Disulfide‐centered poly(methyl acrylates): Four different stimuli to cleave a polymer

A series of poly(methyl acrylates) incorporating a disulfide center was subjected to four different stimuli to cleave the S S bond, as the weakest member of the chain. Four polymers with molecular weights in the range of 25–93 kDa were synthesized via Cu‐based atom transfer radical polymerization starting from a difunctionalized disulfide‐containing initiator. In tetrahydrofuran, the labile disulfide center was cleaved directly by mechanical force generated by ultrasound irradiation, whereas in chloroform, competing cleavage was also triggered by radical species that were generated by non‐mechanical sonolysis. A reductive cleavage was observed upon treatment with reducing agent, and a clean reversal of the cleavage via oxidation could be obtained if the terminal bromide was first removed via hydrogenolysis. This modified polymer allowed studies on a fourth strategy for the cleavage of the disulfide, namely, dynamic covalent exchange reactions with a fluorine labeled small molecule. While partially based on established protocols, this comparative study underscores the versatility of the disulfide bond for applications in stimuli‐responsive and adaptive materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1404–1411     

Controlling supramolecular polymerization through multicomponent self‐assembly

The self‐assembly into supramolecular polymers is a process driven by reversible non‐covalent interactions between monomers, and gives access to materials applications incorporating mechanical, biological, optical or electronic functionalities. Compared to the achievements in precision polymer synthesis via living and controlled covalent polymerization processes, supramolecular chemists have only just learned how to developed strategies that allow similar control over polymer length, (co)monomer sequence and morphology (random, alternating or blocked ordering). This highlight article discusses the unique opportunities that arise when coassembling multicomponent supramolecular polymers, and focusses on four strategies in order to control the polymer architecture, size, stability and its stimuli‐responsive properties: (1) end‐capping of supramolecular polymers, (2) biomimetic templated polymerization, (3) controlled selectivity and reactivity in supramolecular copolymerization, and (4) living supramolecular polymerization. In contrast to the traditional focus on equilibrium systems, our emphasis is also on the manipulation of self‐assembly kinetics of synthetic supramolecular systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 34–78     

Recent progress on cyclic polymers: Synthesis,bioproperties, and biomedical applications

Polymer topologies exert a significant effect on its properties, and polymer nanostructures with advanced architectures, such as cyclic polymers, star‐shaped polymers, and hyperbranched polymers, are a promising class of materials with advantages over conventional linear counterparts. Cyclic polymers, due to the lack of polymer chain ends, have displayed intriguing physical and chemical properties. Such uniqueness has drawn considerable attention over the past decade. The current review focuses on the recent progress in the design and development of cyclic polymer with an emphasis on its synthesis and bio‐related properties and applications. Two primary synthetic strategies towards cyclic polymers, that is, ring‐expansion polymerization and ring‐closure reaction are summarized. The bioproperties and biomedical applications of cyclic polymers are then highlighted. In the end, the future directions of this rapidly developing research field are discussed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1447–1458     

Polymerization of supramonomers: A new way for fabricating supramolecular polymers and materials

Supramolecular polymers and materials are attracting more and more attention nowadays due to their dynamic properties such as reversibility, stimuli-responsiveness and self-healing. Conventionally, bifunctional or multi-functional monomers are first covalently synthesized, followed by the supramolecular complexation to form supramolecular polymers and materials. Recently, we have proposed the supramonomer concept to construct supramolecular polymers and materials in a different way. Supramonomers are bifunctional or multi-functional monomers fabricated by noncovalent synthesis, but can undergo traditional covalent polymerization. In this highlight article, we will summarize and discuss the fabrication of supramonomer and covalent polymerization methods of supramonomers; fabrication of multi-responsive supramolecular polymers from supramonomers; and fabrication of supramolecular materials from supramonomers. It is highly anticipated that the supramonomer concept will enrich the methodology towards supramolecular polymers and materials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 604–609     

Synthesis and character of novel polycarbonate for constructing biodegradable multi‐stimuli responsive delivery system

Here, we design a novel triple‐stimuli‐sensitive graft copolymer assembly which responds to the changes in temperature, reducing agent, and light. The graft copolymer consists of thermo‐responsive tetraethylene glycolyl poly(trimethylene carbonate) (P(MTC‐4EG)) as backbone and light‐sensitive poly(2‐nitrobenzyl methacrylate) (PNBM) as side chain linked by an intervening disulfide bond. In aqueous solution, the polymer can self‐assemble into micelle with thermo‐sensitive shell (P(MTC‐4EG)), light‐sensitive core (PNBM), and disulfide linker. The assemblies in response to stimuli were revealed by dynamic light scatting (DLS) and transmission electron microscopy (TEM). The drug release behaviors of Nile Red (NR)‐loaded carriers were also valued with stimuli from temperature, reducing agent, and light. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3583–3592     

A facile and controllable synthesis of dual‐crosslinked elastomers based on linear bifunctional polydimethylsiloxane oligomers

Dual‐crosslinked supramolecular elastomers with the hybrid network consisting of hydrogen bonds and covalent bonds combine the reversibility of hydrogen bond and mechanical properties of covalent crosslinking network. In this article, isocyanate mixture is used as curing agent to prepare dual‐crosslinked elastomer based on bifunctional polydimethylsiloxane under mild condition. This method can effectively build up a hybrid network with the designed structure. A series of elastomers with same hydrogen bond density and variable covalent crosslinking degree are obtained. Swelling measurements and ¹H‐NMR spectra confirm the feasibility and controllability of curing method, the increasing of bifunctional isocyanate give rise to higher covalent crosslinking degree, improving the solvent resistance. The studies on viscoelastic property show that the introduction of an irreversible covalent crosslinking network stabilize the hybrid network, restrain the chain movement. The mechanical and self‐healing property studies reveal that the covalent crosslink significantly reinforce the whole network, while the reparable strength seems to mainly depend on the hydrogen bond density. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3760–3768     

Comb‐like polymers pendanted with elastin‐like peptides showing sharp and tunable thermoresponsiveness through dynamic covalent chemistry

Comb‐like polymers carrying two elastin‐like polypeptide (ELP) pendants in each repeat unit were synthesized. The densely attached peptide chains afford these polymers with sharp thermally induced phase transitions, and their lower critical solution temperature (LCST) can be varied with molecular weights, solution pH and salt concentrations. Through amino terminals in ELP pendants, oligoethylene glycol (OEG)‐based dendrons cored with aldehyde were attached to the polymers through dynamic covalent imines. By virtue of dynamic characteristics of these novel dendronized polymers, their LCSTs can be tuned significantly by dendron coverage to shift from that dominated by ELPs to that dominated by OEG dendrons. Furthermore, dendron coverage can be enhanced obviously by the thermally induced phase transitions or greatly by freezing the polymer aqueous solutions. The work provides a convenient methodology to improve thermoresponsiveness of ELPs through polymer topology and to switch their properties through dynamic covalent chemistry. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3379–3387     

Applications of shear‐induced polarized light imaging (SIPLI) technique for mechano‐optical rheology of polymers and soft matter materials

A new experimental method for studying the mechano‐optical rheology of polymeric liquids and soft matter materials is presented. The method is based on a combination of rotational rheology and a recently developed optical technique—shear‐induced polarized light imaging (SIPLI). The method provides a unique opportunity to monitor a complete sample view during rheological measurements in plate–plate and cone‐and‐plate geometry. Applications of the method are presented including simultaneous SIPLI and the rheology of the oriented lamellar phase of block copolymers and liquid crystals as well as a study of the thermally induced reversible transformation of worm‐like micelles to spherical micelles. In addition, a direct relation between the shish formation and the polymer melt viscosity upturn during flow‐induced crystallization of semi‐crystalline polymers is demonstrated. An application of SIPLI for quantitative birefringence measurements is also shown. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2151–2170     

Self‐sustained actuation from heat dissipation in liquid crystal polymer networks

Liquid crystal polymer networks (LCNs) lead the research geared toward macroscopic motion of materials. These actuators are molecularly programed to adapt their shape in response to external stimuli. Non‐photo‐responsive thin films of LCNs covered with heat absorbers (e.g., graphene or ink) are shown to continuously oscillate when exposed to light. The motion is governed by the heat dissipated at the film surface and the anisotropic thermal deformation of the network. The influence of the LC molecular alignment, the film thickness, and the LC matrix on the macroscopic motion is analyzed to probe the limits of the system. The insights gained from these experiments provide not only guidelines to create actuators by photo‐thermal or pure photo‐effects but also a simple method to generate mechanical oscillators for soft robotics and automated systems. © 2018 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1331–1336     

Multi‐Stimuli‐Responsive Polymer Materials: Particles,Films, and Bulk Gels

Stimuli‐responsive polymers have received tremendous attention from scientists and engineers for several decades due to the wide applications of these smart materials in biotechnology and nanotechnology. Driven by the complex functions of living systems, multi‐stimuli‐responsive polymer materials have been designed and developed in recent years. Compared with conventional single‐ or dual‐stimuli‐based polymer materials, multi‐stimuli‐responsive polymer materials would be more intriguing since more functions and finer modulations can be achieved through more parameters. This critical review highlights the recent advances in this area and focuses on three types of multi‐stimuli‐responsive polymer materials, namely, multi‐stimuli‐responsive particles (micelles, micro/nanogels, vesicles, and hybrid particles), multi‐stimuli‐responsive films (polymer brushes, layer‐by‐layer polymer films, and porous membranes), and multi‐stimuli‐responsive bulk gels (hydrogels, organogels, and metallogels) from recent publications. Various stimuli, such as light, temperature, pH, reduction/oxidation, enzymes, ions, glucose, ultrasound, magnetic fields, mechanical stress, solvent, voltage, and electrochemistry, have been combined to switch the functions of polymers. The polymer design, preparation, and function of multi‐stimuli‐responsive particles, films, and bulk gels are comprehensively discussed here.     

Grubbs-inspired metathesis in the Moore group

Alkene metathesis has proven to be a powerful method for carbon carbon bond formation, particularly in the field of polymer and materials science. The availability of various tailor-made catalysts not only enables the synthesis of well-defined polymers but facilitates the development of functional, stimuli–responsive materials. This highlight, dedicated to Professor Robert Grubbs on his 75th birthday, focuses on the various research efforts in our group utilizing both alkene and alkyne metatheses and the interesting materials derived from them. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2935–2948     

Shape memory ionomers

This review is focused on the use of ionomers in shape memory polymers. Ionomers are polymers that contain less than ∼15% ionic groups. The incompatibility between the ion-pairs and the polymer backbone drives microphase separation producing dispersed ionic aggregates, which can physically crosslink the polymer. Shape memory polymers are responsive materials that can be deformed to program a temporary shape and then recovered on application of an external stimulus. Through the review of the main types of ionomers used in shape memory polymers, polyurethanes and polyester ionomers, polyolefin and polyaromatic ionomers, and perfluorosulfonic acid ionomers (i.e., Nafion^®) it will be shown that ionomers can produce robust thermoplastic shape memory polymers and in many cases impart unique properties which allow advanced shape memory materials to be obtained including antibacterial, high temperature, and multishape memory polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1389–1396     

Degradable Hybrid Materials Based on Cationic Acylhydrazone Dynamic Covalent Polymers Promote DNA Complexation through Multivalent Interactions

The design of smart nonviral vectors for gene delivery is of prime importance for the successful implementation of gene therapies. In particular, degradable analogues of macromolecules represent promising targets as they would combine the multivalent presentation of multiple binding units that is necessary for achieving effective complexation of therapeutic oligonucleotides with the controlled degradation of the vector that would in turn trigger drug release. Toward this end, we have designed and synthesized hybrid polyacylhydrazone‐based dynamic materials that combine bis‐functionalized cationic monomers with ethylene oxide containing monomers. Polymer formation was characterized by ¹H and DOSY NMR spectroscopy and was found to take place at high concentration, whereas macrocycles were predominantly formed at low concentration. HPLC monitoring of solutions of these materials in aqueous buffers at pH values ranging from 5.0 to 7.0 revealed their acid‐catalyzed degradation. An ethidium bromide displacement assay and gel electrophoresis clearly demonstrated that, despite being dynamic, these materials are capable of effectively complexing dsDNA in aqueous buffer and biological serum at N/P ratios comparable to polyethyleneimine polymers. The self‐assembly of dynamic covalent polymers through the incorporation of a reversible covalent bond within their main chain is therefore a promising strategy for generating degradable materials that are capable of establishing multivalent interactions and effectively complexing dsDNA in biological media.     

A New Dynamic Covalent Bond of Se?N: Towards Controlled Self‐Assembly and Disassembly

A new kind of Se? N dynamic covalent bond has been found that can form between the Se atom of a phenylselenyl halogen species and the N atom of a pyridine derivative, such as polystyrene‐b‐poly(4‐vinylpyridine). This Se? N dynamic covalent bond can be reversibly and rapidly formed or cleaved under acidic or basic conditions, respectively. Furthermore, the bond can be dynamically cleaved by heating or treatment with stronger electron‐donating pyridine derivatives. The multiple responses of Se? N bond to external stimuli has enriched the existing family of dynamic covalent bonds. It can be used for controlled and reversible self‐assembly and disassembly, which may find potential applications in a number of areas, including self‐healing materials and responsive assemblies.     

Small changes with big effects: Tuning polymer properties with supramolecular interactions

For biological polymers like DNA and proteins, supramolecular interactions dictate the folding and assembly of the polymer chains. Advances in synthetic polymer chemistry enable the synthesis of polymers of defined length and composition, but the field has yet to reach the same level of sophistication as nature's polymers. However, the incorporation of just a few supramolecular interactions into a synthetic polymer chain can drastically change the manner in which the polymer assembles and interacts, thereby altering the properties of a polymeric material. This highlight will focus on approaches wherein a low‐density of supramolecular functionalities (<10 wt %) were used per polymer chain. How the selection of the appropriate supramolecular functionality (based on the directionality and strength of the interaction), along with the location of these groups on a polymer chain, can afford a spectrum of material properties has been highlighted. At one end, the supramolecular motif can dramatically alter the elasticity of a material, and at the other, the motif can have a more subtle effect like increasing the stability of a micelle. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 457–472     

Imine‐based dynamic polymer networks as photoprogrammable amine sensing devices

Here, we describe a “smart” polymeric material, which is able to readily detect and discriminate amine vapors. The dynamic imine‐based network can be conveniently prepared by mixing a commercially available, amino‐functionalized polysiloxane with small amounts of a diarylethene dialdehyde. The photoswitchable crosslinker allows for reversible imprinting of custom‐designed patterns on the polymer surface with (sun)light and thus enables noninvasive information storage in the material, which before, during, and after amine exposure can readily be decoded with commonly used smartphone apps. This feature along with the self‐healing nature of the dynamic polymer, an easy recycling and manufacturing procedure, and the overall low cost and toxicity render this material advantageous to develop low‐cost and practical amine sensing devices for the broad public. © 2019 The Authors. Journal of Polymer Science Part A: Polymer Chemistry published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2378–2382     

Spontaneous crosslinking of poly(1,5‐dioxepan‐2‐one) originating from ether bond fragmentation

The spontaneous reaction of unsaturated double bonds induced by the fragmentation of ether bonds is presented as a method to obtain a crosslinked polymer material. Poly(1,5‐dioxepan‐2‐one) (PDXO) was synthesized using three different polymerization techniques to investigate the influence of the synthesis conditions on the ether bond fragmentation. It was found that thermal fragmentation of the ether bonds in the polymer main chain occurred when the synthesis temperature was 140 °C or higher. The double bonds produced reacted spontaneously to form crosslinks between the polymer chains. The formation of a network structure was confirmed by Fourier transform infrared spectrometry and differential scanning calorimetry. In addition, the low molar mass species released during hydrolysis of the DXO polymers were monitored by ESI‐MS and MALDI‐TOF‐MS. Ether bond fragmentation also occurred during the ionization in the electrospray instrument, but predominantly in the lower mass region. No fragmentation took place during MALDI ionization, but it was possible to detect water‐soluble DXO oligomers with a molar mass up to approximately 5000 g/mol. The results show that ether bond fragmentation can be used to form a network structure of PDXO. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7258–7267, 2008     

Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. II. Influence of side‐chain structure

Solution‐crystallization is studied for two polyfluorene polymers possessing different side‐chain structures. Thermal analysis and temperature‐dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X‐ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar‐zigzag chain conformation termed the β‐phase, which is observed for certain linear‐side‐chain polyfluorenes, is necessary for the formation of so‐called polymer‐solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side‐chains prevents formation of the β‐phase conformation and results in non‐solvated, i.e. melt‐crystallization‐type, polymer crystals. Unlike non‐solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β‐phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter‐chain interactions. The presented results clarify the fundamental differences between the β‐phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1492–1506     

Self‐assembly of supramolecular polymers into tunable helical structures

There is growing interest in the design of synthetic molecules that are able to self‐assemble into a polymeric chain with compact helical conformations, which is analogous to the folded state of natural proteins. Herein, we highlight supramolecular approach to the formation of helical architectures and their conformational changes driven by external stimuli. Helical organization in synthetic self‐assembling systems can be achieved by the various types of noncovalent interactions, which include hydrogen bonding, solvophobic effects, and metal‐ligand interactions. Since the external environment can have a large influence on the strength and configuration of noncovalent interactions between the individual components, stimulus‐induced alterations in the intramolecular noncovalent interactions can result in dynamic conformational change of the supramolecular helical structure thus, driving significant changes in the properties of the materials. Therefore, these supramolecular helices hold great promise as stimuli‐responsive materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1925–1935, 2008