Triblock copolymer assembly

Pochan DJ, Chen Z, Cui H et al. (2004) Toroidal triblock copolymer assemblies. Science 306 94-97... 

Ma, Q. G. and Remsen, E. F. (2002). Chemically induced supramolecular reorganization of triblock copolymer assemblies Trapping of intermediate states via a shell-crosslinking methodology. Proc. Natl. Acad. Sci. USA, 99, 5058-63. 

Only one membrane conformation is possible for AB copolymers, where the B blocks constitute the hydrophobic layer and the A blocks extend outwards on both sides of the membrane. In the case of the ABA triblocks, the hydrophobic B block either can form a loop so as the hydrophilic A chains are on the same side of the membrane (U shape) or can stretch forming a monolayer with the two A blocks at the opposite sides of the membrane (I shape), while for the BAB type only, the U-shaped conformation is possible since the hydrophobic B chain ends must assemble into a membrane and the hydrophilic A blocks must form a loop. Similar conformations can be anticipated for ABABA pentablock copolymers. The same general rules apply when a third chemically different block is added, and although the overall geometry is the same, multiblock copolymers have an extra level of control over the polymersome structure introduced by the extra interaction between the blocks. Hence, ABC triblock copolymers assemble into asymmetric or Janus membranes, and the resulting vesicles are characterised by different external and internal surface chemical properties. Even more interestingly, ABCA tetrablocks (where A is hydrophilic and B and C are both hydrophobic) have been shown to... 

Most work on amphiphilic conjugates to date is based either on simple a-helix-fotming polypeptides or on complex pro-teins/enzymes. Work on biomolecular building blocks that fall between the two aforementioned groups, such as the coiled-coil protein tertiary stmctural motif, has been lim-ited. " Recendy, a heterodimer coiled-coil was used to noncovalendy link PEG and PS blocks and the resultant amphiphilic PEG-peptide-PS triblock copolymer assembled into thermoresponsive micellar assemblies in aqueous solution that transformed from rodlike micelles to spherical micelles upon heating, as depiaed in Figure 10(b). ... 

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

Recently, we have also prepared nanosized polymersomes through self-assembly of star-shaped PEG-b-PLLA block copolymers (eight-arm PEG-b-PLLA) using a film hydration technique [233]. The polymersomes can encapsulate FITC-labeled Dex, as model of a water-soluble macromolecular (bug, into the hydrophilic interior space. The eight-arm PEG-b-PLLA polymersomes showed relatively high stability compared to that of polymersomes of linear PEG-b-PLLA copolymers with the equal volume fraction. Furthermore, we have developed a novel type of polymersome of amphiphilic polyrotaxane (PRX) composed of PLLA-b-PEG-b-PLLA triblock copolymer and a-cyclodextrin (a-CD) [234]. These polymersomes possess unique structures the surface is covered by PRX structures with multiple a-CDs threaded onto the PEG chain. Since the a-CDs are not covalently bound to the PEG chain, they can slide and rotate along the PEG chain, which forms the outer shell of the polymersomes [235,236]. Thus, the polymersomes could be a novel functional biomedical nanomaterial having a dynamic surface. 

In the last few years there have been new creative methods of preparation of novel hydrophilic polymers and hydrogels that may represent the future in drug delivery applications. The focus in these studies has been the development of polymeric structures with precise molecular architectures. Stupp et al. (1997) synthesized self-assembled triblock copolymer, nanostructures that may have very promising applications in controlled drug delivery. Novel biodegradable polymers, such as polyrotaxanes, have been developed that have particularly exciting molecular assemblies for drug delivery (Ooya and Yui, 1997). 

Poly(lactic acid) (PLA) has also been added to poly(SA) via melt polycondensation to produce the triblock copolymers poly(lactic acid-Wock-sebacic acid-Wock-lactic acid) (P(LA-block-SA-block-LA)) by Slivniak and Domb (2002). The PLA (d-, l-, and dl-) was incorporated by acetylation and addition to the PSA synthesis. They showed the formation of stable stereocomplexed particles with increased melting points and reduced solubility, and studied the degradation and drug release characteristics of the same (Slivniak and Domb, 2002). The stereocomplexes self-assemble as a consequence of the chirality in the PLA portions of the chains (Slivniak and Domb, 2002). 

Figure 1. Schematic of self-assembly assisted polypolymerization (SAAP) of triblock copolymers in a selective solvent for the synthesis of long multiblock copolymers with a controllable chain sequence and block length. [3 5]...

In the past several years, we have used the SAAP method to prepare different long multiblock copolymers. Generalities about the preparation and characterization of different end-functionalized triblock copolymers are first outlined. Then, the micellization of triblock copolymer as well as coupling efficiency with and without self-assembly method are discussed. 

The peak in /(Rh) located at 3-4 nm represents individual triblock copolymer chains. At 29 °C, an additional peak appears indicating the self-assembly of the triblock copolymer chains. Pentanediol (H0(CH2)50H) was added as the linking agent to couple each two functional ends of the triblock copolymer chains in the presence of pyridine. The resultant multiblock heteropolymer chains have a structure like (PMMA-/)-PS-/)-PMMA-c-)n, where c denotes the linking agent, pentanediol. The structure can also be written as (PMMA-Z>-PS)n, in which the PMMA block is twice longer than that in the initial triblock PMMA-Z>-PS-Z>-PMMA copolymer chain because each two PMMA blocks are connected together in the resultant multiblock copolymer. 

Figure 2. Typical hydrodynamic radius distributions (/(RjO) of individual triblock PMMA-Z>-PS-Z>-PMMA copolymer chain end-capped with oxalyl chloride in a solvent mixture of methyl acetate and acetonitrile (10/1, v/v) at 45 °C and the aggregates formed via the self-assembly of the triblock copolymer chains at 29 °C, where the triblock copolymer concentration is 1 x 10 4 g / mL.[35]...

Before the coupling reaction, the self-assembly of PI-Z>-PS-Z>-PI triblock copolymer chains in w-hcxane was investigated by LLS. Figure 7 shows typical hydrodynamic radius distributions (/(Rh)) of individual PI-Z>-PS-Z>-PI triblock chains in THF, a good solvent for both the PI and PS blocks, and the core-shell micelles formed via the self-assembly of the triblock copolymer chains in -hexane, a solvent selectively good for the PI block. The shifting of the peak from... 

Figure 6. SEC profiles of PI-6-PS-6-PI triblock copolymer chains end-capped with butyl bromide group (SI44) before and after the coupling reaction in n-hexane, with the self-assembly as well as in THF without the self-assembly.

In contrast, when the coupling reaction was performed in THF without the self-assembly, the SEC profile contains only two peaks, namely, a dimer peak with Mw 1.2 x 104 and precursor triblock copolymer peak. It shows that the coupling reaction stops after two triblock copolymer chains are linked together. This is because for longer copolymer chains (Mw > 104 g/mol), the chain ends are likely wrapped and hidden inside the chains coiled in a good... 

Figure 2.2 Representation of formation of (a) micellar and (b) inverse micellar vesicular assemblies from diblock and triblock copolymers, respectively.

Jang et al. (2004) observed that ABA triblock copolymers composed of a docosyl chain, a rigid aromatic segment, and a flexible PEO dendrimer assemble into a hexagonal columnar or body-centered cubic structure in the solid state for the first- and second-generation dendrons, respectively (Fig. 11.27). 

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