Ring‐Opening Polymerization of Lactide with Group 3 Metal Complexes Supported by Dianionic Alkoxy‐Amino‐Bisphenolate Ligands: Combining High Activity, Productivity, and Selectivity

Abstract

A series of new alkoxy-amino-bis(phenols) (H2L 1-6) has been synthesized by Mannich condensations of substituted phenols, formaldehyde, and amino ethers or diamines. The coordination properties of these dianionic ligands towards yttrium, lanthanum, and neodymium have been studied. The resulting Group 3 metal complexes have been used as initiators for the ring-opening polymerization of rac-lactide to provide poly(lactic acid)s (PLAs). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers, together with the linear natures of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. Complex [Y(L6){N(SiHMe2)2}(THF)] (17) polymerized rac-lactide to heterotactic PLA (Pr = 0.90 at 20 degrees C) and meso-lactide to syndiotactic PLA (Pr = 0.75 at 20 degrees C). The in situ formation of [Y(L6)(OiPr)(THF)] (18) from 17 and 2-propanol resulted in narrower molecular weight distributions (PDI = 1.06). With complex 18, highly heterotactic PLAs with narrow molecular weight distributions were obtained with high activities and productivities at room temperature. The natures of the ligand substituents were shown to have a significant influence on the degree of control of the polymerizations, and in particular on the tacticity of the polymer.

Synthesis, structure and catalytic behavior of yttrium complexes bearing a diaminobis(phenolate) ligand

“…The metal center is coordinated by three oxygen atoms and three nitrogen atoms to form a slightly distorted octahedron. The overall molecular geometry of complex 2 is similar to the geometries observed in alkoxy-aminobis (phenolate) yttrium or lanthanide complexes [6,7,10] and in LLnNPh 2 (THF) [15] In complex 2, the Y-O and Y-N(amido) bond lengths are slightly longer than the corresponding Y-O(phenoxy) and Y-N(amido) bond lengths found in complex 1. [10] .…”

“…It can be seen that complex 2 is an efficient initiator for the ROP of L-lactide, allowing full conversion of 150-500 equiv of L-lactide in 15-80 min at 70℃ in toluene, eventually offering high number-average molecular weight (M n ). The catalytic activity of complex 2 is much higher than the activities of amino-aminobis (phenolate) yttrium alkyl complex [19] and the carbonbridged bis(phenolate) lanthanide methoxides [18] , but somewhat lower than those of alkoxy-aminobis (phenolate) yttrium amides [6,7] . As shown in Figure 3, there is a linear relationship between the number-average molecular weights and the molar ratios of monomer to initiator, and the molecular weight distributions (PDIs) of the resulting polymers are relatively narrow, which indicated a somewhat controlled polymerization achieved with this initiator [3,7] .…”

“…Of these recent advances in catalyst design, the application of bridged bis(phenolate) groups as ancillary ligands in groups 3, 4 and lanthanide chemistry has received increasing attention [4,5] . As a result, several well-defined single-site yttrium complexes bearing amino-bridged bis(phenolate) ligands have been developed recently, which have been found to be efficient initiators for ROP of rac-lactide [6] and stereoselective ROP of rac-lactide [6−8] and rac--butyrolactone [5,8,9] . It is worthy to note that all of these reported yttrium complexes were prepared via alkane and amine elimination routes, i.e.…”

Unprecedented Polymerization of Trimethylene Carbonate Initiated by a Samarium Borohydride Complex: Mechanistic Insights and Copolymerization with ε‐Caprolactone

“…[1][2][3][4][5][6][7] Aliphatic polyesters and polycarbonates, as well as their copolymers, are indeed highly attractive as biomaterials for their biocompatibility and bioresorbability. They are widely used in drug-delivery systems or in tissue engineering.…”

Polymerization of rac-lactide catalyzed by group 4 metal complexes containing chiral N atoms

“…Most known catalysts do not bias toward different enantiomers in the ROP of rac-LA, thereby furnishing atactic amorphous PLA. In the last decade, many complexes of Ca [8], Mg [9,10], Zn [11,12], Sn [13], Al [5,6], Sc [14], Y [15,16], Ge [17], Ti [18,19], and Zr [20], have been found to be good initiators for the ROP of rac-LA to heterotactic-rich PLA, while the reports on efficient catalysts to isotactic-rich PLA are limited to salen and salan aluminum complexes [2,3,[21][22][23][24].…”