What is the primary structural feature necessary for a molecule to make it useful in a condensation polymerization reaction ?

Explanation

To determine the primary structural feature necessary for a molecule to be useful in a condensation polymerization reaction, we need to consider the characteristics of the monomers involved in this type of reaction.

### Step-by-Step Solution:

1. **Understanding Condensation Polymerization**:
- Condensation polymerization is a type of polymerization where two monomers react to form a polymer, accompanied by the loss of a small molecule, typically water or alcohol.

2. **Identifying the Required Structural Feature**:
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