Hydrophilic-hydrophobic block copolymer

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

Preparation of polyfethylene oxide) (PEO) and poly(arylene ether) based hydrophilic-hydrophobic block copolymer is of special interest because PEO has been proven to be particularly reliable and versatile for the surface modification of biomaterials. The first poly(ediylene oxide)-/ /oc/c-polysulfonc (PEO-fc-PSF) copolymers were reported by Aksenov et al.217 They employed diisocyanate chemistry to link hydroxy-terminated sulfone oligomers and polyfethylene... 

Hydrophilic ends, of surfactants, 22 725 Hydrophilic libers, 9 158 11 168 Hydrophilic flavor compounds, 11 551 Hydrophilic fumed silica, 22 368 Hydrophilic head group, 24 137 Hydrophilic-hydrophobic block copolymers, 20 485... 

Hydrophilic surfaces, 1 584—585 Hydrophilic/tunably hydrophilic/ hydrophobic block copolymers, 20 485-487 Hydrophilite, 5 785t Hydrophobic additives, in paper manufacture, 18 113 Hydrophobic alkoxy silanes, as silylating agents, 22 697... 

In ionic block copolymers, micellization occurs in a solvent that is selective for one of the blocks, as for non-ionic block copolymers. However, the ionic character of the copolymer introduces a new parameter governing the structure and properties of micellar structures. In particular, the ionic strength plays an important role in the conformation of the copolymer, and the presence of a high charge density leads to some specific properties unique to ionic block copolymers. Many of the studies on ionic block copolymers have been undertaken with solvents selective for the ionic polyelectrolyte block, generally water or related solvents, such as water-methanol mixtures. However, it has been observed that it is often difficult to dissolve ionic hydrophilic-hydrophobic block copolymers in water. These dissolution problems are far more pronounced than for block copolymers in non-aqueous selective solvents, although they do not always reflect real insolubility. In many cases, dissolution can be achieved if a better solvent is used first and examples of the use of cosolvents are listed by Selb and Gallot (1985). 

FIGURE 13.6 Preparation of hydrophilic-hydrophobic block copolymer 34 through the coupling reaction between precursor 32 and the amino ketal 33. Block copolymer 24 is designed to associate and then dissociate upon hydrolysis of the ketal to give the more hydrophilic block copolymer 35 [99]. 

Mori, H., Hirao, A., Nakahama, S., Senshu, K. Synthesis and surface characterization of hydrophilic-hydrophobic block copolymers containing poly(2,3-dihydroxypropyl methacrylate). Macromolecules 27, 4093 100 (1994)... 

Controlled structure polymeric betaines were reported for the first time only recently (286). The first examples were those prepared from the post-polymerization modification of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), and its block copolymers, which had been prepared imder GTP conditions (286,287). Initial reports detailed the modification of hydrophilic-hydrophobic block copolymers, but this was subsequently extended to the selective modification of diamino hydrophilic-hydrophilic block copolymers (276). 

Hydrophilic/Tunably Hydrophilic/Hydrophobic Block Copolymers. The next level of complexity for amphiphilic block copolymers are those comprised of one block which is permanently hydrophilic with the second block being tunably hydrophilic/hydrophobic, ie under a set of conditions A, the second block is readily water soluble and thus the block copolymer exists as unimers, but upon the application of a certain stimulus (change in pH or temperature for example) to condition B the second block becomes hydrophobic. Provided appropriate block copolymer compositions are employed this will lead to self-assembly. Additionally, such behavior is typically completely reversible. While there are now numerous examples of such smart self-assembly these types of systems have not been... 

Micellar-drug formulation is another delivery system based on hydrophilic-hydrophobic block copolymers that self-assemble in aqueous solution into micellar particles with a hydrophobic core and a hydrophilic shell. NK012 is an example of micellar formulation of SN 38 (11) developed by Nippon Kayaku which is in Phase 1 clinical trial [287-289]. 

This situation is quite opposite to that reported for the micellization in aqueous medium, for low molecular weight surfactants as well as for hydrophilic-hydrophobic block copolymers in water. Typical examples are those reported by different authors for PEO-PPO-PEO block copolymers for which the micellization is an entropy driven process [ 114,135,136). According to Liu etal. [137] this phenomenon is mainly the consequence of hydrophobic interactions and changes of the water structure in vicinity of the polymer chains. 

Hydrogels have received increasing interest for biomedical and consumer products application [98]. PLA and PEG hydrophilic/hydrophobic block copolymers are especially promising for soluble hydrophilic/hydrophobic system that becomes an insoluble microsphere when injected into the body as drug release systems [99]. The hydrolysis and biodegradation of these copolymers are subjects of ongoing research. 

Emulsion polymerization has become an important process for the production of a large number of industrial polymers in the form of polymer colloids or latexes. They are the base of adhesives, paints and especially of waterborne coatings. An interest has been developed in recent years in emulsion polymerization systems in which the classical low molecular weight surfactaints are replaced by polymeric surfactants, either hydrophilic-hydrophobic block and graft copolymers (1-4) or functionalized oligomers (5). 

Figure 7.1 Typical structures of hydrophilic-hydrophobic block and graft copolymers. A hydrophilic block B hydrophobic block.

Figure 7.4 Schematic representation of micellar stmctures in aqueous medium for A-B hydrophilic-hydrophobic diblock copolymers, poly A being the hydrophilic block. R core radius, L shell (corona) thickness. From Ref. [15]. Copyright 2003, with permission from Elsevier.

Billot, J.P., Douy, A., and Gallot, B. (1976) Synthesis and structural study of block copolymers with a hydrophobic polyvinyl block and a hydrophilic polypeptide block Copolymers polystyrene/poly(L-lysine) and polybutadiene/ poly(L-lysine). Makromolekulare Chemie-Macromolecular Chemistry and Physics, 111, 1889-1893. 

Permanent and permanent sculpted nanostructures are stable architectures. They can therefore be further chemically processed to yield more complex structures. For example, we have covalently linked different stable block copolymer nanostructures to yield coupled nanostructures. The end coupling of a hydrophobic nanotube with a hydrophilic nanotube yielded an amphiphilic nanotube diblock structure (Yan et al., 2004). The attachment of hydrophobic block copolymer nanotubes by their ends to the surfaces of hydrophilic nanospheres bearing carboxyl groups yielded a supersurfactant (Liu et al., 2003). 

Polymeric micelles are made up of hydrophilic and hydrophobic block copolymers in the form of a sphere in which the hydrophobic portions form the core and the hydrophilic regions form the shell. The hydrophobic structures in the middle of the... 

Polyall lene Oxide Block Copolymers. The higher alkylene oxides derived from propjiene, butylene, styrene (qv), and cyclohexene react with active oxygens in a manner analogous to the reaction of ethylene oxide. Because the hydrophilic oxygen constitutes a smaller proportion of these molecules, the net effect is that the oxides, unlike ethylene oxide, are hydrophobic. The higher oxides are not used commercially as surfactant raw materials except for minor quantities that are employed as chain terminators in polyoxyethylene surfactants to lower the foaming tendency. The hydrophobic nature of propylene oxide units, —CH(CH2)CH20—, has been utilized in several ways in the manufacture of surfactants. Manufacture, properties, and uses of poly(oxyethylene- (9-oxypropylene) have been reviewed (98). 

Certain block copolymers have also found appHcation as surfactants (88). Eor example, AB or ABA block copolymers in which one block is hydrophilic and one block is hydrophobic have proven useful for emulsifying aqueous and non-aqueous substances and for wetting the surface of materials. Examples of such surfactants are the poly(propylene oxide- /oi / -ethylene oxide) materials, known as Pluronics (BASC Wyandotte Co.). 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

Recently, unique vesicle-forming (spherical bUayers that offer a hydrophilic reservoir, suitable for incorporation of water-soluble molecules, as well as hydrophobic wall that protects the loaded molecules from the external solution) setf-assembUng peptide-based amphiphilic block copolymers that mimic biological membranes have attracted great interest as polymersomes or functional polymersomes due to their new and promising applications in dmg delivery and artificial cells [ 122]. However, in all the cases the block copolymers formed are chemically dispersed and are often contaminated with homopolymer. 

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