Predicting physical-chemical properties of compounds from molecular structures by recursive neural networks

In this paper, we report on the potential of a recently developed neural network for structures applied to the prediction of physical chemical properties of compounds. The proposed recursive neural network (RecNN) model is able to directly take as input a structured representation of the molecule and to model a direct and adaptive relationship between the molecular structure and target property. Therefore, it combines in a learning system the flexibility and general advantages of a neural network model with the representational power of a structured domain. As a result, a completely new approach to quantitative structure-activity relationship/quantitative structure-property relationship (QSPR/QSAR) analysis is obtained. An original representation of the molecular structures has been developed accounting for both the occurrence of specific atoms/groups and the topological relationships among them. Gibbs free energy of solvation in water, Delta(solv)G degrees , has been chosen as a benchmark for the model. The different approaches proposed in the literature for the prediction of this property have been reconsidered from a general perspective. The advantages of RecNN as a suitable tool for the automatization of fundamental parts of the QSPR/QSAR analysis have been highlighted. The RecNN model has been applied to the analysis of the Delta(solv)G degrees in water of 138 monofunctional acyclic organic compounds and tested on an external data set of 33 compounds. As a result of the statistical analysis, we obtained, for the predictive accuracy estimated on the test set, correlation coefficient R = 0.9985, standard deviation S = 0.68 kJ mol(-1), and mean absolute error MAE = 0.46 kJ mol(-1). The inherent ability of RecNN to abstract chemical knowledge through the adaptive learning process has been investigated by principal components analysis of the internal representations computed by the network. It has been found that the model recognizes the chemical compounds on the basis of a nontrivial combination of their chemical structure and target property.     

Prediction of Polymer Properties from their Structure by Recursive Neural Networks

Summary: We propose a new approach for predicting polymer properties from structured molecular representations based on recursive neural networks. To this aim, a structured representation is designed for the modeling of polymer structures. This representation can also account for average macromolecule characteristics. Preliminarily, this model is applied to the calculation of the T_g of (meth)acrylic polymers with different stereoregularity.

Representation of poly(methyl methacrylate) as a chemical tree and unfolding of the encoding process through its structure.     

NMR studies on the stereospecific polymerization of methyl vinyl ether. Part II. Polymerization by sulfuric acid–aluminum sulfate complex: Enantiomorphic catalyst sites model

Highly crystalline poly(methyl vinyl ether) (PMVE) was produced in toluene in a temperature range of 0 to ?20°C. with the use of sulfuric acid–aluminum sulfate complex (SA catalyst). It was found from the NMR spectra that these polymers contained more than 50% of the triad isotactic fraction and the melting point of the unfractionated polymer was about 130°C. However, PMVE containing a large amount of the isotactic fraction was insoluble in nitromethane, so the triad tacticity of highly crystalline PMVE could not be quantitatively determined. The molecular weight of PMVE increased with increasing conversion and increasing polymerization temperature. This behavior is different from that in metal halide catalysts. Also, the stereoregularity of PMVE decreased with increasing monomer concentration. However, addition of a polar solvent and increasing the polymerization temperature had little effect on the stereoregularity of the polymer. The increase in the isotactic fraction at high catalyst concentration and the difference in the monomer composition in the copolymerization of methyl vinyl ether with 2-chloroethyl vinyl ether by SA catalyst from that obtained by BF₃·O(C₂H₅)₂ suggest that the absorption of MVE on a catalyst surface is an important step in the propagation step by SA catalyst. The fraction of the triad tacticity calculated from the enantiomorphic catalyst sites model⁸ coincided with the experimental results. This fact shows that the steric structure of the adding monomer is determined only by the nature of the catalyst irrespective of the nature of a growing chain end. It is concluded, on considering also the results of the previous paper, that completely different factors can control the steric structure of a polymer even for the same monomer when different catalysts are used.     

Characterization and study of microwave activation effects on the polystyrene tacticity

¹³C Nuclear magnetic resonance proved to be an advantageous tool to determine the stereoregularity of polystyrene polymers. The latter was achieved through the analysis of the signal of the quaternary carbon and that of the carbon-p in the aromatic ring too. Styrene was polymerized through microwaves and conventional heating activation using two different polymerization techniques: emulsion and bulk. Microwave activation was performed in a mono-modal type device under the following experimental conditions: various initiator concentrations, an average irradiation power of 50?W, temperature of 70°C, and using a batch reactor for emulsion and bulk experiments. The results obtained in these experiments were compared with those obtained by conventional heating activation polymerization under the same initiator concentration and temperature conditions. Microwave-activated reactions resulted in shorter reaction times and higher yields. The tacticity of the polymer samples was not significantly altered, which lead to the conclusion that, in this case, the stereoregularity of polystyrene was not influenced by microwave irradiation.     

Etude de la chloration du polychlorure de vinyle en fonction de sa tacticite

Polyvinyl chloride samples of various tacticity have been prepared and photochlorinated in CCl₄ for 4 hr. The chlorination degree correlates directly with the degree of syndiotacticity. Initial PVC with syndiotacticity about 56 per cent, which can be regarded as alternate sequences of syndiotactic and isotactic diads, is quickly chlorinated. The mechanism of chlorine attack on the macromolecula is directly connected with the stereoregularity.     

立构规整性对PAN大分子天生缠结及溶解性的影响

对各种立构规整性的聚丙烯腈(PAN)样品的大分子天生缠结和溶解性能进行研究发现,当立构规整性相同时,PAN样品随着分子量的增加,其天生缠结指数(EI)增加,溶解性降低;对于分子量和化学构造相近的样品,PAN样品随着分子链等规立构(mm)的含量增加,其EI降低,溶解性也降低。天生缠结指数与溶解性并不总是表现出一致性。     

基于改进排序遗传算法的径向基函数神经网络色谱峰解析

构造了以塔板模型为基函数的径向函数神经网络（P－RBFNN），为了使P－RBFNN具有结构重组能力，又在网络学习算法中引入鲁棒（Rubust)和随机全局最优的两阶段排序遗传算法：结构学习和进化。P－RBFNN结合改进的排序遗传算法很适合组分数未知的色谱（含重叠）峰解析。     

Evaluation of molecular mass and tacticity of polyvinyl alcohol by non-equilibrium capillary electrophoresis of equilibrium mixtures of a polymer and a dye

Non-equilibrium capillary electrophoresis of equilibrium mixtures (NECEEM) has been used to characterize polyvinyl alcohol (PVA). Commercial PVA samples with different molecular masses, from M(w)=15 up to 205 kDa, were used. According to the (13)C NMR spectra, the samples also differed in tacticity (stereoregularity). Mixtures of PVA and the anionic azo-dye Congo Red (CR) were injected in the presence of a borate buffer. The electropherograms gave a band and a peak due to the residual PVA-CR complex and the excess dye, respectively, plus a superimposed exponential decay due to the partial dissociation of the complex during migration. The stoichiometry of the PVA-CR complex, q=[monomer]/[dye], reached a maximum, q(sat), which depended on both M(w) and tacticity of PVA. Thus, q(sat) decreased from a molar ratio of ca. 4.9 to 3.6 at increasing M(w) values, this variation also being largely dependent on tacticity. A similar dependence of the electrophoretic mobility of the complex on both M(w) and tacticity was also observed. A possible explanation, based on the formation of a stack of CR ions inside the PVA-CR complex, was proposed and discussed. Finally, at increasing M(w) values, the stability constant of the complex increased slightly, and the pseudo-first order dissociation rate of the complex decreased, this later parameter also showing a dependence on both M(w) and tacticity.     

13C NMR sequence determination and modelling of polypropylene oils

Propylene can be polymerized using a variety of catalysts under different reaction conditions. In the general case, the polymer may exhibit both stereochemistry (tacticity) and regiochemistry (normal and inverted propylene enchainment). When the molecular weight is low, chain ends also become important. The NMR spectra of the resulting polymers can be very complex. In this work, the spectra are studied with the help of analytical (model fitting) and synthetic (computer simulation) approaches in conjunction with a tetrapolymerization model. The approaches described herein are applied to regioirregular polypropylene, particularly low-molecular-weight irregular polypropylene samples (polypropylene oils) made with homogeneous catalysts. Complete spectral assignments and analyses are provided for these oils. Studies of polymer main chain structure provide information on polymer chain propagation; analyses of chain ends provide information on initiation, chain transfer, and termination.     

Structure–property relationships of poly(vinyl alcohol). I. Influence of polymerization solvents and temperature on the structure and properties of poly(vinyl alcohol) derived from poly(vinyl acetate)

The solubility properties of poly(vinyl alcohol) (PVA) vary with the method of preparation of the poly(vinyl acetate) (PVAc) from which it is derived. PVAc was prepared with free-radical catalysts over a range of temperatures from ?78 to 90°C. with solvents of varying chain-transfer ability. The corresponding PVA samples varied in their resistance to dissolution in water. Their high-resolution proton nuclear magnetic resonance spectra showed on differences in tacticity. Data on 1,2-diol content showed only minor differences. Hence, the increase in resistance of PVA to dissolution in water arising from changes in chain-transfer activity of the solvent used in vinyl acetate polymerization is largely attributable to a decrease in molecular weight, and the increase in resistance of PVA to dissolution in water arising from a decrease in the temperature of the vinyl acetate polymerization is largely attributable to a decrease in both long and short branches. Evidently, with polar polymers having small side groups, tacticity is not the only factor influencing property variation; that is, variations in stereoregularity influence more the crystallinity of the sample as measured by density or x-ray methods than the ultimate crystallizability under conditions of mechanical and thermal treatment. In this regard polar polymers having small side groups differ from nonpolar polymers.     

采用阴离子、基团转移及自由基聚合方法合成旋光性聚甲基丙烯酸薄荷酯

近年来, 旋光性高分子的广泛应用及其特有的功能已引起了广泛关注, 尤其在手性记忆功能材料[1～3]、液晶及手性催化等方面[4～6]皆表现出良好的应用前景. 用旋光性单体合成旋光性聚合物是最常用的方法之一. 早在20世纪70年代, 就有关于聚甲基丙烯酸薄荷酯(PMnMA)的研究[7], 但有关配体参与的阴离子聚合, 基团转移聚合(GTP)及其立构规整性的研究还未见报道.     

Polymerization of β-cyanopropionaldehyde. V. Anionic polymerization initiated by benzophenone–alkali metal complexes

Anionic polymerization of β-cyanopropionaldehyde was studied with use of benzophenone-monosodium, -disodium, and -dilithium complexes as initiators. The resulting poly(cyanoethyl)oxymethylene was compared with that obtained by cationic and coordinated initiators previously reported. Polymer of higher stereoregularity but lower molecular weight was formed in the present system. The marked influence of the initiator concentration on the polymer yield and stereoregularity is explained on the basis of the difference in the degree of association of the alcoholate ion pair, i.e., the associated ion pair may form stereoregular polymer and the nonassociated or less-associated ion pair may form a large amount of amorphous atactic polymer. The initiation with benzophenone-dialkali metal complex was found to be bond-formation type. Chain transfer with active hydrogen of β-cyanopropionaldehyde frequently occurs.     

Structure–Property relationships of poly(vinyl alcohol). III. Relationships between stereo-regularity,crystallinity, and water resistance in poly(vinyl alcohol)

Structure–property relationships of poly(vinyl alcohol) have been studied by measuring the crystallinity and water resistance of films derived from samples of varying, known tacticity. Crystallinities of unannealed and annealed films were examined by means of density, infrared, and x-ray measurements. Higher tacticity did not lead to higher crystallinity. The apparent order of crystallizability was atactic ≧ syndiotactic-rich ? isotactic-rich. Water resistance of these films was determined by measuring the swelling index at 30°C. and solubility at 70 and 130–140°C. Water resistance increases as tacticity increases, with syndiotactic-rich PVA exhibiting the highest water resistance. Since water resistance also increases with crystallinity, both stereoregularity and crystallinity must be considered when evaluating structure–solubility relationships. Differential thermal and thermogravimetric analyses of these samples are also presented, together with a correlation of tacticity index as measured by an infrared technique with that of an NMR technique.     

Characterization of Interdiffusion between PVDF and Stereoregular PMMA by Using ATR-FTIR Spectroscopy

In this study we investigated the interdiffusion between PVDF and PMMA below the melting temperature of PVDF by ATR-FTIR spectroscopy. The influence of the stereoregularity of different PMMA samples was studied. The PMMA tacticity showed a significant influence on the kinetics of the diffusion. Syndiotactic PMMA diffuses faster than isotactic and atactic PMMA which can be explained from the difference of chains stiffness between the two stereoisomers.     

Triad tacticity of polyacrylonitrile

The high-resolution NMR spectra of polyacrylonitrile-β,β-d₂ prepared by radical polymerization were determined, and the stereoregularity of the polymer was studied. The NMR spectra of methine protons of polyacrylonitrile-β,β-d₂ in dimethyl sulfoxide-d₆ and a mixture of nitromethane-d₃ and ethylene carbonate showed three partially resolved multiplets. The deuterium-decoupled spectra of the polymer were measured, and three well resolved peaks were observed in the two solvents and dimethylformamide-d₇. These three peaks were analyzed by comparison with the NMR spectra of model compounds and polyacrylonitrile-α-d, and they were assigned to isotactic, heterotactic, and syndiotactic triads with decreasing magnetic field. This order seems to be unchanged in other solvents. Triad stereoregularity of the polymer was determined according to the assignment. Polymerizations of acrylonitrile-β,β-d₂ by radical initiators between ?78°C and 60°C were explained by the Bernoulli trial propagation step. The polymers had an atactic structure, independent of polymerization temperature. This shows that in free-radical polymerization of acrylonitrile, the chain end is not represented as having any particular stereochemistry. Other stereochemical control is necessary to produce tactic polymers. The triad tacticity of isotactic polyacrylonitrile was also determined.     

Increasing molecular weight parameters of a helical polymer through polymerization in a chiral solvent

In this communication, we will demonstrate that polymerization in a chiral solvent can affect the molecular weight distribution of the product by perturbing the balance of the P and M helical screw senses of the growing chains. Specifically, for the Wurtz-type synthesis of polymethylphenylsilane (PMPS) in either (R) or (S)-limonene, the weight-average molecular weight of the products (average Mw = 80 000) was twice that of PMPS synthesized in (R/S)-limonene (average Mw = 39 200). Peturbation of the helical segmentation along the polymer chains leads to a reduction in the rate of occurrence of a key termination step. This the first time that a chiral solvent has been demonstrated to have such an effect on a polymerization process in affecting molecular weight parameters in contrast to affecting tacticity.     

Stereoregular poly(methacrylic acids)

A convenient method is described for the preparation of isotactic and syndiotactic poly(trimethylsilyl methacrylates) by using the monomer trimethyl silyl methacrylate and butyllithium initiation in toluene and tetrahydrofuran, respectively. The structure of these polymers enables complete hydrolysis to the corresponding poly(methacrylic acids), which were characterized with respect to tacticity and molecular weight. The asymmetric induction in toluene produced 89% isotactic polymer, while that in tetrahydrofuran gave polymer <90% syndiotactic and heterotactic in terms of triads. A method of fractionation of the polyelectrolytes by gel-permeation chromatography on a preparative scale was shown to be applicable.     

Tacticity measurements on polypropylene sulphide by 13C-NMR spectroscopy

Determination of tacticity of polypropylene sulphide is studied by NMR. The measurements made by ¹³C-NMR are compared with those obtained by ¹H 100 MHz NMR on corresponding selectively deuterated polymers. The good correlation between the two sets of results shows that stereoregularity of polypropylene sulphide can be easily found with 5 per cent precision.     

Environmental effects in free-radical polymerizations. Part III. Vinyl chloride and n-butyraldehyde

The properties of poly(vinyl chloride) samples prepared by a free-radical process in the presence of n-butyraldehyde have been studied from the point of view of polymer tacticity, branching, molecular weight, and relative crystallinity. The postulate of a polymer radical–aldehyde complex, invoked to explain the increased crystallinity, was tested. The polymers had a lower degree of polymerization and branching than normal, and these parameters rather than increased syndiotacticity were responsible for the high degree of crystallinity. Both molecular weight and branching affect the crystallinity, since polymer samples prepared in the presence of various transfer agents with similar molecular weights were less crystalline than those prepared in aldehyde, but yet more crystalline than high molecular weight bulk polymer. Polymers prepared in aldehyde had a lower degree of branching than those formed in other transfer agents. It was concluded that aldehyde was effective in increasing the crystallinity of poly(vinyl chloride) in these two ways, and so appeared to be unique among the transfer agents. There was no evidence for assuming any complexing between polymer radicals and aldehyde.