How many among the following are biodegradable polymers?
(i) Polyhydroxybutyrate (PHB)
(ii) PHBV
(iii) SBR
(iv) Polyglycolic acid
(v) Bakelite
(vi) Thiokol
(vii) Nylon-2-Nylon-6
(vii) PMMA
(ix) Nylon-6, 6.

Answer

To determine how many of the given polymers are biodegradable, we will analyze each polymer based on its chemical structure and functional groups. Biodegradable polymers typically have functional groups that allow them to be broken down by microorganisms.

### Step-by-Step Solution:

1. **Identify each polymer and its properties:**
- (i) **Polyhydroxybutyrate (PHB)**: This polymer has an ester functional group, which makes it biodegradable.
- (ii) **Polyhydroxybutyrate valerate (PHBV)**: Similar to PHB, it also contains an ester functional group, thus it is biodegradable.
- (iii) **Styrene-butadiene rubber (SBR)**: This is a synthetic rubber that does not decompose easily; hence, it is non-biodegradable.
- (iv) **Polyglycolic acid (PGA)**: This polymer contains an acid functional group, making it biodegradable.
- (v) **Bakelite**: This is a thermosetting plastic with a phenolic structure, which is non-biodegradable.
- (vi) **Thiokol**: This polymer contains sulfur and has functional groups that allow for biodegradation, making it biodegradable.
- (vii) **Nylon-2-Nylon-6**: This polymer has both amide and acid groups, which makes it biodegradable.
- (viii) **Polymethyl methacrylate (PMMA)**: This is a non-biodegradable polymer.
- (ix) **Nylon-6,6**: This polymer is also non-biodegradable.

2. **Count the biodegradable polymers:**
- From the analysis, the biodegradable polymers are:
- PHB
- PHBV
- Polyglycolic acid (PGA)
- Thiokol
- Nylon-2-Nylon-6

Therefore, there are **5 biodegradable polymers** in total.

### Final Answer:
**5 biodegradable polymers**