Polymerization micellar

The square-root dependence of [AVA] supports the conventional (instantaneous) bimolecular termination of the growing radicals and/or the higher nuclea-tion activity of the present system compared to the micellar model [9,10]. The high rate of polymerization resembles the polymerization in particles and was discussed in terms of the micellar polymerization considering that the kt was extremely low. The linear dependence of Rp on the macromonomer concentration (the first-order dependence of [tC4-(EO)24-MA] or [tC4-(EO)24-VB]) was attributed to the linear dependence of the number of micelles on the macromonomer... 

This result supports the micellar polymerization mechanism, i.e., the stronger hydrophobic intermolecular interactions between PEO molecules favors the... 

Ito s group [83] reported the micellar polymerization mechanism was operative during the radical polymerization of PEO macromonomers in cyclohexane and water under similar reaction conditions. The reaction medium has an important effect on the polymerization behavior of macromonomers. Cyclohexane was chosen as a nonpolar type of solvent. The polymerization was found to be independent of the lengths of p-alkyl group (R) and the PEO chain in benzene. On the other hand, the rate of polymerization in cyclohexane increased with increasing number of EO units. This may be attributed to the formation of aggregates (micelles) and/or compartmentalization of reaction loci,i.e., polymerization in distinct aggregates (polymer particles). The C12-(EO)14-MA macromonomer polymerized faster in bulk than in benzene but far slower than in water. 

Hydrophobically modified PNIPAM-seg-St segmented copolymers can be prepared by evenly inserting short styrene segments (stickers) into a PNIPAM chain backbone using the micellar polymerization. In this method, hydrophobic styrene (St) monomers is first solubilized inside small micelles made of surfactant, hexadecyltriethylammonium bromide (CTAB). KPS and TMED can be used to initiate the polymerization of hydrophibc NIPAM monomers dissolved in the continuous aqueous medium. When the free radical end of a growing PNIPAM chain enters a micelle, styrene monomers entrapped inside start to react to form a short hydrophobic segment (sticker). In this way, the coming-in-and-out of different micelles of each free-radical chain end can connect short styrene blocks on a PNIPAM chain. 

There are a number of different factors which may affect the level of uptake and the energetics of adsorption from solution the chemistry and electrical properties of the solid surface and the molecular/micellar/polymeric structure of the solution must all be taken into account. Whenever possible, a study of both adsorption isotherms and enthalpies of displacement is worthwhile, but it is often necessary to complement these measurements with others including electrophoretic mobilities, FI7R spectra-and various types of microscopy. 

One approach to the formation of CEP nanoparticles is through the use of micellar polymerization and microemulsion techniques. The advantage of such an approach is that the particle size can be predehned by establishing the appropriate size and geometry of the templating micelle (Figure 2.16). 

Particles of the enzymatically synthesized phenolic polymers were also formed by reverse micellar polymerization. A thiol-containing polymer was synthesized by peroxidase-catalyzed copolymerization of p-hydroxythiophenol and p-ethylphenol in reverse micelles [70], CdS nanoparticles were attached to the copolymer to give polymer-CdS nanocomposites. The reverse micellar system was also effective for the enzymatic synthesis of poly(2-naphthol) consisting of qui-nonoid structure [71], which showed a fluorescence characteristic of the naphthol chromophore. Amphiphilic higher alkyl ester derivatives were enzymatically polymerized in a micellar solution to give surface-active polymers at the air-water interface [72, 73]. 

In the model system, mercaptoethanol is required to prevent production of very high MW PLMA—a MW higher than can be reasonably assumed to occur during copolymerization with AAm. If the model of micellar polymerization of LMA into AAm discussed in the beginning of this chapter is correct, this need for a CTA can be explained as follows. From the point of view of LMA polymerization, AAm acts to end the propagation of the PLMA radical in one micelle and, after radical propagation in the aqueous phase (during which time AAm is added) to initiate polymerization of the LMA in another micelle. If it is desired to keep the LMA from different micelles as separate polymer molecules, then a CTA is necessary terminate the PLMA radical in each micelle. Without such termination the PLMA radical appears to transfer between micelles. 

Novel polymerization techniques were used to synthesize new macro-molecules that consisted of a water-soluble backbone unth small amounts of hydrophobic functionality. Micellar polymerization is based on the capability of surfactant micelles to solubilize hydro-phobic molecules into an aqueous medium it was used to copolymerize acrylamide and hydrophohically substituted acrylamide monomers. A critical aspect of these polymerizations was the incorporation of the hydrophobic monomer into the water-soluble polymers. A method that used the UV chromophore of newly synthesized N-aryl substituted acrylamides was developed to quantify incorporation at the low levels of hydrophobe normally used about 1 mol %). The synthesis of the substituted acrylamides, the UV technique, and results obtained with it are discussed. 

The RAM polymers were synthesized with the micellar polymerization technique, in which the hydrophobic monomer was solubilized into the aqueous medium with a surfactant SDS. The polymerizations were initiated with potassium persulfate at 50 C. In some cases, a sample of the reaction... 

Originally, the hydrophobic monomer was thought to be relatively inaccessible, solubilized within the micelle. Apparently, the double bond of the hydrophobe is readily available to attack by the propagating radical moiety. More detailed study is required to fully understand incorporation during micellar polymerization. 

A critical aspect of the micellar polymerization of hydrophobically substituted acrylamides and water-soluble monomers is the incorporation of the hydro-phobic monomer into the resultant water-soluble polymers. Because of the low levels of hydrophobic monomer (<1 mol %), previous analyses were unsuccessful in measuring hydrophobe incorporation into the polymer. By using the UV chromophore of newly synthesized N-aryl-substituted acrylamides, a method was developed to quantify incorporation at the low levels of hydrophobe normally used. In all the cases examined, the hydrophobic monomer was incorporated at close to feed levels with complete conversion to polymer. Incorporation varied as a function of conversion therefore, the resultant polymers were compositionally heterogeneous. Research is continuing to gain a better understanding of hydrophobe incorporation. 

Polymer Synthesis. Copolymers of alkylacrylamide (R) and acrylamide (AM), which we called RAM, were prepared with a micellar polymerization technique (4). A micellar surfactant solution was used to disperse the hydrophobic alkylacrylamide monomer into an aqueous phase that contained acrylamide. The monomers were polymerized with a standard free-radical initiator (e.g., potassium persulfate) or a redox initiator to yield the desired random copolymer. Varied temperature and initiator concentrations were used to provide polymers of different molecular weights. Polymerizations were taken to essentially complete conversion. Compositions, in terms of hydrophobe level reported in this chapter, were based on amounts charged to the reactor. Further details on the synthesis and structure of these RAM polymers... 

Polymerizations performed with an emulsifier above its critical micelle concentration with all the monomer solubilized within the micelles and without any monomer present as emulsion droplets may be described as micellar polymerizations [62]. Although such systems can never produce a high yield of polymer per unit volume they are advantageous if it is desired to use photochemical initiation, these being transparent whereas emulsions are opaque. Micellar polymerizations can help to elucidate the mechanism of emulsion polymerizations. They are useful practically when it is desired to copolymerize hydrophilic and hydrophobic monomers to synthesize associative thickeners [63,64]. 

Both synthetic (biodegradable not) and natural polymers have been proposed and te.sted as drug delivery systems (104,105). It was Speiscr( 104.106) who first prepared spherical capsules made of a polymeric material capable of being loaded with active drugs by entrapment or adsorption. The method was based on the so-called micellar polymerization of. such monomers as acrylamide or methyl methacrylate. Since those first contributions, the number of monomers potentially useful in the field, as well as the polymerization routes employed have grown almost exponentially, and so have the fields of application in pharmaceutical dosage. 

Fig. 2 (a-c) Physical polymer-network cross-linking provided by mixed micelles in hydrogels formed via hydrophobic interactions in surfactant solutions. Mixed micelles are formed by aggregation of hydrophobic blocks of per-se hydrophilic polymers and surfactant alkyl tails, (b) Nonionic polymer and ionic surfactant gel system at the state of preparation. For clarity, charges are not shown, (c) Ionic polymer and oppositely charged surfactant gel system after extraction of free micelles, (d) Structure of the hydrophobic monomers used in the micellar polymerization... 

Alkvlacrvlamide-acrvlamide Polymers. Copolymers of alkylacrylamide (R) and acrylamide (AM), which we called RAM, were prepared using a micellar polymerization technique. ... 

Hydrophobically associating acrylamide based polymers were explored as a means of alleviating the salt sensitivity observed in the block systems. A micellar polymerization technique was developed to enable preparation of random copolymers of acrylamide and N-n-alkylacrylamide. When these copolymers were dissolved in an aqueous solvent, the hydrophobic groups associated to minimize their exposure to water. The hydrophobic associations provided an additional dimension to polymer molecular weight and chain expansion by ionic groups for the control of aqueous fluid rheology. 

Hydrophobically associating acrylamide copolymers can be prepared by micellar polymerization. These copolymers have short blocks of hydrophobic groups randomly distributed in the backbone. A recent paper reviews the major advances in this area (229). 

Copolymer synthesis suffers the same viscosity restrictions as the homopolymers when the comonomers are sufficiently water-soluble to produce water-soluble copolymers. When water-insoluble comonomers are used it is possible to resort to cosolvents with water or surfactants in micellar polymerizations to effect solubility to a limited degree, but this is only useful at low molecular weight and is undesirable environmentally. Higher molecular weight copolymers are usually made by emulsion, inverse emulsion, or suspension polymerization. 

There are other less widely used syntheses of associative polymers including acid monomer grafting to poly(ethylene oxide) (170,171), inverse emulsion polymerization (172-174), suspension polymerization (175), micellar polymerization (176,177), and solid-phase extrusion polymerization (178). 

Photopolymerization in micellar systems is useful for the synthesis of polymers displaying high molecular weights [57]. The model of photopolymerization used to describe a micellar polymerization does not differ from the one in bulk or solution photopolymerization [79]. 

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