Polymerisation reactions dispersed-phase

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

The purpose of this paper is to summarise some of the advances which we have made in recent years in developing the theory of compartmentalised free-radical polymerisation reactions. These developments either have been, or will be, published and discussed in detail elsewhere. By the term "compartmentalised free-radical polymerisation reaction" we mean a free-radical polymerisation which is taking place within a large number of separate reaction loci. These loci are dispersed in a contiguous external phase. 

Polymerisation. Emulsified droplets containing a monomer can react with a second monomer soluble in the continuous phase to form a membrane at the interface (i.e. diamine reacting with a acid dichloride). This is called interfacial polymerization. Many derivative methods can be set-up from this method, using pre-polymers in place of monomers, inversing the continuous and dispersed phases, developing a radical reaction. Covering all possible methods is not possible here. 

Polymerisation in disperse media has some advantages over other supen-sion or mass processes. In particular, this type of polymerisation yields high molecular weights at rapid reaction rates. This is due to the fact that reaction sites are subdivided and monomer concentrations are high. Latexes obtained in this way contain a high proportion of solids (50% polymer), and yet have low viscosity. Indeed, viscosity is not a function of polymer molecular weight. To a first approximation, it only depends on the volume fraction of the disperse phase and interparticle interactions. Finally, latexes are model systems for fundamental studies on colloidal dispersions. 

Emnlsion polymerisation is influenced by system turbulence due to its heterogeneous nature. System effective agitation is necessary to keep the particles in dispersed phase, to prevent flocculation, to improve mass and heat transfer, phenomena that influence the reaction mechanism and kinetics as well as the final product properties and the properties of the product based on achieved latex. The influence of agitation on properties of the product using the obtained latex is presented, with emphasis on viscosity and rheological behaviour. Also presented is the influence of system initial rheology scaling up. 18 refs. 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

Methodological and experimental approaches, along with original results provided below for the model of the fast chemical reaction of liquid-phase electrophilic (cationic) isobutylene polymerisation are general and applicable to other fast liquid-phase processes. They turned out to be fruitful for the description of various chemical processes as well as nonpolymerisation reactions, especially of those with mass exchange (extraction, mixing, dispersion, and so on) as an important factor [27-32],... 

Ruckenstein and Sun [89] have used inverted emulsion polymerisation for the synthesis of PANI rubber composites using an isooctane-toluene mixture and water to form the emulsion and using ammonium persulfate as the oxidant. Inverse emulsion polymerisation consists of an aqueous solution of the monomer, which is emulsified in a non-polar organic solvent and the polymerisation is initiated with an oil-soluble initiator. The reaction is carried out in a heterogeneous system in which the reaction takes place in a large number of reaction loci dispersed in a continuous external phase. 

A rapid and low cost method was developed for direct analysis of residual monomer concentration of acrylamide from inverse-emulsion reactions. Inverse-emulsion polymerisations involve the dispersion of a water-soluble monomer in aqueous solution in a continuous organic phase. The addition of a low-medium hydrophilic-lyophilic balance steric stabiliser and continuous agitation is required to maintain emulsification. 19 refs. 

Applications of the TCR have included single phase liquid reactions reactions between inunisdble liquids dispersion of solids in liquids aystaUisations polymerisations electrochemistry fermentation photochanical reactions emulsion polymerisation synthesis of silica particles heterogeneous catalytic reactions and liquid-liquid extractions. 

The mechanism of dispersion polymerisation has been discussed in detail in the book edited by Barrett [11]. A distinct difference between emulsion and dispersion polymerisation may be considered in terms of the rate of reaction. As mentioned above, with emulsion polymerisation the rate of reaction depends on the number of particles formed. However, with dispersion polymerisation, the rate is independent of the number of particles formed. This is to be expected, since in the latter case polymerisation initially occurs in the continuous phase, whereby both monomer and initiator are soluble, and the continuation of polymerisation after precipitation is questionable. Although in emulsion polymerisation the initial monomer initiation reaction also occurs in the continuous medium, the particles formed become swollen with the monomer and polymerisation may continue in these particles. A comparison of the rate of reaction for dispersion and solution polymerisation showed a much faster rate for the former process [11]. 

Classical methods of emulsion polymerisation employ an aqueous phase containing water and surfactant plus initiator to which is added the monomers to be polymerised. Commercial polymerisations utilising acrylic monomers normally use a pre-emulsion technique in which the monomer mixture has been dispersed in the water phase with a surfactant. This pre-emulsion is added continuously during the course of the reaction to the water phase contaiiting further surfactant. 

Polymerisation in emulsion, in which the monomer is (a) dispersed in monomer droplets stabilized by an adsorbed layer of soap molecules (Fryling and Harrington, 1944, Kolthoff and Dale, 1945, Price and Adams, 1945, Siggia et ah, 1945, Vinograd etal, 1944) (b) solubilised in the soap micelles (Harkins, 1945, McBain, 1942, McBain and Soldate, 1944) which exist in an aqueous soap solution of sufficient concentration and (c) molecularly dissolved in the water. The amount of polymer formed in the droplets, in the micelles, and in solution will depend upon the way in which the monomer and catalyst are distributed in the three existing phases the monomer phase, the soap micelle phase, and the water phase - and possibly also upon the accessibility and reactivity of the monomer in these three phases. In certain aqueous soap emulsions, such as styrene, dichlorostyrene, or isoprene, the amount of molecularly dissolved monomer is small and, therefore, the reaction will occur preponderantly either in the monomer droplets or in the soap micelles. If the polymer formation occurs preponderantly in the micellar phase, one is inclined to speak of a typical emulsion polymerisation. If, however, polymerisation takes place to a considerable extent both in the monomer droplets and the soap micelles, the case is intermediate between suspension and emulsion polymerisation. There also exist emulsion... 

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