Increasing Production Rates of High MW Polystyrene

Styrene monomer will spontaneously or auto-polymerize and must be inhibited to prevent reaction during transport and storage. Polymerization is initiated by the generation of free radicals either by the reaction of the styrene with itself ( auto-initiation ) or by means of a peroxide initiator ( chemical initiation ). Radicals rapidly propagate by reaction with monomer and ultimately terminate by coupling with another growing radical or by transferring the radical to a small molecule to start a new chain (chain transfer). 

Two key variables to be controlled in this very simple process are production rate and product molecular weight. Polystyrene is commercially produced in 

Modern Styrenic Polymers Polystyrene and Styrenic Copolymers. Edited by J. Scheirs and D. B. Priddy 0) 2003 John Wiley Sons Ltd 

Temperature is a key control parameter for adjusting production rate. Increasing the temperature increases both the propagation rate constant (according to an Arrhenius correlation) and the concentration of radicals due to increased rates of initiation. Another means of increasing rate is by the addition of chemical initiators to increase the radical concentration. Heat removal capabilities of the system provide a practical limitation to production rate since the reaction is exothermic and the rate increases with temperature, the reaction will run away if the cooling system is inadequate. 

As before, temperature increases all of the rates and the radical concentration (note that termination rate is proportional to the square of the radical concentration). The end result is that product MW is inversely proportional to temperature and therefore inversely proportional to production rate. MW can also be decreased by the addition of chain transfer agents or chemical initiators. 

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Another type of initiator that has been evaluated for increasing polystyrene production rates are the multifunctional peroxides. Examples include 2,2-bis [4,4-bis(tert-butylperoxy)cyclohexyl]propane (I) [9], peroxyfumaric acid, 0,0-te/Y-butyl O-butyl ester (II) [10], ter t-butyl peritaconate (III) [11], and poly (monopercarbonates) (IV) (Figure 7.4) [12]. Although all of these initiators indeed show extremely fast production rates of high MW polystyrene, they all suffer from a flaw, i.e. the polystyrene produced is branched and special precautions must be taken to keep the continuous bulk polymerization reactors from fouling [13]. This is likely why none are currently used commercially for polystyrene manufacture. 

A very unique approach to increasing the production rate of high MW polystyrene was recently developed by Dow researchers. They discovered that the rate-MW curve for auto-initiation polymerization of styrene can be significantly... 

The inverse relationship between rate and MW traditionally presents a problem for the economic production of high MW polystyrene products owing to their slow production rates. The production rate of high MW products is generally increased by the use of peroxides. The addition of a simple monofunctional peroxide such as tert-butyl perbenzoate results in about a 15 % production rate increase over the use of auto-initiation. The use of difunctional peresters [2] and perketals [3] results in >30% rate increases over auto-initiation. However, these... 

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