Polymerization coupled to the oscillations of the Belousov-Zhabotinskii reaction

William Walter West


The mechanism of polymerization coupled to the oscillations of the Belousov-Zhabotinskii, BZ, reaction has been elucidated for acrylonitrile, n-butyl acrylate, and t-butyl acrylate. The failure of other vinyl monomers such as methacrylic acid, vinyl acetate, and several derivatives of acrylonitrile to polymerize is explained. The polymers produced in the BZ reaction were characterized by NMR, FTIR, and XRF. The structural information obtained from the characterization was used to determine the mechanism. The effects of the addition of monomer to the BZ reaction were also used to determine how the monomers interacted with the BZ reaction. Acrylonitrile was found to polymerize in phase with the oscillations of the BZ reaction. The polymerization is initiated by malonyl radicals and then proceeds by a normal free radical mechanism. Poly(acrylonitrile) is terminated by combination with malonyl radicals and bromous radicals. The bromous ester is quickly hydrolized in the acid BZ reaction to leave an alcohol endgroup. The malonyl endgroups are stable. The presence of a 8 wt.% (6 mol %) bromine in the BZ reaction produced poly(acrylonitrile) is explained by standard light induced halogenation of saturated compounds in solution. The presence of acrylonitrile in the BZ reaction inhibits the oscillations for a period of time that is proportional to the amount of acrylonitrile added and the temperature. The addition of bromide removes the inhibition period. The addition of bromomalonic acid does not affect the inhibition period. Activation energies for the quiescent period and oscillatory periods were calculated from the temperature experiments. The activation energies were compared to literature values for the BZ reaction and to a control BZ reaction. The poly(acrylonitrile) produced has a bimodal molecular weight distribution. Two symmetrical distributions are obtained from the GPC of the polymer produced. The high molecular weight material has a MW of $10\sp6$ relative to poly(styrene). The lower MW is $10\sp3.$ t-Butyl acrylate polymerizes in the BZ reaction in phase with the oscillations of the BZ reaction. The polymerization occurs only when the malonyl radical concentration is high. Bromous radicals will not initiate polymerization, and the malonyl radical is shown to be the initiating species. The absence of bromine in the BZ produced poly(t-butyl acrylate) lends support to a mechanism involving the penetration of the monomer droplet by malonyl radicals or the initiation of polymerization on the surface of a droplet. The polymerization then occurs within the droplet. Termination is by combination between radicals. The MWD of the polymer is symmetrical and has a maximum about 5500 relative to poly(styrene). n-Butyl acrylate is susceptible to de-esterification in the BZ reaction and this limits the polymer yield severely. Characterization of the polymer shows that it polymerizes according to the same mechanism as t-butyl acrylate except that the polymer will de-esterify to poly(acrylic acid)/poly(n-butyl acrylate) copolymer. The acid groups on the polymer can react with cerium to degrade the polymer. The monomer is very susceptible to degradation once it has been de-esterified to acrylic acid. The MW of the polymer was 3400 relative to poly(styrene).