What are thermosetting polymers ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Thermosetting Polymers**:
Thermosetting polymers are a type of polymer that, once they are set into a given shape, cannot be remolded or reheated to change their form. They are characterized by their infusibility, meaning they do not melt upon heating.

2. **Cross-Linking**:
These polymers have a network of cross-links between their molecular chains. Cross-linking refers to the bonding between different polymer chains, which creates a three-dimensional structure. This structure is what gives thermosetting polymers their strength and rigidity.

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