Chain-end Scission Research Articles

Molecular-level kinetic modelling for the pyrolysis of high-density polyethylene in a two-stage process

A two-stage process has been developed for the pyrolysis of high-density polyethylene to obtain light olefins, where probable mechanisms for each stage pyrolysis were proposed in our previous study. This work aims to establish molecular-level kinetic models for the first-stage pyrolysis at a lower temperature and for the second-stage pyrolysis at a higher temperature, respectively, to predict the overall reaction performances in accordance to the role of detailed mechanisms. A structure-oriented lumping (SOL) method based molecular-level kinetic model can effectively describe the first-stage pyrolysis, while a detailed molecular-level kinetic model is developed for the second-stage pyrolysis to track the evolutions of all alkanes, olefins, and diolefins, with special focus on the formation of light olefins. The established separate models can achieve the automatic generation of complex reaction networks and present good agreement with experimental data obtained from the first- and second-stage. In addition, the first-stage model indicates that the random scission plays an important role in the pyrolysis process at lower temperatures, while the chain-end scission mechanism dominates the second-stage reaction to produce light olefins at higher temperatures. As a whole, the molecular-level modelling scheme behaves excellently at a wide range of temperature, which provides guidance for upgrading the pyrolysis process.

How temperature-induced depolymerization and plasticization affect the process of structural relaxation

The self-plasticization, i.e., the increase in the polymer segmental mobility by the inclusion of its own monomer, has a major impact on the structural, thermal, and mechanical properties of the polymer. Differential scanning calorimetry (DSC) was used to investigate the influence of thermally induced self-plasticization on the structural relaxation of polydioxanone (PDX). Depolymerization (based dominantly on the end-chain scission mechanism) was found to be controlled by the depolymerization temperature Td as well as the actual number of re-melting cycles (while keeping the time spent at Td constant). PDX samples with the glass transition temperature (Tg) ranging from −52 (highly plasticized) to −13 °C (virgin) were prepared. The DSC data were described in terms of the Tool-Narayanaswamy model; a consistent structural relaxation behavior associating the degree of plasticization with Tg was identified. The activation energy first decreased with plasticization from 430 kJ mol−1 to 210 kJ mol−1 in the Tg range of −40 to −13C, which is consistent with the plasticization-caused spacing-apart of the polymer chains resulting in larger free volume and increased freedom for the relaxation movements. For the highly plasticized PDX samples, the activation energy increased from 210 kJ mol−1 to 310 kJ mol−1, which appears to be associated with the possible segregation of the portion of the plasticizer into a discrete phase. The width of the relaxation times distribution increased with plasticization as a consequence of the plasticizer loosening the polymeric chains and enabling a wider variety of the segmental movement. The plasticization also leads to a higher dependence of the segmental relaxation movements on their current physico-chemical and steric surrounding.

Valorizing disposable face masks into non-condensable hydrocarbon gases via optimal reactor configurations in thermal pyrolysis

Pyrolysis studies often overlook the potential of pyrolytic gas as a valuable hydrocarbon resource, thereby hindering the production of advanced carbonaceous nanomaterials (CNM). This study aims to explore various configurations of a multi-zone tubular reactor to upcycle disposable face masks into non-condensable hydrocarbons as potential CNM precursors. The highest gas yield (56.2 wt%) was achieved using a 3-zone configuration (Z-1,3,4), attributed to the end chain scission mechanism. The increased abundance of light hydrocarbons (≤C10) in the resulting liquid confirms a greater extent of end chain scission in the optimum reactor configuration with cold zone retainment. Employing this configuration, different face mask types, including 3ply (polypropylene-based), KF94 (polypropylene-based) and N95 (polyester-based), were utilized as individual feedstocks or in various ratios as mixtures to investigate the resultant composition of hydrocarbon gases. This composition was compared with existing literature studies to identify suitable hydrocarbon gas compositions for CNM production. Mixing 3ply and KF94 face masks produced a consistent gas composition predominantly comprising propylene (62.3 – 66.9%) and ethane (15.4 – 16.8%) via thermal pyrolysis at 500 ℃ with a constant heating rate of 20 ℃/min. However, the inclusion of N95 face masks increased CO2 level fivefold, diminishing the gas quality as a CNM precursor. These findings underscore the necessity of segregating polyester face mask material from polypropylene face mask material prior to pyrolysis to ensure a consistent quality and yield of CNM precursors.

A reaction-diffusion framework for hydrolytic degradation of amorphous polymers based on a discrete chain scission model

Hydrolytic degradation of polymers involves the scission of long chain molecules, leading to molecular weight reduction and mass loss. The precise degradation response however depends on the scission probability of individual bonds along the polymer backbone. In particular, bonds near the chain ends are considered to be more susceptible to hydrolysis than inner bonds. In this paper, we incorporate a discrete chain scission model that can handle arbitrary bond scission probabilities within a continuum reaction-diffusion framework. Overall hydrolysis kinetics (including autocatalysis) is described independently of the chain scission model. By decoupling the description of the chain scission mechanism from kinetics, our framework enables the identification of the chain scission mechanism from molecular weight reduction and mass loss curves commonly reported in experimental degradation studies. We further propose a reduced continuum model which is better suited for large-scale simulations while retaining the predictive capability of the full discrete-continuum model. The model capability is illustrated in representative case studies based on experimental data from the literature for different materials and geometries. Statement of significanceMany models have been proposed to predict the evolution of molecular weight and mass loss in biodegradable polymers undergoing hydrolytic degradation. However, existing models remain limited in their ability to describe the degradation mechanism, autocatalytic kinetics and short chains diffusion simultaneously. Moreover, existing models often rely on empirical relations and a large number of fitting parameters. Here, we propose a conceptually simple discrete-continuum mathematical framework with a small number of parameters which all have a clear physical meaning. Model calibration against experimental data is simplified, and further provides insights into the degradation mechanisms at play, namely random scission, chain-end scission, or a combination of both. The framework can serve as a basis for future generalisations, including a description of evolving crystallinity, or other degradation mechanisms, such as thermal oxidation or photo-degradation.

Simulation of irreversible and reversible degradation kinetics of linear polymers using sectional moment method

In the field of polymer reaction engineering, accurately simulating polymer degradation process is beneficial to understand the reaction kinetics, which guides the design, operation, and control of the degradation reactor. Herein, a sectional strategy based on the method of moments with a newly defined critical chain length was proposed to simulate the degradation process of linear polymers with low computational cost. By accounting for the concentration of species with low molar mass (e.g., dimer), the evolution of average properties and concentration polymer species during linear polymer degradation were simulated with high accuracy through this model. In addition, the evolution of molar mass distribution was approximated by using the as-developed model. Sensitivity analysis was performed for simultaneous random scission and irreversible/reversible chain-end scission processes. Results show that monomer yields, average and distributed molecular properties can be tuned by the rate coefficients of random scission (kr), chain-end scission (ke), and its reverse reaction (k-e). Finally, kinetic insights into the polylactic acid alcoholysis system were gained by the as-developed MoM-based model, which demonstrates the accuracy and applicability of the model. This work provides a new method for simulating linear polymer degradation, and thus revealing the kinetic evolution during degradation.

Thermochemical recycling of end-of-life and virgin HDPE: A pilot-scale study

Industrial-scale application of end-of-life plastic pyrolysis still faces significant problems, such as a lack of detailed knowledge on degradation mechanisms, parameters affecting the degradation, and formation pathways of the primary pyrolysis products. Today, the degradation mechanisms based on radical chain scission are insufficiently understood to explain the pyrolysis chemistry from feedstock-to-product comprehensively. In this study, the impact of operating conditions on the degradation mechanism is evaluated by pyrolyzing end-of-life and virgin high-density polyethylene (HDPE) in a continuous pilot-scale unit at temperature and pressure ranges of 450–504 ºC and 0.1–2 bara. The pyrolysis products were analyzed based on the detailed product composition obtained using comprehensive two-dimensional gas chromatography (GC×GC). A simplified kinetic mechanism was proposed to describe the main production pathways of the various components by considering the weakest points along the polymer chain. The results showed that the chain-end scission mechanism is the main mechanism in the HDPE pyrolysis process, even at low temperatures and pressures in the studied ranges. The pyrolysis of virgin HDPE under sub-atmospheric pressure, 0.1 bara, at 464 ºC reactor temperature, yields the highest concentration of linear hydrocarbons in the pyrolysis oil (93.2 wt%). At higher pressure and temperature, the cyclic and branched hydrocarbons had a higher share of up to 17.4 wt% compared to 6.8 wt% at vacuum pressure and lower temperature. Interestingly, the pyrolysis of end-of-life HDPE at atmospheric pressure and 450 ºC led to more cyclic and branched hydrocarbons (sum: 22.1 wt%), as opposed to that of virgin HDPE which is more prone to the production of linear hydrocarbons at the studied conditions. Regarding the additives and contaminants, a large amount of different metals and halogen atoms in the ppm range were detected in end-of-life HDPE, of which a small amount was still found in the pyrolysis oil.

Atomic-scale insight into thermal decomposition behavior of polypropylene: A ReaxFF method

In this study, reactive force field molecular dynamics (ReaxFF MD) was implemented for the first time on a large polypropylene (PP) system, thermal decomposition characteristics of polypropylene (PP) during both heating and constant temperature process were investigated, and the pyrolysis mechanisms of PP were discussed. At the beginning of the reaction, random C-C bond scission of long PP chains resulted in the formation of a full range of products. In the heating stage, with the increase of the temperature, the product composition gradually shifted to light components and the number of propylene was significantly higher than that of other products. In the constant temperature stage, the long-term high temperature led to the secondary reactions in the system, and the content of heavier components increased. The observation of two partial reaction pathways indicated that both random scission of polymer links and chain-end scission mechanisms played a role in the thermal decomposition of PP. The analysis of pyrolysis kinetics based on simulation data suggested that the Contracting Cylinder model would be suitable for PP pyrolysis, and the activation energy for overall degradation of PP was in good agreement with the previous work. The current results would be helpful in illustrating the pyrolysis processes of PP and understanding its reaction mechanisms.

Thermal Decomposition of Anionic, Zwitterionic, and Cationic Polyfluoroalkyl Substances in Aqueous Film-Forming Foams.

In this study, we investigated thermal decomposition mechanisms of cationic, zwitterionic, and anionic polyfluoroalkyl substances, including those present in aqueous film-forming foam (AFFF) samples. We present novel evidence that polyfluoroalkyl substances gave quantitative yields of perfluoroalkyl substances of different chain lengths during thermal treatment. The results support a radical-mediated transformation mechanism involving random-chain scission and end-chain scission, leading to the formation of perfluoroalkyl carboxylic acids such as perfluorooctanoic acid (PFOA) from certain polyfluoroalkyl amides and sulfonamides. Our results also support a direct thermal decomposition mechanism (chain stripping) on the nonfluorinated moiety of polyfluoroalkyl sulfonamides, resulting in the formation of perfluorooctanesulfonic acid (PFOS) and other structurally related polyfluoroalkyl compounds. Thermal decomposition of 8:2 fluorotelomer sulfonate occurred through end-chain scission and recombination reactions, successively yielding PFOS. All of the studied polyfluoroalkyl substances began to degrade at 200-300 °C, exhibiting near-complete decomposition at ≥400 °C. Using a high-resolution parent ion search method, we demonstrated for the first time that low-temperature thermal treatments of AFFF samples led to the generation of anionic fluoroalkyl substances, including perfluoroheptanesulfonamide, 8:2 fluorotelomer sulfonic acid, N-methyl perfluorooctane sulfonamide, and a previously unreported compound N-2-propenyl-perfluorohexylsulfonamide. This study provides key insights into the fate of polyfluoroalkyl substances in thermal processes.

On the use of sectional techniques for the solution of depolymerization population balances: Results on a discrete-continuous mesh

To study the discrete bond-breaking phenomena of depolymerization, the use of a fully continuous Population Balance Equation (PBE) is inadequate to embody all the inherent characteristics of the process, thus resulting in the need for a discrete-continuous mesh. Here, the performance of the fixed pivot technique (FPT) and the cell average technique (CAT) in approximating discrete depolymerization was extensively compared and evaluated. Both methods show different accuracy depending on the breakage mechanisms. For chain-end scission, the FPT and the CAT solutions coincide and satisfactorily predict the population densities and moments. We identified a previously-not-reported issue of a precipitous drop in the number density at the boundary of discrete and continuous region specifically for chain-end scission. We fixed this problem by modifying the particle allocation functions at the boundary points. For random scission, by introducing modifications that mimic the inherently discrete bond-breaking depolymerization process, both techniques predict the population densities and moments accurately at a very coarse mesh, even though the performance of the CAT pales in comparison with the FPT. In all assessed cases, the FPT is more computationally efficient and easily implemented. The assessments in this present work intend to provide a clear-cut direction to efficient and economical modelling of depolymerization processes.

Relation between Surface Area and Surface Potential Change during (co)Polyesters Degradation as Langmuir Monolayer

ABSTRACTPolyhydroxyalkanoates (PHAs) are degradable (co)polyesters synthesized by microorganisms with a variety of side-chains and co-monomer ratios. PHAs can be efficiently hydrolyzed under alkaline conditions and by PHA depolymerase enzymes, altering their physicochemical properties. Using 2D Langmuir monolayers as model system to study the degradation behavior of macromolecules, we aim to describe the the interdependency between the degradation of two PHAs and the surface potential, which influences material-proteins interaction and cell response. We hypothesize that the mechanism of hydrolysis of the labile ester bonds in (co)polyesters defines the evolution of the surface potential, owing to the rate of accumulation of charged insoluble degradation products. The alkaline hydrolysis and the enzymatically catalyzed hydrolysis of PHAs were previously defined as chain-end scission and random-scission mechanisms, respectively. In this study, these two distinct scenarios are used to validate our model. The surface potential change during the chain-end scission of poly(3-R-hydroxybutyrate) (PHB) under alkaline conditions was compared to that of the enzymatically catalyzed hydrolysis (random-scission) of poly[(3-R-hydroxyoctanoate)-co-(3-R-hydroxyhexanoate)] (PHOHHx), using the Langmuir monolayer technique. In the random-scission mechanism the dissolution of degradation products, measured as a decrease in the area per molecule, was preceded by a substantial change of the surface potential, provoked by the negative charge of the broken ester bonds accumulated in the air-water interface. In contrast, when chains degraded via the chain-ends, the surface potential changed in line with the dissolution of the material, presenting a kinetic dependent on the surface area of the monolayers. These results provide a basis for understanding PHAs degradation mechanism. Future research on (co)polymers with different main-chain lengths might extend the elucidation of the surface potential development of (co)polyesters as Langmuir monolayer.

Preparation of modified citrus pectin (MCP) using an advanced oxidation process with hydroxyl radicals generated by UV-H2O2

An advanced UV-H2O2 oxidation system was developed to prepare modified citrus pectin (MCP) from citrus pectin. A rhamnogalacturonan I (RG-I) enriched MCP fraction with the weight-average molecular weight (Mw) of 44.9 kDa and a MCP recovery rate of 91.2% was obtained under a condition of 30 mM H2O2 and UV irradiation for 5 h. The kinetic study implied that UV-H2O2 oxidation process is more efficient in reducing the Mw than the most commonly used approaches. By comparison of the chemical compositions of MCP and relevant intermediates during continuous UV irradiation, we proposed a functional mechanism of the UV-H2O2 oxidation process. Pectin depolymerization started with an initial random scission, followed by main chain scission through selective oxidative degradation of galacturonic acid linkages, and ended with a chain-end scission. In a minor extent, β-elimination and de-esterification occurred simultaneously throughout the process.

Thermal degradation of waste plastics under non-sweeping atmosphere: Part 1: Effect of temperature, product optimization, and degradation mechanism

Continuous generation of plastic waste has prompted substantial research efforts in its utilization as a feedstock for energy generation. Pyrolysis has emerged as one of the best waste management technique for energy extraction from the plastic waste. The objective of this work is to investigate the effect of operating temperature on the liquid product yields in the pyrolysis process by non-isothermal heating. Non-catalytic thermal pyrolysis of waste polyethylene (PE) [high density polyethylene (HDPE)], waste polypropene (PP), waste polystyrene (PS), waste polyethylene terephthalate (PET) and mixed plastic waste (MPW) was carried out in a non-sweeping atmosphere in a semi-batch reactor at four different temperatures 450, 500, 550, and 600 °C. The minimum degradation temperature of the mixed and individual plastics was obtained using a thermogravimetric apparatus (TGA) at a heating rate of 20 °C/min. The TGA results show that all plastics degrade in a single step and the degradation temperatures of PS > PET > PP > HDPE, while mixed plastic degradation indicates two distinct degradation steps. Further, a waste polymer shows a lower degradation temperature than the virgin polymer. The degradation of HDPE is found to produce the maximum oil yield with minimum solid residue. The degradation of PET results in the highest amount of solid and benzoic acid as crystals and gas with no oil. Degradation of mixed plastic causes oil yield in the intermediate range of pyrolysis of individual plastic wastes. Overall, 500 °C is observed to be an optimum temperature for the recovery of low-density pyrolytic oil with the highest liquid yield. The degradation of PE and PP is found to be caused by random chain scission followed by inter and intramolecular hydrogen transfer. The degradation of PS occurs by side elimination or end chain scission followed by β-scission mechanism. The degradation of mix plastics results from random chain scission followed by β-scission mechanism. The effect of temperature on oil and gas recovery as well as recovery time was also assessed.

The influence of pH on the molecular degradation mechanism of PLGA

ABSTRACTPoly[(rac-lactide)-co-glycolide] (PLGA) is used in medicine to provide mechanical support for healing tissue or as matrix for controlled drug release. The properties of this copolymer depend on the evolution of the molecular weight of the material during degradation, which is determined by the kinetics of the cleavage of hydrolysable bonds. The generally accepted description of the degradation of PLGA is a random fragmentation that is autocatalyzed by the accumulation of acidic fragments inside the bulk material. Since mechanistic studies with lactide oligomers have concluded a chain-end scission mechanism and monolayer degradation experiments with polylactide found no accelerated degradation at lower pH, we hypothesize that the impact of acidic fragments on the molecular degradation kinetics of PLGA is overestimated. By means of the Langmuir monolayer degradation technique, the molecular degradation kinetics of PLGA at different pH could be determined. Protons did not catalyze the degradation of PLGA. The molecular mechanism at neutral pH and low pH is a combination of random and chainend-cut events, while the degradation under strongly alkaline conditions is determined by rapid chainend cuts. We suggest that the degradation of bulk PLGA is not catalyzed by the acidic degradation products. Instead, increased concentration of small fragments leads to accelerated mass loss via fast chain-end cut events. In the future, we aim to substantiate the proposed molecular degradation mechanism of PLGA with interfacial rheology.

Towards improved predictions for the enzymatic chain-end scission of natural polymers by population balances: The need for a non-classical rate kernel

Enzymatic chain-end depolymerization is commonly employed for the transformation of biomass into important products. To date, investigation on the validity of the rate kernel which is critical to model success, has been conveniently avoided. Through a case study with extensive confrontation with experimental data, we uncover this critical relationship by inspecting every minute detail in the mechanistic modelling procedure. Using a newly proposed shape-evolving function for the rate kernel, model calibration reveals that the commonly employed constant rate kernel is inappropriate for modelling the scission step, and that the apparent rate kernel of hydrolysis resembles an endothermic activation energy barrier function. Facilitated by the adoption of this non-classical rate kernel, good predictions are attained by the model at different hydrolysis conditions with a global parameter set. Being the first to predict distributed data, the methodology here serves as a guide for future studies on the enzymatic disruption of polymeric biomass, i.e. for guiding substrate and enzyme structure modifications.

Valorization of packaging plastic waste by slow pyrolysis

Low and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) are three most common polyolefins profusely used as packaging materials and abundantly found in the plastic waste stream. These plastic waste samples were collected from household waste and converted into plastic derived oil (PDO) by low temperature (300°C to 400°C) slow pyrolysis (long isothermal holding time) in a semi-batch reactor. The PDO samples obtained had shown variation in their compositions and fuel properties based on the pyrolysis temperature. PDO from the pyrolysis of PP has high octane number (∼92) and low viscosity. Noticeably, the PDO samples obtained at low temperature pyrolysis are lighter with low viscosity, high octane number and having high calorific values. 1H NMR analysis revealed that the oil samples mostly consist of paraffinic and olefinic hydrocarbons. Simulated distillation (SimDist) of PDO indicated that the liquid products resemble the characteristic closer to middle distillate of petroleum fraction having very low pour point and flash point. The temperature with long pyrolysis time also influenced the evolved gas composition and yield. Trace amount of hydrogen, carbon monoxide and carbon dioxide were present in the gaseous product along with various hydrocarbon gases ranging from C1–C5. The degradation mechanism follows end chain scission which produces monomer units whereas random scission results most of the hydrocarbon products. Subsequent reactions like radical recombination and inter or intra molecular hydrogen transfer results in the formation of most of the olefinic components.

Depolymerization kinetics for thermoplastic polyurethane elastomer degradation in subcritical methanol

The depolymerisation kinetics of thermoplastic polyurethane elastomer (TPU) in subcritical methanol was investigated. The molecular weight distributions (MWDs) of the samples were measured with gel permeation chromatography (GPC) and analysed through Origin 8.5 peak fit module, and continuous distribution kinetics was employed to describe the degradation procedure. Experimental and modelling results indicated that the degradation procedure was two stage owing to the existence of weak linkages between soft and hard domains. During degradation, weak linkages primarily completed the fracture, and strong linkages underwent scission throughout the process. The fracture of TPU chain involved random and chain-end scissions. With continuous distribution kinetics, random scission was divided into two stages by the time when weak linkages were totally disrupted. The activation energies in stages 1 and 2 were 181 and 241 kJ/mol. Chain-end scission was only related to strong linkages. The activation energies of the three kinds of monomers---4,4′-methylene diphenyl carbamate (MDC), 1,4-butanediol (BDO) and dimethyl adipate (DMA) were 137.0, 122.5 and 224.9 kJ/mol, respectively.

Lipase-mediated degradation of poly-ε-caprolactone in toluene: Behavior and its action mechanism

Lipase-catalyzed hydrolysis of poly(ɛ-caprolactone) (PCL) in toulene was investigated. PCL with number-average molecular weight (Mn) 10,000 g mol−1 was hydrolyzed using immobilized Candida antarctica lipase B (CALB). The increase in PCL concentration led to a decrease in degradation rate. Enhanced rate was observed when reaction temperature was increased from 30 to 50 °C. Enzymatic chain scission of PCL yielded cyclic dicaprolactone, tricaprolactone, tetracaprolactone and oligomers with Mn less than ∼1000 g mol−1. Catalytic formation of cyclic lactones via back-biting mechanism in low water content environment was attributed to CALB. Its hydrolysis of PCL displayed consecutive random- and chain-end scission with time from detailed thermal, molecular weight and structural analyses. Apparent activation energy, Ea for hydrolysis was 45 kJ mol−1 i.e. half of that reverse reaction. Dicaprolactone and oligomers from hydrolysis readily re-polymerized to produce mid-range polymer with Mn 1400 g mol−1 after 36 h in the same reaction medium.

Oligo(lactic acid)n-Paclitaxel Prodrugs for Poly(ethylene glycol)-block-poly(lactic acid) Micelles: Loading, Release, and Backbiting Conversion for Anticancer Activity.

Poly(ethylene glycol)-block-poly(d,l-lactic acid) (PEG-b-PLA) micelles are nanocarriers for poorly water-soluble anticancer agents and have advanced paclitaxel (PTX) to humans due to drug solubilization, biocompatibility, and dose escalation. However, PEG-b-PLA micelles rapidly release PTX, resulting in widespread biodistribution and low tumor exposure. To improve delivery of PTX by PEG-b-PLA micelles, monodisperse oligo(l-lactic acid), o(LA)8 or o(LA)16, has been coupled onto PTX at the 7-OH position, forming ester prodrugs: o(LA)8-PTX and o(LA)16-PTX, respectively. As expected, o(LA)n-PTX was more compatible with PEG-b-PLA micelles than PTX, increasing drug loading from 11 to 54%. While in vitro release of PTX was rapid, resulting in precipitation, o(LA)n-PTX release was more gradual: t1/2 = 14 and 26 h for o(LA)8-PTX and o(LA)16-PTX, respectively. Notably, o(LA)8-PTX and o(LA)16-PTX in PEG-b-PLA micelles resisted backbiting chain end scission, based on reverse-phase HPLC analysis. By contrast, o(LA)8-PTX and o(LA)16-PTX degraded substantially in 1:1 acetonitrile:10 mM PBS, pH 7.4, at 37 °C, generating primarily o(LA)2-PTX. The IC50 value of o(LA)2-PTX was ∼2.3 nM for A549 human lung cancer cells, equipotent with PTX in vitro. After weekly IV injections at 20 mg/kg as PEG-b-PLA micelles, o(LA)8-PTX induced tumor regression in A549 tumor-bearing mice, whereas PTX delayed tumor growth. Surprisingly, o(LA)8-PTX caused less toxicity than PTX in terms of change in body weight. In conclusion, o(LA)n acts as a novel promoiety, undergoing backbiting conversion without a reliance on metabolizing enzymes, and o(LA)n-PTX improves PTX delivery by PEG-b-PLA micelles, providing a strong justification for clinical evaluation.

Application of the Direct Quadrature Method of Moments for the modelling of the enzymatic hydrolysis of cellulose: I. Case of soluble substrate

The modelling of the enzymatic hydrolysis of cellulosic polymers is investigated through a population balance approach. Both Endoglucanase (EG) and Exoglucanase (CBH) activities are taken into account. EG achieves random attacks along cellulosic chains and cleaves the β-glycosidic bonds whereas CBH produces cellobiose molecules by chain-end scission mechanism. The EG activity is modelled as a pure breakage while the CBH activity is assimilated to an erosion process with a specific product (cellobiose). In the two cases, the inhibition of the cellulases activity by the end-product is incorporated. The population balance equation (PBE) accounting for breakage processes is solved using the Direct Quadrature Method of Moments (DQMOM) coupled to a distribution reconstruction technique based on the Maximum Entropy (ME) principle in order to track the time evolution of the chain length distribution (CLD) during the hydrolysis reaction. The β-glucosidase activity transforming the produced cellobiose into glucose is modelled as a Michaelis–Menten type kinetic with a competitive inhibition effect and solved simultaneously with the PBE. The numerical results show the time-evolution of the CLD during the hydrolysis reaction as well as the rate of conversion of the substrate into simple sugars. These results are in concordance with those predicted analytically. The synergistic action of the EG and CBH is highlighted and discussed and the inhibition effect is investigated. The approach is promising by its accuracy and fastness for the analysis of dynamic experimental data of the enzymatic hydrolysis reaction.

Millisecond Pulsed Films Unify the Mechanisms of Cellulose Fragmentation

The mechanism of crystalline cellulose fragmentation has been debated between classical models proposing end-chain or intrachain scission to form short-chain (molten) anhydro-oligomer mixtures and volatile organic compounds. Models developed over the last few decades suggest global kinetics consistent with either mechanism, but validation of the chain-scission mechanism via measured reaction rates of cellulose has remained elusive. To resolve these differences, we introduce a new thermal-pulsing reactor four orders of magnitude faster than conventional thermogravimetic analysis (106 vs 102 °C/min) to measure the millisecond-resolved evolution of cellulose and its volatile products at 400–550 °C. By comparison of cellulose conversion and furan product formation kinetics, both mechanisms are shown to occur with the transition from chain-end scission to intrachain scission above 467 °C concurrent with liquid formation comprised of short-chain cellulose fragments.