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 dodec yl acrylate (DA)-dodecyl methacrylate (DMA) and DA-methyl acrylate (MA) sys tems has been studied by exclusively applying laser-assisted techniques. Th e ratio of termination to propagation rate coefficients, k(t)/k(p), is meas ured by the SP-PLP technique, and k(p) is deduced from PLP-SEC. k(t) is det ermined up to fairly large degrees of overall monomer conversion, at least up to 40%. The discussion of the present paper focuses on the initial plate au region of constant k(t), which extends up to about 15% in MA homopolymer izations 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 tur ns out to be excellently applicable. It is particularly noteworthy that plo tting 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 b ar, 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 t hese measurements should not be valid at other reaction conditions includin g ambient pressure.