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

Abstract

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 (rLA = 1.31; rCL = 0.99) and the similar average lengths of the caproyl and lactidyl sequences (LCL = 2.34; LLA = 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

Random copolymerization of l‐lactide and ε‐caprolactone by aluminum alkoxide complexes supported by N2O2 bis(phenolate)‐amine ligands

“…The steric effect significantly reduces the reactivity ratio gap of ε‐CL and LA in the copolymerization. The successful work of Nomura motivated several research groups attempting to achieve random copolymer with practically Bernoullian distributions . Moreover, Ma and Kan exhibited that the rigid backbone in the salen ligand could create a crowded environment around the metal center which reduces the rate of LA and affords the random CL/LA copolymerization .…”

Fluorinated β‐Ketoiminate Zinc Complexes: Synthesis, Structure and Catalytic Activity in Ring Opening Polymerization of Lactide

“…[3] β-Ketoiminate metal complexes showed excellent activities to the ROP of cyclic esters under mild reaction condition in solution. [4] In recent years we reported on homo-and heteroleptic βketoiminate zinc complexes and showed that heteroleptic phenoxide complexes LZnOAr (type A, scheme 1) are highly active catalysts for the ROP of LA. [5] In addition, β-diketiminate complexes LZnR (R = alkyl, alkoxide OR, amide N(SiMe 3 ) 2 ) were also found to be active catalysts for the ROP of cyclic esters.…”

Comparative Experimental and Theoretical Study of Mg, Al and Zn Aryloxy Complexes in Copolymerization of Cyclic Esters: The Role of the Metal Coordination in Formation of Random Copolymers

“…Due to the contrasting physical properties of polylactide (PLA) and poly-ε-caprolactone (PCL), random copolymers of LA and εCL have generally proved useful for the development of polymer materials with desired mechanical characteristics and biodegradation rate [ 33 , 34 , 35 , 36 , 37 , 38 ]. However, the inherent difference in reactivity of LA and εCL usually leads to the formation of diblock or gradient copolymers, since in most cases LA was polymerized primarily [ 39 , 40 , 41 , 42 , 43 , 44 , 45 ]. Occasionally, only PLA was formed when the mixtures of LA and εCL were used as a comonomer feed [ 46 , 47 ].…”

“…The first approach ( Scheme 2 B) is to adjust the reactivity ratios of LA and εCL to nearly equal with a formation of the random copolymer in the course of living ROP; this approach can be potentially implemented via the catalyst’s design. This approach had been realized, more or less successfully, when used with sterically hindered complexes of Al [ 44 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ], Zn [ 43 , 62 , 63 , 64 , 65 , 66 ], Ti [ 67 , 68 , 69 , 70 , 71 , 72 , 73 ], Zr [ 70 , 71 ], Hf [ 70 ], Mo [ 74 ], Mg or alkali-earth metals [ 45 , 75 ] and lanthanides [ 76 , 77 , 78 ]. The formation of random copolymers with the catalysis by metal chelates was usually attributed to higher inhibition of the coordination of LA relative to εCL due to higher steric hindrance for LA coordination [ 50 , 79 ].…”