Meaning
Parasitic chemical pathways occurring alongside main polymer chain propagation alter molecular weight distribution and introduce unwanted functional groups into step-growth polymers. Secondary chemical pathways, collectively categorized as polycondensation side reactions, form cyclic oligomers, ether linkages, linear dimers, and volatile degradation products during polyester or polyamide synthesis. The boundary of these reactions spans reactor synthesis and high-temperature melt re-processing, stopping where thermal degradation dominates over chain extension.
Byproduct Formation
Ether formation during esterification introduces diethylene glycol units into polyethylene terephthalate backbones, lowering glass transition temperature and resin melting point. Uncontrolled polycondensation side reactions generate volatile acetaldehyde, which taints packaged beverages and reduces thermal stability during bottle blow moulding. Adjusting catalyst concentration and reaction temperature suppresses these competing chemical routes.
Chain Scission
High melt temperatures accelerate thermal scission, creating vinyl ester groups and free carboxyl end groups. Accumulation of carboxyl ends from polycondensation side reactions catalyzes further hydrolytic degradation during melt processing, reducing melt strength in sheet extrusion. Vacuum extraction removes light volatile byproducts, shifting chemical equilibrium toward linear chain growth.
Polymer Quality
Polymer purity relies on suppressing unwanted chemical branches during melt processing. Excessive polycondensation side reactions broaden melt viscosity ranges, causing inconsistent filling in multi-cavity injection moulds.
