Substances that modify and increase the rate of a reaction without being consumed in the polymerization process are known as polymerization catalysts. Polymerization catalysts include Ziegler–Natta catalyst, single-site catalyst, metallocene catalysts, and reaction initiator catalysts. These catalysts usually do not have well defined molecular structures. Metallocene catalysts help in refining and designing the structure of polymers. Metallocene catalysts offer characteristics such as improved clarity in film, high impact strength and toughness, and better melt characteristics on account of the control over molecular structure. Transition metals such as titanium and zirconium are also used as polymerization catalysts. Polymerization catalysts are usually made up of zeolites, metals (base metals and precious metals), enzymes, organometallic materials, and chemical compounds (peroxides, acids, amines, etc.) among others. Certain end-user industries such as packaging, building & construction, electrical & electronics, household goods, automobiles, and sports goods are utilizing polymerization catalysts in their manufacturing processes.