Tunable Relaxation in Polyborosiloxane Networks via Hydrosilylation with Dual-Functional Cross-Linkers

Polyborosiloxane networks are a versatile class of polymeric materials valued for their unique viscoelastic properties, dynamic bonding behavior, and thermal stability. However, precisely tuning their relaxation dynamics without relying on reinforcing fillers remains a key challenge. In this work, we report the design and synthesis of a new generation of dynamic borosiloxane cross-linkers that enable tunable viscoelastic performance in filler-free polyborosiloxane networks. These cross-linkers were synthesized cleanly without byproducts in one step from a broad range of commercially available boronic acids, yielding functional boronate esters that are compatible with hydrosilylation-based network formation. By tuning the alkyl or aryl substituent attached to boron, the cross-linkers control the rate of stress relaxation in polyborosiloxane networks spanning several orders of magnitude, and combining cross-linkers with mismatched kinetics within a single network enables independently tunable, dual-mode relaxation behavior. In summary, this strategy offers a modular and efficient approach to tailor network dynamics, advancing the rational design of adaptive silicon-based materials.