DFT Investigation of Polyethylene-co-vinyl Acetate: Kinetics of Initiation and Propagation, Copolymer Composition, and Unit Sequence Distribution

We conducted a kinetic investigation of initiation and propagation of copolymerization between ethylene and vinyl acetate (VAc) based on the density functional theory. Two initiators azodiisobutyronitrile and dibenzoyl peroxide were compared, and the former tends to initiate ethylene, while the latter tends to initiate β C of VAc. A calibration was performed to validate the calculation. For monomers, the reactivity follows the order α C of VAc < β C of VAc ≈ C of ethylene, while for radicals, the order is tail radical < alkyl radical < head radical. Reactivity ratios were discussed, indicating that the copolymerization is close to ideal copolymerization, especially around 430 K, and temperature is unfavorable for VAc content in the product. The relationship between monomer composition, copolymer composition, conversion, and temperature is discussed based on the kinetics calculated. Additionally, the unit sequence distribution was predicted, and the number of repetitive ethylene units can be 8–10, while VAc units often appear as 1–2. Our work gives highly reliable and comprehensive evaluation of propagation kinetics for secondary radicals in EVA copolymerization and explores the inherences of kinetic results by energy decomposition analysis. Compared with the literature, our calculation employed conformation search to average conformational deviations and considered pressure dependence, and all self- and cross-propagation modes for secondary radicals have been considered for the first time. Besides, reactivity inherences were illustrated from orbital, dispersion, repulsion, electrostatic, and entropic interactions.