Modification of side chain terminals of PEGylated molecular bottle brushes—A toolbar of molecular nanoobjects

Water soluble molecular brush with poly(glycidyl methacrylate) (PGMA) as the main chain and mono-allyl terminated poly(ethylene glycol) (PEG) as the densely grafted side chains was efficiently synthesized via grafting-onto approach by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Then highly efficient post-modification of the ends of the side chains was realized through UV light induced thiol–ene radical addition. Thus polymer brushes with various side chain terminal functional groups including hydroxyl group, carboxyl group, amino group, amino acid, sulfonic group and glucose unit were obtained.     

Functionalization of two-component molecular networks: recognition of Fe³⁺

Two-component supramolecular networks have been constructed with a symmetric triphenylene derivative with three carboxyl groups (sym-TTT) and melamine. Two kinds of hydrogen bonds with different strength are involved in the multi-component self-assembly, one is H-bond between carboxyl group of sym-TTT and melamine, the other is intermolecular H-bond between melamine molecules. These interactions drive a structural transformation from close-packed network to hexagonal network with active amino groups inside of the cavity. Scanning tunneling microscopy (STM) measurements reveal that the functionalized network of sym-TTT/melamine could recognise Fe(3+). These results could be helpful for designing functionalized molecular networks by multi-component self-assembling strategy.     

Synthesis and characterization of boron‐containing molecular sieves

The synthesis, physico-chemical characterization and catalytic activity of boron-containing molecular sieves (boralites) are reviewed and discussed. Incorporation of boron in the silica framework of several phases has been assessed by structural and spectroscopic evidence. Compared with the aluminosilicate parent compounds, boralites display weaker acid strength sufficient, however, for catalyzing several reactions. This revised version was published online in August 2006 with corrections to the Cover Date.     

β-Galactosidase-instructed formation of molecular nanofibers and a hydrogel

Here we report the first example of using β-galactosidase to trigger the formation of cell compatible, supramolecular nanofibers, which ultimately may lead to a new approach for the development of soft nanotechnology.     

Dual stimuli-responsive polymers derived from α-amino acids: Effects of molecular structure,molecular weight and end-group

Four kinds of well defined poly(N-methacryloyl-L-amino acid)s were prepared via reversible addition fragmentation chain transfer polymerization. The effects of molecular structure, molecular weight, and end-group on their responsive properties in water were investigated. We found that the monomer structure is critical for the polymer to exhibit stimuli-responsive properties in water; only polymers derived from aspartic acid showed defined dual thermo- and pH-responsive properties. To study the effects of molecular weight and end group on the lower critical solution temperature behavior of poly(N-methacryloyl-L-β-isopropyl aspartic acid) (PMAIPAC), a series of PMAIPAC homopolymers with controlled molecular weights and narrow polydispersities were prepared and treated with excess radical initiators to give defined end groups. The cloud point of PMAIPAC was found to increase with the increase of solution pH, due to the ionization of carboxylic acid groups, and decrease with the increase of molecular weight. The structure of end groups also affects the cloud point of PMAIPAC particularly for low molecular weight samples at low pH.     

Structure of ultrahigh molecular weight polyethylene–air counterflow flame

The combustion of ultrahigh molecular weight polyethylene (UHMWPE) in airflow perpendicular to the polyethylene surface (counterflow flame) was studied in detail. The burning rate of pressed samples of UHMWPE was measured. The structure of the UHMWPE–air counterflow flame was first determined by mass spectrometric sampling taking into account heavy products. The composition of the main pyrolysis products was investigated by mass spectrometry, and the composition of heavy hydrocarbons (C₇—C₂₅) in products sampled from the flame at a distance of 0.8 mm from the UHMWPE surface was analyzed by gas-liquid chromatography mass-spectrometry. The temperature and concentration profiles of eight species (N₂, O₂, CO₂, CO, H₂O, C₃H₆, C₄H₆, and C₆H₆) and a hypothetical species with an average molecular weight of 258.7 g/mol, which simulates more than 50 C₇—C₂₅ hydrocarbons were measured. The structure of the diffusion flame of the model mixture of decomposition products of UHMWPE in air counterflow was simulated using the OPPDIF code from the CHEMKIN II software package. The simulation results are in good agreement with experimental data on combustion of UHMWPE.     

Exploring the roles of molecular structure on the β-crystallization of polypropylene random copolymer

To investigate effects of molecular structures on β-crystallization of polypropylene random copolymer (PPR), PPR-A and PPR-B with similar molecular weight and distribution but significantly different polymorphic behavior were used. Wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FT-IR), solvent fractionation combined with successive self-nucleation and annealing (SSA) and Gel Permeation Chromatography (GPC) were applied. Results revealed that after addition of same amount of β-nucleating agent (β-NA), large amount of β-phase is obtained in PPR-A (K _β  = 0.603), while no β-phase can be seen from PPR-B; Structure characterizations revealed that they have similar molecular weight but different total amount and distribution of ethylene comonomer: the total content of ethylene comonomer of PPR-A is higher compared with PPR-B, but its ethylene comonomer distribution is less uniform. In this way, PPR-A possesses high molecular weight high isotactic fraction, which cannot be seen from PPR-B due to its more uniform comonomer distribution. Therefore, the presence of highly isotactic PP (HPP) is the key factor in β-crystallization of PPR. For verification, HPP with low or high molecular weights are respectively added into β-nucleated PPR-B. Results revealed that HPP was effective in enhancing β-crystallization of PPR-B; higher molecular weight of HPP was more favorable and more effective.     

Catalytic application of mesoporous molecular sieves to vapor‐phase Beckmann rearrangement of cyclohexanone oxime

The vapor‐phase Beckmann rearrangement of cyclohexanone oxime to ɛ‐caprolactam catalyzed by mesoporous molecular sieves was studied. The proton‐form mesoporous molecular sieves, H‐MCM‐41 and H‐FSM‐16, exhibited extremely high activity, selectivity and stability for ɛ‐caprolactam formation when 1‐hexanol was used as diluent. The weak acid sites and/or surface silanol groups of mesoporous molecular sieves play an important role in the selective ɛ‐caprolactam formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface modification of ultra-high molecular weight polyethylene fibers by γ-radiation-induced grafting

A technique for grafting acrylic polymers on the surface of ultra-high molecular weight polyethylene (UHMWPE) fibers utilizing ⁶⁰Co gamma radiation at low dose rates and low total dose has been developed. Unlike some of the more prevalent surface modification schemes, this technique achieves surface grafting with complete retention of the exceptional UHMWPE fiber mechanical properties. In particular, poly(butyl acrylate) and poly(cyclohexyl methacrylate) were successfully grafted onto UHMWPE fibers with no loss in tensile properties. The surface and tensile properties of the fibers were evaluated using Fourier transform infrared/photoacoustic spectroscopy (FTIR/PAS), X-ray photoelectron spectroscopy (XPS), and tensile tests. The reinforcement efficiency of untreated, polymer-grafted, and plasma-treated UHMWPE fibers in polystyrene and a poly(styrene-co-butyl acrylate-co-cyclohexyl methacrylate) statistical terpolymer was characterized using mechanical tensile tests. The thermoplastic matrix composites were prepared with 4 wt% discontinuous (10 mm), randomly distributed UHMWPE fibers. An approximate 30% increase in composite strength and modulus was observed for poly(cyclohexyl methacrylate)-grafted fibers in the terpolymer and polystyrene matrices. A comparable improvement was realized with the plasma-treated fibers. On the other hand, poly(butyl acrylate) grafts induced void formation, i.e. energy dissipation through plastic deformation and volume expansion at the fiber/matrix interface in terpolymer composites. The latter resulted in a 75% increase in the elongation to failure. The effect of polymer grafts on fiber/matrix adhesion is discussed in terms of the graft and matrix chain interactions and solubility, graft chain mobility, and fracture surface characteristics as determined by scanning electron microscopy (SEM).     

Catalytic monoepoxidation of butadiene over titanium silicate molecular sieves TS‐1

Titanium silicate molecular sieves TS‐1 have been prepared under hydrothermal conditions with tetrapropylammonium hydroxide as structure‐directing agent. The structures of TS‐1 were characterized by means of XRD, TEM, FT‐IR, DR UV‐Vis, ICP‐AES, and N₂ adsorption/desorption techniques. Its catalytic performances in the epoxidation of butadiene with hydrogen peroxide were investigated; 3,4‐epoxy‐1‐butene was obtained with high selectivity. The influences of various parameters such as temperature, solvent, and titanium content on the activities of the catalysts were studied systematically. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tuning mesoporous molecular sieve SBA-15 for the immobilization of α-amylase

The present work describes the immobilization of α-amylase over well ordered mesoporous molecular sieve SBA-15 with different pore diameters synthesized by post synthesis treatment (PST) hydrothermally after reaction at 40°C. The materials were characterized by N₂ adsorption–desorption studies, small angle X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. Since α-amylase obtained from Bacillus subtilis has dimensions of 35 × 40 × 70 Å it is expected that the protein have access to the pore of SBA-15 (PST-120°C) with diameter 74 Å. The pore dimension is appropriate to prevent considerable leaching. The rate of adsorption of the enzyme on silica of various pore sizes revealed the influence of morphology, pore diameter, pore volume and pH.     

Production of carbon molecular sieves by plasma treated activated carbon fibers☆

Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO₂ and CH₄. The CO₂/CH₄ selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO₂/CH₄ selectivity (26) was observed for C₂H₄/N₂ plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO₂, thereby altering CO₂/CH₄ selectivity.     

Rapid generation of molecular complexity in the Lewis or Brønsted acid-mediated reactions of methylenecyclopropanes

Although they are highly strained, methylenecyclopropanes (MCPs) are readily accessible molecules that have served as useful building blocks in organic synthesis. MCPs can undergo a variety of ring-opening reactions because the release of cyclopropyl ring strain (40 kcal/mol) can provide a thermodynamic driving force for reactions and the π-character of the bonds within the cyclopropane can afford the kinetic opportunity to initiate the ring-opening. Since the 1970s, the chemistry of MCPs has been widely explored in the presence of transition metal catalysts, but less attention had been paid to the Lewis or Br?nsted acid mediated chemistry of MCPs. During the past decade, significant developments have also been made in the Lewis or Br?nsted acid mediated reactions of MCPs. This Account describes chemistry developed in our laboratory and by other researchers. Lewis and Br?nsted acids can be used as catalysts or reagents in the reactions of MCPs with a variety of substrates, and substituents on the terminal methylene or on the cyclopropyl ring of MCPs significantly affect the reaction pathways. During the past decade, we and other researchers have found interesting transformations based on this chemistry. These new reactions include the ring expansion of MCPs, cycloaddition reactions of MCPs with aldehydes and imines, cycloaddition reactions of MCPs with nitriles in the presence of strong Br?nsted acid, radical reactions of MCPs with 1,3-dicarbonyl compounds, intramolecular Friedel-Crafts reactions of MCPs with arenes, acylation reactions of MCPs, and the reaction of MCPs with 1,1,3-triarylprop-2-yn-1-ols or their methyl ethers. These Lewis or Br?nsted acid mediated reactions of MCPs can produce a variety of new compounds such as cyclobutanones, indenes, tetrahydrofurans, and tetrahydroquinolines. Finally, we have also carried out computational studies to explain the mechanism of the Br?nsted acid mediated reactions of MCPs with acetonitrile.     

Polymer precursor synthesis of novel ZrC–SiC ultrahigh-temperature ceramics and modulation of their molecular structure

In this study, ZrC–SiC composite ceramics were prepared with varying Zr/Si molar ratios using sol–gel method. Composites were characterized by Fourier-transform infrared spectroscopy (FT–IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). FT–IR analysis confirmed macromolecular network structure of composites, in which the precursor is composed of polyvinyl butyral (PVB) as main chain, silane molecules are interlinked via –OH moieties in PVB side chains, and Zr atoms are crosslinked with Si in corresponding proportion. Ceramic precursor begins to decompose at a temperature exceeding 1300 °C and is completely transformed into ZrC–SiC composite ceramics with corresponding Zr/Si molar ratio at 1600 °C. Raman spectroscopy and TEM results reveal that after annealing at 1600 °C, ZrC powder uniformly covers surface of SiC ceramics, and high-crystallinity graphite carbon covers ZrC powder.     

Use of molecular simulation for evaluating adsorption equilibrium of inhalation anaesthetic agents on metal–organic frameworks

Metal–organic frameworks (MOFs) were studied as alternatives to zeolites and activated carbon for adsorptive removal of wasted inhalation anaesthetic agents (IAA). Monte Carlo simulation was used to predict equilibrium adsorption isotherms of IAA on selected MOFs. Rather than generic forcefields (FFs), the all-atom FF parameters published by Arcario were used for IAA modelling. Continuous fractional component Monte Carlo (CFCMC) proved crucial for speedy simulation of large molecules. We found that allocating 70% probability to the CFlambdaSwap move gave optimum fits between simulation and experiment. The simulations provided us with an insight into the adsorption mechanisms of IAA in these structures. Heats of adsorption, Brauner-Emmet-Teller (BET) surface area, and total pore volume were deduced to be the crucial parameters for low, medium, and high range of relative pressures in the isotherm. Therefore, the chromium atoms in MIL-101-Cr are better adsorbers of IAA than MIL-100-Al at lower pressures despite the similarities in terms of the type of linkers and topology. Our simulation results corroborated the earlier published studies on the self-association behaviour of sevoflurane molecules based on the experimental isotherms reported for MOF-177-Zn. Finally, the high polarity of IAA is thought to explain good low-pressure simulation/experiment data agreement for the MOFs possessing coordinatively unsaturated sites (CUS) despite using generic DREIDING FF for the framework atoms. Our in-house parsing code helped realize that the grand-canonical Monte-Carlo simulation speed is not the same for all pressure points but decreases for higher pressure points. This can be explained by increased density of the adsorbates making successful trial moves less probable.     

Preparation and properties of neodymium based polybutadiene with narrow relative molecular mass distribution

具有窄相对分子质量分布的钕系聚丁二烯的制备与性能张志强1,刘峰1,林曙光1,董为民2,石路颖1,姜连升2,张学全2(1.中国石化锦州石化股份有限公司技术中心,辽宁锦州121001;2.中国科学院长春应用化学研究所,吉林长春130022)采用Nd(vers)3/Al(i-Bu)2H/Al(i-Bu)2Cl催化体系,合成了相对分子质量分布指数为2.53的聚丁二烯,考察了Nd(vers)3和Al(i-Bu)2H用量对该聚丁二烯相对分子质量分布及其门尼黏度的影响规律。结果表明,具有窄相对分子质量分布的聚丁二烯的加工行为良好,其300%定伸应力、拉伸强度和扯断伸长率明显高于宽相对分子质量分布的钕系聚丁二烯BR9100-41和镍系聚丁二烯BR9000。     

The effects of PCL diol molecular weight on properties of shape memory poly(ε-caprolactone) networks

Shape memory poly(ε-caprolactone) (PCL) networks with different molecular weights of PCL diol were prepared via thiol–ene reaction in this work. The highly efficient thiol–ene reaction ensured a uniform distribution of PCL chains between crosslinker, contributing well-defined network architecture with good mechanical and shape memory properties. ¹H NMR spectra were used to confirm that PCL diol was completely converted into acrylate-terminated PCL. Gel content experiment and Fourier-transform infrared showed that almost all samples exhibited virtually the complete crosslinking network due to the highly selective thiol–ene reaction between acrylate-terminated PCL and tetrathiol crosslinker. Differential scanning calorimetry and X-ray diffraction tests revealed that the melting temperature and crystallinity of the prepared PCL networks by using high molecular weight of PCL diol displayed a higher result compared to ones using low molecular weight. The dynamic mechanical analyzer results revealed that the storage modulus of the network dropped evidently across the thermal transition, these characteristics of the PCL network indicated that all exhibited a good shape memory effect. The shape fixing ability was kind of being affected by the PCL diol molecular weight, while the shape recovery ratio of all samples can almost reach 99% irrespective of the length of PCL diol. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47055.     

High molecular weight copolymers of l-lactide and ε-caprolactone as biodegradable elastomeric implant materials

Summary High molecular weight copolymers of L-lactide and -caprolactone have been synthesized by ring opening copolymerization with stannous octoate as catalyst. The good mechanical properties of the 50/50 copolymers make it a suitable material for biomedical applications such as nerve guides etc., where degradation of the elastomeric implant is required. In contrast to the frequently used MDI containing polyurethanes, degradation products of the P(LLA--CL) are non toxic. The use of such a material is therefore preferable.     

Control of molecular weight distribution for polypropylene obtained by commercial Ziegler–Natta catalyst: effect of temperature

Polymerization of propylene was carried out by using MgCl₂-supported TiCl₄ catalyst in conjunction with triethylaluminium (TEA) as cocatalyst. The effect of polymerization temperature on polymerization of propylene was investigated. The catalyst activity was influenced by the polymerization temperature significantly and the maximum activity of the catalyst was obtained at 40 °C. With increasing the polymerization temperature, the molecular weight of polypropylene (PP) drastically decreased, while the polydispersity index (PDI) increased. The effect of the two-stepwise polymerization procedure on the molecular weight and molecular weight distribution of PP was studied and the broad PDI of PP was obtained. It was also found that the PDI of PP could be controlled for propylene polymerization through regulation of polymerization temperature. Among the whole experimental cases, the M _w of PP was controlled from 14.5 × 10⁴ to 75.2 × 10⁴ g/mol and the PDI could be controlled from 4.7 to 10.2.