One recently-proposed mechanism for these compounds relies on the interaction of two substituents to protect the reaction site from attack by Lewis bases.  
One recently-proposed mechanism for these compounds relies on the interaction of two substituents to protect the reaction site from attack by Lewis bases.  Besides that "trigger" mechanism, another proposed mechanisms for these compounds include Greene-Rooney. Early experiments using these compounds used magnesium sulfate for "support", though nowadays magnesium chloride is considered better-suited.  The most accepted mechanism makes use of a a metal-alkyl intermediate forming inside one substituent's coordination sphere, and is named for Cossee and Arlman. These compounds' function is aided by the Phillips catalyst - that function is reacting with terminal carbon-carbon double bonds to make products such as polyethylene.  FTP, name this class of organometallic titanium-aluminium catalysts named for their German and Italian discoverers.
One recently-proposed mechanism for these compounds relies on the interaction of two substituents to protect the reaction site from attack by Lewis bases.  
Ziegler-Natta catalysts