Correlation of model compounds and laterally attached side-chain liquid-crystalline polynorbornenes with an 11-carbon spacer

Poly{5-[[[11'-[2 ",5 "-bis[2-(3'-fluoro-4'-n-alkoxyphenyl)ethynyl]phenyl]undecyl]oxy]carbonyl]bicyclo[2.2.1]hept-2-ene}s exhibit an enantiotropic nematic mesophase when n = 2-12. In contrast to the corresponding reference polymers with a 1-carbon spacer, none of these polymers with an 11-carbon spacer crystallize if their molecular weight is sufficiently high. Although the corresponding 1,4-bis[(3'-fluoro-4'-n-alkoxyphenyl)ethynyl]-2-undecylbenzenes exhibit only a monotropic nematic mesophase when n = 2, 4, 6-11, their isotropization temperatures are nearly identical to those of the polynorbornenes, and they are therefore appropriate models of the polymers. The polymers (DPn = 14-158, pdi = 1.14-1.37) were prepared by ring-opening metathesis polymerization of 5-{[[11'-[2 ",5 "-bis[2-(3'-fluoro-4'-n-alkoxyphenyl)ethynyl]phenyl]undecyl]oxy]carbonyl}bicyclo[2.2.1]hept-2-enes in THF using Mo(CHCMe2-Ph)(N-2,6-(Pr2Ph)-Pr-i)((OBu)-Bu-t)(2) as the initiator. Their thermotropic behavior becomes independent of molecular weight at approximately 30 repeat units.