Schematic view of a polycondensation reaction

Polycondensation is considered to be a step growth reaction . The process often (but not always) needs a catalyst which is usually a metal salt or a combination of metal salts. 

The degree of polymerisation is generally lower than in the case of chain polymerisation (between 1000 and 10000) due to inherent process characteristics. The molecule grows step by step at a relatively slow rate. The growth proceeds slowly from monomer to dimer, trimer, etc. until full sized macromolecules are formed only at very high conversion rates towards the end of the reaction time as illustrated by the Table 2.1  

Generally, polycondensation reactions are carried out either in bulk or in organic solvents. 

The control of oxygen is important not only for safety reasons, but also for product quality. Oxygen causes side reactions resulting in products which discolour the end-product and increase the concentration of low molecular weight products. These parts either remain in the product or have to be removed and sent for waste treatment, for instance incineration. The high reaction temperature at the end of the reaction may also lead to degradation products, which also cause discoloration. Localised heat spots have to be avoided. 

The build-up of solid layers in the inside of the reactors or heat exchangers also occurs in these reactions (see Section 2.3.2.1). 

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