Gamma radiation‐initiated polymerization of styrene at high pressure. II. Chain termination

Abstract

The pressure dependence of the termination rate constant kt for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although kt is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in kt at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, kt is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible.

High-Conversion Free-Radical Bulk Polymerization of Styrene: Termination Kinetics Studied by Electron Spin Resonance, Fourier Transform Near-Infrared Spectroscopy, and Gel Permeation Chromatography

“…This conversion range has been assigned to termination rate control by segmental diffusion. 35,[37][38][39] The absolute value obtained by Buback et al 35,36 is in reasonable agreement with the value obtained here at low conversion. The value of k t at low conversion has been proposed to be proportional to P n -a as a result of chain-length dependence (a is a positive constant).…”

“…In agreement with previous k t measurements for St by Yamada et al (ESR), Shen et al (ESR), and Buback et al , (PLP), k t remains close to constant up to approximately 20% conversion. This conversion range has been assigned to termination rate control by segmental diffusion. ,− The absolute value obtained by Buback et al , is in reasonable agreement with the value obtained here at low conversion. The value of k t at low conversion has been proposed to be proportional to - a as a result of chain-length dependence ( a is a positive constant) .…”

Termination Kinetics of Methyl Acrylate and Dodecyl Acrylate Free-Radical Homopolymerizations up to High Pressure

“…The majority of investigations carried out so far are on styrene. The pressure dependence of styrene k t has been studied by Nicholson and Norrish, [30] Ogo et al, [31] Moore et al, [32] and by Buback and Kuchta. [28] The temperature dependence of styrene k t has been investigated by Matheson et al, [33] by Mahabadi and O'Driscoll [34] and by Buback and Kuchta.…”