Condensation polymerization is the process by which two or more chemically different monomers are polymerized to form a cross-linked polymer along with the production of a by-product such as water or ammonia. This type of polymerization occurs through the formation of covalent bonds between the monomers, resulting in the removal of a small molecule as a by-product.

Here is a detailed explanation of the process of condensation polymerization:

1. Definition:
Condensation polymerization is a reaction where two different monomers react by forming covalent bonds and eliminating a small molecule as a by-product.

2. Monomers:
Condensation polymerization involves two or more chemically different monomers. These monomers have functional groups that are capable of reacting with each other, such as alcohol groups (-OH) and carboxylic acid groups (-COOH).

3. Reaction:
The process begins with the reaction between the functional groups of the monomers. For example, in the case of a polyester, the reaction occurs between an alcohol group of one monomer and a carboxylic acid group of another monomer. This reaction forms an ester linkage, resulting in the elimination of a water molecule as a by-product.

4. Polymerization:
As the reaction proceeds, more monomers join the growing polymer chain through similar condensation reactions. The polymerization continues until all the monomers are consumed or until the desired polymer length is achieved.

5. Cross-linking:
In some cases, the condensation polymerization process can lead to the formation of cross-linked polymers. Cross-linking occurs when the polymer chains are connected through additional covalent bonds, resulting in a three-dimensional network structure. This enhances the mechanical properties of the polymer, such as strength and toughness.

6. By-product:
During condensation polymerization, a small molecule, such as water or ammonia, is produced as a by-product. The formation of this by-product is a result of the elimination of functional groups from the monomers during the reaction.

7. Examples:
Condensation polymerization is commonly used in the production of various polymers, including polyesters, polyamides, and polyurethanes. For instance, the condensation reaction between ethylene glycol and terephthalic acid produces polyethylene terephthalate (PET), which is widely used in the manufacture of bottles and fibers.

In conclusion, condensation polymerization is a process in which two or more chemically different monomers react to form a cross-linked polymer, along with the generation of a by-product such as water or ammonia. This type of polymerization is important in the synthesis of various polymers, offering a wide range of applications in industries such as packaging, textiles, and coatings.