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What Are Polyaddition Reactions?

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Polyaddition reactions are similar to polycondensation reactions because they are also step reactions, however without splitting off low molecular weight by-products. The reaction is exothermic rather than endothermic and therefore cannot be stopped at will. Typical for polyaddition reaction is that individual atoms, usually H-atoms, wander from one monomer to another as the two monomers combine through a covalent bond. The monomers, as in polycondensation reactions, have to be added in stoichiometric amounts. These reactions do not start spontaneously and they are slow.

Polyaddition does not play a significant role in the production of thermoplastics. It is commonly encountered with cross-linked polymers. Polyurethane, which can be either a thermoplastic or thermosets, is synthesized by the reaction of multi-functional isocyanates with multifunctional amines or alcohol. Thermosetting epoxy resins are formed by polyaddition of epoxides with curing agents, such as amines and acid anhydrides.
In comparing chain reaction polymerization with the other two types of polymerization the following principal differences should be noted: Chain reaction polymerization, or simply called polymerization, is a chain reaction as the name implies.

Only individual monomer molecules add to a reactive growing chain end, except for recombination of two radical chain ends or reactions of a reactive chain end with an added modifier molecule. The activation energy for chain initiation is much grater than for the subsequent growth reaction and growth, therefore, occurs very rapidly.

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