A type of catalyst used in the synthesis of polymers such as high-density polyethylene from olefins (alkenes). They are based on compounds of titanium (p. 420) such as titanium (IV) chloride (TiCl₄) and organoaluminium compounds such as triethyl-aluminium (Al(C₂H₅)₃). They are named after the German chemist Karl Waldemar Ziegler (1898–1973) and the Italian chemist Giulio Natta (1903–79). They were jointly awarded the Nobel Prize in Chemistry in 1963.