Three kinds of bimetallic Schiff base titanium complexes with different substituent moieties on organic ligands were synthesized. Ring-opening polymerizations (ROP) of lactides were carried out by using these titanium complexes as catalysts. The polymerization data and kinetic studies showed that complex 1a had the highest activity and complex 2a had the lowest activity for ROP of lactides. Moreover, all these three newly synthesized bimetallic titanium complexes showed higher polymerization activity and better molecular weight control than their monometallic counterparts reported in our previous work.

A series of novel biodegradable polyurethanes (PELU) based on poly(ethylene glycol)-b-polylactide copolymers were prepared by chain extension with isophorone diisocyanate (IPDI). The PELUs were used to toughen polylactide (PLA) by melt blending. The results of DSC and SEM uniformly indicated that the PELUs were partially compatible with PLA and the PLA segments in PELU could effectively improve the compatibility between PLA and PELU. PELU as a plasticizer could significantly improve the toughness of PLA materials and remain their high strength and modulus. When the PELU content was 10–20 wt%, the elongation at break of PLA/PELU-40/ADR (0.4 wt%) and PLA/PELU-50/ADR (0.4 wt%) reached up to 250%–350%. When the PELU content was 20 wt%, the tensile strength and modulus of PLA blends containing PELU-30, PELU-40 and PELU-50 maintained 35–38 MPa and 1300–1500 MPa, respectively. The moisture absorption of the PLA materials enhanced because of blending with PELU containing PEG segments, but the hydrolytic degradation property of PLA materials was little affected by this.

Poly(ether urethane)s (PEU), including PEUI15 and PEUH15, were prepared through chain-extension reaction of poly(ethylene glycol) (PEG-1500) using diisocyanate as a chain extender, including isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI). These PEUs were used to toughen polylactide (PLA) by physical and reactive blending. Thermal, morphological, mechanical and aging properties of the blends were investigated in detail. These PEUs were partially compatible with PLA. The elongation at break of the reactive blends in the presence of triphenyl phosphate (TPP) for PLA with PEUH15 or PEUI15 was much higher than that of the physical blends. The aging test was carried out at -20 °C for 50 h in order to accelerate the crystallization of PEUs. The PEUs in the PLA/PEU blends produced crystallization and formed new phase separation with PLA, resulting in the declined toughness of blends. Fortunately, under the aging condition, although PEUH15 in blends could also form crystallization, the reactive blend of PLA/PEUH15/TPP(80/20/2) had higher toughness than the other blends. The elongation at break of PLA/PEUH15/TPP(80/20/2) dropped to 287% for the aging blend from 350% for the original blend. The tensile strength and modulus of PLA/PEUH15/TPP blend did not change obviously because of the crystallization of PEUH15.

In this study, a series of monodispersed poly(L-lactide) (PLLA) were synthesized by the ring-opening polymerization with Schiff base aluminum catalyst, and the effects of the number-average molecular weight (Mn) on the crystallization and melting behaviors of PLLA were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The total crystallization rate of PLLA was Mn-dependent, which reached the maximum value for PLLA with Mn of 18.6 kg/mol. In addition, when Mn of PLLA was 18.6 kg/mol, the melting enthalpy (ΔHm) showed a maximum value (87.1 J/g), which was the highest reported value till now. The critical temperature for change of crystal formation from δ- to α-form crystals increased in the isothermal crystallization process with Mn increasing. In the reheating procedure, high-Mn PLLA demonstrated a small exothermal peak prior to the dominant melting peak, corresponding to crystal transition from δ- to α-form, but low-Mn PLLA didn’t show the peak of crystal transition. These different crystallization and melting behaviors were attributed to the different chain mobility of PLLA with different Mn.

•PEG/PLLA block copolymers with different architectures were synthesized.•The different architectures did not alter the crystal structures of PEG and PLLA.•The effect of architectures was markedly on the crystallization behavior of PEG and PLLA.•A mode of the confined environment of PEG in the PEG/PLLA block copolymers with different architectures was suggested.PEG/PLLA block copolymers bearing various number of arms were synthesized. The molecular weights of PLLA in the block polymers were determined by 1H NMR and GPC. For 1-arm, 2-arm, 4-arm PEG/PLLA block copolymers, DSC and WAXD results indicated that the different architectures and compositions of the block copolymers didn't alter the structures of PEG and PLLA crystallites, but markedly affected the crystallinities and melting temperatures of PEG and PLLA. The critical Mn, PLLA of each arm decreased with the number of arms increasing (Mn, MPEG5-PLLA > Mn, 2PEG10-PLLA > Mn, 4PEG20-PLLA), when the PEG crystallites were not detected. As Mn, PLLA of each arm was similar in the copolymers, Tm, PEG and XPEG decreased gradually with the number of arms increasing, and the similar regularities also displayed for PLLA. The crystallization behavior and the variation architectures of PEG/PLLA copolymers can provide primary data to the applications in the drug delivery and industry parts.