Synthesis, characterization and biodegradable studies of 1,3-propanediol based polyesters

A series of biodegradable polyesters were synthesized from dicarboxylic acids and 1,3-propanediol catalyzed by transestrification polycondensation reaction in the bulk. The structure, average molecular weights and physical properties of the resulting aliphatic polyesters were characterized by ¹H NMR, FT-IR, solution viscosity, GPC, DSC and TGA. Homopolyesters show higher degree of crystallinity, melting and thermal stability in comparison to copolyesters. The biodegradability of the polyesters was determined by monitoring the normalized weight loss of polyester films with time in phosphate buffer (pH 7.2) without and with Rhizopus delemar lipase at 37 °C. The rate of enzymatic degradation of homopolyesters follows the path PPSu > PPAd > PPSe. PPSe did not show significant weight loss in presence of enzyme which may be due to its highest degree of crystallinity and melting point compared to the PPSu, PPAd and copolyesters. In the soil burial degradation polyester sample showed severe surface degradation by the attack of microorganism.     

Studies on the enzymatic hydrolysis of polyesters I. Low molecular mass model esters and aliphatic polyesters

The bio-catalysed cleavage of ester bonds in low molecular mass model esters and aliphatic polyesters was studied in detail with the aim to gain improved information about the underlying mechanism and the parameters controlling polyester degradation. Among various hydrolytic enzymes the lipase of Pseudomonas species (PsL) was chosen for the investigations. In the heterogeneous phase system the specific hydrolysis rate of the esters was constant as long as free substrate surface was available. In addition to aliphatic low molecular mass model esters, also cycloaliphatic and aromatic esters were cleaved by PsL, indicating that a steric hindrance of the enzymatic ester cleavage is not the predominant controlling factor in polyester degradation. However, the cleavage rates of the aliphatic model esters are larger by more than an order of magnitude. For aliphatic polyesters the temperature difference between the melting point of the polymer and the temperature where degradation takes place (ΔT_mt), turned out to be the primary controlling parameter for polyester degradation with the lipase. Only if ΔT_mt<30 °C, a measurable enzymatic degradation rate is found. ΔT_mt can be regarded as a measure of the mobility of the polyesters chains in the crystalline domains, necessary for the access of the esters to the active site of the lipase. Though aliphatic homopolyesters are seemingly very similar with regard to their chemical structure and reactivity of the ester bonds, their enzymatic degradation rates still differ significantly even at the same ΔT_mt. These differences have obviously to be attributed to small changes in the chemical structure, as, for instance, the C number of the aliphatic diacid.     

Synthesis and comparative biodegradability studies of three poly(alkylene succinate)s

Poly(alkylene succinates) were synthesized from succinic acid and aliphatic diols with 2 to 4 methylene groups by melt polycondensation. DSC, ¹H NMR, WAXD and molecular weight measurements were used to characterise the polymers. Biodegradability studies of polyesters with the same average molecular weight, included enzymatic hydrolysis for several days using Rhizopus delemar lipase at pH 7.2 and 30 °C. DSC traces of biodegraded polyesters revealed that hydrolysis affected mainly the amorphous material. For all polyesters an increase in glass transition, melting point and heat of fusion was recorded. In the first days of enzymatic hydrolysis, fast rates of mass loss were observed accompanied by a rapid reduction of intrinsic viscosity and molecular weight, thus indicating a mixed endo- and exo-type hydrolysis mechanism. Afterwards, it turned to an exo-type mechanism, taking place in the crystalline phase, since after 15-25 days of enzymatic hydrolysis molecular weight was stabilized, while mass loss kept on decreasing though in a slower rate. End-group analysis revealed that carboxyl and hydroxyl groups increased due to ester bonds' scission. The biodegradation rates of the polymers decreased following the order PPSu > PESu ≥ PBSu and it was attributed to the lower crystallinity of PPSu compared to other polyesters, rather than to differences in chemical structure. Finally, a simple theoretical kinetic model was developed and Michaelis-Menten parameters were estimated.     

Nanocomposites of aliphatic polyesters: An overview of the effect of different nanofillers on enzymatic hydrolysis and biodegradation of polyesters

In the present review the findings concerning the effect of nanofillers to biodegradation and enzymatic hydrolysis of aliphatic polyesters were summarized and discussed. Most of the published works are dealing with the effect of layered silicates such as montmorillonite (unmodified and modified with organic compounds), carbon nanotubes and spherical shape additives like SiO₂ and TiO₂. The degradation of polyester due to the enzymatic hydrolysis is a complex process involving different phenomena, namely, water absorption from the polyesters, enzymatic attack to the polyester surface, ester cleavage, formation of oligomer fragments due to endo- or exo-type hydrolysis, solubilization of oligomer fragments in the surrounding environment, diffusion of soluble oligomers by bacteria and finally consumption of the oligomers and formation of CO₂ and H₂O. By studying the published works in nanocomposites, different and sometimes contradictory results have been reported concerning the effect of the nanofillers on aliphatic polyesters biodegradation. Most of the papers suggested that the addition of nanofillers provokes a substantial enhancement of polyester hydrolysis due to the catalyzing effect of the existed reactive groups (–OH and –COOH), to the crystallinity decrease, to the higher hydrophilicity of nanofillers and thus higher water uptake, to the higher interactions, etc. However, there are also some papers that suggested a delay effect of nanofillers to the polyesters degradation mainly due to the barrier effect of nanofillers and the lower available surface for enzymatic hydrolysis.     

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, ¹H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E = 157 kJ/mol while the second mechanism is a first-order reaction with E = 221 kJ/mol, as calculated by the fitting of experimental measurements.     

Degradation behaviors of polyester monolayers at the air/water interface: Alkaline and enzymatic degradations

The degradation kinetics of Langmuir monolayer films of a series of biodegradable polyesters has been studied to investigate the effect of degradation medium, alkalinity and enzymes. The degradation behavior of polyester monolayers strongly depended on both degradation medium and surface pressure. As the surface pressure was increased, the degradation rates of poly(l-lactide) (PLLA) and poly[(R)-3-hydroxybutyrate] (P(3HB)) increased in both degradation media. When monolayers were exposed to an alkaline subphase, the degradation of PLLA monolayers occurred at relatively low surface pressures; the PLLA monolayers were hydrolyzed at pH 10.5 regardless of surface pressure, while the alkaline degradation of P(3HB) monolayer occurred over a constant surface pressure of 7 mN/m at pH 11.8. These results have been explained by the difference in hydrophilic/hydrophobic balance of the polymers; PLLA is more hydrophilic than P(3HB). In contrast, the enzymatic degradations of both polymer monolayers occurred at higher constant surface pressures than those of the alkaline treatment; 7 mN/m for PLLA and 10 mN/m for P(3HB). This behavior was attributed to the enzymes being much larger than the alkaline ions: the enzymes need a larger contact area with the submerged monolayers to be activated.     

FTIR analysis of hydrolysis in aliphatic polyesters

Infrared (IR) spectroscopy was adopted to study the hydrolytic degradation of films of aliphatic polyesters in alkaline environment. A measurable increase with the time of immersion of the absorbance of the peak centered at about 1570 cm⁻¹ was observed. Analysis of the IR spectra showed that the integrated peak area in that region can be used to quantify changes in the concentration of degradation products and thus to provide indications regarding the kinetic constant of the hydrolysis reaction. It was found that the hydrolysis of ester bonds proceeds linearly with time, and this result suggests that the controlling mechanism is the chemical reaction rather than water diffusion. The results also show that degradation rate increases with increasing polydispersity.     

Preparation of poly(lactide-co-glycolide-co-caprolactone) nanoparticles and their degradation behaviour in aqueous solution

The tri-component copolymer poly(lactide-co-glycolide-co-caprolactone) (PLGC) was synthesized to prepare nanoparticles by the modified spontaneous emulsification solvent diffusion method (modified-SESD method); and the method was also modified by using the Tween60 instead of poly(vinyl alcohol) (PVA) as dispersing agent. The obtained nanoparticles have spherical shape and good particle distribution with mean size in the range from 100 to 200 nm. The in vitro degradation behaviour of PLGC nanoparticles was investigated. It was found that PLGC nanoparticles could remain stable during the degradation with no agglomeration. Compared with PLA and PLGA nanoparticles, the degradation rate of PLGC nanoparticles is faster. After 9 weeks of hydrolysis, the M_n of PLGC is less by 10% of the original M_n. The mean radius of the nanoparticles increases from 68 nm to 80 nm continuously during the first stage, and after 4 weeks of degradation, the particles' size decreases gradually from 80 nm to about 40 nm. These results suggest that the PLGC nanoparticles may show degradation-controlled drug release behaviour and seem to be a promising drug delivery system.     

The phase behavior of tetramethyl bisphenol-A polyarylate/aliphatic polyester blends

The phase behavior of blends of tetramethyl bisphenol-A polyarylate (TMPAr) with various linear aliphatic polyesters characterized by the ratio of aliphatic carbons to ester groups in the repeating unit, CH₂/COO = 3 ∼ 9, was examined by differential scanning calorimetry and dynamic mechanical analysis. TMPAr/aliphatic polyester blends prepared by solvent casting were found to be miscible when the CH₂/COO ratio of aliphatic polyesters was larger than 4 and up to 9. The thermodynamic interaction parameter, B for the miscible blends was determined by the analysis of the depression of the melting point of polyester using the Hoffman-Weeks method. From the analysis of the heat of mixing data using a binary interaction model, it was concluded that strong unfavorable intramolecular interaction exists between the  CH₂ and  COO units in aliphatic polyesters and that four substituted methyl groups produces more favorable effects on the miscibility TMPAr with aliphatic polyesters. © 1998 John Wiley & Sons, Inc. J Polym Sci 36 : 201–212, 1998     

Synthesis and characterization of elastic aliphatic polyesters from sebacic acid, glycol and glycerol

A series of biodegradable polyesters have been prepared from sebacic acid (SA), glycol (Go) and glycerol (Ge) through a two-step process. First the linear prepolymers were prepared from SA and Go, then crosslinked polyesters were obtained from the prepolymer and Ge with different molar ratios. The resulting samples were characterized by Fourier Transform Infrared Spectrum (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Differential Scanning Calorimetry (DSC). Dynamic Contact Angle tests (DCA) and mechanical tests were also investigated. The enzymatic degradation studies were performed at 37 °C in phosphate buffer solution with porcine pancreas lipase. The resultant polyesters were transparent, flexible, insoluble in organic solvents, and the surfaces of the polyesters were hydrophilic. Young’s modulus, tensile strength, glass transition temperature (T_g) and the degree of enzymatic degradation increased with increasing the content of Ge. It was also worth noticing that the surface content of -COOC- groups was a key factor in the enzymatic degradability.     

Use of UV-visible spectroscopy to monitor nitrocellulose degradation in thin films

Activation energies for nitrocellulose (NC) degradation have been determined from Arrhenius plots constructed using first-order rate constants measured at 40, 50 and 60 °C. The rate constants were obtained by monitoring the absorbance (A) at a wavelength in the visible region of an anthraquinone dye dispersed in NC thin films. The dye acts as a stabilizer and is slowly depleted as a result of its reaction with NO_x from the breakdown of the nitrate ester groups on NC. The data produced two linear regions in the first-order plots of ln(A₀/A_t) vs aging time. The first-region is attributed to the reaction of the dye with NO_x desorbed from the NC surface. The activation energy (∼73.5 kJ mol⁻¹) is in line with that found for NO_x surface desorption processes. The second linear region is thought to be due to the reaction of NO_x from the breakdown of the nitrate ester groups on the NC molecule. The activation energy (∼104.0 kJ mol⁻¹) is consistent with that for nitrate ester hydrolysis. The use of UV-visible spectroscopy has in this way made it possible to monitor the degradation of NC non-destructively without the need for stabilizer extraction and analysis.     

Water-soluble polyesters from long chain alkylesters of citric acid and poly(ethylene glycol)

Long chain aliphatic alcohols have been used as model compounds to develop a preparative method for a water-soluble material, which could be a carrier for triacontanol, a highly hydrophobic plant growth regulator. New polyesters from long chain aliphatic (C = 12, 18 and 22) mono-1-alkyl citrates and poly(ethylene glycol) were synthesized and characterized by NMR spectroscopy. The polyester containing the triacontyl moiety was obtained from mono-1-triacontyl citrate, which was synthesized from the corresponding alcohol extracted from the Agave fourcroydes. The molecular weight of the polyesters depends on experimental conditions during synthesis such as reaction time, atmosphere, catalyst concentration and temperature. The reaction is second order in the early stage of the polyester synthesis. The reaction rate constant is independent of the length of the aliphatic chain, but it decreases with increasing of the poly(ethylene glycol) employed. Turbidity measurements have been used to study the polyester solubility. Solubility characteristics were found to depend on the of poly(ethylene glycol), the aliphatic-chain length and the value of for the polyester. These preparations could potentially be used to release triacontanol.     

Thermal degradation mechanism of poly(ethylene succinate) and poly(butylene succinate): Comparative study

Two aliphatic polyesters that consisted from succinic acid, ethylene glycol and butylene glycol, —poly(ethylene succinate) (PESu) and poly(butylene succinate) (PBSu)—, were prepared by melt polycondensation process in a glass batch reactor. These polyesters were characterized by DSC, ¹H NMR and molecular weight distribution. Their number average molecular weight is almost identical in both polyesters, close to 7000 g/mol, as well as their carboxyl end groups (80 eq/10⁶ g). From TG and Differential TG (DTG) thermograms it was found that the decomposition step appears at a temperature 399 °C for PBSu and 413 °C for PESu. This is an indication that PESu is more stable than PBSu and that chemical structure plays an important role in the thermal decomposition process. In both polyesters degradation takes place in two stages, the first that corresponds to a very small mass loss, and the second at elevated temperatures being the main degradation stage. The two stages are attributed to different decomposition mechanisms as is verified from the values of activation energy determined with iso-conversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures, is auto-catalysis with activation energy E = 128 and E = 182 kJ/mol and reaction order n = 0.75 and 1.84 for PBSu and PESu, respectively. The second mechanism is nth-order reaction with E = 189 and 256 kJ/mol and reaction order n = 0.68 and 0.96 for PBSu and PESu, respectively, as they were calculated from the fitting of experimental results.     

Thermal degradation of aliphatic hyperbranched polyesters and their derivatives

In this study results of thermal degradation of aliphatic hyperbranched polyesters, AHBP, and their derivatives, determined by non-isothermal thermogravimetric analysis in inert atmosphere (N₂) are presented. The thermal stability of linear polyester PHPA (polyhydroxypivalic acid), additionally synthesized from hydroxypivalic acid, was also studied. AHBP samples, from second to tenth pseudo-generation, were synthesized starting from 2,2-bis(hydroxymethyl)propionic acid and di-trimethylolpropane. Modification of some selected AHBP samples was accomplished with the propionyl and benzoyl chloride, as well as with stearic acid. Thermal degradation of AHBP samples starts in the region between 250 °C and 275 °C and it ends around 430 °C. The thermal stability of AHBP samples increases with the number of end groups in the macromolecule, as well as with the modification of end groups with stearic acid and propionyl chloride. An AHBP sample of the fourth pseudo-generation, where all -OH end groups are modified with benzoyl chloride, shows lower thermal stability than the corresponding unmodified sample. The thermal stability of the linear polyester PHPA is lower than the thermal stability of the AHBP samples of the similar molar mass. The activation energies of thermal degradation for all synthesized AHBP samples were also calculated.     

New Strategy for Controlling Biodegradability of Biodegradable Polyesters by Enzyme‐Catalyzed Surface Grafting

A novel preparation method for the core‐shell type biodegradable polyesters or biodegradable materials grafted with biodegradable polyesters was developed by alkaline surface treatment of biodegradable polyester films and subsequent enzymatic polymerization of aliphatic lactones, one example of which is shown in this study, i.e., the preparation of poly(L ‐lactide) (PLLA) film grafted with poly(ε‐caprolactone). It is revealed that only alkaline surface treatment or the combination of alkaline surface treatment and enzyme‐catalyzed grafting, the former and the latter, respectively accelerating and delaying the enzymatic degradation of PLLA, will give PLLA materials having a wide variety of biodegradability. Also, the specificity of the enzyme used for hydrolysis could be used to confirm the grafted chain species.

     

聚羟基烷酸酯酶降解动力学研究

研究了羟基丁酸 羟基戊酸共聚物 (PHBV)在脂肪酶中的降解行为 ,用滴定法测定降解速度并进行酶促反应动力学研究 .探讨了降解速度与酶浓度和底物浓度的数学关系和Michaelis Menten常数 ,从实验上和理论上证实了PHBV的物理形态和几何尺寸对酶降解过程的影响 ,以及实验数据与非均相动力学模型的拟合     

Preparation, characterization and enzyme inhibition of methylmethacrylate copolymer nanoparticles with different hydrophilic polymeric chains

Methylmethacrylate copolymer nanoparticles with different hydrophilic chains were prepared by the free radical polymerization of methylmethacrylate with N-isopropylacrylamide (NIPAAm), N-methacrylic acid (MAA), N-trimethylaminoethylmethacrylate chloride (TMAEMC) or N-dimethylaminoethylmethacrylate hydrochloride (DMAEMC). These particles were characterized by particle size and zeta potential. The polymerization conditions were shown to influence the particle size and surface charge. Particle sizes of MMA-NIPAAm nanoparticles after 3 h of reaction reached constant level at 180 nm. An increasing amount of total monomer (0.5-5%) would result in the nanoparticles of particle size of 115-204 nm for 30% NIPAAm of the total monomer. In the same range of 5-40% NIPAAm of the total monomer, the particle size decreased from 280 to 170 nm. The concentration of the initiator APS up to a concentration of 0.2% for MMA-TMAEMC and 0.1% for MMA-NIPAAm showed no effect on the particle size of the final nanoparticle suspensions, while higher concentration would lead to aggregation in the polymerization process. MMA-NIPAAm nanoparticles were pH-dependent in zeta potential at pH 1-12 values reducing from 12.2 mV to −16.8 mV, respectively. Nanoparticles were incubated with pepsin and trypsin at 37 °C for 20 min and their enzyme inhibition was determined. The activity of pepsin decreased to 27% in the presence of MMA-NIPAAm nanoparticles, and MMA-MAA nanoparticles reduced the activity of trypsin to 39%, respectively.     

Rate constants for 1,n-hydrogen transfer reactions in some amino acid derived radicals

Absolute rate constants for 1,n-hydrogen atom transfers in some substituted amino acid derived radicals have been determined in benzene through the use of competitive kinetic experiments. Radicals derived from methyl N-(2-iodobenzoyl)-N-(tert-butyloxycarbonyl)glycinate, -alaninate, -leucinate, -tert-leucinate and -phenylglycinate undergo intramolecular 1,5-hydrogen atom transfer to afford the corresponding α-amino acid ester radicals with rate constants in the range: 1.0-4.3 × 10⁷ s⁻¹ at 80 °C. Where abstractable hydrogen atoms exist in the amino acid side-chain, 1,6- and 1,7-translocations are competitive processes.     

Synthesis and characterization of 1,3-bis(2-hydroxyethoxy) benzene based saturated and unsaturated polyesters

Polyesterification of adipic acid and maleic anhydride with 1,3-bis(2-hydroxyethoxy)benzene (HER) in the presence of toluene-4-sulphonic acid was carried out using melt condensation technique. The structural characterization of the synthesized polyesters had been carried out using Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (¹H NMR) spectroscopic methods. The thermal properties of the polyesters were studied using differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The activation energies for the thermal degradation of the polyesters were calculated by the method of Dharwadkar and Kharkhanavala and discussed. The char residue value at 600 °C indicated maleic anhydride based polyester is thermally more stable compared to the adipic acid based polyester. The mechanism of degradation of these polyesters is discussed.     

Water absorption and degradation characteristics of chitosan-based polyesters and hydroxyapatite composites

Blends of chitosan and biodegradable synthetic aliphatic polyesters (polycaprolactone, poly(butylene succinate), poly[(butylene succinate)-co-adipate], poly[(butylene terephthalate)-co-adipate], and poly(lactic acid)) were injection-molded. These samples were immersed in isotonic solution at 37 degrees C for a period of 60 d. The water uptake and the degradation properties, as measured by the loss in tensile strength, were evaluated as a function of time. In this study, the rate and the equilibrium water uptake were proportional to the amount of chitosan in the blend. The addition of HA to chitosan and polyester significantly reduced the equilibrium water uptake. The water uptake did not follow the classical Fickian phenomena and could be expressed by a two-stage sorption non-Fickian diffusion model. Contact angle measurement was used to quantify the changes in surface hydrophilicity as a function of chitosan and polyester composition. The glycerol contact angle decreased with increasing synthetic components in the blend. The blends and composites also showed increased degradation, as quantified by a loss in their mechanical properties, with increase in natural content. The degradation of properties was directly related to the water uptake of the blends; the higher the water uptake, the higher the degradation. Pure polyesters, while having low water uptake, nevertheless showed significant degradation by a precipitous drop in the strain at break. Among the polyesters, poly(lactic acid) displayed maximum degradation, while polycaprolactone displayed the least.