Poly(2-oxazoline)s and click chemistry: A versatile toolbox toward multi-functional polymers

Polyoxazolines with Cholesterol Lipid Anchor for Fast Intracellular Delivery

Due to the increasing challenges posed by the growing immunity to poly(ethylene glycol) (PEG), there is growing interest in innovative polymer-based materials as viable alternatives. In this study, the advantages of lipids and polymers are combined to allow efficient and rapid cytoplasmic drug delivery. Specifically, poly(2-methyl-2-oxazoline) is modified with a cholesteryl hemisuccinate group as a lipid anchor (CHEMSPOx). The CHEMSPOx is additionally functionalized with a coumarin group (CHEMSPOx-coumarin). Both polymers self-assembled in water into vesicles of ≈100 nm and are successfully loaded with a hydrophobic model drug. The loaded vesicles reveal high cellular internalization across variant cell lines within 1 h at 37 °C as well as 4 °C, albeit to a lesser extent. A kinetic study confirms the fast internalization within 5 min after the sample's addition. Therefore, different internalization pathways are involved, e.g., active uptake but also nonenergy dependent mechanisms. CHEMSPOx and CHEMSPOx-coumarin further demonstrate excellent cyto-, hemo-, and membrane compatibility, as well as a membrane-protecting effect, which underlines their good safety profile for potential biological intravenous application. Overall, CHEMSPOx, as a lipopolyoxazoline, holds great potential for versatile biological applications such as fast and direct intracellular delivery or cellular lysis protection.

Unravel the Tangle: Atomistic Insight into Ultrahigh Curcumin-Loaded Polymer Micelles

Amphiphilic ABA-triblock copolymers, comprised of poly(2-oxazoline) and poly(2-oxazine), can solubilize poorly water-soluble molecules in a structure-dependent manner forming micelles with exceptionally high drug loading. All-atom molecular dynamics simulations are conducted on previously experimentally characterized, curcumin-loaded micelles to dissect the structure-property relationships. Polymer-drug interactions for different levels of drug loading and variation in polymer structures of both the inner hydrophobic core and outer hydrophilic shell are investigated. In silico, the system with the highest experimental loading capacity shows the highest number of drug molecules encapsulated by the core. Furthermore, in systems with lower loading capacity outer A blocks show a greater extent of entanglement with the inner B blocks. Hydrogen bond analyses corroborate previous hypotheses: poly(2-butyl-2-oxazoline) B blocks, found experimentally to have reduced loading capacity for curcumin compared to poly(2-propyl-2-oxazine), establish fewer but longer-lasting hydrogen bonds. This possibly results from different sidechain conformations around the hydrophobic cargo, which is investigated by unsupervised machine learning to cluster monomers in smaller model systems mimicking different micelle compartments. Exchanging poly(2-methyl-2-oxazoline) with poly(2-ethyl-2-oxazoline) leads to increased drug interactions and reduced corona hydration; this suggests an impairment of micelle solubility or colloidal stability. These observations can help driving forward a more rational a priori nanoformulation design.

Non-activated Esters as Reactive Handles in Direct Post-Polymerization Modification

Non-activated esters are prominently featured functional groups in polymer science, as ester functional monomers display great structural diversity and excellent compatibility with a wide range of polymerization mechanisms. Yet, their direct use as a reactive handle in post-polymerization modification has been typically avoided due to their low reactivity, which impairs the quantitative conversion typically desired in post-polymerization modification reactions. While activated ester approaches are a well-established alternative, the modification of non-activated esters remains a synthetic and economically valuable opportunity. In this review, we discuss past and recent efforts in the utilization of non-activated ester groups as a reactive handle to facilitate transesterification and aminolysis/amidation reactions, and the potential of the developed methodologies in the context of macromolecular engineering.

Conformational Parameters and Hydrodynamic Behavior of Poly(2-Methyl-2-Oxazoline) in a Broad Molar Mass Range

In this work, we report our results on the hydrodynamic behavior of poly(2-methyl-2-oxazoline) (PMeOx). PMeOx is gaining significant attention for use as hydrophilic polymer in pharmaceutical carriers as an alternative for the commonly used poly(ethylene glycol) (PEG), for which antibodies are found in a significant fraction of the human population. The main focus of the current study is to determine the hydrodynamic characteristics of PMeOx under physiological conditions, which serves as basis for better understanding of the use of PMeOx in pharmaceutical applications. This goal was achieved by studying PMeOx solutions in phosphate-buffered saline (PBS) as a solvent at 37 °C. This study was performed based on two series of PMeOx samples; one series is synthesized by conventional living cationic ring-opening polymerization, which is limited by the maximum chain length that can be achieved, and a second series is obtained by an alternative synthesis strategy based on acetylation of well-defined linear poly(ethylene imine) (PEI) prepared by controlled side-chain hydrolysis of a defined high molar mass of poly(2-ethyl-2-oxazoline). The combination of these two series of PMeOx allowed the determination of the Kuhn-Mark-Houwink-Sakurada equations in a broad molar mass range. For intrinsic viscosity, sedimentation and diffusion coefficients, the following expressions were obtained: η=0.015M0.77, s0=0.019M0.42 and D0=2600M-0.58, respectively. As a result, it can be concluded that the phosphate-buffered saline buffer at 37 °C represents a thermodynamically good solvent for PMeOx, based on the scaling indices of the equations. The conformational parameters for PMeOx chains were also determined, revealing an equilibrium rigidity or Kuhn segment length, (A) of 1.7 nm and a polymer chain diameter (d) of 0.4 nm. The obtained value for the equilibrium rigidity is very similar to the reported values for other hydrophilic polymers, such as PEG, poly(vinylpyrrolidone) and poly(2-ethyl-2-oxazoline), making PMeOx a relevant alternative to PEG.

Poly(2-oxazoline)s as Stimuli-Responsive Materials for Biomedical Applications: Recent Developments of Polish Scientists

Poly(2-oxazoline)s are the synthetic polymers that are the products of the cationic ring-opening polymerization (CROP) of 2-oxazoline monomers. Due to their beneficial properties, from which biocompatibility, stealth behavior, high functionalization possibilities, low dispersity, stability, nonionic character, and solubility in water and organic solvents should be noted, they have found many applications and gained enormous interest from scientists. Additionally, with high versatility attainable through copolymerization or through post-polymerization modifications, this class of polymeric systems has been widely used as a polymeric platform for novel biomedical applications. The chemistry of polymers significant expanded into biomedical applications, in which polymeric networks can be successfully used in pharmaceutical development for tissue engineering, gene therapies, and also drug delivery systems. On the other hand, there is also a need to create ‘smart’ polymer biomaterials, responsive to the specified factor, that will be sensitive to various environmental stimuli. The commonly used stimuli-responsive biomedical materials are based mostly on temperature-, light-, magnetic-, electric-, and pH-responsive systems. Thus, creating selective and responsive materials that allow personalized treatment is in the interest of the scientific world. This review article focuses on recent discoveries by Polish scientists working in the field of stimuli-responsive poly(2-oxazoline)s, and their work is compared and contrasted with results reported by other world-renowned specialists.

Quaternized Sodium Alginate-g-Ethyl-Oxazoline Copolymer Brushes and Their Supramolecular Networks via Hydrogen Bonding

Novel copolymer brushes of quaternized sodium alginate-g-(2-ethyl-2-oxazoline)_n are achieved by the grafting reaction of 2-ethyl-2-oxazoline (EOX) from benzyl bromide groups in quaternized sodium alginate (QSA). The average number of (EOX)_n structural units is mediated from 1 to 5 by changing the molar ratio of the EOX monomer to benzyl bromide side groups. There exists obvious microphase separation between the QSA backbone and (EOX)_n segments in the copolymer brushes due to their thermodynamic incompatibility. The strong hydrogen-bonding interaction between -OH groups in the backbone and N─C═O groups in (EOX)_n segments is helpful for the construction of reversible supramolecular networks. The copolymer brushes show low cytotoxicity for HeLa cells and good antibacterial properties for Escherichia coli and Staphylococcus aureus for the contribution of hydrophilic (EOX)_n segments and antibacterial activity of the quaternary ammonium. The antiprotein behavior of polymer surfaces is improved after rearrangement of (EOX)_n segments by tetrahydrofuran (THF) vapor induction. These copolymer brushes have good prospects for biomedical applications.

Differences and similarities between mono-, bi- or tetrafunctional initiated cationic ring-opening polymerization of 2-oxazolines

Cationic ring-opening polymerization (CROP) is an interesting synthesis technique to obtain well-defined polymers with narrow molar mass distribution (MMD).

Protected amine-functional initiators for the synthesis of α-amine homo- and heterotelechelic poly(2-ethyl-2-oxazoline)s

Screening a series of protected amine cationic ring-opening polymerization initiators revealed the commercially available N-(3-bromopropyl)phthalimide as the most suitable to achieve defined polymers with high degree of amine functionalization.

Compartmentalization and Photoregulating Pathways for Incompatible Tandem Catalysis

This contribution describes an advanced compartmentalized micellar nanoreactor that possesses a reversible photoresponsive feature and its application toward photoregulating reaction pathways for incompatible tandem catalysis under aqueous conditions. The smart nanoreactor is based on multifunctional amphiphilic poly(2-oxazoline)s and covalently cross-linked with spiropyran upon micelle formation in water. It responds to light irradiation in a wavelength-selective manner switching its morphology as confirmed by dynamic light scattering and cryo-transition electron microscopy. The compartmental structure renders distinct nanoconfinements for two incompatible enantioselective transformations: a rhodium-diene complex-catalyzed asymmetric 1,4-addition occurs in the hydrophilic corona, while a Rh-TsDPEN-catalyzed asymmetric transfer hydrogenation proceeds in the hydrophobic core. Control experiments and kinetic studies showed that the gated behavior induced by the phototriggered reversible spiropyran to merocyanine transition in the cross-linking layer is key to discriminate among substrates/reagents during the catalysis. The smart nanoreactor realized photoregulation to direct the reaction pathway to give a multichiral product with high conversions and perfect enantioselectivities in aqueous media. Our SCM catalytic system, on a basic level, mimics the concepts of compartmentalization and responsiveness Nature uses to coordinate thousands of incompatible chemical transformations into streamlined metabolic processes.

Cell-Mediated Immunoreactivity of Poly(2-isopropenyl-2-oxazoline) as Promising Formulation for Immunomodulation

Poly(2-isopropenyl-2-oxazoline) (PIPOx) represents a functional polymer with high potential for drug delivery, tissue engineering, and immunomodulation. The immunomodulatory efficiency of the PIPOx formulation has been studied in vitro following splenic cells and RAW 264.7 macrophages exposition. The cell-specific immunomodulative effect on production of Th1, Th2, Th17, and Treg signature cytokines has been demonstrated. The impact on the functionality of PIPOx-sensitized RAW 264.7 macrophages was assessed by cell phagocytosis. Time- and concentration-dependent cell internalization and intracellular organelles colocalization of fluorescently labeled PIPOx has been examined. The in vitro results demonstrated the PIPOx bioavailability and the capability of triggering immune cell responses resulting in the induced production of cell-specific signature interleukins, important prerequisite properties for future potential biomedical applications.

Thioacetate-Based Initiators for the Synthesis of Thiol-End-Functionalized Poly(2-oxazoline)s

New functional initiators for the cationic ring-opening polymerization of 2-alkyl-2-oxazolines are described to introduce a thiol moiety at the α terminus. Both tosylate and nosylate initiators carrying a thioacetate group are obtained in multigram scale, from commercial reagents in two steps, including a phototriggered thiol-ene radical addition. The nosylate derivative gives access to a satisfying control over the cationic ring-opening polymerization of 2-ethyl-2-oxazoline, with dispersity values lower than 1.1 during the entire course of the polymerization, until full conversion. Cleavage of the thioacetate end group is rapidly achieved using triazabicyclodecene, thereby leading to a mercapto terminus. The latter gives access to a new subgeneration of α-functional poly(2-oxazoline)s (butyl ester, N-hydroxysuccinimidyl ester, furan) by Michael addition with commercial (meth)acrylates. The amenability of the mercapto-poly(2-ethyl-2-oxazoline) for covalent surface patterning onto acrylated surfaces is demonstrated in a microchannel cantilever spotting (µCS) experiment, characterized by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS).

Injectable biocompatible poly(2-oxazoline) hydrogels by strain promoted alkyne-azide cycloaddition

Poly(2-alkyl-2-oxazoline) (PAOx) hydrogels are tailorable synthetic materials with demonstrated biomedical applications, thanks to their excellent biocompatibility and tunable properties. However, their use as injectable hydrogels is challenging as it requires invasive surgical procedures to insert the formed hydrogel into the body due to their nonsoluble 3D network structures. Herein, we introduce cyclooctyne and azide functional side chains to poly(2-oxazoline) copolymers to induce in situ gelation using strain promoted alkyne-azide cycloaddition. The gelation occurs rapidly, within 5 min, under physiological conditions when two polymer solutions are simply mixed. The influence of several parameters, such as temperature and different aqueous solutions, and stoichiometric ratios between the two polymers on the structural properties of the resultant hydrogels have been investigated. The gel formation within tissue samples was verified by subcutaneous injection of the polymer solution into an ex vivo model. The degradation study of the hydrogels in vitro showed that the degradation rate was highly dependent on the type of media, ranging from days to a month. This result opens up the potential uses of PAOx hydrogels in attempts to achieve optimal, injectable drug delivery systems and tissue engineering.

Combining CROP and ATRP to synthesize pH-responsive poly(2-ethyl-2-oxazoline-b-4-vinylpyridine) block copolymers

Herein, we present the synthesis and characterization of block copolymers based on the biocompatible and stealth polymer poly(2-ethyl-2-oxazoline) and the polydentate ligand and pH-responsive poly(4-vinylpyridine).

One-pot synthesis of amphiphilic multiblock poly(2-oxazoline)s via para-fluoro-thiol click reactions

A clickable initiator, pentafluoro benzyl bromide, has been investigated for the cationic ring opening polymerization of poly(2-oxazolines).

Stoichiometric Control over Partial Transesterification of Polyacrylate Homopolymers as Platform for Functional Copolyacrylates

Only recently, post-polymerization modification reactions of unactivated polyacrylates have been emerging as an attractive alternative to utilizing reactive monomers, enabling the synthetic upcycling of these widely applied polymers. Within this contribution, the triazabicyclodecene-catalyzed transesterification of polyacrylates is reported, including the reaction kinetics and the broad scope for macromolecular design of functional copolyacrylates. More specifically, the transesterification is performed under equilibrium conditions with a set of primary alcohols whereby the reaction kinetics and the obtained conversion as a function of stoichiometric excess of alcohol are evaluated. The results show that the obtained conversion is dependent on the polarity of the solvent and of the alcohol. Through this approach, the transesterification degree can be accurately controlled by stoichiometry, enabling the precise modulation of the macromolecular structure. Finally, the utility of this approach is demonstrated to incorporate functional side chains that are incompatible with radical polymerization, to facilitate Diels-Alder and thiol-ene reactions, enabling access to a broad range of functional materials from simple polyacrylate homopolymer precursors.

Systematic Study of Perfluorocarbon Nanoemulsions Stabilized by Polymer Amphiphiles

Perfluorocarbon (PFC) nanoemulsions, droplets of fluorous solvent stabilized by surfactants dispersed in water, are simple yet versatile nanomaterials. The orthogonal nature of the fluorous phase promotes the formation of nanoemulsions through a simple, self-assembly process while simultaneously encapsulating fluorous-tagged payloads for various applications. The size, stability, and surface chemistry of PFC nanoemulsions are controlled by the surfactant. Here, we systematically study the effect of the hydrophilic portion of polymer surfactants on PFC nanoemulsions. We find that the hydrophilic block length and identity, the overall polymer hydrophilic/lipophilic balance, and the polymer architecture are all important factors. The ability to modulate these parameters enables control over initial size, stability, payload retention, cellular internalization, and protein adsorption of PFC nanoemulsions. With the insight obtained from this systematic study of polymer amphiphiles stabilizing PFC nanoemulsions, design features required for the optimal material are obtained.

Thermal and crystalline properties of poly(2-oxazoline)s

The review gathers together data concerning the influence of poly(2-substituted-2-oxazoline)s structure on their thermal and crystalline properties, and how this relationship can be adjusted in controlled manner.

Alendronate-Functionalized Poly(2-oxazoline)s with Tunable Affinity for Calcium Cations

A library of poly(2-oxazoline)s functionalized with controllable amounts of alendronate, hydroxyl, and carboxylic acid side groups was successfully synthesized to create novel polymers with tunable affinity for calcium cations. The affinity of alendronate-containing polymers for calcium cations was quantified using isothermal titration calorimetry. Thermodynamic measurements revealed that the Ca²⁺-binding affinity of these polymers increased linearly with the amount of alendronate functionalization, up to values (K_Ca²⁺ = 2.4 × 10⁵ M^–1) that were about 120-fold higher than those for previously reported polymers. The calcium-binding capacity of alendronate-functionalized poly(2-oxazoline)s was exploited to form robust hydrogel networks cross-linked using reversible physical bonds. Oscillatory rheology showed that these hydrogels recovered more than 100% of their initial storage modulus after severe network destruction. The versatile synthesis of alendronate-functionalized polymers and their strong and tunable affinity for calcium cations render these polymers promising candidates for various biomedical applications.

Controlling nanoemulsion surface chemistry with poly(2-oxazoline) amphiphiles

Emulsions are dynamic materials that have been extensively employed within pharmaceutical, food and cosmetic industries. However, their use beyond conventional applications has been hindered by difficulties in surface functionalization, and an inability to selectively control physicochemical properties. Here, we employ custom poly(2-oxazoline) block copolymers to overcome these limitations. We demonstrate that poly(2-oxazoline) copolymers can effectively stabilize nanoscale droplets of hydrocarbon and perfluorocarbon in water. The controlled living polymerization of poly(2-oxazoline)s allows for the incorporation of chemical handles into the surfactants such that covalent modification of the emulsion surface can be performed. Through post-emulsion modification of these new surfactants, we are able to access nanoemulsions with modified surface chemistries, yet consistent sizes. By decoupling size and surface charge, we explore structure-activity relationships involving the cellular uptake of nanoemulsions in both macrophage and non-macrophage cell lines. We conclude that the cellular uptake and cytotoxicity of poly(2-oxazoline)-stabilized droplets can be systematically tuned via chemical modification of emulsion surfaces.

Amidation of methyl ester side chain bearing poly(2-oxazoline)s with tyramine: a quest for a selective and quantitative approach

Three new amidation approaches are evaluated to incorporate tyramine on methyl ester functional poly(2-oxazolines).

Side chain variations radically alter the diffusion of poly(2-alkyl-2-oxazoline) functionalised nanoparticles through a mucosal barrier

Functionalised nanomaterials are gaining popularity for use as drug delivery vehicles and, in particular, mucus penetrating nanoparticles may improve drug bioavailability via the oral route. To date, few polymers have been investigated for their muco-penetration, and the effects of systematic structural changes to polymer architectures on the penetration and diffusion of functionalised nanomaterials through mucosal tissue have not been reported. We investigated the influence of poly(2-oxazoline) alkyl side chain length on nanoparticle diffusion; poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline), and poly(2-n-propyl-2-oxazoline) were grafted onto the surface of thiolated silica nanoparticles and characterised by FT-IR, Raman and NMR spectroscopy, thermogravimetric analysis, and small angle neutron scattering. Diffusion coefficients were determined in water and in a mucin dispersion (using Nanoparticle Tracking Analysis), and penetration through a mucosal barrier was assessed using an ex vivo fluorescence technique. The addition of a single methylene group in the side chain significantly altered the penetration and diffusion of the materials in both mucin dispersions and mucosal tissue. Nanoparticles functionalised with poly(2-methyl-2-oxazoline) were significantly more diffusive than particles with poly(2-ethyl-2-oxazoline) while particles with poly(2-n-propyl-2-oxazoline) showed no significant increase compared to the unfunctionalised particles. These data show that variations in the polymer structure can radically alter their diffusive properties with clear implications for the future design of mucus penetrating systems.

Renewable (Bis)pyrrolidone Based Monomers as Components for Thermally Curable and Enzymatically Depolymerizable 2-Oxazoline Thermoset Resins

In this study we describe the synthesis of bis(pyrrolidone) based dicarboxylic acids from itaconic acid and their application in 2-oxazoline resins for fully renewable thermoset materials. The monomers are obtained using a bulk aza-Michael addition of a diamine and two itaconic acid molecules using a catalytic amount of water. The monomers can be isolated in high purity after recrystallization, though their yield proved to be highly dependent on the selected diamine spacer length: In general, only the dicarboxylic acids containing diamines with an even number of methylene spacers are isolated in high yields. Through NMR, GPC, and FTIR analysis we demonstrate that these bis(pyrrolidone) based dicarboxylic acids exhibit significantly enhanced curing rates in 2-oxazoline resins compared to resins containing aliphatic dicarboxylic acids such as sebacic acid. Overall, we demonstrate that the rate of 2-oxazoline ring-opening addition with carboxylic acid functionalities is determined by the used dicarboxylic acid, whereas the ring-opening addition of the 2-oxazoline functionality with amide groups is determined by the used bis(2-oxazoline) compound. The thermosets obtained after curing proved to be readily plasticized by water, opening up possibilities for enzymatic degradation.