Styrene-acrylonitrile Copolymerization Research Articles

Micro- and Macromixing in Polymerization Reactors

Abstract The paper deals with the influence of the local mixing state on the product properties in a mass styrene-acrylonitrile copolymerization stirred tank reactor in steady state. Experimental investigations at different stirrer speeds have been carried out in a specially constructed bench scale reactor. The idealized models for microfluid and complete segregation were used to calculate the polymer mass fraction, the weight average polymerization degree and the molecular nonuniformity at the outlet of the reactor. The apparatus operated as an “ideally mixed” continuous stirred tank reactor under all chosed conditions as demonstrated by measurements of the residence time distributions. The results are: the segregation model describes the experimentals at low stirrer speeds and the microfluid model is appropriate at high agitator revolutions. Polymer mass fraction, weight average polymerization degree and molecular nonuniformity as functions of average residence time calculated on the basis of both the idealized models indicate intersection points. A high extent of microfluid must not tend to decrease the molecular nonuniformity. The agitator speed can be limited in such a way that it is just sufficiently high to approximate the residence time distribution of an “ideally mixed” continuous stirred tank reactor in order to avoid dead spots and channelling. Segregation is not dangerous for the reactor performance and the product properties.

Influence of thermal runaway in styrene–acrylonitrile bulk copolymerization revealed by computational fluid dynamics modeling

AbstractThe computational fluid dynamics (CFD) and kinetic‐based moment methods coupled approach is adopted to simulate the bulk copolymerization of styrene–acrylonitrile (SAN) in a stirred tank reactor. Numerical simulations are carried out to investigate the impacts of impeller speed, monomer ratio, initiator ratio, and initial reaction temperature on the copolymerization process and product properties. Particularly, the Chaos theory is selected as a criterion for evaluating the occurrence of the thermal runaway. The Flory's and Stockmayer's distributions are employed to calculate chain length distribution and copolymer composition distribution of copolymer. The simulation results highlight that the appearance of thermal runaway can be postponed by properly increasing the rotation speed, decreasing the initiator loadings, initial acrylonitrile contents and initial reactor temperature. Furthermore, significant differences exist in the product properties that predicted by the ideal and non‐ideal models, which demonstrates that the temperature heterogeneity plays a crucial role in SAN copolymerization. This study could offer references for the safe operation and design of polymerization processes.

Transition structures and reaction barriers in the styrene–acrylonitrile copolymerization system according to quantum mechanical calculations

AbstractThe geometries and electronic properties of substrates, transition structures (TS), and product radicals in modeled elementary propagation reactions were studied for the styrene–acrylonitrile monomer system by use of quantum‐mechanical calculations: (DFT/B3‐LYP/6–31G(d), ROMP2/6–311+G(3df,2p)//DFT/B3‐LYP/6–31G(d), and DFT/B3‐LYP/6–311+G(3df,2p)//DFT/B3‐LYP/6–31G(d)) and for some parameters, the high‐level composite method G3 (Gaussian‐3, G3/MP2). Activation enthalpies (ΔHact) and reaction enthalpies (ΔHr) for modeled propagation reactions at 298.15 K were evaluated. The enthalpy of activation energy (ΔHact, kJ/mol) for the investigated elementary reactions rises for the B3‐LYP calculation in the following order: (CH3A•+S) < (CH3A•+A) < (CH3S•+A) < (CH3S•+S). For three propagation reactions, (CH3A•+A), (CH3A•+S), and (CH3S•+A), correlation between reaction enthalpy and enthalpy of activation suggests weak or negligible polar effects reflecting the Evans–Polanyi relation. However, from the electron affinities and ionization energies values data, it is not excluded that at least for [CH3A•+S[b]] and [CH3S•+A[b]] reactions, nucleophilic and electrophilic polar effects, respectively, can also be expected. The dependencies between TS geometries, electronic parameters, and enthalpic effects suggest the presence of a steric factor in all TS, including its exceptionally high contribution to the activation enthalpy for the CH3S•+S addition. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1827–1844, 2005

Evaluation of styrene–acrylonitrile copolymerization thermal stability and runaway behavior

Evaluation of thermal stability and runaway behavior of any exothermic chemical system is of great importance for the design and operation of a chemical process. The evaluation process should be based on a thorough investigation of the reaction chemistry including reaction pathways, thermodynamic, and kinetic parameters. When addressing the reactivity hazards of any reacting system, the dominant pathway(s) should be identified. Identifying the main reaction pathway under specific conditions will lead to a better thermodynamic and kinetic characterization of the reacting system. In this article, the thermal stability and runaway behavior of styrene–acrylonitrile copolymerization reaction system in bulk is evaluated. Traditional thermal analysis techniques (calorimetric analysis) are combined with computational quantum chemistry methods and empirical thermodynamic–energy correlations. Reaction pathways are identified from the theoretical approach and verified by experimental measurements. The results of this analysis are compared to literature data for this system.

Penultimate unit effects in the free‐radical copolymerization of styrene with acrylonitrile according to theoretical thermochemistry

AbstractPenultimate unit effects in the free‐radical copolymerization of styrene with acrylonitrile were investigated by the consideration of the theoretical thermochemistry of three subsequent propagation steps in the copolymerization process at 298 K and the electronic properties of the relevant reactants. The total energies, zero‐point energies incorporating a 0.96 scale factor, and thermal enthalpy corrections for all optimized structures were computed with the B3LYP density functional theory and the 6‐311G(d,p) basis set. The penultimate unit effect on the enthalpy of reaction for elementary propagation reactions ranged from −1.2 to 2.7 kcal/mol. The enthalpies of elementary copolymerization propagation reactions showed that penultimate unit effects depended not only on the γ substituent itself but also on the terminal unit of the growing radical and the monomer being attached. The exothermicity of the addition of radicals, varying in the penultimate unit for a given monomer, was lower for more polar radicals and smaller Mulliken charges at the radical atom, except for radicals with a CN substituent placed in the γ position when they reacted with acrylonitrile. For the latter system, the repulsive interactions between the CN substituent and nitrile group of the monomer being added contributed to the reaction enthalpy. Almost no penultimate unit effect was detected upon spin distribution at the radical atom, and this probably indicated the absence of the independent implicit penultimate model. The results obtained strengthen the concept of the inseparability of implicit and explicit penultimate unit effects in radical copolymerization. However, it appears that for the styrene–acrylonitrile copolymerization system, the explicit penultimate model prevails over the implicit penultimate model. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3592–3603, 2002

Kinetics and mechanism of styrene-acrylonitrile copolymerization in micro-emulsion

Stable and transparent poly(styrene-acrylonitrile) latexes were produced by the polymerization of styrene-acrylonitrile in ternary o/w micro-emulsions containing sodium dodecyl sulfate as an anionic surfactant. Kinetics of copolymerization was studied at different temperatures using different concentrations of potassium persulfate and hydrogen peroxide/ascorbic acid. The suitability of existing model for homopolymerization was examined for styrene-acrylonitrile copolymerization after appropriate modification. The latexes were characterized for particle size and number of particles by dynamic light scattering and TEM. The isolated products were characterized by 1H and 13C NMR as well as by thermal analysis. The overall size of particles was found to be between 15 and 20 nm. © 2000 Society of Chemical Industry

Identification of cis- and trans-1-phenyl-2-cyano-cyclobutane in the thermal copolymerization of styrene and acrylonitrile

A complete one- and two-dimensional nuclear magnetic resonance (NMR) study was carried out on the styrene-acrylonitrile (SAN) dimer fraction of a sample of SAN thermal copolymer. A crude sample of SAN oligomers was first fractionated by distillation and then the dimers were isolated by thin-layer chromatography. The structures and stereochemistry of these compounds were then determined by NMR and the presence of disubstituted cyclobutanes was pointed out. This suggests that the Flory mechanism of initiation must be taken into account in spontaneous styrene-acrylonitrile copolymerization.

The effect of solvent on the styrene-acrylonitrile copolymerization

The copolymerization of styrene and acrylonitrile has been examined in acetonitrile and toluene and in bulk. All the systems display a significant penultimate unit effect. The comparison of the bulk system with these involving toluene and acetonitrile reveals a relatively small solvent effect. The bulk and toluene systems behave very similarly in the high styrene region ( X s > 0.3), while the bulk and acetonitrile systems behave very similarly in the high acrylonitrile region ( X s < 0.3). The solvents, therefore, behave as effective analogues of styrene and acrylonitrile, which are the “solvents” in the bulk copolymerization. The relationships between the copolymer compositions and triad fractions for the polymers prepared in each system have been assessed for evidence consistent with partitioning of monomer between polymer and solvent phases. Similar relationships were found for the triads independent of the solvent, a result which may be indicative of a “bootstrap” effect.

Determination of reactivity ratios from analysis of triad fractions—analysis of the copolymerization of styrene and acrylonitrile as evidence for the penultimate model

A numerical analysis of the relationship between the composition of the monomer feed and that for the resultant copolymer (at low conversion) is the method usually adopted for determination of copolymerization reactivity ratios. However, the six triad fractions provide much more information about a copolymer than does the single copolymer composition. In most of the cases where both sets of information can be reliably obtained, the copolymer composition can be determined with slightly greater experimental accuracy than the triad fractions. Despite the somewhat lower accuracy of the individual triad fractions, in some cases the “best” model of the copolymerization and the corresponding reactivity ratios can be determined with higher precision from the triad fractions, because of the extra information provided by these data. A direct numerical analysis of the triad fractions for the styrene-acrylonitrile copolymerization in terms of the penultimate model has yielded the reactivity ratios and their joint confidence regions for the bulk polymerization at 60°. The analysis shows that the data can be satisfactorily described by the penultimate model, whereas statistical tests demonstrate that the terminal model will not adequately represent the polymerization. The reactivity ratios are determined with higher precision from the triad fractions than from the copolymer compositions.

ChemInform Abstract: MODEL COPOLYMERIZATION REACTIONS. DIRECT OBSERVATION OF A “PENULTIMATE EFFECT” IN A MODEL STYRENE‐ACRYLONITRILE COPOLYMERIZATION

AbstractDie aus den entsprechenden Hg‐bromiden durch Reduktion mit NaBH4 in O2‐freiem DMF erhaltenen Radikale (I) ergeben nach ihrer konkurrierenden Addition an die Olefine (II) und (III) Produktverhältnisse von (IVa)/(Va) = 24.5‐26.2, (IVb)/(Vb) = 22.6 und (IVc)/(Vc) = 6.8.

Model copolymerization reactions. Direct observation of a "penultimate effect" in a model styrene-acrylonitrile copolymerization

Forty years after the publication of the classical kinetic scheme for radical copolymerization, there is still debate concerning the factors that control composition and comonomer sequence in some copolymer chains. Interest in this problem arises not only from an intrinsic desire to understand the copolymerization process but also from the realization that alternative kinetic models of copolymerization predict very different copolymer chain structures. For example, Hill, O’Donnell, and O’Sullivan have shown in their analysis of styrene-acrylonitrile copolymerization that the best terminal, penultimate, and complex-participation models predict very similar copolymer compositions but very different comonomer sequences; in a copolymer prepared from an equimolar monomer mixture, the number fraction of styrene sequences which consist of an isolated styrene unit is predicted to be 0.74 for the terminal model, 0.61 for the penultimate model, and 0.18 for the unrestricted complex-participation model. Clearly the synthesis of well-defined copolymer structures requires a precise understanding of copolymerization mechanism.

Some thermodynamic aspects in emulsion copolymerization

Emulsion polymerization proceeds in a heterogeneous medium; in copolymerization, due to discrepancy in water solubility and interactions with the resulting copolymers, monomers are partitioned in a complex way between droplet, water and particle phases. As monomer swollen polymer particles are the main loci of polymerization, it is of great interest to know the monomer feed within the particle phase to understand the behaviour of this copolymerization whatever may be the process and the experimental conditions. Two approaches are examined in this lecture to investigate emulsion copolymer and colloid properties of the latexes. A first, simple one is based on experimental partition coefficients and a second one is based on more theoretical thermodynamic considerations (volume fractions, interaction χ parameters …). The latter gives a more general insight of emulsion processes and allows to investigate influence of many parameters like monomer-monomer, monomer-polymer, monomer-emulsifier interactions, interfacial tension, particle size, surface charge density… Computer programs are derived which give predictions in rather good agreement with experiments. The thermodynamic approach allows to study crosslinking effect as well as competition between polymerization rate and monomer diffusion rate if some control by diffusion does occur. Owing to these computer programs it becomes possible to correlate many experimental parameters. Most of experimental date are relative to styrene-acrylonitrile copolymerization.

Mechanical properties of products of thermocatalytic and radiolytic styrene–acrylonitrile copolymerization

AbstractThe mechanical properties of styrene (S)–acrylonitrile (AN) mixtures, ranging from 20 to 80 wt % S, polymerized by thermocatalytic and radiolytic techniques were studied. Maximum compressive and tensile strength was obtained for the mixture containing 60 wt % styrene. The hardness increased with styrene concentration up to 40 wt % and then remained nearly constant. Radiolytic copolymerization gave stronger copolymers than thermal copolymerization since irradiation enhances crosslinking. For the same composition, as the dose increases, the strength increases to a maximum and then decreases due to competing rates of crosslinking and degradation.

A Kinetic Study of Free-Radical Styrene-Acrylonitrile Copolymerization by Differential Scanning Calorimeter

Abstract The isothermal free-radical copolymerization of styrene and acrylonitrile was examined in the temperature range of 333-373 °K. Initial rate studies of styrene homopolymerization were conducted and agreed favorably with values in the literature. Initial rate studies for acrylonitrile solution polymerization in DMF were also performed. Initiator decomposition rate constants measured in the presence of AN and styrene monomer are reported for AIBN, BP, and DTBP. Rate functions for the full spectrum of comonomer compositions initiated by AIBN and benzoyl peroxide are reported. Several copolymerization kinetic models were tested and found to be inadequate. Conversion histories were found to be consistent with observed initial rates and follow a simple pseudohomopolymerization kinetic model. Autoacceleration was observed and found to increase in severity with increased AN feed compositions and decreased reaction temperatures.

The termination stages in the styrene-acrylonitrile copolymerization

The termination stage of the styrene-acrylonitrile (ST-AN) copolymerization is being studied. The rotating sector method has been used to measure the average lifetime of the radicals for various monomer ratios. These results and the experimentally found initiation rates were used to determine the true rate constants of termination in the system. The Melville constants Φ and termination constants respectively have been calculated from the Walling and North equation. The termination is controlled in our system by diffusion even at low conversions, and the termination rate constant depends on the composition of the monomer mixture.

Acrylonitrile Copolymerization. VIII. Radical Determination by the Spin-Trapping Technique

Abstract The technique of spin trapping of polymer radicals by 2-methyl-2-nitrosopropane has been applied to the study of acrylonitrile copolymerization. Provided the concentration of the spin trapping agent is kept low, and the samples are kept at low temperature during storage before obtaining the ESR spectra, quantitative studies of styrene-acrylonitrile copolymerization are possible. However, the spin-trapping reaction is a competitive one, and the trapping efficiency is dependent on the reactivity of the radicals present. In the case of acrylonitrile copolymerization with either vinyl chloride or vinyl acetate, these two last radicals are trapped preferentially. Furthermore, the cyclization reaction observed with many acrylonitrile copolymerization is nearly eliminated in the presence of the spin-trapping agent.

Styrene—Acrylonitrile Copolymerization: Effect of Remote Units on the Reactivity of the Styrene-Ended Radicals

Abstract Application of the gas-liquid chromatographic method to the analysis of the process of styrene-acrylonitrile radical copolymerization, especially with a monomer mixture rich in styrene, provided evidence for penultimate and antepenultimate effects. Methods are given for the determination of the corresponding reactivity ratios: r A ≈ 0, r AS = 0.55, r ASS = 0.50, r SSS = 0.25