Oxygen-Initiated Free-Radical Polymerization of Alkyl Acrylates at High Temperatures

Abstract

Molecular oxygen has been reported to be a strong inhibitor at low temperatures. Here, it is shown that the introduction of oxygen gas into the free-radical polymerization of n-butyl acrylate (nBA) results in high conversion of the monomer in a very short time at temperatures above 140 °C without the use of any conventional initiators. By conducting first-principles calculations based on density functional theory, this work for the first time identifies and kinetically studies the most-likely reactions via which molecular oxygen contributes to polymer chain initiation. It provides theoretical and experimental evidence that molecular oxygen acts as a catalyst in alkyl acrylate free-radical polymerization at high temperatures. 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.

High Power Sunlight‐Simulated UV‐Induced Radical Polymerization: Self‐Initiation and Self‐Crosslinking

“…While oxygen can also be involved in initiation process (Scheme 1, Oxygen initiation), a triplet diradical intermediate can be generated from solvated oxygen reacting with an alkyl acrylate monomer. [ 22 ]…”

“…It has been reported that in the presence of a very small amount of molecular oxygen and in the absence of any thermal initiators, free‐radical polymerization of acrylate monomer occurs sustainably and proceeds to a very high monomer conversion. [ 22 ] In high power UV‐induced polymerization, oxygen can also contribute to polymer chain initiation (mechanism 3 in Scheme 1). However, the participation of oxygen in polymerization may inhibit initiation, [ 24 ] when the amount of dissolved oxygen was reduced, less chain termination can happen, leading to shorter chains in open air system.…”

“…While oxygen can also be involved in initiation process (Scheme 1, Oxygen initiation), a triplet diradical intermediate can be generated from solvated oxygen reacting with an alkyl acrylate monomer. [22] In HPSS-UV-ATRP, the formation of activators can happen through two mechanisms (Scheme 1): 1) Addition reaction between monomers and CuBr 2 , which allows a halogen-exchange mechanism; [23] 2) UV reduction of added CuBr 2 . [17b]…”

Additive-Free Method for Enhancing the Volume Phase Transition Rate in Light-Responsive Hydrogels: A Study of Micro-Nano Bubble Water on PNIPAM-co-AAc Hydrogels

“…The mechanical properties of gels differ significantly between degassed and bubble water, affecting the kinetics and control of polymerization reactions [48,49]. While oxygen can serve as a free radical initiator [50,51], it may also quench radicals, inhibiting polymerization [52,53]. However, this oxygen-radical interplay insufficiently accounts for the observed variations in the elastic modulus due to effervescent water aging.…”