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ABSTRACT:The syntheses and properties of dendritic and hyperbranched aromatic polyamides and polyimides are reviewed. In addition to conventional stepwise reactions for dendrimer synthesis, an orthogonal/double-stage convergent approach and dendrimer syntheses with unprotected building blocks are described as new synthetic strategies for dendritic polyamides. Hyperbranched polyamides and polyimides composed of various repeating units are presented. Besides the self-polycondensation of AB 2 -type monomers, new polymerization systems with AB 4 , AB 8 , A 2 ϩ B 3 , and A 2 ϩ BBЈ 2 monomers have been developed for hyperbranched polyamides and polyimides. The copolymerization of AB 2 and AB monomers is discussed separately with respect to the effects of branching units on the properties.

Two novel B3 monomers, tri(phthalic anhydride) and tri(phthalic acid methyl ester), were synthesized. Hyperbranched polyimides were prepared by A2 + B3 polymerizations of (a) 1,4-phenylenediamine (A2) and tri(phthalic anhydride) (B3) (method A) and (b) 1,4-phenylenediamine (A2) and tri(phthalic acid methyl ester) (B3) (method B) in a 1:1 molecular ratio. Gelation was effectively avoided in the A2 + B3 polymerization by method B and a dramatic inherent viscosity increase at the critical polymerization concentration was observed. The high viscosity phenomena, generally observed in the preparation of hyperbranched polymers through the A2 + B3 approach, are elucidated by the hyperbranched structure (dendritic, linear, and terminal content) characterization for polyimides with different viscosities. The self-standing films were successfully prepared from the hyperbranched precursors by the casting method. The results indicate that the weight-average molecular weight of hyperbranched precursors are ranged from 33 600 to 125 000 and their inherent viscosities are varied from 0.17 to 0.97 dL/g. The degree of branching (DB) of hyperbranched polyimides is estimated to be 0.52−0.56 by 1H NMR measurement. Their glass transition temperatures measured by differential scanning calorimetry (DSC) range from 212 to 236 °C. The 5% weight loss temperatures of films, measured by thermogravimetric analysis (TGA), are around 500 °C. Their tensile storage modulus by dynamic mechanical thermal analysis (DMA) attains 4.0 Gpa, similar to that of their linear analogues.

Previous experiments from our group [Macromolecules 1998, 31, 5964] have established that thermal polymerization of 3,5-bis(4-aminophenoxy)benzoic acid (monomer 1) gave a hyperbranched aromatic polyamide (polymer 1). Here we show that thermal polymerization of methyl 3,5-bis(4-aminophenoxy)benzoate (monomer 2), having the same reaction behavior as that of monomer 1, gives a hyperbranched aromatic polyamide (polymer 2). In addition, the direct polycondensation of monomer 1 was conducted in the presence of triphenyl phosphite and pyridine as condensing agents in NMP to give a hyperbranched aromatic polyamide (polymer 3). The structures of the resulting polymers were confirmed to be identical by IR, 1H NMR, and 13C NMR. All three polymers were soluble in DMF, DMAc, NMP, DMSO, and 2-methoxyethanol. The inherent viscosity of the polymers in DMF ranged from 0.17 to 0.19 dL/g. Absolute molecular weights (M w) determined by laser light scattering for polymers 1, 2, and 3 were 74 600, 47 800 and 36 800, respectively, and the corresponding polydispersities were 2.6, 3.2, and 1.8. The glass transition temperatures (T g) of polymers 1, 2, and 3 were 200, 180, and 200 °C, respectively. End-capping reactions of the terminal amino groups in polymer 3 was easily accomplished with several kinds of acid chlorides. Thermal properties and solubility of the polymers changed after end-capping reactions.