The Simmons-Smith reaction uses iodomethylzinc iodide, formed from the reaction of diiodomethane and zinc coated with copper, to add across alkenes and form cyclopropanes. This reaction proceeds through a stereospecific syn-addition. Ziegler-Natta catalysts, often a combination of titanium chloride and an aluminum alkyl, are used to catalyze the coordination polymerization of alkenes. The mechanism involves the alkyl aluminum activating the titanium chloride, which then inserts into the alkene to propagate the growing polymer chain.