Effect of temperature in the free radical polymerization of acrylic acid I am working with the model of a free radical polymerization process in a tubular reactor. In the simulations, I observe that by keeping the feed flow of the initiator (sodium persulfate) and the monomer (acrylic acid) constant and just by increasing the jacket temperature, the concentration of the monomer (for example of the product) increases, even though the reaction is exothermic.
I have used the method of moments to model the chain length distribution of polymers. Change of viscosity has not been modeled.

My question is: Is the increase in the monmoer concentration by the increase of temperature, an expected behavior in polymerization? If so, do you know some literature that I can learn more about this?
Yes, this is expected.

With higher temperature, there is a larger concentration of radicals in the polymerization mixture, and initially the polymerization goes faster than at lower temperatures.  This leads to a faster increase in viscosity, which slows the ability of molecules to diffuse in solution.  This means it is more difficult for unreacted monomer to find growing radicals, and for unreacted initiator to find monomer.  Radicals are highly reactive so when they can't react the have a habit of engaging in termination reactions which are nonproductive regarding conversion.

This is normally called autoacceleration, and is fairly well described in Odian's 'Principles of Polymerization'.  Generally, you do not need to heat radical polymerizations beyond the bare minimum to decompose your thermal initiator.