Styrene autoinitiation

Acid inhibition of styrene autoinitiation may also play a role. A review by Jianying summarizes various acidic additives which have since been... 

The use of SGI allowed the miniemulsion polymerization of styrene to proceed at temperatures helow 100 G." Even though only 60% monomer conversion was obtained after 24 h, probably due to the PRE and a negligible rate of thermal autoinitiation of styrene at such temperatures, polymers exhibited rather low PDls. 

For TEMPO-mediated styrene miniemulsions, polymerization rates are nearly independent of the water solubility of the nitroxide even if partition coefiScients differ significantly (e.g., TEMPO and 4-hydroxy-TEMPO). ° The reason for this surprising result is that thermal autoinitiation of styrene dominates the phase partitioning behavior. In the absence of thermal initiation with monomers such as BA, the rate of polymerization is significantly faster in systems with more water-soluble TEMPO derivatives (4-hydroxy-TEMPO) compared to systems with TEMPO. Zetterlund and Okubo ° corroborated these findings. 

When the acyclic SG1 nitroxide was selected instead of TEMPO, the miniemulsion polymerisation of styrene could be performed at 90° C in the presence of AIBN as a monomer-soluble radical initiator (Lansalot et al, 2000). However, even after 24h the monomer conversion was incomplete owing to the persistent radical effect leading to the release of a high free nitroxide concentration. With the slow thermal autoinitiation of styrene at 90°C and the good stability of SGI in the studied medium, no consumption of the nitroxide excess could be achieved under such experimental conditions. Consequently, although the polymers exhibited good molar mass characteristics, the polymerisation was too slow to be really viable. 

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