| dc.description.abstract | Hyperbranched polymers (HBPs) are highly branched macromolecules that exhibit excellent solubility, low melt viscosity, and they possess a multitude of modifiable peripheral functional groups.1 Unlike their structurally perfect analogues, namely dendrimers, HBPs possess a large number of defects resulting from a statistically governed growth process. The densely branched architecture in HBP hinders chain entanglement as a result they have poor mechanical properties. One way to facilitate chain entanglement in the HBP structures is to increase the chain length between two adjacent branching points; this would lead to a special type of HBPs which are called segmented HBPs. A second approach would be to install interacting units on the numerous peripherally located terminal groups; the nature and strength of the interaction would be expected to modulate the mechanical properties of HBPs.
My thesis first focuses on the development of novel clickable aromatic donor-acceptor (DA) pairs that can be readily incorporated into polymer structures. Naphthalene monoimide and dinitro naphthalene monoimide were synthesized as acceptor molecules, whereas the carbazole and 3, 6-dimethoxy carbazole were designed as donor molecules; the main advantage of these being that they can be easily modified to install a single reactive handle, like an azide. The interaction strength of these DA pairs was investigated using UV-Vis and 1H-NMR spectroscopic techniques, and the results were compared with the standard symmetrical DA pairs, namely dialkoxynaphthalene (DAN) donor and naphthalene diimide (NDI) acceptor. Blends of HBPs carrying the new D and A units led to brittle samples, hence these DA pairs were clicked onto a low Tg polymer, namely a n-butyl acrylate-based copolymer, and the charge-transfer (CT) interactions in the polymeric systems were examined in solution and thin films. To understand the effect of CT interactions on the mechanical properties, thin films of blends and copolymers carrying the D and A units, was studied by nanoindentation; it was evident that the CT interaction strength between the D-A pair impacted their hardness and modulus.
In a second study, peripherally clickable HBPs bearing peripherally located propargyl groups was used as a multifunctional scaffold to generate an amphiphilic crosslinked network using PEG diazides of different lengths that helped vary the crosslink density; in addition, the unutilized propargyl groups were used to create a molecular jacket around the triazole rings that served as ligands to coordinate catalytic metal-ions, like Cu. The Cu-loaded hydrogel effectively catalysed the alkyne-azide click reaction of various small molecules, and the molecular jacket was shown to influence the reaction rate, depending on the relative polarity of the jacket and the reactant; furthermore, large biomacromolecules, like human serum albumin protein, could also be catalytically clicked with PEG segments.2 This concept was extended to design a reaction vessel wherein the catalytic hydrogel film is covalently anchored to the inner walls; different metal ions, like Cu, Pd, and Au, were then incorporated into the hydrogel film. These vials served as a catalytic reaction vessel for different reactions, such as CuAAC, Suzuki coupling and Au-nanoparticle catalysed reduction; to carry out the reaction, reactants were simply stirred in these vessels, the products were poured out and the vail reused multiple times after a simple solvent wash.
Finally, in a different vein, but one that utilizes D-A interactions, a linear polymer containing dialkoxy-carbazole donors linked via a flexible oligoethylene segment was prepared via azide-alkyne click polymerization using suitably derivatized monomers, and upon interaction with an acceptor molecule linked to an ammonium group, it sets up a 2-point interaction that leads to a pleated structure. An important feature of this design is the availability of the functionalizable N-atom on the carbazole ring for installing desirable units that would become ordered when the chain adopts a folded conformation. | en_US |
