Composition Drift Research Articles

Emulsion co- and terpolymerization of styrene, methyl methacrylate, and methyl acrylate. I. Experimental determination and model prediction of composition drift and microstructure in batch reactions

In Part I of this series the reactivity ratios of the comonomer pair methyl acrylate-methyl methacrylate were determined with low-conversion bulk polymerizations. It was shown that the binary reactivity ratios of the systems styrene-methyl acrylate, styrene-methyl methacrylate, and methyl acrylate-methyl methacrylate describe composition drift in low-coversion bulk terpolymerizations with these monomers reasonably well. A computer model was developed to simulate the composition drift in emulsion co- and terpolymerizations. The composition drift in two batch emulsion copolymerization systems (styrene-methyl acrylate and methyl acrylate-methyl methacrylate) and one emulsion terpolymerization system (styrene-methyl acrylate-methyl methacrylate) was investigated both experimentally and with the model. Experimental results were compared with model calculations. The copolymer chemical composition distributions (CCD) were determined with gradient polymer elution chromatography (GPEC®). This technique was also used for the first time to obtain information about the extent of composition drift in emulsion terpolymerizations. Cumulative terpolymer compositions were determined with 3H-NMR as a function of conversion and with this information the three-dimensional CCD was obtained. The composition drift was analyzed with respect to free radical copolymerization kinetics (reactivity ratios) and monomer partitioning. It was shown that in most emulsion copolymerizations the composition drift is mainly determined by the reactivity of the monomers and to a lesser extent by monomer partitioning, except in systems where there is a large difference in water solubility. The model predictions for cumulative terpolymer composition as a function of conversion and the three-dimensional terpolymer CCD showed excellent agreement with the experiments. The GPEC® elution chromatogram of the terpolymer was found to be in accordance with the predicted CCD and the experimentally determined CCD. © 1996 John Wiley & Sons, Inc.

Emulsion co- and terpolymerization of styrene, methyl methacrylate and methyl acrylate. II. Control of emulsion terpolymer microstructure with optimal addition profiles

An optimal addition profile for the preparation of a chemically homogeneous emulsion terpolymer of styrene, methyl methacrylate, and methyl acrylate was determined using a recently developed model for describing composition drift in emulsion co- and terpolymerizations. TRISEPS, described in Part I of this series. The model uses recently published simplified equations to describe monomer partitioning and the terminal model for describing terpolymer composition. The optimal addition rate profile was determined from the calculated optimal addition profile with a purely empirical and iterative method. With gradient polymer elution chromatography (GPEC®) the homogeneity and/or heterogeneity of the terpolymers prepared in the iterative series of experiments could be determined and compared to the heterogeneity of the corresponding batch terpolymer described in Part I. It was shown that a homogeneous terpolymer could be obtained indicating that the simplified equations for monomer partitioning and the terminal model for terpolymer composition describe the system adequately. It was also shown that GPEC® was useful in the determination of the optimal addition rate profile. © 1996 John Wiley & Sons, Inc.

Emulsion co‐ and terpolymerization of styrene, methyl methacrylate, and methyl acrylate. I. Experimental determination and model prediction of composition drift and microstructure in batch reactions

Emulsion co‐ and terpolymerization of styrene, methyl methacrylate, and methyl acrylate. I. Experimental determination and model prediction of composition drift and microstructure in batch reactions

Swelling and Mechanical Properties of Crosslinked Hydrogels Containing N-Vinylpyrrolidone

Copolymers of 2-hydroxyethyl methacrylate/N-vinyl-2-pyrrolidone (HEMA/NVP) and methyl methacrylate (MMA)/NVP were prepared in the presence of varying amounts of ethylene glycol dimethacrylate (EGDMA) and methylene diacrylamide (MDA) as crosslinkers by photopolymerisation. The resultant solid polymers were swollen to equilibrium in water at 293 K to produce hydrogels. These hydrogels were characterised by soluble fraction and equilibrium water content. The gels were also characterised by compression—strain measurements, which enabled the calculation of Young's modulus and effective crosslink density. The differences in these properties of HEMA/NVP and MMA/NVP polymer series and the effects of MDA versus EGDMA as a crosslinker were explained in terms of compositional drift of polymerisation, heterogeneous crosslinking and hydrophilicity/hydrophobicity of the components involved. In comparison with EGDMA, MDA was found to be more effective in reducing the soluble fraction of the polymers studied and to produce less rigid networks when swollen.

The Dual Role of Fibrinogen as Inhibitor and Nucleator of Calcium Phosphate Phases: The Importance of Structure

Constant composition and free drift methods have been used to investigate the abilities of human serum albumin (HSA) and fibrinogen to influence calcium phosphate precipitation. Both molecules inhibit hydroxyapatite (HAP) crystal growth when present in the solution phase. Fibrinogen, when immobilized at hydrophobicized germanium or silica surfaces, is able to nucleate calcium phosphate phases; at clean germanium or silica surfaces, it appears to be inactive. The apparent configuration of fibrinogen molecules at germanium or silica surfaces on which octadecyltrichlorosilane (OTS) was deposited probably exposes more negative sites for participation in nucleation.

Reactivity ratios of ethylene/α-olefin in gas phase reactors

Experimental determinations of reactivity ratios for heterogeneous copolymerizations are highly dependent upon experimental conditions such as different solubilities and composition drift of the comonomers due to their different reactivities. An integrated form of the copolymerization equation is proposed to accommodate these variations. Experimental conditions are discussed to obtain more accurate values of reactivity ratios taking gas phase ethylene/propylene copolymerization as an example.

Allyl reactivity in group transfer copolymerization

AbstractStatistical copolymers of allyl methacrylates with methyl and ethyl methacrylates are synthesized. Moderate conversions are used. Reactivity ratios are calculated by linearised methods which take composition drift into account as well as by non‐linear least‐squares approximation. For the allyl methacrylate/methyl methacrylate system, rallyl values obtained by various methods are between 1,62 and 1,60 and rmethyl values between 0,36 and 0,34. For the allyl methacrylate/ethyl methacrylate system, rallyl values are between 1,37 and 1,16 and rethyl values between 0,40 and 0,47.

On‐line gas chromatography and densimetry to obtain partial conversions of both monomers in emulsion copolymerization

AbstractMonitoring and controlling composition drift is an important issue in emulsion copolymerization. Due to the heterogeneity of the polymerization system often combined with nonideal kinetics, predicting copolymer composition and changes in feed composition as a function of time is not straightforward. Therefore, accurate and fast on‐line determination of partial conversions of the separate monomers is a key to understanding and controlling the copolymer system studied. For this reason on‐line densimetry, resulting in overall weight conversion, is combined with on‐line gas chromatography, resulting in the overall ratios of the residual monomer. Combining these two on‐line data gives the partial conversion of each monomer as a function of time without the need of an internal standard. The determination of partial conversion of monomers in batch emulsion copolymerization from on‐line gas chromatography and on‐line densimetry is illustrated for the monomer system methyl acrylate‐vinyl acetate. © 1995 John Wiley & Sons, Inc.

The effect of composition drift and copolymer microstructure on mechanical bulk properties of styrene‐methyl acrylate emulsion copolymers

AbstractIn order to determine the influence of composition drift and copolymer microstructure on the mechanical bulk properties of styrene ‐methyl acrylate copolymers, several copolymers were produced by emulsion copolymerization. Three different average compositions were used. By performing the copolymerizations under batch and semicontinuous conditions with two different monomer addition strategies (starved conditions and optimal addition) it was possible to control composition drift and to produce copolymers with different microstructures (chemical composition distributions). All these copolymers were processed in a way that ensured that the original particle structure was lost before the polymers were tested. It was found that composition drift had an influence on the mechanical properties (Young's modulus, maximum stress, elongation at break). This influence could be understood very well on the basis of present knowledge about structure‐mechanical properties relationships. In the case of homogeneous copolymers maximum stress and elongation at break are dependent on the molecular weight, and only weakly dependent on the chemical composition, and Young's modulus is independent of chemical composition and molecular weight in the range of compositions investigated, as expected. In the case of heterogeneous copolymers, the influence of copolymer microstructure on Young's modulus, maximum stress and elongation at break is very large. Depending on the extent of control of composition drift during the polymerizations, phase separation was observed in the processed polymers, and the presence of a rubber phase affected the properties profoundly.

Rate-Determining Kinetic Mechanisms in the Seeded Emulsion Copolymerization of Styrene and Methyl Acrylate

A kinetic study of the seeded emulsion copolymerization of styrene (S) and the relatively water-soluble methyl acrylate (MA) was carried out so as to investigate the mechanisms of radical entry into latex particles, radical exit from the latex particles, and the fate of radical species in the aqueous phase. Three seed latices of differing size were used, two of poly(S-co-MA) latices and one poly(S) latex. This way of determining kinetic parameters of emulsion copolymerization on a homopolymer seed (heteroseeded polymerization) is possible because the techniques used involve only a relative small change in conversion. This means that phase separation will not be a problem at the high monomer concentrations used and that composition drift is negligible. Conditions were such that the system followed zero-one kinetics (Smith-Ewart cases 1 and 2), which means that no latex particles contain more than one growing chain. Data were obtained for the steady-state rate of polymerization with persulfate as initiator and for the non-steady-state relaxation kinetics in γ-radiolysis initiation experiments. The latter kinetics are dominated by radical loss mechanisms such as radical exit and termination. It was possible to obtain a value for the chain transfer constant of styrene-terminated radicals to methyl acrylate from the molecular weight distributions. It was found that this copolymer system is retarded at very low conversions, possibly by oxygen. The data can be explained with the following mechanisms. Radical exit occurs via the same transfer - diffusion mechanism as found in a number of other systems, i.e., transfer of the radical activity to monomer(s) and subsequent desorption into the aqueous phase. These desorbed monomeric radicals re-enter the particles and either terminate or propagate therein. The entry efficiency of persulfate is low, with 50-95% of the persulfate-derived radicals undergoing termination in the aqueous phase. This is in accord with the mechanism for entry that states that the rate-determining events are aqueous-phase propagation and termination; radicals of a critical degree of polymerization enter a particle irreversibly and instantaneously. At high fractions of S in the S-MA system, although MA polymerizes quickly (high propagation rate constant), the occasional addition of S in the aqueous phase slows the propagation rate down considerably. Thereby it also decreases the probability of the attainment of a sufficiently high degree of polymerization for surface activity, perhaps 10-20 monomer units with the relatively water-soluble MA.

Compositional heterogeneity effects in hydrophobically associating water-soluble polymers prepared by micellar copolymerization

An aqueous micellar radical copolymerization technique has been used to prepare water-soluble copolymers of acrylic acid/styrene, acrylamide/styrene and acrylic acid/ethylphenylacrylamide. In this process, the hydrophobic monomer (1–2 mol% based on the monomer feed) is dispersed within surfactant micelles while the water-soluble monomer is dissolved in the aqueous medium. This synthesis leads to block copolymers in which the number and length of the hydrophobic blocks vary with the degree of conversion. Kinetic studies have shown that the compositional heterogeneity results from the balance between the values of the reactivity ratios of the monomer pairs and the micellar effect inherent to the technique. The latter always leads to an enhanced apparent reactivity for the hydrophobic monomer. It is shown that an optimization of the process can be achieved which eventually corrects for the drift in the average copolymer composition with conversion. The thickening ability of the hydrophobically modified water-soluble copolymers is directly affected by the extent of their compositional heterogeneity. Analysis of the data provides some criteria for selecting appropriate systems with good controllable rheological properties.

Determination of the reactivity ratios of methyl acrylate with the vinyl acetate, vinyl 2,2‐dimethyl‐propanoate, and vinyl 2‐ethylhexanoate

AbstractThe course of composition drift in copolymerization reactions is determined by reactivity ratios of the contributing monomers. Since polymer properties are directly correlated with the resulting chemical composition distribution, reactivity ratios are of paramount importance. Furthermore, obtaining correct reactivity ratios is a prerequisite for good model predictions. For vinyl acetate (VAc), vinyl 2,2‐dimethyl‐propanoate also known as vinyl pivalate (VPV), and vinyl 2‐ethylhexanoate (V2EH), the reactivity ratios with methyl acrylate (MA) have been determined by means of low conversion bulk polymerization. The mol fraction of MA in the resulting copolymer was determined by 1H‐NMR. Nonlinear optimization on the thus‐obtained monomer feed–copolymer composition data resulted in the following sets of reactivity ratios: rMA = 6.9 ± 1.4 and rVAc = 0.013 ± 0.02; rMA = 5.5 ± 1.2 and rVPV = 0.017 ± 0.035; rMA = 6.9 ± 2.7 and rV2EH = 0.093 ± 0.23. As a result of the similar and overlapping reactivity data of the three methyl acrylate–vinyl ester monomer systems, for practical puposes these data can be described with one set of reactivity data. Nonlinear optimization of all monomer feed–copolymer composition data together resulted in rMA = 6.1 ± 0.6 and rVEst = 0.0087 ± 0.023. © 1994 John Wiley & Sons, Inc.

Model prediction of batch emulsion copolymerization as a function of conversion: A sensitivity analysis

AbstractRecently a model has been developed capable of predicting absolute monomer concentrations and their ratios in the polymer, aqueous, and monomer droplet phases as a function of conversion in batch emulsion copolymerizations without using any adjustable parameters. In this article the sensitivity of model predictions of composition drift toward deviations of 10% in all model parameters (maximum swellabilities of monomer in the polymer phase, water solubilities, reactivity ratios, and monomer and polymer densities) was estimated using the monomer combination methyl methacrylate‐styrene as an example. From the sensitivity analysis it can be concluded that the reactivity ratios are the most important parameters affecting composition drift. The effects of deviations in maximum swellabilities and monomer and polymer densities on composition drift can be neglected, while the water solubility is important only in those cases where the amount of monomer in the aqueous phase cannot be neglected as compared with the total monomer amount. © 1994 John Wiley & Sons, Inc.

Prediction of polymer composition in batch emulsion copolymerization

AbstractMonomer partitioning in emulsion copolymerization plays a key role in determining composition drift and polymerization rates. The combination of recently developed thermodynamically based monomer partitioning relationships with mass balance equations, makes predictions of monomer partitioning in emulsion copolymerizations possible in terms of monomer mole fractions and monomer concentrations in the particle and aqueous phases. Using this approach, the effects of monomer to water ratios and polymer volumes on the monomer mole fraction within the polymer particle phase in a nonpolymerizing system at thermodynamic equilibrium can be determined. Comparison of these monomer partitioning predictions with experiments for the monomer system methyl acrylate—vinyl acetate shows good agreement. Furthermore, composition drift occurring in a polymerizing system as a function of conversion can be predicted if the assumption is made that equilibrium is maintained during reaction. Comparison of predictions with experimental results for emulsion copolymerizations of the monomer systems methyl acrylate—vinyl acetate and methyl acrylate—indene shows good agreement. © 1994 John Wiley & Sons, Inc.

Monomer‐to‐water ratios as a tool in controlling emulsion copolymer composition: The methyl acrylate–indene system

AbstractA drifting copolymer composition as a function of conversion is an aspect typical of copolymerization. Reducing this so‐called composition drift in batch copolymerizations will lead to a decrease in chemical heterogeneity of the copolymers formed. For monomer systems in which the more water‐soluble monomer is also the more reactive one, theory predicts that composition drift in emulsion copolymerization can be reduced or even minimized by optimizing the monomer‐to‐water ratio. The monomer combination methyl acrylate–indene (MA–Ind) meets the requirements needed to minimize composition drift in batch emulsion copolymerization. Therefore, this monomer combination is chosen as a model monomer system in order to verify this theoretical prediction. Reactivity ratios needed for model predictions have been determined by low conversion bulk polymerization, resulting in rMA = 0.92 ± 0.16 and rInd = 0.086 ± 0.025. Furthermore, emulsion copolymerization reactions at the same monomer mole fraction are performed at different monomer to water ratios. From the good agreement between experiments and theoretical predictions for MA–Ind, it was concluded that control and even minimization of composition drift in batch emulsion copolymerization for monomer systems in which the more water‐soluble monomer is also the more reactive one is indeed possible by changing the initial monomer‐to‐water ratio of the reaction mixture provided that the reactivity ratios of both monomers are not too far from unity. © 1994 John Wiley & Sons, Inc.

Compensating the composition drift in reactivity ratio calculations for copolymerizations carried to high conversions

AbstractThe effect of composition drift on the accuracy of linearised methods of determining the reactivity ratios in copolymerization reactions is calculated. It is shown that, when the reactivity ratios differ widely, even at a very low conversion of 5% the drift causes large errors in the ratios found with these methods. However, by using an appropriate average of the initial and the final monomer ratios, the errors due to the composition drift can be significantly reduced, so that linearised methods with averaging can be used with confidence at moderate (30–50%) conversions.

Preparation and characterization of some linear copolymers as precursors to thermoplastic hydrogels

The free radical copolymerization of N-vinyl-2-pyrrolidone (VP) with methyl methacrylate (MMA) was carried out to high conversion using 2,2′-azobisbutyronitrile (AIBN) in hexane at 70°C in the presence of a graft dispersant polymer based on poly(lauryl methacrylate). A stepwise semi-continuous procedure was employed in order to suppress the drift in composition during the conversion. The compositions of the copolymers, determined by i.r. and 13C-NMR, ranged from 23 to 73 wt% VP and the variation of T g with composition accorded well with the Fox equation. These linear copolymers were completely soluble in benzyl alcohol, chloroform, 1,2-dichloroethane, dioxane and DMF, but in other liquids they were insoluble, partially soluble or swollen depending on the composition. Osmometry and light scattering measurements on solutions showed that the copolymers were considerably polydisperse in molar mass but homogeneous in chemical composition. Dry films cast from solution become partially soluble in water when the VP content is >62 wt%. However, at lower contents of VP, the copolymers swell in water to produce white opaque, elastic hydrogels in which the hydrophobic sequences are considered to behave similarly to chemical crosslinks.

Synthesis of high refractive index acrylic copolymers

A series of brominated carbazole-containing monomers with a polymerizable acrylic or methacrylic functionality were synthesized and characterized. These monomers were polymerized thermally and characterized by differential scanning calorimetry (DSC) and refractive index via Brewster's angle. The monomer 4-(1,3,6,8-tetrabromo-9-carbazolyl)-1-butyl acrylate (4b) was copolymerized with methyl methacrylate (M2) yielding the reactivity ratiosr1= 1.08 and r2= 0.93; these values were used in the Skeist procedure to predict compositional drift with conversion. The Alfrey–Price Q and e values for (4b) were 0.86 and 0.46, respectively. The copolymers were all colourless and transparent, despite the absence of an azeotrope for this comonomer pair, and their refractive indices were found to lie between those of the constituent homopolymers. Copolymers rich in methyl methacrylate (MMA) were found to be soluble, yet those rich in 4b were found to be intractable.

Emulsion copolymerization: Kinetics model and reactor performance

AbstractThe Dow process for producing perfluorinated ionomeric membranes includes several emulsion copolymerizations involving gaseous tetrafluoroethylene and a second liquid phase monomer. The choice of the organic phase monomer depends on the desired product. The emulsion copolymerization reactor model was developed by extending the Smith‐Ewart‐Gardon theory for emulsion polymerization processes. Population balance techniques and Flory‐Huggins solution theory were applied. The resulting coupled partial differential equations were solved using the method of characteristics. The reactor model, with minimal adjustable parameters, predicts most polymerization results, including molecular weight, reaction rates in the three process stages, latex particle size, polymer composition, and the composition drift as a function of reaction time. The analysis and reactor model is used in the manufacturing process to set process conditions to obtain the desired properties in the polymer product.

Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching

AbstractThe full moment equations and equations using pseudo‐kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number‐ and weight‐average molecular weights (M̄n and M̄w) were compared with those found using the pseudo‐kinetic rate constant method (PKRCM). The results show that the weight‐average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ1˙ and φ2˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo‐kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo‐kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.