Potassium N-arylbenzimidates as readily accessible and benign (pre)catalysts for the ring opening polymerization of ε-CL and L-LA

Abstract

Eight examples of potassium N-arylbenzimidates, [K2{OCPhN(2-R1,6-R2,4-R3C6H2)}2]n (R1 = R2 = R3 = H K1; R1 = R2 = H, R3 = Me K2; R1 = R2 = H, R3 = OMe K3; R1 = R2 = H, R3 = F K4; R1 = R2 = H, R3 = Cl K5; R1 = R2 = H, R3 = NO2K6; R1 = R3 = H, R2 = Me K7; R1 = R2 = Me, R3 = H K8), have been prepared and characterized by spectroscopic and analytical techniques. In addition, the molecular structures of K6(3THF) and K7 were determined and shown to consist of two-dimensional coordination polymers involving combinations of π-aryl and NO2-/N-donor interactions. In the presence of benzyl alcohol (BnOH), K1 - K8 all showed high efficiency for the ROP of ε-caprolactone (ε-CL) and L-lactide (L-LA), affording linear polyesters (PCL and PLA) with BnO or CH3O end-groups (the latter derived from the quenching solvent). By contrast, lower conversions were achievable when the runs were performed in the absence of BnOH. Mechanistic studies have indicated that for runs performed with BnOH, monomer activation by benzyl alkoxide and N-arylbenzimidate were competitive pathways for the ROP of both cyclic esters. Furthermore, the nature of the N-aryl substituents influenced the ROP of ε-CL and L-LA in distinct ways. For example, K1 and K6 showed lower conversions of ε-CL than that observed for K2 - K5, K7 and K8, while the conversions of L-LA using K1 - K3 and K6 exceeded that shown by K4, K5, K7 and K8. In addition, the type of solvent greatly affected their catalytic efficiency with hexane and toluene proving superior to THF and CH2Cl2 in the ROP of ε-CL. Conversely, for L-LA almost quantitative conversions were observed in THF, CH2Cl2 and toluene while hexane was less effective. Furthermore, kinetic studies of the ROP of L-LA performed at different run temperatures highlighted the beneficial effect of higher temperature.