Diels–Alder Polymers
The Diels–Alder reaction has been used to form many polymeric materials. One such material, for instance, forms from a reaction of diethynyl benzene with cyclopentadienone [194–196]. The products, phenylated polyphenylenes, reach molecular weights of Mn up to 40,000:
where x = 1,2. These polymers are amorphous and form clear films that are stable in air up to 550C. Another example is condensation of 2-vinyl-1,3-butadiene with p-benzo-quinone. The product is a ladder structure, or a double-stranded polymer [197]:
This double-stranded polymer, shown above, is soluble only in hexafluoro isopropyl alcohol [198] and is infusible. When, however, some of the rings are made flexible by condensing 2-vinyl-1,3 butadiene with a large ring bis fumarate, the solubility improves [198]. Some stable polymers also form in 1,3-dipole addition reactions [190]. Bis-sydnones, for instance, condense with diacetylene to form pyrazole rings in the polymer backbone. The reactions presumably proceed through Diels–Alder intermediates [199]:
Similarly, bis-sydnones condense with quinone [199]:
The resultant polymers are not high in molecular weights and only slightly soluble in solvents like dimethylformamide. Their powders decompose near 420C in air and near 500C in a nitrogen atmosphere. 1,3-Dipolar additions of bisnitrileimines (generated from tetrazoles with dienes or with di nitriles) result in formations of polypyrazoles and polytriazoles [200]. Some examples of these reactions are:
By this condensation, high molecular weight polymers can form [200]. They decompose near 500C in air or in nitrogen atmosphere. Cyclopentadienone derivatives condense by the Diels–Alder reaction in homopolycycloaddition. The reactions involve a series of steps consisting of initial cycloaddition, followed by loss of carbon monoxide through an expulsion of a bridge carbonyl group [201]:
The products are ladder polymers of varying molecular weights. They loose approximately 30% [201] of their weight at 700C in a nitrogen atmosphere. Another example is condensation of bis dienophiles with dienes [202]:
Only low molecular weight polymers form, however, together with some insoluble, possibly cross- linked material. Bis maleimides also condense with bisfulvene [202]. The products of these condensations depolymerize reversibly. Also, reactions of maleic anhydride with dienes, like bicycloheptene, bicyclohexene, dicyclopentenyl ether, and dicyclohexenyl ether, yield soluble, low molecular weight polymers [204]:
Photo-cycloaddition reactions also form low molecular weight polymers with heterocyclic rings in the backbone. The reactions are photoinitiated with benzophenone to obtain repeated 2л + 2л photo- cycloadditions of bismaleimides [205]:
where, R (CH2)x. These polymeric materials form transparent, flexible films [205].
Diels-Alder reaction can also be used to modify the chemical structures of some aromatic polyamides. This improves their heat stability, raises their T, value, and makes them rigid at higher temperatures [203].
