The species which can best serve as an initiator for the cationic polymerization is:

Options

A. `LiAIH_(4)`
B. `HNO_(3)`
C. `AICI_(3)`
D. BuLi

Answer

To determine which species can best serve as an initiator for cationic polymerization, we need to identify a Lewis acid among the given options. Cationic polymerization is typically initiated by Lewis acids, which are electron pair acceptors.

### Step-by-Step Solution:

1. **Understanding Cationic Polymerization**:
- Cationic polymerization is a type of polymerization that involves the formation of a cationic species that initiates the polymer chain growth.

2. **Identifying Lewis Acids**:
- Lewis acids are defined as substances that can accept an electron pair. Therefore, we need to look for a species that can act as a Lewis acid among the options provided.

3. **Analyzing the Options**:
- Let's analyze the given options:
- **Option A: AlCl3**
- Aluminum chloride (AlCl3) has an incomplete octet and can accept an electron pair in its vacant p orbital. Thus, it acts as a Lewis acid.
- **Option B: HNO3**
- Nitric acid (HNO3) is a protic acid that can release H+ ions but does not accept electron pairs. Therefore, it is not a Lewis acid.
- **Option C: Butyl Lithium**
- Butyl lithium is a nucleophile and acts as a base due to the presence of a negative charge. It does not act as a Lewis acid.
- **Option D: LiAlH4**
- Lithium aluminum hydride (LiAlH4) is a hydride donor and acts as a nucleophile. It cannot accept electron pairs, so it is also not a Lewis acid.

4. **Conclusion**:
- Among the options analyzed, **AlCl3** is the only species that can act as a Lewis acid and thus can serve as an initiator for cationic polymerization.

### Final Answer:
The species which can best serve as an initiator for the cationic polymerization is **AlCl3**.