Catalyst-transfer polymerization (CTP) is a living, chain-growth method for accessing conjugated polymers with control over their length and sequence. Typical catalysts utilized in CTP are either Pd or Ni complexes with bisphosphine or N-heterocyclic carbene ancillary ligands. More recently, diimine-ligated Ni complexes have been employed; however, in most cases nonliving pathways become dominant at high monomer conversions and/or low catalyst loading. We report herein an alternative Ni diimine catalyst that polymerizes 3-hexylthiophene in a chain-growth manner at low catalyst loading and high monomer conversion. In addition, we elucidate the chain-growth mechanism as well as one chain-transfer pathway. Overall, these studies provide insight into the mechanism of conjugated polymer synthesis mediated by Ni diimine catalysts.

ABSTRACTSynthesizing conjugated polymers via catalyst-transfer polymerization (CTP) has led to unprecedented control over polymer sequence and molecular weight. Yet many challenges remain, including broadening the monomer scope and narrowing the molecular weight dispersities. Broad polymer dispersities can arise from nonliving pathways as well as slow initiation. Previously, slow initiation was observed in Ni-mediated CTP of phenylene monomers. Although precatalysts with faster initiation rates have been reported, the rates still do not exceed propagation. Herein a second- and third-generation of reactive ligands are described, along with a simple method for measuring initiation rates. A precatalyst with an initiation rate that exceeds propagation is now reported, however, the resulting polymer samples still exhibit broad dispersities, suggesting that slow initiation is not the most significant contributing factor in Ni-mediated phenylene polymerizations. In addition, initiation rates measured under authentic polymerization conditions revealed that both exogenous triphenylphosphine and an ortho-trifluoroethoxy substituent on the reactive ligand have a strong influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1530–1535

The stimuli-responsive nature of molecular gels makes them appealing platforms for sensing. The biggest challenge is in identifying an appropriate gelator for each specific chemical or biological target. Due to the similarities between crystallization and gel formation, we hypothesized that the tools used to predict crystal morphologies could be useful for identifying gelators. Herein, we demonstrate that new gelators can be discovered by focusing on scaffolds with predicted high aspect ratio crystals. Using this morphology prediction method, we identified two promising molecular scaffolds containing lead atoms. Because solvent is largely ignored in morphology prediction but can play a major role in gelation, each scaffold needed to be structurally modified before six new Pb-containing gelators were discovered. One of these new gelators was developed into a robust sensor capable of detecting lead at the U.S. Environmental Protection Agency limit for paint (5000 ppm).

Catalyst-transfer polycondensation (CTP) is a relatively new method for synthesizing conjugated polymers in a chain-growth fashion using transition metal catalysis. Recent research has focused on screening catalysts to broaden the monomer scope. In this effort, small molecule reactions have played an important role. Specifically, when selective difunctionalization occurs, even with limiting quantities of reaction partner, it suggests an associative intermediate similar to CTP. Several new chain-growth polymerizations have been discovered using this approach. We report herein an attempt to use this method to develop chain-growth conditions for synthesizing poly(2,5-bis(hexyloxy)phenylene ethynylene) via Sonogashira cross-coupling. Hundreds of small molecule experiments were performed and selective difunctionalization was observed with a Buchwald-type precatalyst. Unexpectedly, these same reaction conditions led to a step-growth polymerization. Further investigation revealed that the product ratios in the small molecule reactions were dictated by reactivity differences rather than an associative intermediate. The lessons learned from these studies have broad implications on other small molecule reactions being used to identify new catalysts for CTP.

Polymerizing electron-deficient arenes in a controlled, chain-growth fashion remains a significant challenge despite a decade of research on catalyst-transfer polycondensation. The prevailing hypothesis is that the chain-growth mechanism stalls at a strongly associated metal–polymer π-complex, preventing catalyst turnover. To evaluate this hypothesis, we performed mechanistic studies using thiazole derivatives and identified approaches to improve their chain-growth polymerization. These studies revealed a surprisingly high barrier for chain-walking toward the reactive C–X bond. In addition, a competitive pathway involving chain-transfer to monomer was identified. This pathway is facilitated by ancillary ligand dissociation and N-coordination to the incoming monomer. We found that this chain-transfer pathway can be attenuated by using a rigid ancillary ligand, leading to an improved polymerization. Combined, these studies provide mechanistic insight into the challenges associated with electron-deficient monomers as well as ways to improve their living, chain-growth polymerization. Our mechanistic studies also revealed an unexpected radical anion-mediated oligomerization in the absence of catalyst, as well as a surprising oxidative addition into the thiazole C–S bond in a model system.

Small molecule gelators are serendipitously discovered more often than they are designed. As a consequence, it has been challenging to develop applications based on the limited set of known materials. This synopsis highlights recent strategies to streamline the process of gelator discovery, with a focus on the role of unidirectional intermolecular interactions and solvation. We present these strategies as a series of tools that can be employed to help identify gelator scaffolds and solvents for gel formation. Overall, we suggest that this guided approach is more efficient than random derivatization and screening.

Gradient sequence copolymers of 3-hexylthiophene (90 mol%) and 3-(6-bromohexyl)thiophene (10 mol%) were synthesized by catalyst transfer polycondensation. Post-polymerization conversion of the side-chain bromides into azides and subsequent Cu-catalyzed azide–alkyne cycloaddition installed C60-functional groups. Comparing blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) with and without the gradient copolymer additive revealed that, when the gradient copolymer was present, micron-scale phase separation was not observed even after prolonged thermal annealing times. In addition, the PCBM was still able to quench the P3HT emission after thermal annealing, indicating that the donor–acceptor interfacial area is maintained. Together, these data suggest that the gradient copolymers are an effective compatibilizer for P3HT/PCBM physical blends. This stabilized film morphology led to stable power conversion efficiencies (PCE) of the corresponding bulk heterojunction solar cells even upon extended thermal annealing. Nevertheless, the short circuit current and fill factor were reduced when the gradient copolymer was present, leading to a lower PCE. Overall, these gradient copolymer additives represent a promising tool for inhibiting micron-scale phase separation and producing robust polymer/fullerene-based solar cells.

A generalizable method for detecting protease activity via gelation is described. A recognition sequence is used to target the protease of interest while a second protease is used to remove the residual residues from the gelator scaffold. Using this approach, selective assays for both MMP-9 and PSA are demonstrated.

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

Correction for ‘Modifying a known gelator scaffold for nitrite detection’ by Danielle M. Zurcher et al., Chem. Commun., 2014, 50, 7813–7816.

The relationship between chemical structure and gelation ability was examined for a series of nine Hg-containing compounds. Both solid-state properties (dissolution enthalpies/entropies and packing structure) and gel properties (strength, morphology, cation selectivity, and anion tolerance) were examined. Overall, the results reveal a complex relationship between chemical structure and properties. The remediation potential of these Hg-triggered gelations was also investigated, revealing that >98% of the Hg2+ in water can be removed through gel formation.

Small molecule competition experiments were performed to determine whether Ni-catalyzed Kumada cross-coupling reactions proceed through an intramolecular oxidative addition. Indeed, preferential intramolecular oxidative addition was observed for all four complexes when stoichiometric quantities of competitive agent were present. At higher concentrations of competitive agent, the intramolecular pathway was still preferred when bidentate, electron-rich ligands were utilized, suggesting that these ligands promote the formation and reactivity of the key intermediate. To determine whether a similar pathway is involved in the polymerizations, (4-bromo-2,5-bis(hexyloxy)phenyl)magnesium bromide was polymerized in the presence and absence of competitive agent. The number-average molecular weights were lower and the molecular weight distributions were broadened substantially when competitive agent was present, consistent with the presence of competing intermolecular pathways. Because bidentate, electron-rich ligands suppressed these undesired intermolecular reactions, these ligands should lead to improved polymerization catalysts.

A gradient sequence copolymer containing 3-hexylthiophene (3HT) and 3-(6-bromohexyl)thiophene (3BrHT) with a linear change in comonomer composition was synthesized via a controlled, chain-growth semi-batch method. For comparison, random and block copolymers with the same molecular weight and comonomer ratio (1:1) were prepared. All three copolymers exhibited similar molecular weight (Mn ∼ 32 kDa), low dispersity (Đ < 1.2) and high regioregularity (>99%), suggesting that any differences among the three copolymers can be attributed to the different copolymer sequences. The optical and thermal properties, as well as the thin film morphologies, of the gradient copolymer were compared to the random and block copolymers and the physical blend of the homopolymers. The physical blends showed extensive micron-scale phase separation by AFM and TEM. Adding the gradient copolymer to the blend resulted in a dramatic reduction in the domain size. Moreover, the domain size decreased as the amount of the gradient copolymer increased, suggesting that the copolymer is compatibilizing the polymer blend. By comparison, the random and block copolymers were less effective compatibilizing agents, which indicates that the gradient sequence copolymer is well suited to tailor the morphology of immiscible polymer blends.

Nickel(II) complexes with varying reactive ligands, which were designed to selectively accelerate the initiation rate without influencing the propagation rate in the chain-growth polymerization of π-conjugated monomers, were investigated. Precatalysts with electronically varied reacting groups led to faster initiation rates and narrower molecular weight distributions. Computational studies revealed that the reductive elimination rates are largely modulated by the ability of the two reacting arenes to stabilize the increasing electron density on the catalyst during reductive elimination. Overall, these studies provide insight into a key mechanistic step of cross-coupling reactions (reductive elimination) and highlight the importance of initiation in controlled chain-growth polymerizations.

The recent discovery of a living, controlled chain-growth method for synthesizing π-conjugated polymers has ignited the field and led to the development of many new materials. This Perspective focuses on the mechanistic underpinnings of the synthetic transformation, highlighting the controversial hypotheses and supporting data. A critical analysis of the literature revealed that the monomer scope remains largely limited to electron-rich monomers at this time. Last, a brief overview of some exciting new materials accessed via this method is provided.

The role of ligand-based electronic effects was investigated in the Ni-catalyzed polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. The catalyst with the most electron-donating ligand outperformed the other catalysts by providing polymers with narrower molecular weight distributions. This result is attributed to both a suppression of competing reaction pathways (e.g., chain transfer and termination) as well as a relative acceleration of precatalyst initiation compared to propagation. Further studies revealed that, for all three catalysts, precatalyst initiation is slower than propagation, despite the fact that they exhibit the same rate-determining steps (i.e., reductive elimination) and have similar catalyst resting states. These results suggest that better control over the polymer molecular weight, end-functionality and sequence can be obtained with electron-rich catalysts, such as those described herein.

A convenient and portable triacetone triperoxide (TATP) sensor was developed utilizing a thiol-to-disulfide oxidation to trigger a solution-to-gel phase transition. Using this method, TATP can be detected visually without any instrumentation.

A modular system for detecting protease activity via enzyme-triggered gel formation is described. Protease-specific recognition sequences are utilized to achieve enzyme specificity. Artificial blood clotting is demonstrated by activating endogenous thrombin to trigger gelation in fibrinogen-deficient blood plasma.

The effect of polymeric additives on molecular gelation was explored using poly(acrylic acid) and pyridine-based gelators. A significant reduction in the critical gel concentration (cgc) and an increase in gel strength were observed when the polymer was present during gel formation. Detailed studies revealed that the polymer is adsorbing onto the growing fibers, reducing the growth rates, and leading to thinner fibers. These and other morphological changes lead to improved gel properties by increasing the number of fiber–fiber entanglements. Several other polymers were briefly examined and these studies revealed that polymer structure is important. The polymer containing a complementary functional group relative to the gelator (e.g., H-bond donor/acceptor) provided the lowest cgc.

The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. Three different Ni(L-L)Cl2 catalysts were synthesized using commercially available bis(dialkylphosphino)ethane ligands with varying steric properties. One of these catalysts (Ni(depe)Cl2) outperformed the others for this polymerization. The polymer characterization data were consistent with a chain-growth mechanism. Rate and spectroscopic studies revealed a rate-limiting reductive elimination for both initiation and propagation with Ni(depe)Cl2. In contrast, less hindered Ni(dmpe)Cl2 and more hindered Ni(dcpe)Cl2 were ineffective polymerization catalysts; NMR spectroscopic studies indicated that competing decomposition and uncontrolled pathways intervene. For other monomers, Ni(depe)Cl2 performed similar to the conventional catalysts. Copolymerization studies revealed that block copolymers could be effectively prepared. Overall, these studies indicate that altering the ligand-based steric properties can have a significant impact on the chain-growth polymerization.

A series of aryl trihydroxyborate salts were synthesized and found to form gels in benzene. The compounds were thermally unstable and readily underwent protodeboronation in solution and the solid state. Gelation could be induced without decomposition via sonication. Subsequent characterization studies revealed an unusual dependence of gel properties on alkyl chain length.

The relationship between thermodynamic dissolution parameters (enthalpy and entropy) and gelation ability was examined for two different classes of compounds in three different solvent systems. In total, 11 dipeptides and 19 pyridines were synthesized and screened for gelation in aqueous and organic solvents, respectively. The dissolution parameters were determined from the variable-temperature solubilities using the van’t Hoff equation. These studies revealed that the majority of gelators had higher dissolution enthalpies and entropies compared to nongelators, consistent with the notion that gelators have stronger intermolecular interactions and more order in the solid state. The dissolution parameters were also found to be solvent-dependent, suggesting that solvent–solute interactions are also important in gelation. Overall, these results indicate that converting nongelators into gelators is attainable when structural modifications or a change in solvent lead to increases in the dissolution parameters.

A new gelator was discovered by identifying molecular scaffolds exhibiting 1D intermolecular interactions in the solid-state and synthesizing derivatives. Gelation can be triggered by adding Hg(OAc)2 to a precursor molecule. The in situ gelation is selective for Hg2+ over other metals.

This paper describes a graduate-level class project centered on editing chemistry-related entries in Wikipedia. This project enables students to work collaboratively, explore advanced concepts in chemistry, and learn how to communicate science to a diverse audience, including the general public. The format and structure of the project is outlined and assessment metrics are discussed. A panel survey of current students provided an evaluation of the effectiveness of this project in contributing to the learning objectives of the course. Last, a discussion of the challenges involved in implementing this project is provided.Keywords (Audience): Graduate Education/Research; Upper-Division Undergraduate; Keywords (Domain): Curriculum; Public Understanding/Outreach; Keywords (Pedagogy): Collaborative/Cooperative Learning; Communication/Writing; Internet/Web-Based Learning; Keywords (descriptor): CLIC;

The relationship between molecular structure and gelation ability was investigated for a series of pyridine-based compounds. Nineteen compounds were synthesized and screened for gelation. Of these, eight were discovered to be gelators for a variety of organic solvent/water combinations. Solubility studies on the bulk powders revealed that gelators and nongelators are indistinguishable based on their room temperature solubilities. Likewise, X-ray diffraction experiments indicated that the presence (or absence) of 1D intermolecular interactions does not correlate with gelation ability. van’t Hoff analyses of the temperature-dependent solubilities revealed that the majority of gelators have higher dissolution enthalpies and entropies than nongelators. In combination, these data suggest a complex relationship between molecular structure and gelation ability.

The mechanisms for Ni(dppp)Cl2-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. A combination of rate and spectroscopic studies revealed that transmetalation is the rate-determining step of the catalytic cycle for both monomers. 31P NMR spectroscopic studies revealed that a Ni(II)−aryl halide and a Ni(II)−thienyl halide are the catalyst resting states. In addition, LiCl was found to alter the arene polymerization rates. These results are different than those previously obtained with an alternative catalyst (Ni(dppe)Cl2) and suggest that the ligand has a strong mechanistic influence on the polymerization.

The mechanisms for Ni(dppe)Cl2-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the monomer concentration. 31P NMR spectroscopic studies of the reactive organometallic intermediates suggest that the resting states are unsymmetrical NiII−biaryl and NiII−bithiophene complexes. In combination, the data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was found to have no effect on the rate-determining step or molecular weight distribution in the arene polymerization.

Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined π-conjugated copolymers with narrow molecular weight distributions, defined end-groups, and specific sequences. In particular, a π-conjugated copolymer with a novel linear gradient sequence was prepared. Compared to the analogous block and random copolymers, the gradient copolymer displayed unique optical and thermal properties as well as thin-film morphology. Moreover, the gradient copolymer exhibited an intermediate extent of phase separation into thiophene-rich and selenophene-rich domains compared to the block and random copolymers. Because this gradient sequence provides access to new solid-state properties, these materials should be further explored in applications where phase-separated morphology is important (e.g., solar cells, transistors, and light-emitting diodes).

A generalizable method for detecting protease activity via gelation is described. A recognition sequence is used to target the protease of interest while a second protease is used to remove the residual residues from the gelator scaffold. Using this approach, selective assays for both MMP-9 and PSA are demonstrated.

Correction for ‘Modifying a known gelator scaffold for nitrite detection’ by Danielle M. Zurcher et al., Chem. Commun., 2014, 50, 7813–7816.

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

A modular system for detecting protease activity via enzyme-triggered gel formation is described. Protease-specific recognition sequences are utilized to achieve enzyme specificity. Artificial blood clotting is demonstrated by activating endogenous thrombin to trigger gelation in fibrinogen-deficient blood plasma.

A convenient and portable triacetone triperoxide (TATP) sensor was developed utilizing a thiol-to-disulfide oxidation to trigger a solution-to-gel phase transition. Using this method, TATP can be detected visually without any instrumentation.

A new gelator was discovered by identifying molecular scaffolds exhibiting 1D intermolecular interactions in the solid-state and synthesizing derivatives. Gelation can be triggered by adding Hg(OAc)2 to a precursor molecule. The in situ gelation is selective for Hg2+ over other metals.

Small molecule competition experiments were performed to determine whether Ni-catalyzed Kumada cross-coupling reactions proceed through an intramolecular oxidative addition. Indeed, preferential intramolecular oxidative addition was observed for all four complexes when stoichiometric quantities of competitive agent were present. At higher concentrations of competitive agent, the intramolecular pathway was still preferred when bidentate, electron-rich ligands were utilized, suggesting that these ligands promote the formation and reactivity of the key intermediate. To determine whether a similar pathway is involved in the polymerizations, (4-bromo-2,5-bis(hexyloxy)phenyl)magnesium bromide was polymerized in the presence and absence of competitive agent. The number-average molecular weights were lower and the molecular weight distributions were broadened substantially when competitive agent was present, consistent with the presence of competing intermolecular pathways. Because bidentate, electron-rich ligands suppressed these undesired intermolecular reactions, these ligands should lead to improved polymerization catalysts.

The role of ligand-based electronic effects was investigated in the Ni-catalyzed polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. The catalyst with the most electron-donating ligand outperformed the other catalysts by providing polymers with narrower molecular weight distributions. This result is attributed to both a suppression of competing reaction pathways (e.g., chain transfer and termination) as well as a relative acceleration of precatalyst initiation compared to propagation. Further studies revealed that, for all three catalysts, precatalyst initiation is slower than propagation, despite the fact that they exhibit the same rate-determining steps (i.e., reductive elimination) and have similar catalyst resting states. These results suggest that better control over the polymer molecular weight, end-functionality and sequence can be obtained with electron-rich catalysts, such as those described herein.

Nickel(II) complexes with varying reactive ligands, which were designed to selectively accelerate the initiation rate without influencing the propagation rate in the chain-growth polymerization of π-conjugated monomers, were investigated. Precatalysts with electronically varied reacting groups led to faster initiation rates and narrower molecular weight distributions. Computational studies revealed that the reductive elimination rates are largely modulated by the ability of the two reacting arenes to stabilize the increasing electron density on the catalyst during reductive elimination. Overall, these studies provide insight into a key mechanistic step of cross-coupling reactions (reductive elimination) and highlight the importance of initiation in controlled chain-growth polymerizations.

Gradient sequence copolymers of 3-hexylthiophene (90 mol%) and 3-(6-bromohexyl)thiophene (10 mol%) were synthesized by catalyst transfer polycondensation. Post-polymerization conversion of the side-chain bromides into azides and subsequent Cu-catalyzed azide–alkyne cycloaddition installed C60-functional groups. Comparing blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) with and without the gradient copolymer additive revealed that, when the gradient copolymer was present, micron-scale phase separation was not observed even after prolonged thermal annealing times. In addition, the PCBM was still able to quench the P3HT emission after thermal annealing, indicating that the donor–acceptor interfacial area is maintained. Together, these data suggest that the gradient copolymers are an effective compatibilizer for P3HT/PCBM physical blends. This stabilized film morphology led to stable power conversion efficiencies (PCE) of the corresponding bulk heterojunction solar cells even upon extended thermal annealing. Nevertheless, the short circuit current and fill factor were reduced when the gradient copolymer was present, leading to a lower PCE. Overall, these gradient copolymer additives represent a promising tool for inhibiting micron-scale phase separation and producing robust polymer/fullerene-based solar cells.