Chain Length Dependent Termination in Butyl Acrylate Free-Radical Polymerization Studied via Stationary and Pulsed Laser Initiated RAFT Polymerization

Abstract

The chain-length dependence of the termination rate coefficient, kt, in butyl acrylate free-radical polymerization has been determined by two independent methods, RAFT-SP-PLP and RAFT-CLD-T, both employing control of radical chain length by reversible addition fragmentation chain transfer (RAFT) polymerization. Within RAFT-SP-PLP, the polymerization induced by a laser single pulse is monitored via near-IR spectroscopy with a time resolution of microseconds. In RAFT-CLD-T, isothermal reaction rate measurements are carried out via DSC under stationary polymerization conditions. The resulting kt data refer to the situation of living/controlled radical polymerization, where both radical chain length and monomer conversion increase during the course of the reaction. The RAFT-SP-PLP measurements were carried out at 60 °C and two pressures, 5 and 1000 bar. The RAFT-CLD-T experiments were run at ambient pressure and at two temperatures, 60 and 80 °C, respectively. In absolute value, the termination rate coefficients for identical pressure and temperature deduced from the two methods differ by less than a factor of 2. For the dependence of kt on chain length, i, almost identical information is provided by the two techniques. The chain-length dependence of kt may be described by the power-law expression kt(i) = kt(1,1)i-α with, however, α being different for short-chain and long-chain radicals. RAFT-SP-PLP yields α1 = 1.25 for the short-chain regime from 1 < i < 30, and α2 = 0.22 for chain lengths above i = 50. RAFT-CLD-T results in α1 = 1.04 and α2 = 0.20 in identical chain length regimes. kt(1,1) values are found to be close to 1 × 109 L mol-1 s-1.