Statistical thermodynamics in the framework of the lattice fluid model, 1. Polydisperse polymers of special distribution

The Gibbs free energies and equations of state of polymers with special molar mass distributions, e.g., Flory distribution, uniform distribution and Schulz distribution, are derived based on a lattice fluid model. The influence of the polydispersity (or the chain length) on the close-packed mass density, the close-packed volume of a mer and the mer-mer interaction energy or the scaling temperature is discussed. The diagrams of the Gibbs free energies as a function of temperature and chain length are simulated with a computer. The results suggest that a polydisperse polymer is thermodynamically more stable than the corresponding monodisperse polymer and that the thermodynamical properties of a polydisperse polymer are identical with those of the corresponding monodisperse polymer when the average degree of polymerization is sufficiently high.     

Postgel properties in the statistical crosslinking of heterochains. II. Free‐radical crosslinking copolymerization

The heterochain crosslinking theory is applied to postgel behavior in the free‐radical crosslinking copolymerization of vinyl and divinyl monomers. In this context, the crosslinked polymer formation can be viewed as a system in which the primary chains formed at different times are combined in accordance with the statistical chain‐connection rule governed by the chemical reaction kinetics. Because the primary chains are formed consecutively, the number of chain types N must be extrapolated to infinity, N → ∞. Practically, such extrapolation can be conducted with the calculated values for only three different N values. The analytical expressions for the weight fraction and average molecular weights of the sol fraction are derived for the general primary chain length distribution function in free‐radical polymerization. Illustrative calculations show that the obtained results agree with those from the Monte Carlo method, and that the postgel properties in free‐radical crosslinking copolymerization systems could be significantly different from those in randomly crosslinked systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2342–2350, 2000     

(Supercritical) Ammonia for Recycling of Thermoset Polymers

Chemical recycling of thermosetting bisphenol-A-polycyanurate, triphenylisocyanurate and of N-phenylmaleimide (pPMI) as well as 4,4′-methylenbis- (4-N-phenylmaleimide) (pBMI) has been studied. Polycyanurate thermosets undergo ammonolytic degradation to the level of oligomeric soluble products already at room temperature, complete degradation to bisphenol-A and melamine, however requires supercritical conditions (160 °C) and longer reaction times. Polyimides prepared by free radical polymerisation after ammonolysis at 160 °C give the corresponding amines and linear polymers with unsubstituted imide and diamide units. SEC of the polymer analogous (with respect to the carbon chain) reaction products and of linear pPMI shows that the polymers have a polymodal molar mass distribution with a high molar mass and an oligomeric fraction.     

Deterioration of ancient cellulose paper,Hanji: evaluation of paper permanence

Hanji paper, the paper material traditionally used in Korea, is in the focus of the present aging and mechanistic study. As raw materials and historic recipes for paper making are still available for Hanji today, specimen resembling historical material at the point of production can be prepared. While from that starting point, historical material had taken the path of natural aging, newly prepared samples—prepared according to both historic and current recipes—were artificially aged, and both aging modes can be compared. For the first time, an in-depth chemical and mathematical analysis of the aging processes for Hanji is presented. The aging of Hanji paper, resulting in hydrolysis and oxidation processes, was addressed by means of selective fluorescene labeling of oxidized groups in combination with gel permeation chromatography, providing profiles of carbonyl and carboxyl groups relative to the molar mass distribution. Starting Hanji showed the highest molecular weight (>1,400 kDa) ever reported for paper. We have defined two critical parameters for comparison of the paper samples: half-life DP (the time until every chain is split once on average) and life expectancy (the time until an average DP of failure is reached and no further mechanical stress can be tolerated). The two values were determined to be approximately 500 and 4,000 years, respectively, for the Hanji samples, provided there is no UV radiation. The rate of cellulose chain scission under accelerated aging (80 °C, RH 65 %), was about 600 times faster than under natural conditions. In addition, cellulose degradation of Hanji paper under accelerated aging condition was about 2–3 times slower than that of historical rag paper as those used in medieval Europe.     

Theoretical predictions of chemical degradation reaction mechanisms of RDX and other cyclic nitramines derived from their molecular structures

Analysis of environmental degradation pathways of contaminants is aided by predictions of likely reaction mechanisms and intermediate products derived from computational models of molecular structure. Quantum mechanical methods and force-field molecular mechanics were used to characterize cyclic nitramines. Likely degradation mechanisms for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) include hydroxylation utilizing addition of hydroxide ions to initiate proton abstraction via 2nd order rate elimination (E2) or via nucleophilic substitution of nitro groups, reductive chemical and biochemical degradation, and free radical oxidation. Due to structural similarities, it is predicted that, under homologous circumstances, certain RDX environmental degradation pathways should also be effective for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and similar cyclic nitramines. Computational models provided a theoretical framework whereby likely transformation mechanisms and transformation products of cyclic nitramines were predicted and used to elucidate in situ degradation pathways.     

双烯烃橡胶复分解-加氢制备端羧基聚烯烃

采用烯烃复分解法,以双烯烃橡胶为原料,在Grubbs II代催化剂和链转移剂(马来酸)的作用下制得相应的端羧基聚二烯烃,通过对甲苯磺酰肼/三正丙胺试剂对其进一步加氢得到端羧基聚烯烃.主要研究了反应时间、反应温度、橡胶中双键/催化剂摩尔比、橡胶中双键/链转移剂摩尔比等因素对产物分子量及分子量分布的影响.通过核磁共振氢谱(1H-NMR)和碳谱(13C-NMR)、红外光谱(FTIR)、凝胶渗透色谱仪(GPC)、热重分析(TGA)和示差扫描量热分析(DSC)对产物的结构和性能进行了测试表征.结果表明,通过调整橡胶中双键/催化剂的摩尔比或橡胶中双键/链转移剂的摩尔比可以调控产物的分子量.另外,采用该方法制得的端羧基聚丁二烯具有较高的反式1,4-结构含量,与原料相比其顺式1,4-结构含量大幅下降,从而对产物的性能产生一定影响;而以异戊橡胶为原料时并没有观察到该现象.端羧基聚二烯烃经加氢反应后转变成端羧基聚烯烃,具有更好的热稳定性.该方法合成步骤简单,产物分子量可控,为功能材料的制备提供了新的可能.     

Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene

The thermal oxidation of an unvulcanized, unstabilized polyisoprene rubber (IR) has been studied in the 40-140 °C temperature range. Ageing was monitored by FTIR determination of double bonds and carbonyl groups, mass uptake measurement, and weight average molar mass determination. A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the diversity of initiation processes and the existence of inter- and intramolecular radical additions to double bonds, was built. The kinetic model derived from this scheme is composed of seven differential equations to be solved in discrete thickness layers to take into account the kinetic control by oxygen diffusion. This system was numerically solved using a Matlab program dedicated to stiff systems of differential equations. The elementary rate constants and other kinetic parameters were then determined from experimental data, using an inverse approach. A set of physically reasonable parameter values was obtained, thus allowing us to envisage lifetime predictions at low temperature (long term). The results led to observations difficult to make from classical analytical studies, for instance the predominance of bimolecular hydroperoxide decomposition among other initiation modes or the competition between intermolecular hydrogen abstraction and intramolecular addition of peroxy radicals to double bonds.     

A Molecular Mechanism of Fenton Oxidation Degradation of m-Xylene

Density functional theory(DFT) quantum chemical method was used to study the process of meta-xylene Fenton oxidation. The EHOMO energies of meta-xylene molecules are higher, so they have better ability to provide electron and prone to the nucleophilic reaction. M-xylene mainly reacts with OH free radical in addition reactions. And the position of C(6) is most likely to react with the OH free radical, rather than the C(3), which is the most difficult to occur. According to all the above results, the most likely reaction mechanism of advanced oxidation degradation of meta-xylene is determined.     

Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.     

Synthesis of densely branched poly(methyl methacrylate)s via ATR copolymerization of methyl methacrylate and ethylene glycol dimethacrylate

Densely branched poly(methyl methacrylate)s have been synthesized by copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) using atom transfer free radical polymerization (ATRP). By employing the phenyl and benzyl esters of 2-bromo-2-methylpropionic acid as the initiators with 2,2-bipyridyl and Cu(I)Cl it has been possible to use high field ¹H nuclear magnetic resonance spectroscopy to evaluate in some detail the composition and structure of the branched PMMAs obtained. Parallel molar mass size exclusion chromatographic analysis using a multi-angle light scattering detector with a refractive index detector (MALS/SEC) has allowed the branched architecture of the products to be confirmed. Rather remarkably, high yields of branched PMMAs can be obtained without crosslinking using MMA/EGDMA molar feed ratios of up to 5/1 by appropriate adjustment of the molar feed of initiator. In particular by maintaining the EGDMA/initiator molar feed ratio ∼1/1 fully soluble products can be obtained that are densely branched since this feed ratio ensures that on average each living primary chain initiated contains on average only one branching EGDMA segment. As might be expected this controlled free radical process offers better control in the synthesis of branched polymer than the corresponding system we have reported using conventional free radical polymerization, and unlike the latter which requires the use of a chain transfer agent, the ATRP system requires no additional chain regulating component. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2375–2386, 2007     

分子筛催化剂-亚硫酸盐体系降解水中对乙酰氨基苯酚

过渡金属诱导亚硫酸盐体系产生硫氧自由基是一种新型的高级氧化技术,比常规的高级氧化过程(基于羟基自由基等)有更优越的性能.我们对过渡金属离子活化过硫酸盐和亚硫酸盐的硫氧自由基生成链式反应过程、自由基用于氧化有机污染物的国内外研究现状进行了大致总结,并以SBA-15介孔分子筛为载体,采用浸渍法制备Co-SBA-15固相催化剂,将此固相催化剂用于活化亚硫酸钠,产生强氧化性自由基,以此来降解制药废水中典型污染物对乙酰氨基酚(APAP).通过正交试验,探讨了pH、温度、钴离子与亚硫酸根的比值(简称钴硫比)、气体含氧量等参数对污染物降解效果的影响.对催化剂进行表征,对体系中产生的自由基类型进行检测,结合实验结果和文献资料对降解产物进行了分析,探究了APAP的降解原理.     

Solvation free energies of amino acid side chain analogs for common molecular mechanics water models

Quantitative free energy computation involves both using a model that is sufficiently faithful to the experimental system under study (accuracy) and establishing statistically meaningful measures of the uncertainties resulting from finite sampling (precision). In order to examine the accuracy of a range of common water models used for protein simulation for their solute/solvent properties, we calculate the free energy of hydration of 15 amino acid side chain analogs derived from the OPLS-AA parameter set with the TIP3P, TIP4P, SPC, SPC/E, TIP3P-MOD, and TIP4P-Ew water models. We achieve a high degree of statistical precision in our simulations, obtaining uncertainties for the free energy of hydration of 0.02-0.06 kcal/mol, equivalent to that obtained in experimental hydration free energy measurements of the same molecules. We find that TIP3P-MOD, a model designed to give improved free energy of hydration for methane, gives uniformly the closest match to experiment; we also find that the ability to accurately model pure water properties does not necessarily predict ability to predict solute/solvent behavior. We also evaluate the free energies of a number of novel modifications of TIP3P designed as a proof of concept that it is possible to obtain much better solute/solvent free energetic behavior without substantially negatively affecting pure water properties. We decrease the average error to zero while reducing the root mean square error below that of any of the published water models, with measured liquid water properties remaining almost constant with respect to our perturbations. This demonstrates there is still both room for improvement within current fixed-charge biomolecular force fields and significant parameter flexibility to make these improvements. Recent research in computational efficiency of free energy methods allows us to perform simulations on a local cluster that previously required large scale distributed computing, performing four times as much computational work in approximately a tenth of the computer time as a similar study a year ago.     

Population-based models of thermoplastic degradation: Using optimization to determine model parameters

A comparison of different model forms for non-oxidative thermoplastic degradation rate is presented. A physically meaningful model is derived from a three-step radical depolymerization mechanism. A moment-based approximation to the population balance equations is derived and implemented to predict the thermal degradation of polyethylene (PE) and poly(methyl methacrylate) (PMMA). In the absence of accurate molecular dynamical calculations to predict the controlling model parameters, it is necessary to use experimental data to find the best fit parameter values. A sequential quadratic programming algorithm (SQP) was used to find the set of parameters that minimized the error between the model and sets of non-isothermal thermogravimetric (TG) data at different heating rates for PE and PMMA.     

不同聚合机理的“平均聚合度”公式的分析讨论

平均聚合度和聚合度分布是决定聚合物产品质量的一个重要指标。平均聚合度直接表征聚合产品的分子量的大小,它的大小又决定高分子材料的性能,因此,研究平均聚合度具有重要的理论意义和实际意义。聚合反应按机理分类,可分为链型聚合和逐步聚合,链型聚合又分为自由基聚合、离子聚合、配位聚合,逐步聚合又分为缩聚和加聚反应。本文以自由基聚合、离子聚合、配位聚合以及缩聚的聚合机理为主线,对不同聚合机理对应的平均聚合度的公式进行推导和讨论,并加深理解,掌握其内涵。旨在提高学生对平均聚合度的认识和把握。     

Enantiospecific Trifluoromethyl‐Radical‐Induced Three‐Component Coupling of Boronic Esters with Furans

In the presence of trifluoromethylsulfonium reagents, boronate complexes derived from 2‐lithio furan and non‐racemic secondary and tertiary alkyl or aryl boronic esters undergo deborylative three‐component coupling to give the corresponding 2,5‐disubstituted furans with excellent levels of enantiospecificity. The process proceeds via the reaction of boronate complexes with a trifluoromethyl radical, which triggers 1,2‐metallate rearrangement upon single‐electron oxidation. Alternative electrophiles can also be used in place of trifluoromethylsulfonium reagents to effect similar three‐component coupling reactions.     

Modeling of elementary reactions and kinetics of radical‐initiated methyl methacrylate polymerization in the presence of ferrocene

On the basis of quantum chemical modeling, a kinetic scheme of methyl methacrylate polymerization initiated by benzoyl peroxide in the presence of ferrocene was proposed. The process runs by mechanism, which includes the reactions of free radical polymerization, and the reactions leading to formation and operability of two type coordination active sites that are capable of converting into each other. On the basis of the proposed scheme, a kinetic model was developed. This model quantitatively described the following: the experimentally determined time dependences of the methyl methacrylate conversion, the conversion dependencies of the number‐average and weight‐average molar masses of poly(methyl methacrylate), the stereoregularity values of poly(methyl methacrylate), and the time dependencies of the methyl methacrylate conversion upon its polymerization on poly(methyl methacrylate) macroinitiators obtained in radical‐initiated polymerization in the presence of ferrocene. As a result of solving the inverse kinetic problem, the parameters of temperature dependences of the reaction rate coefficients of the proposed kinetic scheme were found.     

Free‐radical copolymerization of ethyl α‐hydroxymethylacrylate with methyl methacrylate by reversible addition–fragmentation chain transfer

The controlled free‐radical homopolymerization of ethyl α‐hydroxymethylacrylate and copolymerization with methyl methacrylate were performed in chlorobenzene at 70 °C by the reversible addition–fragmentation chain transfer polymerization technique with 2,2′‐azobisisobutyronitrile as the initiator. 2‐Phenylprop‐2‐yl dithiobenzoate and 2‐cyanoprop‐2‐yl dithiobenzoate were used as chain‐transfer agents in the homopolymerization, whereas only the former was used in the copolymerization. All reactions presented pseudolinear kinetics. The effect of the monomer feed ratio on the copolymerization kinetics was examined. The conversion level decreased when the proportion of ethyl α‐hydroxymethylacrylate in the monomer feed was larger. Kinetic studies indicated that the radical polymerizations proceeded with apparent living character according to experiments, demonstrating an increase in the molar mass with the monomer conversion and a relatively narrow molar mass distribution. All copolymers were statistical in chain structure, as confirmed by determinations of the monomer reactivity ratios. The monomer reactivity ratios were determined, and the Mayo–Lewis terminal model provided excellent predictions for the variations of the intermolecular structure over the entire conversion range. Additionally, the chemical modification of poly(ethyl α‐hydroxymethylacrylate) was carried out to introduce glucose pendant groups into the structure. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5618–5629, 2006     

Polymerization of free secondary amine bearing monomers by RAFT polymerization and other controlled radical techniques

This work describes the polymerization of the free secondary amine bearing monomer 2,2,6,6‐tetramethylpiperidin‐4‐yl methacrylate (TMPMA) by means of different controlled radical polymerization techniques (ATRP, RAFT, NMP). In particular, reversible addition‐fragmentation chain transfer (RAFT) polymerization enabled a good control at high conversions and a polydispersity index below 1.3, thereby enabling the preparation of well‐defined polymers. Remarkably, the polymerization of the secondary amine bearing methacrylate monomer was not hindered by the presence of the free amine that commonly induces degradation of the RAFT reagent. Subsequent oxidation of the polymer yielded the polyradical poly(2,2,6,6‐tetramethylpiperidinyloxy‐4‐yl methacrylate), which represents a valuable material used in catalysis as well as for modern batteries. The obtained polymers having a molar mass (M_n) of 10,000–20,000 g/mol were used to fabricate well‐defined, radical‐bearing polymer films by inkjet‐ printing. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Hydroxyl radical in aqueous solution: Computer simulation

Molecular dynamics simulations of hydroxyl radical in water are carried out by use of a classical simple point charge extended (SPC/E) water model and a similar point charge model for hydroxyl radical. Structural and dynamical properties are studied along the coexistence curve of SPC/E water at 298, 373, 473, 573, and 633 K and above its critical point at 683, 733, 783, and 833 K with density fixed at 0.3 g/cm3. Dramatic changes in the diffusion dynamics of water and hydroxyl radical near the critical point are related to the reorganization of the three-dimensional structure of water around hydroxyl radical, as revealed by the study of the spatial distribution functions. This study helps us understand the kinetics of oxidation reactions in high-temperature water.     

The effect of 1,3-diphenylisobenzofuran on the photo-oxidative degradation of cis-1,4-polybutadiene

1,3-Diphenylisobenzofuran (DPBF) is easily photo-oxidized by two mechanisms viz free radical oxidation and singlet oxygen oxidation. The final products of DPBF oxidation by these two mechanisms are the same. Using light in the range 280–480 nm, DPBF is an effective sensitizer of photooxidative degradation of polybutadiene in the solid and in solution. In a system with methylene blue (MB) in methanol-benzene solution (0.5:9.5) where free radicals from MB and ¹O₂ are formed during irradiation with visible light, DPBF is oxidized by both ¹O₂ and free radical mechanisms. DPBF cannot stop free radical degradation of PB initiated by MB radicals in MB-methanol-benzene solution. These results show that the DPBF is an ineffective stabilizer for polydienes against ¹O₂ and free radical oxidation. It rather acts as a sensitizer for photo-oxidation of polydienes.