Amphiphilic diblock copolymer

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... 

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). 

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). 

The dispersion polymerization of styrene in supercritical CO2 using amphiphilic diblock copolymers to impart steric stabilization has been investigated. Lipophilic, C02-insoluble materials can be effectively emulsified in carbon dioxide using amphiphilic diblock copolymer surfactants. The resulting high yield (> 90%) of polystyrene is obtained in the form of a stable polymer colloid comprised of submicron-sized particles (Canelas et al., 1996). 

Zhang, X., J. K. Jackson, and H. M. Burt. 1996a. Development of amphiphilic diblock copolymers as micellar carriers of paclitaxell.nt. J. Pharm.132 195-206. 

However, polyphosphazene micelles are known. These are formed by sonication of aqueous suspensions of amphiphilic diblock copolymers—macromolecules in which one block is hydrophilic and the other is hydrophobic. Both blocks may be phosphazene-based, or one block can be a phosphazene and the other an organic polymer. Polymers of this type are assembled via the living cationic polymerization discussed earlier. The polymers shown as 3.95-3.99 illustrate the range of structures that have been studied and the size of the micelles formed.237-240... 

The formation of polymersomes from water in- oil-in-water drops. Initially, a double emulsion consisting of single aqueous drops within drops of a volatile organic solvent ( oil ) is prepared using a microcapillary device. Amphiphilic diblock copolymers dissolved in the middle phase assemble into monolayers at the oil-water interfaces. Evaporation of the solvent then leads to the formation of polymer bilayers (polymersomes). 

An amphiphilic diblock copolymer spread from a solution of organic solvent onto the water surface, normally were found to form a stable monolayer [129], The surface monolayer has been successfully transferred onto a substrate by the Langmuir - Blodgett technique. Some times the surface pressure - area isotherms exhibited a plateau region, suggesting a structural change taking place on the water surface at specific pressures. 

One of the first step toward understanding the surface behavior of these systems is to check the monolayer formation at the air - water interface. A system as an amphiphilic diblock copolymer for example, from the initial studies of them,... 

So far, micelles and vesicles of amphiphilic block copolymers with two different blocks have been described. In this section the work on amphiphilic block copolymers and block copolyampholytes composed of three different blocks will be reviewed. Much less work has been carried out on these systems and there are less systematic studies available. Focus will be laid on block copolymers with at least one polyelectrolyte block. While in the case of amphiphilic diblock copolymers questions like the influence of block lengths on the size of micellar aggregates have been studied in great detail, in ternary block copolyampholytes other properties have attracted greater interest, such as the influence of the block sequence on the solution properties and aggregate formation. 

Jiang Y, Chen T et al (2005) Effect of polydispersity on the formation of vesicles from amphiphilic diblock copolymers. Macromolecules 38 6710-6717... 

From this method, telechelic and amphiphilic diblock copolymers containing hydrophilic methacrylic acid sequences and hydrophobic styrene segments were obtained [205]. 

Su W, Han K, Luo Y et al. (2007) Pormation and photoresponsive properties of giant microvesicles assembled from azobenzene-containing amphiphilic diblock copolymers. Mac-romol Chem Phys 208 955-963... 

Wang G, Tong X, Zhao Y. (2004) Preparation of azobenzene-containing amphiphilic diblock copolymers for light-responsive miceUar aggregates. Macromolecules 37 8911-8917... 

The masked disilene procednre was nsed by Saknrai and co-workers to synthesize two samples of diblock copolymers of 1,1-dimethyl-2,2-dihexylsilane (MHS) and 2-(trimethylsilyloxy)ethyl methacrylate, which differed only in the relative lengths of their blocks. Hydrolysis of the trimethylsilyl protecting groups gave the corresponding amphiphilic diblock copolymers, poly(l,l-dimethyl-2,2-dihexyldisilene)-fe-poly(2-hydroxyethyl methacrylate) (PMH S-fc-PHEMA), depicted in Pig. 22 [48]. 

Fig. 22 Synthesis of amphiphilic diblock copolymers of poly(l,l-dimethyl-2,2-dihexyldisilene)-i>-poly(2-hydroxyethyl methacrylate). Reproduced with permission from [48] Sanji et al. (1999) Macromolecules 32 5718. American Chemical Society...

The amphiphilic diblock copolymer has been synthesized in the group of Stephan Forster (University of Hamburg). It consists of a polystyrene sulfonic acid) block of 144 monomers and a hydrophobic poly(ethylethylene) block of 136 monomers. It is called PSSH-PEE in the following discussion. Synthetic details are described elsewhere [18]. The films of the diblock copolymers are made from pure aqueous polymer solutions without any further surfactant. The concentration of the diblock copolymers is given in gL 1-... 

Amphiphilic diblock copolymers act as a surfactant and stabilize free-standing films. They are assumed to adsorb at the interface by analogy with low-molecular-weight surfactants The hydrophobic part is collapsed at the interfaces and the hydrophilic part is directed towards the film core (Fig. 2a). Investigations of the structure at a single liquid interface (air/water) show that the amphiphilic diblock copolymers present polymer brushes which are anchored by the hydrophobic block at the interface [22, 23], This structure is also assumed at the film surfaces. Fig. 3 shows the disjoining pressure iso-... 

Block co-polymers Self-assembled mask Parallel, fast, small size, no resist, solvents, etc. Material constraints, no long-range order, cleanliness New amphiphilic, diblock copolymers, multifunctional structures... 

Amphiphilic diblock copolymers based on 2-oxazoline derivatives with chiral diphosphine 187 were prepared (Scheme 3.61) and used in the asymmetric hydrogenation of methyl (Z)-(z-acelarnido cinnamate 188 in water to give the (R)-phenylalanine derivative 189 in 85% ee [124]. The polymeric catalyst could be recycled. This result illustrated the advantages of using amphiphihc copolymers for the efficient transformation of a hydrophobic substrate in water. 

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