Amphiphilic Block Copolymers in Aqueous Solutions

Nonionic Amphiphilic Block Copolymers in Aqueous Solution. 89... 

Nonionic amphiphilic block copolymers in aqueous solution are typically formed of a water-soluble hydrophilic block, e.g., PEO, PMVE, PNIPAM, linked to a hydrophobic block, e.g., PPO, PBO, PS, PMMA. 

A further important application of CD is the preparation of relatively low polydispersity polystyrenes (compared to radical CD polymerization) using the water-soluble RAFT reagent 3-benzylsulfanyl thiocarbonylsulfanylpropionic acid (TTC) complexed in aqueous Me-P-CD solution (Fig. 28) [65], This method also allows the direct synthesis of amphiphilic block copolymers in aqueous solution without compatibility issues. 

Self-organization of such amphiphilic block copolymers in aqueous solutions indicated the formation of vesicles. Stabilization of vesicles was attained by cross-linking chain extension of the PNIPAAm block using hexamethylene diacrylate [64]. Multifunctional micelles for cancer cell targeting, distribution, and anticancer drug delivery have been prepared using PNIPAAm-c )-methacrylic acid-g-PDLA and diblock copolymers [65]. 

Taking into consideration the latest research interest in biological applications and green chemistry, the amphiphilic block copolymers in aqueous solution have been studied more often. 

The self-assembly of amphiphilic block copolymers in aqueous solutions has attracted considerable interest not only because of their unique properties but also due to their widespread application possibilities in technical and especially biomedical areas. As far as representative examples of such systems are concerned, a categorisation into nonionic and ionic containing copolymers is feasible. 

Kositaz, M. J., Bohne, C., Alexandridis, P., Hatton, T. A., and Holzwarth, J. F. 1999. Dynamics of micro-and macrophase separation of amphiphilic block copolymers in aqueous solution. Macromolecules 32 5539-5551. 

Ge Z, Liu S (2009) Supramolecular self-assembly of nonlinear amphiphilic and double hydrophilic block copolymers in aqueous solutions. Macromol Rapid Commun 30 1523-1532... 

KIM Kim, C., Lee, S.C., Kang, S.W., Kwon, I.C., and Jeong, S.Y., Phase-transition characteristics of amphiphilic poly(2-ethyl-2-oxazoline)/poly(e-caprolactone) block copolymers in aqueous solutions J. Polym. Sci., Part B Polym. Phys., 38, 2400, 2000. 

Abstract We present an overview of statistical thermodynamic theories that describe the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers in dilute solution. Block copolymers with both strongly and weakly dissociating (pH-sensitive) ionic blocks are considered. We focus mostly on structural and morphological transitions that occur in self-assembled aggregates as a response to varied environmental conditions (ionic strength and pH in the solution). Analytical theory is complemented by a numerical self-consistent field approach. Theoretical predictions are compared to selected experimental data on micellization of ionic/hydrophobic diblock copolymers in aqueous solutions. 

Figure 11.15 Schematic phase diagram for the phase behaviour of amphiphilic block copolymers represented by ABA-type triblock copolymers in aqueous solution.

One important class of block copolymers are amphiphilic block copolymers that have affinities for two different environments. These two blocks interact very differently with the environment due to their chemical nature, and they behave distinctively in solution. The aforementioned differences can induce microphase separation of an amphiphilic block both in aqueous media and in organic solvents. Two basic processes can be distinguished for block copolymers in solvent media, namely micellization and gelation. Micellization occurs when the block copolymer is dissolved in a large amount of a selective solvent for one of the blocks. In this case, the polymer chains tend to organize themselves in a diversity of structures, from... 

As already mentioned, there has been an extensive amount of studies on the self-assembly of amphiphilic block copolymers in selective solvents over the years. In this section, we will try to give representative examples of these studies divided into two main categories regarding the solvation medium, i.e. amphiphilic copolymers in organic solvents or in aqueous solutions. The latter can be further distinguished in nonionic or ionic containing copolymers. These examples will be limited to the more commonly studied di- and triblock linear copolymers, but concise reviews on the self-assembly of amphiphilic copolymers with more complex non-linear chain architectures can be found elsewhere [98-100]. 

The formation of polymeric capsules can also be achieved by the cross-linking of self-assembled amphiphilic block copolymers [85]. The hydrophobic section of the polymer in an aqueous solution will tend to aggregate on the interior of the micelle, whereas the hydrophilic ends will form the outer shell of the micelle. If the hydrophilic end is appropriately functionalized, it can be cross-linked, giving a polymeric shell. The overarching concept is shown in Figure 5.10. 

Recently, many studies have focused on self-assembled biodegradable nanoparticles for biomedical and pharmaceutical applications. Nanoparticles fabricated by the self-assembly of amphiphilic block copolymers or hydrophobically modified polymers have been explored as drug carrier systems. In general, these amphiphilic copolymers consisting of hydrophilic and hydrophobic segments are capable of forming polymeric structures in aqueous solutions via hydrophobic interactions. These self-assembled nanoparticles are composed of an inner core of hydrophobic moieties and an outer shell of hydrophilic groups [35, 36]. 

Fig. 30 Types of nanocarriers for drug delivery, (a) Polymeric nanoparticles polymeric nanoparticles in which drugs are conjugated to or encapsulated in polymers, (b) Polymeric micelles amphiphilic block copolymers that form nanosized core-shell structures in aqueous solution. The hydrophobic core region serves as a reservoir for hydrophobic drugs, whereas hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble.

Amphiphilic Block Copolymers with One Ionic Block in Aqueous Solution... 

For some applications, it is desirable to lock the micellar structure by cross-Hnking one of the micellar compartments, as discussed previously in Sect. 2.6. Cross-Hnked core-shell-corona micelles have been prepared and investigated by several groups as illustrated by the work of Wooley and Ma [278], who reported the cross-linking of PS-PMA-PAA micelles in aqueous solution by amidation of the PAA shell. Very recently, Wooley et al. prepared toroidal block copolymer micelles from similar PS-PMA-PAA copolymers dissolved in a mixture of water, THF, and 2,2-(ethylenedioxy)diethylamine [279]. Under optimized conditions, the toroidal phase was the predominant structure of the amphiphilic triblock copolymer (Fig. 19). The collapse of the negatively charged cylindrical micelles into toroids was found to be driven by the divalent 2,2-(ethylenedioxy)diethylamine cation. 

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Torchilin et al. synthesized an iodine-containing amphiphilic block-copolymer consisting of iodine-substituted poly-L-lysine which is able to form micelles in aqueous solution [37]. The two components of the block-copolymer were methoxy-poly(ethylene glycol) propionic acid (MPEG-PA) with a molecular weight of 12 kDa and poly[ ,M-(2,3,5-triiodobenzoyl)]-L-lysine. The particle size of the micelles was approx. 80 nm, and the iodine concentration was 20 mg mL . Biodistribution studies in rats showed significant and prolonged enhancement of the aorta, the liver and spleen. 

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