Key polymer properties are substantially related to the polymer molecular weight distribution (MWD). Modeling polymerization processes based on MWD with kinetic-based approach involves the mass balances for all chain types and for all possible chain lengths. These balances exhibit a large-scale dynamic system which is computationally expensive to obtain numerical solutions. Moreover, these models are hard to apply for dynamic optimization. In this work, a reduced order model of dynamic MWD is proposed with a novel idea to reformulate and decouple the population balances based on nullspace projection method. The reformulation first separates the fast equilibrium of living chains from the accumulative dead chains, and then decomposes the coupling in the living chain lengths by further derivation. Dynamic simulation and optimization results demonstrate that the reduced model with manageable scale benefits computational efficiency, which provides a perspective for on-line applications. (C) 2019 Elsevier Ltd. All rights reserved.