Unravelling the dynamic crosslinking mechanism in polyborosiloxane

This study offers a comprehensive investigation into the mechanism behind the dynamic crosslinking of polyborosiloxane (PBS). Despite this material being well known for over 70 years, the origin of PBS's unique viscoelastic properties has been a topic of debate in the literature. Through combined FTIR and solid-state 29Si and 11B MAS NMR analyses, this study provides evidence that the formation of Si–O–B dynamic covalent bonds, along with their associative exchange with neighboring hydroxyl-bearing species (free silanol, water, alcohol, etc.), drives the gelation and viscoelastic behavior of PBS. Results show no indication of hydrogen or dative bonding, instead the low energy barrier for formation and breakage of Si–O–B bonds allows for easy exchange at room temperature. Moreover, the study finds that the viscoelastic properties of PBS can be adjusted by the choice of boron B–O functionality, leading to n-functional dynamic crosslinking through Si–O–B bonds. This research provides a clear understanding of the mechanism of dynamic crosslinking in PBS, resolving long-standing controversies in the field.