UNSYMMETRICAL 3-DIMENSIONAL MACROMOLECULES - PREPARATION AND CHARACTERIZATION OF STRONGLY DIPOLAR DENDRITIC MACROMOLECULES
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The preparation and characterization of a series of strongly dipolar monodisperse dendritic molecules ranging in molecular weight from 604 to 10 530 are described. The molecules are designed to achieve high dipole moments through the placement of electron-withdrawing cyano groups and electron-donating benzyloxy groups at segmentally opposed regions of the chain ends of the dendrimers. This is accomplished through application of the convergent growth approach in the preparation of dendritic benzyl ether fragments bearing the appropriate chain ends, followed by stepwise attachment to a linear difunctional core molecule, 4,4-dihydroxybiphenyl. For comparison purposes, a series of symmetrical unfunctionalized dendritic benzyl ethers with molecular weights from 791 to 13 526 were also prepared. Dipole moment values were calculated from measurements of capacitance and refractive index. The dipole moments of the functionalized dendrimers were much larger than those for the symmetrical structures and increased with increasing molecular weight. In contrast to the behavior of linear polymers, plots of dipole moment vs molecular weight for the dendritic molecules were not linear. This is attributed to steric effects that affect the shape of the dendritic structures with the adoption of an increasingly globular shape as the molecular size increases. This shape change affects the net contribution of a large number of polar functionalities to the overall vector addition of the dipole moment. 1993, American Chemical Society. All rights reserved.