Symmetrical bn Monomers

Systematic studies of a2 -I- b3 polycondensations began with the work of W. Smith [32], who reported in 1901 on polycondensations of glycerol and phthalic 

Several research groups reported on syntheses and characterization of hb polyimides [51-58].The first contribution came from Okamoto et al. [52], who used the triamine (a) (Formula 10.1) in combination with commercial tetracarb-oxylic anhydrides ((c) and (d)) as monomers in a2/b3 ratios of 1 1 and 2 1 at low concentrations (0.025 mol/L). These feed ratios had the purpose to generate either hb polyimides rich in b end groups or rich in a groups and to reduce the risk of cross-linking. The same feed ratios and anhydride (d), but triaminopyrimidine (b) were used by Liu et al. [52]. A third triamine ((e) in Formula 10.1) served as b3 monomer in the work of Chen and Yin [53-55]. The alternative concept, namely the polycondensation of a trifunctional anhydride ((f) in Formula 10.1) with commercial diamines was explored by the research group of Kakimoto and Jikei [56-58]. Again, equimolar feed ratios and low monomer concentrations 

Another approach to polyesters having only few branching points consist of the copolycondensation of a2 -I- b2 monomers with small amounts of a3 or b3 monomers. In addition to trimesic acid or its trimethylester, the multifunctional alcohols outlined in Pormula 10.2 served as branching monomers. This approach was intensively studied by several research groups to modify the physical and mechanical properties of commercial polyesters such as PET, PBT, or poly (butylene succinate). A detailed discussion of syntheses, properties, and applications presented in the excellent review of Long et. al [64]. 

HB polyarylenes were synthesized by two research groups via different C-C coupling methods. Smet et. al [65]. studied the condensation of isatins with a trifiinctional phenyl ether (see Formula 10.3). Trifluoromethane sulfonic acid served as catalyst and the low monomer concentration prevented gelation. This approach is, in principle, a variant of the acid catalyzed aldehyde-phenol polycondensation developed more than 100 years before for the production of Novolac and Bakelite (see Chap. 2). Possum et al. [66]. studied Pd-catalyzed polycondensations of 1,4-benzene diboronic acid with tri(4-bromophenyl)phosphioxide (see Formula 10.3). 

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