Diblock amphiphiles

Theoretical and experimental investigations dealing with the polyelectrolyte adsorption and the structure of polyelectrolyte liquid films have been carried out for more than twenty years in order to give a better understanding of emulsion stabilization mechanisms by polyelectrolytes on a microscopic scale. In this section, we describe the structure of liquid films composed of three kinds of polyelectrolytes the diblock amphiphilic polyelectrolytes, which lead to the formation of brushes, the homopolyelectrolytes, which form a semicrystalline structure within the films, and finally the amphiphilic random polyelectrolytes. Special attention is given to the charged monomer layer thickness at interfaces. 

Nowak A.P., Breedveld V., Pakstis L., Ozbas B., Pine D.J., Pochan D., and Deming T.J. Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles. Nature, 417, 424, 2002. 

Yang CY, Song BB, Ao Y et al (2009) Biocompatibility of amphiphilic diblock copolypeptide hydrogels in the central nervous system. Biomaterials 30 2881-2898... 

Kim W, Thevenot J, Ibarboure E et al (2010) 5elf-assembly of thermally responsive amphiphilic diblock copolypeptides into spherical micellar nanoparticles. Angew Chem Int Ed 49 4257 260... 

The facility to introduce well-defined chain ends has been used to prepare star polymers557 and diblocks via reaction with macromolecular aldehydes.558 The synthesis of amphiphilic star block copolymers has also been described using a cross-linking agent.559 560 A similar strategy has recently... 

Finally, we have designed and synthesized a series of block copolymer surfactants for C02 applications. It was anticipated that these materials would self-assemble in a C02 continuous phase to form micelles with a C02-phobic core and a C02-philic corona. For example, fluorocarbon-hydrocarbon block copolymers of PFOA and PS were synthesized utilizing controlled free radical methods [104]. Small angle neutron scattering studies have demonstrated that block copolymers of this type do indeed self-assemble in solution to form multimolecular micelles [117]. Figure 5 depicts a schematic representation of the micelles formed by these amphiphilic diblock copolymers in C02. Another block copolymer which has proven useful in the stabilization of colloidal particles is the siloxane based stabilizer PS-fr-PDMS [118,119]. Chemical... 

Fig. 6. Chemical structures of amphiphilic diblock copolymeric stabilizers PS-6-PFOA (top) and PS-6-PDMS (bottom) [118]...

Arimura H, Ohya Y, Ouchi T (2005) Formation of core-shell type biodegradable polymeric micelles from amphiphilic poly(aspartic acid)-Wock-polylactide diblock copolymer. Biomacromolecules 6 720-725... 

Nolte and coworkers reported on the formation of micelles with a helical superstructure from AB amphiphilic diblock copolymers prepared from an amine-end-capped polystyrene that was used as the initiator for the polymerization of various dipeptide-derived isocyanides (Fig. 16). 

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

Scheme 6.10 Synthesis of the amphiphilic star polymers with a hyperbranched core and amphiphilic diblock graft arms [59].

A polymer is considered to be a copolymer when more than one type of repeat unit is present within the chain. There are a variety of copolymers, depending on the relative placement of the different types of repeat units. These are broadly classified as random, block, graft, and alternating copolymers (see Fig. 2.1 for structural details Cheremisinoff 1997 Ravve 2000 Odian 2004). Among these stmctures, block copolymers have attracted particular attention, because of their versatility to form well-defined supramolecular assemblies. When a block copolymer contains two blocks (hydrophobic and hydrophilic), it is called an amphiphilic diblock copolymer. The immiscibility of the hydrophilic and lipophilic blocks in the polymers provides the ability to form a variety of assemblies, the stmctures and morphologies of which can be controlled by tuning the overall molecular weight and molar ratios of the different blocks (Alexandridis et al. 2000). 

In addition to utilizing natural ODNs in block copolymers, ODN mimics have also been studied using polymers with the capability of forming hydrogen bonds. Bazzi and colleagues (Bazzi and Sleiman 2002 Bazzi et al. 2003) have reported diblock and triblock amphiphilic polymers with diamidopyridine (DAP) and dicarboximide moieties as molecular recognition units and studied their micelle formation properties (Chart 2.4). 

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

Nieiengarten J-F, Eckert J-F, Rio Y, del Pilar Carreon M, Gallani J-L, GuiUon D. Amphiphilic diblock dendrimers synthesis and incorporation in Langmuir and Langmuir-Blodgett films. J Am Chem Soc 2001 123 9743-9748. 

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