Copolymerization propagation kinetics of dimethyl itaconate and styrene: Strong entropic contributions to the penultimate unit effect

In this work, pulsed-laser polymerization has been utilized to measure average propagation rate coefficients, 〈k_p〉, in the bulk copolymerization reaction of dimethyl itaconate and styrene at 20 °C. The measured 〈k_p〉 values were substantially higher (up to a factor of 3) than those predicted by the terminal model. This unusual result is attributed to strong entropic penultimate unit effects on the DMI-terminal radicals. We discuss the significance of this result particularly for other copolymerization systems where there are large size differences between the monomers and for systems where the comonomer substituents are similar (for instance, acrylate-methacrylate pairs), where the enthalpic penultimate unit effect is diminished and the entropic effect may become observable.