Abstract

Abstract

Abstract

Novel metal complexes with a single catalytic site and less transesterification seem to provide alternative efficient synthetic approaches to preparing new biodegradable and biologically responsive materials with well-defined structures. In this study, we rationally designed a new category of aluminum metal complexes bearing a bulky Salen ligand and diverse steroidal alkoxy moieties to synthesize novel biodegradable aliphatic polyesters end-capped with steroidal building blocks. At first, three new aluminum metal complexes (9–11) were synthesized with good yields of 80–90%, bearing cholesterol and diosgenin derivatives as functional alkoxy moieties. By means of nuclear magnetic resonance (NMR) spectrometry, matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI–FTMS), and Fourier transform infrared spectrometry, the molecular structures of 9–11 were characterized. Furthermore, new biodegradable aliphatic polyesters, poly(ε-caprolactone) and poly(δ-valerolactone) end-capped with diverse steroidal moieties, were synthesized through the ring-opening polymerization of ε-caprolactone and δ-valerolactone catalyzed by these new metal complexes under 100 °C in toluene, and they were also characterized by gel permeation chromatography, NMR, MALDI–FTMS, differential scanning calorimetry, and thermogravimetric analysis. Very narrow molecular weight distributions were revealed for these new polymer products, and their thermal crystallization and stability strongly depended on the degree of polymerization of the polyester building blocks and the distinct steroidal moieties. Because of the nature of the steroidal moieties, these biodegradable polymers may pave a path to new possibilities as potential biomaterials. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2045–2058, 2006

In this work, new investigations on the effect of comonomer sequential structure on the thermal and crystallization behaviors and biodegradability have been implemented for the biodegradable poly(butylene succinate-co-butylene terephthalate) (PBST) as well as aliphatic poly(butylene succinate) (PBS). At first, these copolyesters were efficiently synthesized from dimethyl succinate and/or dimethyl terephthalate and 1,4-butanediol via condensation polymerization in bulk. Subsequently, their molecular weights and macromolecular chain structures were analyzed by gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy. By means of differential scanning calorimeter (DSC) and wide-angle X-ray diffractometer (WAXD), thermal and crystallization behaviors of these synthesized aromatic–aliphatic copolyesters were further explored. It was demonstrated that the synthesized copolyesters were revealed to have random comonomer sequential structures with thermal and crystallization properties strongly depending on their comonomer molar compositions, and that crystal lattice structures of the new crystallizable copolyesters shifted from the monoclinic crystal of semicrystalline PBS to triclinic lattice of the poly(butylene terephthalate) (PBT) with increasing the terephthalate comonomer composition, and the minor comonomer components were suggested to be trapped in the crystallizable component domains as defects. In addition, the enzymatic degradability was also characterized for the copolyesters film samples. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1635–1644, 2006

Summary: In situ self-assemblies of new biodegradable triblock PLLA-b-PBS-b-PLLA and PDLA-b-PBS-b-PDLA have been investigated in acetonitrile solution. At first, two series of PLLA-b-PBS-b-PLLA and PDLA-b-PBS-b-PDLA, respectively denoted as the P and Q triblock copolyester series, were prepared with fixed PBS block ( = 6.9 kDa) and diverse enantiomeric PLLA/PDLA blocks. Further, their chemical structures and thermal properties were characterized by means of titration, nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), polarimeter, wide-angle X-ray diffraction (WAXD) and thermal analytical instruments. When mixing the synthesized enantiomeric copolyester pairs denoted as P₁/Q₁ – P₈/Q₈ in acetonitrile solution at 60 °C, in situ self-assemblies were found to happen for the P₄/Q₄ to P₈/Q₈ pairs, bearing longer enantiomeric PLA block lengths. DSC and WAXD analysis of the self-assembled microparticles demonstrated that PLLA/PDLA racemic crystals were formed for the P₅/Q₅ – P₈/Q₈ systems, as evidenced by their melting points over 200 °C, and a new X-ray diffraction peak detected at 2θ = 11.8°. Moreover, morphological studies by scanning electron microscopy (SEM) indicated the formation of disk- or platelet-like microparticles. It was noted that the diameters of the microparticles self-assembled in situ decreased from 1.28–1.50 μm down to 480–660 nm, through tailoring the enantiomeric PLA block length. Other factors, such as a central PBS block, the enantiomeric block length and the preparation conditions were suggested to play important roles in the in situ self-assembly of these enantiomeric triblock copolyesters. These results provide a facile way to self-assemble hydrophobic, biodegradable microparticles, through tuning the important van der Waals stereocomplexation interactions between two enantiomeric blocks in solution.

Summary: In our previous study [J. Yang, L. Jia, L. Yin, J. Yu, Z. Shi, Q. Fang, A. Cao, Macromol. Biosci.2004, 4, 1092.], new biodegradable copolymers of diblock methoxy poly(ethylene glycol)-block-poly(ε-caprolactone) and methoxy poly(ethylene glycol)-block-poly(δ-valerolactone), and triblock poly(ε-caprolactone)-block-poly(ethylene glycol)-block-poly(ε-caprolactone) and poly(δ-valerolactone)-block-poly(ethylene glycol)-block-poly(δ-valero-lactone) bearing narrow molecular weight distributions and well-defined block architectures were reported to be prepared with our original aluminum metal complex templates. This work will continue to report new investigations on their water solubility, and reversible thermal responsive micellization and solution to gel transition in distilled water. Among the new synthesized copolymers (P1–P23), seven diblock or triblock samples (P3, P6, P7, P11, P12, P19, and P21) with higher hydrophilic building block populations were revealed to be water soluble under ambient temperature. By means of UV spectrophotometer attached with a thermostat, important parameters as critical micellization mass concentrations (CMCs) and critical micellization temperatures (CMTs) were characterized for these new amphiphile dilute aqueous solution with the aid of an lipophilic organic dye probe of 1,6-diphenyl-1,3,5-hexatriene (DPH). Furthermore, the critical gelation temperatures (CGTs) were simultaneously investigated for these water-soluble block copolymers via a tube tilting method. It was found that the CMC, CMT, and CGT were strongly affected by the population and nature of the hydrophobic building blocks, and a higher hydrophobicity of the new amphiphilic block copolymer finally led to lower CMC and CMT, and higher CGT. In addition, the salts of KBr and NaCl were found to play as a salt-out effect on the solution to gel transition for the diblock P6 and triblock P11, exhibiting an interesting tunable gelation temperature close to 35–42 °C. These results will pave new possibility for the synthesized block structural amphiphiles as potential biomaterials to be applied in vivo.

Two novel aluminum metal complexes (2 and 3) bearing salen ligands were in situ prepared from trimethyl aluminum (AlMe₃), methanol, and (R,R)-N,N′-bis(salicylidene)-1,2-diaminocyclohexane with original synthetic strategies, and a preliminarily resoluted (R,R)-1,2-diaminocyclohexane was applied as a synthetic precursor. By means of Fourier transform infrared spectrometry, NMR spectrometry, mass spectrometry, and single-crystal X-ray diffractometry, 2 and 3 were revealed to be distinct molecular structures with corresponding yields of 85 and 10%, respectively. Further studies via ²⁷Al NMR techniques and single-crystal X-ray diffraction indicated that dimeric metal complex 3 appeared in the six-coordinated state, whereas there was a dynamic equilibrium transition between the five- and six-coordinated states for metal complex 2 in a CDCl₃ solution. The more stable dimeric metal complex (3) exhibited two inequivalent aluminum metal centers coordinated to nitrogen atoms attributed to two different salen ligands, and this was different from the reported salen aluminum complex structures. Furthermore, 2 and 3 were employed as candidate catalysts for the ring-opening polymerization (ROP) of some important biodegradable aliphatic polyesters and polycarbonates, including poly(ϵ-caprolactone) (PCL), poly(δ-valerolactone), poly(trimethylene carbonate), and poly(2,2-dimethyl trimethylene carbonate). The synthetic results indicated that both metal complexes efficiently catalyzed ROP at 100 °C in an anisole solution, and 3 showed much better controlled characteristics of ROP than 2. Very narrow molecular weight distributions close to 1.21 for PCL were detected with 3 as the ROP catalyst. In addition, a catalytic mechanism study confirmed that ROP catalyzed by these metal complexes was in good agreement with the commonly accepted coordination polymerization reported for aluminum triiso [Al(OⁱPr)₃] and stannous octanoate [Sn(Oct)₂]. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 373–384, 2005

Summary: The chemical preparation of structurally well-defined biodegradable amphiphilic block copolymers is now of great interest for biomedical applications and the fundamental mimetic study of biomacromolecule self-assembly. For this purpose, in this study, (R,R)-N,N′-bis(3-tert-butylsalicylidene)-1,2-cyclohexanediamine 2 as a ligand was first synthesized from 1,2-cyclohexanediamine (DACH) and was allowed to further react with AlMe₃, leading to a precursor compound 3. Then, the novel five-coordinated aluminum metal complexes 4–6 and 7–8 were prepared with good yields of 80–90%, bearing various molar mass monofunctional methoxy-poly(ethylene glycol) MPEG and difunctional poly(ethylene glycol) PEG as the alkoxy moieties, respectively. By means of nuclear magnetic resonance spectrometry (NMR), mass spectrometry (MALDI-FTMS) and Fourier Transform infrared spectrometry (FT-IR), new metal aluminum complexes 4–8 were characterized as having distinct chemical structures. Utilizing the synthesized metal complexes 4–8 as novel coordination polymerization catalytic templates, biodegradable amphiphilic MPEG-b-PCL, MPEG-b-PVL, PCL-b-PEG-b-PCL and PVL-b-PEG-b-PVL were synthesized with good control of the molecular weight distribution via the ring opening polymerization of ε-caprolactone and δ-valerolactone monomers at 100 °C in toluene. In addition, the chemical and crystalline structures and the thermal properties of these block biodegradable copolymers were analyzed by means of NMR, gel permeation chromatography (GPC), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). It was found that the melting points and crystallinities of the block copolymers synthesized strongly depended on the molecular structures of the polyether and polyester building blocks. Only one glass transition stage was detected, indicating good chain/segmental miscibility between the hydrophilic MPEG/PEG and hydrophobic PCL/PVL blocks in the non-crystalline regions. Moreover, TGA analysis exhibited typical two-step decomposition profiles with the weight-loss percentages in good agreement with block compositions from NMR calculations.