Semiladder structure

A ladder (or two-strand) structure, as shown above, is one that has an uninterrupted sequence of rings joined one to another at two connecting atoms. A semiladder structure, on the other hand, is one in which there are single bonds interconnecting some of the rings. Polyimide (XIV) obtained by polycondensation of pyromellitic anhydride (XII) and m-phenylenediamine (XV) is an example of semiladder polymer. 

An alternative to a linear polymer is a branched one. The branches can be long or short. Low-density polyethylene, for instance, can have both short and long branches. Linear and branched molecules are illustrated in Fig. 1.1a and b. Branched polymers can also be star or comb shaped (Fig. 1.1c and d). In addition to the above, polymer molecules can also be double stranded. Such polymers are called ladder polymers (Fig. 1. le). It is also possible for polymers to have semiladder structures (Fig. l.lf). 

Trimellitic anhydride (TMA) is an important raw material for high temperature resistant polymers such as polyesterimide, poly-amideimide etc. The cyclic anhydride group in the TMA molecule offers a reaction site for imide formation and the carboxyl group for synthesis of ester or amide linkages. The polycondensation of TMA with glycol and/or diamine results in the formation of semiladder structures in the polymer, which is responsible for high temperature resistance. 

It may be safely stated, in spite of the amount of material publi ed, that there is still much to do in the area of ladder pol3uners. The initial excitement over the myriad possibilities for the technological applications of such materials as are implicit goals in the work described here has been dampened somewhat by disappointment and experimental difficulty. It is to be hoped that the more prosaic task of evaluation will find as many investigators willing to undertake it as did the problems of synthesis. An important step yet to be undrrtaken is the rigorous examination of the effects of structure on thermal properties. Attempts must be made to determine accurate tl extent of ladder formation in many of these pofymers, e.g., by the study of soluble, semiladder structures. 

Spiro polymers are also sometimes classified as ladder polymers, and molecules in which the ladder structure is interrupted by periodic single bonds are called semiladders. Consisting entirely of fused ring structures, ladder polymers possess very rigid chains with excellent thermal stability. 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

Other examples of semiladder-like polymers include the poly(benzoxazole)s, poly(phenylene-13,4-oxadiazole)s, poly(quinoxaline)s, and poly(s-triazine)s. The high-temperature performance of these and other structures discussed are shown in Table 15.8. 

FIGURE 1.1. Shapes of polymeric molecules, (a) linear polymer, (b) branched polymer, (c) star>shaped polymer, (d) comb-shaped polymer, (e) ladder polymei (f) semiladder polymer, and (g) network structure. 

We report here morphological, electrochemical and spectroelectrochemical studies of semiladder poly(benzobisimidazobenzophenanthroline) (BBB), ladder poly(benzobisimidazobenzophenanthroline) (BBL) and poly(2,2 -[10-mediyl-3,7-phcnothiazylene]-6,6 -bis[4-phenylquinoline]) (PPTZPQ) e structures are shown in Figure 2. We also describe the construction and characterization of die electro-optical properties of all-polymer electrochromic devices using PPTZPQ as one electrode and BBL, BBB or V2O5 as die counter electrode. 

Heterocyclic polymers yield materials with outstanding high-temperature performance see Table 7.2 (2). Many of these polymers have ladder or semiladder chemical structures. If one bond in a ladder structure is broken by heat or oxidation, the chain may retain its original molecular weight. If a single carbon atom chain is oxidized, usually the chain is degraded, with concomitant loss of properties. 

Space limitations do not permit the description of other varieties of rigid chain macromolecules, such as semiladder and spiro structures, which are of lesser current commercial importance. 

---