Synthesis and Comparison of Hyperbranched Aromatic Polyimides Having the Same Repeating Unit by AB2 Self-Polymerization and A2 + B3 Polymerization

Abstract

A new AB2 monomer, designed from a template of the repeating unit of the hyperbranched polyimides by nonideal A2 + B3 polymerization, was successfully prepared via a multistep synthesis. Hyperbranched polyimides, having the same repeating unit as that by nonideal A2 + B3 polymerization, were prepared from this new AB2 monomer by direct self-polycondensation in the presence of diphenyl (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate (DBOP). Hyperbranched polyimides, with a molecular weight of 1.11 × 104 to 1.73 × 105 and an inherent viscosity of 0.12−0.17 dL/g, were obtained. As-prepared hyperbranched polyimides were soluble in DMF, DMAc, DMSO and NMP. By 1H NMR analysis, the degree of branching (DB) of the prepared hyperbranched polyimides was estimated to be around 0.50. By thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement, their 5% weight loss temperatures and glass transition temperatures were found to be in the ranges 445−460 and 155−161 °C, respectively. A comparison between the hyperbranched polyimides by AB2 self-polymerization and nonideal A2 + B3 polymerization reveals that their physical properties, such as viscosity, thermal stability, and glass transition temperature, etc., are different from each other. Especially, a comparison between the plots of intrinsic viscosity [η] vs molecular weight Mw elucidates that their molecular shapes are also different each other. The examination of the chain entanglement and intermolecular interaction by employing a freeze-extracting technique reveals that hyperbranched polyimides by nonideal A2 + B3 polymerization have a certain extent of chain entanglement and intermolecular interaction. A comprehensive analysis finally suggests that hyperbranched polyimides by AB2 self-polymerization have a compact highly branching structure, while that by nonideal A2 + B3 polymerization have a low branching density topology.