What are cross - linked polymers? Give one example of this type.

Explanation

### Step-by-Step Solution:

1. **Definition of Cross-Linked Polymers**:
- Cross-linked polymers are defined as polymers where linear polymer chains are interconnected or linked together to form a three-dimensional network structure. This linking can occur through various types of bonds, such as covalent bonds.

2. **Structure of Cross-Linked Polymers**:
- In cross-linked polymers, the linear chains are not just free-standing; they are chemically bonded at various points, creating a network that provides structural integrity and stability. This three-dimensional structure is what distinguishes cross-linked polymers from linear polymers.

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