Physical classification of reaction systems

Polycondensation reactions are conducted under a variety of reaction conditions. The initial features of kinetic control may vary considerably according to the system used. For example, some are limited primarily by the kinetics of the chemical reactions, whereas others have as the limiting rate processes the diffusion of the reactants. The former are those in 

In most practical situations the reaction system is not chosen because of its influence on the reaction kinetics. Economic feasibility is most often the dominating factor in this choice. Where adequate process control can be obtained, melt or a combination of melt and solid-state polymerization may be preferred for obvious reasons. When solvents must be used, they are often selected primarily on a combination of polymer solubility and cost. 

Each of the main types of polycondensation systems will be reviewed briefly, with emphasis on the primary kinetic features. Additional details may be found in earlier publications [5, 6]. 

Polymerization in the melt is widely used commercially for the production of polyesters, polyamides, polycarbonates and other products. The reactions are controlled by the chemical kinetics, rather than by diffusion. Molecular weights and molecular weight distributions follow closely the statistical calculations indicated in the preceding section, at least for the three types of polymers mentioned above. There has been much speculation as to the effect of increasing viscosity on the rates of the reactions, without completely satisfactory explanations or experimental demonstrations yet available. Flory [7] showed that the rate of reaction between certain dicarboxylic acids and glycols was independent of viscosity for those materials, in the range studied. The viscosity range had a maximum of 0.3 poise, however, far below the hundreds of thousands of poises encountered in some polycondensations. 

Solution polycondensation, i.e., where the monomers and polymer are soluble in the solvent(s), is also kinetically controlled. It is closely related mechanistically to reactions in the melt, but the choice of the solvent may influence both the rate and equilibrium, and hence the molecular weight. This process is utilized commercially in the production of a variety of organic coatings and certain solution-spun fibres, e.g., certain spandex types and also very high-melting aromatic polymers. 

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