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This study synthesized a series of titanium iminophenoxide complexes and investigated their suitability as catalysts for the ring-opening polymerization of L-lactide ( L-LA) and ε-caprolactone (CL). Complexes with bidentate ligands demonstrate higher catalytic activity than their tridentate counterparts since the third coordination atom needs to contend with L-LA and CL. Differences in the geometric framework of bidentate ligands also influence the catalytic activity. Type II ligands ( N, N-trans form of Ti complex) prevent the coordination of monomers to Ti thereby decreasing the initiation rate. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 [ABSTRACT FROM AUTHOR]

• AMM associated Al complexes had higher activity than that of ketiminate ligands. • Steric hindrance groups in the ligands revealed the greater catalytic activity. • Electron-withdrawing groups in the ligands revealed the greater catalytic activity. • Ligands with chelating group reduced the reactivity of ring-opening polymerization. A series of Al complexes bearing diethyl 2-((arylamino)methylene)malonate and diethyl 2-((alkylamino)methylene)malonate ligands (AMM ligands) were synthesized and their application in the ring-opening polymerization of ε -caprolactone was studied. All the Al complexes bearing AMM ligands exhibited higher catalytic activity (approximately 2.8–16.5 fold) than those Al complexes bearing ketiminate ligands did, except OiPrAlMe 2 (2 fold lower catalytic activity). In addition, Al complexes with steric hindrance or electron-withdrawing groups in the ligands revealed the greater catalytic activity, and the chelating effect reduced the reactivity of ring-opening polymerization. [ABSTRACT FROM AUTHOR]

• Comparison study of CL polymerization by six-membered ring Al complexes. • Al complexes bearing aminophenolate ligands revealed the greatest catalytic activity. A series of 6-membered ring aluminum (Al) complexes bearing Schiff base, benzotriazole phenolate, aminophenolate, carboxamide phenolate, and carboxylate phenolate were synthesized, and their applications in ε-caprolactone (CL) polymerizations were studied. The decreasing catalytic rates for CL polymerization were ranked as follows: Al complexes bearing aminophenolate > benzotriazole phenolate > carboxylate phenolate > Schiff base > carboxamide phenolate. The Al complex bearing aminophenolate (LBu-prop) revealed an appreciably higher (10–100 fold) polymerization rate and a shorter induction period than the other 6-membered ring Al complexes in the CL polymerization. Further investigation of the steric effect in the phenolate ring of aminophenolate ligands demonstrated that Al complexes with the tert -butyl groups on 2,4 positions of the phenolate ring had the highest catalytic activity, followed by that of the cumyl groups, and it was also discovered that the hydrogen substituents were responsible for the largest decrease in the catalytic activity of Al complexes. The bulky substituents on the amino group increased the catalytic activity of associated Al complexes as well as the aminophenolate ligands with the third chelating atoms on the amino group. However, the aminophenolate ligands with the fourth chelating atoms on the amino group slightly decreased the catalytic activity. [ABSTRACT FROM AUTHOR]

• Ethyl salicylate-bearing Zn complexes exhibited a high polymerization rate during the polymerization of CL and LA. • The positive effect of substituents at the ortho position of phenolates was observed in CL polymerization. • Electronic donating substituents on phenolates enhanced the catalytic activity of Zn complexes in LA polymerization. • Bulky groups at the ortho position facilitated the detachment of the bridging alkoxide group to form the intermediate II-CL. • Ligands with electron-withdrawing groups may stabilize the intermediate trip. A series of zinc complexes bearing ethyl salicylates were synthesized, and their catalytic activity in the polymerization of ε -caprolactone (CL) and l -lactide (LA) was investigated. In CL polymerization, the catalytic activity of Zn complexes was promoted by ligands with the substituents at the ortho position of phenolates, such as the 2-methyl group of L6MeZnEt (40 min, 94% conversion at 25 °C, [CL] = 2.0 M) and the 2,4-di‑ tert ‑butyl groups of LBuZnEt (55 min, 93% conversion at 25 °C, [CL] = 2.0 M). In LA polymerization, the catalytic activity was promoted by ligands with electron-donating substituents, such as the 4-methyl group of L4MeZnEt (55 min, 90% conversion at 50 °C, [LA] = 0.5 M) and the 2,4-di‑ tert ‑butyl group of LBuZnEt (60 min, 90% conversion at 50 °C, [LA] = 0.5 M). Density functional theory calculations revealed that the polymerization requires the detachment of a bridging alkoxide group from one Zn center to form an intermediate II-CL ; bulky ligands may facilitate this unfavored dissociation process (from I-CL to II-CL). In LA polymerization, ligands with electron-withdrawing groups increase the Lewis acidity of Zn centers; the detachment of coordination atoms from the Zn atom is difficult; hence, the carbonyl group of ring-opening LA back binds to the Zn center to form a low-energy intermediate trip and then hinders another LA coordination. Ethyl salicylate-bearing Zn complexes exhibited a high polymerization rate during the polymerization of CL and LA. The positive effect of substituents at the ortho position of phenolates was observed in CL polymerization; by contrast, electronic donating substituents on phenolates enhanced the catalytic activity of Zn complexes in LA polymerization. DFT calculations revealed that bulky groups at the ortho position of phenolates facilitated the detachment of the bridging alkoxide group from the Zn center (I-CL) to form the intermediate II-CL in CL polymerization. However, the carbonyl group of ring-opening LA back binds to the Zn center to form the low-energy intermediate trip and then hindered another LA coordination. Ligands with electron-withdrawing groups may reduce the electron density on the Zn atom and increase the interaction between the Zn atom and the carbonyl group that stabilized the intermediate trip. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Multinuclear metal complexes improve the catalytic activity of catalysts for ROP of cyclic esters. • Multinuclear metal catalysts are cooperative processing of coordination-insertion reaction by metal centers. • Multinuclear metal catalysts improve the catalytic activity through the electronic (or inductive) effect. • An emphasis on comparing mononuclear and multinuclear metal catalysts was reported. Multinuclear metal complexes can effectively improve the catalytic activity of catalysts for ring-opening polymerization (ROP) of cyclic esters. If the catalytic metal centers are close enough to each other, they divide the labor of coordination-insertion reactions in polymerization processes. If the catalytic metal centers are too far apart, and the ligands connecting the catalytic metal centers can have resonance, their positive charge properties affect each other and thus affect the catalytic polymerization reaction in the same manner that electron-withdrawing groups do. This review focused on recent reports of multinuclear metal catalysts in ROP that exhibited higher catalytic activity than did mononuclear metal catalysts. The dilemma faced by the development of novel multinuclear metal complexes, and the strategies and the attention for designing catalysts are also provided. [ABSTRACT FROM AUTHOR]

[Display omitted] In this study, a series of mononuclear complexes and dinuclear aluminum complexes bridged by 1,3- and 1,4-substituted phenyl ligands were synthesized, and their activities for the ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated. All the dinuclear aluminum complexes exhibited higher catalytic activity than did the corresponding mononuclear Al complexes (2–6-fold for the k obs values), and most of the dinuclear Al complexes bearing 1,3-substituted phenyl ligands exhibited higher activity than did the corresponding dinuclear Al complexes bearing 1,4-substituted phenyl ligands (1.5–3-fold for the k obs values). The only exceptions were the Al complexes bearing benzothiazole ligands, for which the dinuclear Al complex with 1,4-bis(benzothiazol-2-yl)benzene exhibited higher catalytic activity than did the corresponding 1,3- bis(benzothiazol-2-yl)benzene complex. This higher activity of the dinuclear Al complexes was attributed to one of the Al atoms in the dinuclear Al system being regarded as an electron-withdrawing group. This group influenced the other Al atom through associated aromatic ligands to enhance the catalytic activity for the ROP of CL. [ABSTRACT FROM AUTHOR]

A series of titanium (Ti) compounds bearing 2,6-bis(o -hydroxyalkyl)pyridine ligands were synthesized and investigated as catalysts for the ring-opening polymerization of ε-caprolactone (CL) and L -lactide (LA). The Ti complexes with electron-withdrawing groups in pyridine rings or steric bulky groups in hydroxymethyl groups reduced the catalytic activity in CL and LA polymerizations, and Me L Ph -TiOPr 2 exhibited the highest catalytic activity ([CL] = 0.5 M, [CL]:[Cat] = 100:8, 60 °C, 7.5 h, conversion = 93%; [LA] = 0.5 M, [LA]:[Cat] = 100:8, 60 °C, 3.5 h, conversion = 98%). The density functional theory calculation results revealed that CL interacted with the ligand through weak hydrogen bonds, and then approached the Ti center. After being attacked by isopropyl oxide, the carbonyl group of CL bonded to the Ti center. After the rearrangement and ring opening of CL ring, CL was converted to poly-ε-caprolactone. Image 1 • Ti compounds bearing 2,6-bis(o -hydroxyalkyl)pyridine ligands were synthesized. • LA and CL polymerizations were investigated. • Me L Ph -TiOPr 2 exhibited the highest catalytic activity. [ABSTRACT FROM AUTHOR]

A series of aluminum complexes bearing heterocyclic amine ligands were synthesized, and their application in ε -caprolactone (CL) and L -Lactide (LA) polymerization and copolymerization were studied. The CL polymerization results revealed that ligands containing the higher electronegativity atom increased the polymerization rate of CL polymerization. Among these Al complexes, NON-AlMe 3 bearing benzofurazan ligand was the best catalyst in CL polymerization ([ NON-AlMe 3 ] = 10 mM, conversion = 94% after 4 min at room temperature). In LA polymerization, the catalytic trend of Al complexes was random, and NCN-AlMe 3 bearing 1-benzylbenzimidazole ligand was the best catalyst ([ NCN-AlMe 3 ] = 10 mM, conversion = 90% after 45 min at 80 °C). The selectivity of the production of a PLA- gradual -PCL copolymer could be controlled by different Al complexes. Our study revealed that the four-coordinated trimethyl Al complexes bearing heterocyclic amine ligands were easily synthesized and constitute an ideal catalyst design in CL polymerization. Image 1 • NON-AlMe 3 was the best catalyst in CL polymerization. • NCN-AlMe 3 was the best catalyst in LA polymerization. • PLA- gradual -PCL copolymer could be controlled by different Al complexes. [ABSTRACT FROM AUTHOR]

A series of pyridine and polypyridine-zinc complexes were synthesized, and their application in ε-caprolactone and l -lactide polymerization was studied. In ε-caprolactone polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a higher polymerization rate than did diethyl zinc (2–10 times). In l -lactide polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a considerably higher polymerization rate than did diethyl zinc (2–220 times). According to density functional theory results, the ortho -hydrogens of polypyridine ligand can co-activate the carbonyl group of monomers with zinc metal center to increase the catalytic activity of zinc catalysts. Image 1 • The polypyridne-zinc complexes were synthesized. • 1,10-Phenanthroline derivatives enhanced the catalytic activity of Zn complexes. • Hydrogens of polypyridine co-activate the carbonyl group of monomers. [ABSTRACT FROM AUTHOR]

The ring-opening polymerizations of ε-caprolactone (CL) using 5- and 6-membered ring Zn complexes bearing Schiff base as catalysts were comparatively studied. The 5-membered ring Zn complexes exhibited a considerably higher polymerization rate than the 6-membered ring Zn complexes (2–12 fold for CL polymerization) did. X-ray data revealed that 5-membered ring L 5-Ph 2 Zn creates larger empty space around the Zn center than 6-membered ring L 6-Bn 2 Zn. This allows higher interaction between the initiator and Zn complex and easy CL coordination. Image 1 • The 5-membered ring Zn complexes exhibited a considerably higher polymerization rate. • X5-membered ring L5-Ph 2 Zn creates larger empty space around the Zn center than 6-membered ring L6-Bn 2 Zn [ABSTRACT FROM AUTHOR]