Detailed analysis of termination kinetics in radical polymerization

Abstract

AbstractStudies into the termination step of radical polymerization have mostly been carried out via chain‐length‐averaged properties. With the advent of pulsed laser polymerization (PLP) induced by single pulses (SPs), the analysis of termination rate coefficients kt has significantly improved. In conjunction with highly time‐resolved near‐infrared (NIR) detection of monomer consumption, SP‐PLP‐NIR allows for the measurement of chain‐length‐averaged termination rate coefficients as a function of temperature, pressure and degree of monomer conversion. With the addition of a suitable reversible addition−fragmentation chain−transfer agent, this technique even allows for the determination of chain‐length‐dependent termination rate coefficients kti,i, where i refers to the chain length of two terminating radicals of identical size. The decay of kti,i with chain length is more pronounced at small radical size below a crossover chain length ic. At larger chain lengths, kti,i decreases to a weaker extent. Thus, the so‐called composite model is perfectly suited for representing kti,i behaviour across the entire chain‐length range. A further clear improvement of kti,i determination is provided by the SP‐PLP‐EPR method, which directly measures the type and concentration of radicals via microsecond time‐resolved electron paramagnetic resonance (EPR) spectroscopy. This EPR‐assisted technique is particularly valuable for polymerizations where secondary radicals undergo backbiting and thus two types of radicals, secondary and tertiary, are simultaneously present, as is the case with acrylic monomers. © 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.