Two tubular microreactor geometries—straight tube reactor (STR) and coil flow inverter reactor (CFIR)—are numerically studied with unmixed feed condition for the synthesis of poly(methyl methacrylate) and poly(styrene) by free radical polymerization. A new transformation rendering model dimensionless in terms of concentration is used during computational fluid dynamics modeling. For decoupled case of constant fluid thermo-physical properties (FTPP) (density, viscosity, and thermal conductivity), significant differences between the two types of reactor are observed. CFIR is found to achieve higher number-average chain lengths (DPn) while keeping polydispersity index (PDI) lower compared to STR. For the case of coupled FTPP variables with conversion (XM) and temperature, XM and DPn are found to have similar trends as in decoupled case but have systematically higher values while PDI values follows a different trend. CFIR is found to show better mixing compared to STR under similar conditions, thus allowing a better control over PDI under unmixed feed condition. An anomalous case of increased mixing in STR at low diffusion coefficient for coupled case is also observed. Overall, the importance of modeling the coupled case is clearly emphasized. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim