Termination kinetics in free-radical bulk copolymerization: The systems dodecyl acrylate dodecyl methacrylate and dodecyl acrylate methyl acrylate

The termination kinetics in free-radical bulk copolymerization of the dodecyl acrylate (DA)-dodecyl methacrylate (DMA) and DA-methyl acrylate (MA) systems has been studied by exclusively applying laser-assisted techniques. The ratio of termination to propagation rate coefficients, k(t)/k(p), is measured by the SP-PLP technique, and k(p) is deduced from PLP-SEC. k(t) is determined up to fairly large degrees of overall monomer conversion, at least up to 40%. The discussion of the present paper focuses on the initial plateau region of constant k(t), which extends up to about 15% in MA homopolymerizations and up to 70% in DA homopolymerizations. Among the existing models for copolymerization k(t), the treatment that considers penultimate units and uses the geometric mean approximation to estimate cross-termination turns out to be excellently applicable. It is particularly noteworthy that plotting log k(t) vs monomer mole fraction (f(i)) yields a linear correlation for each of the two copolymerization systems. The excellent fit of k(t,copo) by the penultimate unit model provides some evidence that steric effects significantly contribute to termination rate control in (meth)acrylate free-radical polymerization at low and moderate degrees of monomer conversion. The entire set of experiments has been carried out at high pressure (1000 bar, 40 degrees C) where the signal-to-noise ratio of the SP-PLP experiments is very satisfactory. No reason is seen why the conclusions derived from these measurements should not be valid at other reaction conditions including ambient pressure.