Comparison of Polymerization Methods

Bulk polymerization offers real hazards. The thermal conductivities of monomers and polymers are low, and the viscosity buildup limits heat transfer by forced convection. Removal of unreacted monomer from the final product is difficult because of the low surface-to-volume ratio. However, the level of impurities can be held down by use of low initiator levels and diligent monomer removal. 

Solution polymerization offers easier temperature control because of (1) added heat capacity of solvent and (2) lower viscosity. Removal of last traces of solvent and unreacted monomer can be difficult. Level of impurities can be very low, since the initiator residues can be extracted. 

As stated earlier, suspension polymerization is essentially a bulk polymerization carried out in droplets. Temperature control can be complicated by the unstable nature of the suspension. Agitation is critical. Often, as the viscosity within the beads rises, the reaction rate increases suddenly (Trommsdorff effect). This leads to a surge in heat generation, which does not usually occur in solution or emulsion polymerization. However, the viscosity of the continuous phase (water) does not change during the reaction, so that heat removal from the reactor is easier than in bulk polymerization. Monomer recovery parallels that in emulsion polymerization. Polymer recovery is simple and usually leads to lower impurity levels than with emulsion polymers. 

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