Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc Complexes for the Ring-Opening Polymerization of Cyclic Esters

Three novel aminotroponiminate (ATI) zinc complexes I–III (I = [(Ph₂)­ATI]­Zn–N­(SiMe₃)₂, II = [(C₆H₃-2,6-C₂H₅/Ph)­ATI]­Zn–N­(SiMe₃)₂, and III = [(C₆H₃-2,6-CH­(CH₃)₂/Ph)­ATI]­Zn–N­(SiMe₃)₂) were synthesized and tested in the ring-opening polymerization of the lactones β-rac-butyrolactone (BBL) and rac-lactide (LA). The ligands, with two of them literature unknown, were readily obtained via a three-step synthesis from tropolone. Forming a five-membered metallacycle with zinc, the complexes were further structurally examined via single-crystal X-ray analysis and compared with that of the established, 6-ringed β-diiminate (BDI) complex IV ([CH­(CMeNPh)₂]­Zn–N­(SiMe₃)₂). The influence of the varying metallacycle ring size on the polymerization was evaluated. In situ IR measurements indicate a higher catalytic activity of the novel ATI complexes I–III for BBL compared with the BDI system IV. The activity and degree of control were further improved by an in situ generated alkoxy initiating group generated after the addition of 2-propanol. An enhanced initiator efficiency allowed the synthesis of polymers with controlled molecular weights and narrow polydispersities. Furthermore, II and III exhibited a high activity in the ring-opening polymerization of rac-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by the addition of 2-propanol.