Thiol-epoxy 'click' chemistry: a focus on molecular attributes in the context of polymer chemistry.

Base-catalyzed ring-opening reaction of epoxides with the thiol nucleophiles is useful in the preparation and post-polymerization modification of synthetic polymers. Due to its many beneficial characteristics, this process is referred to as the thiol-epoxy 'click' reaction. In this article, our aim is to discuss the fundamental attributes of this process by tracing our own steps in the field. We initially address the aspects of efficiency, regio-selectivity, stoichiometry, and reaction conditions with the help of linear, hyperbranched, graft, dendritic, and cross-linked poly(ß-hydroxy thioether)s. A special emphasis is placed on hydrogel synthesis and photopolymerization on surfaces. Subsequently, quenching of the alkoxide anion is considered which is a critical step in the formation of the ß-hydroxy thioether linkage upon completion of reaction. The amenability of further reaction on the hydroxy and thioether groups through esterification and sulfur alkylation is then discussed. Initially, post-gelation/fabrication modification of sulfide linkages is considered to obtain cationic sulfonium hydrogels and zwitterionic photopatterned networks with antibacterial and antibiofouling properties, respectively. A post-synthesis functionalization strategy is then described to access same centered and segregated main-chain poly(ß-hydroxy sulfonium)s as potent antibacterial materials. In side-chain polysulfides, the sequential post-synthesis modifications involving poly(glycidyl methacrylate) scaffolds can lead to the formation of amphiphilic homopolymers. The application of such materials is discussed in the arena of siRNA delivery. Finally, concerns relating to the formation of disulfide defects and open research goals such as study of the orthogonality of the reaction are addressed.

Uniquely sized nanogels via crosslinking polymerization.

Nanogels are very promising carriers for nanomedicine, as they can be prepared in the favorable nanometer size regime, can be functionalized with targeting agents and are responsive to stimuli, i.e. temperature and pH. This induces shrinking or swelling, resulting in controlled release of a therapeutic cargo. Our interest lies in the controlled synthesis of functional nanogels, such as those containing epoxide moieties, that can be subsequently functionalized. Co-polymerization of glycidyl methacrylate and a bifunctional methacrylate crosslinker under dilute conditions gives rise to well-defined epoxide-functional nanogels, of which the sizes are controlled by the degree of polymerization. Nanogels with well-defined sizes (polydispersity of 0.2) ranging from 38 nm to 95 nm were prepared by means of controlled radical polymerization. The nanogels were characterized in detail by FT-IR, DLS, size exclusion chromatography, NMR spectroscopy, AFM and TEM. Nucleophilic attack with functional thiols or amines on the least hindered carbon of the epoxide provides water-soluble nanogels, without altering the backbone structure, while reaction with sodium azide provides handles for further functionalization via click chemistry.

Micellar Assembly and Disassembly of Organoselenium Block Copolymers through Alkylation and Dealkylation Processes.

The aim of this work is to demonstrate that the alkylation and dealkylation of selenium atoms is an effective tool in controlling polymer amphiphilicity and, hence, its assembly and disassembly process in water. To establish this concept, poly(ethylene glycol)-block-poly(glycidyl methacrylate) was prepared. A post-synthesis modification with phenyl selenolate through a base-catalyzed selenium-epoxy 'click' reaction then gave rise to the side-chain selenium-containing block copolymer with an amphiphilic character. This polymer assembled into micellar structures in water. However, silver tetrafluoroborate-promoted alkylation of the selenium atoms resulted in the formation of hydrophilic selenonium tetrafluoroborate salts. This enhancement in the chemical polarity of the second polymer block removed the amphiphilic character from the polymer chain and led to the disassembly of the micellar structures. This process could be reversed by restoring the original amphiphilic polymer character through the dealkylation of the cations.

Main-Chain Polysulfonium Salts: Development of Non-Ammonium Antibacterial Polymers Similar in Their Activity to Antibiotic Drugs Vancomycin and Kanamycin.

Typically, quaternary ammonium polymers are employed for antibacterial purposes. However, a century of use has led bacteria to develop resistance to such materials. Therefore, attention is now turning toward other cationic moieties. In this context, the present work explores sulfur-based main-chain cationic polymers. The results indicate that sulfonium polymers with a ß-hydroxy motif do not suffer from structural instability issues as is commonly observed in cationic polythioethers. Furthermore, they can be highly effective toward important Gram-positive bacterial strains such as Mycobacterium smegmatis, a model organism to develop drugs against rapidly spreading tuberculosis infections. More importantly, however, more challenging Gram-negative strains such as Escherichia coli can also be targeted by the polysulfoniums with equal effectiveness. Interestingly, side-chain sulfonium polyelectrolytes are observed to be devoid of any significant antibacterial activity. Finally, a comparison with kanamycin and vancomycin suggests the present polymers to be similarly effective as the bactericidal antibiotic drugs. Overall, these results indicate the effectiveness of the main-chain trivalent ß-hydroxy sulfonium motif for the development of novel antibacterial polymers with a non-ammonium structure.

Aggregation-free and high stability core-shell polymer nanoparticles with high fullerene loading capacity, variable fullerene type, and compatibility towards biological conditions.

Fullerenes have unique structural and electronic properties that make them attractive candidates for diagnostic, therapeutic, and theranostic applications. However, their poor water solubility remains a limiting factor in realizing their full biomedical potential. Here, we present an approach based on a combination of supramolecular and covalent chemistry to access well-defined fullerene-containing polymer nanoparticles with a core-shell structure. In this approach, solvophobic forces and aromatic interactions first come into play to afford a micellar structure with a poly(ethylene glycol) shell and a corannulene-based fullerene-rich core. Covalent stabilization of the supramolecular assembly then affords core-crosslinked polymer nanoparticles. The shell makes these nanoparticles biocompatible and allows them to be dried to a solid and redispersed in water without inducing interparticle aggregation. The core allows a high content of different fullerene types to be encapsulated. Finally, covalent stabilization endows nanostructures with stability against changing environmental conditions.

Selenonium Polyelectrolyte Synthesis through Post-Polymerization Modifications of Poly (Glycidyl Methacrylate) Scaffolds.

Atom transfer radical polymerization of glycidyl methacrylate monomer with poly(ethylene glycol)-based macroinitiators leads to the formation of reactive block copolymers. The epoxide side-chains of these polymers can be subjected to a regiospecific base-catalyzed nucleophilic ring-opening reaction with benzeneselenol under ambient conditions. The ß-hydroxy selenide linkages thus formed can be alkylated to access polyselenonium salts. 77Se-NMR indicates the formation of diastereomers upon alkylation. In such a manner, sequential post-polymerization modifications of poly(glycidyl methacrylate) scaffolds via selenium-epoxy and selenoether alkylation reactions furnish practical access to poly(ethylene glycol)-based cationic organoselenium copolymers.

Polyselenonium salts: synthesis through sequential selenium-epoxy 'click' chemistry and Se-alkylation.

With the help of amphiphilic homopolymers, this work explores three new avenues in polymer chemistry: (i) the 'click' nature of the selenium-epoxy reaction, (ii) alkylation of the seleno-ethers as a means to prepare cationic polyelectrolytes, and (iii) the antibacterial activity of polyselenonium salts.

Disulfides as mercapto-precursors in nucleophilic ring opening reaction of polymeric epoxides: establishing equimolar stoichiometric conditions in a thiol-epoxy 'click' reaction.

The base-catalyzed oxirane ring opening reaction with thiol nucleophiles is frequently employed for post-polymerization modification of polymeric glycidyl scaffolds. Due to various beneficial attributes, it is often referred to as a 'click' reaction. However, the tendency of the free thiol molecules to undergo oxidative dimerization through the formation of a disulfide bond under ambient conditions results in partial consumption of the sulfhydryl precursors. Therefore, an excess of the thiol precursors is typically used to counterbalance the side-reaction. This violates the equimolar stoichiometry conditions required for 'click' reactions in the context of polymer synthesis. Here, we show that commercially available disulfides can be used to generate the necessary thiolate nucleophiles in situ through the reduction of the SS-bond with sodium borohydride. Such activation strategy eliminates the sulfhydryl oxidation mechanism to disulfides and ensures that the post-synthesis functionalization of epoxy polymers can be performed under equimolar 'click' conditions.

Photoinduced Proton-Transfer Polymerization: A Practical Synthetic Tool for Soft Lithography Applications.

Proton-transfer photopolymerization through the thiol-epoxy "click" reaction is shown to be a versatile new method for the fabrication of micro- and nanosized polymeric patterns. In this approach, complexation of a guanidine base, diazabicycloundecene (DBU), with benzoylphenylpropionic acid (ketoprofen) generates a photolabile salt. Under illumination at a wavelength of 365 nm, the salt undergoes a photodecarboxylation reaction to release DBU as a base. The base-catalyzed ring opening reaction then creates cross-linked poly(ß-hydroxyl thio-ether) patterns. The surface chemistry of these patterns can be altered through alkylation of the thio-ether linkages. For example, a reaction with bromoacetic acid produces a hitherto unknown sulfonium/carboxylate-based zwitterionic motif that endows antibiofouling capacity to the micropatterns.

Hypersensitive azobenzenes: facile synthesis of clickable and cleavable azo linkers with tunable and high reducibility.

The aim of this work is to show that by increasing the number of donor substituents in a donor/acceptor system, the sensitivity of the azobenzene linkage towards a reductive cleavage reaction can be enhanced to unprecedented high levels. For instance, in a triple-donor system, less than a second constitutes the half-life of the azo (N[double bond, length as m-dash]N) bond. Synthetic access to such redox active scaffolds is highly practical and requires only 1-2 synthetic steps. The fundamental molecular design is also adaptable. This is demonstrated through scaffold functionalization by azide, tetraethylene glycol, and biotin groups. The availability of the azide group is shown in a copper-free 'click' reaction suitable in context with protein conjugation and proteomics application. Finally, the clean nature of the scission process is demonstrated with the help of liquid chromatography coupled with mass analysis. This work, therefore, describes development of cleavable azobenzene linkers that can be accessed with synthetic ease, can be multiply functionalized, and show a clean and rapid response to mild reducing conditions.

Synthesis of thermoresponsive oligo(ethylene glycol) polymers through radical ring-opening polymerization of vinylcyclopropane monomers.

Polyvinylcyclopropanes are an old class of polymers typically known for their low polymerization-induced shrinkage properties. In this work, we show that they are capable of exhibiting a thermally triggered aggregation process in aqueous solutions. The phase transition is sharp, tunable within the temperature range of 25-46 °C, and relatively insensitive to environmental conditions. It is anticipated that this preliminary study will shine new light on polyvinylcyclopropanes and lead to new avenues in their studies and future application.

Antibacterial properties of main-chain cationic polymers prepared through amine-epoxy 'Click' polymerization.

Poly(ß-hydroxyl amine)s are prepared through an amine-epoxy 'click' polymerization process in water under ambient conditions. These materials could be subjected to a post-polymerization protonation/alkylation reaction at the nitrogen atom to yield quaternary ammonium salts in the polymer backbone. The antimicrobial activities indicated that polymers carrying butyl chains at the nitrogen atom are effective towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), as only 10-20 µg mL-1 polymer concentrations are required to inhibit the bacterial growth by >90%. One of the candidates was also found to be effective towards Mycobacterium smegmatis (M. smegmatis) - a model organism to develop drugs against rapidly spreading tuberculosis (TB) infections. The hemolysis assay indicated that a majority of antimicrobial agents did not disrupt red blood cell membranes. The mechanistic studies suggested that cell wall disruption by the cationic polymers was the likely cause of bacterial death.

Thermoresponsive Poly(ß-hydroxyl amine)s: Synthesis of a New Stimuli Responsive Amphiphilic Homopolymer Family through Amine-Epoxy 'Click' Polymerization.

A new synthesis of amphiphilic homopolymers is described. In this synthesis, commercially available and inexpensive primary amines and di-epoxide molecules are utilized as AA- and BB-types of monomers in an amine-epoxy 'click' polymerization process. This process can be carried out in water and at room temperature. It does not require a catalyst or inert conditions and forms no byproducts. Therefore, the polymer synthesis can be carried out in open-air and bench-top conditions and a post-synthesis purification step is not required. The modularity of the synthesis, on the other hand, allows for facile structural modulation and tuning of the thermally triggered aggregation process in the temperature range of 7 to 91 °C. Finally, the underlying principles can be translated from linear architectures to polymer networks (hydrogels).

A Modular and Practical Synthesis of Zwitterionic Hydrogels through Sequential Amine-Epoxy "Click" Chemistry and N-Alkylation Reaction.

In this work, the amine-epoxy "click" reaction is shown to be a valuable general tool in the synthesis of reactive hydrogels. The practicality of this reaction arises due to its catalyst-free nature, its operation in water, and commercial availability of a large variety of amine and epoxide molecules that can serve as hydrophilic network precursors. Therefore, hydrogels can be prepared in a modular fashion through a simple mixing of the precursors in water and used as produced (without requiring any post-synthesis purification step). The gelation behavior and final hydrogel properties depend upon the molecular weight of the precursors and can be changed as per the requirement. A post-synthesis modification through alkylation at the nitrogen atom of the newly formed ß-hydroxyl amine linkages allows for functionalizing the hydrogels. For example, ring-opening reaction of cyclic sulfonic ester gives rise to surfaces with a zwitterionic character. Finally, the established gelation chemistry can be combined with soft lithography techniques such as micromolding in capillaries (MIMIC) to obtain hydrogel microstructures.

Biologically activatable azobenzene polymers targeted at drug delivery and imaging applications.

Molecular design concepts are described for the preparation of azobenzene polymers capable of showing a tunable response to the rat liver microsome-induced side-chain self-immolation process under hypoxic conditions. It is shown that azobenzene nuclei carrying a donor/acceptor substitution pattern are the most active system towards the enzymatically triggered azobenzene cleavage reaction (half-life = t1/2 = 6 min). Their activity is followed by azobenzene nuclei carrying donor/donor (t1/2 = 20 min), electronically non-substituted (t1/2 = 72 min), and acceptor (t1/2 = 78 min) systems. This trend is preserved when a chemical stimulus, sodium dithionite, replaces the biological reducing conditions and demonstrates generality of the findings, and their potential in proteomics procedures. Furthermore, the established design concepts also permit for variation in polymer structure and topology while still maintaining the electronic substitution pattern. The steric constraints or the inherent character (hydrophilic/hydrophobic) of the azobenzene, however, does not alter the fate of the scission reaction. In all cases, the self-immolation process allows the polymer chain to convert from a chemically neutral to a cationic state. This structural transformation can be used as an activation mechanism (in vitro) to gain entry into cells through electrostatic interactions with the oppositely charged cell membrane and to deliver an anticancer drug. Interestingly, polymer structure now plays a role and bottlebrush-like copolymer show higher selectivity and faster cellular uptake. Finally, the best performing polymer allows for structural modulation into a fluorescent imaging probe. In vivo application to mice suffering from colitis confirms accumulation of the imaging probe in the diseased colon and cecum parts of the body where the endogenous microbial flora is known to produce the activation enzyme. This work, therefore, establishes general principles for the molecular design of biologically activatable and cleavable azobenzene-based polymeric scaffolds applicable to delivery and imaging applications.

Proton Transfer Hydrogels: Versatility and Applications.

Proton transfer polymerization between thiol and epoxide groups is shown to be an adaptable and utilitarian method for the synthesis of hydrogels. For instance, the polymerization catalyst can be organic or inorganic, and the polymerization medium can be pure water, buffer solutions, or organic solvents. The gelation mechanism can be triggered at ambient conditions, at a physiological temperature of 37 °C, or through using light as an external stimulus. The ambient and photochemical methods both allow for nanoimprint lithography to produce freestanding patterned thick films. The required thiol- and epoxide-carrying precursors can be chosen from a long list of commercially available small molecular as well as polymeric materials. The water uptake, mechanical, and biodegradation properties of the gels can, therefore, be tuned through the choice of appropriate gelation precursors and polymerization conditions. Finally, the thio-ether groups of the cross-linked networks can be functionalized through a postgelation modification reaction to access sulfonium-based cationic structures. Such structural changes endow antibacterial properties to the networks. In their pristine form, however, the gels are biocompatible and nonadhesive, allowing cancer cells to grow in a cluster formation.

Molecular Tailoring of Poly(styrene-b-methyl methacrylate) Block Copolymer Toward Perpendicularly Oriented Nanodomains with Sub-10 nm Features.

We demonstrate a novel approach for fabricating vertically orientated, sub-10 nm, block copolymer (BCP) nanodomains on a substrate via molecular tailoring of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP, one of the most widely used BCPs for nanopatterning. The idea is to incorporate a short middle block of self-attracting poly(methacrylic acid) (PMAA) between the PS and PMMA blocks, where the PMAA middle block promotes phase separation between PS and PMMA, while maintaining the domain orientation perpendicular to the substrate. The designed PS-b-PMAA-b-PMMA triblock copolymers, which were synthesized via well-controlled anionic polymerization, exhibited order-disorder transition temperatures higher than that of pristine PS-b-PMMA BCPs, indicating the promotion of phase separation by the middle PMAA block. For PS-b-PMAA-b-PMMA BCPs with total molecular weights of 21 and 18 kg/mol, the domain spacing corresponds to 19.3 and 16.7 nm, respectively, allowing us to fabricate sub-10 nm nanodomain structures. More importantly, it was demonstrated that the PMAA middle block, which has a higher surface energy than PS and PMMA, does not significantly alter lateral concentration fluctuations, which are responsible for phase-separation in the lateral direction. This enabled the vertical orientation of microdomains with sub-10 nm feature size on a PS-r-PMMA neutral surface without an additional neutral top layer. We anticipate that this approach provides an important platform for next-generation lithography and nanopatterning applications that require sub-10 nm features over large areas with simple process and reduced cost.

An activatable anticancer polymer-drug conjugate based on the self-immolative azobenzene motif.

Triggered cellular uptake of a synthetic graft copolymer carrying an anticancer drug is achieved through self-immolation of the side-chain azobenzene groups. In this concept, the conjugate is initially chemically neutral and does not possess cell-penetrating function. However, upon cleavage of the azobenzene moieties, a cascade process is initiated that ultimately reveals an ammonium cation in the vicinity of the polymer backbone. Hence, self-immolation results in the transformation of the neutral polymer chain into a polycation. This structural transformation allows the conjugate to be taken up by the cancer cells through favorable electrostatic interactions with the negatively charged phospholipid components of the cell membrane. Once inside the cells, the polymer releases covalently attached doxorubicin in a pristine form through a low pH activated release mechanism. The significance of this approach lies in the sensitivity of the azobenzene group to hypoxic conditions and to the enzyme azoreductase that is secreted by the microbial flora of the human colon and suggests a pathway to targeted drug delivery applications under these conditions.

Perpendicularly Oriented Block Copolymer Thin Films Induced by Neutral Star Copolymer Nanoparticles.

By introducing neutral star copolymers consisting of poly(styrene-r-methyl methacrylate) (PS-r-PMMA) arms, a perpendicular orientation of PS-b-PMMA microdomains in thin films could be achieved without any surface treatment. The star copolymers were synthesized by arm-first method in which short chain arms are cross-linked by employing a multifunctional coupling reagent via atom transfer radical polymerization. To find the optimal neutral composition for the perpendicular orientation, we varied the composition of MMA in PS-r-PMMA arms from 40 mol % to 80 mol %. It was found that the star copolymer having an overall PS and PMMA composition of 59:41 exhibits the well-ordered perpendicular orientation of lamellar structures after thermal annealing. Furthermore, we also prepared the deuterated star copolymers to trace them within PS-b-PMMA films along vertical direction by neutron reflectivity. In this case, it was observed that star copolymers were mainly located at the top surface and bottom interface of the films, thereby effectively neutralizing the surface/interfacial energy differences.

Self-assembly of an interacting binary blend of diblock copolymers in thin films: a potential route to porous materials with reactive nanochannel chemistry.

Self-assembly of a binary mixture of poly(styrene)336-block-poly(4-vinyl pyridine)25 (PS336-b-P4VP25) and poly(ethylene glycol)113-block-poly(4-hydroxy styrene)25 (PEG113-b-P4HS25) is shown to give rise to a cylindrical morphology in thin films through pyridine/phenol-based hetero-complementary hydrogen bonding interactions between the P4VP and P4HS copolymer segments. Removal of the cylindrical phase (PEG-b-P4HS) allowed access to porous materials having a pore surface decorated with P4VP polymer blocks. These segments could be transformed into cationic polyelectrolytes through quaternization of the pyridine nitrogen atom. The resulting positively charged nanopore surface could recognize negatively charged gold nanoparticles through electrostatic interactions. This work, therefore, outlines the utility of the supramolecular AB/CD type of block copolymer towards preparation of ordered porous thin films carrying a chemically defined channel surface with a large number of reactive sites.