Answer:
water
Step-by-step explanation:
Polymerization is a process in which a large number of small molecules called monomers combine to form a macromolecule called a polymer.
One of the methods employed to synthesize polymers is via condensation reaction. Here, when monomers combine, they release small molecules like water as a byproduct.
For example, condensation reaction between monomers containing a diol group and others containing dicarboxylic acid results in the formation of polyesters along with the elimination of molecules of water per repeat unit as shown below:
n(H2N-X-NH2) + n(HOOC-Y-COOH) →[HN-X-NHCO-Y-CO]n + 2nH2O