The Mechanism of Vinyl Polymerization by Dialkylbis(dipyridyl)iron(II)

Kinetics of polymerization of vinyl compounds (acrylonitrile, methacrylonitrile, acrolein, methyl vinyl ketone, methyl acrylate, alkyl methacrylates, and 2-vinylpyridine) with dialkylbis(dipyridyl)iron(II), FeR2(dipy)2 1 (R=CH3, C2H5, n-C3H7), was studied. By comparison with previous kinetic results in which the decomposition of 1 was followed spectroscopically, a polymerization mechanism including a slow initiation process followed by rapid propagation and unimolecular termination processes was proposed. The effects of the concentration of monomer, the addition of dipyridyl and the coordinating ability of solvent on the molecular weight of the polymer were explained by competitive coordination to the iron complex having a growing polymer chain. Results of copoly-merization experiments were explained in terms of competitive coordination and insertion reactions between the monomers involved. A linear relationship was found between log(1⁄r1) values and the logarithms of stability constants of nickel-olefin π-complexes with methacrylonitrile as the reference (M1). From the formation of C2-H5D in the polymerization of β-cis-d1-methyl methacrylate with Fe(C2H5)2(dipy)2 a unimolecular termination process involving abstraction of β-hydrogens of the growing chain by the alkyl group or the iron complex was proposed.