Polylactide (PLA) is a biodegradable, aliphatic polyester derived from lactic acid. It has similar mechanical properties to polyethylene terephthalate, but has a significantly lower maximum continuous use temperature. PLA products can be recycled after use either by remelting and processing the material a second time or by hydrolyzing to lactic acid, the basic chemical. In this review, the technologies for polymerization of the lactic acid and the comparison of physical, thermal and mechanical properties, biodegradability, and biocompatibility of the PLA and copolymers with other similar polymers are described.

A monoethylaluminum Schiff base complex (2) with formula LAlEt (L = N,N'-(2,2-dimethylpropylene)bis(3,5-di-tert-butylsalicylideneimine) was synthesized and employed for the stereoselective ring-opening polymerization of rac-lactide (rac-LA). The complex 2 was characterized by nuclear magnetic resonance, crystal structure, and elemental analysis. It contains a five-coordinate aluminum atom with distorted trigonal bipyramidal geometry in the solid state. In the presence of 2-propanol, 2 showed high stereoselectivity for the polymerization of rac-LA. The polymerization yielded crystalline poly(rac-LA) with a high melting temperature (193-201 degrees C). NMR, differential scanning calorimetry, and wide-angle X-ray diffraction indicated that the poly(rac-LA) was highly isotactic, and a stereocomplex was formed between poly-l- and poly-d-lactide block sequences. By the analysis of electrospray-ionization mass spectrometry and (1)H NMR, the polymer was demonstrated to be endcapped in both terminals with an isopropyl ester and a hydroxy group, respectively. The polymerization was of first order in rac-LA concentration. The relationship between the rac-LA conversion and molecular weights of the polymer was linear so that the polymerization could be well controlled.

A series of pH-responsive random copolymer poly(l-glutamic acid-co-l-lysine) [P(Glu-co-Lys)] were synthesized through the ring-opening polymerization (ROP) of γ-benzyl-l-glutamate N-carboxyanhydride (BLG-NCA) and 3-benzyloxycarbonyl-l-lysine N-carboxyanhydride (ZLys-NCA) and the subsequent deprotection. The chemical structure of the P(Glu-co-Lys)s was confirmed by NMR. Critical aggregation concentration and transmission electron microscopy measurements indicated that the P(Glu-co-Lys)s could self-assemble into aggregates in phosphate buffer. The surface charge of P(Glu-co-Lys) aggregates was greatly affected by the solution's pH and l-glutamic acid/l-lysine ratio because the carboxyl and amino groups present on the P(Glu-co-Lys) aggregates could be protonated or deprotonated to become charged. The pH value of the solution at which the surface charge of the P(Glu-co-Lys) aggregates reversed could be manipulated by the feed ratio of BLG-NCA and ZLys-NCA. In vitro methyl thiazolyl tetrazolium assays demonstrated that negatively charged P(Glu-co-Lys)s were nontoxic and biocompatible. Positive charged P(Glu-co-Lys)s showed some cytotoxicity to Hela cells. Cisplatin (CDDP) was used as a model anticancer drug to evaluate the charge-reversal drug delivery system. By the manipulation of CDDP loading content, the surface charge of the CDDP/P(Glu-co-Lys) nanoparticles could be reversed to positive from negative at tumor extracellular pH (pHe 6.5-7.2). An enhanced drug uptake and inhibition of cancer cell proliferation were observed for the tumoral pHe triggered charge-reversal CDDP/P(Glu-co-Lys) drug delivery system. These indicated that the CDDP/P(Glu-co-Lys) nanoparticles could be used as intelligent drug delivery systems for cancer therapy.

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and 1 H NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)AlO i Pr [(SB-2d) ) 2,2dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)AlO i Pr [(SB-3b) ) 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization. The polymerization kinetics using (SB-3b)AlO i Pr as a catalyst were studied in details, and the experimental results revealed that the rate of polymerization was first-order in [LA] and 1.81th-order with respect to (SB-3b)AlO i Pr, which indicated that the propagating species was not uniform in the system without the protection of steric hindered substituents. Furthermore, the polymerization by initiating with (SB-3b)AlO i Pr could be progressed at low temperatures (0 °C) without the change of stereoselectivity.

Biodegradable poly(ester amide)s have recently been used as biomaterials due to their desirable chemical and biological characteristics as well as their mechanical properties, which are amendable for material processing. In this study, electroactive tetraaniline (TA) grafted poly(ester amide)s were successfully synthesized and characterized. The poly(ester amide)s-graft-tetraaniline copolymers (PEA-g-TA) exhibited good electroactivity, mechanical properties, and biodegradability. The biocompatibility of the PEA-g-TA copolymers in vitro was systematically studied, which demonstrated that they were nontoxic and led to favorable adhesion and proliferation of mouse preosteoblastic MC3T3-E1 cells. Moreover, the PEA-g-TA copolymers stimulated by pulsed electrical signal could serve to promote the differentiation of MC3T3-E1 cells compared with TCPs. Hence, the biodegradable and electroactive PEA-g-TA copolymers possessed the properties in favor of the long-time potential application in vivo (electrical stimulation directly to the desired area) as bone repair scaffold materials in tissue engineering.

No abstract

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 (−OCHMeCH 2 Cl) analysis of polymer via 1 H, 13 C, and two-dimensional (2-D) NMR characterizations.

No abstract