Syntheses of thermotropic liquid crystalline polymers, 2. polyurethanes

Polyurethanes were synthesized by reaction of a rigid molecule, 3,3′‐dimethyl‐4,4′‐biphenyldiyl diisocyanate (DBD), with flexible molecules, oligoethylene glycols (di‐, tri‐, and tetraethylene glycol) or α,ω‐alkanediols, and some of their thermal properties were studied. Poly(DBD‐oligoethylene glycol)s are amorphous polymers which exhibit no mesomorphic phase. Poly(DBD‐alkanediol)s are found to show a mesomorphic phase depending on the number of methylene units in the alkanediol. The polymers having methylene units from five to twelve are crystalline and show a mesomorphic phase. On the contrary, polymers having less than four methylene units do not show a mesomorphic phase since thermal decomposition of the polymers occurs before melting. The transition temperature from the mesomorphic phase to the isotropic liquid decreases with increasing length of the flexible chain of alkanediol. Ternary polymers are obtained by reaction of DBD with a mixture of hexanediol (HD) and dodecanediol (DD). The ternary polymers containing a small amount of HD and DD give a clear mesomorphic phase.