MOLECULAR ENGINEERING OF LIQUID-CRYSTAL POLYMERS BY LIVING POLYMERIZATION .3. INFLUENCE OF MOLECULAR-WEIGHT ON THE PHASE-TRANSITIONS OF POLY(8-[(4-CYANO-4'-BIPHENYL)OXY]OCTYL VINYL ETHER) AND OF POLY(6-[(4-CYANO-4'-BIPHENYL)OXY]HEXYL VINYL ETHER)

The synthesis and living cationic polymerization of 8-[(4-cyano-4'-biphenyl)oxy]octyl vinyl ether (6-8) and of 6-[(4-cyano-4'-biphenyl)oxy]hexyl vinyl ether (6-6) are described. The mesomorphic behavior of poly(6-8) and poly(6-6) with different degrees of polymerization and narrow molecular weight distribution was compared to that of the 6-8 and 6-6 and of 8-[(4-cyano-4'-biphenyl)oxy]octyl ethyl ether (8-8) and 6-[(4-cyano-4'-biphenyl)oxy]hexyl ethyl ether (8-6). 8-8 and 8-6 are the model compounds of the monomeric structural units of poly(6-8) and poly(6-6). 6-8 displays an enantiotropic nematic mesophase, 6-6 and 8-6 exhibit a monotropic nematic mesophase, and 8-8 gives a monotropic nematic and a monotropic s(A) mesophase. Poly(6-8) with a degree of polymerization of 2.1 and poly(6-6)s with degrees of polymerization from 3.3 to 7.3 display an enantiotropic s(A) and an enantiotropic nematic mesophase. Poly(6-8)s with degrees of polymerization from 4.2 to 10.2 and poly(6-6)s with degrees of polymerization from 7.6 to 13.5 display an enantiotropic s(A) mesophase. Poly(6-8)s with degrees of polymerization larger than 12.4 and poly(6-6)s with degrees of polymerization higher than 23.2 display an enantiotropic s(X) (i.e., an unidentified smectic phase) and an enantiotropic s(A) mesophase. Poly(6-8)s and poly(6-6)s with degrees of polymerization from 1 (i.e., 8-8 and 8-6) to 30.0 represent the first examples of side-chain liquid-crystalline polymers that show a continuous change of the mesomorphic-isotropic phase transition (i.e., from nematic-isotropic to smectic-isotropic) on increasing their molecular weight.