Star amphiphilic diblock copolymer

Ionic amphiphilic diblock copolymers are well known to self-assemble into core-corona aggregates (micelles) in aqueous media. The micelle comprises a hydrophobic core formed by nonpolar blocks and a hydrophilic corona built up from polyelectrolyte blocks. The properties of such macromolecular self-assemblies are reviewed in detail elsewhere [57, 58]. In many cases, the micelles are characterized by a spherical morphology. When the radius of the hydrophobic core is considerably smaller than the thickness of the polyelectrolyte corona, such macromolecular self-assemblies are regarded as star-like micelles (Eig. 6b). 

They resemble star-shaped polyelectrolytes with a large number of arms, though the number of arms in such macromolecular self-assemblies might change if the micelles are of dynamic nature, that is, if they are able to change their aggregation numbers with variations in the environmental conditions. Historically, the micelles of ionic amphiphilic diblock copolymers were the first star-like polyionic species involved in interpolyelectrolyte complexation and their IPECs have attracted considerable attention during the recent years. 

Li et al. utilized a solvent-assisted collocation strategy to prepare star-like block copolymers here an amphiphilic diblock copolymer was prepared by sequential monomer addition using a suitably designed RAFT initiator (11), which yielded diblock copolymers with both azide and alkyne groups at the same end of the polymer chain (Scheme 7.27). In this study, they have used either M-isopropylacrylamide (NIPAM) and dimethyl acrylamide (DMA) or styrene and NIPAM to first generate the diblock copolymer the PNIPAM segment in both of them gives the system a temperature-responsive amphiphilic character due the lower critical solution temperature (LCST) behavior of PNIPAM block. 

Amphiphilic star-block copolymers can be prepared by adding a polycyclic diene such as 238 to a living diblock copolymer made by sequential ROMP of (i) the monomer in Table 9 with R = COOSiMe3, and (ii) norbomene. The trimethylsilyl ester groups are then converted to carboxylic acids by soaking the cast film of the polymer in water for 2-3 days to give a product with a hydrophobic core of polynorbomene and a hydrophilic outer layer126,502. 

Living cationic polymerization techniques are also capable of producing well defined star-block copolymers. An approach similar to the DVB method described above for the case of anionic polymerization was employed in order to prepare amphiphilic star-block copolymers [20]. In one case, living diblock copolymers of vinyl ethers and ester-containing vinyl ethers, prepared by the initiating system Hl/Znh in toluene, were reacted with a small amount of a difunctional vinyl ether to produce star shaped block copolymers (Scheme 5). 

A number of theoretical studies have been devoted to analysis of the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers [13-24]. Most of these studies considered copolymers with strongly dissociating (also referred to as quenched ) PE blocks [13-18, 20] and extensively exploited the analogy between the conformation of PE blocks in a corona and that in a spherical PE brush [25-33] or PE stars (see [10] for a review). The micellization and the responsive behavior of nanostructures formed by copolymers with pH-sensitive PE blocks have also been systematically studied in recent years [19, 21-23]. 

QuagUa, F., Ostacolo, L., De Rosa, G., La Rotonda, M.I., Ammendola, M., Nese, G., et al. Nanoscopic core-shell drug carriers made of amphiphilic triblock and star-diblock copolymers. Int. J. Pharm. 324(1), 56-66 (2006)... 

FIGURE 2.1.2 Polymer self-assemblies based on amphiphilic polymers with different architectures, (a) Diblock copolymer, (b) triblock copolymer, (c) graft copolymer, (d) star copolymer, and (e) hyperbranched copolymer [38]. 

Hydrophilic aminoethyl methacrylate blocks can therefore be combined with various hydrophobic blocks as shown also by J6r6me and co-workers [195]. PDMAEMA-PMMA diblock and star-block copolymers, quaternized on the PDMAEMA block with different alkyl halides were synthesized by these authors who could demonstrate that with short alkyl halides the diblock copolymers behave like classical amphiphiles on micellization, whereas their behavior becomes similar to polysoaps in the case of long alkyl halides. 

Yun, J., Faust, R., Szilagyi, L.Sz., Keki, S. and Zsuga, M. (2004) Effect of architecture on the micellar properties of amphiphilic block copolymers comparison of AB linear diblock, A2B2, A3B3 and (AB)3 star block copolymers. /. Macromol Sci. Pure andAppl. Chem., A41,613-627. 

The facility to introduce well-defined chain ends has been used to prepare star polymers557 and diblocks via reaction with macromolecular aldehydes.558 The synthesis of amphiphilic star block copolymers has also been described using a cross-linking agent.559 560 A similar strategy has recently... 

Ring-opening metathesis polymerization was also used recently for the preparation of amphiphilic star-block copolymers [25]. Mo (CH-f-Bu) (NAr) (0-f-Bu)2 was used as the initiator for sequential polymerization of norbornene-type, unfunctionalized and functionalized, monomers. The living diblocks were reacted with endo-ris-endo-hexacyclo- [ 10.2.1.1.3/115>8.02>l 1. O 1-9] heptadeca-6,13-diene, a difunctional monomer in a scheme analogous to the use of DVB in anionic polymerization, to form the central core of the star (Scheme 10). 

The carboxylic moieties of the functional monomers, protected with trimethylsilyl groups during the polymerization reaction, were deprotected by hydrolysis, resulting in amphiphilic star-block copolymers. Molecular weight distributions of the stars were usually higher than 1.2, as determined by SEC, and only in one case was 1.06, although the polydispersities of the Unear diblocks were lower than 1.05. 

Similarly, stable macroanions obtained by the subsequent metalation of the proper end groups such as diphenylvinyl and diphenylmethoxy were used in the living polymerization of tBMA yielding PIB-l7-PtBMA block copolymers with almost quantitative efficiency (Scheme 35). Moreover, amphiphilic polymeric materials can be prepared by hydrolysis of ester moieties of the polymers obtained by this method for instance, amphiphilic PIB-I7-PMAA diblock was prepared by the hydrolysis of the acrylate segment of the suitable precursor copolymer. A series of linear and star copolymers consisting of PIB and PMMA were also prepared. ... 

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