Cyclic bottlebrush polymers were synthesized and used as single molecular templates to prepare donut-shaped hybrid nanoparticles with organo-silica cross-linked and gold nanocluster coordinated internal structures. “Grafting-onto” strategy was adopted to prepare cyclic bottlebrush polymers by combining ring-expansion metathesis polymerization (REMP), reversible addition–fragmentation chain transfer (RAFT) polymerization, and triazolinedione (TAD)-diene click reaction. In this approach, cyclic poly(norbornene imide) backbones with diene side groups (C-PNB-diene) were synthesized based on REMP technique. TAD-terminated diblock copolymer side chains were produced from RAFT polymerization including TAD-terminated poly(3-(triethoxysilyl)propyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PTEPM-b-POEGMA) and TAD-terminated poly(glycidyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PGMA-b-POEGMA). The cyclic bottlebrush polymers 1 and 2 were then prepared by virtue of TAD-diene click reaction to graft TAD-PTEPM-b-POEGMA and TAD-PGMA-b-POEGMA side chains onto C-PNB-diene backbones, respectively. Furthermore, the donut-shaped hybrid nanoparticles with organo-silica cross-linked internal structures were obtained by in situ cross-linking PTEPM domains of the cyclic bottlebrush polymer 1 templates. For the formation of donut-shaped hybrid nanoparticles coordinating gold nanoclusters inside, the cyclic bottlebrush polymer 2 templates were first postmodified to introduce the functional pyridine groups inside the PGMA domains, which were then used as location to coordinate the gold nanoclusters.

•Well-defined cyclic polyesters are prepared via the metal-free homodifunctional bimolecular ring-closure method.•The self-accelerating DSPAAC reaction eliminated the requirement of 1:1 stoichiometry between the two reactants.•Pure cyclic polyesters, including monocyclic homo/block polyesters and bicyclic polyesters, are efficiently prepared.A metal-free and efficient homodifunctional bimolecular ring-closure method was developed specifically for the formation of well-defined cyclic polyesters based on the combination of ring opening polymerization (ROP) and self-accelerating double strain-promoted azide-alkyne cycloaddition (DSPAAC) reaction. In this method, ROP and the following end group modification were used to prepare the azide-terminated liner polyester precursors. The self-accelerating DSPAAC click reaction was then used to prepare the corresponding cyclic polyesters by ring-closing the linear polyester precursors with the small linkers of sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DBA). The distinct advantage of this novel homodifunctional bimolecular ring-closure method lied in the fact that the pure cyclic polyesters could be efficiently prepared by using excess molar amounts of DBA small linkers to ring-close the linear polyester precursors. This was resulted from the self-accelerating property of the DSPAAC ring-closing reaction.Download high-res image (187KB)Download full-size image

Cyclic multiblock polymers with high-order blocks are synthesized via the combination of single-electron transfer living radical polymerization (SET-LRP) and copper-catalyzed azide-alkyne cycloaddition (CuAAC). The linear α,ω-telechelic multiblock copolymer is prepared via SET-LRP by sequential addition of different monomers. The SET-LRP approach allows well control of the block length and sequence as A-B-C-D-E, etc. The CuAAC is then performed to intramolecularly couple the azide and alkyne end groups of the linear copolymer and produce the corresponding cyclic copolymer. The block sequence and the cyclic topology of the resultant cyclic copolymer are confirmed by the characterization of 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography, Fourier transform infrared spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

As the most straightforward synthetic strategy for cyclic polymers in theory, the traditional homodifunctional bimolecular ring-closure methods showed limited success for preparing pure cyclic polymers in practice even after several decades of development. A breakthrough was achieved in this paper to develop a successful homodifunctional bimolecular ring-closure method using a self-accelerating double strain-promoted azide–alkyne click reaction as the intermolecular and subsequent intramolecular coupling reactions. Because of the self-accelerating property of coupling reaction, this novel approach eliminated the usage of equimolar quantities between telechelic polymers and small molecule linkers, which was the prerequisite of traditional homodifunctional bimolecular ring-closure methods for pure cyclic polymers. More importantly, this approach could use an excess amount of small linkers to increase the intermolecular coupling reaction rate, further resulting in a significantly enhanced preparation efficiency of cyclic polymers.

Aggregation-induced emission (AIE) polymer nanoparticles (PNPs) including spherical and cylindrical micelles and vesicles were prepared to have a pH-tunable fluorescence response by virtue of the amphiphilic block copolymer self-assembly technique. The poly(M1)-b-poly(M2-co-M3) block copolymers were prepared from ring-opening metathesis polymerization (ROMP) of norbornene-based monomers M1, M2, and M3, in which the hydrophilic poly(M1) had poly(ethylene glycol) side chains and the hydrophobic poly(M2-co-M3) possessed reactive pentafluorophenyl ester (in M2) and AIE-active tetraphenylethene (in M3) side groups. The spherical and cylindrical micelles and vesicles were then self-assembled from poly(M1)-b-poly(M2-co-M3) with different block ratios in selective solvents of THF/water. This produced self-assembly with hydrophobic microdomains aggregated by poly(M2-co-M3) blocks and dispersing shells formed by hydrophilic poly(M1) blocks. In the presence of reactive pentafluorophenyl ester groups, the hydrophobic microdomains could be crosslinked by reacting with the diamine crosslinkers to chemically fix the self-assembly morphology and produce the stable PNPs. Based on the same activated ester chemistry, stable PNPs with varied morphologies and pH-responsive AIE properties were prepared by further post-functionalizing the crosslinked hydrophobic microdomains. Using N,N-diethylethylenediamine as a post-functionalization agent to introduce the diethylamino groups, the resultant PNPs emitted weak fluorescence when the pH < 4 and strong fluorescence when the pH > 5 in water. Comparatively, using β-alanine as a post-functionalization agent to introduce the carboxylic groups, the resultant PNPs emitted weak fluorescence when the pH > 10 and strong fluorescence when the pH < 8 in water. In addition, the pH-dependant fluorescence “turn on/off” properties were obtained for the post-functionalized PNPs when reversibly varying the pH value of the aqueous solution between 2 and 11.

A series of novel thermoresponsive polymers were developed with hydrophobic polynorbornene backbones and hydrophilic N-alkyl-amide/imide side groups. The preparation method was built on the combination of ring-opening metathesis polymerization (ROMP) and activated ester click chemistry. The polynorbornene templates (P1, P2 and P4) were synthesized by ROMP having pentafluorophenyl ester side groups. The various amine agents were then used to post-modify the precursors of P1, P2 and P4 by a nucleophilic substitution of activated esters, producing a library of well-defined N-alkyl-amide/imide polynorbornenes (P1a–h, P2a–h and P4a–h). The thermoresponsive properties were systematically investigated and quantified by turbidity measurements. By manipulating the types and densities of hydrophilic N-alkyl-amide/imide side groups to balance the hydrophilicity/hydrophobicity of the resultant polynorbornenes, their cloud point temperature (Tcp) in water could be tuned in a wide temperature range between 14.4 °C and 97.6 °C.

The endless molecular topology endows cyclic polymers with fascinating physical properties and applications. As a powerful strategy for the preparation of well-defined cyclic polymers, the current ring-closure methods have an inherent disadvantage of low production efficiency under the classic batch reaction conditions due to the requirement of ultralow reaction concentration for ring-closing linear polymer precursors. Assisted by the continuous-flow technique, we developed an efficient and practical way to ingeniously solve this essential problem and successfully produce cyclic polymers on a large scale by a light-induced ring-closure method for the first time. In addition, due to the large surface-to-volume ratio of the flow reactor, the continuous-flow technique provides the light-induced ring-closing reaction with more uniform light irradiation and transmission, resulting in a significantly increased reaction efficiency compared to that from the classic batch reaction conditions.

An efficient metal-free “grafting onto” method was developed for preparing bottlebrush polymers based on the combination of reversible addition–fragmentation chain transfer polymerization (RAFT) and triazolinedione (TAD)–diene Diels–Alder cycloaddition reaction. In this approach, RAFT and a following postfunctionalization process were used to prepare the polyacrylate backbone with conjugated diene side group in each repeat unit (PHEA–diene) and the various TAD-terminated polymer sides including poly(methyl methacrylate) (PMMA), poly(tert-butyl acrylate) (PtBA), and polystyrene (PS). The TAD–diene Diels–Alder cycloaddition reaction was then employed to efficiently couple the resultant polymer backbone and side chains, which produced the corresponding bottlebrush polymers of PHEA448-g-PMMA27, PHEA448-g-PtBA25, and PHEA448-g-PS25 with a high grafting density above 90% in only 1 min in the presence of slight molar excess (1.2 times) of TAD to diene groups. The quantitative grafting density could be further achieved in less than 10 min coupling reaction. Atomic force microscopy (AFM) characterization visualized the worm-like molecular morphology for all cases.

•Construction of T-shaped and H-shaped topological polymers.•Catalyst free UV-induced strain promoted azide-alkyne cycloaddition reaction.•Reactive polymer with a cyclopropenone-masked dibenzocyclooctyne group in the middle of polymer chain.Topological polymers with T-shaped and H-shaped molecular architecture were built on the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide-alkyne cycloaddition (SPAAC) reaction. In the presence of a cyclopropenone-masked dibenzocyclooctyne functionalized dibromo ATRP initiator, reactive polystyrene (PS) was synthesized to have a cyclopropenone-masked dibenzocyclooctyne group in the middle of polymer chain. After releasing dibenzocyclooctyne from deprotecting cyclopropenone-masked dibenzocyclooctyne under UV irradiation, T-shaped and H-shaped topological polymers were constructed by reacting PS with mono-end and di-end azide functionalized poly(ethylene oxide) (PEO), respectively, based on the SPAAC reaction.

In the present study, we synthesized well-defined tadpole-shaped polystyrene (PS) via the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide-alkyne cycloaddtion (SPAAC) reaction. A di-bromo ATRP initiator (Br-ini-Br) containing cyclopropenone-masked dibenzocyclooctyne group was used to prepare the linear PS with a cyclopropenone-masked dibenzocyclooctyne in the middle of the chain and bromo groups at both ends (Br-PS-Br). Then we used the single electron transfer-nitroxide radical coupling (SET-NRC) reaction to transfer the bromo end groups to azide groups (N3-PS-N3). After UV irradiation, the dibenzocyclooctyne group was quantitatively released, and intramolecularly reacted with alternative azide end group to produce the tadpole-shaped PS based on SPAAC reaction.

Cyclic polymers are a particular class of macromolecules without terminal groups. Most studies has involved their physical properties and polymer composition, while attention has rarely been paid to their emulsification in an oil–water system. Herein we synthesized a cyclic polymer with polystyrene side chains via ring-expansion metathesis polymerization and click-chemistry. This cyclic polymer was compared with linear polystyrene in order to investigate the effect of cyclic topology on preparing porous particles by emulsion templating methods. The contribution of cyclic topology to emulsification originates from the formation of hollow microspheres with the use of cyclic polymer while linear polymer only afforded solid microspheres. With addition of hexadecane as soft template, both cyclic polymer and linear polymer emulsions were successfully converted into porous particles. Superior to linear polymer, cyclic polymer enables the stabilization of emulsion droplets and the tuning of porous morphology. It is revealed that cyclic polymer with nanoring shape tends to assemble at the interfacial area, leading to the Pickering effect that decelerates the macrophase separation. Furthermore, the unique porous feature of polymer particles affords a convenient application for the detection of trace explosive.

A unique method was developed for the preparation of cyclic polymers based on the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide–alkyne cycloaddition (SPAAC) reaction. By virtue of a cyclopropenone-masked dibenzocyclooctyne functionalized ATRP initiator (I-1), well-defined telechelic polystyrene (PS) was synthesized to have a cyclopropenone-masked dibenzocyclooctyne at one polymer chain end and a bromo group at the other. The single electron transfer-nitroxide radical coupling reaction was then used to modify the bromo end group to azide, resulting in the corresponding linear PS precursor. Under UV irradiation on its highly diluted solution, the dibenzocyclooctyne end group was quantitatively released from cyclopropenone-masked dibenzocyclooctyne, which intramolecularly reacted with the azide end group in situ to ring-close the linear PS precursor and produce the corresponding cyclic PS based on the SPAAC click reaction.

A unique ring-closure method was developed specifically focusing on the preparation of cyclic polymers derived from unconjugated vinyl monomers, which was established on the combination of reversible addition–fragmentation chain transfer polymerization (RAFT)/macromolecular design via the interchange of xanthate (MADIX) and light-induced strain promoted azide–alkyne cycloaddition (SPAAC) reaction. By virtue of a specifically designed xanthate agent 1 as the reversible chain transfer agent to polymerize the unconjugated vinyl monomers, well-defined telechelic polymers were produced by the RAFT/MADIX polymerization technique bearing the cyclopropenone-masked dibenzocyclooctyne and azide end groups. Under UV irradiation on a highly diluted solution of the resultant linear polymer precursors, the dibenzocyclooctyne end group was quantitatively released from cyclopropenone-masked dibenzocyclooctyne, which intramolecularly reacted with the azide end group in situ to ring-close the linear polymers and produce the corresponding cyclic polymers based on the SPAAC click reaction. In addition, this novel ring-closure method produced the cyclic polymer chain with a cleavable S–C(S)O bond within the xanthate group, which facilitated the resultant cyclic polymers to be efficiently cleaved back to their linear counterparts under mild aminolysis or reduction reaction conditions.

•ROMP polymers.•Triazolinedione based Alder-ene reaction.•Post-Functionalization of polymers.Triazolinedione (TAD) based Alder-ene reaction was demonstrated as a unique method to efficiently post-functionalize the well-defined polymers derived from living ring-opening metathesis polymerization (ROMP). Using 4-nitrophenyl-1,2,4-triazoline-3,5-dione (p-NO2-PhTAD) as a model TAD compound to react with the double bonds in the backbone of ROMP polymers, the model polymer of poly(N-propyl-5-norbornene-exo-2,3-dicarboximide) (Poly-1) was successfully post-functionalized by grafting the p-NO2-PhTAD functional groups onto the polymer backbone. The TAD based Alder-ene post-functionalization reaction and the resultant p-NO2-PhTAD functionalized Poly-1 were systematically investigated by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). In addition, using a difunctional TAD of 4,4′-(4,4′-diphenylmethylene)-bis-(1,2,4-triazoline-3,5-dione) as a model crosslinking agent, network materials were efficiently produced by crosslinking ROMP derived Poly-1 from TAD based Alder-ene chemistry.

•Cyclopropenone-masked dibenzocyclooctyne end functionalized ATRP initiators.•ATRP of methacrylates, styrenics, and acrylates.•UV-induced SPAAC click chemistry.Two functional atom transfer radical polymerization (ATRP) initiators (I-2 and I-3) were developed bearing a cyclopropenone-masked dibenzocyclooctyne group. ATRP was then explored on three main kinds of monomers for radical polymerization including acrylates, styrenics, and methacrylates based on these novel initiators. By a standard ATRP protocol, the polymerization behavior demonstrated the living characteristics for all three cases and the corresponding well-defined cyclopropenone-masked dibenzocyclooctyne end or middle functionalized polymers were produced conveniently. Since UV irradiation of the cyclopropenone-masked dibenzocyclooctynes could quantitatively release the dibenzocyclooctynes widely used in strain promoted azide-alkyne cycloaddition bioorthogonal click reaction, these novel ATRP initiators and the resultant well-defined polymers should play an important role in the preparation of topological polymers and bio-synthetic polymer conjugates and many other related fields.

Aggregation-induced emission (AIE) amphiphilic block copolymers were developed based on the living ring-opening metathesis polymerization for the first time. By virtue of the block copolymer self-assembly in selective solvents, water soluble fluorescent nano-objects were then prepared with various structures including spherical micelles, cylindrical micelles, and vesicles. This method represents a facile and efficient way to prepare the well-defined AIE polymers and the varied fluorescent nano-objects with controlled structures and functionalities.

A novel ring-closure method was developed to specifically focus on the preparation of water soluble cyclic polymers. The well-defined linear polymers were synthesized by a standard RAFT polymerization using a functional RAFT agent 1. The cyclic polymers were then obtained by virtue of an efficient bromomaleimide-thiol substitution reaction to ring-close the linear precursors. This technique is unique in that it not only produces various well-defined water soluble cyclic polymers with high efficiency and topology purity, but also employs the environmentally benign solvent, water, as the ring-closure reaction media.

Cyclic polymers, as one of the oldest topological polymers, are undergoing resurgence. This is largely ascribed to the significant achievements in modern polymer chemistry. The novel ring-expansion techniques have conveniently produced varied cyclic polymers with highly topological purity and on large scales, which should facilitate their use in the near future. Beyond the monocyclic molecular conformations, the combination of controlled polymerization techniques and click chemistry have established a robust strategy for preparing cyclic polymers with more complex architectures, such as theta, eight, and tadpole shapes. This diversification in cyclic polymer composition and conformation significantly broadens interest in the cyclic polymers. However, compared to the synthesis achievements, the exploration of cyclic polymer property and application are lagging behind. Recently, we explored the ring-expansion metathesis polymerization on various functional ring-strained olefin monomers to produce cyclic functional polymers, which were then used as the building blocks to fabricate cyclic brush polymers and cyclic gel materials and will be discussed here.

•Synthesize PEO-b-PDEAEMA block copolymer by anionic polymerization.•Stable Y(III) and Cu(II) metal ions loaded polymer micelles.•Simple way to prepare Y(III) loaded supramolecular hydrogels in-situ.Poly(ethylene oxide)-block-poly(2-(diethylamino)ethyl methacrylate) (PEO-b-PDEAEMA) diblock copolymer was synthesized by anionic polymerization, whose molecular structure was characterized by 1H NMR and size exclusion chromatography (SEC). The diblock copolymer self-assembled into micelles in nonacid aqueous solution with PEO and PDEAEMA as corona and core respectively. By virtue of the coordinating property of PDEAEMA block to metal ions, the resultant micelles were then used as carriers to load metal ions Y(III) and Cu(II) in the micellar core. The morphology and stability of the metal loaded micelles were characterized by dynamic light scattering (DLS), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The metal loading amounts were determined by elemental analyses, UV spectrometry and titrimetric analysis. In addition, the Y(III) loaded micelles were demonstrated to complex with α-cyclodextrins and form supramolecular hydrogels in-situ. The metal loaded micelles and the resultant supramolecular hydrogels will have potential application for cancer internal radiotherapy.

The dibenzocyclooctyne end functionalized agent 1 was designed as atom transfer radical polymerization (ATRP) initiator. The ATRP was then explored on three types of monomers widely used in free radical polymerization: methyl methacrylate, styrene, and acrylates (n-butyl acrylate and tert-butyl acrylate). The living polymerization behaviors were obtained for the methyl methacrylate and styrene monomers. The SPAAC click reactivity of dibenzocyclooctyne end group were demonstrated by successfully reacting with azide functionalized small chemical agents and polymers. Various topological polymers such as block and brush polymers were produced from strain-promoted azide-alkyne cycloaddition reaction (SPAAC) using the resultant dibenzocyclooctyne end functionalized poly(methyl methacrylate)/polystyrene as building blocks. For the acrylates, however, the polymerization did not hold the living characteristics with the dibenzocyclooctyne end functionalized ATRP initiator 1.

A powerful ring-closure method was developed for the formation of cyclic polymers by combining reversible addition–fragmentation chain transfer polymerization (RAFT) and light-induced Diels–Alder click reaction. The outstanding features of this novel method were demonstrated from the following four aspects. This convenient and efficient technique could produce cyclic polymers in air at room temperature without any other catalyst or stimulus requirements other than a mild UV irradiation. The universality of this method was demonstrated by successfully producing five different types of cyclic homopolymers including polystyrene, poly(methyl methacrylate), poly(tert-butyl acrylate), poly(N,N-dimethylacrylamide), and poly(2-vinylpyridine) and two kinds of block copolymers of poly(methyl methacrylate)-block-polystyrene and poly(methyl methacrylate)-block-poly(tert-butyl acrylate). This is the first time to report a one-pot ring-closure method for the formation of cyclic polymers, in which the crude monomer polymerization solution was directly diluted as precursors and no more requirements were needed to purify and postfunctionalize the linear polymer intermediates. When combing with a batchwise operation, this novel light-induced ring-closure method could significantly improve the notorious disadvantage of low cyclic polymer yield accompanied by the ring-closure strategy.