An unwanted reaction of a drug or other therapeutic. Basically, an adverse side effect.

the reaction is exothermic

The reaction is exothermic.

Exothermic, because the reaction enthalpy must be negative.

With polymerization, the entropy decreases. The Gibbs energy has to be negative. Thus negative reaction enthalpy.

Gibbs energy = reaction enthalpy - temperature*entropy

When a chemical reaction has a negative delta G, the reaction is exothermic because delta G is the change in energy of a system and the change in its entropy. If the effect of a reaction is to reduce G, the process will be spontaneous so delta G is negative.

Hope this helps :)