Prepolymers

Prepolymers are polymeric species that are partially polymerized and terminate with reactive functional groups. They act as intermediates in the manufacture of higher molecular weight polymers and networks. By controlling the degree of polymerization and end-group functionality, manufacturers can tune viscosity, pot life, cure kinetics, and final properties.

Common types include isocyanate-terminated polyurethane prepolymers, epoxy-functional prepolymers (epoxy-terminated or glycidyl ether-containing oligomers), and hydroxyl- or amine-terminated prepolymers used to prepare polyurethanes, polyureas, epoxies, and related systems. They are typically prepared by partial reaction of di- or polyfunctional monomers, by end-capping reactive chains, or by condensation of oligomers with residual reactive groups.

Key characteristics of prepolymers include a defined average molecular weight, a measured functionality (number of reactive end groups), and residual reactivity of the end groups. They may exhibit high viscosity and amine, hydroxyl, or isocyanate end groups, depending on chemistry. Storage stability and moisture sensitivity are important, especially for isocyanate-containing prepolymers, which can continue to cure if exposed to moisture.

Applications span coatings, adhesives, sealants, foams, elastomers, and composites. Prepolymers enable rapid cure and room-temperature processing and are used in spray foams, structural adhesives, and high-performance coatings. They may require careful mixing with curatives to control pot life and crosslink density.