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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Phalcone42 on June 25, 2018, 02:04:37 PM

Title: Deprotonate iPrOH but not terminal alkyne
Post by: Phalcone42 on June 25, 2018, 02:04:37 PM

Hi,

I am attempting anionic polymerization of a monomer containing a terminal alkyne with isopropyl alcohol as an initiator. The reaction is in anhydrous THF. The previous base I used previously to deprotonate the iPrOH is too strong, and will deprotonate the alkyne as well. I am looking to avoid this. Any suggestions as to what base I should use?

Thanks

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: Babcock_Hall on June 25, 2018, 03:27:14 PM

What base did you use, and what is the pK_a of its conjugate acid?

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: wildfyr on June 25, 2018, 04:02:40 PM

I am very skeptical that you can perform an anionic polymerization in the presence of a terminal alkyne. A quick google search for "anionic polymerization alkyne" gave this paper (https://pubs.acs.org/doi/abs/10.1021/ma971849b) which uses terminal alkynes protected with TMS. I saw a few other examples using a similar concept. I think this would be a reasonable thing for you to do, and then you can deprotect them via several different methods.

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: Phalcone42 on June 25, 2018, 04:09:39 PM

@Babcock_Hall :  I used 2.0 M P2-t-Bu base solution in THF. Problem is that I've been unable to find the pKa's of isopropyl alcohol and the alkyne in THF.

@wildfyr : Already thought of that. The conditions for attaching the functional group (via TMS protected Propargyl bromide) to the rest of my molecule are the same conditions used to deprotect the TMS. Attaching the TMS later is also problematic, as the reaction conditions can cause side reactions with my molecule.

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: wildfyr on June 26, 2018, 01:47:11 PM

Ok. I am sorry but to my knowledge a terminal alkyne is fundamentally incompatible with anionic polymerization conditions. And it make sense: the mechanism is predicated on deprotonation of an alkene (such as a styrenic). An alkene is less acidic than an alkyne. If any unprotected terminal alkynes are around, they will be deprotonated before the alkene, and terminate your polymerization.

I still suggest you investigate doing TMS protected alkynes instead of trying to navigate a much more difficult (perhaps impossible?) path. I'm confused that you say that attaching the TMS alkyne is the same conditions used to deprotect TMS.

Perhaps your alkyne can be added by post-polymerization modification?

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: Phalcone42 on June 26, 2018, 03:39:54 PM

@wildfyr :  I am looking into alternate methods of attaching the TMS protected alkyne, but right now I am just looking for a base which is strong enough to form the isopropoxide initiator, without deprotonating the alkyne.

My polymer is a polyether, and no alkenes are deprotonated in this reaction.
Reactions at the polymer chain post polymerization are highly inefficient due to the steric hindrance of the polymer.
Adding TMS later in the monomer synthesis would preferentially attack (protect) the site where polymerization would propagate.

Title: Re: Deprotonate iPrOH but not terminal alkyne
Post by: wildfyr on June 26, 2018, 08:35:34 PM

Is this an anionic ring opening polymerization?