Goldilocks Effect of a Distal Substituent on Living Ziegler−Natta Polymerization Activity and Stereoselectivity within a Class of Zirconium Amidinate-Based Initiators

A new series of cationic zirconium amidinates, [(η5-C5Me5)ZrMe{N(Et)C(R3)N(tBu)}][B(C6F5)4] (R3 = Me, H, Ph, and tBu) (1a−d), were synthesized and their ability to function
as initiators for the stereoselective living Ziegler−Natta polymerization of 1-hexene evaluated.
Whereas 1a is highly active for the isospecific living polymerization of 1-hexene as previously
reported, polymerizations conducted with 1b and 1c both display a significant loss of
stereocontrol, and in the case of 1b, the polymerization is no longer living. Further, 1d was
found to be inactive for polymerization. The Golidlocks effect observed for the distal R3
substituent in this series, i.e., 1b “too small”, 1c “too large”, 1a “just right”, appears to be
steric in origin. Solution and solid state structural studies suggest, however, that two different
mechanisms are most likely operative for the loss of stereocontrol observed for 1b and 1c: 
a low barrier to metal-centered epimerization in the case of 1b and a lack of steric
discrimination at the metal center for olefin binding in the case of 1c.