How many of these are thermoplastic polymers ?
Polyethylene, Polystyrene, Bakelite, Nylon 6, 6, Terylene, Poly (methyl methacrylate)

Explanation

To determine how many of the given polymers are thermoplastic, we need to analyze each polymer based on its structure and properties.

### Step-by-Step Solution:

1. **Definition of Thermoplastic Polymers**:
- Thermoplastic polymers are linear polymers that can be reshaped upon heating. They do not have cross-linking between their chains, which allows them to melt and be reused.

2. **List of Given Polymers**:
- Polyethylene
- Polystyrene
- Bakelite
- Nylon 6,6
- Terylene
- Poly (methyl methacrylate) (PMMA)

3. **Analyzing Each Polymer**:
- **Polyethylene**:
- Structure: Linear structure made from ethylene (CH2=CH2).
- Conclusion: It is a thermoplastic polymer.

- **Polystyrene**:
- Structure: Linear structure made from styrene (C6H5-CH=CH2).
- Conclusion: It is a thermoplastic polymer.

- **Bakelite**:
- Structure: Cross-linked polymer formed from phenol and formaldehyde.
- Conclusion: It is a thermosetting polymer and not reusable.

- **Nylon 6,6**:
- Structure: Linear structure made from hexamethylenediamine and adipic acid.
- Conclusion: It is a thermoplastic polymer.

- **Terylene**:
- Structure: Linear polyester made from ethylene glycol and terephthalic acid.
- Conclusion: It is a thermoplastic polymer.

- **Poly (methyl methacrylate) (PMMA)**:
- Structure: Linear structure, also known as acrylic glass.
- Conclusion: It is a thermoplastic polymer.

4. **Counting the Thermoplastic Polymers**:
- From the analysis:
- Thermoplastic polymers: Polyethylene, Polystyrene, Nylon 6,6, Terylene, PMMA.
- Thermosetting polymer: Bakelite.
- Total thermoplastic polymers = 5.

### Final Answer:
**There are 5 thermoplastic polymers among the given list.**

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