Thermal Properties of Methyl Ester-Containing Poly(2-oxazoline)s

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.

Bactericidal Anti-Adhesion Potential Integrated Polyoxazoline/Silver Nanoparticle Composite Multilayer Film with pH Responsiveness

Bacterial infections occur frequently during the implantation of medical devices, and functional coating is one of the effective means to prevent and remove biofilms. In this study, three different hydrophilic polyoxazolines with carboxyl groups (aPOx: PT1, PT2 and PT3) and bactericidal silver nanoparticles (AgNPs) were synthesized successfully, and an aPOx-AgNP multilayer film was prepared by electrostatic layer-by-layer self-assembly. The effect of charge density and assembly solution concentration was explored, and the optimal self-assembly parameters were established (PT2 1 mg/mL and AgNPs 3 mg/mL). The hydrophilicity of the surface can be enhanced to resist protein adhesion if the outermost layer is aPOx, and AgNPs can be loaded to kill bacteria, thereby realizing the bactericidal anti-adhesion potential integration of the aPOx-AgNP multilayer film. In addition, the aPOx-AgNP multilayer film was found to have the characteristic of intelligent and efficient pH-responsive silver release, which is expected to be used as a targeted anti-biofilm surface of implantable medical devices.

Nucleobase-crosslinked poly(2-oxazoline) nanoparticles as paclitaxel carriers with enhanced stability and ultra-high drug loading capacity for breast cancer therapy

Poly(2-oxazoline) (POx) has been regarded as a potential candidate for drug delivery carrier to meet the challenges of nanomedicine clinical translation, due to its excellent biocompatibility and self-assembly properties. The drug loading capacity and stability of amphiphilic POxs as drug nanocarriers, however, tend to be insufficient. Herein, we report a strategy to prepare nucleobase-crosslinked POx nanoparticles (NPs) with enhanced stability and ultra-high paclitaxel (PTX) loading capacity for breast cancer therapy. An amphiphilic amine-functionalized POx (PMBEOx-NH₂) was firstly prepared through a click reaction between cysteamines and vinyl groups in poly(2-methyl-2-oxazoline)-block-poly (2‑butyl‑2-oxazoline-co-2-butenyl-2-oxazoline) (PMBEOx). Complementary nucleobase-pairs adenine (A) and uracil (U) were subsequently conjugated to PMBEOx-NH₂ to give functional POxs (POxA and POxU), respectively. Due to the nucleobase interactions formed between A and U, NPs formed by POxA and POxU at a molar ratio of 1:1 displayed ultrahigh PTX loading capacity (38.2%, PTX/POxA@U), excellent stability, and reduced particle size compared to the uncross-linked PTX-loaded NPs (PTX/PMBEOx). Besides the prolonged blood circulation and enhanced tumor accumulation, the smaller PTX/POxA@U NPs also have better tumor penetration ability compared with PTX/PMBEOx, thus leading to a higher tumor suppression rate in two murine breast cancer models (E0711 and 4T1). These results proved that the therapeutic effect of chemotherapeutic drugs could be improved remarkably through a reasonable optimization of nanocarriers.

Mutual Effects of Hydrogen Bonding and Polymer Hydrophobicity on Ibuprofen Crystal Inhibition in Solid Dispersions with Poly(N-vinyl pyrrolidone) and Poly(2-oxazolines)

Poly(N-vinyl pyrrolidone) (PVP), poly(2-methyl-2-oxazoline) (PMOZ), poly(2-ethyl-2-oxazoline) (PEOZ), poly(2-n-propyl-2-oxazoline) (PnPOZ), and poly(2-isopropyl-2-oxazoline) (PiPOZ) were used to prepare solid dispersions with ibuprofen (IB), a model poorly-water soluble drug. Dispersions, prepared by solvent evaporation, were investigated using powder X-ray diffractometry, differential scanning calorimetry, and FTIR spectroscopy; hydrogen bonds formed between IB and all polymers in solid dispersions. PMOZ, the most hydrophilic polymer, showed the poorest ability to reduce or inhibit the crystallinity of IB. In contrast, the more hydrophobic polymers PVP, PEOZ, PnPOZ, and PiPOZ provided greater but similar abilities to reduce IB crystallinity, despite the differing polymer hydrophobicity and that PiPOZ is semi-crystalline. These results indicate that crystallinity disruption is predominantly due to hydrogen bonding between the drug molecules and the polymer. However, carrier properties affected drug dissolution, where PnPOZ exhibited lower critical solution temperature that inhibited the release of IB, whereas drug release from other systems was consistent with the degree of ibuprofen crystallinity within the dispersions.

Poly(2-allylamidopropyl-2-oxazoline)-Based Hydrogels: From Accelerated Gelation Kinetics to In Vivo Compatibility in a Murine Subdermal Implant Model

A rapid photo-curing system based on poly(2-ethyl-2-oxazoline-co-2-allylamidopropyl-2-oxazoline) and its in vivo compatibility are presented. The base polymer was synthesized from the copolymerization of 2-ethyl-2-oxazoline (EtOx) and the methyl ester containing 2-methoxycarboxypropyl-2-oxazoline (C₃MestOx) followed by amidation with allylamine to yield a highly water-soluble macromer. We showed that spherical hydrogels can be obtained by a simple water-in-oil gelation method using thiol-ene coupling and investigated the in vivo biocompatibility of these hydrogel spheres in a 28-day murine subdermal model. For comparison, hydrogel spheres prepared from poly(ethylene glycol) were also implanted. Both materials displayed mild, yet typical foreign body responses with little signs of fibrosis. This is the first report on the foreign body response of a poly(2-oxazoline) hydrogel, which paves the way for future investigations into how this highly tailorable class of materials can be used for implantable hydrogel devices.

Adamantane Functionalized Poly(2-oxazoline)s with Broadly Tunable LCST-Behavior by Molecular Recognition

Smart or adaptive materials often utilize stimuli-responsive polymers, which undergo a phase transition in response to a given stimulus. So far, various stimuli have been used to enable the modulation of drug release profiles, cell-interactive behavior, and optical and mechanical properties. In this respect, molecular recognition is a powerful tool to fine-tune the stimuli-responsive behavior due to its high specificity. Within this contribution, a poly(2-oxazoline) copolymer bearing adamantane side chains was synthesized via triazabicyclodecene-catalyzed amidation of the ester side chains of a poly(2-ethyl-2-oxazoline-stat-2-methoxycarbonylpropyl-2-oxazoline) statistical copolymer. Subsequent complexation of the pendant adamantane groups with sub-stoichiometric amounts (0-1 equivalents) of hydroxypropyl β-cyclodextrin or β-cyclodextrin enabled accurate tuning of its lower critical solution temperature (LCST) over an exceptionally wide temperature range, spanning from 30 °C to 56 °C. Furthermore, the sharp thermal transitions display minimal hysteresis, suggesting a reversible phase transition of the complexed polymer chains (i.e., the β-cyclodextrin host collapses together with the polymers) and a minimal influence by the temperature on the supramolecular association. Analysis of the association constant of the polymer with hydroxypropyl β-cyclodextrin via 1H NMR spectroscopy suggests that the selection of the macrocyclic host and rational polymer design can have a profound influence on the observed thermal transitions.

Thermoresponsive Poly(2-propyl-2-oxazoline) Surfaces of Glass for Nonenzymatic Cell Harvesting

As one of the nonenzymatic cell-harvesting technologies, a thermal-responsive surface based on poly(2-oxazoline)s has achieved initial success in supporting the adhesion and thermal-induced detachment of animal cells. However, because of the laborious preparation procedure, this technique was only limited to research purposes. In this work, through using poly(glycidyl methacrylate) (PGMA) as the anchor layer, poly(2-propyl-2-oxazoline)s (PPOx) were grafted onto glass wafers through a facile two-step coating and annealing procedure for nonenzymatic cell harvesting. In the first step, the piranha solution-activated glass wafers were immersed into the chloroform solution of PGMA and then annealed for a given period of time to immobilize PGMA onto the glass wafers through the bonding between epoxy groups and hydroxyl groups. In the second step, the PGMA-coated glass wafers were further immersed into the chloroform solution of carboxyl-functionalized PPOx. After annealing, PPOx were immobilized onto the PGMA layer through the bonding between carboxyl groups and the residual epoxy groups. Atomic force microscopy, X-ray photoelectron spectroscopy, and ellipsometry were used to characterize the modified glass wafers. The results of cytocompatibility evaluation showed that the PPOx-coated glass wafers were almost nontoxic and were able to support the adhesion and proliferation of L929 cells well. By lowering the temperature to 8 °C, L929 and Vero cells were successfully detached from the PPOx-coated glass wafers without any enzymatic treatment. Further cultivation has demonstrated that the cooling procedure had little effect on cell viability, and the cells still retained good viability after harvesting.

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.

Drug–polymer conjugates with dynamic cloud point temperatures based on poly(2-oxazoline) copolymers

A shift in cloud point temperatures of poly(2-oxazoline)/ACE inhibitor polymer drug conjugates occurs on release of the drug.

Novel poly(2-oxazoline) block copolymer with aromatic heterocyclic side chains as a drug delivery platform

Here we report a novel poly(2-oxazoline)-based block copolymer with the aromatic heterocyclic side chains in one block, poly(2-methyl-2-oxazoline)-b-poly(2-N,N-dimethyl-1,3,5-triazine-2,4-diamine-6-ethyl-2-oxazoline) (PMeOx-PcBOx), and demonstrate its potential application as a drug delivery platform. The copolymer was synthesized via the condensation of N,N-dimethylbiguanide with the methyl ester side chain in poly(2-methoxycarboxyethyl-2-oxazoline) block (PMestOx) of the PMeOx-PMestOx diblock copolymer. We confirmed the N,N-dimethylbiguanide condensation with PMestOx and the complete conversion of the side chain to the N,N-dimethyl-1,3,5-triazine-2,4-diamine-6-ethyl moiety by NMR spectroscopy, MALDI-TOF mass spectroscopy, UV-Vis spectroscopy, and titration analysis. The PMeOx-PcBOx copolymer self-assemble into polymeric micelles in aqueous solution. Successful encapsulation into these micelles has been demonstrated for 1) several poorly soluble drugs, such as bruceantin and LY2109761, and 2) dichloro(1,2-diaminocyclohexane)platinum(II) (DachPt). The first class of drugs is incorporated possibly via hydrogen bonding and pi-pi interactions with the PcBOx side groups, while the second one is likely forms coordination bonds with the same side groups. The capability of this new copolymer to solubilize a uniquely diverse set of active pharmaceutical ingredients suggests potential applications in drug delivery.

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).

More Is Sometimes Less: Curcumin and Paclitaxel Formulations Using Poly(2-oxazoline) and Poly(2-oxazine)-Based Amphiphiles Bearing Linear and Branched C9 Side Chains

A known limitation of polymer micelles for the formulation of hydrophobic drugs is their low loading capacity (LC), which rarely exceeds 20 wt%. One general strategy to overcome this limitation is to increase the amphiphilic contrast, that is, to make the hydrophobic core of the micelles more hydrophobic. However, in the case of poly(2-oxazoline) (POx)-based amphiphilic triblock copolymers, a minimal amphiphilic contrast was reported to be beneficial. Here, this subject is revisited in more detail using long hydrophobic side chains that are either linear (nonyl) or branched (3-ethylheptyl). Two different backbones within the hydrophobic block are investigated, in particular POx and poly(2-oxazine) (POzi), for the solubilization and co-solubilization of the two highly water insoluble compounds, curcumin and paclitaxel. Even though high loading capacities can be achieved for curcumin using POzi-based triblock copolymers, the solubilization capacity of all investigated polymers with longer side chains is significantly lower compared to POx and poly(2-oxazine)s with shorter side chains. Although the even lower LC for paclitaxel can be somehow improved by co-formulating curcumin, this study corroborates that in the case of POx and POzi-based polymer micelles, an increased amphiphilic contrast leads to less drug solubilization.

Poly(2-oxazoline)s with pendant cubane groups

The synthesis and properties of poly(2-oxazoline)s with cubane side chains are explored in this work.

Poly(2-oxazoline)-Pterostilbene Block Copolymer Nanoparticles for Dual-Anticancer Drug Delivery

Functional block copolymers based on poly(2-oxazoline)s are versatile building blocks for the fabrication of dual-drug delivery nanoparticles (NPs) for anticancer chemotherapy. Core-shell NPs are fabricated from diblock copolymers featuring a long and hydrophilic poly(2-methyl-2-oxazoline) (PMOXA) block coupled to a relatively short and functionalizable poly(2-methylsuccinate-2-oxazoline) (PMestOXA) segment. The PMOXA block stabilizes the NP dispersions, whereas the PMestOXA segment is used to conjugate pterostilbene, a natural bioactive phenolic compound that is used as lipophilic model-drug and constitutes the hydrophobic core of the designed NPs. Subsequent loading of the NPs with clofazimine (CFZ), an inhibitor of the multidrug resistance pumps typically expressed in a large variety of cancer cells, provides an additional function to their formulation. Optimization of the copolymer composition allows the design of polymer scaffolds showing low toxicity and capable of assembling into highly stable NPs dispersions at physiologically relevant pH. In addition, the incorporation of CFZ increases the stability of the NPs and stimulates their internalization by RAW 264.7 cells.

Thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides

Recent advances in thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides, with a specific focus on structure–property relationships, self-assembly, and applications, are reviewed.