Addition polymerization is a chemical process where small molecules called monomers join together to form a larger molecule known as a polymer. This process typically involves unsaturated monomers that contain double bonds, which open up during the reaction, allowing them to connect with each other. The reaction can be initiated by heat, light, or chemical catalysts, leading to a chain reaction that continues until the monomers are consumed. Common examples of addition polymers include polyethylene, used in plastic bags, and polystyrene, found in foam containers. These materials are widely used in everyday products.