[charzhino]

Basically I am going to be using a condenser in a reaction at 80 degrees under nitrogen. Should I stopper the condenser at the top or leave the reaction system open due to increasing pressure from the vessel? Will this not let air into the reaction mixture therefore retarding my polymerization?

[fledarmus]

Depending on the availability of nitrogen, there are a couple ways to do this. The easiest way is to set up a bubbler on your nitrogen line, so there is a constant but very low flow of nitrogen through the system. Unfortunately this does use up quite a bit of nitrogen over the course of very long reactions.

Another method is to use nitrogen balloon. Put a rubber septum on the top of your condenser, and use a needle to connect it to your nitrogen line (with a bubbler) while you heat the reaction up. Then connect a balloon to a second needle and fill it with nitrogen. Remove the nitrogen line from your septum and replace it with the balloon. It will increase the pressure inside your reaction slightly, but it will also provide the flexibility to absorb increases and decreases in pressure which might otherwise crack your glassware.

All that being said, one of the most successful chemists I ever worked for never bothered with that. She would set up her reaction under nitrogen through a valve at the top of the condenser, and when it reached her desired temperature, she just shut the valve. As long as nothing happens (no leaks in the system, no production or absorption of gasses, no variations in temperature, etc.), the pressure inside the system should stay constant. I never had the faith in my reactions to try that.