Stereoselective polymerization: the chemistry of CaII and YbII: astoundingly similar but not equal!
The prepn. and crystn. of YbII complexes used as stereoselective polymn. catalysts were greatly facilitated by the strong similarity with their calcium analogs. The crystal structures, and the NMR and IR spectra of these Ca/Yb benzyl complexes are nearly identical. Not only a striking similarity in cell parameters but also in mol. geometry is obsd. However, their reactivity in styrene polymn. is completely different: an internally coordinated dibenzylytterbium(II) complex produces polystyrene of high syndiotacticity (r = 94.9%, rr = 90.0 %).
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