Hi Carolyn,

it depends on the application, but generally syringe or other anaerobic methods are advised because of the sensitivity of the initiator. The main problem is that by adding initiator at the beginning you also expose it to all conditions in the prepolymerization mixture and to whatever temperature you use. Even at RT, initiators tend to slowly decompose but since nitrogen/argon purge is usually the last step before sealing the vial/flask and starting the reaction all or most of these radicals are quenched by residual oxygen. On the other hand, even if some radicals initiate polymerization, at lower temperature and in the presence of oxygen it will produce some (useless) oligomers at best. So you lose initiator and may lose monomers for nothing. This justifies the procedure set forth in almost all papers where the initiator solution or solid initiator is kept at low temp and only added during or after inert purge. I hope I could help.

Some part of the question is wrong: In an emulsion polymerization system, you have to use water soluble initiators and you can add them after the miceles formed in the water phase (please add the initiator to the water phase). In a solution polymerization, you use monomer soluble initiators and you may add it at the beginning or after a while according to the procedure to obtain the desired polymer molecular weight. For your case, V50 should be added after the surfactant was dissolved completely and micelles were formed in the water phase.

The greatest single danger is a run-away polymerization that generates (through heat of polymerization) an exponential process that could cause a rupture of the polymerization vessel, and a danger to all in the vicinity. Institutional safety protocols should always carry a warning to keep initiators (and all related compounds like reducing agents and transition metal accelerating agents) separate from the monomer mixture, and separate from each other until one desires to actually start the polymerization process. This preserves the activity of all components, but more importantly allows control of the polymerization rate by separately controlling the addition rate of the active species.

A typical initiator system for an emulsion polymerization might contain a persulfate, a bisulfite, and an iron complex, which results in a redox initiator system that is accelerate by the iron salt.. Control of the polymerization process is of primary importance, and separating the three components of the initiator system from the monomers and from each other gives the maximum control of a strongly exothermic process. This is the first and most important rule. The other rules mentioned by others are important for efficiency and subtle control of the outcome (MW control, for example), but they pale in importance compared to protection of life and property. Please take the safety aspects seriously when engaging in research on polymerization processes!!

We always obliged a neophyte experimenter to first calculate the total heat of polymerization of the monomer mix in the flask, and then subtract the heat that could be absorbed by the water in the monomer-water mix, in order to demonstrate how easily one can boil the water in the flask from the heat of polymerization.

The danger is that this is an exponential process: initially nothing happens, and then only slowly. By the time that one notices a strong rise in temperature, it's too late to do much to control the process. Even dumping in chain transfer agents are inadequate, since they do not stop the polymerization, but only transfer it back to monomer. At the point when the Tromsdorf effect kicks in, one could only retire to a safe distance until the process is over.

Please take the safety aspects very seriously. Handled properly, polymerization processes are of minimal hazard. But handled improperly, they can cause serious damage, injury, and even loss-of-life.

You want monodisperse particles. If the initiator is added at the beginning, polymerization starts before the micelles are properly formed and the size distribution broadens. It the initiator is added at room temperature and the mixture is then warmed, the initial slow decomposition will generate large particles along with smaller ones later. If the micelles are formed properly and the emulsion is at the proper temperature, initiator addition produces an initial burst of radicals that ensures that half the micelles are growing at any one time. If initiator decomposition is rapid enough that at least one radical per micelle every half second is generated, they will all grow at the same rate and you will get a narrow size distribution. It would help if you knew the copolymerization rate constant and the initiator decomposition rate constant so you could ensure that the radical generation is high for the time the system needs to polymerize completely.

Both potassium persulfate and the azo initiator can be added at the beginning of the reaction. But you also can try to add the ca. at 70~80 C, which will be a good comparision.

In general, depending upon the requirement we can add the initiator along with the monomer or after attaining the temperatures of both inititator and reaction mixture(monomer) but better if you add with Syringe by drop wise or with dropping funnel if not other anaerobic methods which depend on the sensitivity of the initiator.

Adding the ca. with a syringe pump will be a good try too. You can set the time range and also the desired flow rate while the reaction is running under an inert gas.In this case you will tend to have a small PDI product.

How can we control the reaction If we put initiator along with the monomer?