Polymerization, CTAB

We distinguish four types of surfactant. Anionic (e.g. SDS), cationic (e.g. CTAB), nonionic (e.g. alkylethylene glycols), and zwitterionic (e.g. phosphatidyl choline). Beside conventional surfactants, Gemini, bolafarm, oligomeric and polymeric surfactants become more and more important. 

Baxendale, Evans and coworkers reported in 1946 that the polymerization of methyl methacrylate (MMA) in aqueous solution was characterized by homogeneous solution kinetics, i.e. where mutual termination of free radicals occurred, in spite of the fact that the polymer precipitated as a separate phase. Increases in the rates of polymerization upon the addition of the surfactant cetyl trimethyl ammonium bromide (CTAB) were attributed to the retardation of the rate of coagulation of particles, which was manifested in a reduction in the effective rate constant for mutual termination,... 

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. 

In a typical reaction, initial concentrations of NIPAM, styrene, CTAB, KPS, and TMEDA are 0.16 M, 5.24 mM, 17.3 mM, 0.34 mM, 0.67 mM, respectively. The styrene content (3.9 mol %) of the resultant segmented PNIPAM-seg-St copolymer can be determined by pyrolysis gas chromatography. The average degree of polymerization between two styrene segments can be over a wide range, mainly depending on the initial NIPAM/styrene ratio. The resultant copolymer can be purified and fractionated by a number of successive dissolution-and- precipitation cycles in a mixture of extremely dried... 

In emulsions, amine hydrochloride constitutes the aqueous phase and acrylic ester the organic phase. Cetyltrimethylanunonium bromide (CTAB) or span/twin (S/T)-type surfactants are used for emulsion polymerization. Solid dispersants such as talc and colloidal silica are often used to stabilize emulsions which are difficult to stabilize with usual surfactants. HydrophiUc colloidal silica (Aerosil 200) drastically increases the stability of some emulsions provided high amounts (up to 10%) of Aerosil are used. Random copolymers containing 10% hydroxyl groups can be used as polymeric dispersants for preparing w/o emulsions. 

The vast majority of miniemulsion polymerizations reported in the literature have been stabilized with anionic surfactants, probably because of the widespread application of anionic surfactants in macroemulsion polymerization, and due to their compatibility with neutral or anionic (acid) monomers and anionic initiators. However, Landfester and coworkers [70, 71] have used the cationic surfactants cetyltrimethyl ammonium bromide (CTAB) and cetyltri-methyl ammonium tartrate for the production of styrene miniemulsions. They report that these surfactants produce similar particle sizes to anionic surfactants used at the same levels. Bradley and Grieser [72] report the use of dodecyltrimethyl ammonium chloride for the miniemulsion polymerization of MMA and BA. 

The polymerization of styrene in Winsor I-like systems by semi-continuous feeding of monomer stabilized by either DTAB, TTAB or CTAB has been systematically investigated by Gan and coworkers [69a]. Rather monodisperse polystyrene microlatexes of less than 50 nm with molecular weights of over one million were obtained at a polymer/surfactant weight ratio of 14 1. The Winsor I-like (micro)emulsion polymerization of styrene stabilized by non-ionic surfactant and initiated by oil-soluble initiators has also been reported very recently [69b]. The sizes of the large monomer-swollen particles decreased with conversion and they merged with growing particles at about 40-50% conversion. 

Nanoparticles of PS (M =1.0xl0 -3.0xl0 mol ) microlatexes (10-30 nm) have also been successfully prepared from their respective commercial PS for the first time [75]. The dilute PS solutions (cyclohexane, toluene/methanol or cyclohexane/toluene) were induced to form polymer particles at their respective theta temperatures. The cationic CTAB was used to stabihze th microlatexes. The characteristics of these as-formed PS latex particles were quite similar to those obtained from the microemulsion polymerization of styrene as reported in literature. These microlatexes could also be grown to about 50 nm by seeding the polymerization of styrene with a monodisperse size distribution of D /Djj=1.08. This new physical method for preparing polymer nano-sized latexes from commercial polymers may have some potential applications, and therefore warrants further study. 

Table 5. Reduction potentials ( red), excited triplet (singlet for NTAB and CTAB) energies, free energy changes (AGet) and quenching rate constants (A q, measured for n-butyltriphenylborate anion) of the light absorbing molecules used as initiators of free-radical polymerization.

The advantage of the emulsion method of polymerization lies in the wide choice of catalysts that may be used. In mass reactions such agents as oil-soluble peroxides, diazoamine derivatives, and diazothioethers must be used. Emulsification permits the use of water-soluble catalysts, as well as mixtures of these with oil-soluble materials. The reaction then occurs at the monomer-water interface, with a high probability that the free radicals formed will immediately react with the adjacent monomer. Use of quarternary emulsifiers such as CTAB or Emulsol 607 in such... 

Preparation and Polymerization of (0/W) Cetyltrimethylammonium Bromide Microemulsion (CTAB-yE) (5-7). An oil in water pE composed of 1.0 g of CTAB, 0.5 g of hexanol, and 1.0 g of 50% styrene-divinylbenzene in 50 mL of water was carefully prepared by slowly adding the water to a stirred mixture of the other components to yield a slightly bluish clear solution. A 0.1% solution (w/w) of initiator AIBN (based on monomer) was then solubilized in the system followed by removal of 02 (by gentle N2 bubbling for 5 min), and finally the system was heated in an oil bath (50°C) until complete polymerization was achieved as determined spectrophotometrically. Proper dilution with water was then made to give a 0.01 M CTAB-P-pE solution P-pE indicates polymerized microemulsion. 

Polymerized Microemulsion Systems. A microemulsion of styrene and divinylbenzene with CTAB + hexanol may readily be made, and subsequently polymerized to form a polymerized microemulsion (5,6,7). This system exhibits two sites of solubilisation for photosystems such as pyrene, one in the surfactant skin layer, and the other in the polymerized styrene-divinylbenzene core. Photochemical reactions induced in the surfactant skin are very similar to those observed in micelles and are not immediately of concern to us at this stage. However, photochemical reactions induced in the rigid polymerized core are of interest, as they essentially confine reactants to a small region of space where movement is restricted as compared to a fluid non-polymerised microemulsion or a micelle. Thus, diffusion is minimised, and it may be possible to investigate reactions which occur over a distance rather than reactions which occur by diffusion. In order to eliminate reactions in the surfactant skin a microemulsion can be constructed which contains cetyl pyridinium chloride in place of CTAB. The pyrene that resides in the surfactant skin layer is immediately quenched by the pyridinium group following excitation. 

Chemicals. N,N-dimethyldodecylamine-N-oxide (DDAO) and cetyl trimethyl-ammonium bromide (CTAB) were purchased from Fluka Chemical Corp. Cetyl pyridinium chloride (CPC) and polyoxyethylene nonyl phenyl ether, with an average degree of polymerization of 10 (NP(EO)2q) were obtained from Hexacel Corp. and... 

The impact of different surfactants (SDS, DOSS, CTAB and hexadimethrine bromide, bile salts °), nonionic and mixed micelles, and additives (neutral and anionic CDs," " tetraalkylammonium salts, organic solvents in EKC separations has been demonstrated with phenol test mixtures. In addition, phenols have been chosen to introduce the applicability of more exotic EKC secondary phases such as SDS modified by bovine serum albumin, water-soluble calixarene, " starburstdendrimers, " " cationic replaceable polymeric phases, ionenes, amphiphilic block copolymers,polyelectrolye complexes,and liposome-coated capillaries. The separation of phenols of environmental interest as well as the sources and transformations of chlorophenols in the natural environment have been revised. Examples of the investigation of phenols by EKC methodologies in aquatic systems, soil," " and gas phase are compiled in Table 31.3. Figure 31.3 illustrates the electromigration separation of phenols by both CZE and EKC modes. 

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