AFM for Nanomechanical Assessment of Polymer Overcoatings on Nanoparticle-Decorated Biomaterials

Nanoparticle adhesion to polymer and similar substrates may be prone to low nano-Newton forces, disrupting the surface bonds and patterning, potentially reducing the functionality of complex surface patterns. Testing this, a functionalised surface reported for biological and medical applications, consisting of a thin plasma-derived oxazoline-based film with 68 nm diameter covalently bound colloidal gold nanoparticles attached within an aqueous solution, underwent nanomechanical analysis. Atomic Force Microscopy nanomechanical analysis was used to quantify the limits of various adaptations to these nanoparticle-featured substrates. Regular and laterally applied forces in the nano-Newton range were shown to de-adhere surface-bound gold nanoparticles. Applying a nanometre-thick overcoating anchored the nanoparticles to the surface and protected the underlying base substrate in a one-step process to improve the overall stability of the functionalised substrate against lower-range forces. The thickness of the oxazoline-based overcoating displayed protection from forces at different rates. Testing overcoating thickness ranging from 5 to 20 nm in 5 nm increments revealed a significant improvement in stability using a 20 nm-thick overcoating. This approach underscores the importance of optimising overcoating thickness to enhance nanoparticle-based surface modifications' durability and functional integrity.

Exploring the Challenges of Characterising Surface Topography of Polymer-Nanoparticle Composites

Nanomechanical testing plays a crucial role in evaluating surfaces containing nanoparticles. Testing verifies surface performance concerning their intended function and detects any potential shortcomings in operational standards. Recognising that nanostructured surfaces are not always straightforward or uniform is essential. The chemical composition and morphology of these surfaces determine the end-point functionality. This can entail a layered surface using materials in contrast to each other that may require further modification after nanomechanical testing to pass performance and quality standards. Nanomechanical analysis of a structured surface consisting of a poly-methyl oxazoline film base functionalised with colloidal gold nanoparticles was demonstrated using an atomic force microscope (AFM). AFM nanomechanical testing investigated the overall substrate architecture's topographical, friction, adhesion, and wear parameters. Limitations towards its potential operation as a biomaterial were also addressed. This was demonstrated by using the AFM cantilever to apply various forces and break the bonds between the polymer film and gold nanoparticles. The AFM instrument offers an insight to the behaviour of low-modulus surface against a higher-modulus nanoparticle. This paper details the bonding and reaction limitations between these materials on the application of an externally applied force. The application of this interaction is highly scrutinised to highlight the potential limitations of a functionalised surface. These findings highlight the importance of conducting comprehensive nanomechanical testing to address concerns related to fabricating intricate biomaterial surfaces featuring nanostructures.

Influence of Hydrophobic Side-Chain Length in Amphiphilic Gradient Copoly(2-oxazoline)s on the Therapeutics Loading, Stability, Cellular Uptake and Pharmacokinetics of Nano-Formulation with Curcumin

Due to the simple one-step preparation method and a promising application in biomedical research, amphiphilic gradient copoly(2-oxazoline)s are gaining more and more interest compared to their analogous block copolymers. In this work, the curcumin solubilization ability was tested for a series of amphiphilic gradient copoly(2-oxazoline)s with different lengths of hydrophobic side-chains, consisting of 2-ethyl-2-oxazoline as a hydrophilic monomer and 2-(4-alkyloxyphenyl)-2-oxazoline as a hydrophobic monomer. It is shown that the length of the hydrophobic side-chain in the copolymers plays a crucial role in the loading of curcumin onto the self-assembled nanoparticles. The kinetic stability of self-assembled nanoparticles studied using FRET shows a link between their integrity and cellular uptake in human glioblastoma cells. The present study demonstrates how minor changes in the molecular structure of gradient copoly(2-oxazoline)s can lead to significant differences in the loading, stability, cytotoxicity, cellular uptake, and pharmacokinetics of nano-formulations containing curcumin. The obtained results on the behavior of the complex of gradient copoly(2-oxazoline)s and curcumin may contribute to the development of effective next-generation polymeric nanostructures for biomedical applications.

Poly(2-isopropenyl-2-oxazoline) as a reactive polymer for materials development

Poly(2-isopropenyl-2-oxazoline) has attracted growing interest as a reactive polymer that can be used as a starting material for the construction of more complex structures.

On the identification and quantification of proton-initiated species in the synthesis of poly(2-alkyl-2-oxazoline)s by high resolution liquid chromatography

Hydrophilic poly(2-oxazoline)s represent a promising alternative to replace poly(ethylene glycol) in the biomedical field. For that purpose, reliable analytical protocols to confirm identity and quantity of impurities are required. In particular, side products deriving from chain transfer reactions occurring during the cationic ring-opening polymerization and incomplete end-capping processes may be present. The analytical approach must hence be capable of separating polymers according to minor changes regarding their end group. We demonstrate that liquid chromatography, relying on a monolithic C18-modified silica column and isocratic as well as gradient elution using water / acetonitrile mixtures and varying detectors, can accomplish such demanding high resolution separations. Poly(2-ethyl-2-oxazoline)s (PEtOx) with acetyl, hydroxyl, and phthalimide ω-end groups were investigated. Identification of side products was achieved through coupling with electrospray ionization mass spectrometry. UV / Vis detection was applied to quantify chain transfer products in PEtOx comprising biphenyl moieties. In addition, gradient elution enabled the separation of PEtOx into macromolecules according to their specific degrees of polymerization in molar mass ranges around 2,000 g mol^-1.

Copolymer chain formation of 2-oxazolines by in situ 1H-NMR spectroscopy: dependence of sequential composition on substituent structure and monomer ratios

In situ 1H NMR characterization of copolymerization reactions of various 2-oxazoline monomers at different molar ratios offers detailed insight into the build-up and composition of the polymer chains. Various 2-oxazolines were copolymerized in one single solvent, butyronitrile, with 2-dec-9'-enyl-2-oxazoline, where the double bond allows for post-polymerization modification and can function as a crosslinking unit to form polymer networks. The types of the monomers and their molar ratios in the feed have a strong effect on the microstructure of the forming copolymer chains. Copolymers comprising 2-dec-9'-enyl-2-oxazoline and either 2-ethyl-, 2-isopropyl-, 2-butyl-, 2-heptyl, 2-nonyl- or 2-phenyl-2-oxazoline, show significant differences in sequential structure of copolymers ranging from block to gradient and random ordering of the monomer units. 1H NMR was found to be a powerful tool to uncover detailed oxazoline copolymerization kinetics and evolution of chain composition.

Poly(2-oxazoline)- and Poly(2-oxazine)-Based Self-Assemblies, Polyplexes, and Drug Nanoformulations-An Update

For many decades, poly(2-oxazoline)s and poly(2-oxazine)s, two closely related families of polymers, have led the life of a rather obscure research topic with only a few research groups world-wide working with them. This has changed in the last five to ten years, presumably triggered significantly by very promising clinical trials of the first poly(2-oxazoline)-based drug conjugate. The huge chemical and structural toolbox poly(2-oxazoline)s and poly(2-oxazine)s has been extended very significantly in the last few years, but their potential still remains largely untapped. Here, specifically, the developments in macromolecular self-assemblies and non-covalent drug delivery systems such as polyplexes and drug nanoformulations based on poly(2-oxazoline)s and poly(2-oxazine)s are reviewed. This highly dynamic field benefits particularly from the extensive synthetic toolbox poly(2-oxazoline)s and poly(2-oxazine)s offer and also may have the largest potential for a further development. It is expected that the research dynamics will remain high in the next few years, particularly as more about the safety and therapeutic potential of poly(2-oxazoline)s and poly(2-oxazine)s is learned.

Synthesis, Structure, Hydrodynamics and Thermoresponsiveness of Graft Copolymer with Aromatic Polyester Backbone at Poly(2-isopropyl-2-oxazoline) Side Chains

New thermoresponsive graft copolymers with an aromatic polyester backbone and poly(2-isopropyl-2-oxazoline) (PiPrOx) side chains are synthesized and characterized by NMR and GPC. The grafting density of side chains is 0.49. The molar masses of the graft-copolymer, its backbone, side chains, and the modeling poly-2-isopropyl-2-oxaziline are 74,000, 19,000, 4300, and 16,600 g·mol⁻¹, respectively. Their conformational properties in nitropropane as well as thermoresponsiveness in aqueous solutions are studied and compared with that of free side chains, i.e., linear PiPrOx with a hydrophobic terminal group. In nitropropane, the graft-copolymer adopts conformation of a 13-arm star with a core of a collapsed main chain and a PiPrOx corona. Similarly, a linear PiPrOx chain protects its bulky terminal group by wrapping around it in a selective solvent. In aqueous solutions at low temperatures, graft copolymers form aggregates due to interaction of hydrophobic backbones, which contrasts to molecular solutions of the model linear PiPrOx. The lower critical solution temperature (LCST) for the graft copolymer is around 20 °C. The phase separation temperatures of the copolymer solution were lower than that of the linear chain counterpart, decreasing with concentration for both polymers.

Thiol-Substituted Poly(2-oxazoline)s with Photolabile Protecting Groups-Tandem Network Formation by Light

Herein, we present a novel polymer architecture based on poly(2-oxazoline)s bearing protected thiol functionalities, which can be selectively liberated by irradiation with UV light. Whereas free thiol groups can suffer from oxidation or other spontaneous reactions that degrade polymer performance, this strategy with masked thiol groups offers the possibility of photodeprotection on demand with spatio-temporal control while maintaining polymer integrity. Here, we exploit this potential for a tandem network formation upon irradiation with UV light by thiol deprotection and concurrent crosslinking via thiol-ene coupling. The synthesis of the novel oxazoline monomer 2-{2-[(2-nitrobenzyl)thio]ethyl}-4,5-dihydrooxazole (NbMEtOxa) carrying 2-nitrobenzyl-shielded thiol groups and its cationic ring-opening copolymerization at varying ratios with 2-ethyl-2-oxazoline (EtOxa) is described. The tandem network formation was exemplarily demonstrated with the photoinitator 2-hydroxy-2-methylpropiophenone (HMPP) and pentaerythritol tetraacrylate (PETA), a commercially available, tetrafunctional vinyl crosslinker. The key findings of the conducted experiments indicate that a ratio of ~10% NbMEtOxa repeat units in the polymer backbone is sufficient for network formation and in-situ gelation in N,N-dimethylformamide.

UV-Induced Cationic Ring-Opening Polymerization of 2-Oxazolines for Hot Lithography

The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.

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.

Nonspherical Nanoparticle Shape Stability Is Affected by Complex Manufacturing Aspects: Its Implications for Drug Delivery and Targeting

The shape of nanoparticles is known recently as an important design parameter influencing considerably the fate of nanoparticles with and in biological systems. Several manufacturing techniques to generate nonspherical nanoparticles as well as studies on in vitro and in vivo effects thereof have been described. However, nonspherical nanoparticle shape stability in physiological-related conditions and the impact of formulation parameters on nonspherical nanoparticle resistance still need to be investigated. To address these issues, different nanoparticle fabrication methods using biodegradable polymers are explored to produce nonspherical nanoparticles via the prevailing film-stretching method. In addition, systematic comparisons to other nanoparticle systems prepared by different manufacturing techniques and less biodegradable materials (but still commonly utilized for drug delivery and targeting) are conducted. The study evinces that the strong interplay from multiple nanoparticle properties (i.e., internal structure, Young's modulus, surface roughness, liquefaction temperature [glass transition (T_g ) or melting (T_m )], porosity, and surface hydrophobicity) is present. It is not possible to predict the nonsphericity longevity by merely one or two factor(s). The most influential features in preserving the nonsphericity of nanoparticles are existence of internal structure and low surface hydrophobicity (i.e., surface-free energy (SFE) > ≈55 mN m^-1 , material-water interfacial tension <6 mN m^-1 ), especially if the nanoparticles are soft (<1 GPa), rough (R_rms > 10 nm), porous (>1 m² g^-1 ), and in possession of low bulk liquefaction temperature (<100 °C). Interestingly, low surface hydrophobicity of nanoparticles can be obtained indirectly by the significant presence of residual stabilizers. Therefore, it is strongly suggested that nonsphericity of particle systems is highly dependent on surface chemistry but cannot be appraised separately from other factors. These results and reviews allot valuable guidelines for the design and manufacturing of nonspherical nanoparticles having adequate shape stability, thereby appropriate with their usage purposes. Furthermore, they can assist in understanding and explaining the possible mechanisms of nonspherical nanoparticles effectivity loss and distinctive material behavior at the nanoscale.

Colloidal Behavior of Cellulose Nanocrystals Grafted with Poly(2-alkyl-2-oxazoline)s

Polymer grafting onto cellulose nanocrystals (CNCs) has been used as a tool to improve CNC dispersion in nonpolar solvents or polymeric matrixes. The grafting of flexible polymer chains onto rigid particle surfaces leads to significant modifications in colloidal behavior. Here, poly(2-alkyl-2-oxazoline)s of well-defined molar mass and narrow molar mass distribution were synthesized by cationic ring-opening polymerization and grafted onto CNC surfaces, where the coupling reaction was favored when partially hydrolyzed polymers were used (reaching 64% reaction yield). The particles grafted with polymer chains could be redispersed in water after freeze-drying, producing stable dispersions, and they were not cell-toxic up to 10 wt % aqueous dispersion. Colloidal stability, nanostructure organization, and rheological behavior of grafted CNC and CNC-grafted CNC mixtures were evaluated. The rheological behavior of grafted nanoparticles, meanwhile, showed new features when compared to original CNC dispersions. Aqueous CNC dispersions showed a liquid crystal nematic organization and rheological behavior characteristic of true gel (at 5 wt %) prior to drying. On the other hand, nanoparticle dispersions behaved as weak gels upon the addition of 10 wt % of CNC-g-(PEtOx95-s-Ei5) under the same conditions. Dispersions of CNC-g-P(PEtOx-s-Ei) particles obtained by redispersion of freeze-dried particles behaved as a fluid, without the presence of the nematic organization. Through oscillatory rheology and time-domain NMR results, it can be concluded that polymer-water interactions are dominant over CNC-water interactions, being responsible for CNC nematic phase disruption. By introducing polymer chains, the introduction of isotropic character modifies water organization, changing the flow behavior of CNC-grafted with poly(oxazoline)s.

Polyoxazoline-Based Bottlebrush and Brush-Arm Star Polymers via ROMP: Syntheses and Applications as Organic Radical Contrast Agents

The synthesis of functional poly(2-alkyl-2-oxazoline) (PAOx) copolymers with complex nanoarchitectures using a graft-through ring-opening metathesis polymerization (ROMP) approach is described. First, well-defined norbornene-terminated poly(2-ethyl-2-oxazoline) (PEtOx) macromonomers (MM) were prepared by cationic ringopening polymerization. ROMP of these MMs produced bottlebrush copolymers with PEtOx side chains. In addition, PEtOx-based branched MMs bearing a terminal alkyne group were prepared and conjugated to an azide-containing bis-spirocyclohexyl nitroxide via Cu-catalyzed azide-alkyne cycloaddition (CuAAC). ROMP of this branched MM, followed by in situ cross-linking, provided PEtOx-based brush-arm star polymers (BASPs) with nitroxide radicals localized at the core-shell interface. These PEtOx-based nitroxide-containing BASPs displayed relaxivity values on par with state-of-the-art polyethylene glycol (PEG)-based nitroxide materials, making them promising as organic radical contrast agents for metal-free magnetic resonance imaging (MRI).

Robust and Biocompatible Functionalization of ZnS Nanoparticles by Catechol-Bearing Poly(2-methyl-2-oxazoline)s

Zinc sulfide (ZnS) nanoparticles (NPs) are particularly interesting materials for their electronic and luminescent properties. Unfortunately, their robust and stable functionalization and stabilization, especially in aqueous media, has represented a challenging and not yet completely accomplished task. In this work, we report the synthesis of colloidally stable, photoluminescent and biocompatible core-polymer shell ZnS and ZnS:Tb NPs by employing a water-in-oil miniemulsion (ME) process combined with surface functionalization via catechol-bearing poly-2-methyl-2-oxazoline (PMOXA) of various molar masses. The strong binding of catechol anchors to the metal cations of the ZnS surface, coupled with the high stability of PMOXA against chemical degradation, enable the formation of suspensions presenting excellent colloidal stability. This feature, combined with the assessed photoluminescence and biocompatibility, make these hybrid NPs suitable for optical bioimaging.

Poly(2-cycloalkyl-2-oxazoline)s: high melting temperature polymers solely based on Debye and Keesom van der Waals interactions

The synthesis of new 2-cycloalkyl-2-oxazoline monomers, namely 2-cyclobutyl (cBuOx), 2-cyclopentyl (cPentOx) and 2-cyclohexyl-2-oxazoline (cHexOx) is described. Polymers thereof are semi-crystalline and reveal unexpectedly high melting transitions.

Block copolymers based on 2-methyl- and 2-phenyl-oxazoline by metallocene-mediated cationic ring-opening polymerization: synthesis and characterization

The cationic polymerization of oxazolines, lactones and vinyl ethers can be efficiently promoted by metallocene complexes activated by floroaryl borates.

Maleimide-functionalized poly(2-ethyl-2-oxazoline): synthesis and reactivity

Poly(2-ethyl-2-oxazoline)s end-functionalized with a maleimide moiety were prepared from azide-terminated PEtOx_x-N₃viacopper-catalyzed azide–alkyne cycloaddition (CuAAC) with an alkyne-bearing maleimide (MI).

Microwave-assisted cationic ring-opening polymerization of 2-oxazolines

Unlike any other polymer class, the (co-)poly(2-oxazoline)s have tremendously benefited from the introduction of microwave reactors into chemical laboratories. This review focuses on the research activities in the area of (co-)poly(2-oxazoline)s prepared by microwave-assisted syntheses and, correspondingly, summarizes the current-state-of the-art of the microwave-assisted synthesis of 2-oxazoline monomers and the microwave-assisted ring-opening (co-)polymerization of 2-oxazolines as well as prominent examples of post-polymerization modification of (co-)poly(2-oxazoline)s. Special attention is attributed to the kinetic analysis of the microwave-assisted polymerization of 2-oxazolines and the discussion of non-thermal microwave effects.

Compartmentalization of Incompatible Catalytic Transformations for Tandem Catalysis

In Nature, incompatible catalytic transformations are being carried out simultaneously through compartmentalization that allows for the combination of incompatible catalysts in tandem reactions. Herein, we take the compartmentalization concept to the synthetic realm and present an approach that allows two incompatible transition metal catalyzed transformations to proceed in one pot in tandem. The key is the site isolation of both catalysts through compartmentalization using a core-shell micellar support in an aqueous environment. The support is based on amphiphilic triblock copolymers of poly(2-oxazoline)s with orthogonal functional groups on the side chain that can be used to cross-link covalently the micelle and to conjugate two metal catalysts in different domains of the micelle. The micelle core and shell provide different microenvironments for the transformations: Co-catalyzed hydration of an alkyne proceeds in the hydrophobic core, while the Rh-catalyzed asymmetric transfer hydrogenation of the intermediate ketone into a chiral alcohol occurs in the hydrophilic shell.

Crystallisation-driven self-assembly of poly(2-isopropyl-2-oxazoline)-block-poly(2-methyl-2-oxazoline) above the LCST

The solution behaviour in water of a polyoxazoline-type block copolymer, namely poly(2-isopropyl-2-oxazoline)-block-poly(2-methyl-2-oxazoline), denoted as P(iPrOx-b-MeOx), above the lower critical solution temperature (LCST) of the PiPrOx block was exploited to induce a temporary or permanent self-assembly. Spherical micelles were first obtained and could be disassembled in a reversible manner when kept for a short period of time (i.e. t < 90 min) above the LCST, and cooled down to room temperature. In contrast, annealing the copolymer solution for more than 90 min at 65 °C induced the crystallisation of the PiPrOx block, as evidenced by wide angle X-ray scattering (WAXS) experiments. This crystallisation-driven self-assembly phenomenon resulted in different morphologies, including spherical and distorted crystallised micelles and micron-size fibers, their relative proportion varies with the annealing time. Formation of micron-size range fiber-like structures might be explained by the re-organization of parent crystallised micelles. The crystal structure, as determined by WAXS, appeared to be identical to that of the PiPrOx homopolymer.

Linear polyethylenimine as (multi) functional initiator for organocatalytic l-lactide polymerization

The preparation of polyethylenimine (PEI)–polylactide (PLA) copolymer structures is promising as these materials may find use in gene and/or drug delivery applications.

Evaluation of poly(2-ethyl-2-oxazoline) containing copolymer networks of varied composition as sustained metoprolol tartrate delivery systems

Segmented copolymer networks (SCN) based on poly(2-ethyl-2-oxazoline) and containing 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, and/or methyl methacrylate segments have been evaluated as potential sustained release systems of the water soluble cardioselective β-blocker metoprolol tartrate. The structure and properties of the drug carriers were investigated by differential scanning calorimetry, attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. Swelling kinetics of SCNs in various media was followed, and the conditions for effective MT loading were specified. MT-loaded SCNs with drug content up to 80 wt.% were produced. The release kinetics of metoprolol tartrate from the systems was studied and it was shown that the conetworks of different structure and composition are able to sustain the metoprolol tartrate release without additional excipients.

Living cationic ring-opening polymerization of 2-oxazolines initiated by rare-earth metal triflates

Lewis acidic rare-earth metal triflates were found to efficiently initiate the cationic ring-opening polymerization of 2-oxazolines in a multi-site propagating fashion.

Cationic Ring-Opening Polymerization of 2-Propyl-2-oxazolines: Understanding Structural Effects on Polymerization Behavior Based on Molecular Modeling

The surprising difference in the cationic ring-opening polymerization rate of 2-cyclopropyl-2-oxazoline versus 2-n-propyl-2-oxazoline and 2-isopropyl-2-oxazoline was investigated both experimentally and theoretically. The polymerization kinetics of all three oxazolines were experimentally measured in acetonitrile at 140 °C, and the polymerization rate constant (k_p) was found to decrease in the order c-PropOx > n-PropOx > i-PropOx. Theoretical free energy calculations confirmed the trend for k_p, and a set of DFT-based reactivity descriptors, electrostatics, and frontier molecular orbitals were studied to detect the factors controlling this peculiar behavior. Our results show that the observed reactivity is dictated by electrostatic effects. More in particular, the charge on the nitrogen atom of the monomer, used to measure its nucleophilicity, was the most negative for c-PropOx. Furthermore, the electrophilicity of the cations does not change substantially, and thus, the nucleophilicity of the monomers is the driving factor for k_p.