Polymers Containing Heterocycles in the Backbone

Polymers containing heterocycles in the backbone include a variety of compounds, as the diversity of heterocyclic molecules is quite large. The polymers from this class may contain groups derived from furan, thiophene, pyrrole, isoindole, benzimidazole, benzothiazole, benzoxazole, quinoxaline, etc. Macromolecules with a ladder backbone containing, for example, a phenoxazine unit in their structure also are known. Amino thermosetting resins from melamine can be considered as polymers containing heterocycles in their structure. 

Table 15.1.1. Polymers with elevated decomposition temperature containing heterocycles in the backbone.

Rapidly developing branches of modern technology put forward increasingly higher requirements from applied polymers. In this connection thermally and chemically resistant polymers have been widely studied [1-11]. Of special interest have been aromatic polymers containing heterocycles in their backbone structures. The interest is accounted for by a complex of valuable properties, sometimes unique physicochemical and mechanical ones, displayed by polymers and related materials [1-23]. 

Ladder polymers have their backbone made from an interrupted series of condensed rings. In such polymers, very frequently the rings contain heteroatoms, and the polymer can be considered as part of the class of polymers with heterocycles in the main chain. One of the first synthesized polymers from this class was obtained from the oxidation and heating of polyacrylonitrile [1, 2]. Multifunctional condensations also can lead to ladder polymers. For example, a poly(phenoxazine) is formed from the reaction of a substituted quinone and a diaminodihydroxybenzene as shown below ... 

In summary, the preparation of polyimidazolinones from polyamides containing a-aminoacid units (3, X = NH) can now be considered to be a general reaction provided that Rz and/or R3 are not hydrogen. When the polyamide has additional secondary or tertiary amine functionality in the backbone, cyclodehydration appears to be exceptionally facile. In the absence of amine functionality however, a catalyst is necessary to promote cyclization. Further studies of this new heterocyclic polymer system are ongoing in our laboratories. 

Epoxy resins are among the most important of the high performance thermosetting polymers and have been widely used as structural adhesives and matrix for fiber composites. Epoxy resins are characterized by the presence of epoxide groups before cure, and they may also contain aliphatic, aromatic, or heterocyclic structures in the backbone. The epoxy group can react with amines, phenols, mercaptans. 

It should be noted that a number of SPC polymers which contain other heterocycles have been prepared, motivated by their promising optical or electrical properties. Examples include pyridine (65) [123], pyrrole (66) [124], oxadiazole (67) [125], selenophene (68) [126], benzo[2,l,3]thiadiazole (69) [127], benzo[2,l,3]selenadiazole (70) [126], perylene bisimide (71) [128], 1,4-diketo- pyrrolo[3,4-c]pyrrole-l,4-dione (72 and 73) [127,129], and triphenyleamine (74) [127] as part of the polymer backbone by SPC (Figure 19c). Specifically for metal complexation, porphyrin [130], difluoroboraindacene [131], bipyridine [132], phenanthroHne [113], terpyridine [133, 134], and the like [123] were embedded in the backbone. In this context, an interesting report was submitted by Rehahn et al., in which l,l -ferrocenyl units were incorporated into a PPP (Figure 22.20). Due to a low-energy barrier for rotation around the Cp-Fe-Cp axes (Cp = cyclopentadienyl), the obtained polymer 75 was assumed to take randomly coiled conformations [135]. 

This part of the review is devoted to PCSs prepared from silicon-carbon cyclic compounds. In the first section, we will consider the polymerization of saturated and unsaturated silicon-carbon heterocycles (SCHs) via a Si-C bond rupture (ringopening polymerization, ROP) and of unsaturated SCHs via a chain C=C bond rupture (ring-opening metathesis polymerization, ROMP). The polymerization of silicon homocycles is beyond the scope of this review since polymers containing only Si atoms in the backbone belong to a special branch of organosiUcon chemistry. 

An oxygen transfer reaction is presumably the origin of the successful photo-cross-linking of several polymers containing heterocyclic N-oxide moieties in the backbone or with N-oxides as additives " as well as for photolithography. "... 

Macromers are then short polymers, which contain an active end group. This end group can be a site of unsaturation, heterocycle, or other group that can further react. Macromers are usually designed as intermediates in the complete synthesis of a polymeric material. These macromers can be introduced as side chains (grafts) or they may serve as the backbones (comonomer) of polymers. The macromers can also act as separate phases. 

Since 1985, a major effort has been devoted to incorporating heterocyclic units within the backbone of poly(arylene etherjs (PAE). Heterocyclic units within PAE generally improve certain properties such as strength, modulus and the glass transition temperature. Nucleophilic and electrophilic aromatic substitution have been successfully used to prepare a variety of PAE containing heteorcyclic units. Many different heterocyclic families have been incorporated within PAE The synthetic approaches and the chemistry, mechanical and physical properties of PAE containing different families of heterocyclic units are discussed. Emphasis is placed on the effect variations in chemical structure (composition) have upon polymer properties. 

Ring-opening polymerization of a number of bicyclic heterocycles provides a series of polymers containing the tetrahydropyran ring (78MI11102). For example, 6,8-dioxabicyclo[3.2.1]octane (125) has been polymerized to stereoregular polymer (126) which has the natural dextran backbone (Scheme 36). Similarly, monomers (127) and (128) can be polymerized to yield polyesters and polyamides, respectively (Scheme 37). Interest in these types of polymer has been spurred by their obvious similarity to polysaccharides and to naturally occurring ionophores. 

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