Polyborosiloxanes (PBSs), Synthetic Kinetics, and Characterization

We synthesized polyborosiloxanes (PBSs) from poly(dimethylsiloxane) (PDMS) and boric acid (BA) by heating a mixture of these compounds to 200 °C. During the reaction the PDMS chains are subject to random scission, upon which the new chain ends get modified by −Si–O–(BO)m(OH)n moieties. This insight is based on gel permeation chromatography (GPC) results on the evolution of the molecular-weight distribution during the modification process, in combination with Fourier transform infrared (FTIR) spectroscopy to identify chemical moieties. We invented a refinement procedure in which the ill-defined polydisperse polar end groups can be converted into predominately borono, −Si–O–B(OH)2, by hydrolysis of boroxane bonds, B–O–B, and subsequent removal of released BA. The resulting PBSs form supramolecular elastomers owing to hydrogen bonding among the end groups. The viscoelastic behavior of the supramolecular elastomers was investigated by dynamic rheometry, and we found that the dynamic moduli increases dramatically upon the modification, even though the mean degree of polymerization decrease. This is explained by the strong interactions between the modified chain ends. Furthermore, we found that by severe dehydration, the boranol groups are converted into boroxane bonds. Upon exposure to ambient humidity, these cross-links are hydrolyzed, and the supramolecular elastomer is recovered.