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

Area of Science:

• Polymer Chemistry
• Chemical Kinetics
• Materials Science

Background:

• Oxygen (O2) is known to inhibit radical polymerization (RP).
• The precise mechanism and threshold at which O2 halts polymerization are not fully understood.
• Understanding O2's role is crucial for controlling polymerization processes.

Purpose of the Study:

• To investigate the abrupt turn-off phenomenon of polymerization in the presence of oxygen.
• To determine the threshold concentration of oxygen ([O2]t) that stops polymerization.
• To explore the conditions for spontaneous or induced re-initiation of polymerization.

Main Methods:

• Studied free radical polymerization (RP) of acrylamide (Am) and sodium styrene sulfonate (SS).
• Investigated Am reversible addition-fragmentation chain-transfer polymerization (RAFT).
• Controlled oxygen flow into reactors and monitored polymerization kinetics.

Main Results:

• Polymerization abruptly stopped at a specific threshold oxygen concentration ([O2]t) in all studied cases.
• Acrylamide demonstrated spontaneous re-initiation by acting as a catalytic radical-transfer agent.
• Other reactions required nitrogen purging to restart, indicating O2 acted as an inhibitor at [O2] > [O2]t.

Conclusions:

• Oxygen acts as a potent inhibitor and chain termination agent in RP and RAFT.
• The high radical-transfer rates to O2 explain the low [O2]t and abrupt polymerization stop.
• Monomer choice significantly influences the ability of polymerization to restart after oxygen exposure.