Final answer:
In a polymerization reaction, the starting molecule is known as a monomer, which binds with other monomers to form a polymer, either through addition or condensation polymerization.
Step-by-step explanation:
The molecule that participates in the polymerization reaction is called a monomer. Polymerization is a chemical process where monomers are chemically bonded together to form a polymer, which is a large molecule with a high molar mass, composed of repeating monomer units. There are two main types of polymerization: addition polymerization where all atoms of the starting monomers are retained in the polymer chain, and condensation polymerization where some atoms of the monomers are lost as small molecules like water.