Which of the following monomer in axcess makes PHBV more flexbile

Options

A. `beta`-hydroxy butanoic acid
B. `alpha`-hydroxy butanoic acid
C. `beta`-hydroxy pentanoic acid
D. `alpha`-hydroxy pentanoic acid

Answer

To determine which monomer in excess makes PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate) more flexible, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of PHBV**:
- PHBV is a copolymer made from two types of monomers: β-hydroxybutanoic acid and β-hydroxyvaleric acid.
- The flexibility of the polymer is influenced by the ratio of these monomers.

2. **Identify the Monomers**:
- The options provided are:
- β-hydroxybutanoic acid
- α-hydroxybutanoic acid
- β-hydroxyvaleric acid
- α-hydroxyvaleric acid

3. **Analyze the Monomers**:
- β-hydroxybutanoic acid contributes to the rigidity of the polymer.
- α-hydroxybutanoic acid is not typically involved in the formation of PHBV.
- β-hydroxyvaleric acid is known to enhance the flexibility of the polymer due to its longer carbon chain compared to β-hydroxybutanoic acid.

4. **Determine the Effect of Excess Monomer**:
- When β-hydroxyvaleric acid is present in excess, it introduces more flexible segments into the polymer chain, leading to an overall increase in flexibility.

5. **Conclusion**:
- Therefore, the monomer that makes PHBV more flexible when present in excess is **β-hydroxyvaleric acid**.

### Final Answer:
The correct answer is **β-hydroxyvaleric acid** (option 3).

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