Precipitation/dispersion polymerization

The first example of nanogel synthesis by direct RAFT polymerization under precipitation/dis-persion polymerization condition was reported by An et al. in 2007 (Figure 54.27). Two types of poly(A,A -dimethylacrylamide)s (PDMAs) bearing a trithiocarbonate group were first synthesized by RAFT solution polymerization and were subsequently used as both stabilizers and RAFT agents for nanogel synthesis by RAFT precipitation/dispersion polymerization. These two types of... 

While fast polymerizations occurred in diethyl-, di Bu- and propylene carbonate (fcp PP = 3 - 3.3 X IQ-2 h- ), the fastest rates were provided by ethylene carbonate, HFIPA ( p PP = 5 - 7 X IQ-2 h i) and especially by dimethyl carbonate (DMC, app j jQ-i ii-i). The conversion, polymerization rate (A p PP), and PDI were largely insensitive to the amount of DMC, = 35 mL, 1 g VDF, 1-12 mL DMC), whereas the rate significantly increased with the amount of VDF pressure and monomer concentration in solution, with 1-4 g VDF, 3-12 mL DMC). These are typical features of heterogeneous polymerizations of gaseous monomers [114], reminiscent of precipitation/dispersion polymerization of PVDF in SCCO2 with [29] and without [115] iodine CTAs. However, while fluorination enhances polymer solubilization, SCCO2 is a very poor PVDF solvent [116], even at high temperatures and pressures. 

Monosized polystyrene particles in the size range of 2-10 /am have been obtained by dispersion polymerization of styrene in polar solvents such as ethyl alcohol or mixtures of alcohol with water in the presence of a suitable steric stabilizer (59-62). Dispersion polymerization may be looked upon as a special type of precipitation polymerization and was originally meant to be an alternative to emulsion polymerization. The components of a dispersion polymerization include monomers, initiator, steric stabilizer, and the dispersion medium... 

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

Polymerization, including radical polymerization, in supercritical C02 has been reviewed.1 6 137 It should be noted supercritical C()2 while a good solvent for many monomers is a very poor solvent for polymers such as the (meth)acrylates and S. As a consequence, with the exception of certain fluoropolymers and polymerizations taken to very low conversion, most polymerizations in supercritical CCb are of necessity precipitation, dispersion or emulsion polymerizations. 

In 1994, we reported the dispersion polymerization of MM A in supercritical C02 [103]. This work represents the first successful dispersion polymerization of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, we took advantage of the amphiphilic nature of the homopolymer PFOA to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymerization (> 3000) in supercritical C02. These polymerizations were conducted in C02 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN were employed as the initiators. The results from the AIBN initiated polymerizations are shown in Table 3. The spherical polymer particles which resulted from these dispersion polymerizations were isolated by simply venting the C02 from the reaction mixture. Scanning electron microscopy showed that the product consisted of spheres in the pm size range with a narrow particle size distribution (see Fig. 7). In contrast, reactions which were performed in the absence of PFOA resulted in relatively low conversion and molar masses. Moreover, the polymer which resulted from these precipitation... 

These block copolymers can act as effective steric stabilizers for the dispersion polymerization in solvents with ultralow cohesion energy density. This was shown with some polymerization experiments in Freon 113 as a model solvent. The dispersion particles are effectively stabilized by our amphi-philes. However, these experiments can only model the technically relevant case of polymerization or precipitation processes in supercritical C02 and further experiments related to stabilization behavior in this sytem are certainly required. 

In dispersion polymerization, the monomer and initiator are dissolved in the continuous phase, which acts as a nonsolvent for the developing polymer. The continuous phase can be organic, aqueous, or a mixture of miscible phases. Two methods of initiation have been employed, including gamma radiation [75] and chemical initiation by potassium perox-odisulphate [76]. As the polymer is formed, it precipitates as nanoparticles. These particles are not polymeric precipitates as in precipitation polymerization. Rather, they are swollen by a mixture of the monomer and the continuous phase [39],... 

Table 11.2.1 Comparison Between Precipitation and Dispersion Polymerizations Defined by Arshady...

In Figure 11.2.1A the process of dispersion polymerization is shown focusing on the formation process of particles. Dispersion polymerization starts from a homogeneous solution, and when oligomeric radicals and polymer, formed in the monomer solution, do not have affinity for the medium, they become insoluble and precipitate. The precipitate is unstable and homoaggregates to become primary particles. Primary particles homoaggregate further until they become stable secondary particles. The mechanism to keep the particles stable depends on what type of stabilizer is used. The propagation processes from nuclei to primaiy particles and from primary to secondary ones does not have to be considered as discontinuous steps. However, it... 

Electrically conducting polymer particles such as polypyrrole and polyaniline could also be prepared by dispersion polymerization in aqueous ethanol (31). The oxidation polymerization of pyrrole and aniline has been carried out at the electrode surfaces so far and formed a thin film of conducting polymer. On the other hand, polypyrrole precipitates as particles when an oxidizing reagent is added to a pyrrole dissolved ethanol solution, which contains a water-soluble stabilizer. In this way electrically conducting polymer particles are obtained and, in order to add more function to them, incorporation of functional groups, such as aldehyde to the surface, and silicone treatment were invented (32). 

Emulsion polymerization a sufficient amount of dispersing agent and mild agitation is employed. This produces small colloidal particles dispersed in the aqueous reaction medium. In this procedure, called aqueous dispersion polymerization, precipitation of the resin particles is avoided. The coagulated dispersion produced by emulsion polymerization is often called a fine-powder or PTFE dispersion. 

An even more striking effect is observed by addition of the surfactant sodiumdodecyl sulfate (SDS) to vesicles (Fig. 23). While monomeric vesicles of (19) and dipalmitoylle-cithin are destroyed by low SDS concentrations, the polymerized vesicles are stable up to 2 - 10 3 mol/1 SDS25). Polymerized vesicle dispersions can be diluted with ethanol without precipitation.231 Polymeric liposomes of (20) are stable in 80% ethanol for weeks. This could also be shown by Regen et al. for polymerized vesicles of the methacryloylic lipids (4) and (6)13141 (Fig. 24) by monitoring the turbidity (absor-... 

Most types of polymerizations can be performed in liquid and supercritical C02. The two major types of polymerizations, chain-growth and step-growth, have been demonstrated in C02. Reviews in the literature (Canelas and DeSimone, 1997b Kendall et al., 1999) have described numerous polymerizations in C02, many of which will not be discussed in this chapter. Since only amorphous or low-melting fluoropolymers and silicones show appreciable solubility at relatively mild temperatures and pressures (T< 100 °C, P<400 bar), only these two classes of polymers can be synthesized by a homogeneous polymerization in C02. All other types of polymers, including semicrystalline fluoropolymers and lipophilic or hydrophilic polymers, must be made by heterogeneous methods, such as precipitation, dispersion, emulsion, and suspension, since the polymers are insoluble in C02 (when T< 100 °C and P<400 bar). Some semicrystalline fluoropolymers and hydrocarbon polymers can be dissolved at more extreme temperatures and pressures and are discussed in Chapter 7 of this book. 

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