Polymerization, dispersion

Dispersion polymerization may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerization. In dispersion emd precipitation polymerization, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerization the initiator is chosen to be soluble in the disperse phase of the monomer. The rate of dispersion polymerization is much faster than precipitation or solution polymerization. The enhancement of the rate in precipitation polymerization over solution polymerization has been attributed to the hindered termination of the growing polymer radicals. 

In most cases increasing the polymeric surfactant concentration (at any given monomer amount) results in the production of a larger number of particles with smaller size. This is to be expected since the larger number of particles with smaller 

In dispersion polymerization [ 115 ], the monomers, the initiator and the stabilizer (or stabilizer precursor) are dissolved in a solvent that is not a solvent for the polymer. Polymerization starts in a homogeneous phase and the polymer precipitates forming unstable nuclei. The nuclei are stabilized by the stabilizer present in the system. This stabilizer may be included in the formulation or formed in the reactor by grafting onto the stabilizer precursor. Nucleation ends when the number of stable polymer particles increases to a point in which all new nuclei are captured by the existing stable particles. Because of the compartmentaliz-ation of the radicals among the polymer particles, the polymerization locus changes from 

Daniel, l.C. (2006) In l.C. Daniel and C. Pichot (eds), Les Latex Synthitiques. Elaboration, Propriitis, Applications. Lavoisier, Paris, pp. 319-329. 

Schmidt-Thiimmes, Schwarzenbach, E. and Lee, D.I. (2002) In D. Urban and K. Takamura (eds). Polymer Dispersions and Their Industrial Applications. Wiley-VCH Verlag GmbH, Weinheim, Germany, pp. 75-101. 

When conventional surfactants are used in dispersion polymerization, difficulties are encountered which are inherent in their use. Conventional surfactants are held on the particle surface by physical forces thus, adsorption/desorption equilibria always exist, which may not be desirable. They can interfere with adhesion to a substrate and may be leached out upon contact with solvent. Surfactant migration affects fQm formation and their lateral motion during particle - particle interactions can cause destabilization of the colloidal dispersion. On the contrary, reactive surfactants contain a polymerizable group thus, they can overcome some of the difficulties encountered with conventioncd surfactants and can also be incorporated into the surface layer of the polymer particles by copolymerization with other unsaturated comonomers. In this manner, these reactive surfactants are bound to the particle surface and therefore they are prevented from subsequent migration. 

Polymerization of amphiphilic macromonomer or copolymerization of hydrophobic or nonpolar monomer with hydrophilic or polar macromonomer leads to the formation of surface active polymers or grafted copolymers. Macromonomers are macromolecules with a polymerizable group (see some examples, PSt denotes polystyrene and PMMA poly (methyl methacrylate))  

These compounds afford a powerful means of designing a vast variety of well-defined comb polymers and graft copolymers. Homopolymerization affords a regular comb polymer since the branches are regularly spaced along the backbone [121]. 

Copolymerization of a conventional monomer with a macromonomer affords well-defined graft copolymers (Fig. 9). This can be performed in a good solvent for both a monomer and its resulting polymer. The branched structure and heterogeneities in MW and composition of graft copolymers prepared in the solution make the classical techniques used to characterize the graft copolymer inefficient. 

One of the fascinating application of macromonomers is in the field of dispersion polymerization. The dispersion polymerization in the presence of suitable stabilizers affords mostly monodisperse submicron- and micron-sized microspheres (particles). The macromonomers are graft-copolymerizaed during copolymerization in the continuous phase and so accumulate on the particle surface, so that the resulting particles are effectively sterically stabilized against flocculation. Amphiphilic copolymers s mthesized by copolymerization of a hydrophobic conventional monomer with a hydrophilic macromonomer and vice verse present all the typical properties of conventional surfactants. They aggregate between themselves and form a micelle in the aqueous or non-aqueous media. The conformation of a micelle formed by PEO-g-PSt polymer in the aqueous medium consists of a hydrophobic PSt core and a hydrophilic PEO shell (Fig. 10). 

Several methodologies for preparation of monodisperse polymer particles are known [1]. Among them, dispersion polymerization in polar media has often been used because of the versatility and simplicity of the process. So far, the dispersion polymerizations and copolymerizations of hydrophobic classical monomers such as styrene (St), methyl methacrylate (MMA), etc., have been extensively investigated, in which the kinetic, molecular weight and colloidal parameters could be controlled by reaction conditions [6]. The preparation of monodisperse polymer particles in the range 1-20 pm is particularly challenging because it is just between the limits of particle size of conventional emulsion polymerization (100-700 nm) and suspension polymerization (20-1000 pm). 

Polymeric steric stabilizer such as poly(vinylpyrrolidone) (PVPo),poly(acrylic acid), poly(hydroxypropyl)cellulose, etc., are used to prepare monodisperse polymer in dispersion polymerization of monomers such as alkyl acrylates and methacrylates, and styrene in polar media. AB and ABA block copolymers are a second type of steric stabilizer which can be used in dispersion polymerization. For example, the poly(styrene-h-ethylene oxide) was recently used by Winnik et al. [6] in the dispersion polymerization of styrene in methanol. 

The nucleation mechanism of dispersion polymerization of low molecular weight monomers in the presence of classical stabilizers was investigated in detail by several groups [2,6,7]. It was, for example, reported that the particle size increased with increasing amount of water in the continuous phase (water/eth-anol), the final latex radius in their dispersion system being inversely proportional to the solubility parameter of the medium [8]. In contrast, Paine et al.[7] reported that the final particle diameter showed a maximum when Hansen polarity and the hydrogen-bonding term in the solubility parameter were close to those of steric stabilizer. 

The graft copolymers were already used for preparation and stabilization of polymer particles by Barrett [1]. He synthesized a poly(12-hydrostearic acid) macromonomer with a methacrylate end group. This macromonomer was copolymerized with MMA to obtain a preformed comb-graft copolymer, which was successfully used as stabilizer in nonaqueous dispersions of MMA. 

Thus the use amphiphilic macromonomers is another method to achieve the particle formation and their subsequent stabilization. Macromonomers can be pre-reacted to form graft copolymers, which are be introduced into the reaction medium afterwards. Macromonomers can also be copolymerized with classical monomers in situ to form graft copolymers. This is a simple and flexible method for producing monodisperse micron-sized polymer particles. Macromonomers can produce ion-free acrylic lattices with superior stability and film forming properties compared to conventional charge stabilized lattices. These non-con- 

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Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

Fig. 3. An aqueous dispersion polymerization process used in the manufacture of acrylic and modacrylic fibers.

Highly porous fabric stmetures, eg, Gore-Tex, that can be used as membranes have been developed by exploiting the unique fibrillation capabiHty of dispersion-polymerized PTFE (113). 

Hexafluoiopiopylene and tetiafluoioethylene aie copolymerized, with trichloiacetyl peroxide as the catalyst, at low temperature (43). Newer catalytic methods, including irradiation, achieve copolymerization at different temperatures (44,45). Aqueous and nonaqueous dispersion polymerizations appear to be the most convenient routes to commercial production (1,46—50). The polymerization conditions are similar to those of TFE homopolymer dispersion polymerization. The copolymer of HFP—TFE is a random copolymer that is, HFP units add to the growing chains at random intervals. The optimal composition of the copolymer requires that the mechanical properties are retained in the usable range and that the melt viscosity is low enough for easy melt processing. 

Tetrafluoroethylene of purity suitable for granular or dispersion polymerizations is acceptable for copolymerization with ethylene. Polymerization-grade ethylene is suitable for copolymerization with tetrafluoroethylene. Modifying termonomers, eg, perfluorobutylethylene and perfluoropropylene, are incorporated by free-radical polymerization. 

Nonaqueous Dispersion Polymerization. Nonaqueous dispersion polymers are prepared by polymerizing a methacryhc monomer dissolved in an organic solvent to form an insoluble polymer in the presence of an amphipathic graft or block copolymer. This graft or block copolymer, commonly called a stabilizer, lends coUoidal stabiUty to the insoluble polymer. Particle sizes in the range of 0.1—1.0 pm were typical in earlier studies (70), however particles up to 15 pm have been reported (71). 

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... 

Only particles of linear or very slightly cross-linked <0.6%) polymers may be produced by dispersion polymerization. Obviously, dispersion polymerization may be used for the production of monosized seed particles, which, after transfer to aqueous conditions, are used for the production of different cross-linked and macroporous particles by the activated swelling and polymerization method. 

In this chapter, the polymerization methods used for the production of uniform latex particles in the size range of O.I-lOO /Ltm are described. Emulsion, swollen emulsion, and dispersion polymerization techniques and their modified forms for producing plain, functionalized, or porous uniform latex particles are reviewed. The general mechanisms and the kinetics of the polymerization methods, the developed synthesis procedures, the effect of process variables, and the product properties are discussed. 

Uniform polymeric microspheres of micron size have been prepared by dispersion polymerization. This process is usually utilized for the production of uniform polystyrene and polymethylmethacrylate microspheres in the size range of 0-1-10.0 /Am. 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

Stable particles in sufficient number, all the oligo-radi-cals and nuclei generated in the continuous phase are captured by the mature particles, no more particles form, and the particle formation stage is completed. The primary particles formed by the nucleation process are swollen by the unconverted monomer and/or polymerization medium. The polymerization taking place within the individual particles leads to resultant uniform microspheres in the size range of 0.1-10 jjLvn. Various dispersion polymerization systems are summarized in Table 4. 

Some typical dispersion polymerization recipes and the electron micrograph of the uniform polymeric particles with Recipe I are given in Table 5 and Fig. 10, respectively. As seen in Table 5, the alcohols or alcohol-water mixtures are usually utilized as the dispersion media for the dispersion polymerization of apolar monomers. In order to achieve the monodispersity in the final product, a costabilizer can be used together with a primary steric stabilizer, which is usually in the polymeric form as in... 

Table 4 Some Examples for Different Dispersion Polymerization Systems...

Table 5 Typical Dispersion Polymerization Recipes Providing Uniform Latex Particles...

We also studied the effect of initiator on the dispersion polymerization of styrene in alcohol-water media by using a shaking reactor system [89]. We used AIBN and polyacrylic acid as the initiator and the stabilizer, respectively. Three different homogenous dispersion media including 90% alcohol and 10% water (by volume) were prepared by using isopropanol, 1-butanol, and 2-... 

The role of initiator concentration on dispersion polymerization can be summarized as follows. The in-... 

Ftgure 11 The electron micrographs of the final products and the variation of the monomer conversion with the polymerization time at different initiator concentrations in the dispersion polymerization of styrene. Initiator concentration (mol%) (a) 0.5, (b) 1.0, (c) 2.0. The original SEM photographs were taken with 2600 x, 2000 x, and 2600 x magnifications for (a), (b), and (c), respectively, and reduced at a proper ratio to place the figure. (From Ref. 93. Reproduced with permission from John Wiley Sons, Inc.)... 

The same PVP series were also tried for the dispersion polymerization of styrene in the ethanol medium by using AIBN as the initiator and aerosol OT as the costabilizer [84]. PVP K-15 usually yielded polymeric particles with a certain size distribution and some coagu-lum. The uniform products were obtained with PVP K-30 and PVP K-90 in the presence of the costabilizer. The tendencies for the variation of the final particle size with the stabilizer concentration and with the molecular weight of the stabilizer were consistent with those obtained for the dispersion polymerization of methyl methacrylate [84],... 

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. 

We have also examined the effect of stabilizer (i.e., polyacrylic acid) on the dispersion polymerization of styrene (20 ml) initiated with AIBN (0.14 g) in an isopropanol (180 ml)-water (20 ml) medium [93]. The polymerizations were carried out at 75 C for 24 h, with 150 rpm stirring rate by changing the stabilizer concentration between 0.5-2.0 g/dL (dispersion medium). The electron micrographs of the final particles and the variation of the monomer conversion with the polymerization time at different stabilizer concentrations are given in Fig. 12. The average particle size decreased and the polymerization rate increased by the increasing PAAc concentra-... 

Uniform polymeric microspheres in the micron size range have been prepared in a wide variety of solvent combinations by dispersion polymerization. The polarity of the dispersion medium is one of the most important... 

Where, x,- is the volume fraction of component /, S, and S/ are the solubility parameters of the homogenous solvent mixture and the component i, respectively. The solubility parameters of some solvents that are widely used as the continuous medium in the dispersion polymerization are given in Table 6. 

Paine et al. [85] extensively studied the effect of solvent in the dispersion polymerization of styrene in the polar media. In their study, the dispersion polymerization of styrene was carried out by changing the dispersion medium. They used hydroxypropyl cellulose (HPC) as the stabilizer and its concentration was fixed to 1.5% within a series of -alcohols tried as the dispersion media. The particle size increased from only 2.0 /itm in methanol to about 8.3 /itm in pentanol, and then decreased back to 1 ixm in octadecanol. The particle size values plotted against the Hansen solubility parameters... 

Almog et al. [80] studied the dispersion polymerization of styrene in different alcohols as the continuous medium by using AIBN and vinyl alcohol-vinyl acetate copolymer as the initiator and the stabilizer, respectively. Their results showed that the final particle size decreased with the alcohol type according to the following order ... 

Okubo et al. [87] used AIBN and poly(acrylic acid) (Mw = 2 X 10 ) as the initiator and the stabilizer, respectively, for the dispersion polymerization of styrene conducted within the ethyl alcohol/water medium. The ethyl alcohol-water volumetric ratio (ml ml) was changed between (100 0) and (60 40). The uniform particles were obtained in the range of 100 0 and 70 30 while the polydisperse particles were produced with 35 65 and especially 60 40 ethyl alcohol-water ratios. The average particle size decreased form 3.8 to 1.9 /xm by the increasing water content of the dispersion medium. 

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