Preparation of Polystyrene by Free-Radical Mechanism

Styrene is one of those monomers that lends itself to polymerization by free-radical, cationic, anionic and coordination mechanisms. This is due to several reasons. One is resonance stabilization of the reactive polystyryl species in the transition state that lowers the activation energy of the propagation reaction. Another is the low polarity of the monomer. This facilitates attack by fi-ee-radicals, differently charged ions, and metal complexes. In addition, no side reactions that occur in ionic polymerizations of monomers with functional groups are possible. Styrene polymerizes in the dark by free-radical mechanism more slowly than it does in the presence of light [149]. Also, styrene formed in the dark is reported to have greater amount of syndiotactic placement [150]. The amount of branching in the polymer prepared by free-radical mechanism increases with temperature [136]. This also depends upon the initiator used [151]. 

The following information has evolved about the free-radical polymerization of styrene  

Styrene polymerizes thermally [151-155]. This is discussed in Chap. 3. 

Oxygen retards polymerizations of styrene. At higher temperatures, however, the rate is accelerated due to peroxide formation [156]. 

The rate of styrene polymerizations in bulk is initially, at low conversions, first order with respect to monomer concentrations. In solution, however, it is a second order with respect to monomer [157]. 

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