Ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium iminophenoxides

Abstract

A series of iminophenoxide ligand precursors [2-(RNCH)C₆H₄OH] (HL¹: R = C₆H₅; HL²: R = 2,6-ⁱPr₂C₆H₃) and [2-(RNCH)-4,6-^tBu₂C₆H₂OH] (HL³: R = C₆H₅; HL⁴: R = 2,6-ⁱPr₂C₆H₃) were synthesized. These compounds reacted with NaN(SiMe₃)₂/15-crown-5 or KN(SiMe₃)₂/18-crown-6 to afford corresponding crown ether complexes of sodium and potassium iminophenoxides (1, (15-C-5)NaL²; 2, (15-C-5)NaL³; 3, (15-C-5)NaL⁴; 4, (18-C-6)KL¹; 5, (18-C-6)KL²; 6, (18-C-6)KL³; 7, (18-C-6)KL⁴). Catalysis of the complexes toward the ring-opening polymerization of rac-lactide was studied. Each of the complexes exhibited high catalytic activity at room temperature. Complexes 2, 3, 6 and 7 showed poor isotactic selectivity and relatively broad molecular weight distributions. Complexes 1 and 5 resulted in more stereoregular polymers with P_m values of 0.58 and 0.66, respectively. Complex 4 led to the best selectivity for isotacticity (P_m = 0.75) when the polymerization was performed in toluene at 0 °C.