Polycarbonates from the polyhydroxy natural product quinic acid.
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abstract
Strategies for the preparation of polycarbonates, derived from natural polyhydroxy monomeric repeat units, were developed for biosourced polycarbonates based on quinic acid. The design and synthesis of regioselectively tert-butyldimethylsilyloxy (TBS)-protected 1,4- and 1,5-diol monomers of quinic acid were followed by optimization of their copolymerizations with phosgene, generated in situ from trichloromethyl chloroformate, to yield protected poly(1,4-quinic acid carbonate) and poly(1,5-quinic acid carbonate). The molecular weights reached ca. 7.6 kDa, corresponding to degrees of polymerization of ca. 24, with polydispersities ranging from 2.0 to 3.5, as measured by SEC using tetrahydrofuran as the eluent and with polystyrene calibration standards. Partially because of the presence of the bicyclic backbone, each regioisomeric poly(quinic acid carbonate) exhibited relatively high glass-transition temperatures, 209 C for poly(1,4-quinic acid carbonate) and 229 C for poly(1,5-quinic acid carbonate). Removal of the TBS-protecting groups was studied under mild conditions to achieve control over potential competing reactions involving polymer degradation, which could include cleavage of lactones within the repeat units, carbonate linkages, or both between the repeat units. Full deprotection was not achieved without some degree of polymer degradation. The regiochemistry of the monomer showed significant impact on the reactivity during deprotection and also on the thermal properties, with the 1,5-regioisomeric polymer having lower reactivity and giving higher T(g) values, in comparison with the 1,4-regioisomer. Each regioisomer underwent a 10-20 C increase in T(g) upon partial removal of the TBS-protecting groups. As the extent of deprotection increased, the solubility decreased. Ultimately, at long deprotection reaction times, the solubility increased and the T(g) decreased because of significant degradation of the polymers.
