# Polycondensation of PMS

This is the reaction that I'm interested in. I reckon this is what occurs:

But how does the reaction proceed from there, there is a lone pair on the oxygen in right side of my picture, but how do you get rid of the CH3 group on that oxygen?

Although I do see a problem here that the structure of my final product was due to my aforementioned reaction scheme would still have RO-(PMS)-OR, where R is methyl group, instead of RO-(PMS)-R. I don't know what to do...

• The more common reaction of this type eleminates acetic acid instead of methanol. Silicon rubber during curing smells like vinegar. I´ll have to look up the specifics ...
– Karl
Apr 17 '20 at 12:35

It is more likely that silicon goes pentacoordinate rather than tricoordinate. If a proton attaches to an oxygen on the siloxane, the silicon goes positive, which will attract an oxygen from: 1) a water, 2) a methanol, or 3) a methyltrimethoxy silane. The second and third options can only reform the original reactants. If water attaches after protonation, forming $$\ce{H2O-Si-OHCH3}$$, methanol could leave, followed by a proton from the water, to give $$\ce{CH3Si(OCH3)2OH}$$. This becomes a fourth attacking option. If this attacks the silicon of a protonated siloxane thru the $$\ce{Si-OH}$$ to give $$\ce{CH3O(H+)-Si-OHSiCH3(OCH3)2}$$, the pentacoordinated silicon can lose methanol, then lose the proton from the oxygen between the silicons, to form $$\ce{CH3(OCH3)2Si-O-SiCH3(OCH3)2}$$, and that's the first step, a dimer.