Dendronized responsivity

In their efforts to construct stimuli-responsive, supramolecular amphiphiles, Frechet et al. [126-129] reported the synthesis of a novel series of AB and ABA block copolymers via the Williamson ether synthesis (e.g., 47, Fig. 21). Polyethylene glycols (PEGs) of different lengths were used as the linear hydrophilic B block while polyaryl ether dendrons of different sizes were used as the hydro-phobic A block. These copolymers were characterized by optical microscopy,... 

H-NMR studies and analysis of sensor responses in the CD spectrum - after addition of sugar as guest to the dendrocleft host - show that the ability of this dendritic host to enantioselectively recognise monosaccharides decreases with increasing generation number of the dendron surrounding the chiral core unit. 

GAO Gao, M., Jia, X., Kuang, G., Li, Y., Liang, D., and Wei, Y., Thermo- and pH-responsive dendronized copolymers of styrene and maleic anhydride pendant with poly(amidoamine) dendrons as side groups. Macromolecules, 42, 4273, 2009. 

These two attributes - responsivity and the loadable interior - of the dendronized polymers can, in principle, be exploited to create smart delivery vehicles in which the release of guest molecules can be triggered by conformational changes of the responsive dendritic envelope. Additionally, unlike conventional polymers. 

Dendronized polymers Distinguishing features o Extended backbone o Defined surface o Loadable interior o Monodisperse diameter o Highly dense and responsive thickness o Large number of end-groups (chargeable) o Resistant against collapse... 

The chapter starts with the synthesis aspects of dendronized polymer research, with details of the three synthetic pathways, assisted by reported examples that help to distinguish the difierent synthetic strategies in terms of their efficiency and fidehty towards generating dendronized polymers of respectable thickness and length. The responsivity of the dendronized polymers - which is an important feature of this unique class of polymers, and may lead to some interesting apphca-tions - is then discussed, followed by details of the single-molecule chemistry and use of dendronized polymers as building blocks for nanoconstruction purposes. 

Advances made during the past few years have made it clear that dendronized polymers possess unique properties, due mainly to their stiffness and responsive thickness, that can be translated into potential applications. Consequently, systematic and detailed investigations of the responsivity of polymers as a function of dendron generation, the chemical nature of dendrons and backbone, the chain length, and the nature of the stimulant will be open targets for future research. 

Thermo-responsive polymers have been used to release encapsulated drugs in response to temperature changes. These polymers typically undergo a rapid and reversible hydration-dehydration at lower critical solution temperature (LCST, from soluble to insoluble) or upper critical solution temperature (UCST, from insoluble to soluble) upon heating. [71] For example, a thermo-responsive polymer, poly (2-isopropyl- 2-oxazoline) (PiPrOx), was conjugated to pH-responsive poly(benzyl ether)dendrons, which exhibited sharp thermal transitions as well as pH-dependent behaviors (Fig. 4a).[105]... 

Fig. 4 Novel drug delivery systems using stimuli-responsive polymers, (a) Morphology changes of poly(2-isopropyl- 2-oxazoline) (PiPrOx) and poly(benzyl ether)dendrons dendritic-linear block copolymers conjugates are Temperature- and pH-dependent.[105] (Copyright 2012, Royal Society of Chemistry) (b) Protein-binding-induced disassembly of dendron-based micelles. This inducible micelle disassembly is selective to the targeted protein and the disassembly can mediate release of the encapsulated therapeutic drugs.[106] (Copyright 2010, American Chemical Society)...

Smith and coworkers have also harnessed related anunonium ion/crown ether interactions to template the assembly of two Boc-protected L-lysine dendrons (Scheme 9)." It was observed that the addition of potassium ions, a competitive gnest of the crown ether hosts, triggers disassembly of the snpramolecniar dendrimer (41) to its component parts (42 and 43). This example of stimuli-responsive dendritic architectures that release small molecules on demand clearly demonstrates advantages inherent in non-covalent dendrimer synthesis over conventional, covalent approaches. 

---