Ladder chemical structures

Figure 10-6. Chemical structure of the ladder-type poly(/xi/u-pheny-Icne). X represents u methyl-group and R and R are /i-hcxyl aud 1,4-dccylphcnyl, respectively.

In Figure 8-1 we show the chemical structure of m-LPPP. The increase in conjugation and the reduction of geometrical defects was the main motivation to incorporate a poly(/ -phenylene)(PPP) backbone into a ladder polymer structure [21]. Due to the side groups attached to the PPP main chain excellent solubility in nonpolar solvents is achieved. This is the prerequisite for producing polymer films of high optical quality. A detailed presentation of the synthesis, sample preparation,... 

Figure 7.10 Chemical structure of a monomeric pseudorotaxane electropolymerized ladder polymer (Buey and Swager 2000).

Moreover, the differences in the segmental anisotropy of ladder polymers may be related to the differing extent of the defectiveness of their double-chain structure (see p. 100). This can be demonstrated by a comparison of the anisotropy of various samples of ladder polyphenylsiloxanes with an identical chemical structure of monomer units but with the values of ([n]/[r ])oo and, hence, of Oj - 2 which may differ by a factor of 2.5. In fact, if angle bonds are introduced into a cyclic chain a partially ladder structure results which greatly decreases the equilibrium chain rigidity. 

Fig-l The steady state absorption and emission spectra of (a) methyl ladder type polymer, MeLPPP (b) poly[9,9-di-n-(2 ethylhexyl)fluorene] (PF2/6, n 60) (c), pentafluorene (d) trifluorene and (e) monofluorene measured in dilute toluene solution are compared. The insets depict the chemical structures of each. All spectra were measured in dilute MCH solution... 

Chart l.n Chemical structure of a ladder-type poly(p-phenylene). 

A new class of polymers with high free volumes has been introduced recently by Budd et al. [290]. The molecular structure of these polymers contains sites of contortion (e.g. spiro-centers) within a rigid backbone (e.g. ladder polymer). A typical chemical structure is shown in Fig. 7.5 A. 

FIGURE 5.7 Absorption, photoluminescence emission, and photoinduced absorption spectrum of a polymer film of ladder-type polymer with pentaphenyl segments. The inset shows the actual chemical structure. 

Stabilization process which is carried out in air (oxidative stabilization) constitutes the first and very important operation of the conversion of the PAN fiber precursor to carbon as well as activated carbon fiber. During stabilization, the precursor fiber is heated to a temperature in the range of 180-300 °C for over an hour. Because of the chemical reactions involved, cyclization, dehydrogenation, aromatization, and oxidation and cross linking occur and as a result of the conversion of CH N bonds to C=N bonds fully aromatic cyclized ladder type structure forms [36, 49]. 

Polyimidazopyrrolone (ladder pyrrone, polypyrrolone) n. An aromatic, heterocyclic polymer that results from the reaction of an aromatic dianhydride with a tetramine. Due to the double-chain or ladder-hke structure, these polymers have outstanding resistance to radiation, chemicals, and heat (no weight loss to 550°C). However, this structure also makes them difficult to process. To overcome this difficulty pyrrone pre-polymers in the form of solutions and salt-like powders have been made available. The powders can be molded under conditions that complete the cychzation or conversion of the ladder-hke molecular structure during the molding cycle. The cychzation reaction generates water, which must be removed from the part. 

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. 

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