Modeling Spin-Forbidden Monomer Self-Initiation Reactions in Spontaneous Free-Radical Polymerization of Acrylates and Methacrylates

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.