Phenylethynyl and Phenol End-Capping Studies of Polybiphenyloxydiphenylsilanes for Cross-Linking and Enhanced Thermal Stability
journal contributionposted on 14.06.2011, 00:00 by Kerry Drake, Indraneil Mukherjee, Khalid Mirza, Hai-Feng Ji, Yen Wei
Polyaryloxydiphenylsilanes have been studied for decades and are known to be stable at high temperatures. Polybiphenyloxydiphenylsilane was synthesized to further study its high-temperature characteristics. However, condensation reactions between dichlorosilanes and biphenol produced polybiphenyloxydiphenylsilanes that underwent uncontrolled cross-linking through silanol end-groups, when heated to high temperatures (around 275 °C). End-capping the polymers with phenoxy groups (phenol as end-capping agent) to prevent cross-linking or with ethynyl containing end-groups to allow for controlled cross-linking resulted in improved thermal stability compared to the uncapped polymer, which was verified using rheology. Two synthetic routes were developed to end-cap the polymer with phenylethynyl containing substituents. Successful end-capping using lithiumphenylacetylide and 4[(4-fluorophenylethynyl)]phenol was verified chemically by 13C NMR, FTIR, and Raman analysis. Capping was further confirmed by the cessation of growth in molecular weight after addition of the capping reagents as measured by GPC, which led to enhanced high-temperature melt stability relative to that of comparable molecular weight uncapped materials as measured by parallel plate rheometry. Exothermic peaks consistent with ethynyl curing reactions were observed by differential scanning calorimetry (DSC) analysis of ethynyl-capped polymers. This work demonstrates how end-capping can be used to control the reactivity and thermal behavior of a polymer that has high-temperature applications.