Preparation of high‐molecular‐weight poly(L‐lactic acid)‐based polymers through direct condensation polymerization in bulk state

Poly(L ‐lactic acid‐co‐succinic acid‐co‐1,4‐butanediol) (PLASB) was synthesized by a direct condensation copolymerization of L ‐lactic acid, succinic acid (SA), and 1,4‐butanediol (BD) in bulk state using titanium(IV) butoxide (TNBT) as a catalyst. Weight average molecular weight (M_w) of PLASB increased from 3.5 × 10⁴ to 2.1 × 10⁵ as the content of SA and BD went up from 0.01 to 0.5 mol/100 mol of L ‐lactic acid (LA). PLASB having M_w in the range from 1.8 × 10⁵ to 2.1 × 10⁵ showed tensile properties comparable to those of commercially available poly(L ‐lactic acid) (PLLA). In sharp contrast, homopolymerization of LA in bulk state produced PLLA with M_w as low as 4.1 × 10⁴, and it was too brittle to prepare specimens for the tensile tests. M_w of PLASB synthesized by using titanium(IV)‐2‐ethyl(hexoxide), indium acetate, indium hydroxide, antimony acetate, antimony trioxide, dibutyl tin oxide, and stannous‐2‐ethyl 1‐hexanoate was compared with that of PLASB obtained by TNBT. Ethylene glycol oligomers with different chain length were added to LA/SA in place of BD to investigate effect of chain length of ethylene glycol oligomers on the M_w of the resulting copolymers. Biodegradability of PLASB was analyzed by using the modified Sturm test. Toxicity of PLASB was evaluated by counting viable cell number of mouse fibroblast cells that had been in contact with PLASB discs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 466–472, 2006     

Effect of poly(ethylene glycol) on the solid‐state polymerization of poly(ethylene terephthalate)

Poly(ethylene glycol) (PEG) and end‐capped poly(ethylene glycol) (poly(ethylene glycol) dimethyl ether (PEGDME)) of number average molecular weight 1000 g mol^?1 was melt blended with poly(ethylene terephthalate) (PET) oligomer. NMR, DSC and WAXS techniques characterized the structure and morphology of the blends. Both these samples show reduction in T_g and similar crystallization behavior. Solid‐state polymerization (SSP) was performed on these blend samples using Sb₂O₃ as catalyst under reduced pressure at temperatures below the melting point of the samples. Inherent viscosity data indicate that for the blend sample with PEG there is enhancement of SSP rate, while for the sample with PEGDME the SSP rate is suppressed. NMR data showed that PEG is incorporated into the PET chain, while PEGDME does not react with PET. Copyright © 2005 Society of Chemical Industry     

Calorimetric analysis of the multiple melting behavior of poly(L‐lactic acid)

This article contains a detailed calorimetric analysis of the multiple melting behavior of poly(L ‐lactic acid) (PLLA) in dependence of crystallization conditions. PLLA crystals formed upon primary crystallization have a greater tendency to reorganize into more stable structures during the heating scan that leads to fusion. Depending on crystallization temperature, one or multiple melting endotherms and/or reorganization exotherms can be evidenced. This complex melting behavior arises from the fusion of a certain amount of the original crystals (already partially perfected during the heating scan), followed by recrystallization and final melting of more perfect crystals, partly grown during primary crystallization, and partly formed through the reorganization processes occurring during the heating scan. A detailed map of the melting behavior of PLLA is described in this contribution. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3145–3151, 2006     

Blends of poly(L‐lactic acid) with poly(ω‐pentadecalactone) synthesized by enzyme‐catalyzed polymerization

Poly(ω‐pentadecalactone) (PPDL) was synthesized by enzyme‐catalyzed polymerization. The molecular weight of the PPDL was about 35,000. Opaque poly(L ‐lactic acid) (PLLA)/PPDL blend films were created by the solvent casting technique. The addition of PPDL led to PLLA crystallization. Furthermore, the addition of PPDL with PLLA increased both the Young's modulus [pure PLLA : 0.67 GPa, PLLA/PPDL (70/30 wt %) : 1.01 GPa] and the PLLA glass transition temperature. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Plasma‐induced solid‐state polymerization modified poly(tetrafluoroethylene) membrane for pervaporation separation of aqueous alcohol mixtures

Acrylamide (AAm) solid state polymerization was induced using argon plasma to improve the pervaporation performance of poly(tetrafluoroethylene) (PTFE) membranes (PTFE‐g‐PAAm) in aqueous alcohol mixtures. The surface morphology, chemical composition, and hydrophilicity changes in the PTFE and PTFE‐g‐PAAm membranes were investigated using ATR‐FTIR, SEM, AFM, X‐ray photoelectron spectroscopy, and water contact angle measurements. The surface hydrophilicity rapidly increased with increasing Ar exposure time, but decreased after longer Ar exposure time because of the degradation in the PTFE‐g‐PAAm membrane grafted layer. Compared with the hydrophilicity of the pristine PTFE membrane (water contact angle = 120°), the argon plasma induced acrylamide (AAm) solid‐state polymerization onto the PTFE surface (water contact angle = 43.3°) and effectively improved the hydrophilicity of the PTFE membrane. This value increases slowly with increasing aging time and then reaches a plateau value of about 50° after 10 days of storage under air. The pervaporation separation performances of the PTFE‐g‐PAAm membranes were higher than that of the pristine PTFE membrane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:909–919, 2006     

Phase behavior and morphology in blends of poly(L‐lactic acid) and poly(butylene succinate)

Blends of poly(L ‐lactic acid) (PLA) and poly(butylene succinate) (PBS) were prepared with various compositions by a melt‐mixing method and the phase behavior, miscibility, and morphology were investigated using differential scanning calorimetry, wide‐angle X‐ray diffraction, small‐angle X‐ray scattering techniques, and polarized optical microscopy. The blend system exhibited a single glass transition over the entire composition range and its temperature decreased with an increasing weight fraction of the PBS component, but this depression was not significantly large. The DSC thermograms showed two distinct melting peaks over the entire composition range, indicating that these materials was classified as semicrystalline/semicrystalline blends. A depression of the equilibrium melting point of the PLA component was observed and the interaction parameter between PLA and PBS showed a negative value of ?0.15, which was derived using the Flory–Huggins equation. Small‐angle X‐ray scattering revealed that, in the blend system, the PBS component was expelled out of the interlamellar regions of PLA, which led to a significant decrease of a long‐period, amorphous layer thickness of PLA. For more than a 40% PBS content, significant crystallization‐induced phase separation was observed by polarized optical microscopy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 647–655, 2002     

Telechelic poly(L‐lactic acid) for dilactide production and prepolymer applications

The suitability of different types of telechelic poly(lactic acid) (PLA) copolymers for dilactide production and prepolymer products was evaluated. L ‐lactic acid (L ‐LA) was copolymerized with 1,4‐butanediol, pentaerythritol, adipic acid, or 1,2,3,4‐butanetetracarboxylic acid (1,2,3,4‐BTCA). The influence of branching, the choice of catalyst, and the type of terminal groups on the properties and the thermal stability of the end product was determined. Carboxyl‐termination of PLA was shown to lead to higher molar masses than hydroxyl‐termination. The observed differences in the molar masses were explained by the lower thermal stability of the hydroxyl‐terminated PLA, as evidenced by the faster depolymerization rate of the hydroxyl‐terminated polymers and their higher tendency to undergo racemization. Sn(Oct)₂ was found to be a more effective copolymerization catalyst than Fe(OAc)₂ in terms of the final molar masses obtained. It was additionally found that the amount of chains not attached to the comonomers decreased toward longer polymerization times and was typically higher for the hydroxyl‐terminated copolymers. The results suggest that predominant carboxyl‐termination would increase the thermal stability of PLA polymers, whereas hydroxyl‐termination could be utilized to increase the production speed and efficiency of dilactide. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and characterization of star‐shaped poly(ethylene glycol)‐block‐poly(L‐lactic acid) copolymers by melt polycondensation

Compared with linear diblock or triblock poly(ethylene glycol)‐block‐poly(L ‐lactic acid) copolymer (PEG‐b‐PLLA), star‐shaped PEG‐b‐PLLA (sPEG‐b‐PLLA) copolymers exhibit smaller hydrodynamic radius and lower viscosity and are expected to display peculiar morphologies, thermal properties, and degradation profiles. Compared with the synthesis routine of PEG‐b‐PLLA form lactide and PEG, the traditional synthesis routine from LA and PEG were suffered by the low reaction efficiency, low purity, lower molecular weight, and wide molecular weight distribution. In this article, multiarm sPEG‐b‐PLLA copolymer was prepared from multiarm sPEG and L ‐lactic acid (LLA using an improved method of melt polycondensation, in which two types of sPEG, that is, sPEG₁ (four arm, Mn = 4300) and sPEG₂ (three arm, Mn = 3200) were chosen as the core. It was found the molecular weight of sPEG‐b‐PLLA could be strongly affected by the purity of LLA and sPEGs, and the purification technology of vacuum dewater and vacuum distillation could help to remove most of the impurities in commercial available LLA. The polymers, including sPEG and sPEG‐b‐PLLA with varied core (sPEG₁ and sPEG₂) and LLA/sPEG feeding ratios, were characterized and confirmed by ¹H‐NMR and ¹³C‐NMR spectroscopy, Fourier transform infrared spectroscopy (FT‐IR) and gel permeation chromatography, which showed that the terminal hydroxyl group in each arm of sPEGs had reacted with LLA to form sPEG‐b‐PLLA copolymers with fairly narrow molecular weight distribution. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Relationship between the crystallization behavior of poly(ethylene glycol) and stereocomplex crystallization of poly(L‐lactic acid)/poly(D‐lactic acid)

Poly(l ‐lactic acid) (PLLA) is a good biomedical polymer material with wide applications. The addition of poly(ethylene glycol) (PEG) as a plasticizer and the formation of stereocomplex crystals (SCs) have been proved to be effective methods for improving the crystallization of PLLA, which will promote its heat resistance. In this work, the crystallization behavior of PEG and PLLA/poly(d ‐lactic acid) (PDLA) in PLLA/PDLA/PEG and PEG‐b‐PLLA/PEG‐b‐PDLA blends has been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both SCs and homocrystals (HCs) were observed in blends with asymmetric mass ratio of PLLA/PDLA, while exclusively SCs were observed in blends with approximately equal mass ratio of PLLA/PDLA. The crystallization of PEG was only observed for the symmetric blends of PLLA_39k/PDLA_35k/PEG_2k, PLLA_39k/PDLA_35k/PEG_5k, PLLA_69k/PDLA_96k/PEG_5k and PEG‐b‐PLLA_31k/PEG‐b‐PDLA_27k, where the mass ratio of PLLA/PDLA was approximately 1/1. The results demonstrated that the formation of exclusively SCs would facilitate the crystallization of PEG, while the existence of both HCs and SCs could restrict the crystallization of PEG. The crystallization of PEG is related to the crystallinity of PLLA and PDLA, which will be promoted by the formation of SCs. © 2017 Society of Chemical Industry     

Reactive blending of poly(L‐lactic acid) with poly(ethylene‐co‐vinyl alcohol)

Poly(L ‐lactic acid) (PLLA) was blended with poly(ethylene‐co‐vinyl alcohol) (EVOH) in the presence of an esterification catalyst to induce reaction between the hydroxyl groups of EVOH and the terminal carboxylic group of PLLA. Nascent low‐molecular‐weight PLLA, obtained from a direct condensation polymerization of L ‐lactic acid in bulk state, was used for the blending. Domain size of the PLLA phase in the graft copolymer was much smaller than that corresponding to a PLLA/EVOH simple blend. The mechanical properties of the graft copolymer were far superior to those of the simple blend, and the graft copolymer exhibited excellent mechanical properties even though the biodegradable fraction substantially exceeded the percolation level. The grafted PLLA reduced the crystallization rate of the EVOH moiety. Melting peak temperature (T_m) of the PLLA phase was not observed until the content of PLLA in the graft reaction medium went over 60 wt %. The modified Sturm test results demonstrated that biodegradation of EVOH‐g‐PLLA took place more slowly than that of an EVOH/PLLA simple blend, indicating that the chemically bound PLLA moiety was less susceptible to microbial attack than PLLA in the simple blend. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 886–890, 2005     

Synthesis and thermal properties of poly(methyl methacrylate)‐poly(L‐lactic acid)‐poly(methyl methacrylate) tri‐block copolymer

Poly(methyl methacrylate)‐poly(L ‐lactic acid)‐poly(methyl methacrylate) tri‐block copolymer was prepared using atom transfer radical polymerization (ATRP). The structure and properties of the copolymer were analyzed using infrared spectroscopy, gel permeation chromatography, nuclear magnetic resonance (¹H‐NMR, ¹³C‐NMR), thermogravimetry, and differential scanning calorimetry. The kinetic plot for the ATRP of methyl methacrylate using poly(L ‐lactic acid) (PLLA) as the initiator shows that the reaction time increases linearly with ln[M]₀/[M]. The results indicate that it is possible to achieve grafted chains with well‐defined molecular weights, and block copolymers with narrowed molecular weight distributions. The thermal stability of PLLA is improved by copolymerization. A new wash‐extraction method for removing copper from the ATRP has also exhibits satisfactory results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Crystallization and solid‐state polymerization of poly(aryl ester)s

The crystallization and solid‐state polymerization (SSP) of poly(aryl ester)s was investigated. Oligomers with different end‐groups were prepared by degradation of commercially available poly(aryl ester)s. The SSP of these oligomers was carried out after crystallization and under reduced pressure, in the presence of various catalysts. Polymers were characterized by means of their inherent viscosities and thermal properties. It has been found that Ti(OⁱPr)₄ was a better catalyst for SSP. The structures and morphologies of semicrystalline poly(aryl ester)s were investigated by X‐ray diffraction and differential scanning calorimetry (DSC). Copyright © 2004 Society of Chemical Industry     

Biodegradable poly(L‐lactic acid)/TiO2 nanocomposites: Thermal properties and degradation

Poly(L ‐lactic acid)‐titanium dioxide nanocomposites (with various loadings of TiO₂: 0.5, 1, 2, 5, and 10 wt %) were produced by solution casting method. The influence of TiO₂ on thermal properties and crystallinity of PLA was investigated by DSC and FTIR spectroscopy. The TiO₂ nano filler has no significant influence on the characteristic temperatures (T_g, T_c, and T_m), but has high impact on the crystallinity of these systems. The degree of crystallinity X_c significantly increases for PLA nanocomposites loaded with up to 5 wt % of TiO₂, while for 10 wt % load of TiO₂ it drops below X_c of the pure resin. The degradation of the prepared composites was evaluated hydrolytically in 1N NaOH, enzymatically in α‐amylase solutions, and under UV irradiation. The catalytic effect of TiO₂ nano particles on the degradation processes under UV light exposure (λ = 365 nm) and hydrolytic degradation was confirmed with the increase of the filler content. The opposite effect was identified in enzymatic degradation experiments. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrospun poly(L‐lactic acid)/hydroxyapatite composite fibrous scaffolds for bone tissue engineering

Poly(L ‐lactic acid) (PLLA) is one of the most studied synthetic biodegradable polymeric materials as a bone graft substitute. Taking into account the osteoconductive property of hydroxyapatite (HAp), we prepared fibrous matrices of PLLA without and with HAp particles in amounts of 0.25 or 0.50% (w/v, based on the volume of the base 15% w/v PLLA solution in 70:30 v/v dichloromethane/tetrahydrofuran). These fibrous matrices were assessed for their potential as substrates for bone cell culture. The presence of HAp in the composite fibre mats was confirmed using energy dispersive X‐ray spectroscopy mapping. The average diameters of both neat PLLA and PLLA/HAp fibres, as determined using scanning electron microscopy, ranged between 2.3 and 3.5 µm, with the average spacing between adjacent fibres ranging between 5.7 and 8.5 µm. The porosity of these fibrous membranes was high (ca 97–98%). A direct cytotoxicity evaluation with L929 mouse fibroblasts indicated that the neat PLLA fibre mats released no substance at a level that was toxic to the cells. The presence of HAp particles at 0.50% w/v in the PLLA fibrous scaffolds not only promoted the attachment and the proliferation of MC3T3‐E1 mouse pre‐osteoblastic cells, but also increased the expression of osteocalcin mRNA and the extent of mineralization after the cells had been cultured on the scaffolds for 14 and 21 days, respectively. The results obtained suggested that the PLLA/HAp fibre mats could be materials of choice for bone tissue engineering. Copyright © 2009 Society of Chemical Industry     

Synthesis and characterization of poly(o‐toluidine) doped with organic sulfonic acid by solid‐state polymerization

Poly (o‐toluidine) (POT) salts doped with organic sulfonic acids (β‐naphthalene sulfonic acid, camphor sulfonic acid, and p‐toluene sulfonic acid) were directly synthesized by using a new solid‐state polymerization method. The FTIR spectra, ultraviolet visibility (UV–vis) absorption spectra, and X‐ray diffraction patterns were used to characterize the molecular structures of the POT salts. Voltammetric study was done to investigate the electrochemical behaviors of all these POT salts. The FTIR and UV–vis absorption spectra revealed that the POT salts were composed of mixed oxidation state phases. All POT salts contained the conducting emeraldine salt (half‐oxidized and protonated form) phase; the pernigraniline (fully oxidized form) phase is predominant in POT doped with β‐naphthalene sulfonic acid, and POT doped with p‐toluene sulfonic acid had the highest doping level. The X‐ray diffraction patterns showed that the obtained POT doped with organic sulfonic acids were lower at crystallinity. The conductivity of the POT salts were found to be of the order 10^?3‐10^?4 S/cm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1630–1634, 2005     

Isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s prepared by melt polycondensation

The isothermal cold crystallization and melting behaviors of poly(L ‐lactic acid)s (PLLAs, weight average molecular weight, M_w, 6000–80,000) prepared via melt polycondensation were studied with differential scanning calorimeter in this work. It is found that the crystallization rate increased with decreasing M_w, reached a maximum at M_w of ca. 21,000 and then decreased again. The crystallinity of PLLA can be controlled in the range 30–50% by crystallization temperature (T_c) and time to fulfill the requirement of subsequent solid state polycondensation. The melting behavior strongly depends on T_c. The samples crystallized at high T_c melted with a single peak but those crystallized at low T_c melted with double peaks. The higher melting point (T_mH) kept almost constant and the lower melting point (T_mL) increased clearly with T_c. But the T_mL changed in jumps and a triple melting peak appeared at the vicinity of a characteristic crystallization temperature T_b, possibly because of a change of crystal structure. The equilibrium melting temperature of PLLA with M_w of 21,300 was extrapolated to be 222°C with nonlinear Hoffman‐Weeks method. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Photodegradation of Poly(L‐lactic acid): Effects of Photosensitizer

Summary: Amorphous and crystallized poly(L ‐lactic acid) (PLLA‐A and PLLA‐C, respectively) films with different contents of N,N,N′,N′‐tetramethyl‐1,4‐phenylenediamine (TMPD) as a photosensitizer were prepared, and the effects of the addition of TMPD on the photodegradation of PLLA films were investigated. It was found that the addition of TMPD effectively enhanced the photodegradation of PLLA films and thereby decreased their molecular weight of PLLA films regardless of their crystallinity, and that PLLA films with different molecular weights can be prepared by the addition of different amounts of TMPD and subsequent UV irradiation. Too high contents of TMPD however caused the brittleness of PLLA films due to a large decrease in molecular weight. The PLLA chains in crystalline regions as well as those in amorphous regions are photodegradable even at an early stage, in marked contrast to their hydrolytic degradation, where the chains in the amorphous regions are selectively degraded. The basic changes in glass transition, cold crystallization, and melting temperatures (T_g, T_cc, and T_m, respectively) of PLLA films during UV irradiation can be ascribed to low‐temperature annealing effects; i.e., annealing‐induced stabilization in chain packing should have elevated T_g, and annealing‐induced formation of crystallite nuclei should have lowered T_cc and increased T_m. The exceptional large decreases in T_cc and T_m of UV‐irradiated PLLA‐A films and in T_g of UV‐irradiated PLLA‐C films at high TMPD contents are attributable to the large decrease in molecular weight, whereas the exceptional decrease in T_m of PLLA‐C films at high TMPD contents can be due to the folding surface structural change of crystalline regions or to the lattice disorder caused by molecular structural changes.

of PLLA‐A films before UV irradiation and after UV irradiation for 60 h as a function of TMPD content.     

Thermally induced crystallization and enzymatic degradation studies of poly (L‐lactic acid) films

Poly(L ‐lactic acid) (PLLA) films with different crystallinities were prepared by solvent casting and subsequently annealed at various temperatures (T_a) (80–110°C). The effects of crystallinity on enzymatic degradation of PLLA films were examined in the presence of proteinase K at 37°C by means of weight loss, DSC, FTIR spectroscopy, and optical microscopy. DSC and the absorbance ratio of 921 and 956 cm^?1 (A₉₂₁/A₉₅₆) were used to evaluate crystallinity changes during thermally induced crystallization and enzymatic hydrolysis. The highest percentage of weight loss was observed for the film with the lowest initial crystallinity and the lowest percentage of weight loss was observed for the film with highest crystallinity. FTIR investigation of degraded films showed a band at 922 cm^?1 and no band at 908 cm^?1 suggested that all degraded samples form α crystals. The rate of degradation was found to depend on the initial crystallinity of PLLA film and shown that enzymatic degradation kinetics followed first‐order kinetics for a given enzyme concentration. DSC crystallinity and IR absorbance ratio, A₉₂₁/A₉₅₆ ratio, showed no significant changes with degradation time for annealed PLLA films whereas as‐cast PLLA film showed an increase in crystallinity with degradation; this revealed that degradation takes place predominantly in the free amorphous region of annealed PLLA films without changing long range and short range order © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nanocomposites of poly(L‐lysine) and single‐walled carbon nanotubes

BACKGROUND: Single‐walled carbon nanotubes have inspired research owing to their promise in a broad range of applications. The dispersion of carbon nanotubes is of key importance for the utilization of this interesting material for various potential applications. RESULTS: A novel and simple method was developed to fabricate polymer composites with single‐walled carbon nanotubes based on a solid‐state reaction, in which the nanotubes were reacted with poly(L ‐lysine) using high‐speed vibration milling. Fourier transform infrared and UV‐visible spectroscopy as well as thermogravimetry were employed to characterize the novel composites. The morphology and the dispersion of the carbon nanotubes were determined using scanning and transmission electron microscopy. CONCLUSION: The resulting composites were dispersable in water and are expected to have great potential for both molecular‐level studies and device applications of nanotubes. Copyright © 2007 Society of Chemical Industry     

Synthesis of high molecular weight poly(L‐lactic acid) via melt/solid polycondensation: Intensification of dehydration and oligomerization during melt polycondensation

Melt/solid state polycondensation (MP/SSP) is a cost‐effective route for synthesis of high molecular weight poly(L ‐lactic acid) (PLLA). However, the reaction rates in its four stages need to be enhanced greatly and the reaction times to be shortened largely before the MP/SSP technology can be industrialized. In this study, a new catalyst addition policy, i.e., adding TSA at the dehydration stage and SnCl₂·2H₂O at the MP stage, and more appropriate temperature and pressure programs were presented and applied in the MP process of LLA. The presence of TSA from dehydration appeared very effective for speeding up the dehydration and oligomerization stages as well as depressing racemization in the whole MP process. The polymerization degree (X_n) of oligomer was clearly increased, and the reaction time was shortened to a great extent. Direct using reduced pressure was also very helpful for intensifying the dehydration stage, only leading to LLA loss as little as 2%. A PLLA with M_w of 44,000 and optical purity of 96.8% suitable for subsequent SSP was produced after dehydration for 2 h, oligomerization for 2 h and MP for 4 h under appropriate conditions. And an interesting strong dependence of the M_w of final PLLA product on the X_n of the oligomer was observed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011