Inert gas was introduced into a capillary microreactor for intensifying the mixing and reaction of polymer solutions. Operational ranges regarding volumetric flow rates of the gas and liquid phases deserved to be carefully selected for ensuring the process stability. Formation of side-by-side bubble flow in microreactors relevant to special properties of polymer solutions was observed with the use of high-speed camera. The gas introduction obviously promoted the mixing of polymer solutions in microreactors, and accordingly the improvement on the micromixing performance was confirmed by a diazo-coupling reaction system. Various factors such as the gas volume fraction and introduction mode affected gas-liquid two-phase flow patterns and thus the mixing efficiency in microreactors. Simplified mathematic models were developed to evaluate the shear effect and to reveal intensification mechanisms within the gas-liquid slug flow and the annular flow in microreactors. Moreover, the effectiveness of such a mixing intensification strategy for polymer solutions was further demonstrated through the sulfonation of polystyrene (PS) in the capillary microreactor. (C) 2018 Elsevier Ltd. All rights reserved.