Polymerization of a Hydrophobic Monomer

In the conventional emulsion polymerization, monomer droplets are dispersed ip an aqueous phase containing micellar aggregates of surfactant. In this case, the dispersed phase represents a relatively small volume fraction of the system and the micellar aggregates constitute the sites of the polymerization process. In the gel(paste)-like emulsions employed here, the volume fraction of the dispersed phase can be as high as 0.99, and the cells of the concentrated emulsion lead to the polymerized latex particles. 

A typical procedure for the preparation of a concentrated emulsion is as follows. A small amount of an aqueous solution containing sodium dodecyl-sulfate (SDS) was placed in a single neck flask (100 ml capacity) equipped with a mechanical stirrer. Styrene containing the initiator Azobisisobutyronitrile (AIBN) was added to the aqueous solution, with stirring. The whole preparation process of the concentrated emulsion lasted for 10-15 min at room temperature. 

The prepared emulsions were transferred to preweighed centrifuge tubes of 15 ml capacity, which were sealed with rubber septa. A mild centrifugation (1500 rpm, less than 1 min) was employed to pack the concentrated emulsions into the tubes when necessary. Polymerizations were conducted in a temperature-controlled water bath in the presence of air. After polymerization, the concentrated emulsion was dispersed by adding water to the tube and agitating with a spatula. The aqueous system was then poured into methanol and the precipitated polymer was separated by filtration. 

Unlike the conventional emulsion polymerization, in which the polymer latexes grow during polymerization, the size and shape of the latexes in concentrated emulsions is principally determined by the preparation of the gel. 

The bulk phase, reflected in the tail of the GPC curves, increased with NaCl concentration because the double-layer repulsion between the emulsion cells becomes weaker with increasing salt concentration. As a result, the conversion and the weight fraction of polymer formed in the gel phase decrease with increasing NaCl concentration (Fig. 23). 

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The uniform polymeric microspheres in submicron-or micron-size range can also be prepared as seed particles by the soapless emulsion or dispersion polymerization of a hydrophobic monomer like styrene. The uniform seed particles are swollen with the organic phase including functional comonomer, monomer, and oil-soluble initiator at a low temperature in an aqueous... 

M.P. Krafft, L. Schieidknecht, P. Marie, F. Giulieri, M. Schmutz, N. Poulain, E. Nakkache, Fluorinated vesicles allow intrabilayer polymerization of a hydrophobic monomer, yielding polymerized microcapsules, Langmuir 17 (2001) 2872-2877. 

For CRP based on a reversible termination process, the strategy was to synthesize water-soluble macroinitiators (with either an alkoxyamine or an alkyl halide chain-end) by direct CRP or by post-functionalization, before using them as both initiators and stabilizers in emulsion polymerization of a hydrophobic monomer. 

As seen in Section 3.10.3.3.2, the self-assembly can also occur in situ during the surfactant-free polymerization of a hydrophobic monomer initiated with water-soluble (macro) alkoxyamines in aqueorrs dispersed media such as emul-... 

Three arm amphiphilic star block copolymers of IBVE and 2-hydroxyethyl vinyl ether (HOVE) were prepared using the trifunctional initiator 8 with sequential cationic polymerization of two hydrophobic monomers, IBVE and AcOVE. Subsequent hydrolysis of the acetates led to the hydrophilic poly(HOVE) segments [38]. Two types of stars were prepared depending on which monomer was polymerized first three arm star poly(IBVE-h-HOVE), with the hydrophobic part inside and three arm star poly(HOVE-h-IBVE), with the hydrophobic part outside. When IBVE was polymerized first, the experimental conditions were the same as described in Sect. 2.2.1. After reaching quantitative monomer conversion, AcOVE was added and temperature was raised from 0 to 40 °C to accelerate the reaction since this monomer is less reactive than IBVE. When starting with AcOVE as a first block, both polymerizations were carried out at 40 °C. SEC analysis showed that MWDs were narrow for the two steps whatever the se-... 

NIPAAm possesses great potential for medical use due to an LCST close to body temperature. It is also important to note that the LCST of NIPAAm can be altered by copolymerization of other monomers to the poly(NIPAAm) backbone. The addition of a hydrophobic monomer leads to a decrease in the LCST, whereas the addition of a hydrophilic monomer results in an increase in LCST. An example of copolymerization of a hydrophobic monomer to poly(NIPAAm) has been done by Lee et al. (2006). Through free radical polymerization, the group has added... 

The production of such particles usually results fi-om the emulsion copolymerization of a hydrophobic monomer, such as styrene with a water-soluble monomer, such as acrylic acid. Differences in water solubility of the two monomers along with disparate reactivity ratios led to the preparation of particles having a core-shell structure with the hydro-phobic polymer in the core and the water-soluble polymer in the shell layer. It was also found that precipitation polymerization of alkyl(meth)acrylamide, such as N-isopropyl... 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulson polymerization can also be carried out as an inverse emulsion polymerization [Poehlein, 1986]. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively. Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery). 

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. 

An example of the first approach is the integration of hydrogels into nanostructured silica films by addition of a suitable monomer (e.g., methyl methacrylate, /V-isopropyl acrylamide, etc.) and an initiator for radical polymerization to a solution containing a structure-directing surfactant and a prehydrolyzed silica precursor. During self-assembly, the monomers partition within the hydrophobic core of the surfactant mesophase postsynthesis polymerization (for instance, by UV treatment) followed by solvent washing to remove the surfactant template yields a polymer-silica nanohybrid. 

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