Some Aspects of the Reaction between Chitosan and Formaldehyde

Reactions of chitosan (degree of deacetylation 67%, weight-average molecular weight 850,000) with formaldehyde were performed in a dilute aqueous acetic acid solution at a molar ratio of amino groups of the polymer to HCHO of 0.06:O.1 mol/L and at different temperatures (45, 60, and 75°C). In each case the pH of the reaction mixtures was 3.4 during the experiments. The process was investigated by measuring the kinetic changes of the free formaldehyde concentration and the sum of free formaldehyde and methylol groups produced. No gelation of the reaction product was observed. The experimental results led to the conclusion that the methylol groups formed by the addition of HCHO to chitosan functions take part in some intramolecular reactions with amino and/or hydroxyl groups of chitosan. Initially, the concentration of the resultant methylene bridges grows rapidly and then drops again until an equilibrium is established. This rather unusual decomposition of the methylene links once formed without changing the reaction conditions is accompanied by a substantial growth of the amount of methylol derivative whose concentration at the beginning of the process increased rather slowly.     

Using 2D NMR to determine the degree of branching of complicated hyperbranched polymers

Degree of branching (DB) is a crucial structure parameter of hyperbranched polymers, which can be determined by 1H NMR, quantitative 13C NMR, degradative method, etc. However, for complicated hy-perbranched polymers, intricate structure and severe overlap of spectral signals hinder the determina-tion of DB using traditional methods. In this work, the architecture of complicated hyperbranched polymers has been elucidated with the help of 2D NMR techniques. Using such a method, overlapped NMR signals can be well separated into a two-dimensional space, and additional structural information is also available. Correspondingly, quantitative analysis for complicated systems can be realized. De-termination of DBs for three types of complicated hyperbranched polymers synthesized from step-polymerization, self-condensation vinyl polymerization and self-condensation ring-opening po-lymerization is shown as examples.     

Dramatic effect of solvent on the NMR spectral resolution of hyperbranched polyethers

NMR spectroscopy has served as a powerful tool to discriminate between the various types of subunits present in hyperbranched polymers. This discrimination is critical for the determination of degree of branching (DB), which is an important structural parameter that distinguishes hyperbranched structures from dendrimers. However, often times spectral signatures due to the various subunits, namely linear (L), dendritic (D) and terminal (T), are not well‐resolved. We demonstrate here that a dramatic enhancement in the peak resolution could be achieved in a series of hyperbranched polyethers, by recording the proton NMR spectra in an aromatic solvent, such as benzene, instead of the usual CDCl₃. The assignment of the various peaks was confirmed by comparison of the longitudinal relaxation times (T₁) of different protons belonging to the individual subunits. Interestingly, the chemical shift differences between the different types of subunits persist even when fairly long spacer segments separate the branch‐points in these hyperbranched polyethers. This permitted the determination of the DB of an entire series of hyperbranched polymers of varying branching densities without the need for comparison with model compounds. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1474–1480, 2007     

水解聚丙烯酰胺分子量的表征

在 2 5℃、0 .5mol/LNaCl水溶液中 ,测定了 40种水解聚丙烯酰胺样品的特性粘数 ,求出了Mark -Houwink方程常数K、α值。在水解度 2～ 40 %之间 ,发现K、α对水解度具有强烈的依赖性 ,并且特征粘数与水解度的平方根成直线关系。利用本文所求的 K、α值预测本研究的水解聚丙烯酰胺样品分子量 ,其误差在± 8%之间 ;而对文献数据进行处理 ,其平均误差为 11.12 %。     

Degree of branching of the hyperbranched polymers resulted from ab<Subscript>2</Subscript> polycondensation with substitution effect

The analytical expressions of the various structural units and the average degree of branching for the hyperbranched polymers resulted from AB₂ polycondensation with substitution effect were derived by the kinetic mechanism.The reactivity difference between the B group in linear unit and that in terminal group has great effect on the molecular parameters of the products obtained.The concentration of terminal units has a maximum with the increase of the conversion of A groups（x）.The higher the reactivity ratio（r） of linear B group to branched one is,the later the maximum appears and the larger it is.The degree of branching of the hyperbranched polymers obtained is controllable by adjusting the parameters of r and x,which increases with increasing both x and r.     

Asymmetric Aldol Reaction with Formaldehyde: a Challenging Process

The asymmetric aldol reaction with formaldehyde is a fundamental carbon‐carbon bond‐forming reaction in organic synthesis, as well as in the quest of the origin of life, as it is thought to have been the first “molecular brick” involved in the synthetic path to complex sugars. Products of aldol reactions, i.e., the β‐hydroxy carbonyl compounds, are versatile building blocks used to access a great variety of functionalised molecules. The employment of formaldehyde, as a C1 symmetric electrophile, in aldol reactions can be likely considered the most challenging, yet simplest, process to introduce a hydroxymethyl group in an asymmetric fashion. In this account, an overview of the progress achieved in the asymmetric metal‐ and organocatalysed aldol reaction, using readily available formalin or paraformaldehyde sources, is illustrated. Our recent contribution to this area, with the application of asymmetric hydroxymethylation in cascade processes for the synthesis of γ‐butyrolactones, is also shown.

     

Model of hyperbranched polymers formed by monomers A2 and Bg with end-capping molecules

Hyperbranched polymers, HBPs, formed via a stepwise polymerization of A₂, B_g type monomers with the addition of end-capping molecules, AR, were investigated by means of recursive and kinetic models. First, gelation curves were established based on the initial compositions of reactants at various functionalities, g, of monomers B_g. According to this guide, the hyperbranched polymers without gel fraction can be obtained. The molecular structures of HBPs, such as molecular weight and the degree of branching were calculated as related to conversion. It is shown that they can be controlled by the composition of reactants. With the addition of molecules AR, the gelation can be avoided at high conversion, and the distribution of molecular weights of polymers becomes narrower.     

丙烯酰胺水溶性共聚体的醛交联

丙烯酰胺水溶性共聚体的醛交联曹维孝，江必旺，马筑平（北京大学化学与分子工程学院北京１００８７１）关键词　三元共聚体，丙烯酰胺，涤纶基印字版丙烯酰胺的均聚体是当今使用最广的水溶性高分子，在造纸、采油、污水处理、水质净化等方面均有应用［１］．我们在研制涤...     

Theoretical and Experimental Insights of Benzimidazole Catalyzed by the Epoxy–Acrylic Acid Reaction

This study focuses on the experimental and molecular-level investigation of epoxy acrylate formation. Epoxy acrylate vinyl ester resin was prepared by a reaction of diglycidyl ether of bisphenol-A-based epoxy resin and acrylic acid, using benzimidazole as a catalyst. It was confirmed that benzimidazole can effectively catalyze this reaction. FTIR analysis of the product revealed a simple addition esterification reaction between the epoxide group and carboxylic group of acrylic acid excluding the side reactions (e.g., etherification). DFT computational studies were performed to theoretically explore the insights of reaction mechanisms. The calculations revealed that the benzimidazole-catalyzed reaction dominates the uncatalyzed reaction. A comparison of calculated activation energies showed that concerted mechanisms are less significant in such reactions owing to their high activation barriers.     

通过A2+B3反应制备超支化聚芳醚酮荧光材料

用3-二甲氨基苯酚与超支化聚芳醚酮(HPETFDEK-F)的末端氟发生反应,制得荧光超支化聚芳醚酮(FHPETFDEK).采用1HNMR,FTIR,DSC和TGA等方法对所得到的聚合物结构和热性能进行了表征.研究了FHPETFDEK的紫外吸收及荧光发射光谱,发现其具有荧光行为.     

高度支化共聚物的均方回转半径和支化度的关系

对于聚合物的均方回转半径和支化度的关系, 过去鲜有定量研究, 这是由于具有不同支化度的系列同种聚合物样品较难合成之故. 1982年, Kricheldorf等[1]报道了AB2和AB型单体的共缩聚反应, 这种反应可以方便地通过调节AB2和AB型单体的比例来控制产物的支化度.     

Empirical Correlations for Viscosity of Polyacrylamide Solutions with the Effects of Temperature and Shear Rate. II

Apparent viscosity measurements have been made to characterize the effects of shear rate and temperature on the partially hydrolyzed polyacrylamide solutions. The power-law model for the viscosity behavior has been modified to develop empirical correlations that combine effects of shear rate and temperature. Nonlinear regression was performed on the experimental data to develop the proposed correlation. The viscosities of the polymer solutions were measured in the temperature range of 25°C to 80°C, while the shear rate was varied from 1 to 1000 1/s. The proposed correlation should prove supportive for the preliminary selection of the polymers for enhanced oil recovery applications.     

Kinetics of Reaction Bisphenol-S Epoxy Resin with Methylacrylic Acid

The reaction kinetics of bisphenol-S epoxy resin with methyl-acrylic acid in the presence of quaternary ammonium salt catalyst was studied. The reaction rate constants at different temperatures were determined. The reaction is first order with respect to epoxy group, zero order with respect to methylacrylic acid and 0.71 order with respect to quaternary ammonium salt catalyst, respectively. The mechanism of this reaction was discussed.     

Synthesis and characterization of hyperbranched polyurethane-benzyltetrazole

A series of hyperbranched polyurethane-benzyltetrazoles（H-PBTZs） with different linkage structures were synthesized via the polycondensation of hexamethylenediisocyanate as an A₂ type monomer with（4-（1H-tetrazol-5- yl）benzyl）-diethanolamine（TBDEA） as a BB’₂ type monomer in the absence of catalyst at different temperatures.The FTIR, and ¹³C and ¹H-NMR spectroscopy were used to characterize the molecular structures of TBDEA and H-PBTZs as well as the counterpart linear polyurethane-benzyltetrazole（L-PBTZ）.The molecular composition was determined by the reaction selectivity that the isocyanate group reacted with the hydroxyl group in diethanolamine segment or the active hydrogen atom on tetrazole ring.Raising reaction temperature was propitious to the reaction of isocyanate group with the active hydrogen atom on tetrazole ring.The degrees of branching（DB） for H-PBTZs obtained from the ¹H-NMR spectra increased with raising reaction temperature.The wider molecular weight distribution of 1.7-2.9 for H-PBTZs was obtained via GPC analysis.TGA results showed that H-PBTZs had high thermal stability compared with L-PBTZ.     

Some Aspects of the Azide-Alkyne 1,3-Dipolar Cycloaddition Reaction

Russian Journal of Organic Chemistry - Some peculiar features of two most commonly used catalytic systems (Cul and CuSOVsodium ascorbate) controlling the regioselectivity of 1,3-dipolar...     

取代脲促进环氧树脂／双氰胺固化体系反应机理

双氰胺作为环氧树脂的固化剂，由于固化产物具有优良的机械和电性能，广泛应用在汽车、航天及电子等领域中．但由于其固化温度高达180C以上，使应用范围受到很大限制．专利文献曾报道晚衍生物作为环氧树脂／双氰胺固化体系的促进剂，可以使体系的固化温度降低到130～140oC，并且在室温下仍保持一定的潜伏性［‘，’］．在以往的研究中，认为取代脉的促进作用在于其与环氧发生反应生成环状化合物2一心竣烷酮和仲胺，仲胺与环氧基进一步反应生成的叔胺可以催化环氧发生阴离子聚合［’～’］．实验表明，环氧树脂／双氰胺／取代脉体系的固化温…     

环氧树脂/桐油酸酐/蒙脱土纳米复合材料固化动力学

环氧树脂/桐油酸酐/蒙脱土纳米复合材料固化动力学;固化反应;DSC     

Reaction of epoxy resin and hyperbranched polyacids

The condensation reaction between two different epoxy resins and a hyperbranched polyester (MAHP) [poly(allyloxy maleic acid‐co‐maleic anhydride)] was studied. We compared two kinds of diglycidyl ether bisphenol A type of epoxy resins with different molecular weights, that is, epoxy resin GY240 (M = 365 g/mol) and GT6064 (M = 1540 g/mol) in this reaction. The results showed a marked difference in their reaction pattern in terms of ability to form crosslinked polymer networks with MAHP. For the former low‐molecular‐weight epoxy resin, no crosslinking could be observed in good solvents such as THF or dioxane within the set of reaction conditions used in this study. Instead, polymers with epoxide functional degrees between 0.34 and 0.5 were formed. By contrast, the latter high‐molecular‐weight epoxy resin, GT6064, rapidly produced highly crosslinked materials with MAHP under the same reaction conditions. The spherical‐shape model of hyperbranched polymer was applied to explain this difference in reaction behavior. Hence, we have postulated that low‐molecular‐weight epoxy resins such as GY240 are unable to crosslink the comparatively much bigger spherically shaped MAHP molecules. However, using high‐molecular‐weight epoxy resins greatly enhances the probability of crosslinking in this system. Computer simulations verified the spherical shape and condensed bond density of MAHP in good solvents, and submicron particle analysis showed that the average MAHP particle size was 9 nm in THF. Furthermore, the epoxy‐functionalized polyesters were characterized by ¹H NMR and FTIR, and the molecular weights and molecular‐weight distributions were determined by size‐exclusion chromatography. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4457–4465, 2000     

Some Aspects of Selectivity in the Reaction of Glycosyl Donors*

Some advantages, disadvantages, and anomalies of various donors in glycosidations are discussed. By studying several two‐component donor/acceptor‐diol reactions, it is shown that regiopreferences are not very sensitive to the type of donor used. However, in competitive glycosidations within a given type of donor and between different types of donor, it is shown that regio‐ and chemoselectivities must be indexed to donor reactivity.