Recent advances in the syntheses of radical‐containing macromolecules

Tailor‐made polymers containing specific chemical functionalities have ushered in a number of emerging fields in polymer science. In most of these next‐generation applications the focus of the community has centered upon closed‐shell macromolecules. Conversely, macromolecules containing stable radical sites have been less studied despite the promise of this evolving class of polymers. In particular, radical‐containing macromolecules have shown great potential in magnetic, energy storage, and biomedical applications. Here, the progress regarding the syntheses of open‐shell containing polymers are reviewed in two distinct subclasses. In the first, the syntheses of radical polymers (i.e., materials composed of non‐conjugated macromolecular backbones and with open‐shell units present on the polymer pendant sites) are described. In the second, polyradical (i.e., macromolecules containing stabilized radical sites either within the macromolecular backbone or those containing radical sites that are stabilized through a large degree of conjugation) synthetic schemes are presented. Thus, the state‐of‐the‐art in open‐shell macromolecular syntheses will be reported and future means by which to advance the current archetype will be discussed. By detailing the synthetic pathways possible for, and the inherent synthetic limitations of, the creation of these functional polymers, the community will be able to extend the bounds of the radical‐containing macromolecular paradigm. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1875–1894     

Elucidation of the electrochemical behavior of phenothiazine-based polyaromatic amines

Polyarlylamines with discrete redox active groups in the polymer backbone represent a promising class of cathode materials for electrical energy storage applications. In this area, our group recently reported a set of phenothiazine-based polymers that exhibit both high capacities and power densities. In order to rationally improve the properties of these electrode materials, a fundamental understanding of their electrochemical properties is indispensable. Herein, we probe the electrochemical behavior of our phenothiazine-based systems by synthesizing small molecule analogs using C–N cross-coupling. Additionally, electropolymerization of a class of these small molecule phenothiazines yields thin films that were then characterized with an electrochemical quartz crystal microbalance. Analysis of these materials provides insights into the number of electrons accessed from each repeat unit in our polymer backbone during electrochemical cycling, as well as counter ion transport dynamics.     

Conjugated Nanohoop Polymers based on Antiaromatic Dibenzopentalenes for Charge Storage in Organic Batteries

With their bent π-systems, cyclic conjugation and inherent cavities, conjugated nanohoops are attractive for organic electronics applications. For ease of processing and morphological stability, an incorporation into polymers is desirable, but to date was hampered with few exceptions by synthetic difficulties. We herein present a unique strategy for the synthesis of conjugated nanohoop polymers using a dibenzo[a,e]pentalene (DBP) as central connector. We demonstrate this versatility by synthesizing three electronically diverse copolymers with dithienyldiketo(pyrrolopyrrol), fluorene and carbazole comonomers, and report the first donor-acceptor nanohoop polymer. Optoelectronic investigations reveal the prevalence of cyclic or linear conjugation, depending on the comonomer unit, and ambipolar electrochemical properties through the antiaromatic character of the DBP units. As the first report on using conjugated nanohoops for charge storage as positive electrode materials, we show a significant improvement in battery performance in a nanohoop-containing polymer compared to an equivalent nanohoop-free reference polymer. We believe this study will pave the way for the synthesis of a diverse range of nanohoop polymers and further stimulate their exploration for charge storage in batteries.     

Bioconjugates of smart polymers and proteins: synthesis and applications

Over the past 20 years we have been deeply involved with the synthesis and applications of stimuli-responsive polymer systems, especially polymer-biomolecule conjugates. The work of Toyoichi Tanaka has been a constant inspiration for our work and this article is dedicated to him. This article summarizes the research that we have carried out along with many collaborators on polymer-protein conjugates. We include conjugates prepared by random polymer conjugation to lysine amino groups, and also those prepared by site-specific conjugation of the polymer to specific amino acid sites that are genetically-engineered into the known amino acid sequence of the protein. We describe the preparation and properties of thermally-sensitive, random conjugates to enzymes and several affinity recognition proteins. We have also prepared site-specific conjugates to streptavidin. with temperature-sensitive polymers, pH-sensitive polymers, and light-sensitive polymers. The preparation of these conjugates and their many fascinating applications are reviewed in this article.     

Progress of Indeno-type Organic Diradicaloids

Organic diradicaloids have unusual open-shell nature and properties and are promising materials for organic electronics, spintronics, energy storage and nonlinear optics. In this review, we focus on indeno-type organic diradicaloids and summarize their molecular design and synthesis, as well as topological structures, open-shell characters and diradical properties. The molecular systems are classified into indenofluorenes and diindenoacenes, indeno-based molecules with one-dimensional, two-dimensional and unique topological structures, and heterocyclic indeno-based molecules. By constructing these various topological π-skeletons with tunable conjugation modes and variation of atomic composition, their key open-shell parameters, such as diradical characters and singlet-triplet energy gaps, along with the optical, electronic and magnetic properties, as well as stabilities are efficiently modulated. More attention may be paid to accurate computational analysis, rational design and synthesis, and novel functions of indeno-type diradicaloids, which will promote the development of radical chemistry and materials.     

Hydrothermal Generation of Conjugated Polymers Using the Example of Pyrrone Polymers and Polybenzimidazoles

Various polyimides and polyamides have recently been prepared via hydrothermal synthesis in nothing but H₂O under high‐pressure and high‐temperature conditions. However, none of the prepared polymers feature a truly conjugated polymer backbone. Here, we report on an expansion of the synthetic scope of this straightforward and inherently environmentally friendly polymerization technique to the generation of conjugated polymers. Selected representatives of two different polymer classes, pyrrone polymers and polybenzimidazoles, were generated hydrothermally. We present a mechanistic discussion of the polymer formation process as well as an electrochemical characterization of the most promising product.     

Sterically-encumbered ionenes as hydroxide ion-conducting polymer membranes

Hydrogen/oxygen-based electrochemical energy conversion cells that operate under highly alkaline conditions deploy inexpensive electrocatalysts compared to their acidic counterparts. Solid polymer electrolyte (SPE) cells offer a reduced system footprint, and an additional reduction in capital cost. Alkaline membrane, SPE systems are attractive because they offer a synergistic combination of the two cost savings. Durable, hydroxide-conducting SPEs operating in highly caustic media are lacking because organic molecules and polymers, particularly their cationic derivatives, are inherently unstable to caustic conditions. This review focuses on an emerging class of alkaline SPE's, Ionenes: polymers that incorporate cations directly into the polymer backbone. The purpose of this opinion piece is to highlight the fact that fixed cationic charges may be incorporated into the polymeric backbone to provide membranes with high conductivity, mechanical strength, and stability, and to dispel the widely-held view that distancing a cationic group from the polymer backbone is a necessary requirement.     

Preparation, photophysics, and electrochemistry of segmented comonomers consisting of thiophene and pyrimidine units: new monomers for hybrid copolymers

An efficient coupling route to novel pi-conjugated comonomers consisting of pyrimidine, thiophene, and bithiophene units was developed. The novel pi-donor-acceptor-donor and pi-donor-acceptor-acceptor-donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman homocoupling reactions. Photophysical investigation of these alternating pi-donor and acceptor compounds indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%. Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative to its metal-free counterpart.     

Photochemically degradable polymers containing metal-metal bonds along their backbones: the effect of stress on the rates of photochemical degradation

The syntheses of polymers that have metal-metal bonds along their backbones are described. The polymers are photodegradable because the metal-metal bonds homolyze when irradiated with visible light. The photochemical reactions of the polymers in solution are identical to the photochemical reactions of the discrete metal-metal bonded dimers. Typical reactions include metal-metal bond disproportionation and metal radical capture, for example, by chlorine atom abstraction from carbon tetrachloride. The polymers are also photochemically degradable in the solid state; thin films of the polymers degrade when irradiated with visible light in the presence of oxygen or in the absence of oxygen if the polymer backbone has a built-in radical trap. The origin of tensile stress-induced rate enhancements in the photodegradation of polymers was studied using the polymers with metal-metal bonds along their backbones and with built-in radical traps. By eliminating the need for external oxygen to act as a radical trap, the experimentally challenging problem of diffusion-controlled oxidation kinetics was avoided. Analysis of plots of quantum yields for degradation vs. stress reveals that stress increases the separation of the radical fragments produced by photolysis. An increased separation leads to less radical-radical recombination, which increases the efficiency of degradation. Quantitative knowledge of the factors that control polymer degradation rates will eventually allow synthesis of an ideal photodegradable polymer - one that has a tunable onset of degradation and that degrades quickly once degradation has started.     

Precursor synthesis strategy for polycyclic aromatic conjugated polymers on the application of supercapacitors

Polycyclic aromatics (PCAs) possess excellent photoelectric properties, but the construction of such compounds has been a quite challenging subject of study, mainly due to very low solubility. Herein we report a precursor synthesis strategy for polycyclic aromatic conjugated polymers. A soluble precursor polymer, that containing fusible “double U-shaped aromatic” (DUA) and perylenetetracarboxydiimide (PDI) units, was firstly synthesized by Suzuki coupling. The stereo aromatic units in polymer backbone were found to be converted into polycyclic aromatic units, i.e. hexa-peri-hexabenzocoronene (HBC), by chemical or electrochemical oxidation, which resulted in a formation of insoluble polycyclic aromatic conjugated polymers. The electrochemical oxidations that occurred at the interface of electrode and solution exhibited higher cyclization reactivity and leads to the formation of high quality films on the electrode surface. Characterization by Raman and UV-visible (UV-Vis) spectroscopy validated the successful formation of this HBC structure. Some potential applications of such thin films are being explored, and here we focus on the characteristics of supercapacitors based on their excellent electrochemical properties.     

Effect of Extended π‐Conjugation Structure of Donor–Acceptor Conjugated Copolymers on the Photoelectronic Properties

New donor–acceptor conjugated copolymers based on alkylthienylbenzodithiophene (BDTT) and alkoxynaphthodithiophene (NDT) have been synthesized and compared with their benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based analogues to investigate the effect of the extended π conjugation of the polymer main chain on the physicochemical properties of the polymers. A systematic investigation into the optical properties, energy levels, field‐effect transistor characteristics, and photovoltaic characteristics of these polymers was conducted. Both polymers demonstrated enhanced photovoltaic performance and increased hole mobility compared with the BDT‐based analogue. However, the BDTT‐based polymer (with π‐conjugation extension perpendicular to main chain) gave the highest power conversion efficiency of 5.07 % for the single‐junction polymer solar cell, whereas the NDT‐based polymer (with π‐conjugation extension along the main chain) achieved the highest hole mobility of approximately 0.1 cm² V^?1 s^?1 based on the field‐effect transistor; this indicated that extending the π conjugation in different orientations would have a significant influence on the properties of the resulting polymers.     

Systematic investigation of benzodithiophene- and diketopyrrolopyrrole-based low-bandgap polymers designed for single junction and tandem polymer solar cells

The tandem solar cell architecture is an effective way to harvest a broader part of the solar spectrum and make better use of the photonic energy than the single junction cell. Here, we present the design, synthesis, and characterization of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4-1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic properties. Single junction solar cells were fabricated, and the polymer:PC(71)BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3-6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells.     

Molecular engineering tuning optoelectronic properties of thieno[3,2-<Emphasis Type="Italic">b</Emphasis>]thiophenes-based electrochromic polymers

Thieno[3,2-b]thiophene (TT) monomers end-capped with 3,4-ethylenedioxythiophene (EDOT) moieties are electropolymerized to form π-conjugated polymers with distinct electrochromic (EC) properties. Steric and electronic factors (electron donor and acceptor substituents) in the side groups of the TT core, as well as the structure of the polymer backbone strongly affect the electrochemical and optical properties of the polymers and their electrochromic characteristics. The studied polymers show low oxidation potentials, tunable from–0.78 to +0.30 V (vs. Fc/Fc⁺) and the band gaps from 1.46 to 1.92 eV and demonstrate wide variety of color palettes in polymer films in different states, finely tunable by structural variations in the polymer backbone and the side chains. EC materials of different colors in their doped/dedoped states have been developed (violet, deep blue, light blue, green, brown, purple-red, pinkish-red, orange-red, light gray, cyan and colorless transparent). High optical contrast (up to 79%), short response time (0.57–0.80 s), good cycling stability (up to 91% at 2000 cycles) and high coloration efficiency (up to 234.6 cm² C^–1) have been demonstrated and the influence of different factors on the above parameters of EC polymers have been discussed.     

Synthesis and electrochromic properties of a new family of methacrylate polymers containing pendant oligothiophenes

A new family of methacrylate polymers containing pendant terthiophene, quaterthiophene, quinquethiophene, and sexithiophene was synthesized by the radical polymerization of the corresponding methacrylate monomers, and the electrochemical and electrochromic properties were investigated. The anodic oxidation of these polymers in the presence of tetra‐n‐butylammonium perchlorate as a supporting electrolyte produced radical‐cation salts of pendant oligothiophenes with ClO as a dopant. The electrochemically oxidized polymers had partially crosslinked structures resulting from the coupling reaction of pendant oligothiophene radical cations, the extent of crosslinking significantly decreasing with the increasing conjugation length of the pendant oligothiophenes. Comparative spectroelectrochemical studies of the monomers in solution and the polymers as solid films showed that π‐dimer formation of oligothiophene radical cations took place more readily for the polymer films than for the monomers in solution. This new family of methacrylate polymers containing pendant oligothiophenes constitutes a new class of potential electrochromic materials, undergoing reversible, clear color changes on electrochemical oxidation and reduction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2471–2484, 2003     

Intramolecular hydrogen-bond-assisted planarization of asymmetrically functionalized alternating phenylene-pyridinylene copolymers

We report on the synthesis and characterization of a series of asymmetrically functionalized amphiphilic polymers with alternating pi-donor units (e.g., substituted benzene) and pi-acceptor units (e.g., pyridine) along the polymer backbone. The purpose of our present work involves incorporation of functional groups along the main chain to form intrachain hydrogen bonds, which promote planarization of the polymer backbone, and to fine-tune the optical properties. The structure-property relationship of polymers P1-P6 was investigated by means of analytical methods, such as FTIR spectroscopy, 1H and 13C NMR spectroscopy, UV/Vis spectroscopy, fluorescence spectroscopy, gel permeation chromatography, thermogravimetric analysis, cyclic voltammetry, and X-ray powder diffraction. All polymers were soluble in common organic solvents, and the optical and fluorescence spectra of the polymers showed significant changes according to the formation (P4, P5) or absence (P6) of intramolecular hydrogen bonding along the polymer backbone. Moreover, the 2,6- or 3,5-linkage of the pyridine rings in P5 and P6, respectively, reduced the conjugation along the polymer backbone and this is reflected in their optical properties. The optical properties of the polymers were influenced by the addition of acid (P1-P6), base (P4-P6), and metal ions (e.g., Cu2+, Fe3+, Ag+, Ni2+, Pd2+, Mn2+, Zn2+, Mg2+, and Pr3+). Such polymers could be used in various applications, including sensors and stimuli-responsive displays.     

Energy Transport along Conjugated Polymer Chains with Extended Radiative Lifetimes: A Theoretical Study

The ability to improve exciton diffusion lengths is a key issue in optimizing many opto‐electronic devices based on conjugated polymers. On the basis of quantum‐chemical calculations, we investigate a strategy consisting of extending the radiative lifetime of energy carriers through incorporation along the polymer backbone of repeating units with forbidden optical transition. The results obtained for poly(p‐phenylenebutadiyne), PPE, and poly(p‐triphenylenebutadiyne), PTPE, show that the larger number of hops performed by the electronic excitations during their lifetime in PTPE is compensated by the smaller hopping length (associated with the reduced conjugation length), so that similar on‐chain diffusion lengths are predicted in both polymers.     

Unusual Internal Electron Transfer in Conjugated Radical Polymers

Nitroxide‐containing organic radical polymers (ORPs) have captured attention for their high power and fast redox kinetics. Yet a major challenge is the polymer's aliphatic backbone, resulting in a low electronic conductivity. Recent attempts that replace the aliphatic backbone with a conjugated one have not met with success. The reason for this is not understood until now. We examine a family of polythiophenes bearing nitroxide radical groups, showing that while both species are electrochemically active, there exists an internal electron transfer mechanism that interferes with stabilization of the polymer's fully oxidized form. This finding directs the future design of conjugated radical polymers in energy storage and electronics, where careful attention to the redox potential of the backbone relative to the organic radical species is needed.     

Programming Photodegradability into Vinylic Polymers via Radical Ring-Opening Polymerization

Incorporation of photolabile moieties into the polymer backbone holds promise to remotely-control polymer degradation. However, suitable synthetic avenues are limited, especially for radical polymerizations. Here we report a strategy to program photodegradability into vinylic polymers by exploiting the wavelength selectivity of photocycloadditions for radical ring-opening polymerization (rROP). Irradiation of coumarin terminated allylic sulfides with UVA light initiated intramolecular [2+2] photocycloaddition producing cyclic macromonomers. Subsequent RAFT-mediated rROP with methyl acrylate yielded copolymers that inherited the photoreactivity of the cyclic parent monomer. Irradiation with UVB initiated efficient photocycloreversion of the coumarin dimers, causing polymer degradation within minutes under UVB light or days under sunlight exposure. Our synthetic strategy may pave the way to insert photolabile linkages into vinylic polymers, tuning degradation for specific wavelengths.     

Conformation Control of Conjugated Polymers

In the past few decades, conjugated polymers have aroused extensive interest in organic electronic applications. The electrical performance of conjugated polymers has a close relationship with their backbone conformation. The conformation of the polymer backbone strongly affects the πelectron delocalization along polymer chains, the energy band gap, interchain interactions, and further affects charge transport properties. To realize a rigid coplanar backbone that usually possesses efficient intrachain charge transport properties and enhanced π–π stackings, such conformation control becomes a useful strategy to achieve high-performance (semi)conducting polymers. This minireview summarizes the most important polymer structures through conformation control at the molecular level, and then divides these rigid coplanar conjugated polymers into three categories: 1) noncovalent interactions locked conjugated polymers; 2) double-bond linked conjugated polymers; 3) ladder conjugated polymers. The effect of the conformation control on physical nature, optoelectronic properties, and their device performance is also discussed, as well as the challenges of chemical synthesis and structural characterization.