Five Years of ChemRxiv: Where We Are and Where We Go From Here.

ChemRxiv was launched on August 15, 2017 to provide researchers in chemistry and related fields a home for the immediate sharing of their latest research. In the past five years, ChemRxiv has grown into the premier preprint server for the chemical sciences, with a global audience and a wide array of scholarly content that helps advance science more rapidly. On the service's fifth anniversary, we would like to reflect on the past five years and take a look at what is next for ChemRxiv.

Five Years of ChemRxiv: Where We Are and Where We Go from Here.

ChemRxiv was launched on August 15, 2017, to provide researchers in chemistry and related fields a home for the immediate sharing of their latest research. In the past five years, ChemRxiv has grown into the premier preprint server for the chemical sciences, with a global audience and a wide array of scholarly content that helps advance science more rapidly. On the service's fifth anniversary, we would like to reflect on the past five years and take a look at what is next for ChemRxiv.

Unprecedented Sequence Control and Sequence-Driven Properties in a Series of AB-Alternating Copolymers Consisting Solely of Acrylamide Units.

Herein, we report a method to synthesize a series of alternating copolymers that consist exclusively of acrylamide units. Crucial to realizing this polymer synthesis is the design of a divinyl monomer that contains acrylate and acrylamide moieties connected by two activated ester bonds. This design, which is based on the reactivity ratio of the embedded vinyl groups, allows a "selective" cyclopolymerization, wherein the intramolecular and intermolecular propagation are repeated alternately under dilute conditions. The addition of an amine to the resulting cyclopolymers afforded two different acryl amide units, i.e., an amine-substituted acryl amide and a 2-hydroxy-ethyl-substituted acryl amide in alternating sequence. Using this method, we could furnish ten types of alternating copolymers; some of these exhibit unique properties in solution and in the bulk, which are different from those of the corresponding random copolymers, and we attributed the differences to the alternating sequence.

Orthogonal Folding of Amphiphilic/Fluorous Random Block Copolymers for Double and Multicompartment Micelles in Water.

Here, we report orthogonal folding and self-assembly systems of amphiphilic/fluorous random block copolymers for double core and multicompartment micelles in water. For this, we developed the precision folding techniques of polymer chains via the selective self-assembly of the pendant groups. Typically, A/C-B/C random block copolymers were designed: Hydrophobic dodecyl groups (A) and fluorous fluorinated octyl groups (B) were introduced into the respective blocks, while hydrophilic poly(ethylene glycol) chains (C) were randomly incorporated into all the segments. By controlling the chain length and composition of the respective blocks, the copolymers induce orthogonal single-chain folding in water to form double-compartment micelles comprising hydrophobic and fluorous cores. The copolymers were site-selectively folded in a fluoroalcohol to result in tadpole unimer micelles comprising a hydrophobic A/C unimer micelle and an unfolded fluorous B/C chain. Additionally, asymmetric A/C-B/C random block copolymers with short and highly hydrophobic or fluorous segments were effective for multicompartment micelles in water.

Self-Sorting of Amphiphilic Copolymers for Self-Assembled Materials in Water: Polymers Can Recognize Themselves.

Amphiphilic random copolymers bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic alkyl pendants showed dynamic self-sorting behavior, that is, self-recognition, under competitive conditions in aqueous media. The self-sorting universally takes place not only in water but also in hydrogels and on the material surfaces, according to encoded information originating from the primary structure of composition and pendants. Binary blends of the copolymers with different composition or alkyl pendants readily induced composition- or alkyl pendant-dependent self-sorting to simultaneously provide discrete and size-controlled micelles with hydrophobic cores. Surprisingly, the micelles reversibly keep exchanging polymer chains exclusively between identical polymer micelles even in the presence of different counterparts. Owing to the dynamic self-sorting behavior, ABA-triblock copolymers comprising the amphiphilic random copolymer A segments and a hydrophilic PEG chain B segment further provided hydrogels with self-healing yet selectively adhesive properties.

Control of the Alternating Sequence for N-Isopropylacrylamide (NIPAM) and Methacrylic Acid Units in a Copolymer by Cyclopolymerization and Transformation of the Cyclopendant Group.

An alternating copolymer of methacrylic acid and N-isopropyl acrylamide (NIPAM) was synthesized by selective cyclopolymerization of a special divinyl monomer and transformation of repeating cyclo-units in the resultant cyclopolymer. Crucial to the breakthrough is the monomer design in view of two types of cleavable bonds (3° ester and activated ester) in the pendant group of the monomer and the lower reactivity ratio of the two double bonds (methacrylate and electron-poor acrylate) for the polymerizable groups. The thus-obtained cyclopolymer was transformed into the alternating copolymer by transformation of the activated ester into amide by isopropyl amine and cleavage of the 3° ester by trifluoroacetic acid. The resultant copolymer showed thermal and pH response in aqueous solution that was different from the 1:1 random copolymer as well as the homopolymers.

Nanostructured Materials via the Pendant Self-Assembly of Amphiphilic Crystalline Random Copolymers.

Versatile self-assembly systems to nanostructured materials in both solid and solution were developed with common amphiphilic random copolymers bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic crystalline octadecyl pendants. The copolymers efficiently induced precision self-assembly of the pendants to provide not only core-crystalline, thermoresponsive micelles and vesicles in water and reverse micelles in hexane but also sub-10 nm lamellar or spherical microphase separation structure in solid. Typically, the solid random copolymers with 50-80 mol % octadecyl units formed lamellar structure of a hydrophilic PEG layer and a hydrophobic, crystalline octadecyl layer. Importantly, the domain spacing is about 5 nm, much smaller than that generally obtained with conventional block copolymers. The domain structure is controlled by composition, independent of chain length. The copolymers further gave various thermoresponsive, compartmentalized materials in aqueous and organic media, where the 3D structure can be also controlled by the composition and sample preparation protocols.

Acrylate-Selective Transesterification of Methacrylate/Acrylate Copolymers: Postfunctionalization with Common Acrylates and Alcohols.

Acrylate-selective transesterification of methacrylate/acrylate copolymers with alcohols was developed for a site-selective postfunctionalization technique of polymers without using specific monomers. Importantly, a common methyl acrylate efficiently works as a selective modification unit via transesterification coupled with a titanium alkoxide catalyst. The acrylate-selective transesterification is achieved owing to less steric hindrance of the carbonyl groups that are attached to the main chain without an α-methyl group. Typically, the acrylate pendants of dodecyl methacrylate/methyl acrylate (MA) random copolymers were selectively transesterified with benzyl alcohol (BzOH). The conversion of the pendent esters into benzyl esters proportionally increased with MA contents. Additionally, various alcohols were applicable to this MA-selective transesterification system.

Compartmentalization Technologies via Self-Assembly and Cross-Linking of Amphiphilic Random Block Copolymers in Water.

Orthogonal self-assembly and intramolecular cross-linking of amphiphilic random block copolymers in water afforded an approach to tailor-make well-defined compartments and domains in single polymer chains and nanoaggregates. For a double compartment single-chain polymer, an amphiphilic random block copolymer bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dodecyl, benzyl, and olefin pendants was synthesized by living radical polymerization (LRP) and postfunctionalization; the dodecyl and benzyl units were incorporated into the different block segments, whereas PEG pendants were statistically attached along a chain. The copolymer self-folded via the orthogonal self-assembly of hydrophobic dodecyl and benzyl pendants in water, followed by intramolecular cross-linking, to form a single-chain polymer carrying double yet distinct hydrophobic nanocompartments. A single-chain cross-linked polymer with a chlorine terminal served as a globular macroinitiator for LRP to provide an amphiphilic tadpole macromolecule comprising a hydrophilic nanoparticle and a hydrophobic polymer tail; the tadpole thus self-assembled into multicompartment aggregates in water.

Cyclopolymerization of Cleavable Acrylate-Vinyl Ether Divinyl Monomer via Nitroxide-Mediated Radical Polymerization: Copolymer beyond Reactivity Ratio.

Cyclopolymerization of a divinyl monomer, where two different vinyl groups, that is, acrylate and vinyl ether, are connected via an ester bond, was performed under diluted condition with nitroxide-meditated radical polymerization (NMP). Both vinyl groups were consumed at almost same rate under suitable condition, although the inherent cross-propagation ability between the two vinyl groups are pretty low in radical copolymerization. Furthermore, the polymerization was controlled to some extent to give polymers of unimodal molecular weight distributions. The results obviously differed from copolymerization and homopolymerization with vinyl monomers that constitutes the divinyl monomer, 2-methoxyethyl acrylate and 2-acetoxyethyl vinyl ether. Structural analyses indicated formation of the cyclopolymer but the cyclo-efficiency was imperfect indicating that some units of olefinic dangling were incorporated. Eventually, the ester bonds of the cyclo units were cleaved to convert into the copolymer consisting of acrylic acid and 2-hydroxy ethyl vinyl ether and the composition ratio (DPacryl/DPVE) was 55:45. The copolymer showed higher glass transition temperature than that estimated from the composition ratio and Tg values of the homopolymers, which is likely due to the formation of quasi-cyclopolymer between carboxylic acid and hydroxy groups aligned in alternating fashion.

Alternating Sequence Control for Carboxylic Acid and Hydroxy Pendant Groups by Controlled Radical Cyclopolymerization of a Divinyl Monomer Carrying a Cleavable Spacer.

By utilizing features of the hemiacetal ester (HAE) bond: easy formation from vinyl ether and carboxylic acid and easy cleavage into different functional groups (-COOH and -OH), we achieved control of the alternating sequence of two functional pendant groups of a vinyl copolymer. Methacrylate- and acrylate-based vinyl groups were connected through HAE bonds to prepare a cleavable divinyl monomer, which was cyclo-polymerized under optimized conditions in a ruthenium-catalyzed living radical polymerization. Subsequent cleavage of the HAE bonds in the resultant cyclo-pendant led to a copolymer consisting of alternating methacrylic acid and 2-hydroxyethyl acrylate units as analyzed by 13 C NMR spectroscopy. The alternating sequence of -COOH and -OH pendants specifically provided a lower critical solution temperature (LCST) in an ether solvent, which was not observed with the random copolymer of same composition ratio.

Sequence Analysis for Alternating Copolymers by MALDI-TOF-MS: Importance of Initiator Selectivity for Comonomer Pair.

A special initiator for metal-catalyzed living radical polymerization facilitates sequence analyses by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) of alternating copolymers from styrene and maleimide derivatives. The initiator is a malonate-based alkyl halide (DEMM-Br), in which two ester groups are attached on the carbon neighboring to bromide, and poor electron density of the radical species allows determination of next unit to the initiator in resultant alternating copolymers due to the selective initiation to styrene derivative. Thanks to the well-defined α-end group, sequence of the oligomeric products via radical copolymerization of PMS and EMI with DEMM-Br can be more simply analyzed by MALDI-TOF-MS, and indeed the following are clarified: the crossover propagation is almost perfectly controlled regardless of the injection ratio; a minor error event of the disordered alternating sequence containing St-St sequential unit could take place; the minor error can be suppressed with an excess amount of maleimide.

Terminal-Selective Transesterification of Chlorine-Capped Poly(Methyl Methacrylate)s: A Modular Approach to Telechelic and Pinpoint-Functionalized Polymers.

Terminal-selective transesterification of chlorine-capped poly(methyl methacrylate)s (PMMA-Cl) with alcohols was developed as a modular approach to create telechelic and pinpoint-functionalized polymers. Being sterically less hindered and electronically activated, both the α-end ethyl ester and ω-end methyl ester of PMMA-Cl were efficiently and selectively transesterified with diverse alcohols in the presence of a titanium alkoxide catalyst, while retaining the pendent esters intact, to almost quantitatively give various chlorine-capped telechelic PMMAs. In sharp contrast to conventional telechelic counterparts, the telechelic polymers obtained herein yet carry a chlorine atom at the ω-terminal to further work as a macroinitiator in living radical polymerization. The iterative process of living radical polymerization and terminal-selective transesterification successfully afforded unique pinpoint-functionalized polymers where a single functional monomer unit was introduced into the desired site of the polymer chains.

A strategy for sequence control in vinyl polymers via iterative controlled radical cyclization.

There is a growing interest in sequence-controlled polymers toward advanced functional materials. However, control of side-chain order for vinyl polymers has been lacking feasibility in the field of polymer synthesis because of the inherent feature of chain-growth propagation. Here we show a general and versatile strategy to control sequence in vinyl polymers through iterative radical cyclization with orthogonally cleavable and renewable bonds. The proposed methodology employs a repetitive and iterative intramolecular cyclization via a radical intermediate in a one-time template with a radical-generating site at one end and an alkene end at the other, each of which is connected to a linker via independently cleavable and renewable bonds. The unique design specifically allowed control of radical addition reaction although inherent chain-growth intermediate (radical species) was used, as well as the iterative cycle and functionalization for resultant side chains, to lead to sequence-controlled vinyl polymers (or oligomers).

Iterative Radical Addition with a Special Monomer Carrying Bulky and Convertible Pendant: A New Concept toward Controlling the Sequence for Vinyl Polymers.

Herein we propose a new concept to control sequence for vinyl polymers. A tertiary alkyl methacrylate monomer carrying both adamantyl and isopropyl groups (IPAMA) is very unique to allow control of single unit addition with an alkyl halide initiator for metal-catalyzed living radical polymerization due to the exceptional bulkiness. After control of the single unit addition, the bulkiness can be removed via acidolysis to further convert into the ester pendant with less bulky and nontertiary alcohol. The resultant adduct can be used as an initiator for the next single unit addition of IPAMA and the terminal ester can be selectively hydrolyzed followed by esterification similar to the first process. Namely, the cycle consisting of "radical addition of IPAMA", "acidolysis of the IPAMA side group", and "esterification of resultant carboxylic acid" can be repeated to construct sequence well-defined poly(oligo-)methacrylates. In this letter, results of the cycle and the iterative process with the special methacrylate monomer are actually demonstrated as well as the scope of applicable alcohols for the esterification process toward sequence control with functional units.

Discussion on "Aperiodic Copolymers".

In a recent Viewpoint (ACS Macro Lett. 2014, 3, 1020), J.-F. Lutz brought to the community's attention the need for more informative nomenclature, especially with respect to macromolecules with prescribed but not repeating sequences of monomers. Lutz proposes the use of the term "aperiodic" for this situation. In this Viewpoint, we comment on the need for such nomenclature and offer some alternatives for consideration.

Star Polymer Gels with Fluorinated Microgels via Star-Star Coupling and Cross-Linking for Water Purification.

Two types of star polymer gels containing perfluorinated microgels were created as purification materials to separate polyfluorinated surfactants (e.g., perfluorooctanoic acid) from water. One macrogel is prepared by the radical coupling of fluorine and/or amine-functionalized microgel star polymers alone, while another is done by the radical cross-linking of the star polymers with poly(ethylene glycol) methyl ether methacrylate. Importantly, the reactive olefin remaining within the microgel cores was directly employed for both coupling and cross-linking reactions. Swelling properties of star polymer gels were effectively controlled by the latter cross-linking technique. Analyzed by small-angle X-ray scattering, a star-star coupling gel typically consists of a three-dimensional network where star polymers are sequentially connected with the microgels at the constant interval of about 20 nm. Owing to the fluorous and acid/base cooperative interaction, star polymer gels carrying fluorine/amine-functionalized microgels efficiently captured polyfluorinated surfactants in water and successfully afforded the removal from water via simple mixing and filtration.

Shuttling Catalyst for Living Radical Miniemulsion Polymerization: Thermoresponsive Ligand for Efficient Catalysis and Removal.

In this report, we demonstrate the use of a thermoresponsive ligand for the ruthenium-catalyzed living radical polymerization of butyl methacrylate (BMA) in miniemulsion. A phosphine-ligand-functionalized polyethylene glycol chain (PPEG) in conjunction with a Cp*-based ruthenium complex (Cp*: pentamethylcyclopentadienyl) provided thermoresponsive character as well as catalysis for living polymerization: the complex migrated from the water phase to the oil phase for polymerization upon heating and then migrated from the oil to water phase when the temperature was decreased to quench polymerization. Consequently, simple treatment (i.e., water washing or methanol reprecipitation) yielded metal-free polymeric particles containing less than 10 µg/g (by ICP-AES) of ruthenium residue.

LCST-Type Phase Separation of Poly[poly(ethylene glycol) methyl ether methacrylate]s in Hydrofluorocarbon.

Poly[poly(ethylene glycol) methyl ether methacrylate]s [poly(PEGMA)s] sharply and reversibly exhibited lower critical solution temperature (LCST)-type phase separation in 2H,3H-perfluoropentane (2HPFP). The cloud points decreased from 52 to 41 °C with increasing the PEG pendant length [-(CH2CH2O)mCH3: m = 4.5, 9, 19]. The cloud point was precisely controlled via the addition of perfluoroalkanes (e.g., perfluorooctane) to the 2HPFP solution: typically, it was inversely proportional to the amount of perfluorooctane in the mixture. The unique thermoresponsive solubility further afforded the temperature-mediated micellization of a block copolymer of PEG19MA and methyl methacrylate (MMA) in 2HPFP to uniquely give a PEG-core micelle with PMMA shell. Therefore, the LCST phase separation properties in the hydrofluorocarbon would open new vistas for thermoresponsive polymeric materials.

Fluorous microgel star polymers: selective recognition and separation of polyfluorinated surfactants and compounds in water.

Immiscible with either hydrophobic or hydrophilic solvents, polyfluorinated compounds (PFCs) are generally "fluorous", some of which have widely been employed as surfactants and water/oil repellents. Given the prevailing concern about the environmental pollution and the biocontamination by PFCs, their efficient removal and recycle from industrial wastewater and products are critically required. This paper demonstrates that fluorous-core star polymers consisting of a polyfluorinated microgel core and hydrophilic PEG-functionalized arms efficiently and selectively capture PFCs in water into the cores by fluorous interaction. For example, with over 10 000 fluorine atoms in the core and approximately 100 hydrophilic arms, the fluorous stars remove perfluorooctanoic acid (PFOA) and related PFCs in water from 10 ppm to as low as a parts per billion (ppb) level, or an over 98% removal. Dually functionalized microgel-core star polymers with perfluorinated alkanes and additional amino (or ammonium) groups cooperatively recognize PFOA or its ammonium salt and, in addition, release the guests upon external stimuli. The "smart" performance shows that the fluorous-core star polymers are promising PFC separation, recovery, and recycle materials for water purification toward sustainable society.