The influence of micro-mixing on polymerization

Several polymerization processes are carri out in single liquid phase systems. The most widespread process of this type is the high pressure polymerization of ethylene (for low-density polyethylene). Other well Imown examples are the newest high temperature versions of processes for the polymerization of ethylene with Ziegler-type catalysts (for high density polyethylene), the preparation of some synthetic rubbers (polybutadiene, polyisoprene, ethylene-propylene co- and ter-polymers), the bulk polymerization of styrene, etc. All these processes have in common that the reaction product is a polymer melt or solution, that is relatively viscous. Since at the same time the intrinsic reaction rates are usually quite high, the conversion rates are often limited by diffusion. These processes are usually carried out in stirred reactors, for which the effects of micro-mixing have to be taken into account. 

In polymerization processes carried out in batch, the viscosity of the polymer/monomer solution will increase progressively with the conversion of the monomer, because the polymer concentration and its molecular mass both go up. Therefore, the effects of diffusion limitation may increase progressively or even appear quite suddenly at a certain degree of conversion. 

In continuous single-phase polymerization processes, carried out in a mixed reactor, the viscosity of the reaction mixture is always high. This may have several effects the incoming streams of initiator and monomers are not mixed sufficiently rapidly with the reactor contents, and the polymerization proceeds constantly under conditions of diffusion limitation. 

A few examples will be discussed below. The influence of backmixing (residence time distribution combined with micro-mixing) is discussed in section 13.3.2. 

Polyaddition rates entirely determined by micro-mixing 

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