C-13 NMR STRUCTURAL STUDIES OF A SOLUBLE POLYDIACETYLENE POLY(4BCMU)

The chromism of a soluble polydiacetylene, poly(4BCMU), whose side chain substituents are (CH2)4OOCNHCH2COO(CH2)3CH3, was studied by use of high-resolution, solid-state13C NMR spectroscopy. The solid-state thermochromic transition (104 °C; DSC at 10 °C/min) resulted in a more localized electronic state for the backbone, extended side chains with more trans bond character, and retention of the hydrogen-bonding network between side chains, results similar to those observed for the thermochromic transition of poly(ETCD), whose side chains are (CH2)4OOCNHCH2CH3. Unlike poly(ETCD), the thermochromic transition observed in poly(4BCMU) was irreversible, and poly(4BCMU) cooled from the red phase or molten state to room temperature displayed two distinct backbone structures. Melting poly(4BCMU) (132 °C; DSC at 10 °C/min) caused a weakening of the hydrogen bonds or possibly the formation of intermolecular hydrogen bonds. A solid, red gel of poly(4BCMU) in toluene-d8 and a liquid, yellow-orange solution of poly(4BCMU) in CDCl3 were found to have backbone electronic states observable by solid-state13C NMR. The absence of both the butatrienic form of backbone conjugation and a trans to cis isomerization of the backbone double bonds was demonstrated. Backbone electronic and motional states and the conformations, motions, and hydrogen bonding of the side chains of poly(4BCMU) in the solid state and in solvents were found to be consistent with a model that suggests the thermochromic phase transition in polydiacetylenes may result from the strain placed on the backbone by its side chains. © 1990, American Chemical Society. All rights reserved.