Poloxamers structure

It is important to recognize that the following analytical methods essentially determine EO-PO ratio ( H NMR, IR, cleavage methods) or even simply alkylene oxide content (compleximetric methods) of the analyte, and as such are not specific quantitative or qualitative methods for poloxamers, since EO-PO copolymers of a different structure (for instance, random copolymers, or PO-EO-PO block copolymers) may respond to the methods in a way indistinguishable from poloxamers. The principal technique that permits definitive identification of a sample as a poloxamer is C NMR, which allows structural details, such as the distribution of EO and PO units along the polymer chain, to be elucidated [10]. 

Micelle-like junction zones are formed by methylcel-lulose and polyethyloxylene polypropyloxylene block copolymers (poloxamers). Although the polymers differ in chemical structure, both have hydrophobic regions in their chains the di- and trimethyl-o-glucose residues of methylcellulose and the polypropyloxylene block of poloxamer. Another feature common to the... 

At relatively high concentrations (>20%), poloxa-mers form thermoreversible gels however, they gel on heating rather than cooling The amphiphilic nature supports the gelling mechanism of poloxamers, where micelle-like junction zones form at or above room temperature. The junction zones consist of large populations of micelle-like structures, which apparently form a viscous, liquid crystalline phase. Poloxamers can also form gels in dilute hydroalcoholic solutions. 

Besides the asymmetry between monolayers in cytomembranes, two of the more obvious differences between cubic phases and membranes are the unit cell size and the water activity. It has been argued that tire latter must control the topology of the cubic membranes [15], and hence tiiat the cubic membrane structures must be of the reversed type (in the accepted nomenclature of equilibrium phase behaviour discussed in Chapters 4 and 5 type II) rather than normal (type I). All known lipid-water and lipid-protein-water systems that exhibit phases in equilibrium with excess water are of the reversed type. Thus, water activity alone cannot determine the topology of cubic membranes. Cubic phases have recently been observed with very high water activity (75-90 wt.%), in mixtures of lipids [127], in lipid-protein systems [56], in lipid-poloxamer systems [128], and in lipid A and similar lipopolysaccharides [129,130]. 

Pluronic block copolymers (also known by their nonproprietary name poloxamers ) consist of ethylene oxide (EO) and propylene oxide (PO) blocks arranged in a basic A-B-A structure E0 , -P03,-E0 c. This arrangement results in an amphiphilic copol3rmer, in which the number of hydrophilic EO(x) and hydrophobic PO(y) units can be altered. The structural formula of Pliironic block copolymers is shown in Figure 1. Table 1 presents a list of selected Pluronic copol3nners... 

Pluronics, also known as poloxamers, are a class of synthetic block copolymers which consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), arranged in an A-B-A triblock structure, thus giving PEO-PPO-PEO (Fig. 11.7) (Batrakova and Kabanov 2008). They can be found either as liquids, pastes or solids (Ruel-Gariepy and Leroux 2004). Due to their amphiphilic characteristics (presence of hydrophobic and hydrophilic components), pluronics possess surfactant properties which allow them to interact with hydrophobic surfaces and biological membranes (Batrakova and Kabanov 2008). Being amphiphilic also results in the ability of the individual block copolymers, known as unimers, to combine and form micelles in aqueous solutions. When the concentration of the block copolymers is below that of the critical micelle concentration (CMC), the unimers remain as molecular solutions in water. However, as the block copolymer concentration is increased above the CMC, the unimers will self-assemble and form micelles, which can take on spherical, rod-shaped or lamellar geometries. Their shapes depend on the length and concentration of the block copolymers (i.e. EO and PO), and the temperature (Kabanov et al. 2002). Micelles usually have a hydrophobie eore, in this case the PO chains, and a hydrophilic shell, the EO ehains. 

FIGURE 13.8 Synthesis scheme of poloxamer networks from poloxamer macromer (A) and structure of chitosan/poloxamer SIPNs (B). (From Kim, I.Y. et al., Int. J. Pharm., 341, 35, 2007. With permission.)... 

When poloxamers are introduced into water at a low concentration, the soluble species are only isolated hydrated molecules. When the concentration of a copolymer is increased at constant temperature above the critical micelle concentration (CMC), micelles composed of several molecules are formed. In aqueous medium, such micelles are endowed with a core-shell structure composed of a hydrophobic core and a hydrophilic shell. The equilibrium is illustrated in Figure 4.2. 

The poloxamers are a series of copolymers composed of ty o polyoxyethylene blocks separated by a polyoxypropylene block (Lundsted, 1954 Lundsted, 1976 Schmolka, 1991). Although all poloxamers are composed of the same U o monomers, they vary in total molecular tveight, polyoxypropylene to polyoxyethylene ratio, and surfactant properties. The poloxamers all have the general structure ... 

Figure 9.4. Cryo-TEM images and proposed interpretation of the colloidal structures of lipid nanoparticles with different lipid matrixes and stabilized with poloxamer 188. NEmu-10% MCT emulsion, SLN glyceryl behenate. NLC-1.0 and NLC-3.0 glyceryl behenate/MCT mixture 9 1 and 7 3 (w/w). Reprinted from J. Control. Rel., Vol. 95, K. Jores et ah, Investigations on the structure of solid lipid nanoparticles (SLN) and oil-loaded solid Upid nanoparticles by photon correlation spectroscopy, field-flow fractionation and transmission electron microscopy, 217-227. Copyright (2004), with permission from Elsevier.

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