Copolymerization controlling emulsion

In copolymerization, control of the copolym composiricm can be obtained when applying monomer addition profiles. These monomer addition profiles either can be based on the direct translation of on-line measurements to monomer addition steps (controlled ccxnposiritxi reactor) or the profiles can be predicted by emulsion copolymoization models on a conversion basis. The required conversion-time relation is then obtained by on-line measurements. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Continuous emulsion copolymerization processes for vinyl acetate and vinyl acetate—ethylene copolymer have been reported (59—64). CycHc variations in the number of particles, conversion, and particle-size distribution have been studied. Control of these variations based on on-line measurements and the use of preformed latex seed particles has been discussed (61,62). 

SBR is produced by addition copolymerization of styrene and butadiene monomers in either emulsion or solution process. The styrene/butadiene ratio controls the glass transition temperature (To) of the copolymer and thus its stiffness. T ... 

The copolymerization occurs in an aqueous emulsion. When free radicals are used, a random copolymer is obtained. Alternating copolymers are produced when a Zieglar-Natta catalyst is employed. Molecular weight can be controlled by adding modifiers and inhibitors. When the polymerization reaches approximately 65%, the reaction mixture is vacuum distilled in presence of steam to recover the monomer. 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

The reactions of tert-alkyl hydroperoxides with ferrous ion generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, c.g., butadiene-styrene. Alkyl hydroperoxides are among tile most drermally stable organic peroxides. However, hydroperoxides are sensitive to chain decomposition reactions initiated by radicals and/or transition-metal ions. Such decompositions, if not controlled, can be autoaccelerating and sometimes can lead to violent decompositions when neat hydroperoxides or concentrated solutions of hydroperoxides are involved,... 

Several methodologies for preparation of monodisperse polymer particles are known [1]. Among them, dispersion polymerization in polar media has often been used because of the versatility and simplicity of the process. So far, the dispersion polymerizations and copolymerizations of hydrophobic classical monomers such as styrene (St), methyl methacrylate (MMA), etc., have been extensively investigated, in which the kinetic, molecular weight and colloidal parameters could be controlled by reaction conditions [6]. The preparation of monodisperse polymer particles in the range 1-20 pm is particularly challenging because it is just between the limits of particle size of conventional emulsion polymerization (100-700 nm) and suspension polymerization (20-1000 pm). 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

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. 

We will describe its use for controlling the styrene-acrylonitrile emulsion copolymerization system. Results concerning copolymer compositions, molecular characteristics and particle sizes will be compared to the corresponding ones from batch or semi-continuous processes. 

The batch-suspension process does not compensate for composition drift, whereas constant-composition processes have been designed for emulsion or suspension reactions. It is more difficult to design controlled-composition processes by suspension methods. In one approach (155), the less reactive component is removed continuously from the reaction to keep the unreacted monomer composition constant. This method has been used effectively in VDC—VC copolymerization, where the slower reacting component is a volatile and can be released during the reaction to maintain constant pressure. In many other cases, no practical way is known for removing the slower reacting component. 

Usually or most widely applied, polymer latexes are made by emulsion polymerization [ 1 ]. Without any doubt, emulsion polymerization has created a wide field of applications, but in the present context one has to be aware that an inconceivable restricted set of polymer reactions can be performed in this way. Emulsion polymerization is good for the radical homopolymerization of a set of barely water-soluble monomers. Already heavily restricted in radical copolymerization, other polymer reactions cannot be performed. The reason for this is the polymerization mechanism where the polymer particles are the product of kinetically controlled growth and are built from the center to the surface, where all the monomer has to be transported by diffusion through the water phase. Because of the dictates of kinetics, even for radical copolymerization, serious disadvantages such as lack of homogeneity and restrictions in the accessible composition range have to be accepted. 

Hernandez-Barajas J, Hunkeler DJ (1997) Inverse-emulsion copolymerization of acrylamide and quaternary ammonium cationic monomers with block copolymeric surfactants copolymer composition control using batch and semi-batch techniques. Polymer 38(2) 449—458... 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

Mention has been made of the fact that the polar character of polymer surfaces is strongly affected by the ionic polymer end groups that are residues of initiator-derived ion radicals, when persulfates are used in emulsion polymerizations. Variation of the initiator type between those that yield ionic and nonionic end groups is an effective way to control particle stability and avoid complications due to migration of surfactant from one polymer surface to another [25]. This method can also be supplemented by copolymerization with polar monomers to affect surface hydrophilicity. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

In emulsion polymerizations semibatch operation provides better control of the particle size of the product. The properties of the product polymers can be modified, also, by continuous or intermittent changes in the composition of the monomer feed in emulsion copolymerizations, where a given monomer can be preferentially concentrated in the interior or on the surface of the final particles, as described in Chapter 8. 

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