Choice of photo-excitation conditions in xanthate-supported photo-iniferter (XPI) RAFT polymerization

Abstract

The use of photo-controlled reversible deactivation radical polymerization (RDRP) techniques offers several advantages regarding the control of polymerization processes and material synthesis. Reversible addition-fragmentation chain-transfer (RAFT) polymerization is particularly interesting in this context as the chain transfer agent (CTA) used in these processes can be activated by light directly in a photo-iniferter (PI) process. We have recently introduced a method where a xanthate (a particularly active iniferter) is combined with a second CTA resulting in a xanthate supported (X)PI-RAFT process. Herein conducted DFT calculations suggest a significant difference in orbital contributions to the β-scission process when comparing xanthates and trithiocarbonates (TTC), offering an explanation for the improved performance of xanthate. Experimentally, we investigate how the choice of wavelength, light intensity and irradiation setup influences the control over the polymerization and in particular its livingness (end group fidelity). This is probed via the synthesis of multiblock and diblock copolymers where livingness is essential and can be accessed via deconvolution of the SEC traces. While a change in wavelength to more selectively activate the xanthate while not exciting the π-π* transition of the TTC does not seem to improve livingness, the light intensity was found to have a tremendous impact, with oversaturation being detrimental for chain end fidelity. Also using a flow-chemistry setup did not help to overcome this limitation. As such, the choice of light intensity was isolated as a tremendously important factor in XPI-RAFT polymerization.