Continuous free radical polymerization, mixing

An example of the importance of mixing effects in chemical reactors continuous free radical polymerization. One might now ask the question are segregation effects really important in practice or is micromixing "a solution in search of a problem"... 

CO2 is completely nonflammable. This property provides a tremendous advantage for some traditionally hazardous chemical processes and reactions. For example, fluoroethylene monomers used for the production of tetrafluoroethylene (TFE) are rendered nonexplosive when mixed with CO2. In addition, highly reactive free-radical polymerization of these monomers can be carried out directly in a supercritical CO2 continuous phase. 

There are several product quality reasons for favoring flow reactors. If the life of a growing chain is small, as in free-radical polymerizations, a perfectly mixed CSTR will give the lowest polydispersity and the narrowest composition distribution for copolymers. Heat and mass transfer are best accomplished in flow systems. Thus the continuous mode is preferred for vinyl addition polymers where there is a large exotherm. It is also preferred for condensation polymers where the by-product must be removed to overcome an equilibrium limitation and for situations in general where a small molecule, typically solvent or unreacted monomer, must be removed as part of a clean-up operation. 

In continuous polymerization the initiator and the monomer are often introduced separately, particularly in free radical polymerization. When the mixing of each of these streams with the reactor contents is not sufficiently rapid, compared to the propagation reaction, different initiator/monomer ratios will exist in various zones in the reactor. Obviously, this will cause a broadening of the molecular mass distribution. It will also reduce the effectivity of the initiator, since in the zones with high initiator concentration, more free radicals will combine. 

Low-density polyethylene (density = 0.915-0.935 g cm ) has long been manufactured by free-radical polymerization using continuous autoclave reactors. The autoclave reactor shown schematically in Fig. 3a is a typical multizone ethylene polymerization reactor. The reactor is typically a vertical cylindrical vessel with a large LID ratio. The reacting fluid is intensely mixed... 

As mentioned previously, most continuous anionic polymeri tion studies have been conducted at relatively low temperatures ( < 50 °C). Even then, mixing kinetics have been of considerable concern due to the fast polymerization kinetics. In the recent anionic polymerization studies of Priddy and Pirc [1,73], the polymerization temperature range of 80-140 °C was studied (typical free radical temperature range). At these temperatures, the polymerization kinetics are extremely fast Also, the high polymerization temperature results in significant thermal termination of active polystyryl chains. Kem et aL [74] found that the termination reaction involved liberation of lithium hydride (1) and was first order. They found the apparent rate constant K at 65, 93, and 120°C are 0.15, 0.78, and 1.3 h respectively. 

A different approach was taken by Touhsaent et al. [2081. These authors synthesized two polymers, one of which formed a network, by simultaneous independent reactions in the same container. They have indicated that intercrosslinking reactions are eliminated by combining free radical (acrylate) and condensation (epoxy) polymerization. By this method, they modified an epoxy resin with poly(n-butyl acrylate) polymer. They have found that a two-phase morphology developed, consisting of co-continuous rubber domains (about 0.1—0.5 p-m) within the epoxy resin. The dimensions of the dispersed rubber phase domains and the extent of molecular mixing between the two components were found to depend on the relative reaction rates (or gel time) with respect to the rate of phase separation. Better mechanical properties resulted when the extent of molecular mixing was minimized and heterophase semi-IPNs were produced. 

Nowakowsky et al. [23] disclosed a continuous solution polymerization wherein an aqueous monomer mixture was polymerized by free radicals in a special reactor which prevented back mixing of the polymerized gel with the fresh monomer being fed continuously to the reactor. An aqueous mix of 25 wt.% partially neutralized (75%) acrylic acid, 0.8% MBA (BOM) and 0.8% ammonium persulfate (BOM) was fed to the reactor concurrently with a 0.3% solution of sodium bisulfite in water. Relative feed rates were 200 1. Reactor temperature was maintained at 60-63°C at a pressure of 200 mbar for 40 minutes. The gel was then dried in a kneader at ITO C and 150 mbar, yielding a dry product with a swelling capacity of 46 g/g in 0.9% NaCI solution, with a sol fraction of 11%. 

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