Free radical polymerization with catalytic chain transfer: Using NMR to probe the strength of the cobalt–carbon bond in small molecule model reactions

Catalyzed Chain Transfer in Vinyl Acetate Polymerization Mediated by 9-Oxyphenalenone Cobalt(II) Complexes

The vinyl acetate (VAc) radical polymerization initiated by V-70 at 30 °C in the presence of [Co^II(OPN)₂] (OPN = deprotonated 9-oxyphenalenone), 1, leads to PVAc of lower molecular weight (MW) than expected for organometallic-mediated radical polymerization (OMRP), whether reversible termination or degenerate transfer conditions are used. This represents the first clear evidence of catalyzed chain transfer (CCT) in VAc polymerization. The bis-pyridine adduct [Co^II(OPN)₂(py)₂], 2, shows a marginally lower polymerization rate and an increased CCT activity relative to 1, whereas the activity decreases with marginal effect on the polymerization rate upon addition of excess py. However, raising the temperature to 80 °C (with AIBN as initiator) led to a low MW polymer even in the presence of a large py excess. The CCT was confirmed by ¹H NMR characterization of the chain ends and by a MALDI-TOF MS analysis of the recovered polymer. The collective trends are consistent with greater CCT activity for the 5-coordinate complex [Co^II(OPN)₂(py)] relative to 1 and 2. The presence of py association/dissociation equilibria relating these three complexes was confirmed by a ¹H NMR investigation.

Simple, chemoselective, catalytic olefin isomerization

Catalytic amounts of Co(Sal^tBu,tBu)Cl and organosilane irreversibly isomerize terminal alkenes by one position. The same catalysts effect cycloisomerization of dienes and retrocycloisomerization of strained rings. Strong Lewis bases like amines and imidazoles, and labile functionalities like epoxides, are tolerated.