Abstract: An esterification or transesterification process comprises the steps of reacting a first polymer with a first alcohol in a reactive extruder to produce a second polymer, while also producing a second alcohol as a by-product. The second alcohol has a lower boiling point than the first alcohol, preferably at least about 100° C. lower. A gas stream is vented from the extruder apparatus that comprises both the first alcohol and the second alcohol. At least part of the first alcohol is condensed from the gas stream, thereby forming a liquid stream, which is then fed back into the reactive extruder. The condensed first alcohol then continues to further the extent of the reaction in the extruder producing the second polymer.