inspector071
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Depolymerization of polydimethylsiloxane
I have been wanting to make some dimethylsiloxane from it's poly form. I'm wanting to use the dimethylsiloxane to then polymerize with boric acid to
make silly putty.
Alternatively, are there some suitable OTC sources of pure dimethylsiloxane? Polydimethylsiloxane can be found in DOT 5 brake fluid, mixed with some
purple dye (which seems to fall out of solution at around 180 C) and a small amount of tributyl phosphate.
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francis
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Hi there,
Unfortunately no such compound exists. Silly putty is made from silanol-chain stopped polydimethylsiloxane (putty with the same characteristics may
be made from any linear polydiorganosiloxane, provided the chains terminate with Si-OH).
The silanol-chain stopped polymer is mixed or milled thoroughly with an organoborate, or boric acid, and heated to 160 deg C.
A process also exists in which diethoxydimethylsilane is reacted with boric acid in HCl and EtOH, and hydrolysis forms the putty with the desired
properties (the second reaction in the image below).
Hydrolysis of the same Starting Material under neutral pH, under cold conditions, forms dimethylsilanediol.
Another process exists, in which the polydiorganosiloxane is reacted to form the Si-OH terminal polymer - and then organoborates are mixed in and the
process is carried out at low temps. it's described in US patent 3213048.
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inspector071
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Thanks for the nice reply. I had found a procedure for making silicone oil from dichlorodimethylsilane and then polymerizing it with 5% boric acid at 180 C. I have some of the brake fluid
I mentioned which contains a high percent of polydimethylsiloxane with a tad bit of dye (which falls out at elevated temperatures leaving a clear
liquid) and a fraction of a percent of tributyl phosphate, of which purpose I do not know. I added to a hundred mL or so of the brake fluid 5% by mass
boric acid. According to the procedure, the mixture should thicken as soon as the boric acid is mixed with the oil, but that did not happen. Some of
it dissolved at room temperature, and all of it dissolved at 180 C, but no thickening ever occured. I was hoping this would be a fun, easy, and also
inexpensive lab but my little experience with polymer chemistry can't figure out why this doesn't work.
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francis
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No problem. It sounds like an interesting experiment, but probably not so simple as we would like.
I think the problem is this:
You need a silanol chain stopped (that is, with Si-OH ends) polydimethylsiloxane, to form the Silly Putty by this method.
(See on page 4 of the procedure you attached).
The brake fluid polymer is probably not silanol chain-stopped. Or it is a mixture of polydimethylsiloxanes, perhaps a small proportion of which are OH
chain stopped.
If you can find a source of OH stopped polydimethylsiloxane, or even a source of some pure silicone oil (say, a brake fluid containing only one type
of the silicon polymer) - you could depolymerise this back to some usable starting compound.
From the starting compound you could form the appropriate dimethyldiethoxy (or dimethyldichloro) silane, which you'd then react with the boric acid to
get the Silly Putty.
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edgecase
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Quote: Originally posted by francis  |
If you can find a source of OH stopped polydimethylsiloxane, or even a source of some pure silicone oil (say, a brake fluid containing only one type
of the silicon polymer) - you could depolymerise this back to some usable starting compound.
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I was reading up on how to break down silicone oils, since I had some glassware I had trouble getting clean, the lab I got it from had wash bottles
full of it, I think they were a bit sloppy with it. I read a paper about using hot NaOH solution, to break it down.
The DOT5 brake fluid would be an interesting OTC source indeed. You can also get dimethicone and cyclomethicone in pure form easily from many online
shops selling ingredients for homemade soaps and bath oils. Would those be useful?
These "silicones" are in *tons* of consumer products... in fact it would be hard to avoid it if you were allergic or were afraid they were a
carcinogen.
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