Synthesis of poly(D,L‐lactic acid) modified by triethanolamine by direct melt copolycondensation and its characterization

Using D ,L ‐lactic acid (LA) and multifunctional group compound triethanolamine (TEA) as starting materials, a novel biodegradable material poly(D ,L ‐lactic acid‐triethanolamine) [P(LA‐TEA)] was directly synthesized by simpler and practical melt polycondensation. The appropriate synthetic condition was discussed in detail. When the molar feed ratio LA/TEA was 30/1, the optimal synthesis conditions were as follows: a prepolymerization time of 12 h; 0.5 weight percent (wt %) SnO catalyst; and melt copolycondensation for 8 h at 160°C, which gave a novel star‐shaped poly(D,L ‐lactic acid) (PDLLA) modified by TEA with the maximum intrinsic viscosity [η] 0.93 dL g⁻¹. The copolymer P(LA‐TEA) as a different molar feed ratio was characterized by [η], Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H‐NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X‐ray diffraction (XRD). Increasing the molar feed ratio of LA/TEA, T_g and M_w increased. However, all copolymers were amorphous, and their T_g (12.2°C–32.5°C) were lower than that of homopolymer PDLLA. The biggest M_w was 9400 Da, which made the biodegradable polymer be potentially used as drug delivery carrier, tissue engineering material, and green finishing agent in textile industry. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Glass transition and mechanical properties of PLLA and PDLLA‐PGA copolymer blends

The change of the glass transition temperatures (T_g) in the blend of poly(L ‐lactic acid) (PLLA) and the copolymers of poly(D,L ‐lactic acid) and poly(glycolic acid) (PDLLA‐PGA) with different D,L ‐lactic acid and glycolic acid composition ratio (50 : 50, 65 : 35, and 75 : 25) was studied by DSC. Dynamic mechanical measurement and tensile testing were performed at various temperatures around T_g of the blend. In the blend of PLLA and PDLLA‐PGA50 (composition ratio of PDLLA and PGA 50 : 50), T_g decreased from that of PLLA (about 58°C) to that of PDLLA‐PGA50 (about 30°C). A single step decrease was observed in the DSC curve around T_g between the weight fraction of PLLA (W(PLLA)) 1.0 and 0.7 (about 52°C) but two‐step changes in the curve are observed between W(PLLA) = 0.6 and 0.3. The T_g change between that of PLLA and that of PDLLA‐PGA and the appearance of two T_gs suggest the existence of PLLA rich amorphous region and PDLLA‐PGA copolymer rich amorphous region in the blend. A single step decrease of E′ occurs at around T_g of the pure PLLA but the two‐step decrease was observed at W(PLLA) = 0.6 and 0.4, supporting the existence of the PLLA rich region and PDLLA‐PGA rich region. Tensile testing for various blends at elevated temperature showed that the extension without yielding occurred above T_g of the blend. Partial miscibility is suggested for PLLA and PDLLA‐PGA copolymer blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2164–2173, 2004     

Syntheses of poly(lactic acid)‐poly(ethylene glycol) serial biodegradable polymer materials via direct melt polycondensation and their characterization

Directly starting from lactic acid (LA) and poly(ethylene glycol) (PEG), biodegradable material polylactic acid‐polyethylene glycol (PLEG) was synthesized via melt copolycondensation. The optimal synthetic conditions, including prepolymerization method, catalyst kinds and quantity, copolymerization temperature and time, LA stereochemical configuration, feed weight ratio m_LA/m_PEG and M_n of PEG, were all discussed in detail. When D ,L ‐LA and PEG (M_n = 1000 Da) prepolymerized together as feed weight ratio m_{D ,l‐LA}/m_PEG = 90/10, 15 h copolycondensation under 165°C and 70 Pa, and 0.5 wt % SnO as catalyst, gave D ,L ‐PLEG1000 with the highest [η] of 0.40 dL/g, and the corresponding MW was 41,700 Da. Using L ‐LA instead of D ,L ‐LA, 10 h polymerization under 165°C and 70 Pa, and 0.5 wt % SnO as catalyst, gave L ‐PLEG1000 with the highest [η] of 0.21 dL/g and MW of 15,600 Da. Serial D ,L ‐PLEG with different feed weight ratio and M_n of PEG were synthesized via the simple and practical direct melt copolycondensation, and characterized with FTIR, ¹H NMR, GPC, DSC, XRD, and contact angle testing. D ,L ‐PELG not only had higher MW than PDLLA, PLLA and L ‐PELG, but also better hydrophilicity than PDLLA. The novel one‐step method could be an alternative route to the synthesis of hydrophilic drug delivery carrier PLEG instead of the traditional two‐step method using lactide as intermediate. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 577–587, 2006     

Syntheses of poly(lactic acid‐co‐glycolic acid) serial biodegradable polymer materials via direct melt polycondensation and their characterization

The optimal synthetic conditions of poly(lactic acid‐co‐glycolic acid) (PLGA) via melt copolycondensation directly from L ‐lactic acid (L ‐LA) and glycolic acid (GA) with a feed molar ratio of 50/50 are discussed; the important drug‐delivery carrier PLGA50/50 is used as a special example. With reaction conditions of 165°C and 70 Pa and with 0.5 wt % SnCl₂ as the catalyst, 10 h of polymerization gave the L ‐PLGA50/50 with the biggest intrinsic viscosity ([η]), 0.1993 dL/g. The optimal synthetic conditions were verified by the synthesis of D,L ‐PLGA50/50 with D,L ‐lactic acid (D,L ‐LA) instead of L ‐LA, but the biggest [η] was 0.2382 dL/g. Under the same synthetic conditions with L ‐LA and D,L ‐LA as starting materials, serial PLGA with different molar feed ratios, including 100/0, 90/10, 70/30, 50/50, 30/70, 10/90, and 0/100, were synthesized via simple and practical direct melt copolycondensation, and their solubilities were investigated. When the glycolic acid feed molar percentage was equal to or more than 70%, solubilities in tetrahydrofuran and CHCl₃ became worse, and some samples were even wholly insoluble. These biodegradable polymers were also systematically characterized with gel permeation chromatography, Fourier transform infrared spectroscopy, ¹H‐NMR spectroscopy, differential scanning calorimetry, and X‐ray diffraction. PLGA synthesized from L ‐LA and D,L ‐LA had many differences in weight‐average molecular weight (M_w), glass‐transition temperature, crystallinity, and composition. When the molar feed ratio of LA to GA was 50/50, both the [η] and M_w values of D,L ‐PLGA were higher than those of L ‐PLGA. With D,L ‐LA as the starting material, the structure of the PLGA copolymer was relatively simple, and its properties were apt to be controlled by its GA chain segment. When the feed molar percentage of the monomer (LA or GA) was more than or equal to 90%, the copolymer was apt to be crystalline, and the aptness was more obvious for the L ‐LA monomer. The composition percentage of GA in PLGA was not only higher than the feed molar percentage of GA, but also, the GA percentage in D,L ‐PLGA was higher than in L ‐PLGA. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 244–252, 2006     

Poly(para‐dioxanone)/poly(D,L‐lactide) blends compatibilized with poly(D,L‐lactide‐co‐para‐dioxanone)

The effect of poly(D ,L ‐lactide‐co‐para‐dioxanone) (PLADO) as the compatibilizer on the properties of the blend of poly(para‐dioxanone) (PPDO) and poly(D ,L ‐lactide) (PDLLA) has been investigated. The 80/20 PPDO/PDLLA blends containing from 1% to 10% of random copolymer PLADO were prepared by solution coprecipitation. The PLADO component played a very important role in determining morphology, thermal, mechanical, and hydrophilic properties of the blends. Addition of PLADO into the blends could enhance the compatibility between dispersed PDLLA phase and PPDO matrix; the boundary between the two phases became unclear and even the smallest holes were not detected. On the other hand, the position of the T_g was composition dependent; when 5% PLADO was added into blend, the T_g distance between PPDO and PDLLA was shortened. The blends with various contents of compatibilizer had better mechanical properties compared with simple PPDO/PDLLA binary polymer blend, and such characteristics further improved as adding 5% random copolymers. The maximum observed tensile strength was 29.05 MPa for the compatibilized PPDO/PDLLA blend with 5% PLADO, whereas tensile strength of the uncompatibilized PPDO/PDLLA blend was 14.03 MPa, which was the lowest tensile strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Microwave‐irradiated ring‐opening polymerization of D,L‐lactide under atmosphere

Poly(D ,L ‐lactide) (PDLLA) was synthesized by microwave‐irradiated ring‐opening polymerization catalyzed by stannous octoate (Sn(Oct)₂) under atmosphere. The effects of heating medium, monomer purity, catalyst concentration, microwave irradiation time, and vacuum level were discussed. Under the appropriate conditions such as carborundum (SiC) as heating‐medium, 0.15% catalyst, lactide with purity above 99.9%, 450 W microwave power, 30 min irradiation time, and atmosphere, PDLLA with a viscosity–average molecular weight (M_η) over 2.0 × 10⁵ and a yield over 85% was obtained. The dismission of vacuum to ring‐opening polymerization of D ,L ‐lactide (DLLA) under microwave irradiation simplified the process greatly. The temperature under microwave irradiation and conventional heating was compared. The largely enhanced ring‐opening polymerization rate of DLLA under microwave irradiation was the coeffect of thermal effects and microwave effects. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2244–2247, 2006     

Effect of stereoisomerism of poly(lactic acid) during neural guide conduit membrane synthesis

Poly(lactic acid) membranes are being developed as biomaterials for several purposes such as artificial implants for peripheral nerve injury, also known as neural guide conduits (NGC). These membranes need to meet standards of mechanical, degradability, and permeability properties, besides dimensional and structural requirements. Among the stereoisomers of polylactides, poly(l ‐lactic acid), and poly(d ,l ‐lactic acid) are the most used as biomaterials, having significant differences in solubility, crystallinity thermal, and mechanical properties. In this work, PLLA and PDLLA were compared for hollow fiber membrane synthesis by liquid induced phase separation. PLLA samples presented 18% of crystallinity, while PDLLA is amorphous. PDLLA and PLLA polymer solutions on N‐methyl‐pyrrolidone presented values of 3428 and 320.2 cP, respectively. In immersion of PLA‐NMP solutions in water, PLLA solution presented instantaneous demixing, while PDLLA showed a 28 s delay on precipitation. The PLLA–NMP–water has a larger demixing region compared to PDLLA–NMP–water system. Hollow fibers of both polymers presented closed external surface with finger‐like macropores morphology in their cross‐sections. PDLLA presented typical liquid–liquid demixing pores while PLLA presented spherulitical crystalline solid–liquid separation structures, which deeply compromised its mechanical properties. PDLLA presented as a good candidate for hollow fiber NGC material, as presented good mechanical resistance in tensile and suture simulating tests. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46190.     

Thermal properties and miscibility of semi‐crystalline and amorphous PLA blends

The focus of this research is the study of the microstructures and miscibility at the interface between semi‐crystalline and amorphous PLAs [poly (l ‐lactic acid)(PLLA) with poly (l ,d ‐lactic acid)(PDLLA), respectively]. The blends are prepared through thermal processing (extrusion and hot‐pressing). To increase the area of interface between PDLLA and PLLA, the fibers from PLLA and PDLLA are used. Thermal and microstructures of the blends were studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), dynamic thermogravimetric analysis(DMA), small‐angle X‐ray diffraction(SAXS) and wide‐angle X‐ray diffraction (WAXD). The two PLAs are miscible in molten state. However, phase separation is detected after various thermal treatments, with PDLLA being excluded from the regions of interlamellar PLLA regions when PDLLA content is low, as determined from X‐ray diffraction studies. The compatibility between the two PLAs is not perfect in the molten state, since enthalpies of the various blends at T_g are lower than any pure PLA material. The semi‐crystalline PLLA fiber can recrystallize alone in the molten amorphous PDLLA, and a higher nuclei density is observed at the interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41205.     

Synthesis of poly(D,L‐lactic acid) modified by cholic acid via direct melt copolycondensation and its characterization

From D,L ‐lactic acid and the natural functional molecule cholic acid (CA), the biodegradable material poly(D,L ‐lactide–cholate) was synthesized via direct copolycondensation. For the CA/lactic acid (LA) molar feed ratio of 1/64, the optimal synthesis conditions were as follows: a prepolymerization time of 8 h, 0.3 wt % SnO catalyst, and melt copolycondensation for 8 h at 160°C, which gave a novel star‐shaped poly(D,L ‐lactic acid) (PDLLA) modified by CA with the maximum weight‐average molecular weight of 5600 Da at a yield of 51.9%. The copolymer poly(D,L ‐lactide–cholate) at different molar feed ratios was characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, gel permeation chromatography, differential scanning calorimetry, thermogravimetry, and X‐ray diffraction. Decreasing the molar feed ratio of CA/LA from 1/15 to 1/128 reduced the average number of CA units embedded in the copolymer from 4 to 1. With 1/15 CA/LA, the copolymer was not a star‐shaped polymer, and its weight‐average molecular weight was the biggest (weight‐average molecular weight = 12,700 Da, weight‐average molecular weight/number‐average molecular weight = 1.68). With 1/32 CA/LA, the copolymer with two CA units was not a star‐shaped polymer either. With 1/64, 1/100, and 1/128 CA/LA, the copolymer mainly had one CA unit core embedded as a normal star‐shaped PDLLA with four arms, and certain crystallinity could be detected. The novel direct copolycondensation method was simple and practical for the synthesis of the asymmetrical star‐shaped PDLLA material, and it was advantageous for this PDLLA material embedded in the special bioactive molecule CA to be applied in the field of drug delivery and tissue engineering. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Water vapor permeability of poly(lactide)s: Effects of molecular characteristics and crystallinity

Amorphous‐made poly(L ‐lactide) [i.e., poly(L ‐lactic acid) (PLLA)], poly(L ‐lactide‐co‐D ‐lactide)[P(LLA‐DLA)](77/23), and P(LLA‐DLA)(50/50) films and PLLA films with different crystallinity (X_c) values were prepared, and the effects of molecular weight, D ‐lactide unit content (tacticity and optical purity), and crystallinity of poly(lactide) [i.e., poly(lactic acid) (PLA)] on the water vapor permeability was investigated. The changes in number‐average molecular weight (M_n) of PLLA films in the range of 9 × 10⁴–5 × 10⁵ g mol^?1 and D ‐lactide unit content of PLA films in the range of 0–50% have insignificant effects on their water vapor transmission rate (WVTR). In contrast, the WVTR of PLLA films decreased monotonically with increasing X_c from 0 to 20%, while leveled off for X_c exceeding 30%. This is probably due to the higher resistance of “restricted” amorphous regions to water vapor permeation compared with that of the “free” amorphous regions. The free and restricted amorphous regions are major amorphous components of PLLA films for X_c ranges of 0–20% and exceeding 30%, respectively, resulting in the aforementioned dependence of WVTR on X_c. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Preparation and evaluation of novel blend microspheres of poly(lactic‐co‐glycolic)acid and pluronic F68/127 for controlled release of repaglinide

The objective of the present work was to study the release behavior of plain and blend microspheres (MS) of PLGA and Pluronic F68/127. In this study, a novel blend MS of poly(D ,L ‐lactic‐co‐glycolic acid) (PLGA) and Pluronic F68/127 (PLF68/127) were prepared by the emulsion–solvent evaporation method. Repaglinide, an antidiabetic drug with a very short half‐life, was successfully encapsulated into the blend MS. Various formulations were prepared by varying the ratio of PLGA and PLF68/127. Drug encapsulation up to 91% was achieved as measured by UV spectroscopy. Scanning electron microscopy showed that MS have smooth surfaces even after incorporation of PLF68/127. Particle size, as measured by using laser light scattering technique, gave an average size ranging from 12 to 47 μm. Differential scanning calorimetry (DSC) was performed to understand the crystalline nature of the drug after encapsulation into MS. DSC revealed the crystalline dispersion in the polymer matrix. In vitro release experiments performed in simulated intestinal fluid, indicated the dependence of release rate on the amount of PLF68/127 present in the MS; slow release was extended up to 153 h. Release data have been fitted to an empirical equation to compute the diffusional exponent (n), which indicated that the release mechanism to be non‐Fickian type. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Poly(dl‐Lactic Acid)‐Based Linear Polyurethanes Capable of Forming Physical Crosslinking via UPy Quadruple Hydrogen Bonding Assembly

Linear shape memory polyurethanes based on poly(dl ‐lactic acid) (PDLLA) macrodiol (PDLLA‐SMPUs) have various advantages such as good processability, biodegradability, shape memory effect, and biocompatibility, yet the insufficient mechanical properties prevent their effective applications in bone repair. 2‐Ureido‐4[1H]‐pyrimidone (UPy) can form strong quadruple hydrogen bonding. Here, a new linear PDLLA‐SMPU containing pendant UPy units (UPy‐p‐PDLLA‐SMPU) is designed and synthesized. The pendant UPy units may dimerize to form physical crosslinking among UPy‐p‐PDLLA‐SMPU chains. As a result, UPy‐p‐PDLLA‐SMPU demonstrates both good processability and significantly higher mechanical properties than the corresponding linear PDLLA‐SMPU without pendant UPys. In addition, UPy‐p‐PDLLA‐SMPU shows excellent shape memory effect near body temperature, with a shape fixity ratio of up to 98.6% and a recovery ratio of up to 92.9%. This work provides a new strategy to design SMPUs integrating the merits of linear and crosslinked polyurethanes, and the obtained UPy‐p‐PDLLA‐SMPU is a promising material for bone tissue repair in view of the mechanical, thermal, and shape memory properties.     

Demonstrating the mechanism and efficacy of water‐induced shape memory and the influence of water on the thermal properties of oriented poly(d,l‐lactide)

Achieving water‐induced shape‐memory property in poly(D,L ‐lactide) (PDLLA), generated by means of advanced processing methods, opens possibilities to develop novel bioresorbable medical devices with shape‐memory properties activated by the human body without external heat. The main phenomena that affect the molecular movements that enable the water‐induced shape‐memory effect in an oriented PDLLA in an aqueous environment at physiological temperature are related to the water driven disruption of the intermolecular dipole‐dipole and/or hydrogen bonding of the oriented PDLLA chains and the subsequent decrease of the glass transition temperature (T_g) to the range of physiological temperature. The diffused water in the polymer matrix decreased the energy needed to finish the glass transition process explaining the higher shape‐recovery rate of the γ‐irradiated PDLLA with respect to the non‐γ‐irradiated PDLLA in an aqueous environment at physiological temperatures. The water‐induced decrease in the T_g was thermally reversible. The efficacy of the generated shape‐memory was tested with PDLLA shape‐memory nails in a pullout test, in which the pullout force of the PDLLA nails increased 360% during a seven day test period in vitro at 37°C. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4209–4218, 2013     

Facile Preparation of Linear Polyurethane from UPy‐Capped Poly(dl‐Lactic Acid) Macrodiol

Linear polyurethanes based on poly(dl ‐lactic acid) (PDLLA) macrodiol are promising materials in tissue engineering, yet their synthesis requires rigorous control on various parameters. A facile way to prepare linear polyurethanes by capping the PDLLA macrodiol (M _n = 4536) with 2‐ureido‐4[1H]‐pyrimidone (UPy) is reported. The obtained low‐molecular‐weight UPy‐capped polyurethane can form flexible, stretchable, and hydrophobic supramolecular films due to the strong and unidirectional quadruple hydrogen bonding of UPy dimers. Tensile tests, shape recovery, and self‐healing observations indicate that, compared with conventional PDLLA macrodiol‐based linear polyurethane (M _n = 48840 and PDI = 1.8), the UPy‐capped polyurethane films have comparable mechanical properties (tensile modulus: 900 ± 38 MPa; ultimate strength: 9.6 ± 0.8 MPa) yet significantly better shape memory and self‐healing properties. These results suggest that the UPy‐capped polyurethane might become an alternative for conventional linear polyurethane as a new biomedical material.     

Melt polycondensation of L‐lactic acid to poly(L‐lactic acid) with Sn(II) catalysts combined with various metal alkoxides

Melt polycondensation of L ‐lactic acid (LA) was examined in the presence of binary catalyst systems consisting of SnCl₂·2H₂O and metal alkoxides as co‐catalysts. Among the co‐catalysts examined, viz (Al(O ⁱPr)₃,Ti(O ⁱPr)₄,Y(O ⁱPr)₃,Si(OEt)₄ and Ge(OEt)₄), Ge(OEt)₄ was found to be the most effective in enhancing the catalytic activity of Sn(II). With an optimized composition of SnCl₂·2H₂O–Ge(OEt)₄, the molecular weight (M_n) of PLLA reached 40 000 Da in a short reaction time (<15 h) at the optimum reaction conditions of 180 °C and 10 Torr. This catalyst system was also superior to the conventional single metal ion catalysts such as Sn(II) in terms of racemization and discolouration of the resultant polymer. The metal alkoxides, added as co‐catalysts, should work as oxo acids that can effectively control the catalytic activity of Sn(II) ion in the direct polycondensation of LA, in a manner similar to that of proton acids. © 2003 Society of Chemical Industry     

Properties and morphology of bioceramics/poly(D,L‐lactide) composites modified by in situ compatibilizing extrusion

Composites of poly(D ,L ‐lactide) (PDLLA) with hydroxylapatite (HA) and PDLLA with tertiary calcium phosphate (TCP) were prepared by in situ modification with methylenediphenyl diisocyanate (MDI) and molded by piston extrusion at temperature between T_g and T_m of PDLLA. Mechanical properties of the composites increased obviously when compared with the unmodified bioactive ceramic particles/PDLLA composites. The effect of MDI contents on mechanical properties of the composites was studied. At the optimum conditions of 1.0/1.0molar ratios of ? NCO groups in MDI to ? OH groups in PDLLA, bending strength 68.4 MPa and bending modulus 2281.5 MPa, were achieved in composite HA/PDLLA/MDI with 15 wt % HA. Both increased by nearly 30% when compared with that of solution cast HA/PDLLA composites. Interfacial adhesion and compatibility between PDLLA and bioactive ceramic particles (HA and TCP) were investigated. Scanning electron microscopy (SEM) indicated that the interface between HA particles and PDLLA was blurred and HA particles were closely surrounded by PDLLA matrix in HA/PDLLA/MDI composites. Oriented fibrils along with longitudinal direction of extrusion die were also observed on the surfaces of HA/PDLLA/MDI composite. It is confirmed that MDI has improved interfacial adhesion and compatibility between HA particles and PDLLA phase. Fibril structures formed in the extrusion, and it contributed a great deal in enhancing the mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4085–4091, 2006     

Polycondensation of lactic acid catalyzed by organic acid anhydrides

Poly(lactic acid) was prepared by the utilization of the acid anhydrides cis‐butenedoic anhydride, phthalic anhydride and pyromellitic dianhydride as catalysts. The effect of the amount of catalyst, temperature and reaction time on the polymerization was investigated in detail. The results show that the acid anhydride catalysts are very efficient in bulk polycondensation, giving poly(D ,L ‐lactic acid) with average molecular weights (M_w) of (1.7–2.3) × 10⁴ in high yield. Copyright © 2007 Society of Chemical Industry     

Design,synthesis, and characterization of a potential flame retardant poly(lactic acid‐co‐pyrimidine‐2,4,5,6‐tetramine) via direct melt polycondensation

Directly starting from d ,l ‐lactic acid (LA) and pyrimidine‐2,4,5,6‐tetramine (PTA), the copolymer P(LA‐co‐PTA) as a novel potential solid compatible polymeric flame retardant is synthesized as designed via melt polycondensation. When the molar feed ratio LA/PTA is 60/1, the optimal synthetic conditions are discussed. After the prepolymerization at 140°C for 8 h, using 0.5 wt % stannous oxide as the catalyst, the melt copolymerization at 160°C for 4 h gives the copolymer with the biggest intrinsic viscosity 0.88 dL g^?1. The structures and properties of P(LA‐co‐PTA)s at different molar feed ratios are characterized by FT‐IR, ¹H‐NMR, ¹³C‐NMR, GPC, XRD, DSC, and TGA. The decomposition temperatures of P(LA‐co‐PTA)s are higher than these of homopolymer poly(d,l ‐lactic acid) (PDLLA). All copolymers have higher char yield than PDLLA, and the more PTA in the feed content, the higher char yield. What's more, there are some residues at 700–800°C, indicating that P(LA‐co‐PTA)s have good charring ability. When the monomer PTA is introduced into polylactic acid by chemical bonding as purine (PU) unit formed during the condensation, both the PTA's relatively higher nitrogen content and the PU's similar structure with flame retardant benzimidazole are beneficial to improve the thermal stability and charring ability, especially the latter. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40275.     

Melt polycondensation of L‐lactic acid catalyzed by 1,3‐dialkylimidazolium ionic liquids

BACKGROUND: In syntheses of biodegradable and bioresorbable polymers, efficient metal‐free catalysts are very desirable as the resulting products may be more biocompatible. As an attempt to find new metal‐free catalysts, 1,3‐dialkylimidazolium salts, the most commonly used ionic liquids or organic melt salts, were used as single‐component catalysts in the melt polycondensation of L ‐lactic acid for the first time. The resulting poly(L ‐lactic acid) (PLLA) was characterized using gel permeation chromatography, ¹³C NMR, DSC and polarimetry. RESULTS: It has been found that less bulky substituents on the imidazolium ring are conducive to catalytic activity. PLLA with molar mass of about 20 000 g mol^?1 was synthesized at high yield (over 70%) in the presence of various 1,3‐dialkylimidazolium salts. The product exhibits satisfactory color (white to slightly yellow), optical purity (89–95%) and crystallinity (40–55%). A possible catalytic mechanism is proposed. CONCLUSION: As compared with the well‐known binary catalyst system SnCl₂ · 2H₂O/toluene sulfonic acid, the catalysts used in this investigation are better in terms of increasing PLLA yield and preventing discoloration and comparable in terms of racemization. Copyright © 2008 Society of Chemical Industry