Selective Oxidation of 2-Hydroxypropyl Ethers of Cellulose and Dextran: Simple and Efficient Introduction of Versatile Ketone Groups to Polysaccharides

Research Progress of the Preparation of Cellulose Ethers and Their Applications: A Short Review

Cellulose ethers (CEs), synthesized through the etherification of cellulose, have emerged as indispensable "green additives" in our modern industries, earning the moniker of industrial "monosodium glutamate" due to their unparalleled multifunctionality. Unlike traditional petroleum-based modifiers, CEs offer a unique combination of renewability, low toxicity, and tunable properties (e.g., water retention, thickening, and stimuli-responsiveness), making them pivotal for advancing sustainable construction practices. This review presents an overview of the preparation methods of various CEs and the applications of CEs especially in concrete and mortars as well as corresponding mechanisms. We systematically analyze the preparation methodologies (homogeneous vs. heterogeneous processes) and highlight the effect of molecular determinants (degree of substitution, molecular weight, functional groups) on the performances of CEs. CEs can enhance the workability and other properties of concrete and mortars primarily by acting as water-retaining and thickening agents to mitigate rapid water loss, improve hydration efficiency and cohesion. The effects of CEs on the delay of hydration and microstructure of concrete and mortars are also analyzed and highlighted. Beyond construction, we reviewed the current and emerging CE applications in biomedicine, tissue-engineering, petroleum industry and food engineering, highlighting their cross-disciplinary potential. This review provides some insights into the structure-property-application relationships of CEs and their brief historical developments, offering guidance for optimizing their utilizations especially in sustainable construction practices.

β-(1 → 4)-Polyglucuronic acids with C2/C3-ketones prepared from regenerated cellulose by catalytic oxidation using solid NaOCl·5H2O

Three commercial regenerated cellulose samples were subjected to TEMPO-catalyzed oxidation using solid NaOCl·5H2O as the primary oxidant for structural analyses of the oxidized products (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl). The regenerated cellulose/water slurries became transparent solutions after oxidation for 60 min. The yields of the oxidized products were almost 100 % when they were isolated as precipitates in ethanol/water mixtures. The solution-state NMR spectra revealed that the oxidized products were almost pure water-soluble β-(1 → 4)-polyglucuronic acids; the reaction conditions described herein ensured the complete oxidation of the C6-OH groups in the regenerated cellulose samples to C6-carboxy groups. However, the solid-state 13C NMR spectra of the oxidized products indicated that C2/C3-ketones (<20 % of the total units) were formed during side reactions, which is characteristic for oxidized products prepared from regenerated cellulose with the C2/C3-glycol structure. These ketones were likely to form intermolecular hemiacetal linkages in the oxidized products. During conductivity titration of the oxidized products, it is necessary to control the sample masses to accurately determine the carboxy contents. The mass-average degree of polymerization decreased from 330 to 890 for the original regenerated cellulose samples to 65-79 for the oxidized products; substantial depolymerization is inevitable during TEMPO-catalyzed oxidation of the regenerated cellulose samples.

All-polysaccharide, self-healing, pH-sensitive, in situ-forming hydrogel of carboxymethyl chitosan and aldehyde-functionalized hydroxyethyl cellulose

In situ forming hydrogels are promising for biomedical applications, especially in drug delivery. The precursor solution can be injected at the target site, where it undergoes a sol-gel transition to afford a hydrogel. In this sense, the most significant characteristic of these hydrogels is fast gelation behavior after injection. This study describes an all-polysaccharide, rapidly in situ-forming hydrogel composed of carboxymethyl chitosan (CMCHT) and hydroxyethyl cellulose functionalized with aldehyde groups (HEC-Ald). The HEC-Ald was synthesized through acetal functionalization, followed by acid deprotection. This innovative approach avoids cleavage of pyran rings, as is inherent in the periodate oxidation approach, which is the most common method currently employed for adding aldehyde groups to polysaccharides. The resulting hydrogel exhibited fast stress relaxation, self-healing properties, and pH sensitivity, which allowed it to control the release of an encapsulated model drug in response to the medium pH. Based on the collected data, the HEC-Ald/CMCHT hydrogels show promise as pH-sensitive drug carriers.

Polysaccharide Aldehydes and Ketones: Synthesis and Reactivity

Polysaccharides are biodegradable, abundant, sustainable, and often benign natural polymers. The achievement of selective modification of polysaccharides is important for targeting specific properties and structures and will benefit future development of highly functional, sustainable materials. The synthesis of polysaccharides containing aldehyde or ketone moieties is a promising tool for achieving this goal because of the rich chemistry of aldehyde or ketone groups, including Schiff base formation, nucleophilic addition, and reductive amination. The obtained polysaccharide aldehydes or ketones themselves have rich potential for making useful materials, such as self-healing hydrogels, polysaccharide-protein therapeutic conjugates, or drug delivery vehicles. Herein, we review recent advances in synthesizing polysaccharides containing aldehyde or ketone moieties and briefly introduce their reactivity and corresponding applications.

Precision Cellulose from Living Cationic Polymerization of Glucose 1,2,4-Orthopivalates

Cellulose serves as a sustainable biomaterial for a wide range of applications in biotechnology and materials science. While chemical and enzymatic glycan assembly methods have been developed to access modest quantities of synthetic cellulose for structure-property studies, chemical polymerization strategies for scalable and well-controlled syntheses of cellulose remain underdeveloped. Here, we report the synthesis of precision cellulose via living cationic ring-opening polymerization (CROP) of glucose 1,2,4-orthopivalates. In the presence of dibutyl phosphate as an initiator and triflic acid as a catalyst, precision cellulose with well-controlled molecular weights, defined chain-end groups, and excellent regio- and stereospecificity was readily prepared. We further demonstrated the utility of this method through the synthesis of precision native d-cellulose and rare precision l-cellulose.

Dess-Martin oxidation of hydroxypropyl and hydroxyethyl cellulose, and exploration of their polysaccharide/polypeptide hydrogels

Oxidation of polysaccharides can provide biomaterials with aldehyde and ketone functional groups, which are particularly useful in biomedical applications, like drug delivery, tissue adhesion and hydrogel preparation. However, despite their potential, only a few such methods have been reported, and achieving selective, quantitative oxidation of polysaccharides remains challenging. Herein we report utilization of a mild oxidant, Dess-Martin periodinane, for the chemoselective oxidation of hydroxypropyl cellulose (HPC) and hydroxyethyl cellulose (HEC). Our findings reveal that the oxidation of HPC is fast, efficient and achieves near-quantitative conversion. Moreover, both Ox-HPC and Ox-HEC exhibit low cell toxicity, and readily form hydrogels by reaction with a polypeptide comprising amino acids with amine-containing a-substituents, α-poly-l-lysine. The peptide/polysaccharide hydrogels display self-healing properties, injectability, and antimicrobial activity, making them highly attractive for biomedical applications including in wound dressings.

Reductive amination of oxidized hydroxypropyl cellulose with ω-aminoalkanoic acids as an efficient route to zwitterionic derivatives

Zwitterionic polymers, with their equal amounts of cationic and anionic functional groups, have found widespread utility including as non-fouling coatings, hydrogel materials, stabilizers, antifreeze materials, and drug carriers. Polysaccharide-derived zwitterionic polymers are attractive because of their sustainable origin, potential for lower toxicity, and possible biodegradability, but previous methods for synthesis of zwitterionic polysaccharide derivatives have been limited in terms of flexibility and attainable degree of substitution (DS) of charged entities. We report herein successful design and synthesis of zwitterionic polysaccharide derivatives, in this case based on cellulose, by reductive amination of oxidized 2-hydroxypropyl cellulose (Ox-HPC) with ω-aminoalkanoic acids. Reductive amination products could be readily obtained with DS(cation) (= DS(anion)) up to 1.6. Adduct hydrophilic/hydrophobic balance (amphiphilicity) can be influenced by selecting the appropriate chain length of the ω-aminoalkanoic acid. This strategy is shown to produce a range of amphiphilic, water-soluble, moderately high glass transition temperature (Tg) polysaccharide derivatives in just a couple of efficient steps from commercially available building blocks. The adducts were evaluated as crystallization inhibitors. They are strong inhibitors of crystallization even for the challenging, poorly soluble, fast-crystallizing prostate cancer drug enzalutamide, as supported by surface tension and Flory-Huggins interaction parameter results.

Potential of ion mobility mass spectrometry in cellulose ether analysis: substitution pattern of hydroxyethyl celluloses

Ion mobility mass spectrometry (ESI-tims-ToF-MS, syringe pump infusion) has been applied to glucose and oligosaccharide ethers derived from hydroxyethyl-methyl celluloses (HEMC) and hydroxyethyl celluloses (HEC) after permethylation and partial depolymerization: by hydrolysis without or with subsequent reductive amination with m-amino benzoic acid (mABA) or by reductive cleavage. As model compounds without tandem substitution methoxyethylated methylcellulose was used. Regioisomeric glucose ethers were separated according to their ion mobility, and positions of substitution could be assigned. Glucose ethers including isomers with tandem substitution showed additional signals with a smaller collision cross-section (CCS) than core-substituted isomers. Positional isomers of cellobiose ethers were only partly resolved due to too high complexity but showed a characteristic fingerprint that might allow classifying samples. Relative intensities of signals of glucose ether isomers could only be quantified in case of ABA derivatives with its fixed charge, while sodium adducts of methoxyethyl ethers showed an influence of the MeOEt position on ion yield. Results were in very good agreement with reference analysis. [M + Na]+ adducts of α- and β-anomers of glucose derivatives were separated in IM, complicating position assignment. This could be overcome by reductive cleavage of the permethylated HE(M)C yielding 1,5-anhydroglucitol-terminated oligosaccharides, showing the best resolved fingerprints of the cellobiose ethers of a particular cellulose ether. With this first application of ion mobility MS to the analysis of complex cellulose ethers, the promising potential of this additional separation dimension in mass spectrometry is demonstrated and discussed.

Kinetics of Periodate-Mediated Oxidation of Cellulose

The oxidation of cellulose to dialdehyde cellulose (DAC) is a process that has received increased interest during recent years. Herein, kinetic modeling of the reaction with sodium periodate as an oxidizing agent was performed to quantify rate-limiting steps and overall kinetics of the cellulose oxidation reaction. Considering a pseudo-first-order reaction, a general rate expression was derived to elucidate the impact of pH, periodate concentration, and temperature on the oxidation of cellulose and concurrent formation of cellulose degradation products. Experimental concentration profiles were utilized to determine the rate constants for the formation of DAC (k1), degradation constant of cellulose (k2), and degradation of DAC (k3), confirming that the oxidation follows a pseudo-first-order reaction. Notably, the increase in temperature has a more pronounced effect on k1 compared to the influence of IO4- concentration. In contrast, k2 and k3 display minimal changes in response to IO4- concentration but increase significantly with increasing temperature. The kinetic model developed may help with understanding the rate-limiting steps and overall kinetics of the cellulose oxidation reaction, providing valuable information for optimizing the process toward a faster reaction with higher yield of the target product.

Novel Para-Aminobenzoic Acid Analogs and Their Potential Therapeutic Applications

A "building block" is a key component that plays a substantial and critical function in the pharmaceutical research and development industry. Given its structural versatility and ability to undergo substitutions at both the amino and carboxyl groups, para-aminobenzoic acid (PABA) is a commonly used building block in pharmaceuticals. Therefore, it is great for the development of a wide range of novel molecules with potential medical applications. Anticancer, anti-Alzheimer's, antibacterial, antiviral, antioxidant, and anti-inflammatory properties have been observed in PABA compounds, suggesting their potential as therapeutic agents in future clinical trials. PABA-based therapeutic chemicals as molecular targets and their usage in biological processes are the primary focus of this review study. PABA's unique features make it a strong candidate for inclusion in a massive chemical database of molecules having drug-like effects. Based on the current literature, further investigation is needed to evaluate the safety and efficacy of PABA derivatives in clinical investigations and better understand the specific mechanism of action revealed by these compounds.

Chemistry of Hydroxypropyl Cellulose Oxidized by Two Selective Oxidants

Along with the increased usage of cellulose in the manufacture of novel materials, those of its derivatives that have good solubility in water or organic solvents have become increasingly important. In this study, hydroxypropyl cellulose (HPC), a cellulosic derivative with distinct features, was utilized to investigate how two of the most-selective oxidation methods currently available in the literature act on the constituent OH groups of both the side chain and the anhydroglycosidic unit in HPC. The oxidation reactions were carried out first using TEMPO, sodium hypochlorite, and sodium bromide, then sodium periodate (NaIO4), for 5 h. A combination of these two protocols was applied. The amount of aldehyde and number of carboxylic groups introduced after oxidation was determined, while the changes in the morphological features of oxidized HPC were, additionally, assessed. Furthermore, utilizing Fourier-transform infrared spectra, X-ray diffraction, and thermogravimetric studies, the chemical structure, crystallinity, and thermal stability of the oxidized HPC samples were examined and compared.

Chitosan as a Canvas for Studies of Macromolecular Controls on CaCO3 Biological Crystallization

A mechanistic understanding of how macromolecules, typically as an organic matrix, nucleate and grow crystals to produce functional biomineral structures remains elusive. Advances in structural biology indicate that polysaccharides (e.g., chitin) and negatively charged proteoglycans (due to carboxyl, sulfate, and phosphate groups) are ubiquitous in biocrystallization settings and play greater roles than currently recognized. This review highlights studies of CaCO₃ crystallization onto chitinous materials and demonstrates that a broader understanding of macromolecular controls on mineralization has not emerged. With recent advances in biopolymer chemistry, it is now possible to prepare chitosan-based hydrogels with tailored functional group compositions. By deploying these characterized compounds in hypothesis-based studies of nucleation rate, quantitative relationships between energy barrier to crystallization, macromolecule composition, and solvent structuring can be determined. This foundational knowledge will help researchers understand composition-structure-function controls on mineralization in living systems and tune the designs of new materials for advanced applications.

Recent advances in polysaccharide-based in situ forming hydrogels

Polysaccharides comprise an important class of natural polymers; they are abundant, diverse, polyfunctional, typically benign, and are biodegradable. Using polysaccharides to design in situ forming hydrogels is an attractive and important field of study since many polysaccharide-based hydrogels exhibit desirable characteristics including self-healing, responsiveness to environmental stimuli, and injectability. These characteristics are particularly useful for biomedical applications. This review will discuss recent discoveries in polysaccharide-based in situ forming hydrogels, including network architecture designs, curing mechanisms, physical and chemical properties, and potential applications.