TOWARD WILLOWLIKE THERMOTROPIC DENDRIMERS

The synthesis and characterization of the AB2 monomers 6-bromo-1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)hexane (8 or TPH-b), 13-bromo-1-(4-hydroxyphenyl)-2-[4-(6-hydroxy-2-naphthalenylyl)phenyl)tridecane (22 or BPNT-b), and 13-bromo-1-(4-hydroxyphenyl)-2-(4-hydroxy-4''-p-terphenylyl)tridecane (30 or TPT-b) are described. The phase-transfer-catalyzed polyetherification of all monomers followed by in situ alkylation of their phenolate chain ends with bromoalkanes, benzyl chloride, or allyl chloride led to the soluble hyperbranched polymers TPH-b-X, BPNT-b-X, and TPT-b-X (where X refers to the nature of their chain ends, for example, when X = Bz = benzyl, X = All = allyl, and, when X = numeral, it represents the number of carbons in the alkyl chain end). The minimum energy conformations of the flexible branching points of these polymers are anti and gauche. The gauche conformer leads to a hyperbranched polymer with a conventional treelike architecture. By analogy with the behavior of the corresponding linear polymers, above their glass transition temperature, these hyperbranched polymers are considered to minimize their free energy by lowering their free volume via a conventional nematic mesophase which is generated by the conformational change of their structural units from gauche to anti. The overall change in the architecture of these hyperbranched polymers via the anti-gauche conformational change of their structural units resembles that of a willow. Regardless of the nature of X, TPH-b-X and BPNT-b-X display only a very narrow enantiotropic nematic mesophase. TPT-b-8 with M(n) = 11 800 and M(w)/M(n) = 2.42 exhibits an enantiotropic nematic mesophase over a range of 82-degrees-C. The degree of branching of TPT-b-All is 0.82. Thus TPT-b is of extreme interest for further investigations on this novel class of hyperbranched polymers and for the synthesis of thermotropic liquid crystalline dendrimers.