Stepladder polymers

For polyimides derived from p-phenylenediamine and a variety of dianhydrides, the order of stability with respect to the dianhydride is pyromellitic dianhydride 3,3, 4,4 -benzophenonetetra-carboxylic anhydride l,3-bis(3,4-dicarboxyphenyl)hexafluoro-propane dianhydride 1,4,5,8-naphthalenetetracarboxylic dianhydride. 

For polyimides derived from pyromellitic dianhydride and a variety of diamines, the order of stability with respect to the diamine is p-linked material m-linked material p-phenylenediamine 1,5-diaminonaphthalene 4,4 -diaminodiphenyl 1,4-diamino-anthracene 1,6-diaminopyrene, i.e., the stability decreases as the number of fused rings in the diamine increases. 

When using diamines of structure H2N—C6H4 X—C6H4—NH2, the stability decreases in the order X = single bond S SO2 CH2 CO SO O. (There are differences of opinion about this order of stability, most likely caused by the use of different experimental conditions.) 

Addition polyimides having reactive end groups (mostly aliphatic in character) show reduced thermal oxidative stability. These end groups are utilized to overcome the processing problems associated with condensation polyimides. 

Polyimides are used as matrix materials for composites, adhesives, fibers, and films. The use as an adhesive is discussed in more detail in the next section. 

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Ladder polymers are also produced by polycondensation reactions of tetrafunctional monomers. If a tetrafunctional monomer is reacted with a bifunctional monomer, as in the formation of polyimides, the derived polymer is referred to as a partial ladder or stepladder polymer. 

PFs, PlFs, and poly(ladder-type pentaphenylene)s have methine bridges, and so are structurally related to LPPP 3. Stepladder polymers have also been prepared with other types of bridges. 

As shown in this review, the properties of polyphenylene-based materials, so as to maximise their potential as active materials in LEDs or polymer lasers, can be controlled by dehberate synthetic design. By incorporating bridges between some or all of the phenylene units to make ladder or stepladder polymers the effective conjugation length of the polymer may be controlled, while the interactions between the chains and the injection of charges may be regulated by careful selection of substituents. By these means it is possible to minimise interchain interactions, which lead to loss of luminescence... 

As described for the corresponding polymers, a powerful strategy for arriving at soluble oligomers with maximum conjugative interaction is incorporation of the PPP backbone into a stepladder - or ladder (see Sect. 2.2.) - framework, in... 

The logical continuation of the stepladder strategy outlined above for minimizing the mutual distorsion of adjacent main chain phenylene units was the incorporation of the complete PPP-parent chromophore into the network of a completely planar ladder polymer. The complete flattening of the conjugated 7T-system by bridging of all the subunits should then lead to maximum conjuga-tive interaction. As with the PTHP 11 systems, alkyl or alkoxy side chains should lead to solubilization of the polymers. 

Polyfluorenes are sometimes referred to as stepladder poly(przra-phenylene) (PPP) polymers as they have a backbone with a planar, ladder-like structure. The first poly-fluorene LED was made by Yoshino and coworkers using poly(9,9-dihexylfluorene) 16 [50]. 

In general, the types of polymers which have the best thermal properties are aromatic in character (often with recurring heterocyclic units), have low hydrogen content, and often have stepladder or ladder structures. Although there are numerous articles in the literature which deal with the effects of structure on stability within a given class of heat-resistant polymers, only a limited number of publications are to be found which compare the stabilities of different classes of heat-resistant polymers under controlled conditions. From Ehler s TGA studies on different classes of heat-resistant polymers, as well as from other sources, a classification can be made of the effects of structure on heat stability for several classes of compounds. For... 

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