Water soluble monomers

The emulsion polymerization system fundamentally includes hydrophobic monomer, water-soluble initiator, emulsifier, and water. Therefore, the initial system of emulsion polymerization is truly an emulsion. 

Deviations with respect to order from the SE theory increase as the monomer water solubility increases. 

Typically, copolymer composition can be manually adjusted by slowly feeding the more reactive monomer in throughout the reaction but this may not be helpful when trying to overcome monomer transport limitations. Therefore, Reimers and Schork [ 102] performed identical copolymerization experiments in miniemulsions, where monomer transport is less significant, in order to determine what effect this would have on the evolution of the copolymer composition. Data on the MMA/VS (and other) copolymerizations indicate that the Schuller equation (and not the Samer adaptation) fits the copolymer composition data. This points to the effect of extremely low monomer water solubility on copolymer composition in macroemulsion polymerization, and the relative insensitivity of miniemulsion polymerization to this effect. 

Monomer Water solubility at 20°C Critical surface tension ( Y ) (mN/m)... 

It is at present clearly impossible to understand all the aspects of these systems. Nevertheless the mechanism and kinetics of some emulsion systems are reasonably well understood—those in which the monomer is water- insoluble and in which the polymer is soluble in the monomer. An outline is given of this mechanism and the kinetics of polymerization are developed on the basis of this mechanism. This theoretical kinetic behavior is then compared with experimental data, both from the literature and from unpublished results. Whenever possible, the influence of monomer water solubility and monomer solubility of the polymer is commented on. These comments are mostly of a qualitative nature and sometimes even speculative. The present state of our knowledge does not permit going beyond such comments, although recently the literature has given a few attempts at quantitative interpretation of emulsion polymerization of water-soluble monomers. 

The relative reactivities of the monomers are important in establishing the strategy for latex synthesis [3]. The monomer water solubility is another important factor in the synthesis of latexes. This not only is a variable related to polyma-polarity, but also directly impacts on particle nucleation during polymer synthesis (see Chapter 5). 

Monomers Water solubility, g/100 g water [5] Boiling point, °C [6] Glass trans. Temp. C Q e Ref. ... 

Domb et al. quantified the effect of monomer water solubility on the in vivo hydrolysis kinetics of polyanhydrides. They subcutaneously implanted polyanhydride matrices (2mmx2 mm x 4 mm) composed of aliphatic diacids, with differing carbon chain lengths, in rats and analyzed the... 

Water Solubility of Vinyl Monomers Water Solubility (25 -50 ), mM... 

Monomer water-solubility can also be a factor in this phenomena because of the initiation and oligomer reactions taking place in the water-phase. 

In mass polymerization bulk monomer is converted to polymers. In solution polymerization the reaction is completed in the presence of a solvent. In suspension, dispersed mass, pearl or granular polymerization the monomer, containing dissolved initiator, is polymerized while dispersed in the form of fine droplets in a second non-reactive liquid (usually water). In emulsion polymerization an aqueous emulsion of the monomer in the presence of a water-soluble initiator Is converted to a polymer latex (colloidal dispersion of polymer in water). 

Guldi D M 1997 Capped fullerenes stabilization of water-soluble fullerene monomers as studied by flash photolysis and pulse radiolysis J. Phys. Chem. A 101 3895-900... 

With this picture in mind, let us consider what happens when monomer is stirred into a surfactant solution-which also contains a water-soluble initiator-above the cmc. 

Cationic monomers are used to enhance adsorption on waste soHds and faciHtate flocculation (31). One of the first used in water treatment processes (10) is obtained by the cyclization of dimethyldiallylammonium chloride in 60—70 wt % aqueous solution (43) (see Water). Another cationic water-soluble polymer, poly(dimethylarnine-fi9-epichlorohydrin) (11), prepared by the step-growth... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Salts are made by reaction of acryhc acid with an appropriate base in aqueous medium. They can serve as monomers and comonomers in water-soluble or water-dispersible polymers for floor pohshes and flocculants. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

In addition to providing fully alkyl/aryl-substituted polyphosphasenes, the versatility of the process in Figure 2 has allowed the preparation of various functionalized polymers and copolymers. Thus the monomer (10) can be derivatized via deprotonation—substitution, when a P-methyl (or P—CH2—) group is present, to provide new phosphoranimines some of which, in turn, serve as precursors to new polymers (64). In the same vein, polymers containing a P—CH group, for example, poly(methylphenylphosphazene), can also be derivatized by deprotonation—substitution reactions without chain scission. This has produced a number of functionalized polymers (64,71—73), including water-soluble carboxylate salts (11), as well as graft copolymers with styrene (74) and with dimethylsiloxane (12) (75). 

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

Table 1. Water Solubilities of Monomers Common to Latex Production...

Initia.tors, The initiators most commonly used in emulsion polymerization are water soluble although partially soluble and oil-soluble initiators have also been used (57). Normally only one initiator type is used for a given polymerization. In some cases a finishing initiator is used (58). At high conversion the concentration of monomer in the aqueous phase is very low, leading to much radical—radical termination. An oil-soluble initiator makes its way more readily into the polymer particles, promoting conversion of monomer to polymer more effectively. 

Almost all synthetic binders are prepared by an emulsion polymerization process and are suppHed as latexes which consist of 48—52 wt % polymer dispersed in water (101). The largest-volume binder is styrene—butadiene copolymer [9003-55-8] (SBR) latex. Most SBRlatexes are carboxylated, ie, they contain copolymerized acidic monomers. Other latex binders are based on poly(vinyl acetate) [9003-20-7] and on polymers of acrylate esters. Poly(vinyl alcohol) is a water-soluble, synthetic biader which is prepared by the hydrolysis of poly(viayl acetate) (see Latex technology Vinyl polymers). 

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