Observation and Modeling of a Sharp Oxygen Threshold in Aqueous Free Radical and RAFT Polymerization

[Image: see text] It is known that oxygen (O(2)) stops radical polymerization (RP). Here, it was found that the reaction turn-off occurs abruptly at a threshold concentration of O(2), [O(2)](t), for both free RP and reversible addition–fragmentation chain-transfer polymerization (RAFT). In some reactions, there was a spontaneous re-start of conversion. Three cases were investigated: RP of (i) acrylamide (Am) and (ii) sodium styrene sulfonate (SS) and (iii) Am RAFT polymerization. A controlled flow of O(2) into the reactor was employed. An abrupt turn-off was observed in all cases, where polymerization stops sharply at [O(2)](t) and remains stopped when [O(2)] > [O(2)](t). In (i), Am acts as a catalytic radical-transfer agent during conversion plateau, eliminating excess [O(2)], and polymerization spontaneously resumes at [O(2)](t). In no reaction, the initiator alone was capable of eliminating O(2). N(2) purge was needed to re-start reactions (ii) and (iii). For (i) and (ii), while [O(2)] < [O(2)](t), O(2) acts a chain termination agent, reducing the molecular weight (M(w)) and reduced viscosity (RV). O(2) acts as an inhibitor for [O(2)] > [O(2)](t) in all cases. The radical-transfer rates from Am* and SS* to O(2) are >10,000× higher than the initial chain propagation step rates for Am and SS, which causes [O(2)](t) at very low [O(2)].