科学过程的多元理解及其对中学化学教学的启示

在不同的语境中,对科学过程的理解主要包括了哲学的过程、历史的过程、心理的过程和实践的过程4种。由此,对科学过程的教育价值也呈现出不同的理解。知识与过程并非对立或分立的,从外部来看,过程是对知识的多维度展开,从内部来看,过程是对知识的建设性重构。在学科教学领域,关注“学生的科学过程”就应该关注知识的内在逻辑和学生的经验背景,把学科思维过程作为化学过程的核心,把多维目标整合在知识教学的过程之中。     

化学计量学在过程分析中的应用及进展

评述了化学计量学方法在生产过程分析中各个方面 ,如过程优化、过程模拟、仪器及仪器校正、过程监测等方面的应用 ,并展望了化学计量学在过程分析中的应用前景     

光化学的初级过程和次级过程

针对物理化学教材中有关光化学初级过程和次级过程、初级过程的量子产率、初级过程的反应速率表示以及是否是零级反应等问题发表了看法。     

过程能力指数对无菌粉针剂生产过程的评估和监控

本文将过程能力分析技术应用于无菌粉针剂生产过程。通过对螺杆分装的参数设定装量情况等的监控和评估,阐述过程能力指数对无菌粉针剂生产过程的评估和监控,以及介绍过程能力分析中的基本概念和理论。结果表明,过程能力指数在药品生产过程中是非常重要,无论是在生产成本控制上还是质量控制上都发挥着重要作用。     

稀土萃取分离过程自动控制研究现状及发展趋势

在简要描述稀土萃取分离生产过程的基础上,综述了目前国内外稀土萃取分离过程中稀土元素成分在线检测的方法、装置及其应用现状;稀土串级萃取分离生产过程的计算机流程模拟以及稀土萃取生产过程的自动控制方法、技术及其应用现状.指出了稀土元素组分含量的软测量方法,以综合生产指标为目标的稀土萃取分离生产过程优化控制方法以及由生产过程管理系统和过程控制系统两层结构组成的稀土萃取分离生产过程综合自动化系统已成为稀土萃取分离生产过程自动化未来发展的方向.     

关于平衡态热力学常见过程的定义

平衡态热力学中的等温过程是系统始态与终态温度相等且等于环境恒定温度的过程;等压过程是系统始态与终态压力相等且等于环境恒定压力的过程;等容过程是系统体积恒定不变的过程。称"系统和环境的温度(或压力)相等并恒定不变的过程为等温(或等压)过程"的说法是不全面的。     

用Excel 2000求解双组分精馏过程的设计型与操作型问题

以精馏过程的设计型与操作型问题为例,介绍Excel 2000的求解过程。计算过程及结果表明,利用Excel 2000解决精馏过程的设计型与操作型问题,不需编程,运算过程简单,结果可靠,具有易学、高效、快速、实用等特点。     

土壤有机氯脱氯转化的界面交互反应*

有机氯杀虫剂、除草剂等难降解有机物是重要的土壤污染物。近年来,有机氯脱氯转化的界面过程已成为土壤环境科学的研究热点。本文综述了土壤有机氯脱氯转化的界面非生物过程、界面生物过程以及界面生物－非生物交互反应过程。界面脱氯转化过程与主要土壤化学过程、土壤根际过程相互关联,该过程中,铁物种循环与铁氧化物的异化还原溶解扮演了重要角色。     

基于过程分析技术和设计空间的金银花醇沉加醇过程终点检测

建立了一种新的基于过程分析技术(PAT)和质量源于设计(QbD)设计空间的中药制药过程终点分析与控制方法.以近红外(NIR)光谱技术为PAT工具, 采集正常操作条件下制药过程的多批次NIR光谱; 采用主成分分析结合移动块相对标准偏差(PCA-MBRSD)法, 确定每一批次过程的理想终点样本(DEPs), 由多批DEPs的光谱信息构成过程终点设计空间; 在过程终点设计空间确定的范围内, 建立多变量统计过程控制(MSPC)模型, 利用多变量Hotelling T²和SPE控制图对过程终点进行判断.应用上述方法, 进行了金银花醇沉加醇过程终点检测研究, 结果表明该方法灵敏、准确, 适宜于中药制药过程终点检测.     

表面接枝季铵盐型高分子材料抗菌过程的特性研究

考察了一种表面接枝季铵盐型高分子材料对水溶液中大肠杆菌的抗菌过程特性,发现此材料可以在短时间内迅速降低溶液中的大肠杆菌的活菌量.用菌体耗氧测定法与扫描电镜观察法研究了其抗菌过程.结果表明,其抗菌过程由吸附和杀灭两个步骤构成,其中吸附过程由纤维与大肠杆菌表面静电作用所控制,是一个快速过程,而杀灭过程则伴随着细胞壁在纤维表面的溶解,是一个慢过程.菌体耗氧测定法与扫描电镜观察法所获得的结果具有一致性,并可广泛应用于研究抗菌纤维的抗菌作用机理及过程特性.     

Thioketone‐Mediated Polymerization with Dithiobenzoates: Proof for the Existence of Stable Radical Intermediates in RAFT Polymerization

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone‐mediated radical polymerization (TKMP) of n‐butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long‐lived intermediates) and intermediate radical termination.     

RAFT polymerization kinetics: How long are the cross‐terminating oligomers?

We extend a new model for the kinetics of reversible addition‐fragmentation chain transfer (RAFT) polymerization. The essence of this model is that the termination of the radical intermediate formed by the RAFT process occurs only with very short oligomeric radicals. In this work, we consider cross‐termination of oligomers up to two monomers and an initiator fragment. This model accounts for the absence of three‐armed stars in the molecular weight distribution, which are predicted by other cross‐termination models, since the short third arm makes a negligible difference to the polymer's molecular weight. The model is tested against experiments on styrene mediated by cyano‐isopropyl dithiobenzoate, and ESR experiments of the intermediate radical concentration. By comparing our model to experiments, we may determine the significance of cross‐termination in RAFT kinetics. Our model suggests that to agree with the known data on RAFT kinetics, the majority of cross‐terminating chains are dimeric or shorter. If longer chains are considered in cross‐termination reactions, then significant discrepancies with the experiments (distinguishable star polymers in the molecular weight distribution) and quantum calculations will result. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3455–3466, 2009     

Parametric Analysis of the Intermediate Concentration in a RAFT Polymerization and its Influence upon the Polymerization Kinetics

Summary: A kinetic analysis of living/controlled radical polymerizations in bulk mediated by RAFT is presented. The main objective is to show how the kinetics of the RAFT process and, in particular, of the RAFT intermediate radical is affecting the overall polymerization rate. Namely, three different cases are analyzed: (i) slow fragmentation of the RAFT intermediate; (ii) cross‐termination of the RAFT intermediate with other radicals; and (iii) slow re‐initiation of the RAFT agent leaving group. Simplified analytical formulas are derived for the time‐dependent concentrations of the involved species as well as for conversion. They are supported by numerical simulations and are qualitatively compared to literature experimental findings. Criteria are also given to judge the influence of the RAFT reaction kinetic rate constants on the different phenomena observed experimentally in RAFT polymerization, namely inhibition and retardation. Since these criteria are given by using non‐dimensional groups, they can be readily applied to a broad spectrum of experimental conditions.

Logarithmic non‐dimensional concentration for the radicals (r) and intermediate radicals (q) versus the non‐dimensional polymerization time ( ).     

Modeling the reversible addition–fragmentation transfer polymerization process

A kinetic model has been developed for reversible addition–fragmentation transfer (RAFT) polymerization with the method of moments. The model predicts the monomer conversion, number‐average molecular weight, and polydispersity of the molecular weight distribution. It also provides detailed information about the development of various types of chain species during polymerization, including propagating radical chains, adduct radical chains, dormant chains, and three types of dead chains. The effects of the RAFT agent concentration and the rate constants of the initiator decomposition, radical addition, fragmentation, disproportionation, and recombination termination of propagating radicals and cross‐termination between propagating and adduct radicals on the kinetics and polymer chain properties are examined with the model. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1553–1566, 2003     

Kinetic model of reversible addition‐fragmentation chain transfer polymerization in microemulsions

A simplified kinetic model for RAFT microemulsion polymerization has been developed to facilitate the investigation of the effects of slow fragmentation of the intermediate macro‐RAFT radical, termination reactions, and diffusion rate of the chain transfer agent to the locus of polymerization on the control of the polymerization and the rate of monomer conversion. This simplified model captures the experimentally observed decrease in the rate of polymerization, and the shift of the rate maximum to conversions less than the 39% conversion predicted by the Morgan model for uncontrolled microemulsion polymerizations. The model shows that the short, but finite, lifetime of the intermediate macro‐RAFT radical (1.3 × 10^?4–1.3 × 10^?2 s) causes the observed rate retardation in RAFT microemulsion polymerizations of butyl acrylate with the chain transfer agent methyl‐2‐(O‐ethylxanthyl)propionate. The calculated magnitude of the fragmentation rate constant (k_f = 4.0 × 10¹–4.0 × 10³ s^?1) is greater than the literature values for bulk RAFT polymerizations that only consider slow fragmentation of the macro‐RAFT radical and not termination (k_f = 10^?2 s^?1). This is consistent with the finding that slow fragmentation promotes biradical termination in RAFT microemulsion polymerizations. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 604–613, 2010     

Effects of poly(2-vinylpyridine) as a template for the radical polymerization of methacrylic acid—II: Influence of initiator concentration

The polymerization of methacrylic acid along an atactic poly(2-vinylpyridine) template was studied by varying the initiator concentration, [I]₀. The concentrations of monomer and template were 0.4 M, the temperature 30°. Reaction rates were determined calorimetrically. The experimental results could be well described by a template polymerization model based on a modified mechanism omitting the requirement of a critical chain length of the oligomer radical prior to its association with the template. This view is in line with the existence of preferential adsorption of monomer by the template. In addition, the different ways of termination were also considered. By applying this kinetic model, the various radical concentrations and rate coefficients could be estimated. The termination rate coefficients for template associated polymer radicals appeared to be about 1000 times smaller than termination rate coefficient for non-associated radicals. Moreover, it was found that the initial polymerization rate has 0.26 order with respect to initiator, signifying a predominance of termination between template associated radicals over that between template associated and non-associated radicals (cross termination).     

A difference of six orders of magnitude: A reply to “the magnitude of the fragmentation rate coefficient”

There has been an ongoing debate regarding the mechanism that causes rate retardation phenomena observed in some reversible addition‐fragmentation transfer (RAFT) polymerization systems. Some attribute the retardation to slow fragmentation of adduct radicals, others attribute it to fast fragmentation coupled with cross‐termination between propagating and adduct radicals. There exists a difference of six orders of magnitude (10^?2 versus 10⁴/s) in the reported values of the fragmentation rate constant (k_f0) for virtually similar RAFT systems of PSt? S? C · (Ph)? S? PSt. In this communication, we explain the estimates of k_f ～ 10⁴/s and the choices of the rate constant in modeling based on experimental polymerization rate and radical concentration data. The use of k_f ～ 10^?2/s in the model results in a calculated adduct radical concentration level of 10^?4 to 10^?3 mol/L, which appears to directly contradict the reported electron spin resonance (ESR) data in the range of <10^?6 mol/L. We hope that this open discussion can stimulate more effort to resolve this outstanding difference. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2833–2839, 2003     

Full Molecular Weight Distribution in RAFT Polymerization. New Mechanistic Insight by Direct Integration of the Equations

The full molecular weight distribution (FMWD) equations in RAFT polymerization are solved by direct integration for three models: intermediate radical termination (IRT), slow fragmentation (SF), and IRT with oligomers (IRTO). It is shown that the quasi steady state approximation (QSSA) is always applicable to living radicals, and also applicable to intermediate radicals in the IRT and IRTO models. The application of the QSSA allows for considerable simplification of the numerical solution of the equations. A marked bimodality is found for the FMWD of the SF model only. This suggests model discrimination by radical trapping experiments followed by GPC measurement of the total MWD.

     

RAFT kinetics revisited: Revival of the RAFT debate

Recently, two electron spin resonance (ESR)‐based methods for the determination of addition and fragmentation rate coefficients in dithiobenzoate‐mediated reversible addition fragmentation transfer (RAFT) polymerization were introduced, one being based on a spin‐trapping method and the other on single‐laser pulse initiation in conjunction with ESR detection at microsecond time resolution. For the RAFT‐intermediate radical fragmentation rate, coefficient data differing by six orders of magnitude were obtained, which cannot be explained by the usual model dependencies, that is the so‐called cross‐termination versus stable intermediate model. Even under consideration of fast cross‐termination in both cases, the large difference persists. Both the experimental designs are thus critically reviewed to identify potential error sources and to explain the vast difference in the individual results. Both techniques appear to be robust and only small interferences could be identified. Finally, recommendations for the refinement of the individual techniques are given to achieve a consistent kinetic picture of the underpinning reaction equilibria. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Polymerization of vinyl acetate retarded by 4-methyl-2,6-di-tert-butylphenol: Kinetic and ESR studies

4-Methyl-2,6-di-tert-butylphenol strongly retards the free radical polymerization of vinyl acetate initiated by azobisisobutyronitrile. The chain transfer constant, estimated from rate data, is 0.020 ± 0.004 at 35°C and does not vary significantly with temperature. Molecular weight data lead to transfer constants of 0.023, 0.020, and 0.024 at 35, 45, and 55°C, respectively. A mean kinetic isotope effect of 9.8 ± 1.0 is observed for the phenol deuterated at the OH group, showing that the main attack of poly(vinyl acetate) radicals on the phenol involves hydrogen abstraction from this group. The activation energy for hydrogen abstraction is estimated to be 7.8 kcal/mole, and the rate constant at 50°C is 160 ± 40 1./mole-sec. The stationary concentration of 4-methyl-2,6-di-tert-butylphenoxyl in the polymerization mixture is proportional to the phenol concentration and is independent of the initiator concentration, as shown by electron spin resonance studies. Cross termination of poly(vinyl acetate) and phenoxy radicals occurs to a greater extent than mutual termination of these radicals. The rate constant for cross termination is close to 1 × 10⁸ 1./mole-sec at 50°C; the activation energy for cross termination is 2.9 ± 1.3 kcal/mole.