Scission Of Ester Bonds Research Articles

Synthesis and Mass Spectrometry Structural Assessment of Polyesteramides Based on ε-Caprolactone and L-Phenylalanine

L-Phenylalanine-ε-caprolactone-based polyesteramides (PCPs) were synthesized via melt polycondensation across a diverse range of molar compositions. The copolymer structure was extensively characterized using nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). NMR analysis confirmed the intercalation of the L-Phenylalanine comonomer units within the polyester backbone. MALDI MS characterization further demonstrated the formation of linear PCP chains with carboxyl end groups. A detailed structural analysis through MALDI MS/MS fragmentation indicated that ester bond scission was the predominant fragmentation mechanism, depicting the polyesteramide sequence in the copolymers. The resulting copolymers were primarily amorphous, except for those with molar compositions of 90/10 and 80/20, which exhibited semi-crystalline structures. Additionally, these PCPs showed an increase in glass transition temperatures with higher amino acid contents and demonstrated good thermal stabilities, as evidenced by a 10% mass loss at elevated temperatures.

Hydrolysis of Acetamide on Low-Index CeO2 Surfaces: Ceria as a Deamidation and General De-esterification Catalyst.

Using DFT calculations and acetamide as the main example, we show that ceria is a potential catalyst for the hydrolysis of amide and similar bonds. The overall reaction is endergonic in the gas phase, yielding acetic acid and ammonia, but is slightly exergonic in the aqueous phase, which facilitates ionization of the products (CH3COO– and NH4+). Neighboring Ce and O sites on the CeO2(111), (110), and (100) facets are conducive to the formation of an activated metastable tetrahedral intermediate (TI) complex, followed by C–N bond scission. With van der Waals and solvation effects taken into account, the overall reaction energetics is found to be most favorable on the (111) facet as desorption of acetic acid is much more uphill energetically on (110) and (100). We further suggest that the Ce–O–Ce sites on ceria surfaces can activate X(=Y)–Z type bonds in amides, amidines, and carboxylate and phosphate esters, among many others that we term “generalized esters”. A Brønsted-Evans–Polanyi relationship is identified correlating the stability of the transition and final states of the X–Z generalized ester bond scission. A simple descriptor (ΣΔχ) based on the electronegativity of the atoms that constitute the bond (X, Y, Z) versus those of the catalytic site (O, Ce, Ce) captures the trend in the stability of the transition state of generalized ester bond scission and suggests a direction for modifying ceria for targeting specific organic substrates.

On the Mechanisms of the Effects of Ionizing Radiation on Diblock and Random Copolymers of Poly(Lactic Acid) and Poly(Trimethylene Carbonate).

This article demonstrates that ionizing radiation induces simultaneous crosslinking and scission in poly(trimethylene carbonate-co-d-lactide) diblock and random copolymers. Copolymer films were electron-beam (EB) irradiated up to 300 kGy under anaerobic conditions and subsequently examined by evaluation of their structure (FT-IR, NMR), molecular weight, intrinsic viscosities, and thermal properties. Radiation chemistry of the copolymers is strongly influenced by the content of ester linkages of the lactide component. At low lactide content, crosslinking reaction is the dominant one; however, as the lactide ratio increases, the ester linkages scission becomes more competent and exceeds the crosslinking. Electron paramagnetic resonance (EPR) measurements indicate that higher content of amorphous carbonate units in copolymers leads to a reduction in free radical yield and faster radical decay as compared to lactide-rich compositions. The domination of scission of ester bonds was confirmed by identifying the radiolytically produced alkoxyl and acetyl radicals, the latter being more stable due to its conjugated structure.

Theoretical investigation of dephosphorylation of phosphate monoesters on CeO2(111)

The dephosphorylation of neutral model phosphate monoesters on CeO2(111), including para-nitrophenyl phosphate and methyl phosphate, has been studied theoretically using self-consistent, periodic density functional theory calculations at the GGA+U-PW91 level. These phosphate monoesters interact strongly with CeO2(111) by forming a bond between the P atom and a surface lattice O atom. A surface-assisted hydrolysis mechanism is proposed for the catalytic dephosphorylation of the phosphate monoesters on CeO2(111), which involves P-O ester bond scission followed by phosphate hydration and product desorption. The energies of the transition states for the P-O ester bond scission are found to follow a linear scaling relation with respect to the energies of the dissociated fragments reasonably well. A nearly spontaneous transfer of one of the H atoms on the phosphate group to the alkoxide group produces the corresponding alcohol. The hydration of the remaining HPO3 group has a maximum activation energy of ca. 1.1 eV in vacuo. Thus although the nature of the alkoxide group affects the activation of the P-O ester bond, it should not affect the overall catalytic activity of CeO2(111) for dephosphorylation because the hydration of the phosphate group is rate-limiting.

Carbodiimide additive to control hydrolytic stability and biodegradability of PLA

Enhanced durability of bio-based polylactic acid (PLA) is one of the prerequisites to be a considered as alternative to petroleum-based polymers in long time application. The effect of bis(2,6-diisopropylphenyl)carbodiimide (BDICDI) additive in various concentration on the extent of hydrolytic stabilization and subsequent degradation kinetics of PLA was studied in the process of abiotic hydrolysis and microbial decomposition under composting conditions. The study showed that BDICDI acts as an efficient stabilizer suppressing the hydrolytic scission of ester bonds of PLA in both degradation processes tested, especially at concentrations above 1.5% w/w. Within the stabilization period which strongly depends on concentrations of BDICDI, suppression of hydrolytic degradation also triggered preservation of thermal and mechanical properties. After the period of stabilization as the dose of stabilizer is depleted, probably via reaction with water molecules and carboxylic groups, the material was hydrolyzed at a comparable or even slightly higher rate than pure PLA. By applying the appropriate amount of BDICDI, the approximate duration of stabilization could be set to suit the desired requirements of the final product.

Quantitative spectroscopic analysis of weathering of polyester-urethane coatings

Transmission FTIR analysis of polyester-urethane coatings (PUC), that were degraded under different accelerated laboratory weathering conditions, are compared. The aim of this comparison is to deepen our insight into the chemical pathways of weathering of polyester-urethane coatings. We monitored the chemical changes for different environments, such as aerobic or anaerobic conditions as well as wet or dry conditions, in order to increase our insight into the effect of each individual stress factor, i.e., photons, oxygen, temperature and water, on the chemical pathways of weathering.We showed that the degradation of urethane groups proceeds via photo-oxidative pathways and that the ester groups mainly degrade via photolytic reactions. The ester bond scission accelerates after an initially-slow-rate stage of weathering in the presence of urethane groups. This is due to an increase in the optical absorptivity of the coating as a result of degradation under anaerobic conditions, as shown before. By means of a kinetic analysis using a combination of FTIR and UV–Vis spectroscopy results (obtained before), we found that a first-order kinetic model can perfectly describe the rate of ester bond scission during the weathering and that the increase in the rate of reaction is due to the increase in the light absorptivity of the coating as a result of degradation. Finally, using the interference fringes of FTIR spectra, we showed that evaporation and water-caused removal of degraded material cause a particularly pronounced decay in the thickness of the coating. In the absence of water spray, the material loss takes place in the same period as urethane groups decompose and stops afterwards, even though the ester bond scission proceeds with higher rates. This supports the hypothesis of photo-oxidative pathways for the urethane group decomposition and photolytic mechanisms for ester bond scission. Dark experiments showed that PUC coatings are highly resistant to hydrolysis and thermal degradation.

A first attempt to simulate oxidization effects on latent track structure in PADC combining the radial dose theory and a radio-oxidation kinetic model

A first attempt is made to mix the radial dose model with a simple radio-oxidation kinetic to simulate the G values for carbonate ester bond scissions in PADC induced by different ions from proton to Xenon with LET ranging from 10 to 1.20 × 104 keV μm−1. Even if some discrepancies exist between experimental and simulated data, this work tends to prove that O2 concentration in PADC rules the observed experimental LET dependence on carbonate ester bond scission in PADC irradiated in air.

Depth-resolved infrared microscopy and UV-VIS spectroscopy analysis of an artificially degraded polyester-urethane clearcoat

Polyester-urethane resins are important candidates for high performance, outdoor coating applications. Despite their relevance, quantitative information regarding the photodegradation of these materials is scarcely available. In the present study, a model polyester-urethane clearcoat without additives is artificially degraded and the changes in optical properties and chemical composition are studied by UV-VIS spectroscopy and FTIR-ATR microscopy, respectively. The change of the optical properties throughout the coating thickness is quantified and interpreted using a newly developed optical model. Chemical changes are quantified in a depth-resolved manner by combining FTIR-ATR microscopy with optical profilometry in order to visualise the time evolution of compositional gradients during photodegradation by accurate assignment of the correct depth position to all recorded ATR spectra. The rate of change for the concentration of several chemical entities in the model polyester-urethane was found to become constant after the initial stage of weathering. The loss of urethane crosslinks in the coating occurs faster and to a much larger extent as compared to ester bond scission. Results from the optical and the chemical characterisation are combined to propose a kinetic model for ester bond photolysis that provides an estimate of the quantum efficiency of this process.

Chemical cross sections induced by ions in solid organic detectors: Experimentation and simulation

Chemical ester bond scission induced by incoming ions in polyethylene terephthalate (PET), bisphenol A polycarbonate (PC), polyallyl diglycol carbonate (PADC), have been systematically determined by FT-IR transmission measurements. The studied ions (H, He, C, Ne, I, Ar, Fe, Kr, Xe) have LET ranging from 10 to 10 000 keV μm−1. We discuss the opportunity to simulate the experimental chemical cross section obtained with an approach based on the dose deposited by the secondary electrons removed by the incoming ion. Such an approach has been already successfully applied for LR115.

Degradability studies of poly(l-lactide) after multi-reprocessing experiments in extruder

Poly(l-lactide), PLLA, belongs to the most widely used biodegradable polyesters. These polymers have attracted great attention in recent years due to their excellent material properties, which give a rise to potential applications in various commercial areas. Previously, PLLA reprocessing in a co-rotating twin-screw extruder was used as a model test for material recycling. It may be assumed basing on this study, that PLLA technological wastes may be suitable for reuse, for example, as an additive for neat PLLA. In order to verify such suitability of reprocessed PLLA its degradability under industrial composting conditions was studied at the static composting open-air pile. Incubation of this material in water at 70 °C (abiotic conditions) was also conducted. The progress of the degradation process was monitored by surface changes and measurements of sample weight loss and changes in the PLLA molecular weight. The molecular level structure of the water soluble degradation products of PLLA samples was determined by electrospray ionization multistage mass spectrometry (ESI-MSn). Lactic acid and its oligomers terminated by hydroxyl and carboxyl end groups were identified as hydrolytic degradation products. The obtained results indicated that in selected environments (industrial composting and incubation under abiotic conditions) hydrolytic degradation via random ester bond scission occurs preferentially. It was demonstrated, that multi-reprocessing of PLLA did not significantly affect the rate of degradation and only slightly affected the disintegration progress of the PLLA samples.

Characterization, biodegradability and blood compatibility of poly[(R)‐3‐hydroxybutyrate] based poly(ester‐urethane)s

Poly(ester-urethane)s (PUs) were synthesized using hexamethylene diisocyanate (HDI) or toluene diisocyanate (TDI) to join short chains (M(n) = 2000) of poly(R-3-hydroxybutyrate) (PHB) diols and poly(epsilon-caprolactone) (PCL) diols with different feed ratios under different reaction conditions. The multiblock copolymers were characterized by nuclear magnetic resonance spectrometer (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), X-ray diffraction (XRD), and scanning electron microscope (SEM). XRD spectra and second DSC heat thermograms of the multiblock copolymers revealed that the crystallization of both PHB and PCL segments was mutually restricted, and, especially, the PCL segment limited the cold crystallization of the PHB segment. The SEM of platelet adhesion experiments showed that the hemocompatibility was affected to some extent by the chain flexibility of the polymers. Hydrolysis studies demonstrated that the hydrolytic degradation of PUs was generated from the scission of their ester bonds or/and urethane bonds. Simultaneously, the rate of ester bond scission was determined to some extent by the crystallization degree, which was further affected by the configuration of polymer chains. These highly elastic multiblock copolymers combining hemocompatibility and biodegradability may be developed into blood contact implant materials for biomedical applications.

Matrix‐assisted laser desorption/ionization time‐of‐flight/time‐of‐flight tandem mass spectra of poly(butylene adipate)

Matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry (MALDI-TOF/TOF-MS/MS) was employed to analyze four poly(butylene adipate) (PBAd) oligomers and to investigate their fragmentation pathways as a continuation of our work on the MALDI-TOF/TOF-MS/MS study of synthetic polymers. MALDI-TOF/TOF-MS/MS analysis was performed on oligomers terminated by carboxyl and hydroxyl groups, methyl adipate and hydroxyl groups, dihydroxyl groups, and dicarboxyl groups. The sodium adducts of these oligomers were selected as precursor ions. Different end groups do not influence the fragmentation of sodiated polyester oligomers and similar series of product ions were observed in all the MALDI-TOF/TOF-MS/MS spectra. According to the structures of the most abundant product ions identified in the present work, three fragmentation pathways have been proposed to occur most frequently in PBAd: beta-hydrogen-transfer rearrangement, leading to the selective cleavage of the --O--CH(2)-- bonds; --CH(2)--CH(2)-- (beta--beta) bond cleavage in the adipate moiety; and ester bond scission.

Degradation of poly(lactic-co-glycolic acid) microspheres: effect of copolymer composition

The in vitro degradation behaviour of a wide range of poly( d,l-lactic-co-glycolic acid) (PLGA) has been examined in terms of degree of degradation and morphological change during an incubation period of up to 53 d. Gel permeation chromatography and differential scanning calorimetry were employed to characterize their degradation profiles. It was found that amorphous PLGA exhibited a transient multiple crystallization behaviour of d- or l-lactic acid oligomers during degradation. This indicated that the hydrolytic scission of ester bonds tends to primarily target the linkage between glycolic acid and d- or l-lactic acid or glycolic acid. In addition, two distinctive glass transition temperatures appeared when these crystallization phenomena occurred, suggesting the transient presence of fast and slowly eroding polymer domains within microspheres during the degradation. This study supports the heterogeneous bulk degradation for PLGA microspheres which has been proposed recently for a large specimen.

Thermal degradation of pentaerythritol diphosphate, model compound for fire retardant intumescent systems: Part II—Intumescence step

The chemical reactions responsible for the intumescent behaviour of pentaerythritol diphosphate (PEDP) have been studied in detail. On heating, condensation of free acidic hydroxy groups of PEDP takes place with elimination of water. This reaction overlaps with scission of ester bonds by pyrolysis or hydrolysis which frees phosphoric acid groups and gives a foamed carbon-rich residue. The mechanism of formation of this char is discussed on the basis of competing reactions of species resulting from ester pyrolysis or hydrolysis. The foaming effect is due to water evolved from condensation of phosphoric to polyphosphoric acid (12% of PEDP) and gaseous products of partial fragmentation of the charring material, e.g. methane, ethylene, propylene, aldehydes (overall 4%). The result of the intumescent process is a physical mixture of phosphoric-polyphosphoric acid with a foamed char characterised by closed spherical cells of diameter ≤100 γm and wall thickness about 10 γm.

Degradation of poly(L-lactide) microcapsule membranes.

Poly (L-lactide) microcapsules were observed to be hydrolytically degraded rapidly in a strongly alkaline solution into lactic acid as the final product. The degradation was accelerated when poly (L-lactide) microcapsules were immersed in solutions of high ionic strengths. The effects of pH and ionic strength of the bulk solution were interpreted in terms of the electric potential distribution in the membrane.The charge distribution in the microcapsule membrane changes during degradation. In the intact poly (L-lactide) microcapsule membrane, the charged groups are considered to be distributed uniformly in it. In the early stage of the degradation process, the zeta potential became more negative with the time elapsed. Analysis of the ionic strength dependence of the zeta potential on the basis of a simple model shows that hydrolytic scission of ester bonds of the polymer chains takes place preferentially at the microcapsule membrane surface to create negative charges localized at the membrane/solution interface. In the later stages of the degradation process, the zeta potential again became less negative. This suggests that liberation of degraded segments takes place in the later stages. It was concluded, therefore, that both the cleavage of the ester bonds and the liberation of degraded segments of poly (L-lactide) molecules start from the surface of poly (L-lactide) microcapsules.

Electrophoretic study of the degradation properties of poly(L-lactide) microcapsules.

The degree of degradation of poly(L-lactide) microcapsules was measured as a function of the time elapsed in solutions of different pH values. To determine how the distribution of poly(L-lactide) molecules in the microcapsule membrane changes during degradation, poly(L-lactide) microcapsules, at various stages of the degradation process in various solutions, were sampled and redispersed in pH 7.6 buffer solutions of different ionic strengths and their zeta potentials determined. In the early stages of the process, the zeta potential became more negative as time elapsed. Analysis of the ionic strength dependence of the surface potential on the basis of a simple model shows that hydrolytic scission of ester bonds in the polymer chains takes place preferentially at the microcapsule membrane surface creating negative charges localized at the membrane/solution interface. In the later stages of the degradation process, the zeta potential again became less negative. This suggests that liberation of degraded segments takes place in the later stages.

Thermal degradation of aromatic polyesters

A study has been made of the high-temperature degradation of polyphenyleneterephthalate and of some compounds modelling individual chain fragments of this polymer. It is shown that degradation of the compounds under study takes place by a chain free-radical mechanism in which a major role is played by ester bond scission as a result of interaction with atomic hydrogen.