Block-copolymers, amphipathic

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). 

Micellar systems based on amphipathic block-copolymers have gained most attention as intravenously administered drug carrier systems over the years. These block-copolymers are composed of a hydrophilic PEG block and a hydrophobic block based on doxorubicin conjugated poly(aspartic acid) or poly(/i-bcnzyl L-aspartate). 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

Piirma I. Synthesis of amphipathic polymeric material block copolymers. In Polymeric Surfactants, Surfactant Science Series. Vol. 42. New York Marcel Dekker, 1992 17-34. 

Fig. 2.2. Schematic lepreseotation of the steric stabilization of a colloidal particle by a amphipathic block copolymer (only some of the stabilizing chains are shown).

We will start our discussion with the grafting to approach, which may be characterized as the simplest. Depending on the nature of the surface of the porous membrane that we want to decorate with a brush, we must choose an end-functionalized polymer chain (or a highly asymmetric block copolymer of an amphipathic nature) that may strongly adsorb from its functionalized part on the inner surface. Then in principle we may use the same method as for the case of a flat surface, i.e., dip the membrane in a solution that contains these polymers and then wait for the full formation of the brush. However, it has been shown [21] that self-assembly inside the pores is prohibited when the ratio of pore diameter to chain radius of gyration... 

Noguchi, S. Nagano, S. Onuma, Y. Matsushita, Y Seki, X. Optical recording material using amphipathic block copolymer phase-separated monolayer, and its manufacturing method. Jpn. Kokai Xokkyo Kobo JP 2006312253, 2006 Chem. Abstr. 2006, 145, 497726. 

Amphipathic molecules such as phospholipids placed in an aqueous solution can also form micelles, which are small spherical stmctures with the hydrophilic heads of the molecules on the surface and a dense core region consisting of the hydrophobic tails (Fig. 28.8). Polymeric micelles are typically 5-100 nm in diameter. They are spontaneously formed above a certain critical concentration, the critical micelle concentration (CMC), by surfactants or block copolymers composed of hydrophilic and hydrophobic regions. The stability of the micelles depends on the CMC. Micelles with high CMC are unstable they may dissociate into unimers and their content may precipitate out. By contrast, micelles with low CMC are more stable. As micelles must be stable upon dilution with a large volume of blood in vivo, their CMC must be very low for them to be effective dmg carriers. The... 

The best steric stabilizers are found empirically to be amphipathic block or graft copolymers. One of the comonomers should generate a homopolymer that is nominally insoluble in the dispersion medium whereas the other should polymerize to form a polymer that is soluble in the dispersion medium. 

Experience shows that the most effective stabilizers are amphipathic block or graft copolymers, which contain anchor groups that are strongly held to the surface and are insoluble in the medium, and stabilizing chains that are soluble in the medium and extended from the surface. High surface coverage is essential. 

The best steric stabilizers are amphipathic block or graft copolymers such as poly(oxyethylene lauryl ether) (Mr 1200). Commercial nonionic surfactants are classified according to the hydrophilic-lipophilic balance (HLB), which scales the relative solubilities of the two components in aqueous and nonaqueous media. The need for the anchor part of the stabilizing molecule can be eliminated if the stabilizing moieties can be covalently bonded to the latex particles. 

As mentioned above, dilute solutions of copolymers in solvents that are good for both components exhibit similar behaviour to homopolymer chains. However, in a selective solvent, whereby the medium is a good solvent for one component, say A, and a poor solvent for the second component B, one part of the amphipathic block or graft separates as a distinct phase, whereas the other stays in solution. The insoluble portion of the amphipathic copolymer will aggregate reversibly to form micelles. It is believed that the polymeric micelles are spherical [4]. The critical micelle concentration of these block and graft copolymers is usually very low. 

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