Pyromellitic dianhydride, polycondensation with

Suspension polycondensation of pyromellitic dianhydride and 3,5-diamino-1,2,4-triazole yielded triazole-containing polyimide beads that were used as a support for Mo02(acac)2 [60]. The resulting catalyst showed high activity and selectivity in the epoxidation of cyclohexene and cycloctene as well as in the epoxidation of noncyclic alkenes such as styrene, 1-octene, and 1-decene with TBHP. The catalyst could be recycled 10 times and activity decreased significantly in the case of 1-octene epoxidation whereas activity remained high in the epoxidation of cyclic alkenes. 

The structure of a ladder polymer comprises two parallel strands with regular crosslinks [Fig. 1.3(e)], as in polybenzimidazopyrrolone (XII), which is made by polycondensation of pyromellitic dianhydride (XIII) and 1,2,4,5-tetraminobenzene (XIV). This polymer is practically as resistant as pyrolitic graphite to high temperatures and high energy radiation. It does not burn or melt when heated but forms carbon char without much weight loss. 

Polyimides with third-order NLO properties have been prepared by Lu et al. [56] by polycondensation pyromellitic dianhydride with either benzoguanamine or 3,3 -diaminobenzophenone under microwave irradiation conditions (Scheme 14.27). The polymers obtained under microwave conditions were characterized by large third-order nonlinearities and time response. 

Lu et al. have obtained poly(amic acid) side-chain polymers by polycondensation of benzoguanamine and pyromellitic dianhydride under microwave irradiation conditions [65-67]. The reactions were performed in a household microwave oven in which 100 mL DMF solution of 33 mmol benzoguanamine and an equimolar amount of pyromellitic dianhydride were stirred and irradiated for 1 h at 60 °C (Scheme 14.31). The resulting poly(amic acid) was precipitated from the solution and then modified to obtain side-chain polymers with fluorescent and third-order NLO properties. 

Polycondensation and imidization of w,w -diaminobenzophenone and pyromellitic dianhydride under microwave radiation was also carried out. The product polyimide was obtained in a two-step process. It is claimed that this product of microwave radiation polymerization compares favorably with a product of conventional thermal polymerization, because it exhibits third-order nonlinear optical coefficient of 1.642 x 10 esu and response time of 24 ps. The third-order optical nonlinearity of this polymer is dependent on the chain length and the molecular structure. 

To improve the thermal stability of these polymers, Biswas and Mitra [349] prepared copolycondensates of carbazole (CBZ) with phthalic anhydride (PtiAn), trimellitic anhydride (TMA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalene tetracar-boxylic dianhydride (NTDA), and benzophenone tetracarboxy dianhydride (BTDA) along with similar copolycondensates of PNVC. The thermal stabihty of copolycondensates was observed to depend on anhydride moiety in the order NDTA > BTDA > PMDA > TMA > PhAn for both the types of materials as given in Table IX. Table X shows the isotherm degradation stability of pairs of polycondensates, and the data confirm the high thermal stability of NTDA over the other materials. 

There is a wide variety of possible structures based on the use of glycerol (v = 3), trimethylolpropane (n = 3), pentaerythritol (n = 4), and sorbitol (n = 6) or their mixture as polyol, reacting not only with phthalic anhydride (w = 2) but also with pyromellitic dianhydride (w = 4), citric acid (v = 4), trimellitic acid (v = 3), or their mixture as polyacid. The most important resins are those obtained from the polycondensation of 0-phthalic anhydride with glycerol, and the resulting network is represented hereafter. 

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