Novel synthesis of high‐molecular‐weight prepolymer of poly(p‐phenylene benzoxazole) in ionic liquids

Using ionic liquids (ILs) as the reaction solvent for the synthesis of prepolymer polyamide of poly(p‐phenylene benzoxazole) (PBO) was investigated. The optimum condition of prepolymer preparation was determined in ILs. A series of 1,3‐dialkylimidazolium ILs were used to be the reaction media of the polycondensation. The relationship between the molecular weight of prepolymer and the structure of ILs was analysed by changing the structure of the cation and species of anion of ILs. In order to prove the feasibility of the transformation, the prepolymer was used to prepare PBO in polyphosphoric acid media, and the conversion process was analyzed. The spinnability of the PBO solution was explored by the preparation of PBO fibers. The basic mechanical properties of PBO single fiber were tested. In a word, using 1,3‐dialkylimidazolium ILs as the reaction solvents was feasible for the synthesis of high‐molecular‐weight PBO prepolymer, which could be a promising PBO preparation method.     

Preparation and properties of copolymer resins based on phenolphthalein benzoxazine–benzoic acid and bisoxazoline

The novel phenolphthalein benzoxazine–benzoic acid (PBB) was synthesized. The structure of the monomer was supported by FTIR, ¹H-NMR, and elemental analysis. The curing behavior of PBB–bisoxazoline (1,3-PBO) resin was monitored by FTIR and differential scanning calorimetry. It was found that PBB–bisoxazoline resin exhibited two-stage polymerization mechanism. The thermogravimetry showed that PBB–bisoxazoline resin had good heat resistance due to the rigidity of PBB polymer chains and the high cross-linking density of copolymer. Furthermore, it seemed that the reaction between PBB and 1,3-PBO also led to an additional cross-linking, which increased the cross-linking density and delayed the decomposition. The cured resin had low water absorption.     

Nanoscopic Visualization of Cross‐Linking Density in Polymer Networks with Diarylethene Photoswitches

The in situ nanoscopic imaging of soft matter polymer structures is of importance to gain knowledge of the relationship between structure, properties, and functionality on the nanoscopic scale. Cross‐linking of polymer chains effects the viscoelastic properties of gels. The correlation of mechanical properties with the distribution and amount of cross‐linkers is relevant for applications and for a detailed understanding of polymers on the molecular scale. We introduce a super‐resolution fluorescence‐microscopy‐based method for visualizing and quantifying cross‐linker points in polymer systems. A novel diarylethene‐based photoswitch with a highly fluorescent closed and a non‐fluorescent open form is used as a photoswitchable cross‐linker in a polymer network. As an example for its capability to nanoscopically visualize cross‐linking, we investigate pNIPAM microgels as a system known with variations in internal cross‐linking density.     

Synthesis of ultra-thin films of aromatic polymers at air/water interface

Ultra-thin films of and precursor polymers for polybenzimidazole (PBI), polybenzoxazole (PBO), or polybenzothiazole (PBT) were formed at air/water interface by spreading monomers and then polymerizing on the water surface. These thin films could be deposited onto appropriate substrates by using the LB method of horizontal lifting. Moreover, the heat-treatment of the built-up films of the precursor polymers of PBI, PBO, and PBT formed the ultra-thin films of high temperature polymers. The resulting ultra-thin films had uniform and controllable thickness, but remained fairly stable when subjected to temperature up to 300°C. They also had good solvent resistance.     

Organotin catalysts in organosilicon chemistry

Organotin compounds are best known in the chemical industry as PVC stabilizers, polyurethane foam formers and antifungal agents. However, they have also been widely used in the silicon industry for decades for curing organosilicon polymers, despite the fact that neither the mechanism through which the siloxane bonds are formed nor their inherent toxicity are completely understood. This review gives an account of the use of organotin compounds in the preparation of diverse polysiloxane‐containing materials via cross‐linking with either organic or inorganic polymers. As they are common ingredients for this application, a brief outline of the preparation of relevant silicon‐containing compounds, as well as their reactivity, are given. In addition, since there is some evidence that stannasiloxanes formed in situ during the reaction play a determinant role as intermediates in the reaction mechanism, an overview of the synthesis and reactivity of Sn‐O‐Si‐containing compounds is also presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Cross‐linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross‐linker

Phosphoric acid doped polybenzimidazole (PBI) membranes have been covalently cross‐linked with dichloromethyl phosphinic acid (DCMP). FT‐IR measurements showed new bands originating from bonds between the hydrogen bearing nitrogen in the imidazole group of PBI and the CH₂ group in DCMP. The produced cross‐linked membranes show increased mechanical strength, making it possible to achieve higher phosphoric acid doping levels and therefore higher proton conductivity. Oxidative stability is significantly improved and thermal stability is sufficient in a temperature range of up to 250°C, i.e. within the temperature range of operation of PBI‐based fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Versatile Supramolecular Cross‐Linker: A Rotaxane Cross‐Linker That Directly Endows Vinyl Polymers with Movable Cross‐Links

A supramolecular cross‐linked cross‐linker, capable of introducing rotaxane cross‐links to vinyl polymers, has been developed for the rational synthesis of polyrotaxane networks. The experimental results reveal that the combination of an oligocyclodextrin (OCD) and a terminal bulky group‐tethering macromonomer (TBM) forms a polymer‐network structure having polymerizable moieties through supramolecular cross‐linking. Radical polymerization of a variety of typical vinyl monomers in the presence of the vinylic supramolecular cross‐linker (VSC) afforded the corresponding vinyl polymers cross‐linked through the rotaxane cross‐links (RCP) as transparent stable films in high yields under both photoinitiated and thermal polymerization conditions. A poly(N,N‐dimethylacrylamide)‐based hydrogel synthesized by using VSC, RCP_DMAAm, displayed a unique mechanical property. The small‐angle X‐ray scattering (SAXS) results, indicating patterns characteristic of a polyrotaxane network, clearly suggested the presence and role of the rotaxane cross‐links. The confirmation of the introduction of rotaxane‐cross‐links into vinyl polymers strongly reveals the significant usefulness of VSC.     

Recyclable polybutadiene elastomer based on dynamic imine bond

Catalyst‐free recyclable polybutadiene (PB) elastomer cross‐linked by dynamic imine bonds is prepared by the reaction between amine functionalized PB and aldehyde cross‐linkers. The dynamic nature of imine bond is investigated by rheometry and creep‐recovery experiments. The cross‐linking degrees are regulated by incorporating different amount of aldehyde, and their influence on the cross‐linked elastomers is investigated in detail. The temperature‐induced imine exchange reactions enable recycling of the cross‐linked PB elastomers and their mechanical properties are almost unchanged after several cycles. It is important to note that the elastomers also show excellent solvent resistance even at high temperature. The good mechanical properties, solvent resistance and recycling ability of the resultant PB elastomer demonstrate the superiority of the imine bonds in the design of recyclable polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2011–2018     

Azide telechelics chain extended by click reaction: Synthesis,characterization, and cross‐linking

Azide telechelics of poly(dimethylsiloxane) (PDMS), polypropylene oxide (PPO), and polyethylene oxide (PEO) were synthesized from the corresponding epoxy telechelics and characterized. These oligomeric azides were chain extended by reaction with bispropargyl ether of bisphenol A (BPEBA) through a copper‐catalyzed azide‐alkyne cycloaddition (CuAAC) reaction. PDMS manifested a faster reaction in contrast to PPO or PEO. The chain‐extended polymers underwent cross‐linking above 170°C through thermal cleavage of residual (terminal) azide groups. This was manifested in their rheograms and was further substantiated by FTIR and NMR spectroscopic analyses. Dynamic mechanical analyses of the cross‐linked polymers exhibited characteristic transitions of hard and soft segments, implying microphase separation in the system. Microscopic evaluation of the thermally cross‐linked sample revealed a porous morphology with microsized to nanosized pores.     

Divergent Synthesis of CF3‐Substituted Allenyl Nitriles by Ligand‐Controlled Radical 1,2‐ and 1,4‐Addition to 1,3‐Enynes

A ligand‐controlled system that enables regioselective trifluoromethylcyanation of 1,3‐enynes has been identified, which provides access to a variety of CF₃‐containing tri‐ and tetrasubstituted allenyl nitriles. We disclose that the involved propargylic radicals can be selectively trapped by (Box)Cu^II cyanide, while the tautomerized allenyl radicals are trapped by (phen)Cu^II cyanide (Box= bisoxazoline, phen=phenanthroline). In addition, the reaction features broad substrate scope and excellent functional group compatibility. Moreover, this protocol represents a novel regioselectivity‐tunable functionalization of 1,3‐enynes via radicals, which we believe will have great implications for the development of catalytic systems for selectivity control in radical and organometallic chemistry.     

Novel Ternary Molecular Composites Prepared by a Sol–Gel Process and Their Conversion into Microcellular Foams

Abstract

A carboxylated polysulfone (C‐PSF) was reacted with 3‐(glycidoxypropyl)trimethoxysilane (GPS) in a sol–gel reaction to form hybrid networks. Specifically, GPS was grafted to the C‐PSF chains at their epoxy ends by ring‐opening reactions and then the methoxy silane groups at their other ends were crosslinked through the usual hydrolysis and condensation reactions. These C‐PSF/GPS cross‐linked binary networks are of interest in their own right, but this type of network was also used in the incorporation of polybenzimidazole (PBI) rigid‐rod polymer to form a C‐PSF/PBI/GPS ternary composite. In this case, the network structures obtained by carrying out the same reaction in the presence of PBI chains successfully suppressed their undesirable segregation within the composite. The transparencies of the preparative solutions and the dried films indicated that the dispersion of PBI was maintained in these environments, presumably at the nano‐ or molecular level. The ternary composites were characterized using Fourier‐transform infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), thermal mechanical analysis (TMA), and thermogravimetric analysis (TGA). Some of these composites were converted to closed‐cell microcellular foams, which were then characterized in a preliminary way with regard to their morphologies.     

Synthesis and characterisation of poly(ester-amide)s from aromatic bisoxazoline precursors

A series of novel aliphatic aromatic poly(ester-amide)s (PEA) have been synthesized by condensation reaction of aromatic bisoxazolines with aliphatic dicarboxylic compounds. These polymers have a number average molecular weight of 20,000-25,000. Depending of the aromatic structure of the bisoxazoline precursor, they are either amorphous or semi-crystalline. A good solubility in aprotic solvents was observed for all PEAs.     

Designing polymer topology by electrostatic self‐assembly and covalent fixation

A novel methodology (electrostatic self‐assembly and covalent fixation) has been proposed for designing unusual polymer topologies such as star polymers, polymacromonomers, dumbbell‐shaped polymers as well as model network polymers. Thus new telechelic polymers having moderately strained cyclic onium salt group as single or both end groups were prepared and subjected to an ion‐exchange reaction to introduce multifunctional carboxylate anions as a counter‐anion. The electrostatically self‐assembled products were then subjected, either directly or after subsequent manipulation, to heat treatment to convert the ionic interaction into the covalent linkage by the ring‐opening reaction to produce a variety of topologically unique polymer architectures in high yields.     

Novel organotin‐containing shell‐cross‐linked knedel and core‐cross‐linked knedel: Synthesis and application in catalysis

A series of novel organotin‐containing core‐cross‐linked knedels and shell‐cross‐linked knedels were first synthesized facilely from poly(styrene)‐b‐poly(acrylate acid) nanoparticles in different selective solvents [Tetrahydrofuran (THF)/H₂O or THF/n‐octane] by using organotin compound 1,3‐dichloro‐tetra‐n‐butyl‐distannoxane as a new cross‐linker. The formation of the 1‐chloro‐3‐carboxylato‐tetra‐n‐butyl‐distannoxane layer in our cross‐linking reaction was supported by Fourier transform infrared (FT‐IR) and inductive coupled plasma emission spectrometer (ICP) analysis of the resulting shell‐cross‐linked knedels and core‐cross‐linked knedels. Transmission electron microscopy (TEM) study showed the spherical morphology and the size of the core‐cross‐linked knedels and shell‐cross‐linked knedel. Especially, the layer structure of the core‐cross‐linked knedels was clearly displayed in TEM image. The increase of extent of cross‐linking lead to the increasing of diameter for the shell‐cross‐linked knedels, whereas there was no significant effect on the core‐cross‐linked knedels. Dynamic light scattering (DLS) measurements gave hydrodynamic diameters of the core‐cross‐linked knedels that were in agreement with the TEM diameters. Moreover, the wall thickness of the shell layer of the core‐cross‐linked knedels could be easily modified by varying the block copolymer composition. Notably, the organotin‐containing core‐cross‐linked knedel exhibited highly efficient catalytic activity for the aqueous esterification reaction under nearly neutral conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Thermolyzable polymer networks and star polymers containing a novel,compact, degradable acylal‐based dimethacrylate cross‐linker: Synthesis,characterization, and thermolysis

A compact, cleavable acylal dimethacrylate cross‐linker, 1,1‐ethylenediol dimethacrylate (EDDMA), was synthesized from the anhydrous iron(III) chloride‐catalyzed reaction between methacrylic anhydride and acetaldehyde. The ability of EDDMA to act as cross‐linker was demonstrated by using it for the preparation of one neat cross‐linker network, four star polymers of methyl methacrylate (MMA), and four randomly cross‐linked MMA polymer networks using group transfer polymerization (GTP). For comparison, the corresponding polymer structures based on the commercially available ethylene glycol dimethacrylate (EGDMA) cross‐linker (isomer of EDDMA) were also prepared via GTP. The number of arms of the EDDMA‐based star polymers was lower than that of the corresponding EGDMA polymers, whereas the degrees of swelling in tetrahydrofuran of the EDDMA‐based MMA networks were higher than those of their EGDMA‐based counterparts. Although none of the EDDMA‐containing polymers could be cleanly hydrolyzed under basic or acidic conditions, they could be thermolyzed at 200 °C within 1 day giving lower molecular weight products. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5811–5823, 2007     

CuII/TEMPO‐Catalyzed Enantioselective C(sp3)–H Alkynylation of Tertiary Cyclic Amines through Shono‐Type Oxidation

A novel strategy for asymmetric Shono‐type oxidative cross‐coupling has been developed by merging copper catalysis and electrochemistry, affording C1‐alkynylated tetrahydroisoquinolines with good to excellent enantioselectivity. The use of TEMPO as a co‐catalytic redox mediator is crucial not only for oxidizing a tetrahydroisoquinoline to an iminium ion species but also for decreasing the oxidation potential of the reaction. A novel bisoxazoline ligand is also reported.     

Synthesis of trialkyl semithioglycolurils from alkylthiourea-glyoxal cyclic adducts and dialkylureas

Condensation of 1-alkyl-4,5-dihydroxyimidazolidine-2-thiones (monoalkylthiourea-glyoxal cyclic adducts) with 1,3-dialkylureas affords novel 1,4,6-trialkylsemithio- glycolurils having non-substituted NH group linked to C=S function. Such compounds can be accessed in two-stage one-pot reaction sequence from alkylthioureas and glyoxal followed by treatment of the resulting adduct with 1,3-dialkylureas     

Preparation and characterization of silicone rubber through the reaction between γ‐chloropropyl and amino groups with siloxane polyamidoamine dendrimers as cross‐linkers

A novel heat‐curable silicone rubber (MCSR/Si‐PAMAM) was prepared by using siloxane polyamidoamine (Si‐PAMAM) dendrimers as cross‐linkers and polysiloxane containing γ‐chloropropyl groups as gums. The chemical cross‐linking occurs through the reaction between Si‐PAMAM dendrimers and polysiloxane containing γ‐chloropropyl groups. The effect of various amounts of cross‐linkers on mechanical properties of MCSR/Si‐PAMAM was discussed in this paper. MCSR/Si‐PAMAM exhibits favorable mechanical properties with a tensile strength of 10.06 MPa and a tear strength of 47.9 kN/m when the molar ratio r of [N‐H]/[CH₂CH₂CH₂Cl] is 1:1. These excellent mechanical properties can be attributed to the formation of concentrative cross‐linking from Si‐PAMAM dendrimers in the cross‐linking networks, along with the introduction of Si–O–Si units in the internal structure of dendrimers. The introduction of Si–O–Si units reduces the steric hindrance of molecular structure, which facilitates the N–H bonds in the interior layers of dendrimers to react with γ‐chloropropyl groups. In addition, thermogravimetric analysis results indicate that MCSR/Si‐PAMAM is thermally stable even at high temperatures in a nitrogen atmosphere. Differential scanning calorimetry analysis reveals that the glass transition peak of MCSR/Si‐PAMAM is not identified in the temperature range −150 to −30°C, only a melting endothermic peak at −40°C.     

Silver‐Catalyzed Oxidative C(sp3)−P Bond Formation through C−C and P−H Bond Cleavage

The silver‐catalyzed oxidative C(sp³)−H/P−H cross‐coupling of 1,3‐dicarbonyl compounds with H‐phosphonates, followed by a chemo‐ and regioselective C(sp³)−C(CO) bond‐cleavage step, provided heavily functionalized β‐ketophosphonates. This novel method based on a readily available reaction system exhibits wide scope, high functional‐group tolerance, and exclusive selectivity.     

Polyhedral Oligomeric Silsesquioxane (POSS) Nanoscale Reinforcement of Thermosetting Resin from Benzoxazine and Bisoxazoline

Summary: The reaction between octaaminophenyl polyhedral oligomeric silsesquioxane (OAPS) and 2,2′‐(1,3‐phenylene)‐bis(4,5‐dihydro‐oxazoles) (PBO) over different temperature ranges was confirmed by FT‐IR spectroscopy. The OAPS was used to modify benzoxazine (BZ) in the presence of PBO. The novel polybenzoxazine (PBZ)‐PBO/OAPS hybrid nanocomposite was prepared by solvent methods. Dynamic mechanical analyses indicated that the nanocomposites exhibited much higher T_g values than the pristine PBZ and PBZ‐PBO resin, and the storage modulus of the nanocomposites was maintained at higher temperatures, although only a small amount of OAPS was incorporated into the systems. Dynamic thermogravimetric analysis showed that the thermal stability of the hybrid was also improved by the inclusion of OAPS.

DMA of PBZ (a), PBZ‐PBO (b), and PBZ‐PBO/OAPS nanocomposites (c–e).