[poly-chips]

Hello,
I'm trying to synthesize a new monomer using an existing di-isocyanate monomer and an existing acrylate monomer.  The acrylate monomer has one hydroxy group, so I should theoretically make 2 urethane bonds from one di-isocyanate monomer and two acrylate monomers.

I am having trouble with the synthesis because urethane synthesis typically proceeds in a matter of minutes, if a catalyst is added (I'm using dibutyltin dilaurate) and if it proceeds under heat.  If I run the reaction at 35-40 Celsius, my FT-IR scans show that the isocyanate peak has very little consumption in 24 hours.  If I bring the mixture up to about 60 degrees, the acrylate monomer becomes unstable and starts a run away reaction. 

Does anyone have any ideas on how to overcome this problem?
Thank you!

[typhoon2028]

Can you try a different catalyst, such as stannous octoate?

[DrCMS]

Blow air into the reaction and run it at 70°C with DBTL.  The oxygen in the air will stop the acrylate double bond from polymerising.

[poly-chips]

DrCMS: I tried the reaction again and bubbled air through the solution.  I kept it at 75 Celsius and watched the temperature with a thermistor.  After 3 hours, the mixture heated up on its own and kicked off the acrylate homo-polymerization.  The temperature rose on it's own to 120 Celsius.  Then something vaporized, probably unreacted (and very toxic) isocyanate monomer. 
This leads me to believe that the urethane forming reaction is exothermic.  Once the solution gets hot enough, the acrylate polymerization starts and starts a runaway reaction.  I'm not sure if there is any way around this besides running the urethane forming reaction at low temperatures for a long time.

typhoon2028: Our labs don't carry stannous octoate, or any other catalysts that I know about.  Is there much difference between catalysts in this reaction?  I've done this reaction before with different monomers and DBTL works fine.

[DrCMS]

Yes the urethane reaction is exothermic.  You need to control the temperature using cooling if required not just sit and watch it heat up by itself. 

I have done reactions like this at scales from 1L to 5000L so I know the procedure works.

Try blowing more air in and add an inhibitor like p-methoxyphenol at say 100-200ppm on the acrylate and hold the temperature at 70-75°C for 2-3hrs then check the isocyanate has reacted by IR or titration.

[typhoon2028]

I used stannous octoate for urethane foam (years ago).  It was a strong catalyst.  I never used DBTL, so I am not familiar how it compares.

I use a crosslinker in polyolefins called divinyl benzene.  To inhibit self/runaway polymerization we use TBC (4-tert-butylcatechol).  We also bubble air, not nitrogen.  Oxygen is needed to hinder vinyl polymerization.