Polymerization in two-phase systems

Molecular Weight Distribution for Polymerization in Two-Phase Systems... 

Rasmussen and co-workers. Chapter 10, have shown that many free-radical polymerizations can be conducted in two-phase systems using potassium persulfate and either crown ethers or quaternary ammonium salts as initiators. When transferred to the organic phase persulfate performs far more efficiently as an initiator than conventional materials such as azobisisobutyronitrile or benzoyl peroxide. In vinyl polymerizations using PTC-persulfate initiation one can exercise precise control over reaction rates, even at low temperatures. Mechanistic aspects of these complicated systems have been worked out for this highly useful and economical method of initiation of free-radical polymerizations. 

To minimize the indane, 99, formation, dimerization was conducted in two-phase systems containing toluenesulfonic acid,354 sulfuric acid,355 356 electrophilic transition-metal complexes,357 the polymeric solid-state acid Nafion,358 359 metal oxide solid-state catalysts such as tungstophosphoric acid,360 various zeolites,361 362 mixed oxides,363 and montmorillonite clay in the presence of organic solvents.364 365 The major limitation of the cationic approach, however, is the unavoidable formation of internal isomer 100. Since isomer 100 is inert in radical polymerization, the lower the content of isomer 100, the higher activity of the 98 mixture. Even in the very best cases, its presence is never less than 5—15%. 

Dinuclear Mn(IV) catalysts were successfully applied for the epoxidation of 4-benzoic acid and styrene in aqueous systems [18], for the epoxidation of styrene and dodecene in two-phase systems [19g] as weU as in methanol and acetone [17d, 19b,g,h], and for the epoxidation of olefins in acetonitrile [19i], aqueous acetonitrile [17f, 19c], acetone [17d, 19b,h], methanol [19i], and acetone-methanol-water mixtures [19j]. Enantioselective epoxidations of olefins have been reported for chiral triazacyclononane derivatives boimd to Mn(OAc2) 4 (H2O) in methanol [19k], Heterogenization of the Mn catalysts on silica for the epoxidation of styrene and cyclohexene, on zeolites for olefin epoxidation, and on a solid MnS04 H2O also for olefin epoxidation [20] has also been described. A polymeric structure bearing a dense arrangement of 1,4,7-triazacyclononane moieties can be synthesized by... 

Thus, the present chapter results have been shown the applicability and usefullness of fractal analysis and strange (anomalous) diffusion conception for description of polymerization reactions, catalyzed by nanofillers. The nanofiller introduction in reactionary mixtnre results in two-phase system formation, where a decisive role will be played by interfacial interactions. The polymerization conversion degree is defined by its active (fractal) time. Hence, the ability to control active time gives the possibility of reaction conrse operation. 

In two-phase systems, absorption of mierowave radiation by dieleetrie heating and ionic conduction enables individual phases to be heated at different rates, potentially affording sizable temperature differenees. Raner et al. (1995) performed a Hoffmann elimination using a two-phase water-chloroform system The reaction carried out in water at 105 °C led to polymerization of the final product. However, the reaction proceeds adequately under microwave irradiation in a two-phase water-chloroform system. The temperatures of the aqueous and oiganie phases were 110... 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

The terminal R groups can be aromatic or aliphatic. Typically, they are derivatives of monohydric phenoHc compounds including phenol and alkylated phenols, eg, /-butylphenol. In iaterfacial polymerization, bisphenol A and a monofunctional terminator are dissolved in aqueous caustic. Methylene chloride containing a phase-transfer catalyst is added. The two-phase system is stirred and phosgene is added. The bisphenol A salt reacts with the phosgene at the interface of the two solutions and the polymer "grows" into the methylene chloride. The sodium chloride by-product enters the aqueous phase. Chain length is controlled by the amount of monohydric terminator. The methylene chloride—polymer solution is separated from the aqueous brine-laden by-products. The facile separation of a pure polymer solution is the key to the interfacial process. The methylene chloride solvent is removed, and the polymer is isolated in the form of pellets, powder, or slurries. 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Emulsion Polymerization. Emulsion polymerization uses soaps and anionic surfactants to create two-phase systems that have having long-term stability. The key steps in a batch emulsion polymerization are the following ... 

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

Polymerization of butyl acrylate was also studied by us in ethyl acetate/water two phase systems (3) using potassium persulfate/quaternary ammonium salts as the initiator system. Under these conditions (a minimum amount of water was used to dissolve the persulfate), it was found that symmetrical quat salts were more efficient than surfactant type quat salts. Also, the more lipophilic quat salts were more efficient. These results prompted us to propose formation of an organic-soluble quaternary ammonium persulfate via typical phase transfer processes. 

In this case, a simple kinetic model could not be derived to explain all the experimental observations. Independently prepared hexadecyl trimethyl ammoniurn persulfate was found to be soluble in toluene/AN mixtures and to catalyze polymerization in this homogeneous system at approximately the same rate as that observed in the two phase system. This result implies that anion exchange at the interface (see below) must be essentially complete under these conditions. Factors which complicated further analysis of the mechanism included a) precipitation of poly(acrylonitri 1 e) during the polymerization ... 

The kinetics of methyl methacrylate (MMA) polymerization in ethyl acetate/water two phase systems was described as being more well-behaved ( ). Using hexadecylpyridinium chloride (HPC) as the phase transfer catalyst, Rp was found to be approximately first order in MMA concentration. In support of a typical phase transfer mechanism, it was found that... 

Very recently, Ghosh and Mandal have reported (20) a thorough kinetic investigation of the polymerization of styrene in the two phase system water/o-dichiorobenzene using potassium persulfate as initiator and tetrabutylammoniurn bromide These studies were conducted at and pH, and found that the rate showed a square root dependence... 

Process in which a polymeric material, consisting of macromolecules differing in some characteristic affecting their solubility, is separated from a polymer-rich phase into fractions by successively increasing the solution power of the solvent, resulting in the repeated formation of a two phase system in which the more soluble components concentrate in the polymer-poor phase. 

It is the intent of this doeument to define the terms most commonly encountered in the field of polymer blends and eomposites. The scope has been limited to mixtures in which the eomponents differ in ehemical composition or molar mass or both and in which the continuous phase is polymeric. Many of the materials described by the term multiphase are two-phase systems that may show a multitude of finely dispersed phase domains. Hence, incidental thermodynamic descriptions are mainly limited to binary mixtures, although they can be and, in the scientific literature, have been generalized to multicomponent mixtures. Crystalline polymers and liquid-crystal polymers have been considered in other documents [1,2] and are not discussed here. 

This review focuses on how such constraints affect the properties of a polymeric material. In the case of one-phase systems, for example, the elastomeric properties in elongation assume particular importance. In the case of two-phase systems in which the chains are constrained by some type of rigid framework or surface,... 

Factors leading to insolubility in water were discussed in Section 10.2.1.1. Provided they are not highly polymerized, such hydrophobic substances are generally soluble in non-aqueous solvents. In a two-phase system formed by an aqueous phase and a second phase immiscible with it a solute will partition between them until its activity is the same in both. The Nernst partition isotherm quantifies this relationship in the form... 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

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