Polyvinyl chloride, also called Polychloroethene or PVC, is a thermoplastic. It turns soft when heated and hard when cooled. Polyvinyl chloride is made by polymerization of the monomer vinyl chloride (chloroethene) CH=CHCl. PVC can be made softer and more flexible by adding plasticizers. Phthalates are often used to soften PVC in this way.