Unimer

Figure I 6.34 Degree of polymerization distribution molar distribution ( ) with number average degree of polymerization (dp = 140 glucose unimers) mass distribution ( ) with weight average degree of polymerization dp = 1242 glucose unimers.

Ribrain (Searle-Endopharm Yamanouchi) Sere (Solvay Pharma) USA Sere (Unimed) wfm... 

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Corresponding author Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Universitit di Messina, Italy. E-mail centi unime.it... 

The electronic state of a hydrogen atom is specified by the quantum numbers n, l and m, so in this case >Q(x) = unim(x) = RniYim 6, 4>). If the spin state of the electron is also to be specified this latter i )a x) must be multiplied by a column vector Xm, where... 

Dept, of Industrial Chemistry and Engineering of Materials and ELCASS (European Laboratory for Catalysis and Surface Science), University of Messina, Salita Sperone 31, 98166, Messina, Italy. E-mail centi unime.it Fax + 39-090-391518... 

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. 

Size-exclusion chromatography (SEC) has been used to characterize the unimer-micelle distribution. However, SEC is not an absolute method and thus requires calibration. Since it is practically impossible to calibrate a SEC apparatus for the unimers and micelles formed by a block copolymer, only indicative MW values can be obtained. Moreover, several authors have noted a strong perturbation of the unimer-micelle equilibrium during SEC experiments even when interaction of the material with the SEC column was minimized [4,61,62],... 

The dynamics of micellar systems is a very important concern that is relevant at several levels. In this respect, the dynamics required for the establishment of the unimer-micelle equilibrium is one aspect. The so-called problem of micelle hybridization that deals with the exchange rate of unimers between different micelles is closely related. Finally, the chain dynamics of polymer blocks in either the core or the corona is another concern that can, however, be linked to some extent to the first two mentioned concepts. These different aspects have been scarcely studied and reviewed by Tuzar and Kratochvil [6,41], and... 

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). 

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

It has been shown that block copolymer micelles are dynamic structures, although they can be kinetically frozen. Unimers can thus escape from micelles and be exchanged with other micelles or be adsorbed on another interface... 

An early illustration of pH-responsive micellization can be found in P2VP-PEO copolymers that exist as unimers at pH < 5 and form micelles at higher pH values, as demonstrated by Webber et al. [165]. Recently, we investigated a P2VP-PEO copolymer with a larger P2VP block [ 166]. Although this copoly-... 

Stimulus-responsive ABC triblock micelles were also investigated by Patrickios et al. for copolymers containing insoluble PEVE blocks, thermore-sponsive PMVE midblocks, and water-soluble PMTEGVE outer blocks with varying block sequences [277]. While in aqueous solutions only unimers were found, and the addition of salt led to aggregates. 

Department of Electronic Engineering, Chemistry and Industrial Engineering University of Messina Messina, Italy e-mail centi unime.it Chapter 16... 

The activation parameters for the (bimolecular) addition and the (unim-olecular) heterolysis steps have been determined [28] for the case of Ri, R2, R3 = H or CH3 and the results are shown in Fig. 1. It is obvious that the heterolysis reaction is entropy controlled which is the consequence of the highly ionic transition state which leads to freezing of water molecules with the concomitant loss of entropy. 

The free-radical clock methodology has been also applied to calibrate unim-olecular radical reactions based on the A h values of Table 3.2 and, when avail-... 

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