Polymerization Reactions in Water

Water offers a number of important properties as a solvent for polymerization reactions. As well as its high polarity, which gives a markedly different miscibility with many monomers and polymers compared to organic solvents, it is nonflammable, nontoxic and cheap. Water also has a very high heat capacity that sustains heat exchanges in a number of very exothermic polymerizations. Largely because of these factors, polymerizations are now widely carried out in aqueous media, and, for example, more than 50% of industrial radical polymerizations are carried out in water [19]. 

Many other metal-catalysed polymerizations may be carried out in water including the copper-catalysed polymerization of methacrylates, the palladium-and nickel-catalysed polymerization of ethene and other alkenes and the rhodium-catalysed polymerization of butadiene [22], 

One of the first reports of polymerization under supercritical conditions was that of the high-pressure production of low-density polyethylene by ICI in the 1930s. 

Supercritical fluids have also been used purely as the solvent for polymerization reactions. Supercritical fluids have many advantages over other solvents for both the synthesis and processing of materials (see Chapter 6), and there are a number of factors that make scCCH a desirable solvent for carrying out polymerization reactions. As well as being cheap, nontoxic and nonflammable, separation of the solvent from the product is achieved simply by depressurization. This eliminates the energy-intensive drying steps that are normally required after the reaction. Carbon dioxide is also chemically relatively inert and hence can be used for a wide variety of reactions. For example, CO2 is inert towards free radicals and this can be important in polymerization reactions since there is then no chain transfer to the solvent. This means that solvent incorporation into the polymer does not take place, giving a purer material. 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by 

In contrast to the free-radical polymerizations, there have been relatively few stndies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(ll)-bisphosphine complexes have been fonnd to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Fignre 10.5) as described in Chapter 5. 

Polymerization Reactions in Alternative Reaction Media SO3K SOgK 

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Acrylate- and methacrylate guanidines (AG and MAG) were prepared with high yield (to 80%) by reaction of acrylic acids and guanidine according to method elaborated by authors of this article and described in work [1], Kinetics of AG and MAG monomers polymerization was studied by dilatometry method in bidistillated water (pH 6.5, 60°C) on low conversion degrees (< 5%) after preliminary degassing of reaction mixtures on vacuum equipment (103 millimeters of mercury). Ammonium persulfate (APS) was used as initiator. The degree of conversion of monomer into polymer was determined on the base of contraction values determined by densimetry method which for GA polymerization reaction in water was 10.8%, and for MAG - 7.0%. Intrinsic viscosities [r ] of polymers were determined IN solution of NaCl in water at 30°C. Relative viscosities r rei of reaction solutions were determined at 30°C. 

Figure 14.1.1 The formation of cellulose during a condensation polymerization reaction in which each new link of glucose monomers releases a water molecule.

The water-soluble ligands described above, together with many others, are used to conduct a wide range of catalytic reactions in water. These reactions include hydrogenation, hydroformylation, oxidation, C-C coupling and polymerization reactions [30], Many of these reactions are discussed in detail in Chapters 7-11. 

Addition. A polymerization reaction in which monomers combine with one another to form a polymer without the formation of small by-products such as water. 

Polymerization in P-cyclodextrin (CD) complexes with monomer offers a route to polymerization, as well as other organic reactions, in water without the need for organic solvents [Ritter and Tabatabai, 2002]. P-Cyclodextrins are toms-shaped, cyclic oligosaccharides obtained by degradation of starch. The hydroxyl groups of the glucose repeat unit of CD are located on the outer surface. This makes the outer surface hydrophilic, whereas the inner surface and cavity are hydrophobic. Water-insoluble monomers become solubilized in water when mixed with CD or CD derivatives because the monomers are absorbed into the cavity. This allows polymerization in aqueous, not organic media, with water-soluble initiators. 

Polymerization reactions in aqueous medium can be carried out in homogeneous solution if the monomers and the polymers are soluble in water as in the case of acrylamide or methacrylic acid (see Examples 3-5,3-9, and 3-35). Since most of the monomers are only sparingly soluble in water, suspension or emulsion techniques have to be applied in these cases. 

Based on experimental results the loci of polymerization are assumed to be the micelles and latex particles. The 3rd power with respect to monomer concentration in Eq. (9) results from the 2nd order polymerization reaction in aqueous solution as well as from the influence of the monomer concentration on the partition equilibrium of the monomer between micelles and monomer/water droplets [13]. This influence is shown in Fig. 11. 

Polyethylene is formed by addition of C2H4 molecules, which are merely linked together without the elimination of any reactant atoms. When other polymers are formed, molecules are linked together, and particular atoms break loose to form additional products. When water (H20) is the additional product, the reaction is called a condensation reaction. Often the reactants are not simple hydrocarbons but are more complex organic molecules. The formation of nylon is a condensation polymerization reaction. In Activity 5.5 students will prepare a condensation polymer and use it to create a macrosculpture. 

Even though monomers are generally quite reactive (polymerizable), they usually require the addition of catalysts, initiators, pH control, heat, and/or vacuum to speed and control the polymerization reaction that will result in optimizing the manufacturing process and final product.74 When pure monomers can be converted directly to pure polymers, it is called the process of bulk polymerization, but often it is more convenient to run the polymerization reaction in an organic solvent (solution polymerization), in a water emulsion (emulsion polymerization), or as organic droplets dispersed in water (suspension polymerization). Often choose of catalyst systems exert precise control over the structure of the polymers they form. They are referred to as stereospecific systems. 

Horclois et al.239 have suggested the use of NMA as a solvent for the reaction of thiosemicarbazide with carbon disulfide to produce 2-amino-5-mercapto-l,3,4-thiadiazole. The reaction proceeds at a lower pressure (atmospheric) and at a lower temperature than the comparable reaction in water. Campbell240 has described the polymerization of acrylonitrile by BF3 in amide solvents including NMA. 

The first drawback in the use of water (the solubility problem) may be overcome by using surfactants, which solubilize organic materials or form emulsions with them in water. Indeed, surfactants have been occasionally used in organic synthesis [3-6]. A successful example is emulsion polymerization [7]. Some late transition metal-catalysed reactions in water have also been conducted in the presence of surfactants or surfactantlike ligands [8-15]. In many other cases, however, large quantities of surfactant molecules compared with the reaction substrates are needed for the desired reactions to proceed efficiently, and thus, the systems are impractical even if water can be used as a solvent. From the viewpoints of practicability and applicability, the surfactant-aided organic synthesis is still at the preliminary stage. 

As a reaction medium for transition rnetal-catalyzed polymerizations, water will, most likely, not be the first choice. The extreme water sensitivity of Ziegler or Phillips catalysts is well known. However, carrying out polymerization reactions in aqueous systems offers unique advantages. Thus, traditional free-radical emulsion and suspension polymerization are carried out on a large scale industrially. A brief review of these established reactions demonstrates some specific properties of polymerizations in aqueous systems. 

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