Reactors for precipitation polymerization

The consequence is, that polymer chains grow from the solid surface and remain attached to it. The polymer becomes rapidly insoluble, and a solid polymer phase encloses the catalyst particle. The formed polymer particles are porous, enabling the dissolved monomer to reach the active sites. In some processes each cat yst particle generates one polymer particle, that will not agglomerate. Due to thecal tensions the catalyst particles often break, but still stay together in one polymer particle. The polymer particle may grow to a size on the order of 100 xm. Its volume may be 10 larger than the catalyst particle. 

When agglomeration can be excluded, and each catalyst particle generates one polymer particle, the precipitation process is completely controlled by the properties of the catalyst, that is prepared separately. 

For designing a reactor on a larger scale, one merely needs to take into account gas dispersion (see sections 4.6.1 and 4.7.22) and heat transfer. Heat can be transferred via the reactor wall, but heat removal via an evaporating solvent is generally to be preferred (sections 8.3.2 and 8.3.3). Both effects can be controll simultaneously in a bubble column type reactor, by adjusting the gas flow rate. This reactor type has the advantage that there are no internals save the gas distributor. In a stirred reactor, the baffles and the impeller shaft are often liable to fouling. 

Recently several types of homogeneous catalyst systems have been developed (usually metal-organic complexes, e.g., metallocenes ) that are used for polymerizations under conditions where the polymer precipitates from the solution. The active chains often have a very long life ( living polymers). In principle, the polymerization starts in the solution. But as the polymer chains grow, they become insoluble and start to precipitate. Nucleation, surface growth an agglomeration may 

even if no more data are available, one should aim at keeping these two factors the same on scale-up (see sections 4.2.2 and 6.3.2). 

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