The first part of this article discusses different partitioning options used in modeling emulsion copolymerization of acrylonitrile and butadiene for the production of acrylonitrile-butadiene rubber (NBR) in batch and continuous reactors. It is observed that the performance of the model using different partitioning methods for estimation of monomer concentration in the different phases (i.e., droplet, aqueous and particle) would be very similar by proper selection of the different parameters of the particular partitioning method. In the second part, the effects of important aspects (during the continuous production of NBR) such as desorption and monomer soluble impurities on polymer properties are discussed. The third part of the paper focuses on procedures and benefits of different kinds of reactor start-ups, such as starting the reactor(s) full of water, half full of water, full of 'batch recipe' or empty. All of the above studies and analysis are targeted towards their effect on principal emulsion polymer characteristics such as number-and weight-average molecular weights, monomer conversion, copolymer composition (bound acrylonitrile), tri- and tetra-functional branching frequencies, and number and average size of the polymer particles. POLYM. ENG. SCI., 51:1909-1918, 2011. (C) 2011 Society of Plastics Engineers