Unimer micelle functionalization

VI. UNIMER MICELLES FUNCTIONALIZED WITH PHOTOACTIVE CHROMOPHORES... 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

Hydrophobic functional-molecules can be incorporated into the higher-order unimer micelles either via chemical bonding of the functional molecules to amphiphilic polymers or by physically solubilizing tiie mol ules into the unimer micelle. For example, when an AMPS-DodMAm copolymer and a hydrophobic small-molecule are dissolved together in ethanol, and the ethanol solution is poured into excess water, then the hydrophobic molecules can be entrapped in a hydrophobic microdomain during the process of the hydrophobic self-association of polymer-bound dodecyl groups. However, it is practically difficult to precisely control the number of the sm l... 

Phenylmethylphenacylsulfonium p-toluenesulfonate (PMPS), an onium salt, is known as a self-destructive electron acceptor which decomposes rapidly into phenylmethylsulfide and a phenacyl rachcal upon accepting an electron (59). When PMPS is added to an aqueous solution of ZnTPP-functionalized unimer micelles, the electron acceptor is localized on the anionic surface of the unimer micelle. The election transfer from the 2HnTPP triplet to PMPS produces a ZnTPP cation radical (ZnTPP ) and a phenacyl radical as transient species (Scheme 1), but reaction between these two transient species, which usually follows the photoinduced forward election transfer in homogeneous solutions, is prevented in the unimer micelle system because the ZnTPP " and phenacyl radical species are separated. Thus, the porphyrin cation radicals can be accumulated in the unimer micelle. 

This chapter deals with a recently developed unimer micelle, a novel intramolecular assembly with a higher-order structure, formed from tailored amphiphilic random copolymers bearing charge and bulky hydrophobes. The first half of this chapter will be devoted to the synthesis of such amphiphilic poly electrolytes and characterization of the unimer micelle. The functionalization of the unimer micelles with chromophoric molecules and some functional properties of these functionalized unimer micelles will be described in some detail in the second half of this chapter. 

The unimer micelles can be functionalized by incorporating various functional groups into the hydrophobic microdomains of the unimer micelles. The incorporation of functional groups can be effected by covalent bonding of functional groups onto the amphiphilic polymers that form unimer micelles. 

Direct dissolution of a functional molecule into the unimer micelle can be an alternative way to functionalization. For the unimer micelles formed from the polymers containing La groups at low concentrations in water, small molecules of amphiphilic nature can be localized in the La microdomain from aqueous solutions. However, hydrophobic microdomains in unimer micelles formed by cyclic hydrophobes, such as Cd, Ad, and 1-Np, are so tightly packed with the rigid hydrophobes that penetration of small mol-... 

The best way to incorporate functional groups in a well-defined number and a uniform distribution into the rigid unimer micelles is to employ ter-polymerization techniques as described in the previous section for labeling the polymers with Py and 2-Np moieties. The functional groups thus incorporated are tightly encapsulated in the hydrophobic microdomains in the unimer micelles. This was termed compartmentalization of functional groups in unimer micelles [24,41,42]. 

Over the last decade, there has been increasing interest in photochemistry in constraining space. If photoactive chromophores are compartmentalized in the hydrophobic microdomains of the unimer micelles, the chromophores will be completely isolated from one another in highly constraining nonpolar microenvironments and protected from the aqueous phase. Owing to these unusual microenvironmental effects, such chromophore-functionalized unimer micelles, unlike the conventional molecular assembly systems, induce a large modification of the photophysical and photochemical behavior of the compartmentalized chromophores. 

Unimer micelles are functionalized with Abz moieties by terpolymeri-zation employing Abz-substituted methacrylamide monomers, which include A -lauryl-4- [4-(methacryloylamino)phenyl]azo benzamide [Abz(La)MAm] and N-cyclododecy 1 - 4 - [4 - (methacryloylamino)phenyl]azo benzamide... 

The critical concentration at which the first micelle forms is called the critical micelle concentration, or CMC. As the concentration of block copolymer chains increases in the solution, more micelles are formed while the concentration of nonassociated chains, called unimers, remains constant and is equal to the value of the CMC. This ideal situation corresponds to a system at thermodynamic equilibrium. However, experimental investigations on the CMC have revealed that its value depends on the method used for its determination. Therefore, it seems more reasonable to define phenomenologically the CMC as the concentration at which a sufficient number of micelles is formed to be detected by a given method [16]. In practical terms, the CMC is often determined from plots of the surface tension as a function of the logarithm of the concentration. The CMC is then defined as the concentration at which the surface tension stops decreasing and reaches a plateau value. 

FIGURE 13.23 Scattered light intensity as a function of temperature showing the unimer-to-micelle transition regionandatypicaltemperaturejumpexperiment. (ReproducedfromGoldmints, I., J. F. Holzwarth, K.A. Smith, andT. A. Hatton. 1997Langmuir13 6130-6134. Wth permission from American Chemical Society.)... 

High-pressure static and dynamic light scattering were used to closely examine the behavior of block copolymers of poly(vinyl acetate) (PVAc) and poly(l,l,2,2-tetrahydroperfluoroalkyl acrylate (PTAN) as a function of C02 density (Buhler et al., 1998). The phase diagram for PVAc-6-TAN shows three distinct phases as a function of polymer concentration and C02 density at a fixed temperature of 45 °C (see Figure 9.1). The block copolymer forms a precipitated phase at low C02 densities, spherical micelles at intermediate C02 densities, and unimers, or free polymer chains in solution, at high densities. The micelles-to-unimer transition was found to be very... 

Hence, under equilibrium conditions, the chemical potential of a unimer must be an increasing function of the aggregation number p, and the grand potential of a micelle, Q(p), must be a decreasing function of p. 

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