This work evaluated the influence of process conditions on the chemical characteristics and yield of polymers based on castor oil. Castor oil maleate and styrene copolymers (MACO-Sty) were produced by bulk polymerization using benzoyl peroxide (BPO) and cobalt naphthenate as free radical initiators and reaction accelerators, respectively. The effects of temperature (100 to 140 °C), molar ratio between styrene and castor oil maleate (2:1 to 4:1), BPO concentration (0.10 to 0.20 wt%), and cobalt naphthenate concentration (0.10 to 0.20 wt%) were evaluated on the number average molecular weight (Mn), weight average molecular weight (Mw), dispersity (Đ), molar fraction of styrene in the copolymer, reaction yield, and viscosity of the copolymer. A wide range of molecular weights (Mw from 15,809 to 38,656) could be produced, with dispersity ranging from 2.0 to 4.8, and high yields into copolymers (> 80%). The physical characteristics ranged from resins of low viscosity (1.583 Pa s) to solid polymers.