Stabilizer polymer polymerization

Some of the most common stabilization—soHdification processes are those using cement, lime, and pozzolanic materials. These materials are popular because they are very effective, plentiful, and relatively inexpensive. Other stabilization—soHdification technologies include thermoplastics, thermosetting reactive polymers, polymerization, and vitrification. Vitrification is discussed in the thermal treatment section of this article and the other stabdization—soHdification processes are discussed below. 

Shirakawa polyacetylene, 444 Siloxanes, polymerization, 239 Size exclusion chromatography, 262-263 Solubility, specialty polymers, 256 Spacers, flexible polymer backbones, 97 Specialty polymers, polar/ionic groups, 256 Stability, polymers, 256 Storage moduli, vs. temperature behavior, 270... 

Figure 5.10 Synthesis of a polymeric shell (top route) or a stabilized polymer core (bottom route). Reproduced with permission from [85].

The great stability of polymerized vesicles can be demonstrated by several experiments (9) dilution of polymer vesicle solutions with 50% of ethanol does not result in a precipitation. Turbidity measured at 300 nm remains the same (28), whereas monomer vesiclesare destroyed under these conditions followed by a considerable decrease of turbidity. A precipitation of stabilized vesicles can, however, be achieved by the addition of salts (KC1), but again it has to be pointed out that the polymeric vesicles... 

It is well known that phosphites or sulfides added to stabilizers of polymeric materials considerably enhance the stabilizing effects. Destruction of polymers is caused by the action of peroxides resulting from oxidation at the defects of polymeric chains. The additives decompose hydroperoxides according to the following equations ... 

Uses. Solvent for polymers polymerization catalyst stabilizer against thermal degradation in polystyrene UV stabilizer in polyvinyl and polyolefin resins... 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

Smith and Ewart calculated the number of particles having been formed at the end of the first stage of polymerization. The number of particles is affected by the initiator decomposition rate (or radical formation rate) and total surface area of emulsifier to stabilize polymer-monomer particles. Smith and Ewart concluded that the number of particles is proportional to the 0.4 power of the initiator concentration and the 0.6 power of the emulsifier concentration, assuming that the surface area of total polymer-monomer particles is equal to the total surface area of emulsifier molecules when the last micelle disappears. 

Reimers, J.L., and Schork, F.J., Robust nucleation in polymer-stabilized miniemulsion polymerization, J. Appl. Polym. Sci., 59, 1833-1841 (1996b). 

Instead of conventional surfactant molecules, amphiphilic water soluble macromonomers, especially PEO macromonomers, have been used extensively as a reactive emulsifier and as steric stabilizer polymer, as summarized in Table 5. Generally speaking, however, the mechanism for the particle nucleation in the emulsion polymerization systems using macromonomers has been poorly established when compared to the dispersion copolymerizations with macromonomers as mentioned earlier. 

One important requirement in replacing a conventional, nonreactive surfactant with a reactive one is that neither the molecular weight nor the particle size distribution of the latex may significantly change. Also, the Surfmer reactivity is important if the Surfmer is too reactive compared to the other monomers in the recipe, it will become partially buried inside the growing polymer particles. This will cause poor stability during polymerization and broadening of the particle size distribution. 

The kinetics of vinyl acetate emulsion polymerization in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly (vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stability, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Additives. Formulation additives do not always have a positive effect on moisture resistance. Therefore, selection of additives must go through a similar assessment process as the base polymer with respect to hydrolytic stability. Any polymeric additive used to modify the properties of the base resin should be considered for its possible effect on moisture resistance and environmental durability. 

The most outstanding feature of alkyl metal catalysts is their stereospecificity. Viewing the entire field in this respect, it appears that a single, common feature is beginning to emerge. This is the coordination of monomer with one part of the catalyst prior to the addition of a partially stabilized polymer chain end. The coordination takes place between jr-electrons or lone-pair electrons of the monomer with vacant orbitals of a metal component. The polymer chain end is fixed in position and partially stabilized by either simple or complex gegen-ions. Such polymerizations are referred to as coordination or coordinated polymerizations to emphasize coordination of monomer. It should be noted that prior usage of these terms frequently implied either coordination of catalyst components or a concerted polymerization mechanism. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

Micron-size particles can also be prepared by polymerization of monomer dispersion in organic solvents (e.g. alcohol) in which the emerging polymer is not soluble. At the beginning of the process, the reaction mixture is homogenous but during the reaction, stabilized polymer particles precipitate. This method offers uniform particles with a diameter of 2-15 pm (40). 

Batch miniemulsion polymerization of MMA using PMMA as the costabilizer was carried out with SLS as the surfactant and KPS as the initiator. Solids content was kept at -30%. A low surfactant level was used with the miniemulsions to ensure droplet nucleation. The initiator concentration of the polymer-stabilized miniemulsion polymerizations was varied from 0.0005 to 0.02 Mjq, based on the total water content. An aqueous phase retarder, (sodium nitrite) or an oil-phase inhibitor (diphenylpicrylhydrazol [DPPH]), was added to both the miniemulsions and the macro emulsions prior to initiation. Particle numbers and rates of polymerization for both systems were determined. 

---