Polymer micelles hydrophilic segments

Hemoglobin and many enzymes are covalent polymers with a globular shape. This shape is enforced by the tendency of hydrophobic amino acids to form a hydrophobic droplet in aqueous solutions solubilized by hydrophilic side-chains around them. The same is true for synthetic block polymers made of hydrophobic and hydrophilic segments. " Spherical biopolymers thus usually appear as micelles, with a core made of organic material. Covalency allows the construction of fully organized micelles, e.g. dendrimeric spheres, where one half has a hydrophilic, the other a hydrophobic surface. Block polymers may not only form micelles, but they may also arrange to form vesicles which entrap a water volume. Such spheres have a thick polymer wall." Both the polymer micelles and vesicles can be removed from solution without collapsing. 

Block copolymers composed of a cationic segment and a hydrophilic segment spontaneously associate with polyanionic DNA to form block copolymer micelles. The distinct feature of the structure is that the core of the polyion complex between DNA and the polycation is coated by a layer of the hydrophilic polymer. The characteristic core-shell structure endows the complex a high colloidal stability and reduced interaction with blood components [12]. 

In addition, polymer micelles have been demonstrated to be more stable and also have a significantly lower cmc than surfactant micelles. Further discussion of surfactant micelles is beyond the scope of this review, and, instead, the reader is directed to a recent review article by Armes. In fact, the polymer building blocks need not be amphiphilic and such phase-separated nanostructures can be formed from completely hydrophobic or lipophilic diblock copolymers that contain two segments with differing solubility (such as polystyrene- -polyisoprene) and hence can undergo phase separation in selective solvents. One example of such completely hydrophobic phase-separated micelles are those reported by Wooley and coworkers, which can be obtained from toluene and acetone solutions of a [polystyrene-a/f-poly(maleic anhydride)]-fc-polyisoprene Iriblock. Conversely, inverse structures are also accessible and are known as reverse micelles. These can be formed by adding a nonsolvent for the hydrophilic block to afford the opposite of a conventional micelle, for which the hydrophilic core is surrounded by a hydrophobic shell in a hydrophobic surrounding media. There have been a handful of reports on the application of these reverse micelles, for example, as nanoreactors and for the extraction of water-soluble molecules. ... 

Most simply, polymer micelles are formed spontaneously via the solution-state self-assembly of amphiphilic multiblock copolymers, which consist of hydrophobic and hydrophilic chain segments (Figure 17.1) (O Reilly et a/., 2006). 

Since 2000, hving radical polymerization has allowed the synthesis of block copolymers with controlled molecular weight and a narrow MWD using many monomer combinations, as shown in Figs. 4 and 5 [71]. Block copolymerization with hydrophilic or thermoresponsive acrylamide derivatives 11,12 has been examined [72-77]. Block copolymers having hydrophiUc segments such as PNlPAM-poly[(meth)acrylic acid] (13) [78-80], PNIPAM-poly(sulfonic acid) (14) [81,82], PNIPAM-poly(2-hydroxyethylacrylate) U5) [83], and PNIPAM- poly](2-dimethylamino)ethyl methacrylate]-co-poly(2-hydroxyethyl methacrylate) (16) [84] were prepared. These formed polymer micelles in response to variation of the temperature. For example, Muller et al. have synthesized PNIPAM-poly(acrylic acid) with low polydis-... 

During the last 14-year period, a significant increase in the number of published papers about micellar systems has been observed (Figure 9.5). Nanostructures from amphiphilic functional copolymers such as di-, tri-, and multiblock copolymers may form micelles spontaneously in aqueous solution at concentrations above the critical micelle concentration. This phenomenon occurs due to hydrophobic interactions, steric repulsion, and solvation. The balance between hydrophobic and hydrophilic segments and a narrow size distribution of the polymers are important parameters to ensure higher efficiency of the nanocarrier [49]. 

Block copolymers that contain segments with different characteristics, such as hydrophobic and hydrophilic blocks, can form phase-separated solid systems, or they can be employed to form micelles or vesicles in aqueous media. Considering that the lengths of each polymer block can be controlled, the opportunities for property variations are almost infinite through such systems. 

The comonomer distribution can be alternated by controlling the synthesis conditions, such as the copolymerization at different reaction temperatures at which the thermally sensitive chain backbone has different conformations (extended coil or collapsed globule). In this way, hydrophilic comonomers can be incorporated into the thermally sensitive chain backbone in a more random or more segmented (protein-like) fashion. On the other hand, short segments made of hydrophobic comonomers can be inserted into a hydrophilic chain backbone by micelle polymerization. One of the most convenient ways to control and alternate the degree of amphiphilicity of a copolymer chain, i.e., the solubility difference of different comonomers in a selective solvent, is to use a thermally sensitive polymer as the chain backbone, such as poly(N-isopropylacrylamidc) (PNIPAM) and Poly(N,N-diethylacrylamide) (PDEA). In this way, the incorporation of a hydrophilic or hydrophobic comonomer into a thermally sensitive chain backbone allows us to adjust the degree of amphiphilicity by a temperature variation. 

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