Highly Selective and Immortal Magnesium Calixarene Complexes for the Ring‐Opening Polymerization of rac ‐Lactide

The lithiation of 1,3‐dipropoxy‐ p‐tert ‐butylcalix[4]arene (LH 2 ) followed by reaction with n BuMgBr in THF resulted in the formation of the hetero‐bimetallic complex [Li(THF)Mg( n Bu)L] ( 1 ). By contrast, the treatment of tripropoxy‐ p‐tert ‐butylcalix[4]arene (L′H) with n Bu 2 Mg afforded a mononuclear complex [L′Mg( n Bu)] ( 2 ). Single‐crystal XRD studies revealed that in both structures the calix[4]arene adopts a cone conformation, and a Li cation resides in the cavity for 1 . Both 1 and 2 were active for the ring‐opening polymerisation of rac‐ lactide. Compound  2 displayed not only exceptional activity (100 equivalents, 3 min, 92 % conversion, BnOH, room temperature) but also high selectivity (probability to form an r dyad, P r =0.85) and exhibited an immortal character in THF. Surprisingly, compound  2 also showed isotactic bias ( P r =0.30–0.36) and an immortal character if toluene was employed as the solvent; 2 D J ‐resolved 1 H NMR spectroscopy was employed for the assignment of the stereoselectivity.