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

Abstract

The bowl‐shaped aluminum alkoxide complexes bearing N2O2 bis(phenolate)‐amine ligands having different side arms as pyridine (1), dimethyl amine (2), and diethyl amine (3) were shown to be highly efficient and well behaved in the homopolymerization and copolymerization of l‐lactide (LA) and ε‐caprolactone (ε‐CL) at 100 °C. The rates of copolymerization are similar for Complexes 1–3 where nearly full conversions were achieved in 60 h for [LA]:[CL]:[Al] ratio of 50:50:1. The minor adjustment of the side arms of the Catalysts 1–3 gave profound differences in the LA/ε‐CL copolymer sequences where tapered, gradient, and highly random structures were obtained in one system, respectively. The chelation of LA to Al metal after ring‐opening process and suitable steric hindrance of the side arms were believed to participate and saturate the aluminum metal centers giving different copolymer structures. The random LA/ε‐CL copolymer structure was confirmed by nuclear magnetic resonance and differential scanning calorimetry analysis. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1635–1644

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

“…The ratio of the integral intensities of these signals allows one to determine the average sequence length (ASL) of PCL fragments in the copolymer by the ratio: ASLC = (ICC + ICL)/ICL (see Table 2). Detailed and consistent analysis of 13 C NMR spectra of LLA/εCL copolymers was reported earlier, the most suitable interval for the detailed analysis of the copolymer's microstructure is the area 169-174 ppm of the signals of carbonyl groups [42,49,52,53,61,62,72]. The view of the 13 C NMR spectra of LLA/εCL copolymers (Figure 4A,B) confirmed the difference in catalytic behavior of the Zn complexes 3 and 4.…”

“…Detailed and consistent analysis of 13 C NMR spectra of l LA/εCL copolymers was reported earlier, the most suitable interval for the detailed analysis of the copolymer’s microstructure is the area 169–174 ppm of the signals of carbonyl groups [ 42 , 49 , 52 , 53 , 61 , 62 , 72 ]. The view of the 13 C NMR spectra of l LA/εCL copolymers ( Figure 4 A,B) confirmed the difference in catalytic behavior of the Zn complexes 3 and 4 .…”

“…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 ].…”

Copolymerization of L-Lactide and ε-Caprolactone promoted by zinc complexes with phosphorus based ligands

“…Generally, although the homopolymerization rate of CL is significantly higher than that of LA (Phomphrai et al, 2010;Chumsaeng et al, 2019), in the copolymerization reactions the net reactivities of these comonomers are inverted (Chandanabodhi and Nanok 2017) therefore block or gradient copolymers are generally produced within a polylactide block is firstly formed. Nevertheless, random copolymers may be obtained as a result of side transesterification reactions which partially randomize the original copolymer composition (Kasperczyk and Bero 1993;Dakshinamoorthy and Peruch 2012;Webster et al, 2013).…”

“…In the literature, some well-characterized metal initiators are reported to directly promote the random copolymerization of LA and CL. The most important examples are metal complexes of aluminum (Florczak and Duda 2008;Pappalardo et al, 2009;Nomura et al, 2010;Li et al, 2012;Pilone et al, 2015;Zhang et al, 2017;Shi et al, 2018;Chumsaeng et al, 2019;Garcia-Valle et al, 2020), yttrium (Hu, Wang et al;Shen et al, 1996;Florczak and Duda 2008;Fadlallah et al, 2017), titanium (Wei et al, 2009;Dakshinamoorthy and Peruch 2012;Lapenta et al, 2015;Sun et al, 2020) or zinc (Darensbourg and Karroonnirun 2010) often bearing phenoxy-based ligands.…”