Regarding cross-linked or network polymers, which of the following statements is incorrect?

Options

A. Examples are bakelite and melamine
B. They are formed from bi-and tri-functional monomers
C. They contain covalent bonds between various linear polymer chains
D. They contain strong covalent bonds in their polymer chains

Explanation

To solve the question regarding cross-linked or network polymers and identify the incorrect statement, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cross-Linked Polymers**:
Cross-linked or network polymers are formed by the interconnection of polymer chains through covalent bonds. This structure gives them unique properties compared to linear polymers.

2. **Analyzing the Options**:
We have four statements to evaluate. Let's analyze each one to determine its correctness.

3. **Option 1**: "For cross-linked polymers, the examples are Bakelite and Melamine."
- **Evaluation**: This statement is correct. Both Bakelite and Melamine are indeed examples of cross-linked polymers formed through the condensation of their respective monomers.

4. **Option 2**: "They are formed from bifunctional and trifunctional monomers."
- **Evaluation**: This statement is also correct. Cross-linked polymers are typically formed from monomers that have two or more functional groups (bifunctional or trifunctional), allowing them to link together.

5. **Option 3**: "They contain covalent bonds between various linear polymer chains."
- **Evaluation**: This statement is correct. Cross-linked polymers have covalent bonds that connect different polymer chains, contributing to their network structure.

6. **Option 4**: "They contain strong covalent bonds in the polymer chain."
- **Evaluation**: This statement is incorrect. While cross-linked polymers do contain covalent bonds, the statement implies that these bonds are solely within the polymer chains. In reality, cross-linking occurs between different chains, not just within a single chain.

### Conclusion:
After evaluating all the options, we find that the incorrect statement is **Option 4**.

### Final Answer:
The incorrect statement regarding cross-linked or network polymers is **Option 4**.