Double stereoselection chain-end and site control

3 Double stereoselection chain-end control and site control 

So far we have looked at chain-end control and site control if they were independent. As already mentioned, a site-control-o /y mechanism does not exist. Since we are, by definition, making a chiral chain-end and since chain-end control does occur as found in achiral catalyst systems, site control must be accompanied by chain-end influences, or a(n) (a)symmetric site may amplify specific chain end influences. Recent results have shown that this is indeed the case [18,19,21,26], The simple explanation given above has to be modified. We limit ourselves to two issues, (a) the stereochemistry of the coordinating propene and (b) reinforcement of the two mechanisms. 

Important. The reinforcement was proven experimentally by the analysis of the stereoregularity of short chains. The enantioselectivity appeared to be very low for single insertions of propene in small alkyl chains. Thus, if the controlling polymer chain is absent, the site control is poor [28], 

---