Emulsion constant composition copolymers

New Design for Producing Constant-Composition Copolymers in Emulsion Polymerization... 

In this paper we would like to describe a new design, based on gas chromatographic analysis of the monomer mixture, for production of constant composition copolymers and its application to emulsion copolymerization. This design was already shortly described and applied to solution copolymerization (3) of methylmethacrylate and vinylidene chloride. Since then, the apparatus was made more simple, more reliable and more accurate. It is actually monitored by an analogic computering system which keeps the ratio of the monomers constant by controlling the addition of one of them. The process based on it can be called corrected batch process because the initial value of this ratio is kept up to the end. 

Guyot, A., Guillot, J., Pichot, C. and Guerrero, L. R. (1981) New design for producing constant-composition copolymers in emulsion polymerization comparison with other processes. Emulsion Polymers and Emulsion Polymerization (eds D.R. Bassett and A.E. Hamielec), ACS, Washington, pp. 415-436. 

Except in very special cases (azeotropic copolymerizations), copolymerization via radical mechanism shows a drift in the composition of the copolymers produced through the polymerization process. Emulsion copolymerization obeys this rule too, although the special features of its mechanism can change the drift process. The most common way to obviate that composition drift is to use the semi-continuous process where, after polymerization has been initiated with a small percent of the total charge (say 10 to 20 %) like in the batch process, most of the charge is added continuously at a much smaller rate (Ra) than the rate (Rp) at the end of the batch period, so that the added charge is polymerized quite instantaneously (J, 2). Then,the composition drift is limited to the initial period and most of the product does possess actually a constant composition. 

Producing a fine porous powder which is easy to blend with compounding ingredients. A smaller amount is polymerized in emulsion and spray-dried for plastisols and organosols. And an even smaller amount is copolymerized with vinyl acetate in organic solution, to produce a uniform copolymer which precipitates at a constant composition and molecular weight. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

Guyot et a/. have described a so-called corrected-batch process for producing copolymers of constant composition by emulsion polymerization. Unlike the power-feed process, the polymerization is carried out in the presence of excess monomer. The principle is to monitor the composition of the unreacted monomer by gas-liquid chromatography, and then to use this information to correct for difference in the rates of copolymerization of the two monomers by controlling the relative rates of feed of the two monomers to the reactor. The objective is to keep the monomer-feed composition constant, and hence also the composition of the copolymer which is being produced instantaneously. 

The semibatch approach, where policies are developed for selective reagent feeds to the reactor, has been extensively elaborated, especially for emulsion polymerization, and in the context of controlling composition during copolymerization reactions [68-72,109]. Discussions are provided in Chapters 4, 7, 12, 17-19, and 21. Sun et al. developed model-based semibatch monomer feeding policies for controlled radical polymerization (CRP) [73, 74]. Vicente et al. [75,76] controlled composition and molecular weight distribution in emulsion copolymerization in an open-loop method by maintaining the ratio of comonomers. Yanjarappa et al. [77] synthesized, via a sanibatch method, copolymers with constant composition for biofunctionalization. General semibatch policies are reviewed by Asua [78]. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Dependence of the Stability (R) of Water-Toluene Emulsions on the Molecular Composition of PTBS (PO),. Copolymers of Constant Molecular Weight... 

Samer [137] studied miniemulsion copolymerization in a single CSTR. Two separate feed streams, miniemulsion (or macroemulsion for comparative studies) and initiator were fed at constant rates into the reactor. SLS was used as the surfactant, HD as the costabilizer, and KPS was the initiator. In the miniemulsion configuration (costabilizer included in recipe), the emulsion stream was continuous. Constant volume was provided by an overflow outlet. Salt tracer experiments were used to validate the ideal mixing model assumed for a CSTR. Total monomer conversion was measured via in-hne densitometry, and copolymer composition via offline NMR. 

It goes without saying that copolymers can also be produced by emulsion polymerization. The composition of a copolymer depends on the ratios of the rate constants (see Section 22.2). If the two monomers have a different solubility in water, therefore, the composition of the products will be altered (in relation to bulk polymerization) even though the monomer composition of the initial mixture is the same in both cases. The different monomer emulsifiabilities thus have a considerable influence on the copolymer... 

A numerical technique that has become very popular in the control field for optimization of dynamic problems is the IDP (iterative dynamic programming) technique. For application of the IDP procedure, the dynamic trajectory is divided first into NS piecewise constant discrete trajectories. Then, the Bellman s theory of dynamic programming [175] is used to divide the optimization problem into NS smaller optimization problems, which are solved iteratively backwards from the desired target values to the initial conditions. Both SQP and RSA can be used for optimization of the NS smaller optimization problems. IDP has been used for computation of optimum solutions in different problems for different purposes. For example, it was used to minimize energy consumption and byproduct formation in poly(ethylene terephthalate) processes [ 176]. It was also used to develop optimum feed rate policies for the simultaneous control of copolymer composition and MWDs in emulsion reactions [36, 37]. 

Assume that an optimum feed flow rate trajectory is searched for the emulsion copolymerization problem described in Section 8.3.3. In this case, also assume that the copolymer composition should be constant throughout the batch. Also assume that the final monomer conversion should be as close as possible to 1, to allow for rninirnization of the residual monomer. In order to achieve the control objectives, one is allowed to manipulate the feed flow rate of monomer 1, which is assumed to be the most reactive monomer, and the initial monomer concentrations. In this particular case, one may write the following objective function ... 

Amodel-based closed-loop controller was designed for maximization of polymer production under safe process condition in emulsion copolymerization processes, while keeping the copolymer composition constant [195]. The interesting feature of the proposed controller was the use of a fuzzy model for design of the optimum reference trajectories. 

As a consequence of the diversified nature of the comonomers, a large number of variants of copolymer composition can be realized, thus achieving a broad variation of properties. The copolymerization can be carried out in the liquid monomer, in a solvent, or in aqueous emulsion. When high molecular mass is desired, solvents with low chain transfer constants (e.g., tert-butanol, benzene, 1,4-dioxane) are preferred. Solution... 

Nomura et al. [74,75] proposed an experimental method to study the competitive particle nucleation mechanisms (micellar nucleation versus homogeneous nucleation) in a given emulsion polymerization system. This approach involves the emulsion copolymerization of relatively hydrophobic styrene with relatively hydrophilic monomers such as methyl methacrylate or methyl acrylate. The composition of copolymer produced during the very early stage of polymerization (far lower than 1% monomer conversion), which reflects the characteristic of copolymer at the locus of particle nucleation, is then determined. Emulsion copolymerization of styrene with methyl methacrylate (or methyl acrylate) was carried out, where sodium dodecyl sulfate was used to stabilize the emulsion polymerization system and where the weight ratio of styrene to methyl methacrylate (or methyl acrylate) was kept constant at 1 1. The experimental results show that the compositions of copolymers obtained from emulsion polymerizations in the presence and absence of monomer-swollen micelles are quite different. This provides supporting evidence of the generally accepted Smith-Ewart theory that micellar nucleation controls the particle nucleation process in the emulsion copolymerization of styrene with... 

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