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.     

Synthesis, characterization and degradation of well-defined crosslinkable aliphatic polyesters end-capped by biomesogenic units

Novel biodegradable-cum-crosslinkable polyesters end-capped by biomesogenic units, cinnamic acid (CA) and ferulic acid (FA), were synthesized via chain-growth polycondensation in solid-liquid phase. The chemical structure of synthesized polymers was characterized by Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance spectroscopy (¹H NMR). The composition of polyesters, which was calculated by ¹H NMR, was in agreement with the feed ratios. The thermal properties and crystallinity of polyesters were measured by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide angle X-ray diffraction (WXRD) and polarizing-light microscopy (PLM). It was found that the polyesters possessed good crystallinity. Furthermore, the obtained polyesters could be crosslinked with methyl methacrylate (MMA), n-butyl acrylate (BA) and styrene (St) under thermal condition. The crosslinked products possessed degradability in phosphate buffer solution at 37 °C, which might be potentially applied as biomaterials.     

Synthesis and characterization of biodegradable aliphatic polyesters using dibutylmagnesium as initiator

Aliphatic polyesters were synthesized via the ring opening polymerization of the corresponding lactones initiated with dibutylmagnesium both in bulk and in solution. The resulting polymers were characterized by 1H, 13C NMR, GPC and XRD. The results indicated that dibutylmagnesium is an effective initiator for the ring opening polymerization of lactones.     

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.     

Macrocycles 27: Cyclic aliphatic polyesters of isosorbide

Numerous polycondensations of isosorbide and suberoyl chloride or other aliphatic dicarboxylic acid dichlorides were performed with pyridine as a catalyst and HCl acceptor. The reaction conditions were varied to optimize both the molecular weight and the fraction of cyclic oligo‐ and polyesters. Furthermore, we attempted to obtain the cyclic monomer by catalyzed back‐biting degradation of the molten cyclic polyesters above 220 °C in vacuo. The polyesters were characterized by viscosity and size exclusion chromatographic measurements as well as matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. In selected cases, mixtures of linear and cyclic polyesters were treated with a hot solution of partially methylated β‐cyclodextrin in methanol. This treatment allowed for a selective extraction of the linear chains up to approximately 5000 Da. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3414–3424, 2003     

Cyclizations in hyperbranched aliphatic polyesters and polyamides

Hyperbranched aliphatic polyesters of 2,2′-bis-(hydroxymethyl) propanoic acid and hyperbranched aliphatic polyamides obtained from new carboxy- and amino-functionalized caprolactams were studied by NMR spectroscopy and MALDI-TOF mass spectrometry. Ring-chain equilibria taking place through intramolecular hydroxy-ester, carboxy-amide or amine-amide interchanges and leading to the formation of cyclic branches or end-groups were found to exert a predominant influence on the molar mass of these hyperbranched polymers. A number of intra- or intermolecular side reactions, such as the formation of ethers in polyesters and the formation of anhydrides, imides, amidines and secondary amines in polyamides were also detected and resulted in polymer crosslinking on prolonged heating. The existence of such ring-chain equilibria and side-reactions make the control of hyperbranched polymer structure much more difficult than generally accepted.     

Synthesis and characterization of biodegradable-cum-crosslinkable well-defined polyesters via chain-growth polycondensation in solid-liquid phase

Well-defined aliphatic polyesters, polyglycolide (PGA), polylactide (PLA) and their copolymers (PLGA) were prepared by chain-growth polycondensation of potassium 2-bromocarboxylates in solid-liquid phase. The polymerization proceeded in a living fashion without any side reactions. The degrees of polymerization (D_p) of polyesters was in agreement with the feed ratio of monomer to initiator. The polymer, which was only composed of heptamer, octamer and nonamer, possessed a narrow molecular weight distribution. Furthermore, the end groups of polymers, such as allyl and phenyl units, were directly yielded during the polymerization for the further modifying and crosslinking. The synthesized polyesters with allyl end groups were successfully crosslinked, and the products possessed biodegradability in phosphate buffer solution at 37 °C.     

Synthesis of aliphatic polyesters by polycondensation using inorganic acid as catalyst

An effective route for the synthesis of aliphatic polyesters made from adipic or sebacic acid and alkanediols, using inorganic acid as a catalyst is reported. The monomer composition, reaction time, catalyst type, and reaction conditions were optimized to yield polyesters with weight average molecular weights of 23,000 for adipic acid and 85,000 for sebacic acid‐based polyesters. The polymers melt at temperatures of 52–65°C and possess melt viscosity in the range of 5600–19,400cP. This route represents an alternative method for producing aliphatic polyesters for possible use in the preparation of degradable disposable medical supplies. Copyright © 2009 John Wiley & Sons, Ltd.     

Inclusion complex formation between alpha-cyclodextrin and biodegradable aliphatic polyesters

Inclusion complexes (ICs) between alpha-cyclodextrin (alpha-CD) and three kinds of biodegradable aliphatic polyesters with different sequence lengths of the monomeric repeating units poly(3-hydroxypropionate) [P(3HP)], poly(4-hydroxybutyrate) [P(4HB)] and poly(epsilon-caprolactone)(PCL) were prepared by mixing a solution of alpha-CD with that of the polymer, followed by stirring. The ICs were obtained as insoluble precipitates and characterized by FT-IR, WAXD and DSC. All measurements showed that the polymer chains of all three kinds of polyester were included into the alpha-CD cavity and formed ICs with different stoichiometries. WAXD patterns and thermal analysis indicated that these ICs possessed a channel structure and the crystallization of the polyester chains was suppressed upon inclusion into the alpha-CD cavity.     

Syntheses of aliphatic polyesters catalyzed by lanthanide triflates

Polycondensations of 1,6‐hexane diol and sebacic acid were conducted in bulk with addition of a lanthanide triflate as acidic catalyst. With exception of promethium triflate all lanthanide triflates were studied. A particularly low molecular weight was obtained with neodym triflate and the best results with samarium triflate. With Sm(OTf)₃ weight average (M_w) values up to 65 kDa (uncorrected SEC data) were achieved after optimization of the reaction conditions. Comparison of these results with those obtained from bismuth, magnesium, and zinc triflates, on the one hand, and comparison with the acidities of all catalysts, on the other, indicates that the esterification mechanism involves complexation of monomer by metal ions. Preparation of multiblock copoly(ether ester)s failed due to insufficient incorporation of poly(tetrahydrofuran) diols. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 170–177, 2009     

Synthesis, characterization, degradation of biodegradable castor oil based polyesters

A new family of castor oil based biodegradable polyesters was synthesized by catalyst free melt condensation reaction between two different diacids and castor oil with d-mannitol. The polymers synthesized were characterized by NMR spectroscopy, FT-IR and the thermal properties were analysed by DSC. The results of DSC show that the polymer is rubbery in physiological conditions. The contact angle measurement and hydration test results indicate that the surface of the polymer is hydrophilic. The mechanical properties, evaluated in the tensile mode, shows that the polymer has characteristics of a soft material. In vitro degradation of polymer in PBS solution carried out at physiological conditions indicates that the degradation goes to completion within 21 days and it was also found that the rate of degradation can be tuned by varying the curing conditions.     

Synthesis of narrow‐polydispersity degradable dendronized aliphatic polyesters

The divergent dendronization of an ?‐caprolactone‐based polymer has been performed to provide access to dendronized polymers with sufficient biocompatibility and degradability for use as drug‐delivery scaffolds. The synthesis was performed through the tin(II) 2‐ethylhexanoate‐catalyzed polymerization of a γ‐functionalized ?‐caprolactone monomer, followed by the divergent growth of pendant polyester dendrons at each repeat unit. The resulting dendronized polymers were obtained up to the fourth generation with molecular weights as high as 80,000 Da and with polydispersities between 1.11 and 1.22. The fourth‐generation hydroxyl‐terminated dendronized polymer was degradable under a variety of aqueous conditions. A comparison of the dendronization approach with a procedure involving the ring‐opening polymerization of a second‐generation dendritic macromonomer reveals that the former procedure is best suited for the preparation of this family of dendronized polyesters because it requires shorter reaction times and affords materials with higher degrees of polymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3563–3578, 2004     

Polyethylene like polymers. Aliphatic polyesters of dodecanedioic acid: 1. Synthesis and properties

A series of aliphatic polyesters has been synthesized starting from 1,12-dodecanedioic acid and aliphatic diols, bearing from 2 to 12 carbon atoms. These polymers, which were fully characterized in terms of chemical structure, molecular weight and thermal behaviour, were obtained as crystalline materials with melting points ranging from 70 to 90 °C and with a relatively high molecular weight. All the monomers used can be obtained from biomasses, as a consequence these materials can be an interesting alternative to synthetic polymers produced from petrochemical processes based on nonrenewable resources.     

Highly branched radial block copolymers via dendritic initiation of aliphatic polyesters

Living ring opening polymerization of ε-caprolactone initiated from the numerous chain-end hydroxymethyl groups of the analogous dendrimeric and hyperbranched polyesters derived from 2,2-bis(hydroxymethyl) propionic acid is described. By controlling the size of the dendritic macromolecule and the molar ratio of ε-caprolactone, a variety of highly branched radial block copolymers are obtained. Comparison of the results obtained for the dendrimeric and hyperbranched initiators demonstrates that the reactivity of the chain-end hydroxymethyl groups in the dendrimer are significantly greater than in the isomeric hyperbranched case. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2793–2798, 1998     

Correlation between chemical and solid-state structures and enzymatic hydrolysis in novel biodegradable polyesters. The case of poly(propylene alkanedicarboxylate)s

A series of seven fast-biodegrading aliphatic polyesters were prepared from 1,3-propanediol and aliphatic diacids with increasing number of methylene units (x). Melting points decreased from PPSu to PPAd and then increased again to PPAz and PPSeb. Crystallization rates and thermal stability increased steadily with increasing x. Glass transition temperatures decreased steadily to PPPim and subsequently increased. Enzymatic degradation of the polymers in the presence of a mixture of Rhizopus delemar and Pseudomonas cepacia lipases was much faster than that of poly(epsilon-caprolactone). All the polyester specimens were almost disintegrated within 36 h. PPSub exhibited the fastest enzymatic hydrolysis rates, PPAd and PPSuc the slowest.     

Thermal decomposition of poly(propylene sebacate) and poly(propylene azelate) biodegradable polyesters: Evaluation of mechanisms using TGA, FTIR and GC/MS

The synthesis, characterization and thermal behavior of two biodegradable aliphatic polyesters poly(propylene azelate) (PPAz) and poly(propylene sebacate) (PPSeb) are described in the present work. The thermal degradation of both polyesters was studied using thermogravimetric analysis (TG) by the determination of their mass losses during heating. From the thermogravimetric curves it can be seen that both polyesters are thermally stable materials since PPAz has its highest decomposition rate at 411.3 while PPSeb at 413.6 °C. From the variation of activation energy (E) with increasing degree of conversion it is found that the polyester's decomposition proceeds with a complex reaction mechanism with the participation of at least two different mechanisms. To evaluate these mechanisms the TG, FTIR and a combination of TG-gas chromatography-mass spectrometry (TG/GC-MS) methods were used. From mass ions detection of formed decomposition compounds, it was found that the decomposition of both polymers takes place, mainly, through β-hydrogen bond scission and secondarily through α-hydrogen bond scission. The main decomposition products are aldehydes, alcohols, allyl, diallyl, and carboxylic acids.     

Assessing the degradation profile of functional aliphatic polyesters with precise control of the degradation products

The pre-polymer poly(but-2-ene-1,4-diyl malonate) (PBM) and a series of PBM-based materials are shown to be degradable under physiological conditions in vitro and they are therefore presented as potential materials for biomedical applications. Four different PBM-based materials are synthesized: a PBM homopolymer, crosslinked PBM with and without spacer, and a triblock copolymer of PBM and PLLA with the PBM as an amorphous middle block. The polymers are subjected to hydrolytic degradation in phosphate-buffered saline at pH = 7.4 and 37 °C. The results show that all the PBM-based materials degrade without a rapid release of acidic degradation products or any substantial lowering of the pH that might jeopardize their biocompatibility.     

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.     

UV curing behaviors and hydrophilic characteristics of UV curable waterborne hyperbranched aliphatic polyesters

A series of waterborne hyperbranched polyesters (WBHPs) endcapped with methacrylic and salt-like groups in different ratios have been investigated as UV curable resins. The kinetic studies of the drying step and UV curing were carried out by FT-IR measurements. The drying of the film of 100 μm thickness was completed in less than 6 hr at 70°C or within 10 hr at 50°C in an oven. The influence of different photoinitiators and their concentrations, extent of unsaturation and acid content of WBHP on final unsaturation conversion was studied. The surface free energy is a critical character, which affects the surface properties of a cured film. So one method based on the measurement of contact angle of a pure liquid on a solid surface was applied to determine the polar and dispersive components of the surface energy of UV cured films. The investigations of surface energy of WBHPs illustrated that those with more acid content and thus higher polar component are more sensitive to water, while those containing less acid content and thus lower polar term are less water sensitive. Moreover, the UV cured films of WBHPs and their blends with commercial waterborne resins (trade name EB 210, EB 2002, EB 11 and IRR 160) have acceptable pendulum hardness varying from 55 to 180 sec. Copyright © 2003 John Wiley & Sons, Ltd.     

脂肪酸酯水解行为及与低聚果糖缔合行为的研究和应用

酯类化合物是酒类产品的主要香味成分,也是化妆品、饮料、食品中主要呈香物质,故对其水解行为的研究对揭示影响酯水解反应因素及提高和稳定上述产品质量,特别是解决酒类产品低度化过程中存在的质量下降问题具有重要的意义[1]。对酯水解反应的研究前人作了大量的工作:Bender与Ma