Preparation of Branched Polystyrene

Most commercial polystyrene is manufactured using radical polymerization because it is the lowest cost chemistry [1]. The low cost is primarily due to the forgiving nature of the chemistry, i.e. monomers and solvents do not need to be highly pure. The economic impact of this attribute is significant. For example, the cost to convert styrene monomer to polymer using anionic polymerization chemistry is about 50% higher than for radical polymerization. This cost 

Modem Styrenic Polymers Polystyrene and Styrenie Copolymers. Edited by J. Scheirs and D. B. Priddy c 2003 John Wiley Sons Ltd 

Clearly the benzylic H-atoms attached to the polystyrene backbone are not as labile as in cumene. This is likely due to the steric effect of the coil configuration of the polymer chain which blocks access of the /er/-butoxy radicals. Nonetheless, some backbone H-atom abstraction from the polystyrene backbone does occur during radical polymerization of styrene. The extent of abstraction is proportional to the concentration of peroxide initiator added to the process. Typically, in commercial continuous bulk polymerization processes the concentration of peroxide initiator is kept below 500 ppm. Also a few percent of a solvent having some chain transfer activity (ethylbenzene) is added to the styrene feed. This is done so that the extent of branching is small. If the concentration of initiator is increased to 500 ppm and/or the chain transfer solvent falls below a certain level, the extent of branching can increase to a level where gels began to appear in the product. The mechanism of 

The main reason that the branching which takes place in suspension polymerization does not lead to reactor fouling is primarily due to the fact that each droplet of monomer dispersed in the continuous water phase is like a tiny isolated reactor. The water phase is continuously in contact with the reactor 

Since the main mode of termination is radical coupling, coupling of the polystyryl macroradicals produced by H-atom abstraction leads directly the formation of a crosslink. 

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