A series of iron(III) chloride complexes based upon Schiff base framework have been synthesized and characterized by mass spectra, elemental analysis, and X-ray crystallography. These bench-stable complexes were for the first time capable as highly efficient catalysts for lactide and ε-caprolactone polymerization in the presence of propylene oxide (PO), greatly surpassing conventional aluminum analogies. Electron-withdrawing substituents as well as elevated temperature boosted the activity while a bulky group on salicylaldehyde moieties abnormally produces the same effect, whereas rigid backbone retarded the reactivity. Polylactide tactics ranging from isotactic to hererotactic enchainment were obtained by tuning the ligand backbone and substituents. The stereoselectivity was confirmed to proceed via a chain-end control mechanism by kinetic studies using different isomers of lactide, and the overall polymerization process was also investigated in detail by the oligomer mass spectrum as well as end group (−OCHMeCH2Cl) analysis of polymer via 1H, 13C, and two-dimensional (2-D) NMR characterizations.

Three pairs of bimetallic Schiff base aluminum complexes (1a–6a) with different tetraamine backbone bridging parts and bulky silyl-substituted moieties on salicylidene were synthesized. Using these complexes as catalysts for ring opening polymerisation (ROP) of racemic-lactide (rac-LA), systematical stereoselectivity and kinetic studies on ROP of rac-LA were carried out. The Gibbs free energy difference between homo-propagation and cross-propagation (ΔG≠) calculated by activation entropy and enthalpy showed that bulky substituents at the 3-position enhanced the stereoselectivity. And the kinetic study results indicated that bulkier substituents with more steric hindrance reduced the ROP reactivity. Moreover, these complexes could also be used for copolymerisation of rac-LA and ε-caprolactone (ε-CL). Copolymers of poly(LA-co-CL) with various monomer ratios were synthesized. Kinetic research on copolymerisation as well as microstructure analysis demonstrated that poly (LA-co-CL) achieved by these complexes were tapered block copolymers.

A series of copolymers of lactide (LA) and ε-caprolactone (ε-CL) with different monomer feed ratios were achieved using three kinds of bimetallic Schiff aluminum complexes as catalysts. The ratios of LA and ε-CL units in different copolymers and the average segments length were determined by NMR analysis. The comparative kinetic study of L-LA/ε-CL and rac-LA/ε-CL copolymerization systems showed that the polymerization rate of LA was faster than ε-CL, and L-LA showed polymerization rate slightly faster than rac-LA. It was inferred that the copolymers achieved by these complexes were gradient copolymers with gradual change in distribution of LA and ε-CL units. The thermal properties of these copolymers were characterized by DSC analysis, which showed that the glass transition temperature (Tg) of these copolymers changed regularly according to the pro-portion change of two structural units.

A number of half-salen aluminum complexes bearing unsymmetrical [ONN]-type ligands were prepared from tridentate dinaphthalene-imine derivatives. These complexes were characterized by 1H and 13C NMR spectroscopy, elemental analysis and single crystal X-ray diffraction analysis. These complexes were employed for rac-lactide and L-lactide polymerization. Upon activation with isopropanol, complex (S)-B6 (R1 = R2 = R4 = H; R3 = F) showed the highest activity (a monomer conversion of 94.6%) amid these aluminum complexes for the ring-opening polymerization of L-lactide; and complex (S)-B2 (R1 = R2 = R3 = H; R4 = tBu) showed the highest stereoselectivity for the ring-opening polymerization of rac-lactide, obtaining a polylactide (PLA) with a Pm of 0.69. The polymerization kinetics utilizing (S)-B6 as a catalyst were researched in detail. The data on the polymerization kinetics revealed that the rate of polymerization was first-order with respect to the monomer and the catalyst. There was a linear relationship between the L-lactide conversion and the number-average molecular weight of PLA.

Four hemi-salen aluminum complexes based on tridentate N, N, O-type binaphthyl-Schiff-base derivatives (1: R = iPr; R1 = H; 2: R = iPr; R1 = Cl; 3: R = 2-adamantyl; R1 = H; 4: R = 2-adamantyl; R1 = tBu) were prepared. These complexes were characterized by 1H, 13C NMR spectroscopy and elemental analysis, these four complexes were employed for ring-opening polymerisation of L-lactide and rac-lactide. Complex 2, which was based on pro-ligand L2 with smaller steric hindrance and electron-withdrawing substituents, displayed the highest activity for ROP of L-lactide among these complexes, and complex 4, which was supported by pro-ligand L4 with the biggest steric hindrance, showed the highest stereoselectivity for the ROP of rac-lactide with partially isotactic polylactide with a Pm of 0.65. Kinetic studies revealed the ROP of L-lactide initiated by complex 3 had first-order dependency on [LA] as well as [Al].

Four titanium complexes based on salen ligands were prepared and employed for the ring-opening polymerisation of lactide. X-ray diffraction analysis revealed that the titanium atom was in a distorted octahedral geometry and the quadridentate ligand adopted β-cis geometry in complex 1.

Two types of polylactide–poly(butylene adipate-co-terephthalate)–polylactide (PLA–PBAT–PLA) tri-block copolymers with different molecular weights (CP1 and CP2) were synthesized as compatibilizers for PLA/PBAT blends. Synergistic effects of CP1 and CP2 on the mechanical and rheological properties of the blends have been studied in detail. The addition of small amounts of CP1 and CP2 remarkably increased the elongation at the break point. 0.5 and 0.5 wt% of CP1 and CP2 led to an increase of elongation by over eightfold. Thermal, morphological and rheological analyses showed that addition of CP1 and CP2 increased the miscibility and interfacial bonding strength between PLA and PBAT, in addition to decreasing the melt viscosity. It was thought that the low-molecular-weight compatibilizer CP1 with high mobility would have a positive effect during the transportation of the high-molecular-weight CP2 from the matrix to the interface. In addition, CP2 played a key role in improving the interaction at the interface.

•A number of Non-Symmetrical Aluminium Salen complexes were synthesized.•These complexes were active in lactide and caprolactone polymerization.•First-ordered kinetics with respect to lactide and caprolactone were observed.Non-symmetrical aluminium salen complexes that contained different substituents were designed and synthesized. All the ligands and their complexes were characterized by 1H, 13C NMR and elemental analysis. These complexes can be used as catalysts to produce polylactide and poly-ε-caprolactone. All polymerizations were living and molar mass distributions were narrow. The Mn(obsd) of the isolated polymers were in good agreement with Mn(calcd). The polymerization rate of electrophilic substituted complex was higher than the non-electrophilic substituted analogies. The bulky substituents with more steric hindrance of the complexes had relatively lower activity. Kinetic studies showed that the polymerizations were both first-ordered with respect to lactide and ε-caprolactone monomers.

A series of Schiff base aluminum(III) complexes bearing morpholinomethyl substituents were synthesized. Comprehensive investigations on their stereoselective and kinetic features in the ring opening polymerization of lactide were carried out. The ring opening polymerization proved to be first-order in the catalyst and the monomer. Linear relationships between the numberaverage molecular weight of the polylactide and the monomer conversion were consistent with a well-controlled polymerization. The propagation rate was strongly affected by morpholinomethyl substituents on the salicylaldehyde moiety.

•A number of half-Salen zinc Schiff-base complexes were synthesized.•These complexes were active in lactide polymerization.•The activity of these complexes was closely related to their structures.A number of unreported zinc complexes based on tridentate O,N,O-type half-Salen ligands were prepared. All the complexes were characterized by 1H, 13C NMR and elemental analysis. These complexes were used as catalysts with 2-propanol for the ring-opening polymerization of l-lactide and rac-lactide. They were effective in lactide polymerization with good conversions. And polymerizations were living with the narrow molar mass distributions. The bulky substituents with relatively larger steric hindrance of the ligands coordinated to the central metal centers had relatively low polymerizing activity.

A series of zinc complexes based on asymmetrical N,N,O-tridentate ligands were prepared via binaphthyl diamine derivatives. These complexes were characterized and employed as catalysts in lactide polymerization. The X-ray diffraction analyses revealed that molecular structures of 1b and 2b were mononuclear complexes with zinc atoms in distorted octahedral geometries. Upon co-catalysis with isopropanol, complex 2a showed the highest activity among these zinc complexes for the ring-opening polymerization of L-lactide, and complex 3a exhibited the highest stereoselectivity for the ring-opening polymerization of rac-lactide affording substantially isotactic polylactide (PLA) with a Pm of 0.62. The polymerization kinetics using 2a as a catalyst was studied in detail. The kinetics of the polymerization results revealed that the rates of polymerization were first-order both in the monomer and the catalyst, and there was a linear relationship between the L-LA conversion and the number-average molecular weight of PLA with a narrow molecular distribution (1.07–1.17). The activation energy (31.49 kJ mol−1) was deduced according to the Arrhenius equation.

A series of unreported aluminum complexes supported by asymmetrically O,N,N,O-quadridentate hemi-salen ligands were synthesized by binaphthyl-imine derivatives. These complexes were characterized and used as catalysts in rac-lactide or l-lactide polymerization. The X-ray diffraction analysis showed that molecular structures of (S)-3 and (rac)-4 were mononuclear coordination compounds with five-coordinated aluminum atoms in the solid state. Using 2-propanol as cocatalyst, complex (S)-6 revealed the highest activity among these aluminum coordination compounds toward the ring-opening polymerization of l-lactide, and complex (S)-2 displayed the highest stereospecificity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide with a Pm of 0.64. The polymerization kinetics using (S)-6 as a catalyst were investigated at great lengths. The kinetics of the polymerization consequences proved that the polymerization was first-order in monomer as well as catalyst. There was a linear dependence between the rac-lactide conversion and the number-average molecular weight of the macromolecules. The PDI values of these macromolecules were in a narrow range (1.04–1.09).

A number of half-salen aluminum complexes bearing unsymmetrical [ONN]-type ligands were prepared from tridentate dinaphthalene-imine derivatives. These complexes were characterized by 1H and 13C NMR spectroscopy, elemental analysis and single crystal X-ray diffraction analysis. These complexes were employed for rac-lactide and L-lactide polymerization. Upon activation with isopropanol, complex (S)-B6 (R1 = R2 = R4 = H; R3 = F) showed the highest activity (a monomer conversion of 94.6%) amid these aluminum complexes for the ring-opening polymerization of L-lactide; and complex (S)-B2 (R1 = R2 = R3 = H; R4 = tBu) showed the highest stereoselectivity for the ring-opening polymerization of rac-lactide, obtaining a polylactide (PLA) with a Pm of 0.69. The polymerization kinetics utilizing (S)-B6 as a catalyst were researched in detail. The data on the polymerization kinetics revealed that the rate of polymerization was first-order with respect to the monomer and the catalyst. There was a linear relationship between the L-lactide conversion and the number-average molecular weight of PLA.

A series of zinc complexes based on asymmetrical N,N,O-tridentate ligands were prepared via binaphthyl diamine derivatives. These complexes were characterized and employed as catalysts in lactide polymerization. The X-ray diffraction analyses revealed that molecular structures of 1b and 2b were mononuclear complexes with zinc atoms in distorted octahedral geometries. Upon co-catalysis with isopropanol, complex 2a showed the highest activity among these zinc complexes for the ring-opening polymerization of L-lactide, and complex 3a exhibited the highest stereoselectivity for the ring-opening polymerization of rac-lactide affording substantially isotactic polylactide (PLA) with a Pm of 0.62. The polymerization kinetics using 2a as a catalyst was studied in detail. The kinetics of the polymerization results revealed that the rates of polymerization were first-order both in the monomer and the catalyst, and there was a linear relationship between the L-LA conversion and the number-average molecular weight of PLA with a narrow molecular distribution (1.07–1.17). The activation energy (31.49 kJ mol−1) was deduced according to the Arrhenius equation.