Synthesis and Characterization of Hyperbranched Polymers

Abstract

The applications of macromolecules are widespread in the modern world and their utilities keep on increasing. Several significant developments in the field of polymeric materials engaged transfer of advantageous characteristics via control of polymer architecture. The interest in hyperbranched polymers (HBPs) stems from the ability to manipulate polymer composition to impart a particular set of traits. This basic research work on HBPs was driven by curiosity and charisma of these extremely elegant and intricate architectures. The present thesis aimed to fabricate HBPs in a one-pot method using new monomers synthesized in this work. All the newly synthesized monomers were subjected to FTIR, 1H, 13 C NMR spectroscopy and elemental analysis. The next goal was to utilize commercially available monomers in the facile synthesis to different families of HBPs. The reactions were stopped before gelation by the optimization of polymerization conditions. A three pronged strategy to HBP was pursued and in each specific strategy AB2, A2 + B3 and 2A2 + CBB′, the shell chemistry of the HBPs was employed to tailor desirable properties. Initially, a novel AB2 monomer, 6-hydroxy-2,4-bis(4′-nitrobenzamide)pyrimidine (NAL), was synthesized and successfully polymerized to hyperbranched polyamide-ethers (HBPAEs), which was subsequently end modified. Afterward, new trifunctional monomers, 1,3,5-tris(4'-hydroxyphenylcarbamoyl)benzene (THPCB), 1,3,5-tris(3',4'- carboxyphenyl)benzene trianhydride (TAn), and 1,3,5-tris(4′- aminophenylcarbamoyl)benzene (TAPCB) were designed, synthesized and efficiently polymerized to hyperbranched polyamide-esters (HBPAE), hyperbranched polyester- imides (HBPEI) and hyperbranched polyamides (HBPAs) respectively, via the polymerization of A2 and B3 monomers without gelation. In these cases a dilute A2 solution was added slowly to a dilute B3 solution to prepare HB samples in the absence of gelation. A new 2A2 + CBB′ approach was undertaken where an A2C dimer was formed initially which can be considered as a new A2B type of monomer. Further reaction among A2C molecules resulted in the formation of hyperbranched polyamide-esters (HBPAEs) containing pyrimidine moieties. Complete structural 1 elucidation of the ensuing HBPs was carried out using FTIR, H and i 13 C NMR spectroscopic analyses. Additionally, all the prepared HBPs were characterized for qualitative solubility test, inherent viscosity, molar mass, thermal stability, glass transition temperature (Tg) and crystallinity. Structure-property correlations were established and it was observed that properties depended on end functionality. The degree of branching (DB) determined for HBPs based on the 2A2 + CBB′ approach was found to be >60 % and 41-56 % for the AB2 and A2 + B3 systems. Molar masses were determined using GPC which showed that the prepared HBPs had moderate Mw values. Thermal analyses for different HBPs systems showed that aliphatic HBPs were less thermally stable and had values of Tg lower than aromatic ones. HBPs contain a distinct multiplicity of peripheral functionalities which offer sites for additional chemical modification or as templates for non-covalent intermolecular interactions. Modification of the end groups in the prepared HBPs was carried out using different modifiers and complete modification was achieved proving that reactive terminal functionalities were easily accessible. The modified polymers displayed good solubility in different organic solvents. The properties of HB systems were compared with their linear analogues based on the same backbone structure. Accordingly, solution viscosity measurements exhibited that HBPs had lower solution viscosity, enhanced solubility and predominantly amorphous character compared to their linear analogues of comparable molar masses. Tg evaluation of HBP samples of different systems were carried out and showed that our HB systems demonstrated a lower Tg than their linear counterparts. Intermolecular hydrogen bonding between pyrimidine nitrogens and the amide NH groups of adjacent molecules provided the basis for material uniqueness. In addition, pyrimidine moieties played a vital role in producing outstanding thermal properties of HBPs; therefore, pyrimidine rings influenced the structural and material characteristics of these HBPs. Future prospects and potential applications of these HBPs are also envisaged.