The reaction to produce the PBS elastomers used in this study from the precursor PDMS-OH polymers proceeds via a condensation reaction with boric acid, forming new B-O-Si bonds (Scheme S1). To remove the water formed and drive the reaction to completion, a Dean-Stark apparatus was used in conjunction with toluene-water azeotropic chemistry. An excess of boric acid was used to terminate all hydroxy groups on the linear PDMS-OH. Unreacted boric acid was removed by redissolving the crude PBS elastomer in hexanes and filtering through a 0.45 μm PTFE syringe filter. Liu et al. studied in detail the effects of reaction temperature on PBS formation, and demonstrated significant chain scission at 200 °C, leading to dramatic molar mass decreases after 1 hour of reaction time and unpredictable crosslinked structures. Using a lower reaction temperature (110 °C) should lead to a more defined elastomer with predictable mechanical properties. Two different PBS batches were synthesised from a low and high molar mass linear PDMS-OH (named LPDMS-OH and HPDMS-OH respectively) precursor. The differences in number average molar mass (Mn) were confirmed via size exclusion chromatography (SEC) in chlorobenzene and found to be 4.6 and 40.0 kg mol -1 respectively (Figure and Table ). According to Fetters et al., the HPDMS-OH molar mass is above Mc of linear PDMS (24.5 kg mol -1 ), where polymer entanglements will largely contribute to a higher viscosity. The resulting polyborosiloxanes synthesised from LPDMS-OH and HPDMS-OH are named LPBS and HPBS respectively. To assess whether the condensation reaction between the linear PDMS-OH and boric acid was successful, FTIR spectroscopy was carried out on the resulting PBS samples after being dried at 120 °C under vacuum overnight (Figure ). Compared to PDMS-OH, both the LPBS and HPBS samples showed a new peak at 1340 cm -1 assigned to the SiO-B stretch observed in literature and according to Kurkin et al. is due to a trigonal planar B(OR)3. When the spectra are normalised to the peak at 1258 cm -1 , which relates to the Si-CH3 stretch on the PDMS backbone, the relative intensity of the peak for LPBS compared to HPBS is an order of magnitude higher (0.14 vs 0.03) indicating a larger number of end group B(OSi)3 crosslinking sites. Alongside the more intense SiB-O stretch, a small broad peak at 3210 cm -1 was also observed for LPBS which was assigned to O-H stretches, suggesting that even drying at 120 °C under vacuum does not lead to complete condensation or the presence of mono and di-borate esters (Figure ). However, the intensity of this stretch is very low compared to the intensity of the SiO-B stretch suggesting only a small number of terminal O-H bonds. To further estimate the boron content in the different PBS samples, B NMR was caried out by dissolving approximately 30 mg of each PBS sample in dry THF (Figure ). The 11 B NMR spectra for both HPBS and LPBS showed peaks at 21.4 and 19.5 ppm. In addition, a smaller peak at 17.7 ppm was present exclusively in the LPBS spectrum. The appearance of peaks at similar chemical shifts, suggest that boron is present in similar chemical environments in both HPBS and LPB and maybe related to the appearance of mono-, di-and tri-borate esters, yet due to the polymer system being so susceptible to moisture we do not go into further detail here. Furthermore, the peak intensities in LPBS were 10-times larger than in HPBS, suggesting that a higher number of B(OSi)3 motifs are present in LPBS compared to HPBS, which is in line with the higher density of hydroxy end-groups present in LPDMS-OH and the previously discussed FTIR results.