- Macrocycles. 19. Cyclization in the nematic phase? Polyesters derived from hydroquinone 4-hydroxybenzoate and aliphatic dicarboxylic acids
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Silylated hydroquinone 4-hydroxybenzoate (HHB) was polycondensed with aliphatic dicarboxylic acid dichlorides (ADADs) in bulk at temperature up to 240 °C. These polycondensations proceeded in the nematic phase, and both transesterification and backbiting degradation were avoided as evidenced by 1H NMR spectroscopy. The nematic LC phases were characterized by optical microscopy and DSC measurements. The MALDI-TOF mass spectra revealed the formation of even-membered cycles (in addition to linear oligomers), whereas odd-membered cycles were almost absent. When the reaction mixture was slightly diluted with 1-chloronaphthalene, so that the polycondensation proceeded in the isotropic phase, odd-membered cycles with a predominance of the cyclic trimer were found. When free HHB was polycondensed with ADADs in 1-chloronaphthalene at 220 °C in 1,2-dichlorobenzene at 190 °C or in the presence of pyridine at 5-10 °C, again odd-membered cycles were found. Obviously, the nematic phase enforces a parallel alignment of polyester chains with strong electronic interaction of neighboring mesogens, so that cyclization can only occur in the case of even-membered chains via hairpin conformations of the aliphatic spacers.
- Kricheldorf, Hans R.,Richter, Martin,Schwarz, Gert
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