Exchange between block copolymer

Fig. 8. Block copolymer vesicles budding-off from a swollen lamellar phase. After separation, there is no further exchange of block copolymers between vesicles, from Ref 50, with permission from ACS Publications Division.

The computational efficiency of this approach allows at the same time models with a resolution close to atomistic and simulations on very large length and timescales.""" For example, these models have been applied to study the interaction and the dynamical exchange of block copolymer chains between a spherical micelle (functioning as drug nanocarrier) and a lipid bilayer (as model of cell surface) also in the presence of drug molecules (iboprufen) in the micelle core. Simulations of 12 nm large micelles with membrane bilayers over several microseconds of simulations could be achieved on home-cluster facilities."" ... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

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. 

Intermolecular chain transfer to polymer leads also to the exchange of segments between macromolecules (scrambling). This may effectively preclude the isolation of block copolymers. This phenomenon is especially pronounced in the polymerization of cyclic acetals. 

The exchange reactions involving amine-terminated or acid-terminated polymer molecules proceed at a faster rate than exchange reactions between amide linkages. Korshak et al. [94b] showed that formation of block copolymers from mixed homopolyamides having chemically blocked end groups was much slower than when the homopolyamides had free carboxyl and free amine end groups. 

These are stable micelles that are formed with polymeric surfactants. Amphiphilic block copolymers such as the pluronics (polyoxyethylene-polyoxypropylene block copolymers) are able to self-assemble into polymeric micelles and hydrophobic drugs may be solubilized within the core of the micelle or, alternatively, conjugated to the micelle-forming polymer. Although micelles are rather dynamic systems that continuously exchange units between the micelle structure and the free units in solution, those composed of polyoxyethylene - poly(aspartic acid) have been found sufficiently... 

Ester-exchange reactions in polyesters (and analogous amide reactions in nylons) that result in a new block copolymer. An example is the formation of a blend between polycarbonate and poly(butylene terephthalate) in which the compatibilizer is formed by a transesterification reaction at elevated temperatures (350 °C), but, when processed at lower temperatures, the systems are not compatible. 

In low-molecular weight surfactants, above the CMC, a constant exchange occurs between aggregated material and that which is free (i.e., unimers) in the aqueous phase. Similarly, with block copolymers, a dynamic equilibrium exists between micelles and nonaggregated chains. The average residence time of a single block copolymer chain within a micelle is of importance because this will dictate the stability of the system. Researchers have used fluorescence techniques to probe such characteristics as the exchange rate between unimers and polymeric micelles [168]... 

In a similar manner, the exchange kinetics have also been investigated for various block copolymers based on dimethylaminoakyl methacrylate and MAA via energy transfer from a donor-labeled core to a dispersed acceptor probe [160]. It was concluded from these measurements that there is a direct link between the hydro-phobicity of the core and the exchange rate constant increasing the hydrophobicity of the copolymer slows down the exchange rate due to less favorable interactions with the aqueous phase. 

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