双分散高分子薄膜中短链对长链缠结和动力学的影响

采用Monte Carlo微观模拟结合原始路径分析的方法,研究了分子量双分散的高分子薄膜中短链的质量分数对长链的缠结和动力学行为的影响.模拟发现,当短链的质量分数较大时,薄膜内缠结点之间的关联性较低,缠结点易于被解开,长链动力学受到链受限程度的控制,随着膜厚的减小单调减慢;当短链的质量分数较小时,薄膜内缠结点之间的关联性较高,缠结点不易被解开,缠结点数目和关联性共同导致长链动力学随膜厚的非单调变化.模拟结果为明晰分子量分布对高分子薄膜中链缠结和动力学行为的影响规律提供了有益参考.     

高分子单链键接Fe_3O_4制备Janus复合纳米颗粒

采用溶剂热法制备油酸保护的Fe_3O_4纳米颗粒,通过硅烷配体交换在颗粒表面引入氨基.基于氨基快速淬灭阳离子活性种,将阳离子聚合制备的活性高分子链如聚(4-氯甲基苯乙烯)键接到氨基改性的Fe_3O_4纳米颗粒表面,制备高分子链/Fe_3O_4复合纳米颗粒.分子量决定键接活性高分子链数目.当高分子流体力学尺寸大于纳米颗粒直径时,空间排斥效应保证单根高分子链键接到Fe_3O_4纳米颗粒(10 nm),得到呈降落伞结构的复合Janus纳米颗粒,高分子链分布在Fe_3O_4颗粒一侧.当高分子链尺寸较小时,多根高分子链键接到纳米颗粒表面.研究了该复合Janus纳米颗粒双亲特性.作为固体乳化剂可容易乳化水/油体系得到稳定乳液.该Janus纳米颗粒在油水界面通过自组装可得到单层薄膜.     

Monte Carlo模拟方法研究管道受限对高分子链缠结的影响

采用基于格子模型的Monte Carlo模拟方法,考察了链长分别为30和250的线形高分子在管道内受限状态下的尺寸及扩散性质.结果表明,当线形高分子的链长低于其缠结链长时,其受限条件下高分子的性质随受限程度的变化而单调变化;而当线形高分子的链长大于缠结链长时,高分子的性质表现出非单调性,进而说明受限具有一定程度的解缠结...     

环形高分子单链塌缩的熔球中间态及其热力学解释

采用动态蒙特卡洛分子模拟研究了环形高分子单链在不良溶剂中发生塌缩转变时可逆地出现具有核-壳结构特征的熔球中间态,发现该结构特征与相同链长的线形单链基本相同,表明其只与链的长短有关,而与链端基的特殊效应无关.本工作将这一现象与单链单晶在其平衡熔点附近出现的类似现象相互关联,采用表面预溶模型来解释单链塌缩出现熔球中间态的热力学机理.分子量越低,熔球越小,表面预溶现象就越显著,塌缩转变随热力学条件变化就越缓慢.实际的高分子体系由于链内拓扑缠结,在表面未必能充分释放片段链,达不到理论预期的平衡态.表面预溶使得相分离临界点或晶体熔点附近在界面厚度方向上存在链单元能量状态不连续分布,这在微观分子水平上与临界界面连续浓度梯度的传统理论处理不一致,为我们深入理解高分子流体界面的微观结构带来帮助.     

CeO2(111)和CeO2(100)的界面调控合成及在Pt(111)上的结构转变

氧化铈基催化材料在催化反应中存在显著的晶面效应,为了在分子尺度上理解其催化化学,需要可控合成具有明确表面结构的氧化铈.因此,我们研究了Pt(111)上氧化铈纳米结构和薄膜的生长.人们通常使用金属-氧化物之间的强相互作用来解释Pt/CeO_x催化剂上的催化过程,然而对于Pt与CeO_x之间的强相互作用仍旧缺乏原子尺度上的了解.我们的结果表明, Pt与氧化铈之间的相互作用可以影响氧化铈的表界面结构,这可能会进而影响Pt/CeO_x催化剂的性质.在Pt(111)上生长的氧化铈薄膜通常暴露CeO_2(111)表面.我们发现Pt(111)表面厚度在三层以内的氧化铈薄膜,其结构是高度动态且随着退火温度升高而变化的,这种动态结构变化可归因于Pt和氧化铈间的界面电子作用.当氧化铈薄膜的厚度增大到三层以上,其负载的氧化铈团簇开始表现出迥异于三层以下氧化铈纳米岛的优异的热稳定性,表明Pt与CeO_x之间的界面电子作用主要影响厚度在三层以内的氧化铈纳米结构.采用常规的反应沉积方法难以获得完全覆盖Pt(111)衬底的规整氧化铈薄膜,而我们通过采取一种两步的动力学限制生长方法,制备出了完全覆盖Pt(111)衬底的氧化铈薄膜.对于Pt(111)上厚度约为3-4层的氧化铈薄膜,在超高真空中于1000 K退火会导致氧化铈薄膜表面形成CeO_2(100)结构.这是因为高温还原促进了c-Ce_2O_3(100)缓冲层的形成,该缓冲层被Pt的界面电子转移以及相匹配的超晶格所稳定,并进一步成为顶层CeO_2(100)结构生长的模板.进一步在900 K的氧气中处理则可将薄膜CeO_2(100)表面完全转变为CeO_2(111)表面.因此, Pt(111)上氧化铈纳米岛和薄膜所展现的结构动态变化是由Pt-CeO_x界面作用与氧化铈层间作用相互竞争所决定.本研究提供了对氧化铈负载Pt催化剂的原子级理解,虽然Pt/CeO_2催化剂活性增强的原因常被简单归结于界面强相互作用,我们的研究在原子尺度上进一步表明Pt/CeO_2在还原条件下易形成界面Ce_2O_3层.此外,本研究提供了不同晶面二氧化铈模型催化剂的构筑方法,可将对氧化铈晶面效应和Pt/CeO_x催化剂的研究推进到分子尺度.     

表面活性剂与高分子链混合体系的模拟

计算机模拟了高分子链对表面活性剂胶束形成过程的影响，以及高分子链构象性质随胶束化过程的变化.结果表明,当高分子链与表面活性剂之间的相互作用强度超过临界值后，高分子链的存在有利于表面活性剂胶束的形成.临界聚集浓度（CAC）与临界胶束浓度（CMC）的比值CAC/CMC随高分子链长的增大和相互吸引作用的增强而减小.在CAC之前，高分子链与表面活性剂分子只有动态的聚集；但在CAC之后，表面活性剂胶束随表面活性剂浓度X的增加而增大，并静态地吸附在高分子链上，形成表面活性剂/高分子聚集体.随着表面活性剂分子的加入，高分子链的均方末端距和平均非球形因子先保持恒定；从X略小于CAC开始， 和快速减小，至极小值后又逐渐增大.模拟结果支持高分子链包裹在胶束表面的实验模型.     

平行板间高分子链构象熵的动力学Monte Carlo模拟

用动力学Monte Carlo方法模拟了受限于两平行板之间的高分子链,并用扫描法计算了链的构象熵S,研究了构象熵相对于自由链的减小量(S0-S)与平行板间距D和高分子链长n的关系.结果证实了de Gennes的自由能标度关系,并给出了标度关系适用的范围.当D非常小时,高分子链受到强烈限制,S0-S与n成正比,表明单链节受到平行板的平均排斥作用力与链长无关.随着D增大,平行板对构象熵的影响越来越弱,单链节受到平行板的平均排斥作用力随链长的增长而增大.当D比较大时,平行板对构象熵的影响近似可以忽略,高分子链构象熵与自由空间中的结果一致.     

界面层中模型高分子链构象的统计理论

高分子自由链在溶液、熔体或固体中的构象有完整的理论和实验工作 ,已是成熟的知识[1～ 4] .近年来 ,受限大分子构象的探索已成为新的热点[5] .界面是一种最简单的限制 ,将对其附近的高分子产生显著的影响 .除理论家的重视以外 ,界面附近高分子构象问题还与许多应用领域相关 .例如 ,液体高分子的表面张力 ,固体高分子的粘附 ,滑移 ,磨蚀等力学特性 ,甚至它们的光学和电学性质都与界面上的组成和分子构象相关[6] .ｄｅＧｅｎｎｅｓ[7] 最早强调界面附近的分子链构象问题的重要性 .他首先从理论上猜测长链的链端若与界面有强吸引作用 ,则它们…     

基板界面对PS/PMMA共混物薄膜相逆转组成比的影响

近年来高分子共混体系中的界面、表面效应逐渐引起了越来越多研究者的兴趣 .人们发现 ,当共混物薄膜厚度减至一定程度时 ,聚合物共混物薄膜中的相形态、相容性及相分离动力学与本体中有较大的不同[1～ 3] .基板界面作用对共混薄膜体系的热力学、动力学行为产生很大的影响 .我们以往的研究 [4 ,5]也发现 ,PP/EVAc(70 /30 )共混体系退火过程中 ,基板界面 (如玻璃 )作用可大大加速分散相(EVAc)粒子的粗化凝聚过程 .本研究用聚甲基丙烯酸甲酯和聚苯乙烯共混物的四氢呋喃溶液在不同基板介质 (如玻璃基板 ,PP基板 )上成膜 ,用相差显微镜观测了…     

用耗散粒子动力学方法研究高分子链取向对形变液滴回缩法测定界面张力值的影响

在利用形变液滴回缩法(DDRM)测量了分子共混体系界面张力的过程中, 要求椭球液滴内高分子链应力松弛速度远快于椭球的回缩速度. 我们建立高分子链取向模型, 用耗散粒子动力学研究高分子链的取向及应力松弛对界面张力测量值的影响. 结果表明, 当高分子链沿着流场方向取向时, 应力是否完全松弛对界面张力测量值的影响较大, 当高分子链取向方向垂直于流场方向时, 应力是否松弛对测量值影响较小.     

二氧化碳和丙烯直接合成甲基丙烯酸的NiPMo/TiO2催化剂的研究

用溶胶-凝胶法以磷钼酸(MPA)的镍盐溶液水解钛酸四丁酯制备了NiPMo/TiO2催化剂.使用ICP、 XRD、 TG-DTA、 IR、 TPD-MS和微反应技术研究了催化剂的化学组成、热稳定性、化学吸附性质和催化反应性能.杂多钼酸盐与TiO2通过O2-在TiO2表面发生了键合.在623 K下,杂多阴离子仍保持原有的Keggin结构.CO2在Lewis酸位Ni(Ⅱ)和Lewis碱位Ni-O-Mo的桥氧协同作用下生成CO2卧式吸附态Ni(Ⅱ)←O-(CO)←(O--Ni).丙烯有多种吸附态在催化剂上吸附.在563 K、 1 MPa和空速1500 h-1的反应条件下,丙烯的摩尔转化率为3.2%,产物MAA选择性为95%.     

Synthesis of 1-benzyloxypyrazin-2(1H)-one derivatives

Different approaches for the synthesis of 1-benzyloxypyrazin-2(1H)-one derivatives from simple amino acids have been investigated. A library of 33 precursors for the preparation of N-hydroxy pyrazinones was obtained in moderate to good yields.     

α-Amino acid derived enaminones and their application in the synthesis of N-protected methyl 5-substituted-4-hydroxypyrrole-3-carboxylates and other heterocycles

A new and simple synthesis of novel N-protected methyl 5-substituted-4-hydroxypyrrole-3-carboxylates, which exist in equilibrium with their 4-oxo tautomers, has been developed in two steps starting from N-protected α-amino acids. The key intermediates are enaminones, which can also be isolated, characterized, and used for the construction of other functionalized heterocycles, before they spontaneously decompose to pyrrole products. 4-Hydroxypyrroles are prone to partial aerial oxidation but can be efficiently alkylated or reduced to stable polysubstituted pyrrolidine derivatives.     

Specific intramolecular aromatic CH insertion of diazosulfonamides

The chemoselectivity in the intramolecular CH insertion of various diazosulfonamides has been experimentally studied. The results reveal that the aliphatic 1,4-, 1,5-, or 1,6-C(sp³)?H insertions of diazosulfonamides are not accessible, while the aromatic 1,5-C(sp²)?H insertion can be realized specifically by adjusting the diazo-adjacent group. In addition, the general chemoselectivities in the intramolecular CH insertions of diazosulfonyl compounds are summarized. Generally, diazosulfones undergo both aromatic 1,5-C(sp²)?H and aliphatic 1,5- and 1,6-C(sp³)?H insertions, while diazosulfonates undergo aliphatic 1,5- and 1,6-C(sp³)?H insertions. However, diazosulfonamides only undergo aromatic 1,5-C(sp²)?H insertion.     

A general and convenient synthesis of 4-(tosylmethyl)semicarbazones and their use in amidoalkylation of hydrogen,heteroatom, and carbon nucleophiles

A general synthesis of previously unknown semicarbazone-based α-amidoalkylating reagents, 4-(tosylmethyl)semicarbazones, has been developed. The synthesis involved three-component condensation of semicarbazones of aliphatic or aromatic aldehydes with the same or other aldehydes and p-toluenesulfinic acid. The scope and limitations of this reaction were investigated. The compounds obtained were demonstrated to be an efficient α-(4-semicarbazono)alkylating agents. They were reacted with H- (sodium borohydride), O- (sodium methylate), S- (sodium phenylthiolate), N- (pyrrolidine, sodium succinimide), P- (trialkyl phosphites), and C-nucleophiles (sodium diethyl malonate) to give the corresponding products of the tosyl group substitution, 4-substituted semicarbazones, including analogues of nitrofurazone. Among the prepared compounds tested in vitro for antibacterial and antifungal activity, three nitrofuryl-containing semicarbazones exhibited high biological activities with minimum inhibitory concentration (MIC) values of 8–32 μg/mL.     

Toward new camptothecins. Part 6: Synthesis of crucial ketones and their use in Friedländer reaction

In the context of the preparation of camptothecin and luotonin A analogs, the synthesis of some key keto-precursors and their use in Friedländer condensation are described. This paper also focuses on the stability of these keto intermediates and emphasizes the major differences between indolizinones and pyrroloquinazolinones series. Noteworthy is also the report of some original structures isolated as by-products of some experiments.     

Synthesis of novel chiral bidentate hydroxyalkyl-N-heterocyclic carbene ligands and their application in palladium-catalyzed Mizoroki–Heck couplings and asymmetric addition of diethylzinc to benzaldehyde

A small library of new chiral bidentate hydroxyalkyl-imidazolium salts 1 is conveniently synthesized on multi-gram scale from inexpensive and commercially available chiral pool amino acids. The corresponding carbenes, generated by deprotonation of imidazolium salts 1, in combination with palladium(II) chloride were tested in the Mizoroki–Heck coupling reaction. The most significant results in terms of yields and reactivities were achieved with low catalyst loading. The catalytic activities of these imidazolium salts were also investigated in the asymmetric addition of diethylzinc to benzaldehyde. The use of MgO nanoparticles as an additive in conjunction with these ligands played a crucial role in increasing the efficiency of these reactions.     

N-Heterocyclic carbene-palladacyclic complexes: synthesis,characterization and their applications in the C-N coupling and α-arylation of ketones using aryl chlorides

N-Heterocyclic carbene-palladacyclic complexes 3 were successfully achieved in a one-pot procedure under mild conditions. The structure of 3a was unambiguously confirmed by X-ray single crystal diffraction and it was an active catalyst in the Buchwald-Hartwig amination and α-arylation of ketones even at very low catalyst loadings (0.01?mol%).     

Iodine-mediated oxidative Pictet-Spengler reaction using terminal alkyne as the 2-oxoaldehyde surrogate for the synthesis of 1-aroyl-β-carbolines and fused-nitrogen heterocycles

An efficient iodine-mediated oxidative Pictet-Spengler reaction in dimethyl sulphoxide (DMSO) using terminal alkynes as the 2-oxoaldehyde surrogate for the synthesis of aryl (9H-pyrido[3,4-b]indol-1-yl)methanones is described. The scope of the protocol includes the total synthesis of Fascaplysin, Eudistomins Y₁ and Y₂. The methodology is extended for preparing pyrrolo[1,2-a]-quinoxaline and indolo[1,5-a]quinoxaline derivatives. The utility of 1-aroyl-β-carbolines was demonstrated by performing palladium-catalyzed β-carboline directed ortho-C(sp2)-H functionalization of the phenyl ring with thiomethyl (SMe) group using DMSO as source and for accessing 4-aryl-canthin-6-ones.     

Facile synthesis of bicyclic 1-arylpyrazol-5-ones

In this Letter, we described a facile method for constructing fused bicyclic 1-arylpyrazol-5-one ring system. We employed various methylene-containing carboxylic acids as the substrates and proved that the pyrazolone ring closure requires activated methylene group in intermediate II. Accordingly, a series of structurally diversified, fused bicyclic 1-arylpyrazol-5-ones was prepared in moderate to high yields using the requisite substrates.