Controlled Radical Polymerization of Vinyl Acetate Mediated by a Bis(imino)pyridine Vanadium Complex

The mechanism of controlled radical polymerization of vinyl acetate using vanadium catalysts is investigated using a range of experimental and computational studies. Optimal control is achieved using the noninnocent bis-(imino)pyridine ligand framework. [BIMPY]VCl3, where BIMPY = 2,6-[(2,6-(Pr2C6H3)-Pr-i)N=C(Me)](2)(C5H3N)), is one of only a few transition metal systems capable of mediating the polymerization of vinyl acetate. Initiation using AIBN at 120 degrees C results in excellent control over poly(vinyl acetate) molecular weights and PDIs to give vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent [BIMPY]VCl3 complex to a radical derived from AIBN generates the active species, [BIMPY]VCl2. This catalyst cannot use the halogen atom transfer equilibrium to control polymerization but can act as a persistent radical and trap the propagating polymer chains through an OMRP reversible termination process. Computational studies support this novel two-step reaction pathway and reveal that the poor control exerted over styrene versus the excellent control observed for vinyl acetate under these conditions is not only dependent on radical reactivity but also due to chelation of the carbonyl group of vinyl acetate to the vanadium center, making the trapping step more favorable. This correlates with an energy difference of just 4 kcal/mol between the reduced [BIMPY]VCl2, and [BIMPY]VCl2R species for vinyl acetate compared to over 20 kcal/mol for styrene. This [BIMPY]VCl3 system can be extended to other vinyl ester monomers, with good control over molecular weights and PDIs obtained for vinyl propionate, vinyl pivalate, and vinyl benzoate.