**4. Conclusions**

The simple, efficient and mild process of hydrolyzing MHMH leads to high-molecularweight PDMS (maximum 153,900 <sup>g</sup>·mol−1) in a controllable and "living" manner. The reactions can be completed in as short a time as 30 min, in two-phase reactions using either bulk water with neat MHMH or MHMH in dry toluene (50 wt%). Under either set of conditions, however, low D4 (1–3%) content is produced and higher values of D5 (2–10%) and particularly D3 (13–33%) are produced. These outcomes are attributed to the particular efficiency of Chojnowski metathesis in organic solvents, even with high water content.

**Supplementary Materials:** Figures S1 and S2: kinetic plots of reaction progress with and without toluene solvent, 1H-NMR and 29Si-NMR and IR characterization of each datapoint on Figure S1, Tables S1 and S2, GC and NMR data for the cold trap with and without toluene solvent (entries 7, 8, Table 1), NMR data for polymer (entry 10, Table 2).

**Author Contributions:** The original concept of this research was proposed by M.A.B. and Y.C. The research was performed by M.L., M.A.B. and M.L. shared the writing activities. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Sciences and Engineering Research Council of Canada.

**Data Availability Statement:** Data is contained within the article or supplementary material.

**Acknowledgments:** We gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Dedication:** Julian Chojnowski is a father of modern silicone polymer chemistry. His scientific prowess and excellence is matched by his generosity. When I asked to use his name in what became the "Piers–Rubinsztajn reaction", he wanted instead to have his colleagues to take all the credit. We acknowledge with gratitude his personal and scientific contributions to our ability to undertake silicone-based science and, for this paper, rename the reaction the Piers–Chojnowski–Rubinsztajn– Kawakami (PCRK) reaction. The metathesis reaction should have his name.

**Sample Availability:** Samples of the compounds are not available from the authors.
