Synthesis of poly(p-phenylene)s having alternating sugar and alkyl substituents by Suzuki coupling polymerization and evaluation of their main-chain conformations

This paper reports synthesis of poly(p-phenylene)s (PPPs) having alternating sugar and alkyl substituents by Suzuki coupling polymerization of a 1,4-dibromobenzene monomer having peracetylated glucose residues with a 1,4-benzene bis(boronic acid) having alkyl chains using Pd(PPh₃)₄ in a mixed solvent of THF and NaHCO₃ aq. at reflux temperature. The polymerization proceeded with the progress of frequent deacetylation, and thus, the crude product was acetylated, followed by the isolation procedures, giving the PPP having alternating peracetylated glucose and alkyl substituents. The structure of the isolated product was confirmed by the ¹H and ¹³C NMR measurements to be the desired PPP derivative. The M_n values were estimated by GPC analysis with DMF as the eluent to be 5400-12,700. The deacetylation of the polymer completely took place using sodium methoxide in methanol/THF. The conformation of the main-chain was evaluated by the CD spectrum in comparison with that of PPP only with the glucose substituents, indicating that the present PPP derivative had the flexible nature of the main-chain by introduction of the alkyl-substituted units between glucose-substituted units. The Suzuki coupling of a 1,4-dibromobenzne monomer having disaccharide substituents with the benzene bis(boronic acid) monomer was also performed under the similar conditions. The product was precipitated from the reaction mixture, which was simply isolated by filtration. The isolated polymer was purified further by reprecipitation into diethyl ether and its structure was a PPP having free disaccharide or monosaccharide residues. This indicated occurrence of complete deacetylation as well as partial degradation of the glycosidic linkages in the disaccharides during the polymerization. The main-chain of the obtained polymer had also the flexible nature. The fluorescence spectra of the obtained PPP derivatives in this study were also measured.