Wei-Yi Lu

Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of l-Lactide

A series of sodium complexes bearing NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized and used as catalysts for the ring-opening polymerization of L-lactide (L-LA). Electronic effects of ancillary ligands coordinated by sodium complexes substantially influence the catalysis, and ligands with electron-donating groups increase the catalytic activity of the sodium complexes for catalyzing L-LA polymerization. In particular, a sodium complex bearing a 4-methoxy group has the highest activity with conversion up to 95% within 30 s at 0 °C and a low polydispersity index of 1.13, whereas the 4-bromo group showed the poorest performance with regard to the catalytic rate of L-LA polymerization in the presence of benzyl alcohol (BnOH). (1)H NMR pulsed-gradient spin-echo diffusion experiments and single-crystal X-ray analyses showed that sodium complexes [L(H)Na(THF)]2 and [L(4-Cl)Na(THF)]2 were dinuclear species in both solution and the solid state. The kinetic results indicated a first-order dependence on each of [[L(4-Cl)Na]2], [l-LA], and [BnOH].

Improvement in Titanium Complexes Bearing Schiff Base Ligands in the Ring-Opening Polymerization of L-Lactide: A Dinuclear System with Hydrazine-Bridging Schiff Base Ligands

A series of titanium (Ti) complexes bearing hydrazine-bridging Schiff base ligands were synthesized and investigated as catalysts for the ring-opening polymerization (ROP) of L-lactide (LA). Complexes with electron withdrawing or steric bulky groups reduced the catalytic activity. In addition, the steric bulky substituent on the imine groups reduced the space around the Ti atom and then reduced LA coordination with Ti atom, thereby reducing catalytic activity. All the dinuclear Ti complexes exhibited higher catalytic activity (approximately 10-60-fold) than mononuclear L(Cl-H)-TiOPr2 did. The strategy of bridging dinuclear Ti complexes with isopropoxide groups in the ROP of LA was successful, and adjusting the crowded heptacoordinated transition state by the bridging isopropoxide groups may be the key to our successful strategy.

Comparative Study of Aluminum Complexes Bearing N,O- and N,S-Schiff Base in Ring-Opening Polymerization of ε-Caprolactone and l-Lactide

A series of Al complexes bearing Schiff base and thio-Schiff base ligands were synthesized, and their application for the ring-opening polymerization of ε-caprolactone (CL) and l-lactide (LA) was studied. It was found that steric effects of the ligands caused higher polymerization rate and most importantly the Al complexes with N,S-Schiff base showed significantly higher polymerization rate than Al complexes with N,O-Schiff base (5-12-fold for CL polymerization and 2-7-fold for LA polymerization). The reaction mechanism of CL polymerization was investigated by density functional theory (DFT). The calculations predicted a lower activation energy for a process involved with an Al complex bearing an N,S-Schiff base ligand (17.6 kcal/mol) than for that of an Al complex bearing an N,O-Schiff base ligand (19.0 kcal/mol), and this magnitude of activation energy reduction is comparable to the magnitude of rate enhancement observed in the experiment. The reduction of activation energy was attributed to the catalyst-substrate destabilization effect. Using a sulfur-containing ligand to decrease the activation energy in the ring-opening polymerization process may be a new strategy to design a new Al complex with high catalytic activity.

The ring-opening polymerization of ε-caprolactone and L-lactide using aluminum complexes bearing benzothiazole ligands as catalysts

A series of aluminum complexes bearing benzothiazole ligands was synthesized and the ring-opening polymerization of e-caprolactone (CL) and L-lactide (LA) using these aluminum complexes as catalysts was studied. The polymerization results revealed that the electron withdrawing groups increased the polymerization rate of the CL and LA polymerization. Steric bulky groups increased the polymerization rate of the CL polymerization but reduced the rate of the LA polymerization. The results also revealed that PLA-gradual-PCL that included PLA-(random-PLA-PCL)-PCL was synthesized by a one-pot synthesis, although the rate of the CL polymerization was higher than that of the LA polymerization. In addition, the results demonstrated that the catalytic activity of the Al complexes bearing benzothiazole ligands was higher than that of other Al complexes bearing heterocyclic amine ligands.

Synthesis, characterization, and catalytic activity of sodium ketminiate complexes toward the ring-opening polymerization of l -lactide

Studies of the ring-opening polymerization of L-lactide (LA) using Na complexes with Schiff base ligands as catalysts have revealed high catalytic activity but poor controllability of the polymer molecular weight. In this study, Na complexes bearing ketiminate ligands instead of Schiff base ligands were synthesized and their application in LA polymerization was tested. The polymerization results revealed that the catalytic activity of Na complexes bearing ketiminate ligands was higher than that of Na complexes bearing Schiff base ligands, but the poor controllability of polymer molecular weight was still a drawback. However, the poor controllability could be improved by means of using a high concentration of initiators in the polymerization system at 0 °C for 1 min. LPy–Na revealed the excellent controllability of polymer molecular weight depending on the ratio of [LA]/[initiator] (the molar averages of the number from 4500 to 35 000) with narrow polydispersity indexes, ranging from 1.29 to 1.35.

Comparative study of ring-opening polymerization of L-lactide and ε-caprolactone using zirconium hexadentate bis(aminophenolate) complexes as catalysts

A series of zirconium bis(aminophenolate) complexes as catalysts for the ring opening polymerization of L-lactide (LA) and e-caprolactone (CL) were investigated. Ligands bearing various chelating groups have a profound influence on the catalysis results. Among them, the thiophen-2-yl methyl group showed the greatest activity while the pyridine-2-yl methyl group showed the worst performance with regard to the rate of CL polymerization. However, the trend was reversed for the rate of LA polymerization. The kinetic results indicated a first-order dependency on [CL] and [LA]. However, the order of the catalyst concentration was different. Polymerization proceeded with second-order dependence on [LOMeZr(OBn)2] for CL but with first-order dependence on [LOMeZr(OBn)2] for LA.

Enhanced Catalytic Activity of Aluminum Complexes for the Ring-Opening Polymerization of ε-Caprolactone

A series of dinuclear aluminum (Al2Pyr2) complexes bridged by two pyrazole ligands were synthesized, and their catalytic activity toward ring-opening polymerization of ε-caprolactone (CL) was investigated. Different types of the Al-N-N-Al-N-N skeletal ring were found among these Al2Pyr2 complexes. The butterfly form, LThio2Al2Me4, exerted the highest catalytic activity for CL polymerization. κ2-CL coordination with both Al centers within the butterfly form LThio2Al2Me4 facilitates the initiation process. Generally speaking, the Al2Pyr2 complexes exhibited substantially higher catalytic activity for CL polymerization than literature examples such as β-diketiminate- or traiaza-bearing aluminum complexes. In fact, the Al2Pyr2 complexes can even carry out CL polymerization at room temperature.

Optimizing ring-opening polymerization of ε-caprolactone by using aluminum complexes bearing amide as catalysts and their application in synthesizing poly-ε-caprolactone with special initiators and other polycycloesters

A series of aluminum complexes bearing amidate ligands, including acylamide, sulfonamide, and aryl carbamate, was synthesized. In addition, the optimization of ring-opening polymerization of e-caprolactone by using these aluminum complexes as catalysts was studied. Polymerization results revealed that steric bulky groups in anilinyl groups decreased the catalytic activity of aluminum sulfonamide complexes but increased that of aluminum–acylamide complexes. Compared with other complexes bearing N-(4-methoxyphenyl) acylamidate and N-(4-methoxyphenyl) p-tolylsulfonamidate, an aluminum complex (MfOMeAlMe2) bearing methyl (4-methoxyphenyl)carbamate had the highest catalytic activity with an ideal molecular weight control and narrow polydispersity index (PDI). Other poly-e-caprolactones with special end chains, such as PEG-200, 2-dimethylaminoethanol, bis(2-hydroxyethyl) disulfide, 2-((2-hydroxyethyl)disulfanyl)ethyl 2-bromo-2-methylpropanoate, and PEG-polyester-bearing disulfide group, were successfully synthesized using MfOMeAlMe2 as the catalyst. The polymerization of δ-valerolactone and 2-bromo-e-caprolactone by using BnOH as an initiator and MfOMeAlMe2 as the catalyst resulted in poly-δ-valerolactone and poly-2-bromo-e-caprolactone, respectively, with a precise molecular weight and a narrow PDI as well as poly-e-caprolactones.

Investigation of the dinuclear effect of aluminum complexes in the ring-opening polymerization of ε-caprolactone

A series of aluminum (Al) complexes bearing hydrazine-bridging Schiff base and salen ligands were synthesized and investigated as catalysts for the ring-opening polymerization of e-caprolactone (CL). The introduction of steric bulky groups increases the catalytic activity of the corresponding mononuclear aluminum complex. However, the opposite phenomenon was observed in dinuclear Al complexes bearing salen ligands because the steric repulsion reduced the cooperative activation mechanism in the dinuclear Al system. Among these Al complexes, LN2Bu-Al2Me4 bearing a hydrazine-bridging Schiff base ligand had the highest catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al complexes bearing salen ligands and mononuclear Al complexes bearing Schiff base ligands. Density functional theory calculations revealed that the mechanism of the coordination of CL to one Al center was initiated by the benzyl alkoxide of another Al center.

Coordinating effect in ring-opening polymerization of ε-caprolactone using aluminum complexes bearing bisphenolate as catalysts

A series of Al complexes bearing diphenolate ligands was synthesized and their application for the ring-opening polymerization of e-caprolactone was studied. Positional variation of the substituent on the aryl ring of the RC*H(4,6-di-t-butylphenol)2 ligand was shown to have a considerable influence on the catalysis result. Complexes with a ortho-substituent showed greater catalytic activity than those with a para-substituent. Substitutions of an aryl moiety by H or methyl groups resulted in the catalytic activity falling between that of the ortho-substitution Al complexes and that of the para ones. Our results demonstrate that the coordinated functional group in the ortho-position of the phenyl ring could increase the catalytic activity. Moreover, X-rays of the structure and DFT analysis revealed that the coordinated functional group in the ortho-position could bridge two Al centers resulting in the transformation of a dinuclear Al complex with bridging benzyl alkoxide into a complex with terminal benzyl alkoxide, further promoting the efficacy of the initiator.