Q. about the Cossee-Arlman mechanism in alkene polymerization

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The discussion centers on the interaction between TiCl4 and triethylaluminum in polymerization processes. Triethylaluminum alkylates TiCl4, resulting in polymeric TiCl3, which then coordinates with ethene. The inquiry raises questions about the oxidation state of titanium, noting that titanium typically has a +4 oxidation state, which is crucial for its catalytic properties. The discussion explores the implications of adding electron density through ethene coordination, suggesting that this could lower the oxidation state of titanium. It also considers whether ethene occupies an empty d orbital on titanium, potentially leading to an unstable intermediate that facilitates olefin insertion, thereby creating another vacant site on the titanium atom for further reactions.
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Starting out with TiCl4 and triethylaluminum, I understand how the triethylaluminum alkylates the TiCl4 and creates a polymeric TiCl3, but then an ethene molecule coordinates with a vacant site on the alkylated TiCl3? I thought titanium's highest oxidation state was +4?
 
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That's what makes it catalytic. It let's go again.


However... think about this. What would happen to the oxidation state of Ti+4 if you added electron density to it (in the form of a double bond)? Would it go up or down?
 
The oxidation state would go down right? When the ethene molecule coordinates with the alkylated TiCl3 does it occupy an empty d orbital on the titanium atom? I suppose that would create an unstable intermediate and open the door for the olefin insertion step to create another vacant site on the titanium atom. Thank you for your help.
 
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