Polyborosiloxanes are used in a variety of fields due to their unique and useful dynamic properties. Traditionally, crosslinked polyborosiloxanes are prepared by incorporating boric acid into siloxane pre-polymers, a process that is time consuming, energy intensive, and challenging due to the immiscibility of the reagents. Here, we report a versatile synthetic method to rapidly cure polyborosiloxane networks via hydrosilylation of chain-end or backbone functionalized polydimethylsiloxane (PDMS) derivatives with an inexpensive trivinylboronate. Networks synthesized from these readily available building blocks cure in ~2 minutes at convenient temperatures (e.g., 90 °C) and exhibit enhanced viscoelastic behavior when compared to traditional polyborosiloxane networks fabricated via the conventional condensation route. By virtue of using efficient hydrosilylation chemistry, another key advantage of this synthetic platform is the ability to synthesize dynamic polyborosiloxanes with different network connectivity by simply using silicones with Si–H moieties placed at the chain ends (telechelic) or distributed throughout the repeat-unit structure (copolymers). The availability of other alkenes amenable to hydrosilylation provides an additional formulation handle to synthesize mixed dynamic–static networks with tunable control over stress relaxation and solvent resistance. In summary, the synthetic approach disclosed herein is a simple and accessible platform for preparing dynamic polyborosiloxanes with tunable material properties.

Chemical Characterization

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra were collected on a Nicolet is10 spectrometer with a diamond ATR accessory. All spectra were collected with 64 scans and a 4 cm–1 resolution in absorbance mode, baseline corrected, and displayed in transmittance mode. Proton nuclear magnetic resonance (NMR) spectra were taken on a Varian Unity Inova AS600 600MHz at 25 °C with 16 scans and a relaxation delay (d1) of 15 seconds. Solution state 11B, 13C, and 29Si NMR spectra were taken on a Bruker Avance NEO 500 MHz at 25 °C. 11B spectra used a d1 value of 2 seconds and 64 scans. 13C used a d1 value of 2 seconds and 256 scans. 29Si used a d1 value of 60 seconds and 32 scans. Solid-state NMR (ssNMR) spectra were taken on a Bruker AVANCE500 WB with magic-angle spinning (MAS) of 14 kHz at 30° pulses. Samples were ground and placed into a 4 mm Bruker zirconia rotor. 29Si ssNMR spectra were captured with a d1 of 30 seconds and 4856 scans. 11B ssNMR spectra were captured with a d1 of 6 seconds and 12248 scans and background corrected.

Rheology

In order to limit hydrolysis and oxygen permeation, rheology experiments were performed immediately after network fabrication.  25 mm discs were punched out of cured networks at 140 °C, immediately transferred onto the 25 mm stainless steel parallel plate within the environmental chamber of the rheometer, and held at the same temperature. Rheology experiments were performed on a strain-controlled TA Instruments ARES-G2 rheometer affixed with a forced convection oven in a nitrogen atmosphere.  Stress relaxation experiments were performed by rapidly applying a 1% strain and monitoring the decay in measured stress over time.

Three samples were measured to ensure consistency (Figures S2, S3, and S4). The applied strain was confirmed to be in the linear viscoelastic regime of each sample (Figure S5). For sample series requiring multiple temperatures, rheology experiments were performed from high to low temperature. Single (Equation 1) and stretched (Equation 2) exponential fits were applied to stress relaxation data measurements for a polyborosiloxane network prepared from a 5 kDa telechelic Si–H functional PDMS precursor. Representative fits are shown in Figure S6, and averaged values with standard deviations are shown in Tables S2 and S3. Normalized modulus values were obtained by normalizing the dataset by the value of the modulus at time 0.1 second.