Facile Generation of Heterotelechelic Poly(2-Oxazoline)s Towards Accelerated Exploration of Poly(2-Oxazoline)-Based Nanomedicine

Controlling the end-groups of biocompatible polymers is crucial for enabling polymer-based therapeutics and nanomedicine. Typically, end-group diversification is a challenging and time-consuming endeavor, especially for polymers prepared via ionic polymerization mechanisms with limited functional group tolerance. In this study, we present a facile end-group diversification approach for poly(2-oxazoline)s (POx), enabling quick and reliable production of heterotelechelic polymers to facilitate POxylation. The approach relies on the careful tuning of reaction parameters to establish differential reactivity of a pentafluorobenzyl initiator fragment and the living oxazolinium chain-end, allowing the selective introduction of N-, S-, O-nucleophiles via the termination of the polymerization, and a consecutive nucleophilic para-fluoro substitution. The value of this approach for the accelerated development of nanomedicine is demonstrated through the synthesis of well-defined lipid-polymer conjugates and POx-polypeptide block-copolymers, which are well-suited for drug and gene delivery. Furthermore, we investigated the application of a lipid-POx conjugate for the formulation and delivery of mRNA-loaded lipid nanoparticles for immunization against the SARS-COV-2 virus, underscoring the value of POx as a biocompatible polymer platform.

Solid-Phase Synthesis as a Tool to Create Exactly Defined, Branched Polymer Vectors for Cell Membrane Targeting

Modern drug formulations often require, besides the active drug molecule, auxiliaries to enhance their pharmacological properties. Tailor-made, biocompatible polymers covalently connected to the drug molecule can fulfill this function by increasing its solubility, reducing its toxicity, and guiding it to a specific target. If targeting membrane-bound proteins, localization of the drug close to the cell membrane and its target is beneficial to increase drug efficiency and residence time. In this study, we present the synthesis of highly defined, branched polymeric structures with membrane-binding properties. One to three hydrophilic poly(ethylene oxide) or poly(2-ethyloxazoline) side chains were connected via a peptoid backbone using a two-step iterative protocol for solid-phase peptoid synthesis. Additional groups, e.g., a hydrophobic anchor for membrane attachment, were introduced. Due to the nature of solid-phase synthesis, the number and order of the side chains and additional units can be precisely defined. The method proved to be versatile for the generation of multifunctional, branched polymeric structures of molecular weights up to approximately 7000 g mol-1. The behavior of all compounds towards biological membranes and cells was investigated using liposomes as cell membrane models, HEK293 and U251-MG cell lines, and red blood cells, thereby demonstrating their potential value as drug auxiliaries with cell membrane affinity.

Hydrolysis of hydrophobic poly(2-oxazoline)s and their subsequent modification via aza-Michael addition

Partially hydrolysed poly(2-oxazoline)s possess unique properties. However, much of the focus in this area has been on water soluble poly(2-oxazoline)s. Where hydrophobic poly(2-oxazoline)s have been used, this is often for selective hydrolysis. However, hydrolysis of very hydrophobic polymers could lead to interesting solution behaviour. Herein, we describe universal conditions for the hydrolysis of poly(2-alkyl-2-oxazoline)s suitable for both hydrophobic and hydrophilic 2-oxazolines. We show that the system utilised gives comparable rates to that of water alone for poly(2-ethyl-2-oxazoline). In addition, poly(2-fatty acid-2-oxazoline) was hydrolysed using the developed system and was found to proceed in a controlled manner allowing the targeting of specific degrees of hydrolysis, albeit much slower than for poly(2-ethyl-2-oxazoline). Finally, we demonstrate the partial functionalisation of poly(2-oxazoline)-poly(ethylene imine) co-polymers via aza-Michael addition.

Living Cationic Ring-Opening Polymerization of Hetero Diels-Alder Adducts to Give Multifactor-Controlled and Fast-Photodegradable Vinyl Polymers

Precise control of multiple structural parameters associated with vinyl polymers is important for producing materials with the desired properties and functions. While the development of living polymerization methods has provided a way to control the various structural parameters of vinyl polymers, the concomitant control of their sequence and regioregularity remains a challenging task. To overcome this challenge, herein, we report the living cationic ring-opening polymerization of hetero Diels-Alder adducts. The scalable and modular synthesis of the cyclic monomers was achieved by a one-step protocol using readily available vinyl precursors. Subsequently, living polymerization of the cyclic monomers was examined, allowing the synthesis of vinyl polymers while controlling multiple factors, including molecular weight, dispersity, alternating sequence, head-to-head regioregularity, and end-group functionality. The living characteristics of the developed method were further demonstrated by block copolymerization. The synthesized vinyl polymers exhibited unique thermal properties and underwent fast photodegradation even under sunlight.

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.

Investigation of cationic ring-opening polymerization of 2-oxazolines in the "green" solvent dihydrolevoglucosenone

For about the last ten years, poly(2-oxazoline)s have attracted significant attention as potential material for biomedical applications in, e.g., drug delivery systems, tissue engineering and more. Commonly, the synthesis of poly(2-oxazoline)s involves problematic organic solvents that are not ideal from a safety and sustainability point of view. In this study, we investigated the cationic ring-opening polymerization of 2-ethyl-2-oxazoline and 2-butyl-2-oxazoline using a variety of initiators in the recently commercialized "green" solvent dihydrolevoglucosenone (DLG). Detailed ¹H NMR spectroscopic analysis was performed to understand the influence of the temperature and concentration on the polymerization process. Size exclusion chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were performed to determine the molar mass of the resulting polymers. Our work shows clearly that the solvent is not inert under the conditions typically used for the cationic ring-opening polymerization, as evidenced by side products and limited control over the polymerization. However, we could establish that the use of the 2-ethyl-3-methyl-2-oxazolinium triflate salt as an initiator at 60 °C results in polymers with a relatively narrow molar mass distribution and a reasonable control over the polymerization process. Further work will be necessary to establish whether a living polymerization can be achieved by additional adjustments.

Grafting density and antifouling properties of poly[N-(2-hydroxypropyl) methacrylamide] brushes prepared by “grafting to” and “grafting from”

Poly(HPMA) brushes prepared by a grafting-from method suppress fouling from blood plasma by an order of magnitude better than the polymer brushes of the same molecular weight prepared by a grafting-to method.

Nitrile-Functionalized Poly(2-oxazoline)s as a Versatile Platform for the Development of Polymer Therapeutics

In recent years, polymers bearing reactive groups have received significant interest for biomedical applications. Numerous functional polymer platforms have been introduced, which allow for the preparation of materials with tailored properties via post-polymerization modifications. However, because of their reactivity, many functional groups are not compatible with the initial polymerization. The nitrile group is a highly interesting and relatively inert functionality that has mainly received attention in radical polymerizations. In this Article, a nitrile-functionalized 2-oxazoline monomer (2-(4-nitrile-butyl)-2-oxazoline, BuNiOx) is introduced, and its compatibility with the cationic ring-opening polymerization is demonstrated. Subsequently, the versatility of nitrile-functionalized poly(2-oxazoline)s (POx) is presented. To this end, diverse (co)polymers are synthesized and characterized by nuclear resonance spectroscopy, size-exclusion chromatography, and mass spectrometry. Amphiphilic block copolymers are shown to efficiently encapsulate the hydrophobic drug curcumin (CUR) in aqueous solution, and the anti-inflammatory effect of the CUR-containing nanostructures is presented in BV-2 microglia. Furthermore, the availability of the BuNiOx repeating units for post-polymerization modifications with hydroxylamine to yield amidoxime (AO)-functionalized POx is demonstrated. These AO-containing POx were successfully applied for the complexation of Fe(III) in a quantitative manner. In addition, AO-functionalized POx were shown to release nitric oxide intracellularly in BV-2 microglia. Thus nitrile-functionalized POx represent a promising and robust platform for the design of polymer therapeutics for a wide range of applications.

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.

Inspired by mussel adhesive protein: hydrophilic cationic copoly(2-oxazoline)s carrying catecholic side chains

The cationic ring-opening copolymerization of 2-ethyl-2-oxazoline and 3,4-dimethoxyaryl-substituted 2-oxazolines yields gradient or random copolymers, which can be converted into adhesive copolymers carrying catecholic and cationic groups.

Poly(2-methyl-2-oxazoline) conjugates with doxorubicin: From synthesis of high drug loading water-soluble constructs to in vitro anti-cancer properties

Poly(2-oxazoline)s represent an emerging class of polymers with increasing potential in biomedical sciences. To date, most of the work on poly(2-oxazoline)-drug conjugates focused on poly(2-ethyl-2-oxazoline) (PEtOx), a biocompatible water-soluble polymer with biological properties similar to polyethylene glycol. However, the more hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) shows better anti-fouling properties than PEtOx and thus indicates greater potential for the construction of polymer therapeutics. Herein, we synthesized for the first time a drug delivery system based on a linear PMeOx with a molar mass that is high enough (40 kDa) to exploit passive accumulation in the tumor by the enhanced permeation and retention effect. The anti-cancer drug doxorubicin is attached to the polymer carrier via an acid-sensitive hydrazone bond, which allows its pH-triggered release in the tumor. The in vitro study demonstrates successful cellular uptake of the PMeOx-doxorubicin conjugate via clathrin-mediated endocytosis, pH-sensitive drug release and high cytotoxicity against B16 melanoma cells. Finally, these properties were critically compared to the analogous systems based on the established PEtOx revealing that the more hydrophilic PMeOx carrier outperforms PEtOx in most of the parameters, showing higher maximal drug loading, superior cellular uptake, better anti-fouling properties, as well as improved in vitro anti-cancer efficiency. The study demonstrates the potential of PMeOx as a versatile platform for synthesis of new drug delivery systems.

Poly(2-amino-2-oxazoline)s: a new class of thermoresponsive polymers

In this report, we describe the synthesis and properties of poly(2-dialkylamino-2-oxazoline)s (PAmOx), a new class of thermoresponsive polymers.

Thermoresponsive hydrogels formed by poly(2-oxazoline) triblock copolymers

Hydrogels are useful materials for drug delivery and tissue engineering. Here, we report the importance of controlling block lengths for making thermoresponsive hydrogels based on ABA triblock copolymers with thermoresponsive outer blocks.