*3.2. Methods*

1H- and 29Si-NMR spectra were recorded on a Bruker Advance 600 MHz nuclear magnetic resonance spectrometer using deuterated solvent chloroform-*d* (Milton, ON, Canada). Chromium(III) acetylacetonate (3 × 10−<sup>3</sup> mol·L−1) was added as a relaxation reagen<sup>t</sup> for some of these measurements.

Gel permeation chromatography was carried out using a Viscotek GPC Max (VE 2001 GPC Solvent/Sample Module) (no longer manufactured). The system was equipped with a Viscotek VE 3580 RI Detector, a Viscotek 270 Dual Detector, and a PolyAnalytik SupeRes PAS-101 (8 mm Å~30 cm) column with a single pore, styrene-divinylbenzene gel, 6 μm particle size (London, Canada). It was additionally calibrated using a single narrow polydispersity polystyrene standard (93 kDa) from Polymer Laboratories. Toluene was used as the eluent at a flow rate of 1.0 mL·min−1.

FTIR data were collected on a Nicolet 6700 FTIR using Thermo Electron's OMNIC software (version 8.0).

GC-MS analyses were performed using an Agilent 6890 N gas chromatograph (Santa Clara, CA, USA) equipped with a DB-17ht column (30 m × 0.25 mm i.d. × 0.15 μm film, J & W Scientific), and coupled to an Agilent 5973 MSD single quadruple mass spectrometer (Santa Clara, CA, USA). One microliter of sample was injected using Agilent 7683 autosampler (Santa Clara, CA, USA) using an injector temperature was 250 ◦C and a carrier gas (helium) flow of 0.8 mL/min. The transfer line was set to 280 ◦C and the MS source temperature was 230 ◦C. The column temperature started at 40 ◦C, raised to 70 ◦C at 5 ◦C/min, raised to 95 ◦C at 10 ◦C/min, raised to 300 ◦C at 40 ◦C/min, and was then held at 300 ◦C for 8 min for a total run time of 21.73 min. Full scan mass spectra between m/z 50 and 800 were acquired, with the MS detector turned off between 2.0–2.8 min for solvent. Please note that the sample is dissolved in propyl formate.

Water concentrations were determined using Karl Fischer titrations (Mettler Toledo DL39 Coulometer) (Columbus, OH, USA) with a one-component system containing a Hydranal Composite solution.

#### *3.3. Experimental Procedures for High-Molecular-Weight PDMS Preparation Using Hydrolysis* ThefollowingparagraphsrefertoentriesinTable1.

#### 3.3.1. One-Shot Addition

B(C6F5)3 stock solution in dry toluene.

To a pre-dried 25.0 mL vial added B(C6F5)3 (0.052 g, 0.01 mmol) catalyst together with dry toluene (1.0156 mL, 0.881 g) to prepare a B(C6F5)3 stock solution in dry toluene (0.01 M).

entry 1 ([BCF]/[SiH] = 0.02 mol%; [OH]/[SiH] = 1; one-shot) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (MHMH) (134 <sup>g</sup>·mol−1, 3.13 g, 0.02 mol) and distilled water (18 <sup>g</sup>·mol−1, 0.402 mL, 0.02 mol) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.01 M, 0.094 mL, 0.0094 mmol). The B(C6F5)3 stock solution was added by an Eppendorf pipette into the flask through opening the septa. Once the B(C6F5)3 stock solution was added, immediately, vigorous bubbling inside of the round bottle flask was observed. The mixture was stirred at room temperature for 3 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate.

entry 2 ([BCF]/[SiH] = 0.02 mol%; [OH]/[SiH] = 3.2; one-shot) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (MHMH) (134 <sup>g</sup>·mol−1, 5.01 g, 0.037 mol) and distilled water (18 <sup>g</sup>·mol−1, 2.144 mL,

0.119 mol) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.01 M, 0.149 mL, 0.0149 mmol). The B(C6F5)3 stock solution was added by an Eppendorf pipette into the flask through opening the septa. Once the B(C6F5)3 stock solution was added, immediately, vigorous bubbling inside of the round bottle flask was observed. The mixture was stirred at room temperature for 3 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate.

entry 3 ([BCF]/[SiH] = 0.1 mol%; [OH]/[SiH] = 1; one-shot) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (M H M H) (134 <sup>g</sup>·mol−1, 1.00 g, 0.0074 mol) and distilled water (18 <sup>g</sup>·mol−1, 0.134 mL, 0.0074 mol) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.01 M, 0.150 mL, 0.015 mmol). The B(C6F5)3 stock solution was added by an Eppendorf pipette into the flask through opening the septa. Once the B(C6F5)3 stock solution was added, immediately, vigorous bubbling inside of the round bottle flask was observed. The mixture was stirred at room temperature for 3 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate. The mixture was stirred at room temperature for 3 h and the reaction was quenched by alumina. The product was collected by filtration through Celite under reduced pressure.

#### 3.3.2. Portion by Portion Addition

entry 4 (overall [BCF]/[SiH] = 0.008 mol%; [OH]/[SiH] = 1; portion by portion adding of B(C6F5)3 catalyst, neat) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (M H M H) (134 <sup>g</sup>·mol−1, 5.01 g, 0.037 mol) and the distilled water (18 <sup>g</sup>·mol−1, 0.67 mL, 0.037 mol) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.001 M, 0.149 mL, 0.00149 mmol, [BCF]/[SiH] = 0.002 mol%, first portion of catalyst). The B(C6F5)3 stock solution was added by an Eppendorf pipette into the flask through opening the septa. Bubbles immediately formed once the catalyst was added. After no bubble formation was observed (throughout the reaction, water droplets always existed, indicating sufficient amount of water), once again, B(C6F5)3 stock solution in dry toluene (0.001 M, 0.149 mL, 0.00149 mmol, [BCF]/[SiH] = 0.002 mol%, second portion of catalyst) was added and vigorous bubbling occurred again. The procedure of adding B(C6F5)3 stock solution in dry toluene (0.001 M, 0.149 mL, 0.00149 mmol, [BCF]/[SiH] = 0.002 mol%) repeated until no bubbles were observed even after the addition of a fresh portion of catalyst; overall, four portions of catalyst were added (0.008 mol%). The mixture was stirred at room temperature for 22 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate.

entry 5 (overall [BCF]/[SiH] = 0.006 mol%; [OH]/[SiH] = 0.73; portion by portion, 1.3 <sup>g</sup>·mL−1, mass of M H M H versus volume of DCM) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (M H M H) (134 <sup>g</sup>·mol−1, 4.12 g, 0.031 mol) and "wet" DCM (3 mL, 72.5 ppm water, 0.016 mmol of water present) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.001 M, 0.119 mL, 0.0012 mmol, [BCF]/[SiH] = 0.002 mol%, first portion of catalyst) followed by a quick addition of the distilled water (18 <sup>g</sup>·mol−1, 0.074 mL, 0.0041 mol, [OH]/[SiH] = 0.13, first portion). The B(C6F5)3 stock solution and water were added by an Eppendorf pipette into the flask through opening the septa and a vigorous bubble formation was observed immediately after the addition of water. Here is how we decided to whether add catalyst or water:

First, we added the distilled water (18 <sup>g</sup>·mol−1, 0.055 mL, 0.0031 mol, [OH]/[SiH] = 0.1, second portion). (i) If bubble formation was observed, the indicated the cease of bubbles originated from insufficient amount of water. (ii) If no bubble formation was observed after the addition of the water indicating the catalyst was insufficient, then B(C6F5)3 stock solution in dry toluene (0.001 M, 0.119 mL, 0.0012 mmol, [BCF]/[SiH] = 0.002 mol%) was added. The above-mentioned procedure repeated until no bubbles formed regardless of the addition of catalyst or water; overall, [BCF]/[SiH] = 0.006 mol%; [OH]/[SiH] = 0.73. The mixture was stirred at room temperature for 5.6 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate.

entry 6 (overall [BCF]/[SiH] = 0.004 mol%; [OH]/[SiH] = 0.75; portion by portion, 1.0 <sup>g</sup>·mL−1, mass of MHMH versus volume of DCM) (Table 1):

To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (MHMH) (134 <sup>g</sup>·mol−1, 3.05 g, 0.023 mol) and "wet" DCM (3 mL, 72.5 ppm water, 0.016 mmol of water present) which was then capped with a septum with a needle with a bubbler open to the atmosphere to balance the pressure. The mixture was stirred for 5–10 min prior to the addition of B(C6F5)3 stock solution in dry toluene (0.001 M, 0.092 mL, 0.00092 mmol, [BCF]/[SiH] = 0.002 mol%, first portion of catalyst) followed by a quick addition of the distilled water (18 <sup>g</sup>·mol−1, 0.062 mL, 0.0034 mol, [OH]/[SiH] = 0.15, first portion). The B(C6F5)3 stock solution and water were added by an Eppendorf pipette into the flask through opening the septa and a vigorous bubble formation was observed immediately after the addition of water. Here is how we decided to whether add catalyst or water: First, we added the distilled water (18 <sup>g</sup>·mol−1, 0.041 mL, 0.0023 mol, [OH]/[SiH] = 0.1, second portion). If bubble formation was observed, this indicated the cessation of bubbles originated from an insufficient amount of water. If no bubble formation was observed after the addition of the water, indicating the catalyst was insufficient, then B(C6F5)3 stock solution in dry toluene (0.001 M, 0.092 mL, 0.00092 mmol, [BCF]/[SiH] = 0.002 mol%) was added. The above-mentioned procedure was repeated until no bubbles formed regardless of the addition of catalyst or water; overall, [BCF]/[SiH] = 0.004 mol%; [OH]/[SiH] = 0.75. The mixture was stirred at room temperature for 2.6 h and the reaction was quenched by alumina for 5 h and the residual water droplet was removed by adding sodium sulfate.

#### 3.3.3. Capturing the Volatiles, Entries 7, 8

To a pre-dried 100.0 mL round-bottomed flask, tetramethyldisiloxane (MHMH) (134 <sup>g</sup>·mol−1, 10.38 g, 77.5 mmol) with distilled water (18 <sup>g</sup>·mol−1, 1.34 mL, 74.4 mmol)(entry 7, Table 1) was added. To the mixture B(C6F5)3 stock solution in dry toluene (0.1 M, 0.298 mL, 0.0298 mmol) was added. The cold trap condenser was submerged in dry ice/acetone with the addition of acetone and was directly connected to the round-bottomed flask through 6 mm O.D. Pyrex tubing. The mixture in the round bottle flask was stirred at room temperature for 3h and the reaction was quenched by alumina and the residual water droplet was removed by adding sodium sulfate with 5h quenching time. Before the work up procedures, the mixtures in the round bottle flask were weighed and characterized by 1H-, 29Si-NMR (SR) followed by filtration through Celite under reduced pressure. The same process was repeated for the experiment that also included toluene (entry 8, Table 1): MHMH (134 <sup>g</sup>·mol−1, 10.32 g, 77.0 mmol) with distilled water (18 <sup>g</sup>·mol−1, 1.34 mL, 74.4 mmol), dry toluene (6.73 mL, 7.76 g) and B(C6F5)3 stock solution in dry toluene (0.1 M, 0.298 mL, 0.0298 mmol).

#### *3.4. Experimental Procedure for Controlled Growth of Linear PDMS Using MHMH and Water* ThefollowingparagraphsrefertoTable 2.

#### 3.4.1. In Neat Water, or in Water/Toluene Mixtures

To a series of pre-dried 25.0 mL vials we added MHMH (134 <sup>g</sup>·mol−1, 1.04 g, 7.8 mmol) with distilled water (18 <sup>g</sup>·mol−1, 0.134 mL, 7.4 mmol). To the mixtures, B(C6F5)3 stock solution in dry toluene (0.1 M, 0.03 mL, 0.003 mmol, respectively) was added and stirred

at room temperature. At time points from 2–180 min alumina was added to quench the reaction at different time intervals together with ~0.5 g of sodium sulfate to remove the residual water. Before the work up procedures, the products were characterized by 1H-, 29Si-NMR (SR) followed by filtration through Celite under reduced pressure. The same process was repeated for the experiments that also included toluene: MHMH (134 <sup>g</sup>·mol−1, 1.04 g, 7.8 mmol) with distilled water (18 <sup>g</sup>·mol−1, 0.134 mL, 7.4 mmol), dry toluene (0.677 mL, 0.585 g) and B(C6F5)3 stock solution in dry toluene (0.1 M, 0.03 mL, 0.003 mmol), respectively, the mixture was then stirred at room temperature (entries 1–9, Table 2).

#### 3.4.2. Chain Extension

To a 10.0 mL round-bottomed flask we added tetramethyldisiloxane (MHMH) (134 <sup>g</sup>·mol−1, 0.015 mL, 0.011 g, 0.085 mmol) with pre-prepared PDMS oil terminated with silanol (21,300 <sup>g</sup>·mol−1, 0.90 g, 0.042 mmol, entry 4, Table 1). To be noted here, to the pre-prepared silanol PDMS we performed Kugelrohr distillation at 100 ◦C under vacuum (635 mmHg) for 60 min before running the GPC to remove the residual water and volatile small molecules. To the mixture, B(C6F5)3 stock solution in dry toluene (0.01 M, 0.0086 mL, 0.086 μmol) was added and stirred at room temperature. After 3 h, alumina was added to the mixture to quench the reaction. The product was collected by filtration through Celite under reduced pressure (Entry 10, Table 2).
