Condensation polymerisation fundamentally requires specific monomer characteristics and reaction conditions to form a polymer chain while simultaneously producing a small molecule by-product.
Key Conditions for Condensation Polymerisation
For condensation polymerisation to occur, the monomers involved must meet crucial structural requirements. This process distinguishes itself from addition polymerisation primarily through the nature of its monomers and the formation of a by-product.
Here are the essential conditions:
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Monomer Structure: Presence of at Least Two Functional Groups
The most critical condition is that each monomer molecule must possess at least two functional groups. These functional groups (e.g., hydroxyl groups (-OH), carboxyl groups (-COOH), amine groups (-NH2), or ester groups (-COOR)) allow the monomers to react with each other and link up. Typically, these functional groups are located "one on each end" of the monomer, enabling linear chain growth. -
Monomer Types: Often Involving Two Different Monomers
Condensation polymerisation frequently involves the reaction between two different types of monomers. Each of these monomer types contributes specific functional groups that react with those of the other monomer to form the polymer backbone. For instance:- A diol (with two -OH groups) reacting with a dicarboxylic acid (with two -COOH groups) to form a polyester (e.g., PET).
- A diamine (with two -NH2 groups) reacting with a dicarboxylic acid (with two -COOH groups) to form a polyamide (e.g., Nylon).
While some condensation polymers can be formed from a single type of monomer (if that monomer has two different functional groups capable of reacting with each other, such as an amino acid forming a polypeptide), the common and defining characteristic, especially when compared to addition polymerisation, is the involvement of two distinct monomer types.
Characteristics of Condensation Polymerisation
The following table summarizes key features, highlighting the conditions and outcomes of condensation polymerisation:
| Feature | Condensation Polymerisation | Addition Polymerisation |
|---|---|---|
| Monomer Types | Two types of monomer are typically involved. | One type of monomer is typically involved. |
| Monomer Structure | Monomers must have two functional groups (one on each end). | Monomers must have a C=C (carbon-carbon double) bond. |
| Products | Two products are formed: the polymer plus many small molecules (usually water). | The only product formed is the polymer. |
The Role of By-product Formation
A defining outcome of condensation polymerisation, which stems from the reaction of the functional groups, is the elimination of a small molecule as a by-product for each new bond formed. This by-product is most commonly water (H₂O), but can also be other simple molecules such as hydrogen chloride (HCl) or methanol (CH₃OH), depending on the specific functional groups involved.
