Micelles random copolymers

As has been described in Chapter 4, random copolymers of styrene (St) and 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) form a micelle-like microphase structure in aqueous solution [29]. The intramolecular hydrophobic aggregation of the St residues occurs when the St content in the copolymer is higher than ca. 50 mol%. When a small mole fraction of the phenanthrene (Phen) residues is covalently incorporated into such an amphiphilic polyelectrolyte, the Phen residues are hydrophobically encapsulated in the aggregate of the St residues. This kind of polymer system (poly(A/St/Phen), 29) can be prepared by free radical ter-polymerization of AMPS, St, and a small mole fraction of 9-vinylphenanthrene [119]. 

This multitude of properties the polymer must possess dictate that better polymer performance will be obtained from materials with complicated structures. Such polymers are complex polymers l) random copolymers, 2) block copolymers, 3) graft copolymers, 4) micellizing copolymers, and 5) network copolymers. There has been a dramatic increase in the past decade in the number and complexity of these copolymers and a sizable number of these new products have been made from natural products. The synthesis, analysis, and testing of lignin and starch, natural product copolymers, with particular emphasis on graft copolymers designed for enhanced oil recovery, will be presented. 

FIGURE 13.2 Schematic representation of block and random copolymer micelles. (Reproduced from Jones, M. C. and J. C. Leroux. 19SELir. J. Pharm. Biopharrr48 101-111. With permission from Elsevier Ltd.)... 

Other copolymers that may show micellization similarto that of BAB triblock copolymers include random copolymers if the hydrophobic units are located near both ends of the polymeric structure (Schild and Tirrell, 1991 Ringsdorf et al., 1992 Chung et al., 1999 Jones and Leroux, 1999), and graft copolymers if the hydrophobic blocks are grafted near both ends of a hydrophilic backbone. 

Amino acid-based norbomene random and block copolymers have been synthesized by Sanda, Masuda et al. [178]. The blocks were constructed with monomers containing either the ester or carboxyl amino acid forms, and C4 was used. While the random copolymers were partially soluble in acetone, the block copolymers were soluble through formation of reverse micelles (Scheme 24). Moreover, the diameter of these aggregates was around 100 nm as measured by DLS and AFM. Amino acid-based ROMP monomers with a different cyclic core, i.e., cyclobutenecarbonyl glycine methyl esters, were polymerized by Sampson et al., leading to head-to-tail-ordered polymers without stereocenters [179]. C6 was used and polydispersities between 1.2 and 1.6 were obtained. 

For polymers, DT is found to be virtually independent of chain length and chain branching, but it is strongly dependent on polymer and solvent composition [84]. For random copolymers, DT varies linearly with monomer composition block copolymers display more complex behavior [111,214]. For segregated block copolymers like micelles, DT seems to be determined by the monomers located in the outer region (see Fig. 18). For particles, DT appears to be both composition and size dependent [215]. 

Morishima, Y., et al. (1995), Characterization of unimolecular micelles of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and methacrylamides bearing bulky hydrophobic substituents, Macromolecules, 28, 2874-2881. 

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

Polymeric micelles are called the new generation of colloid drug delivery systems [28]. In an aqueous medium, block and random copolymers associate into micelles with a structure similar to that of surfactants. Active substances which are insoluble or poorly soluble in water can be built into the lipophilic part of the micelle in two ways i) with dialysis and ii) with the emulsion method (see Figure 18.4). 

The bloek eopolymer of P(/BA-cu-AA)- )-PAzoMA can form core-shell micelles and vesieles depending on the composition and the preparation conditions (Tong et al., 2005 Wang et al., 2004). With the same length of the hydrophobic block of PAzoMA, the aetual hydrolysis degree of tBA on the hydrophilic P(tBA-co-AA) random copolymer turns out to be important since it determines the hydrophilic-... 

Further, the ability to synthesize random copolymers with various hydrocarbon monomers allows the anchor-soluble balance to be tuned while maintaining solubility even with high incorporations of hydrocarbon comonomers [29]. Because of the amphiphilic nature of such copolymers, it was predicted that these materials would selfassemble into micelles consisting of a highly fluorinated corona segregating the lipophilic core from the compressed CO2 continuous phase. Thus, PFOA-F-PS block copolymers were synthesized via controlled free-radical techniques (Fig. 9.3), and it was confirmed (by smaU-angle neutron scattering) that these copolymers spontaneously assemble into multimolecular micelles in solution [40]. In addition to amphiphilic materials, which physically adsorb to the surface of polymer particles in dispersion polymerizations, fluorinated acrylates can be utihzed as polymerizable comonomers in the stabilization of C02-phobic polymer colloids [41]. 

Synthesis of high molecular weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization. The surfactant type and concentration were chosen to provide solubilization of the hydrophobic monomer with preferably one or at most a few hydrophobic monomer groups per micelle. 

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