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

Abstract

A series of iminophenoxide ligand precursors [2-(RN=CH)C6H4OH] (HL1: R = C6H5; HL2: R = 2,6-iPr2C6H3) and [2-(RN=CH)-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 Pm values of 0.58 and 0.66, respectively. Complex 4 led to the best selectivity for isotacticity (Pm = 0.75) when the polymerization was performed in toluene at 0 °C.