Controlling The Conformation Of Polymers In Solution And Synthesis And Characterization Of 'Clickable' Polyesters

Abstract

The thesis constitutes investigations from two distinct areas of research. One part deals with controlling and modulating the conformation of linear polymer in solution. Folding of a polymer chain has been achieved by utilising weak non-covalent interactions interaction like metal ion binding, charge-transfer complex formation and solvophobic effect in tandem. The second part of the thesis deals with synthesis and characterization of end-functionalized polymers prepared by melt-transesterification. The thesis is divided into five chapters. Chapter 1 provides a general introduction on foldamers – a class of polymers that adopts an ordered conformation in solution and various approaches to obtain end-functionalized polymers. Chapter 2 describe the attempts to improve the association constant (based on earlier works reported by Ghosh and Ramakrishnan) between the external folding agent and the polymer repeat unit. The polymer used in this study constitutes an electron deficient pyromellitic dimide units (PDI) linked with a flexible oxyethylene glycol spacer. An electron rich dialkoxy naphthalene (DAN) serves as the folding agent which forms a charge-transfer (C-T) complexation with the electron deficient aromatic units (PDI) in the polymer backbone and effects the folding. The folding agent has the metal ion as its integral part and this aids the interaction between electron-deficient and electron-rich aromatic units by complexing with oxyethylene glycol spacer. Thus folding is due to the synergistic effect of C-T complex formation and metal ion binding. Further a new polymer with larger -surface area of electron acceptor units was prepared with naphthalene dimide (NDI) unit instead of PDI unit which is expected to show higher folding propensity. Chapter 3 explores the possibility of modulating the folding of the donor acceptor (D-A) polymer. A D-A polymer consist of adjacently placed DAN and PDI units linked by an oxyethylene glycol spacer. Folding of the D-A polymer is effected in the presence of suitable metal ion that binds to the oxyethyleneglycol spacer. Random copolymers with segments of alternately placed D-A pairs and segments that is devoid of D-A pairs were prepared. Depending on composition of the random copolymer, the stack length was shown to be modulated as evident from UV-visible and NMR titration experiments. Following a similar approach, a two step folding of the synthetic polymer was demonstrated. The synthesis and characterization of end functionalized polyesters by melt transesterification is discussed in chapter 4. Well defined linear polymer with propargyl group as the end functionalizable group is prepared by the polycondensation of AB type monomer whereas polycondensation of AB2 type monomer leads to peripherally functionalized hyperbranched polymer. Azide-alkyne ‘click’ reactions carried out at the chain end of linear polyester with fluorophores allowed the estimation of the molecular weight by UV-visible and fluorescence spectroscopic method which is compared with estimation from 1H-NMR. Similarly the glass-transistion temperature of hyperbranched polyester is modulated by the peripheral functionalization with various organic azides by ‘click’ reaction. Chapter 5 gives the conclusion and future directions based on the findings from the thesis work.