Heterometallic cooperativity in divalent metal ProPhenol catalysts: combining zinc with magnesium or calcium for cyclic ester ring-opening polymerisation†
Homobimetallic zinc complexes have shown extremely high activity in lactide (LA) ring-opening polymerisation (ROP). In contrast, heterobimetallic ROP catalysts combining zinc with other divalent heterometals such as Mg and Ca are unknown, despite heterometallic cooperativity leading to improved catalyst performance with other heterocombinations. Here, we report the first heterobimetallic LA ROP catalysts featuring only divalent metals, by systematically replacing one Zn centre in a bis-Zn ProPhenol complex with Mg or Ca. The Mg/Zn and Ca/Zn complexes are highly active and outperform their homobimetallic counterparts in rac-LA and ε-caprolactone (ε-CL) ROP. The Ca/Zn complex exhibits the highest activity, converting 48 eq. rac-LA in 5 s and 380 eq. ε-CL in 30 s at room temperature. The activity enhancements are credited to Lewis acidic Mg and Ca enhancing monomer coordination as well as altering the catalyst electronics by increasing the polarity of the Zn–Et initiating group. The heterobimetallic catalysts also display unusual two-step kinetic plots in rac-LA ROP, which is attributed to the initial catalyst preference for the coordination and insertion of one of the rac-LA stereoisomers.