Inorganic Bottlebrush and Comb Polymers as a Platform for Supersoft, Solvent-Free Elastomers

Due to their unique rheological and mechanical properties, bottlebrush polymers are inimitable components of biological and synthetic systems such as cartilage and ultrasoft elastomers. However, while their rheological properties can be precisely controlled through their macromolecular structures, the current chemical spectrum available is limited to a handful of synthetic polymers with aliphatic carbon backbones. Herein we design and synthesize a series of inorganic bottlebrush polymers based on a unique combination of polydimethylsiloxane (PDMS) and polyphosphazene (PPz) chemistry. This non-carbon-based platform allows for simple variation of the significant architectural dimensions of bottlebrush-polymer-based elastomers. Grafting PDMS to PPz and vice versa also allows us to further exploit the unique properties of these polymers combined in a single material. These novel hybrid bottlebrush polymers were cured to give supersoft, solvent-free elastomers. We systematically studied the effect of architectural parameters and chemical functionality on their rheological properties. Besides forming supersoft elastomers, the energy dissipation characteristics of the elastomers were observed to be considerably higher than those for PDMS-based elastomers. Hence this work introduces a robust synthetic platform for solvent-free supersoft elastomers with potential applications as biomimetic damping materials.

Superfluorinated, Highly Water-Soluble Polyphosphazenes as Potential 19F Magnetic Resonance Imaging (MRI) Contrast Agents

"Hot spot" 19F magnetic resonance imaging (MRI) has garnered significant attention recently for its ability to image various disease markers quantitatively. Unlike conventional gadolinium-based MRI contrast agents, which rely on proton signal modulation, 19F-MRI's direct detection has a unique advantage in vivo, as the human body exhibits a negligible background 19F-signal. However, existing perfluorocarbon (PFC) or PFC-based contrast materials suffer from several limitations, including low longitudinal relaxation rates and relatively low imaging efficiency. Hence, we designed a macromolecular contrast agent featuring a high number of magnetically equivalent 19F-nuclei in a single macromolecule, adequate fluorine nucleus mobility, and excellent water solubility. This design utilizes superfluorinated polyphosphazene (PPz) polymers as the 19F-source; these are modified with sodium mercaptoethanesulfonate (MESNa) to achieve water solubility exceeding 360 mg/mL, which is a similar solubility to that of sodium chloride. We observed substantial signal enhancement in MRI with these novel macromolecular carriers compared to non-enhanced surroundings and aqueous trifluoroacetic acid (TFA) used as a positive control. In conclusion, these novel water-soluble macromolecular carriers represent a promising platform for future MRI contrast agents.

Phosphorus and Silicon-Based Macromolecules as Degradable Biomedical Polymers

Synthetic polymers are indispensable in biomedical applications because they can be fabricated with consistent and reproducible properties, facile scalability, and customizable functionality to perform diverse tasks. However, currently available synthetic polymers have limitations, most notably when timely biodegradation is required. Despite there being, in principle, an entire periodic table to choose from, with the obvious exception of silicones, nearly all known synthetic polymers are combinations of carbon, nitrogen, and oxygen in the main chain. Expanding this to main-group heteroatoms can open the way to novel material properties. Herein the authors report on research to incorporate the chemically versatile and abundant silicon and phosphorus into polymers to induce cleavability into the polymer main chain. Less stable polymers, which degrade in a timely manner in mild biological environments, have considerable potential in biomedical applications. Herein the basic chemistry behind these materials is described and some recent studies into their medical applications are highlighted.

Degradable Bottlebrush Polypeptides and the Impact of their Architecture on Cell Uptake, Pharmacokinetics, and Biodistribution In Vivo

Bottlebrush polymers are highly promising as unimolecular nanomedicines due to their unique control over the critical parameters of size, shape and chemical function. However, since they are prepared from biopersistent carbon backbones, most known bottlebrush polymers are non-degradable and thus unsuitable for systemic therapeutic administration. Herein, we report the design and synthesis of novel poly(organo)phosphazene-g-poly(α-glutamate) (PPz-g-PGA) bottlebrush polymers with exceptional control over their structure and molecular dimensions (Dh ≈ 15-50 nm). These single macromolecules show outstanding aqueous solubility, ultra-high multivalency and biodegradability, making them ideal as nanomedicines. While well-established in polymer therapeutics, it has hitherto not been possible to prepare defined single macromolecules of PGA in these nanosized dimensions. A direct correlation was observed between the macromolecular dimensions of the bottlebrush polymers and their intracellular uptake in CT26 colon cancer cells. Furthermore, the bottlebrush macromolecular structure visibly enhanced the pharmacokinetics by reducing renal clearance and extending plasma half-lives. Real-time analysis of the biodistribution dynamics showed architecture-driven organ distribution and enhanced tumor accumulation. This work, therefore, introduces a robust, controlled synthesis route to bottlebrush polypeptides, overcoming limitations of current polymer-based nanomedicines and, in doing so, offers valuable insights into the influence of architecture on the in vivo performance of nanomedicines.

Amino Acid-Based Polyphosphorodiamidates with Hydrolytically Labile Bonds for Degradation-Tuned Photopolymers

Photochemical additive manufacturing technologies can produce complex geometries in short production times and thus have considerable potential as a tool to fabricate medical devices such as individualized patient-specific implants, prosthetics and tissue engineering scaffolds. However, most photopolymer resins degrade only slowly under the mild conditions required for many biomedical applications. Herein we report a novel platform consisting of amino acid-based polyphosphorodiamidate (APdA) monomers with hydrolytically cleavable bonds. The substituent on the α-amino acid can be used as a handle for facile control of hydrolysis rates of the monomers into their endogenous components, namely phosphate and the corresponding amino acid. Furthermore, monomer hydrolysis is considerably accelerated at lower pH values. The monomers underwent thiol-yne photopolymerization and could be 3D structured via multiphoton lithography. Copolymerization with commonly used hydrophobic thiols demonstrates not only their ability to regulate the ambient degradation rate of thiol-yne polyester photopolymer resins, but also desirable surface erosion behavior. Such degradation profiles, in the appropriate time frames, in suitably mild conditions, combined with their low cytotoxicity and 3D printability, render these novel photomonomers of significant interest for a wide range of biomaterial applications.

Poly[bis(serine ethyl ester)phosphazene] regulates the degradation rates of vinyl ester photopolymers

Vinyl esters and carbonates have recently been demonstrated to have considerably lower cytotoxicity than their more commonly used (meth)acrylate counterparts, inspiring their use in the 3D printing of biomaterials. However, the degradation rates of such synthetic photopolymers are slow, especially in the mild conditions present in many biological environments. Some applications, for example, tissue regeneration scaffolds and drug release, require considerably faster biodegradation. Furthermore, it is essential to be able to easily tune the degradation rate to fit the requirements for a range of applications. Herein we present the design and synthesis of hydrolytically degradable polyphosphazenes substituted with a vinyl carbonate functionalized amino acid. Thiolene copolymerization with vinyl esters gave cured polymers which are demonstrated to considerably accelerate the degradation rates of cured vinylester/thiolene polymer scaffolds.

Graphical abstract

Hetero and homo α,ω‐chain‐end functionalized polyphosphazenes

The control of chain‐ends is fundamental in modern macromolecular chemistry for directed one‐to‐one bioconjugation and the synthesis of advanced architectures such as block copolymers or bottlebrush polymers and the preparation of advanced soft materials. Polyphosphazenes are of growing importance as elastomers, biodegradable materials and in biomedical drug delivery due to their synthetic versatility. While controlled polymerization methods have been known for some time, controlling both chain‐ends with high fidelity has proven difficult. We demonstrate a robust synthetic route to hetero and homo α,ω‐chain‐end functionalized polyphosphazenes via end‐capping with easily accessible, functionalized triphenylphosphine‐based phosphoranimines. A versatile thiol‐ene “click”‐reaction approach then allows for subsequent conversion of the end‐capped polymers with various functional groups. Finally, we demonstrate the utility of this system to prepare gels based on homo α,ω‐chain‐end functionalized polyphosphazenes. This development will enhance their progress in various applications, particularly in soft materials and as degradable polymers.

Mechanochemical synthesis of freebase and metal corroles

Herein, we report on the mechanochemical reaction of pyrrole and substituted benzaldehyde precursors to produce freebase corroles and demonstrate the one-pot mechanochemical synthesis of 5,10,15-Tris(4-[Formula: see text]-butylphenyl)corrole (H[Formula: see text]-buPhC), in which both, the condensation and oxidation reactions steps, took place in the ball mill. Moreover, we could achieve the mechanochemical synthesis of copper corroles with decent overall yields of 10–12%. With the mechanochemical approach we could shift the EcoScale obtained from common synthesis procedures to significant more positive values and the E-factor for the mechanochemical copper insertion was lowered by factor of 3.0.

Reversible Speed Regulation of Self-Propelled Janus Micromotors via Thermoresponsive Bottle-Brush Polymers

This work reports a reversible braking system for micromotors that can be controlled by small temperature changes (≈5 °C). To achieve this, gated-mesoporous organosilica microparticles are internally loaded with metal catalysts (to form the motor) and the exterior (partially) grafted with thermosensitive bottle-brush polyphosphazenes to form Janus particles. When placed in an aqueous solution of H2 O2 (the fuel), rapid forward propulsion of the motors ensues due to decomposition of the fuel. Conformational changes of the polymers at defined temperatures regulate the bubble formation rate and thus act as brakes with considerable deceleration/acceleration observed. As the components can be easily varied, this represents a versatile, modular platform for the exogenous velocity control of micromotors.

Reversible Speed Regulation of Self-Propelled Janus Micromotors via Thermoresponsive Bottle-Brush Polymers

Invited for the cover of this issue is the group of Ian Teasdale and Yolanda Salinas at the Johannes Kepler University Linz. The image depicts the self-propelled Janus micromotors reported in this work. Read the full text of the article at 10.1002/chem.202004792.

Diels–Alder cycloaddition polymerization of highly aromatic polyimides and their multiblock copolymers

A novel route to highly aromatic polyimides is presented and is used to form multiblock copolymers which is inherently difficult to achieve via traditional routes for this important polymer family.

Green-light photocleavable meso-methyl BODIPY building blocks for macromolecular chemistry

We report the design of easily accessible, meso-methyl BODIPY monomers and their incorporation into photoclippable macromolecules.

Hybrid Porous Microparticles Based on a Single Organosilica Cyclophosphazene Precursor

Porous organosilica microparticles consisting of silane-derived cyclophosphazene bridges were synthesized by a surfactant-mediated sol-gel process. Starting from the substitution of hexachlorocyclotriphosphazene with allylamine, two different precursors were obtained by anchoring three or six alkoxysilane units, via a thiol-ene photoaddition reaction. In both cases, spherical, microparticles (size average of ca. 1000 nm) with large pores were obtained, confirmed by both, scanning and transmission electron microscopy. Particles synthesized using the partially functionalized precursor containing free vinyl groups were further functionalized with a thiol-containing molecule. While most other reported mesoporous organosilica particles are essentially hybrids with tetraethyl orthosilicate (TEOS), a unique feature of these particles is that structural control is achieved by exclusively using organosilane precursors. This allows an increase in the proportion of the co-components and could springboard these novel phosphorus-containing organosilica microparticles for different areas of technology.

Dual stimuli-responsive polyphosphazene-based molecular gates for controlled drug delivery in lung cancer cells

A switchable silane derived stimuli-responsive bottle-brush polyphosphazene (PPz) was prepared and attached to the surface of mesoporous silica nanoparticles (MSNs). The hybrid polymer with PEG-like Jeffamine® M-2005 side-arms undergo conformational changes in response to both pH and temperature due to its amphiphilic substituents and protonatable main-chain, hence were investigated as a gatekeeper. Safranin O as control fluorophore or the anticancer drug camptothecin (CPT) were encapsulated in the PPz-coated MSNs. At temperatures below the lower critical solution temperature (LCST), the swollen conformation of PPz efficiently blocked the cargo within the pores. However, above the LCST, the PPz collapsed, allowing release of the payload. Additionally, protonation of the polymer backbone at lower pH values was observed to enhance opening of the pores from the surface of the MSNs and therefore the release of the dye. In vitro studies demonstrated the ability of these nanoparticles loaded with the drug camptothecin to be endocytosed in both models of tumor (A549) and healthy epithelial (BEAS-2B) lung cells. Their accumulation and the release of the chemotherapeutic drug, co-localized within lysosomes, was faster and higher for tumor than for healthy cells, further, the biocompatibility of PPz-gated nanosystem without drug was demonstrated. Tailored dual responsive polyphosphazenes thus represent novel and promising candidates in the construction of future gated mesoporous silica nanocarriers designs for lung cancer-directed treatment.

Visible Light Photocleavable Ruthenium-Based Molecular Gates to Reversibly Control Release from Mesoporous Silica Nanoparticles

Herein we present hybrid mesoporous silica nanomaterials (MSN) with visible light-sensitive ruthenium complexes acting as gates. Two different [Ru(bpy)₂L1L2]²⁺ complexes were investigated by grafting [Ru(bpy)₂(4AMP)₂](PF₆)₂ (RC1) and [Ru(bpy)₂(PPh₃)Cl]Cl (RC2) via two or one ligands onto the surface of mesoporous silica nanoparticles (MSNs), to give MSN1-RC1 and MSN2-RC2, respectively. The pores were previously loaded with a common dye, safranin O, and release studies were conducted. The number and position of the ligands were shown to influence the photocages behavior and thus the release of the cargo. Release studies from MSN1-RC1 in acetonitrile showed that in the dark the amount of dye released was minimal after 300 min, whereas a significant increase was measured upon visible light irradiation (ca. 90%). While successful as a photochemically-controlled gated system, RC1 was restricted to organic solvents since it required cleavage of two ligands in order to be cleaved from the surface, and in water only one is cleaved. Release studies from the second nanomaterial MSN2-RC2, where the complex RC2 was bound to the MSN via only one ligand, showed stability under darkness and in aqueous solution up to 180 min and, rapid release of the dye when irradiated with visible light. Furthermore, this system was demonstrated to be reversible, since, upon heating to 80 °C, the system could effectively re-close the pores and re-open it again upon visible light irradiation. This work, thus, demonstrates the potential reversible gate mechanism of the ruthenium-gated nanomaterials upon visible light irradiation, and could be envisioned as a future design of photochemically-driven drug delivery nanosystems or on/off switches for nanorelease systems.

Main-Chain Phosphorus-Containing Polymers for Therapeutic Applications

Polymers in which phosphorus is an integral part of the main chain, including polyphosphazenes and polyphosphoesters, have been widely investigated in recent years for their potential in a number of therapeutic applications. Phosphorus, as the central feature of these polymers, endears the chemical functionalization, and in some cases (bio)degradability, to facilitate their use in such therapeutic formulations. Recent advances in the synthetic polymer chemistry have allowed for controlled synthesis methods in order to prepare the complex macromolecular structures required, alongside the control and reproducibility desired for such medical applications. While the main polymer families described herein, polyphosphazenes and polyphosphoesters and their analogues, as well as phosphorus-based dendrimers, have hitherto predominantly been investigated in isolation from one another, this review aims to highlight and bring together some of this research. In doing so, the focus is placed on the essential, and often mutual, design features and structure-property relationships that allow the preparation of such functional materials. The first part of the review details the relevant features of phosphorus-containing polymers in respect to their use in therapeutic applications, while the second part highlights some recent and innovative applications, offering insights into the most state-of-the-art research on phosphorus-based polymers in a therapeutic context.

Mesoporous Silica Micromotors with a Reversible Temperature Regulated On-Off Polyphosphazene Switch

The incorporation of an extraneous on-off braking system is necessary for the effective motion control of the next generation of micrometer-sized motors. Here, the design and synthesis of micromotors is reported based on mesoporous silica particles containing bipyridine groups, introduced by cocondensation, for entrapping catalytic cobalt(II) ions within the mesochannels, and functionalized on the surface with silane-derived temperature responsive bottle-brush polyphosphazene. Switching the polymers in a narrow temperature window of 25-30 °C between the swollen and collapsed state, allows the access for the fuel H₂ O₂ contained in the dispersion medium to cobalt(II) bipyridinato catalyst sites. The decomposition of hydrogen peroxide is monitored by optical microscopy, and effectively operated by reversibly closing or opening the pores by the grafted gate-like polyphosphazene, to control on demand the oxygen bubble generation. This design represents one of the few examples using temperature as a trigger for the reversible on-off external switching of mesoporous silica micromotors.

Activatable Hybrid Polyphosphazene-AuNP Nanoprobe for ROS Detection by Bimodal PA/CT Imaging

Overproduction of reactive oxygen species (ROS) is often related to inflammation or cancer and can cause tissue damage. Probes that have been previously reported to image ROS typically rely on imaging techniques that have low depth penetration in tissue, thus limiting their use to superficial disease sites. We report herein a novel formulation of hybrid nanogels loaded with gold nanoparticles (AuNP) to produce contrast for computed tomography (CT) and photoacoustics (PA), both being deep-tissue imaging techniques. The polyphosphazene polymer has been designed to selectively degrade upon ROS exposure, which triggers a switch-off of the PA signal by AuNP disassembly. This ROS-triggered degradation of the nanoprobes leads to a significant decrease in the PA contrast, thus allowing ratiometric ROS imaging by comparing the PA to CT signal. Furthermore, ROS imaging using these nanoprobes was applied to an in vitro model of inflammation, that is, LPS-stimulated macrophages, where ROS-triggered disassembly of the nanoprobe was confirmed via reduction of the PA signal. In summary, these hybrid nanoprobes are a novel responsive imaging agent that have the potential to image ROS overproduction by comparing PA to CT contrast.

Dynamic Supramolecular Ruthenium-Based Gels Responsive to Visible/NIR Light and Heat

A simple supramolecular crosslinked gel is reported with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4-vinylpyridine) (P4VP) as a macromolecular ligand. Irradiation of the organogels in H2 O/MeOH with visible and NIR light (in a multiphoton process) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross-links and causing degelation and hence solubilization of the P4VP chains. Real-time (RT) photorheology experiments of thin films showed a rapid degelation in several seconds, whereas larger bulk samples could also be photocleaved. Furthermore, the gels could be reformed or healed by simple heating of the system and restoration of the metal-ligand crosslinks. The relatively simple dynamic system with a high sensitivity towards light in the visible and NIR region make them interesting positive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single- and multiphoton lithography.

Stimuli-Responsive Phosphorus-Based Polymers

This microreview details recent developments in stimuli-responsive polymers with phosphorus in the main-chain, in particular polyphosphazenes and polyphosphoesters. The presence of phosphorus in the polymers endows unique properties onto the macromolecules, which can be utilized for the preparation of materials capable of physically responding to specific stimuli. Achieving the desired responsiveness has been much facilitated by recent developments in synthetic polymer chemistry, in particular controlled synthesis and backbone functionalization phosphorus-based polymers, in order to achieve the required properties and hence responsiveness of the materials. The development of phosphorus-based polymers which respond to the most important stimuli are discussed, namely, pH, oxidation, reduction, temperature and biological triggers. The polymers are placed in the context not just of each other but also with reference to state-of-the-art organic polymers.

Coumarin-Caged Polyphosphazenes with a Visible-Light Driven On-Demand Degradation

Polymers that, upon photochemical activation with visible light, undergo rapid degradation to small molecules are described. Through functionalization of a polyphosphazene backbone with pendant coumarin groups sensitive to light, polymers which are stable in the dark could be prepared. Upon irradiation, cleavage of the coumarin moieties exposes carboxylic acid moieties along the polymer backbone. The subsequent macromolecular photoacid is found to catalyze the rapid hydrolytic degradation of the polyphosphazene backbone. Water-soluble and non-water-soluble polymers are reported, which due to their sensitivity toward light in the visible region could be significant as photocleavable materials in biological applications.

Palladium complexes containing imino phenoxide ligands: synthesis, luminescence, and their use as catalysts for the ring-opening polymerization of rac-lactide

Abstract

The preparation, structural characterization, luminescence, and catalytic activity of three palladium(II) complexes bearing imino phenoxide ligands are reported. The X-ray studies revealed that the complexes are mononuclear with palladium centres coordinated in a square-planar coordination environment. Two of the complexes are emissive in solution at room temperature. The catalytic activities towards the ring-opening polymerization of rac-lactide (rac-LA) were tested. Polymers with moderate molecular weights and relatively broad dispersity (Ð) were obtained. Kinetic studies revealed that the polymerization followed first-order kinetics.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-017-2119-1) contains supplementary material, which is available to authorized users.

Degradable, Dendritic Polyols on a Branched Polyphosphazene Backbone

Herein, we present the design, synthesis, and characterization of fully degradable, hybrid, star-branched dendritic polyols. First multiarmed polyphosphazenes were prepared as a star-branched scaffold which upon functionalization produced globular branched hydroxyl-functionalized polymers with over 1700 peripheral functional end groups. These polyols with unique branched architectures could be prepared with controlled molecular weights and relatively narrow dispersities. Furthermore, the polymers are shown to undergo hydrolytic degradation to low molecular weight degradation products, the rate of which could be controlled through postpolymerization functionalization of the phosphazene backbone.

A pH-sensitive Macromolecular Prodrug as TLR7/8 Targeting Immune Response Modifier

The chemical synthesis and biological activity of novel functionalized imidazoquinoline derivatives (ImQ) to generate Toll-like receptor (TLR) 7/8 specific prodrugs are presented. In vivo activity of ImQs to induce inflammation was confirmed in zebrafish larvae. After covalent ligation to fully biodegradable polyphosphazenes (ImQ-polymer), the macromolecular prodrugs were designed to undergo intracellular pH-sensitive release of ImQs to induce inflammation through binding to endosomal TLR7/8 (danger signal). We showed ImQ dissociation from prodrugs at a pH 5 pointing towards endosomal prodrug degradability. ImQ-polymers strongly activated ovalbumin-specific T cells in murine splenocytes as shown by increased proliferation and expression of the IL-2 receptor (CD25) on CD8+ T cells accompanied by strong IFN-γ release. ImQ prodrugs presented here are suggested to form the basis of novel nanovaccines, for example, for intravenous or intratumoral cancer immunotherapeutic applications to trigger physiological antitumor immune responses.

Metallo-Supramolecular Gels that are Photocleavable with Visible and Near-Infrared Irradiation

A photolabile ruthenium-based complex, [Ru(bpy)2 (4AMP)2 ](PF6 )2 , (4AMP=4-(aminomethyl)pyridine) is incorporated into polyurea organo- and hydrogels via the reactive amine moieties on the photocleavable 4AMP ligands. While showing long-term stability in the dark, cleavage of the pyridine-ruthenium bond upon irradiation with visible or near-infrared irradiation (in a two-photon process) leads to rapid de-gelation of the supramolecular gels, thus enabling spatiotemporal micropatterning by photomasking or pulsed NIR-laser irradiation.

Synthesis and in vivo anticancer evaluation of poly(organo)phosphazene-based metallodrug conjugates

Within this work we aimed to improve the pharmacodynamics and toxicity profile of organoruthenium and -rhodium complexes which had previously been found to be highly potent in vitro but showed unselective activity in vivo. Different organometallic complexes were attached to a degradable poly(organo)phosphazene macromolecule, prepared via controlled polymerization techniques. The conjugation to hydrophilic polymers was designed to increase the aqueous solubility of the typically poorly soluble metal-based half-sandwich compounds with the aim of a controlled, pH-triggered release of the active metallodrug. The synthesized conjugates and their characteristics have been thoroughly studied by means of ³¹P NMR and UV-Vis spectroscopy, ICP-MS analyses and SEC coupled to ICP-MS. In order to assess their potential as possible anticancer drug candidates, the complexes, as well as their respective macromolecular prodrug formulations were tested against three different cancer cell lines in cell culture. Subsequently, the anticancer activity and organ distribution of the poly(organo)phosphazene drug conjugates were explored in vivo in mice bearing CT-26 colon carcinoma. Our investigations revealed a beneficial influence of this macromolecular prodrug by a significant reduction of adverse effects compared to the free metallodrugs.

Oxidation Responsive Polymers with a Triggered Degradation via Arylboronate Self-Immolative Motifs on a Polyphosphazene Backbone

Oxidation responsive polymers with triggered degradation pathways have been prepared via attachment of self-immolative moieties onto a hydrolytically unstable polyphosphazene backbone. After controlled main-chain growth, postpolymerization functionalization allows the preparation of hydrolytically stable poly(organo)phosphazenes decorated with a phenylboronic ester caging group. In oxidative environments, triggered cleavage of the caging group is followed by self-immolation, exposing the unstable glycine-substituted polyphosphazene which subsequently undergoes to backbone degradation to low-molecular weight molecules. As well as giving mechanistic insights, detailed GPC and 1H and 31P NMR analysis reveal the polymers to be stable in aqueous solutions, but show a selective, fast degradation upon exposure to hydrogen peroxide containing solutions. Since the post-polymerization functionalization route allows simple access to polymer backbones with a broad range of molecular weights, the approach of using the inorganic backbone as a platform significantly expands the toolbox of polymers capable of stimuli-responsive degradation.

Preparation of polyphosphazenes: a tutorial review

Poly(organo)phosphazenes are a family of inorganic molecular hybrid polymers with very diverse properties due to the vast array of organic substituents possible. This tutorial review aims to introduce the basics of the synthetic chemistry of polyphosphazenes, detailing for readers outside the field the essential knowledge required to design and prepare polyphosphazenes with desired properties. A particular focus is given to some of the recent advances in their chemical synthesis which allows not only the preparation of polyphosphazenes with controlled molecular weights and polydispersities, but also novel branched architectures and block copolymers. We also discuss the preparation of supramolecular structures, bioconjugates and in situ forming gels from this diverse family of functional materials. This tutorial review aims to equip the reader to prepare defined polyphosphazenes with unique property combinations and in doing so we hope to stimulate further research and yet more innovative applications for these highly interesting multifaceted materials.

Copper(II) complexes with imino phenoxide ligands: synthesis, characterization, and their application as catalysts for the ring-opening polymerization of rac-lactide

ABSTRACT

Four new copper complexes based on bidentate imino phenoxide ligands were synthesized and characterized by IR, UV-Vis spectroscopy, ESI mass spectrometry, single crystal X-ray diffraction, and electrochemistry. The crystal structures revealed that the copper(II) atoms are surrounded by phenolate oxygen and imine nitrogen atoms of two ligands in a distorted square-planar geometry. The existence of ligand-centered, as well as Cu(II)-centered quasi-reversible and reversible redox reactions are observed in the cyclic voltammetry experiments of all the complexes. All complexes are able to catalyze the ring-opening polymerization of rac-lactide yielding polymers with moderate molecular weights and moderately broad molecular weight distributions.

GRAPHICAL ABSTRACT

Macromolecular Pt(IV) Prodrugs from Poly(organo)phosphazenes

The preparation of novel macromolecular prodrugs via the conjugation of two platinum(IV) complexes to suitably functionalized poly(organo)phosphazenes is presented. The inorganic/organic polymers provide carriers with controlled dimensions due to the use of living cationic polymerization and allow the preparation of conjugates with excellent aqueous solubility but long-term hydrolytic degradability. The macromolecular Pt(IV) prodrugs are designed to undergo intracellular reduction and simultaneous release from the macromolecular carrier to present the active Pt(II) drug derivatives. In vitro investigations show a significantly enhanced intracellular uptake of Pt for the macromolecular prodrugs when compared to small molecule Pt complexes, which is also reflected in an increase in cytotoxicity. Interestingly, drug-resistant sublines also show a significantly smaller resistance against the conjugates compared to clinically established platinum drugs, indicating that an alternative uptake route of the Pt(IV) conjugates might also be able to overcome acquired resistance against Pt(II) drugs. In vivo studies of a selected conjugate show improved tumor shrinkage compared to the respective Pt(IV) complex.

Polyphosphazene Based Star-Branched and Dendritic Molecular Brushes

A new synthetic procedure is described for the preparation of poly(organo)phosphazenes with star-branched and star dendritic molecular brush type structures, thus describing the first time it has been possible to prepare controlled, highly branched architectures for this type of polymer. Furthermore, as a result of the extremely high-arm density generated by the phosphazene repeat unit, the second-generation structures represent quite unique architectures for any type of polymer. Using two relativity straight forward iterative syntheses it is possible to prepare globular highly branched polymers with up to 30 000 functional end groups, while keeping relatively narrow polydispersities (1.2-1.6). Phosphine mediated polymerization of chlorophosphoranimine is first used to prepare three-arm star polymers. Subsequent substitution with diphenylphosphine moieties gives poly(organo)phosphazenes to function as multifunctional macroinitiators for the growth of a second generation of polyphosphazene arms. Macrosubstitution with Jeffamine oligomers gives a series of large, water soluble branched macromolecules with high-arm density and hydrodynamic diameters between 10 and 70 nm.

Biodegradable Polyphosphazene Based Peptide-Polymer Hybrids

A novel series of peptide based hybrid polymers designed to undergo enzymatic degradation is presented, via macrosubstitution of a polyphosphazene backbone with the tetrapeptide Gly-Phe-Leu-Gly. Further co-substitution of the hybrid polymers with hydrophilic polyalkylene oxide Jeffamine M-1000 leads to water soluble and biodegradable hybrid polymers. Detailed degradation studies, via ³¹P NMR spectroscopy, dynamic light scattering and field flow fractionation show the polymers degrade via a combination of enzymatic, as well as hydrolytic pathways. The peptide sequence was chosen due to its known property to undergo lysosomal degradation; hence, these degradable, water soluble polymers could be of significant interest for the use as polymer therapeutics. In this context, we investigated conjugation of the immune response modifier imiquimod to the polymers via the tetrapeptide and report the self-assembly behavior of the conjugate, as well as its enzymatically triggered drug release behavior.

Chain-end-functionalized polyphosphazenes via a one-pot phosphine-mediated living polymerization

A simple polymerization of trichlorophosphoranimine (Cl3 P = N-SiMe3 ) mediated by functionalized triphenylphosphines is presented. In situ initiator formation and the subsequent polymerization progress are investigated by (31) P NMR spectroscopy, demonstrating a living cationic polymerization mechanism. The polymer chain lengths and molecular weights of the resulting substituted poly(organo)phosphazenes are further studied by (1) H NMR spectroscopy and size exclusion chromatography. This strategy facilitates the preparation of polyphosphazenes with controlled molecular weights and specific functional groups at the α-chain end. Such well-defined, mono-end-functionalized polymers have great potential use in bioconjugation, surface modification, and as building blocks for complex macromolecular constructs.

Thermoresponsive Polyphosphazene-Based Molecular Brushes by Living Cationic Polymerization

A series of polyphosphazenes with molecular brush type structures have been prepared with controlled molecular weights and narrow polydispersities. The polymers show lower critical solution temperatures (LCST) between 18 and 90 °C, which can be easily tailored by choice of side-substituent to suit the required application. A temperature triggered self-assembly is observed to give stable colloidal aggregates with dimensions in the region of 100-300 nm.

Water-Soluble, Biocompatible Polyphosphazenes with Controllable and pH-Promoted Degradation Behavior

The synthesis of a series of novel, water-soluble poly(organophosphazenes) prepared via living cationic polymerization is presented. The degradation profiles of the polyphosphazenes prepared are analyzed by GPC, 31P NMR spectroscopy, and UV-Vis spectroscopy in aqueous media and show tunable degradation rates ranging from days to months, adjusted by subtle changes to the chemical structure of the polyphosphazene. Furthermore, it is observed that these polymers demonstrate a pH-promoted hydrolytic degradation behavior, with a remarkably faster rate of degradation at lower pH values. These degradable, water soluble polymers with controlled molecular weights and structures could be of significant interest for use in aqueous biomedical applications, such as polymer therapeutics, in which biological clearance is a requirement and in this context cell viability tests are described which show the non-toxic nature of the polymers as well as their degradation intermediates and products.

Applicability of new degradable hypericin–polymer-conjugates as photosensitizers: principal mode of action demonstrated by in vitro models

Novel degradable polymers are shown to be promising carriers for the delivery of hypericin for PDT, significantly enhancing solubility of the free drug without compromising the photoactivity.

Branched Polyphosphazenes with Controlled Dimensions

Using living cationic polymerization, a series of polyphosphazenes is prepared with precisely controlled molecular weights and narrow polydispersities. As well as varying chain length through the use of a living polymerization, amine-capped polyalkylene oxide (Jeffamine) side chains with varied lengths are grafted to the polymer backbone to give a series of polymers with varied dimensions. Dynamic light scattering and size exclusion chromatography are used to confirm the preparation of polymers with a variety of controlled dimensions and thus hydrodynamic volumes. Furthermore, it is demonstrated how the number of arms per repeat unit, and thus the density of branching, can also be further increased from two to four through using a one-pot thiolactone conversion of the Jeffamines, followed by thiol-yne addition to the polyphosphazene backbone. These densely branched, molecular brush-type polymers on a biodegradable polyphosphazene backbone all show excellent aqueous solubility and have potential in drug-delivery applications.

Polyphosphazenes: Multifunctional, Biodegradable Vehicles for Drug and Gene Delivery

Poly[(organo)phosphazenes] are a unique class of extremely versatile polymers with a range of applications including tissue engineering and drug delivery, as hydrogels, shape memory polymers and as stimuli responsive materials. This review aims to divulge the basic principles of designing polyphosphazenes for drug and gene delivery and portray the huge potential of these extremely versatile materials for such applications. Polyphosphazenes offer a number of distinct advantages as carriers for bioconjugates; alongside their completely degradable backbone, to non-toxic degradation products, they possess an inherently and uniquely high functionality and, thanks to recent advances in their polymer chemistry, can be prepared with controlled molecular weights and narrow polydispersities, as well as self-assembled supra-molecular structures. Importantly, the rate of degradation/hydrolysis of the polymers can be carefully tuned to suit the desired application. In this review we detail the recent developments in the chemistry of polyphosphazenes, relevant to drug and gene delivery and describe recent investigations into their application in this field.