More about the stereodependence of DD and LL pair linkages during the ring-opening polymerization of racemic lactide

For the last two decades, mixtures of D - and L-lactides, the DD and LL cyclic dimers of lactic acid enantiomers, were polymerized under various conditions of temperature, time, and initiators. Consequences of the dimeric structure of lactide molecules on the distribution of chiral units and on the properties of resulting PLAs were discussed occasionally. In the meantime, NMR techniques have grown up and are now powerful tools to investigate configurational structures from stereosensitive resonances. Thus far, correlation between experimental data and theoretical n-ades were based on Bernoullian-type enchainments of DD and LL pairs of lactyl units present in PLA stereocopolymer chains obtained by ring-opening polymerization. Analysis of NMR spectra of racemic PLAs of different origins, including literature data, was reconsidered on the basis of the probabilities to form iso- and hetero-dyads during chain propagation. Consideration of equireactivity and stereodependent reactivity between the DD and LL pairs generated from a racemic mixture of D - and L -lactide diastereoisomers led us to the conclusion that DD /LL and LL /DD heterotactic junctions are preferentially formed when one uses Sn octoate and Zn metal as initiator systems, even though completed macromolecules remain predominantly isotactic because of the isodyads inherently present in lactide diastereoisomers. A simple method is proposed to determine whether homo- or cross-addition of DD and LL pairs is favorized in any poly(racemic lactide) in the absence of transesterification rearrangements. This method is based on the comparison of experimental relative weights of the three distinct groups of resonances due to carbonyl carbon atoms with relative weights calculated according to the Bernoullian statistics. It allows the determination of a coefficient of stereoselectivity P_i from which one can easily evaluate the average length of isotactic blocks L_i without any computation approximations. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1651–1658, 1997     

Shapes and Sizes of Linear and Circular Multiple-ring Macromolecules

The shapes and sizes of linear and circular multiple-ring macromolecules in the framework of the Gaussian model have been numerically investigated in terms of shape factors, asphericity and prolateness factors and parameters, and shrinking factors. Simple analytic expressions for the eigenpolynomials of the Kirchhoff or architecture matrices for both linear and circular multi-rings in the limit of an infinitely large individual ring have been obtained via a new recursion method. It is found that for both types of multiple rings, shape asymmetry increases while size decreases as the number of rings increases, and that asphericity and prolateness parameters for a circular 99-ring macromolecule or a doubly stranded closed random walk have stronger dependence on dimensionality of the space in which the molecule is embedded than those of its linear counterpart. © 1997 John Wiley & Sons, Ltd.     

虚拟电梯控制系统建模与仿真

虚拟电梯模型相对于实物模型成本低廉,所以其具有广泛的应用前景。提出了一种通过组态软件设计构建虚拟电梯模型的方法,实现了虚拟电梯模型和软PLC之间的通信,并应用倍福Twincat软PLC实施了对虚拟电梯模型的控制仿真。所提出的方法可以很方便地实现电梯群控模型建模,应用此模型不仅可以实现电梯控制程序的逻辑故障分析而且还可以实现机械故障的模拟。用此方法设计出一个三层的虚拟电梯模型,同时编写并调试了三层电梯的控制程序,实现了Twincat软PLC控制虚拟电梯的正常运行,验证了仿真系统的可用性和控制程序编写方法的合理性。     

FPGA中FIR数字滤波器设计可采用的一种成倍减少硬件资源使用的方法

文章提出了一种新的FIR数字滤波器硬件实现结构。这种实现结构,大大减少了乘法器以及累加器等硬件资源的使用。文章对比讨论了两种FIR数字滤波器硬件实现结构所占用硬件资源的差别,指出了新结构的优势;通过MATLAB及EDA工具的仿真,表明在完成FIR数字滤波方面,新的硬件实现结构的功能与传统结构是相同的。     

计及高比例可再生能源的配网系统储能多场景优化配置

为解决传统方法中可再生能源出力波动大、可再生能源和储能利用效率不高等问题,提出一种计及高比例可再生能源的配网系统储能多场景优化配置方法。通过在配网系统电源侧接入储能,定义配网系统储能配置场景,采用一阶低通滤波,结合波动率期望值确定方法,将低通滤波时间常数作为储能容量配置的关键参数,计算能源发电波动场景储能目标输出功率。采用上下限约束法,计算负荷储能释能场景储能目标输出功率。结合配网系统运行成本,构建储能多场景优化配置模型,求解储能最优功率和容量最优解,获得储能最优配置方案。实验结果表明,所提方法减少了风电出力和光伏出力的最大波动量、最大波动率,可再生能源出力更加平稳,减少了弃风率和弃光率,增加了储能出力,提高了可再生能源和储能利用效率。     

既有公路服务区设施配置适宜性评价模型

随着社会的进步和发展,既有公路服务区设施配置逐渐难以满足司乘人员多元化的功能需求,为实现提高既有公路服务区设施配置适宜性的目的,本文围绕适用于既有公路服务区设施配置适宜性评价指标体系与评价模型展开研究。首先,从设施状况、服务水平、用户体验三方面,构建了涵盖设施完备率、平均拥堵程度、价格满意度等11项的评价指标体系;其次,在采用AHP法、序关系分析法、熵权法、DEMATEL法等单一赋权法获取了各指标主客观权重的基础上,引入最小二乘法最小化组合权重与主客观权重偏差的平方和,确定了各指标的最优组合权重,由此建立了改进云模型;最后,将研究模型应用于四川省某服务区R中,结合正向与逆向云发生器,计算出该服务区的综合评估云,进而确定评价等级为“良”。结果表明,运用改进云模型对既有公路服务区设施配置适宜性进行评价的方法是可行的。     

手性硫代磷酸及其衍生物的研究(Ⅹ)——O-乙基-O-苯基硫代磷酰胺酯甲醇解反应的立体化学

旋光活性O-乙基-O-苯基硫代磷酰胺酯在4.7mol/LHCl-MeOH溶液中进行酸性甲醇解时得到P-N键断裂的构型翻转产物,与MeONa-MeOH的碱性甲醇解得到以PhO基为离去基的构型翻转产物.两种甲醇解反应的机理均可用三角双锥(TBP)中间体概念完满地解释.     

GPRS在组态系统中的应用

GPRS是GSMPhase2 技术,首次在GSM网络中引入分组交换模式,应用分组调度算法,将无线数据通信与数据网络的融合推进了一大步。在企业中应用GPRS技术,采用短信群发,实现快速、安全的数据传输,完成和谐、逼真的组态系统,达到实时的监视和及时的控制效果,以使工业控制系统自动化、智能化水平更高。