SYNTHESIS OF SOLUBLE, LIQUID-CRYSTALLINE ARAMIDS VIA TRANSITION-METAL PI-COMPLEXATION

An organo-soluble, liquid crystalline organometallic polymer, (p-phenyleneterephthalamide)Cr(CO)3 [(PPTA)Cr(CO)3], is prepared by polycondensation of (p-phenylenediamine)Cr(CO)3 with terephthaloyl chloride. Polarizing optical microscopy of this high molecular weight metallo-aramid shows a nematic liquid crystalline texture in N,N-dimethylacetamide (DMAc), demonstrating that metal pi-complexation maintains the lyotropic, rigid-rod character of PPTA. A PPTA copolymer with 50% of the diamine rings Cr(CO)3 pi-complexed is also soluble and lyotropic in DMAc. Tensile properties of films of the 100% Cr(CO)3 complexed aramid are isotropic in the plane of the film, while those of the 50% complexed analog are highly anisotropic. The organometallic substituent is a site for polymer functionalization: exchange of a carbonyl for a trimethylphosphine ligand, and subsequent polymerization affords the phosphine-functionalized (PPTA)Cr(CO)2P(CH3)3, which has a much higher critical concentration for liquid crystal formation. Oxidative decomplexation of the Cr(CO)3 group yields the parent PPTA, demonstrating the reversibility of the organometallic modification approach.