Synthesis of lactide/ɛ‐caprolactone quasi‐random copolymer by using rationally designed mononuclear aluminum complexes with modified β‐ketiminato ligand

A series of novel aluminum complexes containing bulky aryl‐β‐ketiminato ligands [ArNCH C₁₀H₇C₆H₅O]Al(CH₃)₂ ( 3a , Ar = C₆F₅; 3b , Ar = C₆H₅; 3c , Ar = 2,6‐ⁱPr₂C₆H₃) have been synthesized in high yields. These complexes were identified by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X‐ray structural analysis. All the aluminum complexes could efficiently catalyze the ROP of ɛ‐caprolactone (ɛ‐CL) and Lactide (LA) in a controlled manner. It was found that the steric effect of the ligand has less effect on the ROP of CL, while the polymerization rate of L‐LA was suppressed significantly. More interestingly, this kind of catalysts can promote the random copolymerization of ɛ‐CL and L‐LA. The transesterification side reaction and the polymer composition could be adjusted by modulating the electronic and steric effects of the ligand. In paticular, compound 3c could produce quasi‐random copolymers without transesterification side reactions, as indicated by both the values of the reactivity ratios of the two monomers (r_LA = 1.31; r_CL = 0.99) and the similar average lengths of the caproyl and lactidyl sequences (L_CL = 2.34; L_LA = 2.44). Finally, a drug‐random copolymer conjugates could be easily prepared by using 3c , indicating a potential application of 3c in pharmacutical and biomedical field. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 203–212