title: Modeling spin-forbidden monomer self-initiation reactions in spontaneous free-radical polymerization of acrylates and methacrylates.
abstract: A spin-forbidden reaction is a reaction in which the total electronic spin-state changes. The standard transition-state theory that assumes a reaction occurs on a single potential energy surface with spin-conservation cannot be applied to a spin-forbidden reaction directly. In this work, we derive the crossing coefficient based on the Wentzel-Kramers-Brillouin (WKB) theory to quantify the effect of intersystem crossing on the kinetics of spin-forbidden reactions. Acrylates and methacrylates, by themselves, can generate free radicals that initiate polymerization at temperatures above 120 °C. Previous studies suggest that a triplet diradical is a key intermediate in the self-initiation. The formation of a triplet diradical from two closed-shell monomer molecules is a spin-forbidden reaction. This study provides a quantitative analysis of singlet-triplet spin crossover of diradical species in self-initiation of acrylates and methacrylates, taking into account the effect of intersystem crossing. The concept of crossing control is introduced and demonstrated computationally to be a new likely route to generate monoradicals via monomer self-initiation in high temperature polymerization.
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Shi
authors.initials: S
authors.lastname: Liu
authors.forename: Sriraj
authors.initials: S
authors.lastname: Srinivasan
authors.forename: Jianmin
authors.initials: J
authors.lastname: Tao
authors.forename: Michael C
authors.initials: MC
authors.lastname: Grady
authors.forename: Masoud
authors.initials: M
authors.lastname: Soroush
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Rappe