Metal-free synthesis of pyridyl conjugated microporous polymers with tunable bandgaps for efficient visible-light-driven hydrogen evolution

Conjugated microporous polymers (CMPs) with tunable bandgaps have attracted increasing attention for photocatalytic hydrogen evolution. However, the synthesis of CMPs usually needs expensive metal-based catalysts. Herein, we report a metal-free synthetic route to fabricate pyridyl conjugated microporous polymers (PCMPs) via a condensed polymerization between aldehyde and aryl ketone monomers. The PCMPs show widely tunable specific surface areas (347–418 m²/g), which were controlled via changing the used monomers. The PCMPs synthesized using monomers of dialdehyde and diacetylbenzene (diacetylpyridine) in the presence of pyridine exhibited the highest visible-light driven hydrogen evolution rate (9.56 µmol/h). These novel designed PCMPs provide wide adaptability to current materials designed for high-performance photocatalysts in different applications.     

Synthesis of branched polystyrene by ATRP exploiting divinylbenzene as branching comonomer

Branched polystyrenes have been synthesized using atom transfer radical polymerization (ATRP) of styrene in the presence of divinyllbenzene (DVB) as branching comonomer. The synthesis was completed via facile one pot approach. Mole ratio of styrene to DVB in range of 5:1-30:1 was employed to obtain soluble polymers. The kinetics of the polymerization and evolution of polymer compositions were revealed by determining the conversions of reactants by gas chromatography (GC). The growth of molecular weight was monitored by GPC and the results indicate that the branched polymers were formed by self-condensing vinyl polymerization (SCVP) of AB^∗ monomer or macromonomers. The branched structure of the resulting polymers was confirmed by the remarkable discrepancies of the weight average molecular weights determined by GPC and multi angle laser light scattering (MALLS). The specific viscosity of the resulting polymer is also much lower compared with that of linear analogues. The influence of dosage of initiator and catalyst on the yield and molecular weights of the resulting polymers was also investigated.     

Complementary,Cooperative Ditopic Halogen Bonding and Electron Donor-Acceptor π-π Complexation in the Formation of Cocrystals

This study expands and combines concepts from two of our earlier studies. One study reported the complementary halogen bonding and π-π charge transfer complexation observed between isomeric electron rich 4-N,N-dimethylaminophenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the second study elaborated the ditopic halogen bonding of activated pyrimidines. Leveraging our understanding on the combination of these non-covalent interactions, we describe cocrystallization featuring ditopic halogen bonding and π-stacking. Specifically, red cocrystals are formed between the ditopic electron poor halogen bond donor 1-(3,5-dinitrophenylethynyl)-2,4,6-triflouro-3,5-diiodobenzene and each of electron rich pyrimidines 2- and 5-(4-N,N-dimethyl-aminophenylethynyl)pyrimidine. The X-ray single crystal structures of these cocrystals are described in terms of halogen bonding and electron donor-acceptor π-complexation. Computations confirm that the donor-acceptor π-stacking interactions are consistently stronger than the halogen bonding interactions and that there is cooperativity between π-stacking and halogen bonding in the crystals.     

Synthesis of poly(vinyl acetate) with fluorescence via a combination of RAFT/MADIX and “click” chemistry

A novel ω-azido-functionalized RAFT reagent, O-(2-azido-ethyl) S-benzyl dithiocarbonate (AEBDC), was synthesized and subsequently employed to mediate the reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl acetate (VAc) to prepare end-functionalized polymers. The polymerization results showed that the RAFT polymerizations of VAc could be well controlled using AEBDC as the RAFT agent. Number-average molecular weights (M_n GPC) increased linearly with monomer conversion, and molecular weight distributions were relatively narrow. ¹H NMR spectrum of the poly(vinyl acetate) (PVAc) confirmed the existence of functional azido group at the end of the polymers chains. The ω-azido-terminated polymers were coupled by “click” chemistry with a fluorescent alkyne, 7-propinyloxy coumarin, to prepare fluorescent PVAc. The fluorescence properties of the PVAc homopolymers before and after coupling with 7-propinyloxy coumarin in CH₂Cl₂ solution were investigated.     

Acyclic diene metathesis polymerization of tailor-made monomers towards sequence-regulated vinyl copolymers

 Acyclic diene metathesis polymerization (ADMET) enables convenient transfer of sequential information of the designed monomers to the corresponding sequence-regulated copolymers. In this study, two structurally symmetric monomers, M1 and M2, were synthesized via atom transfer radical addition (ATRA) of diethyl meso-2,5-dibromohexanedioate with 1,5-hexadiene and 1,7-octadiene, respectively. Thus, sequenced segment of VB-EA-EA-VB (VB and EA represent vinyl bromide and ethyl acrylate, respectively) was incorporated into the ADMET diene monomers. ADMET polymerization of these two monomers with Grubbs first generation catalyst (Grubbs-I) was performed in CH₂Cl₂ at 40℃ for 5 days under nitrogen purge. Effects of catalyst amount, monomer concentration and methanol precipitation on the M_p and PDI of polymers were investigated by GPC, and the structures of the formed polymers were characterized by NMR. Our results indicate that using 3.0 mol% of Grubbs-I to monomer can afford polymers with high M_p. Moreover, selective precipitation in methanol enables complete removal of low molecular weight components from the crude products. Meanwhile, M2 exhibits higher ADMET polymerization reactivity than M1 due to its capability of suppressing negative neighboring group effect.     

Polymers with advanced structural and supramolecular features synthesized through topochemical polymerization

Polymers are an integral part of our daily life. Hence, there are constant efforts towards synthesizing novel polymers with unique properties. As the composition and packing of polymer chains influence polymer''s properties, sophisticated control over the molecular and supramolecular structure of the polymer helps tailor its properties as desired. However, such precise control via conventional solution-state synthesis is challenging. Topochemical polymerization (TP), a solvent- and catalyst-free reaction that occurs under the confinement of a crystal lattice, offers profound control over the molecular structure and supramolecular architecture of a polymer and usually results in ordered polymers. In particular, single-crystal-to-single-crystal (SCSC) TP is advantageous as we can correlate the structure and packing of polymer chains with their properties. By designing molecules appended with suitable reactive moieties and utilizing the principles of supramolecular chemistry to align them in a reactive orientation, the synthesis of higher-dimensional polymers and divergent topologies has been achieved via TP. Though there are a few reviews on TP in the literature, an exclusive review showcasing the topochemical synthesis of polymers with advanced structural features is not available. In this perspective, we present selected examples of the topochemical synthesis of organic polymers with sophisticated structures like ladders, tubular polymers, alternating copolymers, polymer blends, and other interesting topologies. We also detail some strategies adopted for obtaining distinct polymers from the same monomer. Finally, we highlight the main challenges and prospects for developing advanced polymers via TP and inspire future directions in this area.

This perspective showcases the potential of topochemical polymerization as an effective tool for synthesizing polymers with advanced molecular and supramolecular structures.     

Ring-opening polymerization of macrocyclic aryl ether ketone oligomers containing the 1,2-dibenzoylbenzene moiety

Facile ring-opening polymerization of cyclic aryl ether oligomers containing the 1,2-dibenzoylbenzene moiety to form high molecular weight linear polymers in the presence of a nucleophilic initiator is described. The polymerization can be initiated in the melt in the presence of a nucleophilic initiator such as potassium carbonate, cesium fluoride, and alkali phenoxides. Various alkali phenoxides were investigated as potential nucleophilic initiators. The polymerization reaction rate in the melt increases in the order of K⁺ > Na⁺ > Cs⁺, and in the order of ⁻OPhPhO⁻ > PhO⁻ > PhOPhO⁻ > PhPhO⁻. However, the polymerization in an aprotic dipolar solvent is faster in the presence of cesium phenoxide than in the presence of potassium phenoxide. Polymerization of the cyclic oligomers in solution demonstrates that the ring-opening polymerization proceeds via a chain-growth mechanism and involves a transetherification reaction between linear and cyclic aryl ether oligomers. The ring-chain equilibrium is much more favorable towards linear polymers. Since little or no ring strain exists in the cyclic system, the transetherification reactions are indiscriminate with regards to cyclic or linear chains and the interchain equilibration is also a facile process during polymerization. This intermolecular transetherification has been demonstrated by using low molecular weight aryl ethers to control the molecular weight of the polymer formed via ring-opening polymerization. © 1996 John Wiley & Sons, Inc.     

Evidence for shape selectivity in non-covalently imprinted polymers

The existence of shape selectivity in non-covalent molecularly imprinted polymers (MIPs) has been proven using molecular probes. Twelve secondary amines with different sized side chains were imprinted, and enantioselectivity evaluated by HPLC for each amine on each imprinted polymer. Trends in the quantitative structure-binding relationships (QSBR) revealed two major contributions of cavity structure on selectivity afforded by molecularly imprinted polymers. First, sterics play a dominant role in cases where a molecules structure is too big too fit into an imprinted site formed from a smaller template molecule; e.g. on MIPs made with small templates, large analytes give separation factors (α) close to 1.0 (no selectivity), while small analytes give α values of 1.4. Second, molecular structures that are equal to or smaller than those of the template molecule are selected by maximizing Van der Waals interactions within the MIP binding site. Thus, MIPs made with large analytes give α values up to 2.5, while small analytes on the same MIPs give α values closer to 1.1. Template structure also has an effect on MIP enantioselectivity; e.g. branched structures exhibit a 1.7-fold improvement in separation factors (α) by MIPs made for isopropyl versus propyl derivatives, and cyclohexyl versus hexyl derivatives. Full details of these trends are provided in the text.     

Visible-light-induced N-heterocyclic carbene mediated cascade transformation of N-alkenoxypyridinium salts

While N-alkenoxypyridinium salts are widely used for the synthesis of α-functionalized ketones via umpolung strategy, such approaches are usually limited to special nucleophiles at high temperatures. Herein, we developed an alternative photoinduced N-heterocyclic carbene (NHC)- mediated functionalization of N-alkenoxypyridinium salts with various nucleophiles, including tetramethylammonium azide, secondary amines, aryl and alkyl thiols, and even the challenging C(sp³)-nucleophiles, under mild conditions. A cascade radical-radical coupling/nucleophilic substitution sequence was proposed, wherein the NHC enabled the formation of a photoactive electron donor-acceptor complex for α-iodo ketone synthesis.     

Precise Synthesis of Ultra‐High‐Molecular‐Weight Fluoropolymers Enabled by Chain‐Transfer‐Agent Differentiation under Visible‐Light Irradiation

Ultra‐high‐molecular‐weight (UHMW) polymers display outstanding properties and hold potential for wide applications. However, their precise synthesis remains challenging. Herein, we developed a novel reversible‐deactivation radical polymerization based on the strong and selective fluorine–fluorine interaction, allowing chain‐transfer agents to spontaneously differentiate into two groups that take charge of the chain growth and reversible deactivation of the growing chains, respectively. This method enables dramatically improved livingness of propagation, providing UHMW polymers with a surprisingly narrow molecular weight distribution (?≈1.1) from a variety of fluorinated (meth)acrylates and acrylamide at quantitative conversions under visible‐light irradiation. In situ chain‐end extensions from UHMW polymers facilitated the synthesis of well‐defined block copolymers, revealing the excellent chain‐end fidelity achieved by this method.     

Adsorption isotherms of a molecular imprinted polymer prepared in the presence of a polymerisable template: Indirect evidence of the formation of template clusters in the binding site

The current opinion about molecular imprinted polymers (MIPs) is that their molecular recognition properties are due to the presence of nanocavities formed during a polymerization process developed in the presence of a template molecule. According to this principle, the shape of these nanocavities is complementary to that of the template and non-covalent interactions are established between the binding site and a single template molecule. Nevertheless, there are some experimental indications that the real molecular recognition mechanism involves clusters of template molecules being packed into the binding site. Recently, it has been proposed that template molecules covalently linked to the binding site can act as nucleation points, enhancing the formation of these molecular clusters.We have tested this hypothesis by studying the adsorption isotherms of polymers prepared by imprinting them with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Three different polymers were considered: P0, prepared without the template, P1, whose template was represented by 2,4,5-T molecules, and P2, whose template was 1/3 constituted by the polymerisable 2-(2,4,5-trichlorophenoxyacetoxy)-ethylmethacrylate (2,4,5-TEMA) and 2/3 by 2,4,5-T. The polymers were prepared by thermoinduced polymerization of template mixtures, 4-vinylpyridine and ethylene dimethacrylate. The crushed polymers were packed into HPLC columns and frontal chromatographic runs were performed by eluting the columns with a mobile phase containing variable amounts of 2,4,5-T.The experimental adsorption isotherms were fitted by using several isotherm models, and the Freundlich-Langmuir model was found to give the best fitting in terms of F-test. All the models considered showed a significant difference between the affinity constant values measured for the polymer P1 and P2, with a higher value for the polymer P2 (for Freundlich-Langmuir model: polymer P1, k=(2.00±0.43)×10⁴ M⁻¹; polymer P2, k=(1.93±0.0535)×10⁵ M⁻¹; ratio P2/P1, 9.65±2.09). Such experimental results support the hypothesis that a polymer prepared with a limited amount of template covalently attached to the binding site shows an increased affinity for the template itself.     

A boronate affinity restricted-access material with external hydrophilic bottlebrush polymers for pretreatment of cis-diols in biological matrices

Restricted-access materials (RAMs) with an external hydrophilic bottlebrush polymer layer was prepared for sample pretreatment of bioanalysis.     

Amine‐Reactive Polymers Synthesized by RAFT Polymerization Using an Azlactone Functional Trithiocarbonate RAFT Agent

A new azlactone‐derived trithiocarbonate is prepared and used as a chain‐transfer agent to mediate the reversible addition‐fragmentation chain transfer (RAFT) polymerization of styrene, ethyl acrylate, and N‐isopropyl acrylamide. Well‐defined polymers with controlled molecular weights (M _n = 1000–7000 g mol⁻¹) and narrow molecular weight distributions (PDI = 1.05–1.10) are thus obtained that retain the azlactone functionality at the chain end. The ability of the resulting end‐functionalized polymers to react quantitatively at room temperature with a stoichiometric amount of amino groups with retention of the thiocarbonylthio moiety is ascertained by using 4‐fluorobenzylamine and allylamine.     

A new method for synthesizing hyperbranched polymers with reductive groups using redox/RAFT/SCVP

The novel hyperbranced polymers containing reductive groups were successfully prepared and characterized using redox/reversible addition fragmentation chain transfer(RAFT)/self-condensing vinyl polymerization(SCVP) method. Several redox initiating chemicals such as Cu(III)/―CONH2, Ce(IV)/―CONH2 and Ce(IV)/―OH were chosen to increase the free radical generating rate, and the chain transfer agent(CTA) was used to reduce the molecular chain propagating rate, in order to obtain polymers with high degree of branching. Detailed analyses based on the molecular weight, ? value and the degree of branching of polymers(DB) obtained from 1H-NMR spectra and multi detector size exclusion chromatography(MDSEC) suggested the acquiring of hyperbranced polyacrylamides with Cu(III)/―CONH2 and Ce(IV)/―CONH2 as initiator in the presence of the CTA. Meanwhile, the as-prepared poly(N-hydroxymethyl acrylamide)(PNHAM) with higher DB value(0.48) proved that using Ce(IV)/―OH as the initiator could increase the free radical generating rate and diminish the gap between the propagating rate and the initiation rate during the reaction procedure. In addition, the effect of oxidant concentration on the Mark-Houwink index(?) value and the DB was also studied.     

Simultaneous enhancement of phosphorescence and chirality by host–guest recognition of molecular tweezers

A novel type of host–guest recognition systems have been developed on the basis of a Au(III) molecular tweezer receptor and chiral Pt(II) guests. The complementary host–guest motifs display high non-covalent binding affinity (K_a: ～10⁴ L/mol) due to the participation of two-fold intermolecular π–π stacking interactions. Both phosphorescence and chirality signals of the Pt(II) guests strengthen in the resulting host–guest complexes, because of the cooperative rigidifying and shielding effects rendered by the tweezer receptor. Their intensities can be reversibly switched toward pH changes, by taking advantage of the electronic repulsion effect between the protonated form of tweezer receptor and the positive-charged guests in acidic environments. Overall, the current study demonstrates the feasibility to enhance and modulate phosphorescence and chirality signals simultaneously via molecular tweezer-based host–guest recognition.     

Molecular Engineering of Conjugated Acetylenic Polymers for Efficient Cocatalyst‐free Photoelectrochemical Water Reduction

Conjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene‐ to thiophene‐based. The polarized thiophene‐based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on‐surface Glaser polycondensation, as‐fabricated poly(2,5‐diethynylthieno[3,2‐b]thiophene) on commercial Cu foam exhibits a record H₂‐evolution photocurrent density of 370 μA cm^?2 at 0.3 V vs. reversible hydrogen electrode among current cocatalyst‐free organic photocathodes (1–100 μA cm^?2). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high‐activity organic photoelectrodes.     

A comparative study of two polymorphs of L‐aspartic acid hydrochloride

Two polymorphs of L‐aspartic acid hydrochloride, C₄H₈NO₄⁺·Cl⁻, were obtained from the same aqueous solution. Their crystal structures have been determined from single‐crystal data collected at 100 K. The crystal structures revealed three‐ and two‐dimensional hydrogen‐bonding networks for the triclinic and orthorhombic polymorphs, respectively. The cations and anions are connected to one another via N—H...Cl and O—H...Cl interactions and form alternating cation–anion layer‐like structures. The two polymorphs share common structural features; however, the conformations of the L‐aspartate cations and the crystal packings are different. Furthermore, the molecular packing of the orthorhombic polymorph contains more interesting interactions which seems to be a favourable factor for more efficient charge transfer within the crystal.     

Synthesis and characterization of Fe(II)-coordinated PS-b-P[NIPAM-co-(VBC-Fe-DMAP)] block copolymers

In this work,a new type of block polymers,polystyrene-b-poly[(N-isopropyl acrylamide)-co-(vinyl benzyl chloride)](PS-b-P(NIPAM-co-VBC)),was prepared via reversible addition fragmentation transfer polymerization,then pentacyano(4-(dimethylamino pyridine))ferrate(Fe-DMAP) was attached to VBC units through a quaternization process.The Fe(Ⅱ)-coordinated PS-b-P[NIPAM-co-(VBC-Fe-DMAP)]block copolymers were characterized by ~1H-NMR,FT-IR and TGA.The self-assembly behavior of the block copolymers was also investigated and the micelle morphology was characterized by TEM.It was found that the PS-b-P(NIPAM-co-VBC) block polymer and Fe-coordinated block copolymer could both form spherical micelles in DMF/MeOH mixed solvent.     

RAFT Polymerization of N‐Isopropylacrylamide and Acrylic Acid under γ‐Irradiation in Aqueous Media

Summary: The ambient temperature (20 °C) reversible addition fragmentation chain transfer (RAFT) polymerization of N‐isopropylacrylamide (NIPAAm) and acrylic acid (AA) conducted directly in aqueous media under γ‐initiation (at dose rates of 30 Gy · h⁻¹) proceeds in a controlled fashion (typically, < 1.2) to near quantitative conversions and up to number‐average molecular weights of 2.5 × 10⁵ g · mol⁻¹ for PNIPAAm and 1.1 × 10⁵ g · mol⁻¹ for PAA via two water‐soluble trithiocarbonate chain transfer agents, i.e., S,S‐bis(α,α′‐dimethyl‐α″‐acetic acid)trithiocarbonate (TRITT) and 3‐benzylsulfanylthiocarbonylsulfanyl propionic acid (BPATT). The generated polymers are successfully chain extended, which suggests that the RAFT agents are stable throughout the polymerization process so that complex and well‐defined architectures can be obtained.

An increase of the monomer/CTA ratio leads to an increase of the molecular weight for the RAFT polymerization of NIPAAm under γ‐radiation in water using TRITT at ambient temperature.     

Sieving polymer synthesis by reversible addition fragmentation chain transfer polymerization

Replaceable sieving polymers are the fundamental component for high‐resolution nucleic acids separation in CE. The choice of polymer and its physical properties play significant roles in influencing separation performance. Recently, reversible addition fragmentation chain transfer (RAFT) polymerization has been shown to be a versatile polymerization technique capable of yielding well‐defined polymers previously unattainable by conventional free‐radical polymerization. In this study, a high molecular weight poly‐(N,N‐dimethylacrylamide) (PDMA) at 765 000 gmol^?1 with a polydispersity index of 1.55 was successfully synthesized with the use of chain transfer agent—2‐propionic acidyl butyl trithiocarbonate in a multistep sequential RAFT polymerization approach. This study represents the first demonstration of RAFT polymerization for synthesizing polymers with the molecular weight range suitable for high‐resolution DNA separation in sieving electrophoresis. Adjustment of pH in the reaction was found to be crucial for the successful RAFT polymerization of high molecular weight polymer as the buffered condition minimizes the effect of hydrolysis and aminolysis commonly associated with trithiocarbonate chain transfer agents. The separation efficiency of 2‐propionic acidyl butyl trithiocarbonate PDMA was found to have marginally superior separation performance compared to a commercial PDMA formulation, POP?‐CAP, of similar molecular weight range.