also be prepared by polymerization of monomer dispersion in organic solvents (e.g. alcohol) in which the emerging polymer is not soluble. At the beginning of the process, the reaction mixture is homogenous but during the reaction, stabilized polymer particles precipitate. This method offers uniform particles with a diameter of 2-15 pm (40). 

A similar principle is applied in dispersion polymerization in ionic liquids produced particles are in sub-micron range (41,42). The monomer, initiator and colloidal stabilizer are soluble in the liquid medium, but the obtained polymer is not. Different kinds of ionic liquids may be used, such as for styrene l-butyl-3-methylimidazolium tetrafluoroborate or N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide. In general, radical polymerization in ionic liquids provides higher polymerization rate and higher molecular weights than the process in bulk or organic solvents (homogenous system). 

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