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

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 >10,000x higher than the initial chain propagation step rates for Am and SS, which causes [O-2](t) at very low [O-2].