Dispersion copolymerization

Table 9 The Dispersion Copolymerization Procedures Used for the Synthesis of Uniform Latex Particles...

DSEP direct soapless emulsion polymerization, SSEC seeded soapless emulsion copolymerization, DDC direct dispersion copolymerization, TDSC two-stage dispersion copolymerization, ATES Allyl trietoxysilane, VTES vinyl trietoxysilane, DMAEM dimethylaminoethyl-methacrylate, CMS chloromethylstyrene, GA glutaraldehyde, AAc Acrylic acid Aam Acrylamide HEMA 2-hydroxyethylmethacrylate. 

Styrene monomer was also copolymerized with a series of functional monomers by using a single-step dispersion copolymerization procedure carried out in ethanol as the dispersion medium by using azobisizobu-tyronitrile and polyvinylpyrollidone as the initiator and the stabilizer, respectively [84]. The comonomers were methyl methacrylate, hydroxyethyl acrylate, metha-crylic acid, acrylamide, allyltrietoxyl silane, vinyl poly-dimethylsiloxane, vinylsilacrown, and dimethylamino-... 

Kawaguchi et al. [12] have modified Paine s model for the dispersion copolymerization of amphiphilic PEO macromonomers. The authors have modeled the variation of the particle size and its distribution with reaction conditions. For example, the expressions for the critical conversion (x Xthe particle radius (r), and the surface area (S) occupied by a PEO chain are as follows ... 

Lacroix-Desmazes and Guyot [13] applied Paine s model to the dispersion copolymerization of amphiphilic macromonomers and re-discussed this model in terms of possible incorporation of a new parameter - the chain transfer parameter (Css the chain transfer constant for transfer to solvent-alcohol). The relations for the rate of dead chains (kj) and chain length (CL) are as follows ... 

The rate of dispersion copolymerization of PEO-MA macromonomer and styrene was found to increase with increasing initiator concentration VA - water soluble, DBP (dibenzoyl peroxide) - oil soluble, [PEO-MA] =0.06 mol dnr3, [styrene] =2.13 mol dm-3, in ethanol/water, v/v4/l) [65,66] ... 

The dispersion copolymerization of PEO-MA macromonomer and styrene is presented in Figs. 1 and 2 [70]. The rate-conversion plot is curved with a maximum at very low conversion. In all runs, neither the gel effect nor the stationary interval were observed. The strong decrease of the rate of polymerization with increasing conversion results from a decrease in the monomer concentration at the reaction loci (mainly in the polymer particles). The low monomer concentration in particles is a reason why the gel effect may be operative only at very low conversion. 

The dispersion copolymerization of PEO-MA and PEO-VB macromonomers was found to produce monodisperse polymer particles. The PSD (Dw/Dn) was found to be close to 1.08 and it decreased with increasing the initiator and macromonomer concentration but increased with conversion [69,70]. 

The rate of the dispersion copolymerization of PEO-MA with styrene increased with increasing temperature [72], which was attributed to the increased rate of initiation and particle formation. The overall observed activation energy (E0) for the dispersion copolymerization of PEO-MA or PEO-VB macromonomer and styrene was found to be 48 or 53 kj mol-1. These values are much lower than those (ca. 90-100 kj mol-1 [73]) obtained in the solution polymerization of low molecular weight monomers. 

The preparation of monodisperse micronsized polymer particles by radical dispersion copolymerization of styrene with PEO macromonomers having p-styrylbutyl endgroup (Mn=2000) in methanol/water (90/10, v/v) and modeling of colloidal parameters were shown in [12,78]. The polymer particle diameter varied within the range 0.3-0.5 pm. The dependence of the particle size on the experimental conditions was modeled into the expression... 

The stable polymer dispersions with small-sized polymer particles of diameter >60 nm were prepared by dispersion copolymerization of PEO-MA macromonomer with styrene, 2-ethylhexyl acrylate, acrylic and methacrylic acids, and butadiene at 60 °C [79]. The particle size was reported to decrease with increasing macromonomer fraction in the comonomer feed. Besides, it varied with the type of the classical monomer as a comonomer. Tg of polymer product was found to be a function of the copolymer composition, the weight ratio macromonomer/monomer, and monomer type and varied from 50.6 to 220.4 °C. 

High conversions (close to 100%) can be obtained by the dispersion copolymerization of PEO-MA with butyl acrylate initiated by a water-soluble initiator (VA) [80]. The conversion curves have a shape similar to that for the dispersion copolymerization of PEO-MA with styrene. In runs with AIBN the final conversion was around 90% and/or the polymerization was very slow at high conversion. 

Dispersion copolymerization of PEO-MA macromonomers (Cj-fEO -MA, C1-(EO)48-C6-MA, C1-(EO)48-C10-MA) with MMA was successful in producing very stable PMMA dispersions of micron size [81]. In this case, however, Cr (EO)48-MA was more effective in giving monodisperse particles than C1-(EO)48-C10-MA (the reverse is true with styrene, see above). The particles obtained were found to have uneven surfaces with a number of craters. These results suggest that some compatibility between PMMA and PEO chains and also between PMMA and the medium (methanol/water) may play a role in controlling the particle formation. 

Table 1. Variation of kinetic, molecular weight, and colloidal parameters of dispersion copolymerization of PMA macromonomer and styrene...

Table 2. Dependence of the reactivity ratio r2 and the rate of polymerization in the dispersion copolymerization of PEO-MA macromonomer and styrene (M2) with the monomer feed composition3...

The rate of dispersion (co)polymerization of PEO macromonomers passes through a maximum at a certain conversion. No constant rate interval was observed and it was attributed to the decreasing monomer concentration. At the beginning of polymerization, the abrupt increase in the rate was attributed to a certain compartmentalization of reaction loci, the diffusion controlled termination, gel effect, and pseudo-bulk kinetics. A dispersion copolymerization of PEO macromonomers in polar media is used to prepare monodisperse polymer particles in micron and submicron range as a result of the very short nucleation period, the high nucleation activity of macromonomer or its graft copolymer formed, and the location of surface active group of stabilizer at the particle surface (chemically bound at the particle surface). Under such conditions a small amount of stabilizer promotes the formation of stable and monodisperse polymer particles. 

Table 3. Examples of dispersion copolymerization with macromonomers ...

The technique has been recently extended to polar media, especially alcohols and their mixtures with water as a continuous phase. Kobayashi et al. [104-107] have reported that poly(2-oxazoline) macromonomers such as 34 and 35 are very effective for the dispersion copolymerization with styrene, MMA, and N-vinyl-formamide in methanol, ethanol, and mixtures of these alcohols with water. They reported that the particle size decreased with increasing initial macromonomer concentration and that poly(2-oxazoline) macromonomers graft-copoly-merized are concentrated on the particle surface to act as steric stabilizers. 

Dispersion copolymerizations using poly(ethylene oxide) (PEO) macromonomers 26, 27, 36 and 37 in alcoholic media have been intensively studied by many researchers [38, 108-116]. They afford nearly monodisperse polymeric microspheres of submicron to micron size, covered with PEO chains on their... 

Polyelectrolyte macromonomers 41, 44, 47 and 48 [120, 122, 126, 127] have been also prepared and applied to the dispersion copolymerizations to produce polymeric particles covered with polyelectrolyte chains. Evidently, the dependence of the conformational properties of polyelectrolyte brush chains attached... 

Fi9 5 Schematic picture of a microsphere obtained in emulsion and dispersion copolymerization using macromonomer technique. The grafted chains are exaggerated in size... 

Mechanistic Model of Dispersion Copolymerization with Macromonomers... 

According to the aggregative and coagulative nucleation mechanisms which have been derived originally from the homogeneous nucleation theory of Fitch and Tsai [128], the most important point in the reaction is the instant at which colloidally stabilized particles form. After this point, coagulation between similar-sized particles no longer occurs, and the number of particles present in the reaction is constant. As shown in Fig. 6, the dispersion copolymerization with macromonomers is considered to proceed as follows. (1) Before polymerization, the monomer, macromonomer, and initiator dissolve completely into the... 

Figure 7 shows a comparison of Eq. (27) with the particle radius obtained by dispersion copolymerization of styrene with PEO macromonomer 26 (m=4, n= 45) in methanol-water medium (9/1 v/v). One sees that the experimental particle... 

Table 4. The power law exponents in dispersion copolymerization with PEO macromonomers, R=K[Monomer] a[Macromonomer]b [Initiator]c ...

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. 

Core-shell polystyrene-polyimide high performance particles have been successfully prepared by the dispersion copolymerization of styrene with vinyl-benzyltrimethyl ammonium chloride (VBAC) in an ethanol-water medium using an aromatic poly(amic acid) as stabilizer, followed by imidization with acetic anhydride [63]. Micron-sized monodisperse polystyrene spheres impregnated with polyimide prepolymer have also been prepared by the conventional dispersion polymerization of styrene in a mixed solvent of isopropanol/2-methoxyethanol in the presence of L-ascorbic acid as an antioxidant [64]. 

Dispersion copolymerizations that use the poly(ethylene oxide) (PEG) macromonomers 7-13 in alcoholic media have been intensively studied by... 

Some novel water soluble macromonomers, 24, have been synthesized by the oxyanionic polymerization [149] of 2-(dimethylamino)ethyl, 2-(diisopropyl-amino)ethyl, and 2-(JV-morpholino)ethyl methacrylate, and conducted to dispersion copolymerization of styrene in alcohol media [150]. Sufobetaine-based macromonomer was prepared by the polymer reaction of 24 (R=CH3) with propane sultone, and was found to be useful in the dispersion polymerization of styrene even at high electrolyte levels (up to 1 M NaCl). Ito et al. [151] synthesized new PEO macromonomers with a cationic charge at the co-end, 25, and examined the influence of the charge on the particles size in dispersion copolymerization with styrene in alcohol media. 

In all instances of the dispersion polymerization, amphiphilic graft copolymers produced in a selective solvent for the branches play a crucial role. Schematically, a microsphere obtained by copolymerization in this way with a small amount of macromonomer has a core-shell structure as given in Fig. 2, with the core occupied by the insoluble substrate polymer chains and the shell by the soluble, graft-copolymerized macromonomer chains. The backbone chains of the graft copolymers, which must be insoluble in the medium, serve as the anchors into the core. The following section presents general criteria for the size control of polymeric microspheres by dispersion copolymerization... 

Fig. 2 Schematic description of dispersion copolymerization of styrene with macromonomer...

In remarkable contrast, unusually high exponent values (-1.2) have been obtained in the dispersion copolymerization of a polar monomer, MMA in methanol-water (8 2 and 7 3 v/v) media. The value of the exponent drops to 0.51 when the water content is increased to higher than 40%, as shown in... 

Wu et al. [ 176] studied the surface properties of PS and PMMA microspheres stabilized by the PEO macromonomer 7 (m=l) using dynamic light scattering, and claimed that for PMMA microspheres the surface area occupied by a PEO molecule is nearly twice as large as that for PS microspheres, assuming that 100% macromonomer is copolymerized to attach to the latex surface. However, this is not the case for styrene copolymerization with PEO macromonomers in which only 10% PEO macromonomer was copolymerized [131]. In contrast, it was confirmed that 100% of PEO macromonomers were copolymerized for the MM A and BMA dispersion copolymerization [132, 133]. 

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