C-H Functionalization of Poly(ethylene oxide) - Embracing Functionality, Degradability, and Molecular Delivery

This study presents an organocatalytic C-H functionalization approach for postpolymerization modification (PPM) of poly(ethylene oxide) (PEO). Most of PEO PPM is previously processed at the end hydroxy group, but recent advances in C-H functionalization open a way to modify the backbone position. Structurally diverse carboxylic acids are attached to PEO through a cascade process of radical generation by peroxide and oxidation to oxocarbenium by tertiary butylammonium iodide. Attaching carboxylic acids yields a series of functionalize PEO with acetal units (2-5 mol%) in a backbone, which is not accessible via conventional copolymerization of epoxides. The optimized conditions minimizes the uncontrolled degradation or crosslinking from the highly reactive radical and oxocarbenium intermediate. The newly introduced acetal units bring degradability of PEO as well as delivery of carboxylic acid molecules. Hydrolysis studies with high molecular weight functionalization PEO (Mn = 13.0 kg mol-1) confirm the steady release of fragmented PEO (Mn ∼ 2.0 kg mol-1) and carboxylic acid over days and the process rate is not sensitive to pH variation between pH 5 and 9. The presented method offers a versatile and efficient way to modify PEO with potential energy and medical applications.

Synthesis of polyvinylethylene glycols (PVEGs) via polyetherification of vinylethylene carbonate by synergistic catalysis

An efficient and controllable polyetherification of vinylethylene carbonate (VEC) using diols as initiators is developed. By using a synergistic catalysis with palladium and boron reagents under mild conditions, the polymerization process enables the regioselective production of a series of polyvinylethylene glycols (PVEGs) bearing pendent vinyl groups in high yields with accurate molecular weight control and narrow molecular weight distribution. The utility of PVEGs is demonstrated by the production of functional polyurethanes and post-polymerization modification via thiol-ene photo-click chemistry.

Polyethers Based on Short-Chain Alkyl Glycidyl Ethers: Thermoresponsive and Highly Biocompatible Materials

The polymerization of short-chain alkyl glycidyl ethers (SCAGEs) enables the synthesis of biocompatible polyethers with finely tunable hydrophilicity. Aliphatic polyethers, most prominently poly(ethylene glycol) (PEG), are utilized in manifold biomedical applications due to their excellent biocompatibility and aqueous solubility. By incorporation of short hydrophobic side-chains at linear polyglycerol, control of aqueous solubility and the respective lower critical solution temperature (LCST) in aqueous solution is feasible. Concurrently, the chemically inert character in analogy to PEG is maintained, as no further functional groups are introduced at the polyether structure. Adjustment of the hydrophilicity and the thermoresponsive behavior of the resulting poly(glycidyl ether)s in a broad temperature range is achieved either by the combination of the different SCAGEs or with PEG as a hydrophilic block. Homopolymers of methyl and ethyl glycidyl ether (PGME, PEGE) are soluble in aqueous solution at room temperature. In contrast, n-propyl glycidyl ether and iso-propyl glycidyl ether lead to hydrophobic polyethers. The use of a variety of ring-opening polymerization techniques allows for controlled polymerization, while simultaneously determining the resulting microstructures. Atactic as well as isotactic polymers are accessible by utilization of the respective racemic or enantiomerically pure monomers. Polymer architectures varying from statistical copolymers, di- and triblock structures to star-shaped architectures, in combination with PEG, have been applied in various thermoresponsive hydrogel formulations or polymeric surface coatings for cell sheet engineering. Materials responding to stimuli are of increasing importance for "smart" biomedical systems, making thermoresponsive polyethers with short-alkyl ether side chains promising candidates for future biomaterials.

Polyethylene glycol-derived polyelectrolyte-protein nanoclusters for protein drug delivery

Polyelectrolyte-protein nanocomplexes prepared under mild and simple conditions which could have biological activity arising from protein have emerged as fascinating protein delivery systems. However, common polyelectrolytes have problems of biocompatibility and metabolism in vivo, which may limit their further applications. Herein, a novel polyethylene glycol polyelectrolyte was synthesized and used for carrying protein drugs. Different from previously reported polyelectrolyte-protein nanoclusters, the polyethylene glycol polyelectrolyte-protein nanoclusters avoid organic solvent and protein modification, and the structure and bioactivity of proteins are well preserved. Moreover, the polyethylene glycol polyelectrolyte-protein nanoclusters have good hemocompatibility and biocompatibility. These novel polyethylene glycol polyelectrolyte-protein nanoclusters would provide a potent tool for fabrication of versatile protein drug carriers.

Acetal-Based Functional Epoxide Monomers: Polymerizations and Applications

Protecting group chemistry is essential for various organic transformation and polymerization processes. In particular, conventional anionic ring-opening polymerization (AROP) often requires proper protecting group chemistry because it is typically incompatible with most functional groups due to the highly basic and nucleophilic conditions. In this context, many functional epoxide monomers with proper protecting groups are developed, including the acetal group as a representative example. Since the early introduction of ethoxyethyl glycidyl ether, there is significant development of acetal-based monomers in the polyethers. These monomers are now utilized not only as protecting groups for hydroxyl groups under AROP conditions but also as pH-responsive moieties for biomedical applications, further expanding their utility in the use of functionalized polyethers. Recent progress in this field is outlined from their synthesis, polymerization, and biomedical applications.

The role of poly(2-alkyl-2-oxazoline)s in hydrogels and biofabrication

Poly(2-alkyl-2-oxazoline)s (PAOXAs) have been rapidly emerging as starting materials in the design of tissue engineering supports and for the generation of platforms for cell cultures, especially in the form of hydrogels. Thanks to their biocompatibility, chemical versatility and robustness, PAOXAs now represent a valid alternative to poly(ethylene glycol)s (PEGs) and their derivatives in these applications, and in the formulation of bioinks for three-dimensional (3D) bioprinting. In this review, we summarize the recent literature where PAOXAs have been used as main components for hydrogels and biofabrication mixtures, especially highlighting how their easily tunable composition could be exploited to fabricate multifunctional biomaterials with an extremely broad spectrum of properties.

Well-defined poly(ethylene glycol) polymers as non-conventional reactive tracers of colloidal transport in porous media

HYPOTHESIS

A prominent fraction of mobile organic matter in natural aqueous soil solutions is formed by molecules in sizes that seamlessly exceed the lower end of what is defined as a colloid. The hydrodynamics and the functional diversity of these molecules result in a transport behavior that is fundamentally different from smaller compounds. However, there is a lack of "reactive tracers" that allow for the study of colloidal transport phenomena appropriately. We hypothesize that tailor-made and well-defined synthetic polymers can overcome this limitation.

EXPERIMENTS

We prepared and characterized the hydrodynamic properties of water-soluble poly(ethylene glycol)s (PEG) and studied their adsorption to mixtures of quartz, illite, and goethite in batch and column experiments.

FINDINGS

We used this information to independently predict the transport of PEG with striking agreement to the observed mean breakthrough times in all porous media. As PEG transport can be comprehensively and quantitatively reconstructed, we conclude that functionalized PEGs are promising candidates to be used as tailorable and non-toxic tracers available in the size range of natural organic (macro-)molecules.

Shining Light on Poly(ethylene glycol): From Polymer Modification to 3D Laser Printing of Water Erasable Microstructures

The implementation of stimuli-responsive bonds into 3D network assemblies is a key concept to design adaptive materials that can reshape and degrade. Here, a straightforward but unique photoresist is introduced for the tailored fabrication of poly(ethylene glycol) (PEG) materials that can be readily erased by water, even without the need for acidic or basic additives. Specifically, a new class of photoresist is developed that operates through the backbone crosslinking of PEG when irradiated in the presence of a bivalent triazolinedione. Hence, macroscopic gels are obtained upon visible light-emitting diode irradiation (λ > 515 nm) that are stable in organic media but rapidly degrade upon the addition of water. Photoinduced curing is also applicable to multiphoton laser lithography (λ > 700 nm), hence providing access to 3D printed microstructures that vanish when immersed in water at 37 °C. Materials with varying crosslinking densities are accessed by adapting the applied laser writing power, thereby allowing for tunable hydrolytic erasing timescales. A new platform technology is thus presented that enables the crosslinking and 3D laser printing of PEG-based materials, which can be cleaved and erased in water, and additionally holds potential for the facile modification and backbone degradation of polyether-containing materials in general.

Synthesis of novel polyethers with abundant reactive sites and diverse skeletons based on the ring-opening reaction of D–A cyclopropanes

Based on the ring-opening reaction of D–A cyclopropanes, a facile synthesis of novel polyethers is developed with molecular weights up to 17.7 kg mol⁻¹.

Amino-functional polyethers: versatile, stimuli-responsive polymers

Amino-functional polyethers have emerged as a new class of “smart”, i.e. pH- and thermoresponsive materials. This review article summarizes the synthesis and applications of these materials, obtained from ring-opening of suitable epoxide monomers.

Hydrogels with multiple clickable anchor points: synthesis and characterization of poly(furfuryl glycidyl ether)-block-poly(ethylene glycol) macromonomers

Functional polyacrylamide hydrogels containing multiple furfuryl anchor points for Diels–Alder reactions were prepared employing new macromonomers.

Synthesis of alkynyl-functionalized linear and star polyethers by aluminium-catalyzed copolymerization of glycidyl 3-butynyl ether with epichlorohydrin and ethylene oxide

A novel family of alkynyl-functional linear and star polyethers were prepared by the copolymerization of glycidyl 3-butynyl ether, ethylene oxide and epichlorohydrin catalyzed by i-Bu₃Al/H₃PO₄/DBU.

Multi-olefin containing polyethers and triazolinediones: a powerful alliance

Multi-functional polyethers with ene or diene moieties were prepared via the polymerisation of tailored functional glycidyl ether monomers to create a platform for click chemistry with triazolinediones (TADs).

Design and synthesis of thermoresponsive aliphatic polyethers with a tunable phase transition temperature

A comprehensive study of the synthesis and LCST-type thermoresponsive properties of poly(glycidyl ether) homopolymers and their copolymers is described.

Multihydroxy Polyamines by Living Anionic Polymerization of Aziridines

Acetal-protected and sulfonamide-activated aziridines (Az) have been prepared and polymerized by living anionic polymerization with molecular weight dispersities in most cases below Đ < 1.2 and controlled molecular weights. Three new monomers have been prepared varying in the length of the pendant chain. The resulting double protected polymers can be selectively deprotected in order to release the polyamine or the polyol structures. Detailed structural characterization was performed for all polymers, and chain extension proves their living polymerization behavior and the formation of block copolymers. Thermal analysis can be used in order to follow the deprotection steps. These new protected monomers broaden the scope of the azaanionic polymerization of aziridines and may find useful applications as well-defined functional poly(ethylene imine) derivatives.

Biomaterials-based strategies for salivary gland tissue regeneration

The salivary gland is a complex, secretory tissue that produces saliva and maintains oral homeostasis. Radiation induced salivary gland atrophy, manifested as "dry mouth" or xerostomia, poses a significant clinical challenge. Tissue engineering recently has emerged as an alternative, long-term treatment strategy for xerostomia. In this review, we summarize recent efforts towards the development of functional and implantable salivary glands utilizing designed polymeric substrates or synthetic matrices/scaffolds. Although the in vitro engineering of a complex implantable salivary gland is technically challenging, opportunities exist for multidisciplinary teams to assemble implantable and secretory tissue modules by combining stem/progenitor cells found in the adult glands with biomimetic and cell-instructive materials.

Synthesis and rheological behavior of poly(1,2-butylene oxide) based supramolecular architectures

We describe the synthesis, analysis and rheological behavior of differently functionalized poly(1,2-butylene oxide) based supramolecular polymers.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

Dual-stimuli-responsive microparticles

The need for smart materials in the area of biotechnology has fueled the development of numerous stimuli-responsive polymers. Many of these polymers are responsive to pH, light, temperature, or oxidative stress, and yet very few are responsive toward multiple stimuli. Here we report on the synthesis of a novel dual-stimuli-responsive poly(ethylene glycol)-based polymer capable of changing its hydrophilic properties upon treatment with UV light (exogenous stimulus) and markers of oxidative stress (endogenous stimulus). From this polymer, smart microparticles and fibers were fabricated and their responses to either stimulus separately and in conjunction were examined. Comparison of the degradation kinetics demonstrated that the polymer became water-soluble only after both oxidation and irradiation with UV light, which resulted in selective degradation of the corresponding particles. Furthermore, in vitro experiments demonstrated successful uptake of these particles by Raw 264.7 cells. Such dual-stimuli-responsive particles could have potential applications in drug delivery, imaging, and tissue engineering.

Modular and orthogonal synthesis of hybrid polymers and networks

Biomaterials scientists strive to develop polymeric materials with distinct chemical make-up, complex molecular architectures, robust mechanical properties and defined biological functions by drawing inspirations from biological systems. Salient features of biological designs include (1) repetitive presentation of basic motifs; and (2) efficient integration of diverse building blocks. Thus, an appealing approach to biomaterials synthesis is to combine synthetic and natural building blocks in a modular fashion employing novel chemical methods. Over the past decade, orthogonal chemistries have become powerful enabling tools for the modular synthesis of advanced biomaterials. These reactions require building blocks with complementary functionalities, occur under mild conditions in the presence of biological molecules and living cells and proceed with high yield and exceptional selectivity. These chemistries have facilitated the construction of complex polymers and networks in a step-growth fashion, allowing facile modulation of materials properties by simple variations of the building blocks. In this review, we first summarize features of several types of orthogonal chemistries. We then discuss recent progress in the synthesis of step growth linear polymers, dendrimers and networks that find application in drug delivery, 3D cell culture and tissue engineering. Overall, orthogonal reactions and modulular synthesis have not only minimized the steps needed for the desired chemical transformations but also maximized the diversity and functionality of the final products. The modular nature of the design, combined with the potential synergistic effect of the hybrid system, will likely result in novel hydrogel matrices with robust structures and defined functions.

Multifunctional poly(phosphoester)s with two orthogonal protective groups

2-(2-(Benzyloxy)ethoxy)-1,3,2-dioxaphospholane-2-oxide was developed for the ring-opening polymerization to multifunctional poly(phosphoester)s. An acetal-protected comonomer allows selective release of the hydroxyl groups by hydrolysis and hydrogenation.

Ring-opening polymerization of a 2,3-disubstituted oxirane leading to a polyether having a carbonyl–aromatic π-stacked structure

Ring-opening polymerization of a 2,3-disubstituted oxirane afforded a polyether exhibiting intramolecular charge-transfer (ICT) interactions due to a carbonyl–aromatic π-stacked conformation.

Poly(Ethylene glycol)-based backbones with high peptide loading capacities

Polymer-peptide conjugates are a promising class of compounds, where polymers can be used to overcome some of the limitations associated with peptides intended for therapeutic and/or diagnostic applications. Linear polymers such as poly(ethylene glycol) can be conjugated through terminal moieties and have therefore limited loading capacities. In this research, functionalised linear poly(ethylene glycol)s are utilised for peptide conjugation, to increase their potential loading capacities. These poly(ethylene glycol) derivatives are conjugated to peptide sequences containing representative side-chain functionalised amino acids, using different conjugation chemistries, including copper-catalysed azide-alkyne cycloaddition, amide coupling and thiol-ene reactions. Conjugation of a sequence containing the RGD motif to poly(allyl glycidyl ether) by the thiol-ene reaction, provided a conjugate which could be used in platelet adhesion studies.