SOLUTION POLYMERIZATION

In solution polymerization, the monomer, initiator, and the resulting polymer are all soluble in the solvent or solvent blend. In the case of free radical polymerization, the rate of the reaction is directly proportional to the monomer concentration. Usually, a solution polymerization is started with a high monomer concentration (70% or more) using a minimal amount of catalyst, initiator and a solvent with a low chain transfer constant. As the reaction proceeds, more solvent is added to regulate the viscosity and additional catalyst/initiator is added to adjust the reaction rate. The addition of initiator is important because towards the end of the polymerization the degree of dilution of the monomer is extremely high, and therefore, the efficiency of the initiator is very low. In some cases, it is lost by chain transfer with the solvent or by mutual destruction of the radicals. Thus, several additions of initiator are necessary towards the end of the reaction to achieve a high degree of polymerization.

Adjusting the monomer and intiator concentration is also important for another reasons; the lower the monomer concentration and the higher the initiator concentration, the lower the resulting average molecular weight because there are more reactive sites. Thus a high monomer concentration during the main growth stage results in a higher average molecular weight. A high polymer concentration in solution can also increase branching and crosslinking due to chain transfer.

A homogenous polymerization process has several advantages; the heat of polymerization can be easily dissipated by mixing and by evaporative cooling with or without compression of the vapor and reflux-cooling. Furthermore, the molecular weight can be easily controlled by adding chain transfer agents (halogenated hydrocarbons or thiols) or by adjusting the initiator and monomer concentration. Another advantage is that pure polymers are obtained after evaporation of the solvent.

The solution polymerization has also a number of disadvantages. For example, the solvent can act as a chain transfer agent causing a decrease of the molecular weight. (The lower molecular weight can sometimes be compensated by cross-linking.) Furthermore, the rate of reaction in solution polymerization is often lower than in emulsion polymerization and the residual monomer is higher. To distill unpolymerized monomer can sometimes be difficult unless a satisfactory azeotrope with the solvent exists or might not be economically at all, for example, if no azeotrope with the chosen solvent exists. Another drawback of solution polymerization is the increase of viscosity with increasing degree of conversion, i.e. with increasing molecular weight of the growing polymer chains and solid content.