`MeSiCl` is used during polymerisation of organo silicones because

Options

A. the chain length of organosilicon polymers can be controlled by adding `(CH_(3))_(3)SiCl`
B. `(CH_(3))_(3)SiCl` improves the quality and yield of the polymer
C. `(CH_(3))_(3)SiCl` does not block the end terminal of silicone polymer
D. `(CH_(3))_(3)SiCl` acts as a catalyst during polymerisation

Answer

To solve the question regarding the use of `MeSiCl` during the polymerization of organosilicones, we can analyze the options provided and determine the correct reasoning based on the chemistry involved.

### Step-by-Step Solution:

1. **Understanding the Structure of `MeSiCl`:**
- `MeSiCl` refers to a compound where a silicon atom (Si) is bonded to three methyl groups (Me) and one chlorine atom (Cl). This structure can be represented as `Si(CH3)3Cl`.

2. **Role of `MeSiCl` in Polymerization:**
- During the polymerization of organosilicones, `MeSiCl` can react with silanol groups (Si-OH) present in the starting materials. The reaction typically involves the release of HCl and the formation of siloxane (Si-O-Si) linkages.

3. **Analyzing the Options:**
- **Option 1:** "The chain length of organosilicone polymer can be controlled by adding `MeSiCl`."
- This statement is **correct**. The presence of the methyl groups (CH3) prevents further polymerization by blocking the reactive sites, thus controlling the chain length.

- **Option 2:** "CH3 whole thrice SiCl improves the quality and yield of the polymer."
- This statement is **incorrect**. While `MeSiCl` can influence the properties of the polymer, it does not necessarily improve the yield or quality in a straightforward manner.

- **Option 3:** "CH3 whole thrice SiCl does not block the end terminal of the carbon."
- This statement is **incorrect**. The presence of the methyl groups does block further polymerization at the terminal ends, thus influencing the structure.

- **Option 4:** "Act as a catalyst."
- This statement is **incorrect**. `MeSiCl` does not act as a catalyst; instead, it participates in the polymerization process.

4. **Conclusion:**
- The only correct statement is that the chain length of organosilicone polymer can be controlled by adding `MeSiCl`. This is because the methyl groups prevent further reactions, thus limiting the growth of the polymer chain.

### Final Answer:
`MeSiCl` is used during polymerization of organo silicones because it controls the chain length of the organosilicone polymer by blocking further polymerization.