Theoretical Studies on the Thermodynamic Product Size Distribution in Nucleation−Elongation Polymerization under Imbalanced Stoichiometry

Three different models are developed to calculate the thermodynamic product size distribution in a nucleation−elongation polymerization between a pair of A−A and B−B typed comonomers. These monomers are designed to undergo a single step of nucleation prior to an isodesmic chain elongation, namely, a cooperative, step-growth polymerization with dimerization being an energetically less favored process. Particularly, emphasis is laid on analyzing product distribution under conditions of imbalanced functionality stoichiometry. Consistent results are obtained from independent approaches, mechanistic and statistical, demonstrating that when the mole ratio of the comonomers deviates from unity, at polymerization equilibrium such a nucleation−elongation polymerization generates products of substantially higher molecular weights than those from a corresponding isodesmic system having an identical energetics for chain propagation yet without the nucleation process. This higher molecular weight is shown achieved by retaining a large portion of the excess monomer unreacted at equilibrium and selectively compose product chains with comonomers at a roughly stoichiometric ratio. Essentially, such a polymer−monomer coexisting bimodal distribution is a result from destabilization of the oligomeric species due to the nucleation effect

Oligo- and Polyfluorene-Tethered <i>fac</i>-Ir(ppy)₃: Substitution Effects

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance

Oligo- and Polyfluorene-Tethered <i>fac</i>-Ir(ppy)₃: Substitution Effects

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance

Oligo- and Polyfluorene-Tethered <i>fac</i>-Ir(ppy)₃: Substitution Effects

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance

Oligo- and Polyfluorene-Tethered <i>fac</i>-Ir(ppy)₃: Substitution Effects

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance

Unveiling the Side-Chain Effect on Ionic Conductivity of Poly(ethylene oxide)-Based Polymer-Brush Electrolytes

Polymer-brush architectures have attracted great interests in solid-state electrolytes for battery applications due to the facilitated segmental dynamics and latent capacity to fabricate single ion conductors. The effect of side chain architectures on the ionic conductivity of the polymer-brush electrolytes (PBEs), however, still requires a systematic exploration. This work describes a detailed study on the structure–property relationship between the side chain architectures and the ion-conducting behaviors of PBEs. By means of thermodynamic, spectroscopic, and electrochemical characterizations, factors of both chain length and graft density are investigated to elucidate the mechanism. Our results show that as the chain length increases, the ionic conductivity exhibits first an increase in the short amorphous range and then a drop in the long crystalline range. Moreover, investigation on graft density demonstrates that the amorphous PBEs achieve the highest ionic conductivity at a fully grafted configuration, indicating the significance of branched side chain architecture. For crystalline PBEs, proper regulation of graft density can alleviate the crystallization of the side chains and therefore increase the ionic conductivity. These results would improve our understanding of ion-conducting behaviors in PBEs and provide insights for designing advanced solid polymer electrolytes

Thermoresponsive and Self-Healing Hydrogel Based on Chitosan Derivatives and Polyoxometalate as an Antibacterial Coating

Hospital-acquired infections are a serious threat to the recovery of patients. To prevent such infections, an antibacterial coating is an effective method to eliminate bacterial colonization on healthcare-related surfaces. Herein, we report an antibacterial hydrogel composed of silver-containing polyoxometalate (AgP5W30 POM) and carboxymethyl chitosan (CMC). The silver ion is encapsulated inside the POM cage and demonstrates long-lasting bacteriostasis after repeated exposure to both Gram-positive and Gram-negative bacteria. The chemical structure of chitosan derivatives, as well as the concentration and pH, is studied to tune the mechanical properties of the hydrogel. The hydrogel undergoes a gel–sol transition above the critical temperature and possesses self-healing ability. This hydrogel can be readily coated on the surface of versatile bulk materials, which is especially convenient for porous objects and resists the growth of Staphylococcus aureus, Escherichia coli, and methicillin-resistant S. aureus (MRSA). In summary, we envision that the AgP5W30-CMC hydrogel has great potential to serve as an antibacterial coating to decrease the prevalence of hospital-acquired infections

A Physically Cross-Linked Hydrogen-Bonded Polymeric Composite Binder for High-Performance Silicon Anodes

High-capacity silicon (Si) anodes in lithium-ion batteries require functional binders to accommodate the dramatic volume change and to improve the long-term stability during the charge/discharge cycles. Herein, a physically cross-linked hydrogen-bonded polymeric complex is reported and employed as an efficient binder for high-performance Si anodes. Composed of a blend of two commercially available polymers, poly­(acrylic acid) (PAA) and poly­(ethylene oxide) (PEO), the proposed PAA–PEO binders are synthesized via the solution mixing process. It is revealed that PEO brings in better elasticity and ionic conductivity, but at the expense of lower adhesion properties. With an optimized composition, the PAA–PEO binders show better cycling performance for Si anodes than the pure PAA binder. This study would provide insights for the design of low-cost and efficient binder for electrode materials with huge volume changes

Sequence-Isomerism-Controlled Macromolecular Self-Assembly in Dendritic Rod-Like Molecules

Although in Nature sequence control is widely adopted to tune the structure and functions of biomacromolecules, it remains challenging and largely unexplored in synthetic macromolecular systems due to the difficulties in a precision synthesis, which impedes the understanding of the structure–property relationship in macromolecular sequence isomerism. Herein, we report the sequence-controlled macromolecular self-assembly enabled by a pair of rationally designed isomeric dendritic rod-like molecules. With an identical chemical formula and molecular topology, the molecular solid angle of the dendron isomers was determined by the sequence of the rod building blocks tethered with side chains of different lengths. As a result, entirely different supramolecular motifs of discs and spheres were generated, which were further packed into a hexagonally packed cylinder phase and a dodecagonal quasicrystalline sphere phase, respectively. Given the efficient synthesis and modular structural variations, it is believed that the sequence-isomerism-controlled self-assembly in dendritic rod-like molecules might provide a unique avenue toward rich nanostructures in synthetic macromolecules

Rapid and Efficient Anionic Synthesis of Well-Defined Eight-Arm Star Polymers Using OctavinylPOSS and Poly(styryl)lithium

A new approach has been developed for the preparation of well-defined, eight-arm star polymers via the addition of poly­(styryl)lithium to octavinylPOSS in benzene. The reaction proceeds rapidly to completion (within 5 min for molecular weight of each arm up to 33 kg/mol), forming predominantly eight-arm star polymers. The products were purified by fractionation and fully characterized by ¹H NMR, ¹³C NMR, ²⁹Si NMR, FT-IR, MALDI-TOF mass spectrometry, and size exclusion chromatography. Compared to conventional coupling approaches, this process is found to be less sensitive to the stoichiometry of the reactants and the molecular weight of each arm. A mechanism based on cross-association and intra-aggregate addition is invoked to account for this unusual observation. As evidence, when a polar solvent, tetrahydrofuran, or a strongly coordinating and disassociating Lewis base, tetramethylethylenediamine, was used to dissociate the living polymer chains, star polymers with lower average arm numbers than those of the products synthesized in pure benzene were formed at the same stoichiometry of the reactants. The method has general implications in the understanding of the reactive nature of the living anionic polymerization and may find practical application in the synthesis of functional star polymers of diverse compositions and architectures