Improving the efficiency and sustainability of catalysts for direct arylation polymerization (DArP)

In the past decade, direct arylation polymerization (DArP) has rapidly developed as a sustainable synthetic protocol for cost-effective, atom-economical preparation of conjugated polymers. By circumventing monomer functionalization with toxic transmetallating reagents such as organostannane and organoboron required for Stille-Migita and Suzuki-Miyaura polymerization methods, DArP proceeds through a metal-catalyzed C H activation pathway for the preparation of high-performance conjugated polymer materials. This review evaluates the development of several classes of efficient catalysts/catalytic systems from small-molecule studies to polymerizations, including the mechanisms involved in these transformations and how they inspire catalyst and monomer design for defect-free conjugated polymer synthesis. Recent advances in developing more sustainable first-row transition metal catalysts for DArP are also highlighted, and the fundamental understanding of these efficient and sustainable catalysts should motivate the pursuit for the next generation of catalytic design to enable more effective and environmentally friendly conjugated polymer synthesis.     

Living polymerization of substituted acetylenes

For many years, considerable research efforts have been dedicated to π‐conjugated polymers because of their extraordinary electronic, optical, and structural properties. The employed transition‐metal‐based initiating systems comprise not only simple transition‐metal salts but also rather sophisticated mixtures of two, three, or four compounds and even highly defined single‐component systems such as transition‐metal alkylidene complexes. Extensive fine‐tuning of the electronic and steric properties of initiator–monomer systems eventually allowed the tailor‐made synthesis of conjugated materials via living polymerization techniques. This article focuses on recent developments in the field of the living polymerization of substituted acetylene derivatives. Ill‐defined group 5 and 6 transition metal halide‐based initiators, well‐defined transition‐metal alkylidene complexes, and rhodium(I)‐based systems that induce the living polymerization of numerous substituted acetylenes are reviewed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5723–5747, 2005     

Vinyl Polymerization with Transition Metal Alkyls and Hydrides

Transition metal alkyls and hydrides isolated from Ziegler-type catalyst mixtures serve as appropriate models for coordination polymerization. These transition metal complexes initiate the polymerization of some vinyl compounds and aldehydes. In some cases the monomer-coordinated complexes may be isolated and the interaction of the monomer with the transition metal complexes may be studied. Comparison of the polymerization kinetics of the vinyl compounds with the decay kinetics of the initial transition metal complexes on interaction with the monomer provided important information with respect to the mechanism of coordination polymerization by these complexes. The effect of organoaluminum compounds on the reactivity of the transition metal complexes has been also studied. These transition metal alkyls initiate the polymerization of acetaldehyde to give a polyether-type polymer at -78°C and a polymer with OH groups at room temperature.     

A second transition state for chain transfer to monomer in olefin polymerization promoted by group 4 metal catalysts

beta-Hydrogen transfer (BHT) to monomer is the dominant chain termination pathway for olefin polymerization promoted by group 4 metal catalysts. The transition state (TSA) for BHT studied in earlier work is characterized by a strong metal-hydrogen interaction. Our theoretical study of a series of homogeneous single-site polymerization catalysts reveals the existence of a second transition state (TSC), competitive with TSA, which has no direct metal-hydrogen interaction and strongly resembles that for the main-group metal aluminum. The balance between the two reaction paths is sensitive to choice of metal and ligand structure.     

Poly(aryl ether)s containing o‐terphenyl subunits. II. Random poly(ether sulfone)s

The synthesis of a new A₂X‐type difluoride monomer, N‐2‐pyridyl‐4′,4″‐bis‐(4‐fluorobenzenesulfonyl)‐o‐terphenyl‐3,6‐dimethyl‐4,5‐dicarboxylic imide ( 3 ), is described. The monomer 3 was incorporated into a series of copoly(aryl ether sulfone)s by polymerization of 4,4′‐isopropylidenediphenol and 4,4′‐difluorophenylsulfone. The incorporation of monomer 3 had an observable effect on both the glass‐transition temperature of poly(aryl ether sulfone)s and the tendency for macrocyclic oligomers to form during polymerization. Replacement of the pyridyl imide group via a transimidization reaction with propargyl amine proceeded quantitatively and without polymer degradation. The acetylene containing copoly(aryl ether sulfone) could be crosslinked by simple thermal treatment, resulting in an increase in the glass‐transition temperature and solvent resistance. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 9–17, 2000     

A Self‐Supporting Strategy for Gas‐Phase and Slurry‐Phase Ethylene Polymerization using Late‐Transition‐Metal Catalysts

The polyolefin industry is dominated by gas‐phase and slurry‐phase polymerization using heterogeneous catalysts. In contrast, academic research is focused on homogeneous systems, especially for late‐transition‐metal catalysts. The heterogenization of homogeneous catalysts is a general strategy to provide catalyst solutions for existing industrial polyolefin synthesis. Herein, we report an alternative, potentially general strategy for using homogeneous late‐transition‐metal catalysts in gas‐phase and slurry‐phase polymerization. In this self‐supporting strategy, catalysts with moderate chain‐walking capabilities produced porous polymer supports during gas‐phase ethylene polymerization. Chain walking, in which the metal center can move up and down the polymer chain during polymerization, ensures that the metal center can travel along the polymer chain to find suitable sites for ethylene enchainment. This strategy enables simple heterogenization of catalysts on solid supports for slurry‐phase polymerization. Most importantly, various branched ultra‐high‐molecular‐weight polyethylenes can be prepared under various polymerization conditions with proper catalyst selection.     

Development of catalyst‐transfer condensation polymerization. Synthesis of π‐conjugated polymers with controlled molecular weight and low polydispersity

We describe the development of chain‐growth condensation polymerization for the synthesis of well‐defined π‐conjugated polymers via a new polymerization mechanism, catalyst‐transfer polymerization. We first studied the condensation polymerization of Grignard‐type hexylthiophene monomer with a Ni catalyst as a part of our research on chain‐growth condensation polymerization, and found that this polymerization also proceeded in a chain‐growth polymerization manner. However, the polymerization mechanism involving the Ni catalyst was different from that of previous chain‐growth condensation polymerizations based on substituent effects; the Ni catalyst catalyzed the coupling reaction of the monomer with the polymer, followed by the transfer of Ni(0) to the terminal C? Br bond of the elongated molecule. This catalyst‐transfer condensation polymerization is generally applicable for the synthesis of polythiophene with an etheric side chain and poly(p‐pheneylene), as well as for the synthesis of polyfluorene via the Pd‐catalyzed Suzuki–Miyaura coupling reaction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 753–765, 2008     

Polysilylether: A Degradable Polymer from Biorenewable Feedstocks

The synthesis of polysilylethers (PSEs) using a monomer derived from a biorenewable feedstock is reported. The AB‐type monomer was synthesized from undecenoic acid through hydrosilylation and reduction, and the polymerization was catalyzed by earth‐abundant metal salts. High‐molar‐mass products were achieved, and the degree of polymerization was controlled by varying the amount of an AA‐type monomer in the reaction. The PSEs possess good thermal stability and a low glass‐transition temperature (T_g≈?67 °C). To demonstrate the utility of the PSEs, polyurethanes were synthesized from low‐molar‐mass hydroxy‐telechelic PSEs.     

Topochemical Polymerization of 1,3‐Diene Monomers and Features of Polymer Crystals as Organic Intercalation Materials

From the viewpoint of controlled polymer synthesis, topochemical polymerization based on crystal engineering is very useful for controlling not only the primary chain structures but also the higher‐order structures of the crystalline polymers. We found a new type of topochemical polymerization of muconic and sorbic acid derivatives to give stereoregular and high‐molecular weight polymers under photo‐, X‐ray, and γ‐ray irradiation of the monomer crystals. In this article, we describe detailed features and the mechanism of the topochemical polymerization of diethyl‐(Z,Z)‐muconate as well as of various alkylammonium derivatives of muconic and sorbic acids, which are 1,3‐diene mono‐ and dicarboxylic acid derivatives, to control the stereochemical structures of the polymers. The polymerization reactivity of these monomers in the crystalline state and the stereochemical structure of the polymers produced are discussed based on the concept of crystal engineering, which is a useful method to design and control the reactivity, structure, and properties of organic solids. The reactivity of the topochemical polymerization is determined by the monomer crystal structure, i.e. the monomer molecular arrangement in the crystals. Polymer crystals derived from topochemical polymerization have a high potential as new organic crystalline materials for various applications. Organic intercalation using the polymer crystals prepared from alkylammonium muconates and sorbates is also described.     

Challenge of synthetic cellulose

This article focuses on why and how the chemical synthesis of cellulose was accomplished. The synthesis of cellulose was an important, challenging problem for half a century in polymer chemistry. For the synthesis, a new method of enzymatic polymerization was developed. A monomer of β‐D ‐cellobiosyl fluoride (β‐CF) was designed and subjected to cellulase catalysis, which led to synthetic cellulose for the first time. Cellulase is a hydrolysis enzyme of cellulose; cellulase, inherently catalyzing the bond cleavage of cellulose in vivo, catalyzes the bond formation via the polycondensation of β‐CF in vitro. It is thought that the polymerization and hydrolysis involve a common intermediate (transition state). This view led us to a new concept, a transition‐state analogue substrate, for the design of the monomer. The preparation of cellulase proteins with biotechnology revealed the enzymatic catalytic functions in the hydrolysis and polymerization to cellulose. High‐order molecular structures were in situ formed and observed as fibrils (cellulose I) and spherulites (cellulose II). In situ small‐angle neutron scattering measurements suggested a fractal surface formation of a synthetic cellulose assembly. The principle of cellulose synthesis was extended to the synthesis of other natural polysaccharides, such as xylan and amylose, and unnatural polysaccharides. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 693–710, 2005     

Two‐dimensional polymer synthesis through the topochemical polymerization of alkylenediammonium muconate as a multifunctional monomer

Here we demonstrate a unique two‐dimensional polymer synthesis through topochemical polymerization via polymer crystal engineering, which is useful for controlling and designing the polymerization reactivity as well as the polymer chain and crystal structures. We have succeeded in the synthesis of a sheet polymer through the polymerization of alkylenediammonium (Z,Z)‐muconate as a multifunctional 1,3‐diene monomer in the crystalline state under the irradiation of UV and γ‐rays or upon heating in the dark. The photopolymerization reactivity of several muconates and the structural control of the obtained polymer are described. The stereochemical structure of the polymer and the polymerization mechanism are discussed on the basis of the results of IR and NMR spectroscopy, thermogravimetric measurements, and solid‐state hydrolysis for the transformation into poly(muconic acid). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3922–3929, 2004     

Perspective on metal-mediated polar monomer/alkene copolymerization

A major unsolved problem in polymer synthesis is the design of efficient metal-mediated systems for the copolymerization of alkenes with polar vinyl monomers, such as acrylates and methacrylates. There are several reasons for the absence of efficient transition metal-based insertion copolymerization catalysts. First, following insertion, the ester group of the acrylate coordinates to the metal thereby hindering subsequent monomer coordination. A second reason stems from the preferred 2,1-insertion of acrylates into metal-carbon bonds resulting in the placement of the ester group on the α-carbon. This makes the metal-alkyl species particularly prone to homolysis because of the enhanced stability of the resultant alkyl radical, one that is essentially the same as the propagating species in radical-initiated acrylate polymerization. In this perspective we focus on this issue of facile metal-carbon bond homolysis, especially following acrylate insertion, using examples from our own work. We suggest ways to circumvent these issues, for example forcing 1,2-insertion by imposing steric crowding at the metal. Finally, we discuss the danger of relying on radical traps as probes for polymerization mechanism. Radical traps can react with metal-hydrides and attenuate metal-centered nonradical reactions. However, even when radical traps fail to stop an observed polymerization, it may be wrong to conclude that a nonradical mechanism is at work since the traps can be destroyed under certain reaction conditions.     

Preparation of titanium‐containing polymeric foam for inertial confinement fusion target

In the framework of the increasing need for metal‐doped polymer materials for inertial confinement fusion targets, we report the preparation of low‐density titanium‐containing materials. For this purpose, we developed a new monomer based on hydroxyl and oxime chelation: Ti₃(5‐vinylsalicylaldoximato)₂(Oi‐Pr)₈. We report here the synthesis, characterization and first results of polymerization of this new titanium‐containing monomer. Copyright © 2013 John Wiley & Sons, Ltd.     

Chain-growth polycondensation for well-defined condensation polymers and polymer architecture

The historical development of our research on polycondensation that proceeds in a chain-growth polymerization manner ("chain-growth polycondensation") for well-defined condensation polymers is described. We first studied polycondensation in which change of the substituent effect induced by bond formation drove the reactivity of the polymer end group higher than that of the monomer. In this approach, well-defined aromatic polyamides, polyesters, polyethers, and poly(ether sulfone)s were obtained. The second approach was the study of the phase-transfer polymerization of a solid monomer dispersed in an organic solvent. In this type of polymerization, the solid monomer was physically unable to react with another monomer and was carried with the phase transfer catalyst into the solution phase where it reacted with an initiator and the polymer end group in the solvent in a chain polymerization manner. We also found catalyst-transfer polycondensation as a third approach to chain-growth polycondensation. In the Ni-catalyzed polycondensation of 2-bromo-5-chloromagnesiothiophenes, the Ni catalyst transferred to the polymer end group, and a coupling reaction occurred there to yield a well-defined polythiophene. This chain-growth polycondensation was applied to the synthesis of condensation polymer architectures such as block copolymers, star polymers, graft copolymers, and so on.     

Phototriggered Supramolecular Polymerization

Photo‐initiated supramolecular polymerization of a naphthalenediimide (NDI‐1) derivative containing an ortho‐nitrobenzyl (ONB)‐protected amide group is demonstrated. In a hydrocarbon solvent (methylcyclohexane), it remains as monomer. Upon photo‐irradiation, deprotection of the ONB group produces NDI‐2 with a free amide group, which drives supramolecular polymerization by self‐complementary H‐bonding between the amide groups, leading to gelation. The polymerization rate can be controlled by tuning the energy of the light source. During photopolymerization, a gradual increase in hydrodynamic radius and viscosity is noticed. More interestingly, the morphology of the supramolecular polymer of NDI‐2, produced by photo‐irradiation, was a spherulite, which is in sharp contrast with the fibrillar morphology of NDI‐2 polymer, when assembled spontaneously without a phototrigger. This is ascribed to the ability of the ONB‐caged pro‐monomer (NDI‐1) to act as a chain‐stopper by forming a H‐bonded complex with the active monomer during the growth of the supramolecular polymer under photo‐irradiation.     

Synthesis and thermal properties of disiloxane-arylene polymers by dehydrocoupling polymerization of bis(dimethylsilyl)arylene with water

Disiloxane-arylene polymers having phenylene, biphenylene, and fluorenylene groups as arylene units were synthesized by dehydrocoupling polymerization of corresponding bis(silane) derivatives with water. The reactivity of Si-H was not affected by the structure of aromatic groups in the reaction. The polymers containing biphenylene and fluorenylene units are amorphous and show higher glass transition temperatures than the polymer from 1,4-bis(dimethylsilyl)benzene.     

Chemistry of Guanidinate‐Stabilised Diboranes: Transition‐Metal‐Catalysed Dehydrocoupling and Hydride Abstraction

Herein, we analyse the catalytic boron–boron dehydrocoupling reaction that leads from the base‐stabilised diborane(6) [H₂B(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate) to the base‐stabilised diborane(4) [H₂B(hpp)]₂. A number of potential transition‐metal precatalysts was studied, including transition‐metal complexes of the product diborane(4). The synthesis and structural characterisation of two further examples of such complexes is presented. The best results for the dehydrocoupling reactions were obtained with precatalysts of Group 9 metals in the oxidation state of +I. The active catalyst is formed in situ through a multistep process that involves reduction of the precatalyst by the substrate [H₂B(hpp)]₂, and mechanistic investigations indicate that both heterogeneous and (slower) homogeneous reaction pathways play a role in the dehydrocoupling reaction. In addition, hydride abstraction from [H₂B(hpp)]₂ and related diboranes is analysed and the possibility for subsequent deprotonation is discussed by probing the protic character of the cationic boron–hydrogen compounds with NMR spectroscopic analysis.     

Barbier Hyperbranching Polymerization-Induced Emission from an AB-Type Monomer

Luminescent polymer materials have gained considerable research efforts in the past decades and are generally molecular designed by extending the π system of the polymer main chain or by incorporating chromophores into the polymer chain, which suffer from poor solubility, difficult synthesis, or multi-step procedures. Meanwhile, according to the step-growth polymerization theory, synthesis of hyperbranched polymers from an AB-type monomer is still challenging. Herein, we report a one-pot synthesis of nonconjugated luminescent hyperbranched polymer material via Barbier hyperbranching polymerization-induced emission (PIE) from an AB-type monomer. The key step in the realization of the hyperbranched polymer is bi-functionalization of a mono-functional group. Through a Barbier reaction between an organohalide and an ester group in one pot, bi-functionalization of mono-functional ester is realized through two-step nucleophilic additions, resulting in hyperbranched polytriphenylmethanols (HPTPM). Attributed to through-space conjugation and inter- and intramolecular charge-transfer effects induced by polymer chain, nonconjugated HPTPMs are PIEgens, which are tunable by monomer structure and polymerization time. When all phenyl groups are rotatable, HPTPM is aggregation-induced emission type PIEgen. Whereas, it is aggregation-caused quenching type PIEgen if some phenyl groups are rotation forbidden. Further potential applications of PIEgen are in the fields of explosive detection and artificial light harvesting systems. This work, therefore, expands the monomer library and molecular design library of hyperbranched polymers through “bi-functionalization of mono-functional group” strategy, which eventually expands the preparation library of nonconjugated luminescent polymer materials through one-pot PIE from nonemissive monomer.     

Recent topics of the metallacycles composed of the heavier group 14 elements

This Letter reviews recent advance of metallacycles with chelating Si-, Ge-, and Sn-ligands. Dehydrogenative bond-forming reactions of organosilanes, -germanes, and -stannanes promoted by Pd and Pt complexes afford four- and five-membered metallacycles composed of heavier group 14 elements. It has a couple of advantages such as easier preparation of the starting compounds and reaction procedure than the common metathesis reactions of dianions with transition metal dihalide complexes. These metallacycles are regarded as possible intermediates in catalytic dehydrocoupling polymerizations or as convenient precursors to form discrete oligomers.     

Thermodynamic Parameters of Temperature‐Induced Phase Transition for Brushes onto Nanoparticles: Hydrophilic versus Hydrophobic End‐Groups Functionalization

Quantification of the stimuli‐responsive phase transition in polymers is topical and important for the understanding and development of novel stimuli‐responsive materials. The temperature‐induced phase transition of poly(N‐isopropylacrylamide) (PNIPAm) with one thiol end group depends on the confinement—free polymer or polymer brush—on the molecular weight and on the nature of the second end. This paper describes the synthesis of heterotelechelic PNIPAm of different molecular weights with a thiol end group—that specifically binds to gold nanorods and a hydrophilic NIPAm end group by reversible addition‐fragmentation chain‐transfer polymerization. Proton high‐resolution magic angle sample spinning NMR spectra are used as an indicator of the polymer chain conformations. The characteristics of phase transition given by the transition temperature, entropy, and width of transition are obtained by a two‐state model. The dependence of thermodynamic parameters on molecular weight is compared for hydrophilic and hydrophobic end functional‐free polymers and brushes.