A reactor for an in-situ production of a chemical product in high yield are presented. The reactor, which may be placed in a main solvent, includes a core and a reactor wall around the core. The reactor wall allows controlled permeation of the main solvent to the core. The core contains a reactant that reacts to produce a target product upon being contacted by a main solvent. The target product leaves the reactor at a controlled rate. Because the amount of the main solvent that permeates into the reactor is controlled, a high concentration of the reactant is maintained inside the reactor, resulting in a higher yield of the desired chemical product than if the reactants were directly added to the body of main solvent.