RAFT Polymerization and Thiol Chemistry: A Complementary Pairing for Implementing Modern Macromolecular Design

Dual-pathway chain-end modification of RAFT polymers using visible light and metal-free conditions

We report a metal-free strategy for the chain-end modification of RAFT polymers utilizing visible light. By turning the light source on or off, the reaction pathway in one pot can be switched between either complete desulfurization (hydrogen chain-end) or simple cleavage (thiol chain-end), respectively. The versatility of this process is exemplified by application to a wide range of polymer backbones under mild, quantitative conditions using commercial reagents.

Dual-Function Polymeric HPMA Prodrugs for the Delivery of miRNA

An HPMA-based polymeric prodrug of a CXCR4 antagonist, AMD3465 (P-SS-AMD), was developed as a dual-function carrier of therapeutic miRNA. P-SS-AMD was synthesized by a copolymerization of HPMA with a methacrylamide monomer in which the AMD3465 was attached via a self-immolative disulfide linker. P-SS-AMD showed effective release of the parent AMD3465 drug following treatment with intracellular levels of glutathione (GSH). The AMD3465 was released in the cells and exhibited functional CXCR4 antagonism, demonstrated by inhibition of the CXCR4-mediated cancer cell invasion. Due to its cationic character, P-SS-AMD could form polyplexes with miRNA and mediate efficient transfection of miR-200c mimics to downregulate expression of a downstream target ZEB-1 in cancer cells. The combined P-SS-AMD/miR-200c polyplexes showed improved ability to inhibit cancer cell migration when compared with individual treatments. The reported findings validate P-SS-AMD as a dual-function delivery vector that can simultaneously deliver a therapeutic miRNA and function as a polymeric prodrug of CXCR4 antagonist.

Polymers with acyl-protected perthiol chain termini as convenient building blocks for doubly responsive H2S-donating nanoparticles

H₂S-releasing polymers with an acyl-protected perthiol chain terminus were prepared using a simple, high yielding end-group modification process.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Mild and efficient synthesis of ω,ω-heterodifunctionalized polymers and polymer bioconjugates

Semi-telechelic ω,ω-heterodifunctional polymers and polymer bioconjugates are synthesized under mild conditions using benzotrifuranone.

Addressing the mid-point of polymer chains for multiple functionalization purposes through sequential thiol–epoxy ‘click’ and esterification reactions

A synthetic strategy is devised for the preparation of mid-chain multifunctional polymers.

Glutathione responsive polymers and their application in drug delivery systems

Materials which respond to biological cues are the subject of intense research interest due to their possible application in smart drug delivery vehicles.

RAFT-mediated, visible light-initiated single unit monomer insertion and its application in the synthesis of sequence-defined polymers

In this communication, we report a catalyst-free methodology for single unit monomer insertion (SUMI) into reversible addition–fragmentation chain transfer (RAFT) agents initiated by low intensity visible light.

Novel Self-Assembling Amino Acid-Derived Block Copolymer with Changeable Polymer Backbone Structure

Block copolymers have attracted much attention as potentially interesting building blocks for the development of novel nanostructured materials in recent years. Herein, we report a new type of self-assembling block copolymer with changeable polymer backbone structure, poly(Fmoc-Ser)_ester-b-PSt, which was synthesized by combining the polycondensation of 9-fluorenylmethoxycarbonyl-serine (Fmoc-Ser) with the reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene (St). This block copolymer showed the direct conversion of the backbone structure from polyester to polypeptide through a multi O,N-acyl migration triggered by base-induced deprotection of Fmoc groups in organic solvent. Such polymer-to-polymer conversion was found to occur quantitatively without decrease in degree of polymerization and to cause a drastic change in self-assembling property of the block copolymer. On the basis of several morphological analyses using FTIR spectroscopy, atomic force, and transmission and scanning electron microscopies, the resulting peptide block copolymer was found to self-assemble into a vesicle-like hollow nanosphere with relatively uniform diameter of ca. 300 nm in toluene. In this case, the peptide block generated from polyester formed β-sheet structure, indicating the self-assembly via peptide-guided route. We believe the findings presented in this study offer a new concept for the development of self-assembling block copolymer system.

Stability Enhancing N-Terminal PEGylation of Oxytocin Exploiting Different Polymer Architectures and Conjugation Approaches

Oxytocin, a cyclic nine amino acid neurohypophyseal hormone therapeutic, is effectively used in the control of postpartum hemorrhaging (PPH) and is on the WHO List of Essential Medicines. However, oxytocin has limited shelf life stability in aqueous solutions, particularly at temperatures in excess of 25 °C and injectable aqueous oxytocin formulations require refrigeration (<8 °C). This is particularly problematic in the hot climates often found in many developing countries where daytime temperatures can exceed 40 °C and where reliable cold-chain storage is not always achievable. The purpose of this study was to develop N-terminal amine targeted PEGylation strategies utilizing both linear PEG and polyPEG "comb" polymers as an effective method for stabilizing solution formulations of this peptide for prolonged storage in the absence of efficient cold-chain storage. The conjugation chemistries investigated herein include irreversible amine targeted conjugation methods utilizing NHS ester and aldehyde reductive amination chemistry. Additionally, one reversible conjugation method using a Schiff base approach was explored to allow for the release of the native peptide, thus, ensuring that biological activity remains unaffected. The reversibility of this approach was investigated for the different polymer architectures, alongside a nonpolymer oxytocin analogue to monitor how pH can tune native peptide release. Elevated temperature degradation studies of the polymer conjugates were evaluated to assess the stability of the PEGylated analogues in comparison to the native peptide in aqueous formulations to mimic storage conditions in developing nations and regions where storage under appropriate conditions is challenging.

Reversible Regulation of Thermoresponsive Property of Dithiomaleimide-Containing Copolymers via Sequential Thiol Exchange Reactions

The facile and efficient functionalization of thermoresponsive polymers based on sequential, reversible thiol-exchange reactions is reported. Well-defined dithiomaleimide-containing polymers have been synthesized via Cu(0)-mediated SET-LRP and characterized by ¹H NMR and size exclusion chromatography (SEC). The resulting thermosensitive copolymers were subsequently reacted with various thiols to demonstrate the applicability of the strategy, and the thiol-exchange reaction was found to be very fast and efficient. The cloud point of the prepared copolymers can be continually and reversibly tuned, and desirable functionality can be dynamically exchanged upon sequential addition of functional thiol reagents. Through the substitution by thioglucose, an ON-to-OFF switch for fluorescence of the copolymers along with the generation of a glycopolymer was achieved.

Preparation of Well-Defined Propargyl-Terminated Tetra-Arm Poly(N-isopropylacrylamide)s and Their Click Hydrogels Crosslinked with β-cyclodextrin

As an important class of reversible deactivation radical polymerization (RDRP), reversible addition fragmentation chain transfer (RAFT) polymerization has attracted great attention attributed to its facile and flexible features to prepare well-defined polymers with different complex structures. In addition, the combination of RAFT with click chemistry provides more effective strategies to fabricate advanced functional materials. In this work, a series of temperature responsive tetra-arm telechelic poly(N-isopropylacrylamide)s (PNIPAs) with propargyl end groups were prepared for the first time through RAFT and subsequent aminolysis/Michael addition modification. The temperature sensitivities of their aqueous solutions were researched via turbidity measurement. It was found that the phase transition temperature of obtained PNIPAs increased with their molecular weights ascribed to their distinctions in the hydrophobic/hydrophilic balance. Subsequently, β-cyclodextrin (β-CD) functionalized with azide moieties was used to crosslink the prepared propargyl-terminated tetra-arm PNIPAs through click chemistry, fabricating corresponding hydrogels with thermoresponse. Similar to their precursors, the hydrogels demonstrated the same dependence of volume phase transition temperature (VPTT) on their molecular weights. In addition, the incorporation of β-CD and the residual groups besides crosslinking may provide a platform for imparting additional functions such as inclusion and adsorption as well as further functionalization.

Accessing libraries of bifunctional block copolymers using two distinct pentafluorophenyl moieties

The suitability of pentafluorophenyl (PFP) esters and PFP alkyl moieties for orthogonal dual functionalization of block copolymer segments is evaluated. While amidation followed by para-fluoro–thiol substitution is a viable reaction sequence, thiol-based substitution can occur on PFP esters at a high rate, thereby hampering the viability of the reverse sequence.

Single addition of an allylamine monomer enables access to end-functionalized RAFT polymers for native chemical ligation

A novel method for the introduction of a single protected amine-functional monomer at the chain end of RAFT polymers has been developed to enable native chemical ligation with peptide thioesters.

Synthesis of α,ω-heterotelechelic PVP for bioconjugation, via a one-pot orthogonal end-group modification procedure

A simple one pot orthogonal procedure for synthesizing α-aldehyde, ω-thiol heterotelechelic poly(N-vinylpyrrolidone) (PVP) is introduced.

New biocompatible polymeric micelles designed for efficient intracellular uptake and delivery

New amphiphilic block copolymers are easily synthesised by post-polymerisation modifications of poly(glycidyl methacrylate) chain derivatives. The obtained material, upon dispersion in water, is capable of self-assembling into robust micelles. These nanoparticles, which are also characterised by adaptable stability, were loaded with different thiophene based fluorophores. The photoluminescent micelles were administered to cultured cells revealing a high and rapid internalisation of structurally different fluorescent molecules by the same internalisation pathway. Appropriate pairs of chromophores were selected and loaded into the micelles to induce Förster resonance energy transfer (FRET). The disappearing of the FRET phenomenon, after cell uptaking, demonstrated the intracellular release of the nanoparticle contents. The studied nanomaterial and the loaded chromophores have also shown to be biocompatible and non toxic towards the tested cells.

Transformation of RAFT Polymer End Groups into Nitric Oxide Donor Moieties: En Route to Biochemically Active Nanostructures

Polymers with a terminal S-nitrosothiol moiety were synthesized by modifying the thiocarbonylthio end group formed by reversible addition-fragmentation chain transfer polymerization. Specifically, benzodithioate-terminated poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) was first synthesized by polymerizing OEGMA in the presence of 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid. Sequential treatment with hydrazine hydrate and a stoichiometric amount of nitrous acid resulted in the formation of S-nitrosothiol-terminated polymers. A similar approach was applied to block copolymers of POEGMA incorporating a domain of poly[(N,N-diisopropylamino)ethyl methacrylate], thus, enabling the preparation of pH responsive nitric oxide (NO)-releasing micelles. The micelles possessed substantially modified S-nitrosothiol loss kinetics compared to the hydrophilic homopolymer analogue. Moreover, thiol-triggered degradation of the S-nitrosothiol was significantly slower when the S-nitrosothiol was embedded in a micellar structure. These results demonstrate that it is possible to incorporate nitric oxide donor moieties directly onto a polymer chain end, enabling simple synthesis of biochemically active nanostructures.

Efficient and Chemoselective Synthesis of ω,ω-Heterodifunctional Polymers

We report a strategy for the preparation of semitelechelic polymers containing two distinct functionalities at one chain end by consecutive and chemoselective nucleophilic aromatic substitution reactions on 2,4,6-trichloro-1,3,5-triazine (TCT). Because of its commercial availability, well-defined nature, and ubiquity in biological applications, monomethyl ether poly(ethylene glycol) (mPEG) was chosen to demonstrate the utility of this ω,ω-heterodifunctional end-group modification strategy. TCT-functionalized mPEG underwent highly efficient ω,ω-heterodisubstitution via sequential chemoselective substitution with model thiols and amines. The efficiency of nucleophile conjugation to the polymer end group was confirmed by ¹H NMR spectroscopy and matrix assisted laser desorption-ionization time-of-flight mass spectrometry. In addition, density functional theory calculations provided insight into the importance of nucleophile addition order. This route introduces TCT derivatization as a powerful and facile tool to achieve specific polymeric end-group complexity and efficient heterogeneous functionalization.

Well-defined single polymer nanoparticles for the antibody-targeted delivery of chemotherapeutic agents

Aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization was employed to prepare a series of linear copolymers of N,N-dimethylacrylamide (DMA) and 2-hydroxyethylacrylamide (HEAm) with narrow Đ values over a molecular weight range spanning three orders of magnitude (103 to 106 Da). Trithiocarbonate-based RAFT chain transfer agents (CTAs) were grafted onto these scaffolds using carbodiimide chemistry catalyzed with DMAP. The resultant graft chain transfer agent (gCTA) was subsequently employed to synthesize polymeric brushes with a number of important vinyl monomer classes including acrylamido, methacrylamido, and methacrylate. Brush polymerization kinetics were evaluated for the aqueous RAFT polymerization of DMA from a 10 arm gCTA. Polymeric brushes containing hydroxyl functionality were further functionalized in order to prepare 2nd generation gCTAs which were subsequently employed to prepare polymers with a brushed-brush architecture with molecular weights in excess of 106 Da. These resultant single particle nanoparticles (SNPs) were employed as drug delivery vehicles for the anthracycline-based drug doxorubicin via copolymerization of DMA with a protected carbazate monomer (bocSMA). Cell-specific targeting functionality was also introduced via copolymerization with a biotin-functional monomer (bioHEMA). Drug release of the hydrazone linked doxorubicin was evaluated as function of pH and serum and chemotherapeutic activity was evaluated in SKOV3 ovarian cancer cells.

Dynamic intracellular delivery of antibiotics via pH-responsive polymersomes

Reversible addition-fragmentation chain transfer (RAFT) polymerization was employed to prepare a series of copolymers consisting of 2-hdroxyethyl methacrylate (HEMA) and poly(ethylene glycol) methyl ether methacrylate (FWavg ~ 950 Da) (O950) with variable comonomer compositions and molecular weights for use as polymeric scaffolds. Reactivity ratios for the monomer pair were determined to be 1.37 and 0.290 respectively. To these scaffolds trithiocarbonate-based RAFT chain transfer agents (CTAs) were grafted using carbodiimide chemistry. The resultant graft chain transfer agents (gCTA) were subsequently employed to polymerize dimethylaminoethyl methacrylate (DMAEMA) and (HPMA) between degrees of polymerization (DP) of 25 and 200. Kinetic analysis for the polymerization of DMAEMA targeting a DP of 100 from a 34 arm graft gCTA show linear Mn conversion and pseudo first order rate plots with narrow molecular weight distributions that move toward lower elution volumes with monomer conversion. Đ values for these polymerizations remain low at around 1.20 at monomer conversions as high as 70 %. pH-responsive endosomalytic brushes capable of spontaneously self-assembling into polymersomes were synthesized and a combination of dynamic light scattering (DLS), cryoTEM, and red blood cell hemolysis were employed to evaluate the aqueous solution properties of the polymeric brush as a function of pH. Successful encapsulation of ceftazidime and pH-dependent drug release properties were confirmed by HPLC. Intracellular antibiotic activity of the drug-loaded polymersomes was confirmed in a macrophage coculture model of infection with B. thailandensis and RAW 264.7 cells.

Synthesis of diverse α,ω-telechelic polystyrenes with di- and tri-functionality via tandem or one-pot strategies combining aminolysis of RAFT-polystyrene and a thiol–ene “click” reaction

Diverse new α,ω-telechelic polystyrenes with di-/tri-functionality were synthesized via tandem/one-pot strategies combining aminolysis of RAFT-polystyrene and a thiol–ene “click” reaction.

Thiol-reactive functional poly(meth)acrylates: multicomponent monomer synthesis, RAFT (co)polymerization and highly efficient thiol–para-fluoro postpolymerization modification

Novel Passerini-made pentafluorophenyl-functional (meth)acrylate monomers are (co)polymerized by RAFT and modified quantitatively with a variety of functional primary, secondary, and tertiary thiols.

The synthesis and aqueous solution properties of sulfobutylbetaine (co)polymers: comparison of synthetic routes and tuneable upper critical solution temperatures

With an additional methyl group in the ion bridge, sulfobutylbetaine (co)polymers show significantly higher UCSTs than their more common sulfopropylbetaine counterparts making them a promising class of smart materials.

Synthesis of functionalized poly(vinyl acetate) mediated by alkyne-terminated RAFT agents

Two new xanthates with alkyne functionalities were synthesized for the reversible addition fragmentation chain transfer (RAFT) polymerization of vinyl acetate (VAc).

A guide to the synthesis of block copolymers using reversible-addition fragmentation chain transfer (RAFT) polymerization

The discovery of reversible-deactivation radical polymerization (RDRP) has provided an avenue for the synthesis of a vast array of polymers with a rich variety of functionality and architecture. The preparation of block copolymers has received significant focus in this burgeoning research field, due to their diverse properties and potential in a wide range of research environments. This tutorial review will address the important concepts behind the design and synthesis of block copolymers using reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization is arguably the most versatile of the RDRP methods due to its compatibility with a wide range of functional monomers and reaction media along with its relative ease of use. With an ever increasing array of researchers that possess a variety of backgrounds now turning to RDRP, and RAFT in particular, to prepare their required polymeric materials, it is pertinent to discuss the important points which enable the preparation of high purity functional block copolymers with targeted molar mass and narrow molar mass distribution using RAFT polymerization. The key principles of appropriate RAFT agent selection, the order of monomer addition in block synthesis and potential issues with maintaining high end-group fidelity are addressed. Additionally, techniques which allow block copolymers to be accessed using a combination of RAFT polymerization and complementary techniques are touched upon.

Functional gold nanoparticles for the storage and controlled release of nitric oxide: applications in biofilm dispersal and intracellular delivery

Gold nanoparticles (size 10 nm) were designed to store and release nitric oxide (NO), by functionalizing their surfaces with functional polymers modified with NO-donor molecules. Firstly, block copolymer chains consisting of poly(oligoethylene glycol methyl ether methacrylate)-b-poly(vinyl benzyl chloride) (P(OEGMA)-b-PVBC)) were prepared using RAFT polymerization. The chloro-functional groups were then reacted with hexylamine, to introduce secondary amine groups to the copolymer chains. The block copolymers were then grafted onto the surface of gold nanoparticles, exploiting the end-group affinity for gold - attaining grafting densities of 0.6 chain per nm². The secondary amine functional groups were then converted to N-diazeniumdiolate NO donor molecules via exposure to NO gas at high pressure (5 atm). The NO-bearing, gold nanoparticles were characterized using a range of techniques, including transmission electron microscopy, dynamic light scattering (DLS), thermal gravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The nanoparticles displayed slow release of the nitric oxide in biological media. Proof of potential utility was then demonstrated in two different application areas: Pseudomonas aeruginosa biofilm dispersal and cancer cell cytotoxicity.