Rigidity, aromatic ring substitution

Mullen and coworkers utilized the Diels-Alder reaction to prepare hb-PPs (hb-P25 to hb-P27) with Mw up to 107000 from ethynyl-, propynyl-, and phenylethynyl-substituted tetraphenylcyclopentadienones (Scheme 13) [47]. Despite their high molecular weights and rigid phenyl rings, all the polymers showed good solubility in common aromatic solvents such as toluene and benzene. 

Structurally similar polymers with azo links, Polymers 9 and 10, formed no meso-phases when the rigid aromatic unit contained terminal carbonyl groups, but these formed nematic mesophases when the ester link was reversed The transition temperatures of the azo and azoxy polymers with the same spacer were quite close to each other. Substitution of methyl groups on the phenylene rings of these polymers reduced melting temperatures without adversely affecting nematic mesophase stability... 

However, the stereoselective reductions of the 5-substituted adamantanones 3.50 [CT2, G5, KA4, LL5], and bicyclo-[2.2.2]-octan-2-one 3.51 [MK5] have been interpreted in terms of Cieplak s model [C6, CT3]. Unfortunately, the observed stereoselectivities are low (de <40%), so that it is difficult to dissect the various parameters involved in such reductions [CHS, N2], The electronic contribution to the stereoselectivity of the reduction of substituted 9-benzonorbomenones 3.52 is very important [G5, OTl, OT2] (Figure 3.20). Halogen substituents on the aromatic ring favor syn attack by all hydrides on those very rigid systems. Electrostatic effects have been invoked to interpret these results [PW2, WTl]. 

C=CH) instead of iodine in the )4ra-position of the aromatic rings. The coupling reaction resulted in topologically identical networks composed of rigid arylene-ethynylene struts of different lengths and trifunctional joints of the 1,3,5-substituted benzene, as exemplified below ... 

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