平衡聚合反应产物分子量分布参数的确定

<正> 四氢呋喃的阳离子型聚合反应和α-甲基苯乙烯的阴离子型聚合反应都是平衡聚合反应。Tobolsky等已经对平衡聚合反应动力学作了很多研究,认为该反应的产物具有Flory分布。但尚未确定上述分子量分布函数的参变数与哪些因素有关,因此该公式的应用受到限制。近年来,平衡聚合反应的研究蓬勃展开,其反应动力学理论显然也须进一步完善,以便可以通过反应条件来预测,调节和控制反应产物的平均分子量和分散     

正癸烷加氢裂化集总动力学研究

选用国产３８２５加氢裂化催化剂，对正癸烷加氢裂化动力学进行了研究。根据Ｗｅｅｋ－ｍａｎ集总原理，建立了正癸烷加氢裂化四集总动力学模型。利用Ｍａｒｑｕａｒｄｔ法估计了各反应速度常数，确定了完善的速度表达式。计算了两种压力（６．５和８．５ＭＰａ）下的表观活化能，预测了各反应产物分布，其计算结果与实验值吻合。同时，讨论了空速、温度、压力和反应活化能对产物分布的影响，为重油和渣油加氢裂化集总动力学研究提供     

指数曲线拟合法求一级反应的速率常数

一引言在动力学研究中,为了测定一个一级反应的速率常数k,一般是在实验中记录反应物或产物的某一个与其摩尔浓度成正比的物理量A(如气体的压强,溶液在特定波长下的紫外吸收值等)随时间t的变化,并在已知反应物初始浓度或在反应结束(t→∞)时测得A_∞的条件下,根据一级反应动力学的积分公式: In|A_∞-A_t|=-kt+b ……(1)     

阴离子型共聚合反应产物分子链的序列结构

本文采用非稳态动力学方法研究了阴离子型共聚合反应产物的序列结构。经严格求解动力学微分方程组,推导出在恒定的共聚单体比条件下共聚物的序列分布函数、平均序列长度以及多元组浓度的解析表达式,并将这些结构参数与反应初始条件和时间联系起来,因而可根据聚合条件预测共聚物的微观结构。     

甲烷常压O2部分氧化合成甲醛反应动力学

用Ｍｏ／ＳｉＯ２催化剂在常压流动体系固定床反应器中进行了甲烷部分氧化合成甲醛反应的本征动力学研究，确定了反应途径和反应级数，通过Ｌａｐｌａｃｅ变换计算了各步反应速度常数和活化能，给出了Ｏ２浓度和反应温度对产物分布和最佳空时的影响规律，并预测了不同反应温度下的最佳空时及空时收率（ＳＴＹ）。     

不同尺寸（0.02—0.5μm）单分散聚苯乙烯乳液微球的制备

通过对苯乙烯乳液聚合微观动力学以及聚合过程中胶粒直径及其分布随时间变化的理论分析,并通过实验验证,比较了不同乳化剂种类、不同反应温度和不同单体用量条件下,产物胶乳的粒径分布,发现乳液聚合最终产物的粒径分布与成核期长短没有直接联系,而是取决于自由基进入胶粒的速率常数、稳态增长时间、胶粒中的平均自由基数目和胶粒的体积增长速率,胶乳单分散性随这些参量的增大而提高,从而解释了采用高温、高引发剂浓度以及长时间反应的条件对最终的胶粒尺寸分布的影响。本文还通过实验,找到了在20～500nm范围内控制粒径大小及粒径分布的方法。在20～100nm的范围内,用一步法乳液聚合,通过改变单体用量和乳化剂浓度,制备了一系列粒径的单分散聚苯乙烯胶乳;在100～500nm的范围内,运用种子乳液聚合,通过改变溶胀单体与种子胶乳的用量比,也制得了不同粒径的单分散聚苯乙烯胶乳。     

聚三氟氯乙烯的热裂解动力学和机理的研究

本文报道在4.1型示差精密热天平上装配了微分单元,组成热重-微商热重(TG-DTG)联用装置用于研究聚三氟氯乙烯在空气中的热裂解动力学过程,并运用几种不同的动力学处理方法所求得的结果来评价各种动力学理论的优缺点和说明聚三氟氯乙烯反应程度在5—95％的整个过程中活化能的变化情况。同时根据裂解程度与能量、裂解速率的关系以及裂解产物结构的鉴定,提出了聚合物无规热裂解机理,解释了迄今文献上所报道的裂解产物的结构。结果还证明了Errede提出的键离解能经验公式不适用于聚三氟氯乙烯的裂解机理。     

煤中15种微量元素在燃烧产物中的分配

建立了燃煤中１５种向量元素在悄灰和底灰中含量分布的经验公式，该公式可以估算煤中微量元素在燃烧产物中的分配及传输通量。煤中非挥发性元素大部分存在于底灰和飞灰中，挥发性元素进入大气的量较高。     

PSt-TMI合成及其共聚动力学研究

以BP0为引发剂,甲苯为溶剂,采用溶液聚合法合成了低3-异丙烯基一α,α’-二甲基苄基-异氰酸酯(TMI)含量的PSt-TMI共聚物(wTMI-O．03～O．11),确定了FTIR方法测定共聚物中TMI含量的方法,并对反应动力学与共聚物组成进行了研究。结果表明：总体反应速率与单体浓度成正比,与引发剂浓度O．5次方成正比,80℃下反应时还存在热引发,终止方式为双基终止;反应总体活化能随单体配比中TMI分率增大而升高。引发剂浓度增大则产物分子量减小,分子量分布不变。随反应转化率提高分子量分布指数增大、产物中TMI含量略有下降。GPC串联紫外分析表明,产物中高分子量部分TMI含量高于低分子量部分中TMI含量。     

脉冲激光引发自由基聚合的动力学研究

处理了无链转移时脉冲激光引发自由基聚合中的动力学问题:推导出聚合产物数均和重均分子量的严格数学表达式,给出了链自由基、死聚物及总的聚合产物的归一化的分子量分布函数,计算结果表明:随着单体转化率的上升,各种分子参数,例如数均和重均分子量,以及多分散指数的数值周期性地振荡,且振幅逐渐下降,分子量分布曲线则包含一些特征峰,且随着每次脉冲激光产生的初级自由基浓度的降低,分布曲线峰的数目增加,另外,与歧化终止相比,偶合终止使产物的分子量分布略为变窄.     

Kalman filtering of data from first- and second-order kinetics

The Kalman filter algorithm was used to process data obtained in the simultaneous determination of species following first- and second-order kinetics. The performance of the algorithm in the quantification of chemical components from simulated data was assessed, and the influence of various parameters involved was estimated. The algorithm was applied to the resolution of cysteine-ascorbic acid and glutathione-ascorbic acid mixtures where the ascorbic acid followed pseudo first-order kinetics and the amino acids second-order kinetics in the reaction with the copper(II)-neocuproine system. Some features of the determinations (namely, afforded concentration ratios, accuracy and precision) are discussed.     

The use of digital simulation to improve the cyclic voltammetric determination of rate constants for homogeneous chemical reactions following charge transfers

Cyclic voltammetry (CV) is a very useful electrochemical tool used to study reaction systems that include chemical steps that are coupled to electron transfers. This type of system generally involves the chemical reaction of an electrochemically generated free radical. Published methods exist that are used to determine the kinetics of electrochemically initiated chemical reactions from the measurements of the peak current ratio (i_pa/i_pc) of a cyclic voltammogram. The published method requires working curves to relate a kinetic parameter to the peak current ratio.In the presented work, a digital simulation package was used to obtain improved working curves for specific working conditions. The curves were compared with the published results for the first- and second-order chemical reactions following the charge transfer step mechanisms.According to the presented results, the previously published working curve is reliable for a mechanism with a first-order chemical reaction; however, a change in the switching potential requires a recalculation of the curve. In the case of mechanisms with a second-order step (dimerisation and disproportionation), several different views exist on how the second-order chemical term should be expressed so that different values of the constant are obtained. Parameters such as electrode type, electrode area, electroactive species concentration, switching potential, scan rate and method for peak current ratio calculation modify the working curves and must always be specified.We propose a standardised method to obtain the most reliable kinetic constant values.The results of this work will permit researchers who handle simulation software to construct their own working curves. Additionally, those who do not have the simulation software could use the working curves described here.The revelations of the presented experiments may be useful to a broad chemistry audience because this study presents a simple and low-cost procedure for the study of free radicals that otherwise should be studied with more sophisticated and expensive techniques, such as ESR or pulse radiolysis.     

固定化脲酶催化作用下双醛纤维素对尿素氮的吸附平衡和动力学

研究了在固定化脲酶催化作用下双醛纤维素对尿素氮的吸附平衡和动力学. 吸附过程符合Langmuir方程, 为单分子层化学吸附. 考察了双醛纤维素的氧化度、初始尿素氮浓度、双醛纤维素与固定化脲酶的质量比和温度等对双醛纤维素吸附尿素氮的影响, 结果表明上述因素对尿素氮的吸附均有较显著的影响. 实验数据能很好地拟合准二级吸附速率方程, 说明该吸附过程遵从二级动力学模型. 当氧化度为88%的双醛纤维素与固定化脲酶的质量比为10∶1, 尿素氮浓度为638.3 mg•L^－1时, 由Arrhenius方程求得表观吸附活化能为6.0 kJ•mol^－1, 该吸附过程为吸热反应.     

TEMPO-mediated oxidation of bacterial cellulose in a bromide-free system

A partially C6-carboxylated bacterial cellulose (BC) with a high carboxylate content was prepared in a bromide-free system by using 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) as a catalyst. ART-FTIR, X-ray diffraction, solid ¹³C-NMR, TEM analysis, and reaction kinetics measurements were performed to investigate the oxidation reaction of BC. Results show that C6 carboxylate was formed selectively on the microfiber surface without disrupting its highly ordered nanocrystalline structure. Given the extremely low accessibility of hydroxyl groups in d-anhydroglucopyranose units, the reaction can be described by second-order kinetics with very low reaction rate constants. pH exhibited a significant influence on the oxidation of BC and a higher activity at C6 was observed in a neutral medium.     

Anomalous kinetics in diffusion limited reactions linked to non-Gaussian concentration probability distribution function

We investigate anomalous reaction kinetics related to segregation in the one-dimensional reaction-diffusion system A + B → C. It is well known that spatial fluctuations in the species concentrations cause a breakdown of the mean-field behavior at low concentration values. The scaling of the average concentration with time changes from the mean-field t(-1) to the anomalous t(-1/4) behavior. Using a stochastic modeling approach, the reaction-diffusion system can be fully characterized by the multi-point probability distribution function (PDF) of the species concentrations. Its evolution is governed by a Fokker-Planck equation with moving boundaries, which are determined by the positivity of the species concentrations. The concentration PDF is in general non-Gaussian. As long as the concentration fluctuations are small compared to the mean, the PDF can be approximated by a Gaussian distribution. This behavior breaks down in the fluctuation dominated regime, for which anomalous reaction kinetics are observed. We show that the transition from mean field to anomalous reaction kinetics is intimately linked to the evolution of the concentration PDF from a Gaussian to non-Gaussian shape. This establishes a direct relationship between anomalous reaction kinetics, incomplete mixing and the non-Gaussian nature of the concentration PDF.     

On the mathematical model of flow injection analysis-II. The second order chemical reaction in a straight tube

The mathematical model of flow injection analysis (FIA) for fast second-order chemical reaction in a straight tube is presented. The sample and reagent are initially premixed and the analytical solution for the detection output, i.e. the integral of the product of reaction concentration over the tube cross-section is given. The optimization of FIA is discussed. It is proposed to put the detector in the real FIA systems immediately after the point where the sample and reagent are satisfactorily mixed.     

Applicability of the quasi-adiabatic enthalpimetric method for the study of chemical kinetics

The thermal quasi-adiabatic method and the non-adiabatic one are compared for the study of chemical kinetics. With thermistors as temperature-sensitive elements, and a very simple apparatus, moderately fast reactions (with t(1 2 ) down to 2 sec) and consecutive reactions can be followed, and mixtures analysed with good accuracy. The reproducibility is satisfactory over a wide range of experimental conditions. The calculation of the kinetic parameters, for both first-and second-order kinetics is very simple and requires no calibration of the system. Application of the technique to the study of the alkaline hydrolysis of some alkyl acetates is described.     

25 oC下Na2CO3/Na2SO4/NaCl混合盐的溶解动力学研究

研究了25℃时,搅拌速度,加样量及氯化钠的量等不同实验条件下,Na2CO3, Na2SO4, NaCl混合盐的溶解过程,对不同时刻液相的化学组成进行分析并鉴定其平衡时的固相,结果表明：混合盐溶解前期搅拌速度占主导因素,当溶液中三种盐达到一定浓度后,同离子效应占主导因素,随着加样量的增加,碳酸钠和硫酸钠浓度达到一定程度之后反应生成碳钠矾。结合动力学模型计算得到体系中NaCl的溶解动力学方程,得出在25℃条件下溶解符合Stumm模型,溶解过程属于扩散过程控制,同时还伴随着界面反应和化学反应。     

First- and second-order chemical-electrochemical mechanisms: Extraction of standard potential,equilibrium and rate constants from linear sweep voltammetric curves

Procedures are described for extracting the thermodynamic and kinetic key parameters from cyclic voltammetric data in the case of mechanisms involving a chemical reaction preceding the electron transfer process (CE mechanisms). Both first-order and second-order kinetics conditions for the preceding reactions are considered. These procedures are devised for practical applications mostly in the field of coordination electrochemistry, where CE mechanisms are very frequently encountered.     

Effect of solid volume fraction on aggregation and breakage in colloidal suspensions in batch and continuous stirred tanks

Aggregation and breakage of aggregates of fully destabilized polystyrene latex particles in turbulent flow was studied experimentally in both batch and continuous stirred tanks. Small-angle static light scattering (SASLS) was used to monitor the time evolution of two independent moments of the cluster mass distribution (CMD), namely, the mean radius of gyration and the zero angle scattered light intensity. In addition, information about the structure of the aggregates was obtained in terms of the static light scattering structure factor. It was observed that decreasing the solid volume fraction over more than one order of magnitude resulted in monotonically decreasing steady-state values of both moments of the CMD. Using a combination of batch operation and continuous dilution with particle-free solution in the stirred tank, it was found that the steady-state distributions were fully reversible upon changing the solid volume fraction. These observations indicate that the steady-state CMD in this system is controlled by the dynamic equilibrium between aggregation (with the second-order kinetics in cluster concentration) and breakage (with the first-order kinetics in cluster concentration). In addition, by dilution to very low solid volume fractions, we demonstrate the existence of a critical aggregate size below which breakage is negligible.