Molecular oxygen has been reported to be a strong inhibitor to free radical polymerization at low temperatures, quenching active sites with less reactive peroxide radicals. Laboratory experiments show that introducing oxygen gas at low concentrations into the free-radical polymerization of n-butyl acrylate (nBA) results in high conversion of monomer in a very short time at temperatures above 140 °C without the use of any conventional initiators or solvent. By conducting first-principles calculations based on density functional theory, this work identifies and kinetically studies the most-likely reactions via which molecular oxygen contributes to polymer chain initiation. It provides experimental and theoretical evidence that molecular oxygen acts as a catalyst in alkyl acrylate free radical polymerization at high temperatures. Mechanistic investigations reveal that a triplet diradical intermediate is generated from solvated oxygen reacting with an alkyl acrylate monomer. The intermediate then reacts with another monomer and thermally dissociates from molecular oxygen to proceed toward polymerization. This theoretical finding is supported by laboratory experiments showing that in the presence of a very small amount of molecular oxygen and in the absence of any thermal initiators, free-radical polymerization of nBA occurs sustainably and proceeds to a very high monomer conversion. This work opens a new path for a more economic and sustainable production of higher-quality acrylic polymers using molecular oxygen as an initiator catalyst.

Oxygen-initiated free-radical polymerization of alkyl acrylates at high temperatures

ACS Spring 2022

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