Synthesis of Ladder Polymers Via Soluble Precursor Polymers

Abstract

Although ladder polymers have been known for some time, poor solubility and low molecular weights have prevented processing and have discouraged consideration of these materials for structural and electronic applications. Poor solubility can be associated with strong Van der Waals interactions between delocalized π-electron clouds on adjacent chains. The optimum orbital overlap along the polymer backbone existent for ladder polymers has motivated reexamination of these systems for electronic applications. We have pursued alternatives to the tradiational acid-catalyzed polycondensation of underivatized monomers at elevated temperatures by reacting derivatized (e.g., with alkyl or vinylamine substituents) monomers at reduced temperature in organic solvents (e.g., DMF) to produce open-chain precursors to ladder polymers. These are converted to fully-fused ladder polymers by thermal processing either in solution or as solid state (e.g., thin film) materials. The preparation of precursors of improved solubility greatly enhances processing options and has permitted the utilization of both Langmuir-Blodgett methods and the perparation of optical quality films by casting (including spin casting). By control of thermal processing conditions, both derivatized and underivatized ladder polymers have been prepared. Our approach offers obvious advantages for the investigation of the effect of conformation and of substituents upon electronic properties. The preparation of copolymers with rigid rod and flexible chain segments incorporated in the polymer backbone has also been investigated. For such materials, polymers with fully fused ladder segments can exhibit improved solubility in DMF relative to polymers with open chain segments due to the absence of favorable hydrogen bonding stuctures in the former. Finally, we are investigating the effect of electrochemical polymerization of Langmuir-Blodgett films prepared with derivatized diaminobenzene. Such an approach clearly takes advantage of improved solubility of the derivatized monomer and improved molecular order which can be realized with the Langmuir-Blodgett method. Free radical polymerization may facilitate the realization of improved molecular weight distributions.