Linear and cyclic siloxanes functionalized with polar groups by thiol-ene addition: Synthesis, characterization and exploring some material behaviour

Abstract

1,3-divinyltetramethyldisiloxane (V2), 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3), and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4) are used as hydrophobic substrates to attach carboxyl groups by thermal or photochemical activated thiol-ene addition of 3-mercaptopropionic acid (R1), thioglycolic acid (R2) and thioacetic acid (R3). The reactions occurrence is monitored by IR following the disappearance of specific absorption bands for -S-H and –CH=CH2 bonds. At the end, the structure of the compounds and thioetherification degree of vinyl groups are determined by NMR. Co-existence in the structure of the highly hydrophobic methyl group and the carboxyl or carbonyl group gives the compounds formed an amphiphilic character, as indicated by the calculated hydrophilic-lipophilic balance, making them capable of self-assembling in solution, as the results of the dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) measurements also show. Under normal temperature conditions, these compounds are in the liquid state. The DSC measurements in the range (−150) – (+150) reveal only a glass transition in the negative field and no other thermal transitions or decomposition processes. This behavior could make these compounds containing polar groups useful as solvent-free liquid electrolytes. Dielectric spectra reveal an increase in both dielectric permittivity and conductivity of the thiol-ene addition products as compared with starting vinyl-siloxanes. The increase is even more significant after doping with lithium salt, reaching conductivity values of order 10−4 S/cm.