The numerical investigation of free radical polymerization (FRP) of methyl methacrylate under mixed feed condition at inlet is made for three microtubular reactor geometries, namely, straight tube, coiled tube, and coil flow inverter reactor. The effect of variation of fluid thermophysical properties (FTPP) (density, viscosity, and thermal conductivity) with reaction along with discrete variation of chemical species diffusion coefficient is studied. Predicted values of monomer conversion, number-average chain length (DPn), and polydispersity index (PDI) are found to be systematically lower for variable FTPP case compared to constant FTPP case. But contrary to expectations, results are found to be independent of reactor geometry for either case. Results clearly show the importance of modeling the variation of FTPP especially with respect to conversion. One case of varying diffusion coefficient of each chemical species based on free volume theory with constant FTPP is also simulated. Here again, the results are found to be independent of reactor geometry. Constant thermal conductivity and isothermal reactor condition are found to be good assumptions. This study clearly establishes the need not only for variable FTPP, thus coupling transport processes, but also for incorporating varying diffusion coefficient along with conversion in modeling of FRP in microreactors for better predictions. © 2020 Wiley-VCH GmbH