Binuclear and Mononuclear Aluminum Complexes as Quick and Controlled Initiators of Well‐ordered ROP of Cyclic Esters

Abstract

Described herein is an efficient ring‐opening polymerization (ROP) of rac‐lactide and ε‐caprolactone (ε‐CL) using novel bimetallic and monometallic aluminum complexes supported by functionalized P–N ligands. All the aluminum complexes (1–3) were seen to be active catalysts in the ROP of both rac‐lactide and ε‐CL in toluene at 60 oC. Bimetallic aluminum complex 2, [Al(Me)2{Ph2P(Se)N(CH2)2N‐(CH2CH2)2O}(AlMe3)] containing an uncoordinated Se atom and an auxiliary Al atom within the proximity of another central Al cation was observed to exhibit the highest catalytic activity among all the catalysts studied. Kinetic experiments, comparing bimetallic and related monometallic complexes revealed that bimetallic cooperativity among the two metallic centers in complex 2 plays an essential role in driving such superior reactivity. We synthesized several isoselective polylactides (PLAs) and polycaprolactones (PCLs) with controlled molecular weights and narrow molecular weight distributions and characterized by 1H, 13C NMR, DSC and TGA.