ABSTRACT
Some well-known step-growth polymerizations have been performed through reactive extrusion including the synthesis of thermoplastic polyurethanes, polyamides, polyethylterephtalate, and polyesters. Small amounts of undevolatilized by-products could be used to control the maximum degree of polymerization in polycondensation reactions, but this is undesirable because reactive groups could remain in the final product leading to a material that is unstable during successive processing steps. Controlling the polymerization degree should be obtained by a method leading to a stable and nonreactive product. Step-growth polymerizations in extruders, both polycondensations and polyadditions, are far less investigated than chain growth reactions. For both polycondensation and poly-addition reactions the feeding should be very accurate and stochiometrically correct, because otherwise the conversion and therefore pressure built up will be seriously restricted. Polycondensation reactions are generally mass transfer limited and good devolatilization of the low molecular components is necessary for the reaction to proceed.
