Thermosetting polymer, also known as a thermoset or thermosetting plastic, is a polymer consisting of cross-linked structure or heavily branched molecules.

These polymers are in the soft solid or viscous state and on heating undergo extensive cross-linking in moulds and become irreversibly hard as well as insoluble products.

Properties of thermosetting polymers

• One of the main properties of thermosetting plastics or polymers is that they harden during the moulding process and after solidifying they cannot be softened.
• When the polymers are moulded and given a shape, they acquire a three-dimensional cross-linked structure along with strong covalent bonds that help them retain their strength as well as  structure when the temperatures are set high.
• However, thermoset plastics are brittle and tend to char and burn when heat is applied for a prolonged period. Normally, thermosets decompose before melting. 
• Thermosetting plastics are synthesised by condensation polymerization.
• Thermosetting polymers are formed by condensation polymerization, whereas thermoplastic are plastic polymers formed by the addition polymerization process.  

Examples of thermosetting polymers are: vulcanised rubber, novolac, polyurethane etc.