Termination Kinetics of Surface-Initiated Radical Polymerization Measured by Time-Resolved ESR Spectroscopy after Laser-Pulse Initiation

Abstract

The single-pulse–pulsed-laser polymerization–electron spin resonance (SP-PLP-ESR) method was used to study the termination reaction in polymerization of n-butyl methacrylate (n-BMA) from silica nanoparticles. The polymerization from the surface was initiated by the surface-attached photoinitiator 2,2-dimethoxy-2-phenyl-1-(2-(2-(trimethoxysilyl)­ethyl)­phenyl)­ethane-1-one (DMPTS). The obtained termination rate coefficient at 335 K of k t = (5.4 ± 0.7) × 106 L mol–1 s–1, which could unambiguously be assigned to the termination reaction between two surface-attached macroradicals, is about 2 orders of magnitude smaller than the value found in homogeneous n-BMA polymerization. Surprisingly, no chain-length dependence of the termination rate coefficient could be observed. This fact and temperature-dependent studies of k t, which yield an activation energy that is in perfect agreement with the activation energy of propagation of n-BMA, indicate that the termination between surface-attached macroradicals proceeds under reaction diffusion control, as their translational diffusion is suppressed.

Detailed Kinetic and Mechanistic Insight into Radical Polymerization by Spectroscopic Techniques

“…It should be noted that the SP-PLP-EPR method also allows for studies into termination of radicals in surface polymerization, e.g., from silica nanoparticles. 74 A recent such investigation into butyl methacrylate polymerization indicates that termination of surface-attached macroradicals occurs under reaction-diffusion control.…”

“…However, sample preparation for method 2 is difficult, as introducing significant amounts of polymer and preparing homogeneous polymer–monomer mixtures inside the flat EPR cells pose problems. It should be noted that the SP-PLP-EPR method also allows for studies into termination of radicals in surface polymerization, e.g., from silica nanoparticles . A recent such investigation into butyl methacrylate polymerization indicates that termination of surface-attached macroradicals occurs under reaction-diffusion control.…”

Direct Mapping of RAFT Controlled Macromolecular Growth on Surfaces via Single Molecule Force Spectroscopy

“…Thus, an in-depth understanding of the processes occurring during polymerization directly at the surface is mandatory. Substantial efforts have been made to obtain information on the SI polymerization process: For example, surface termination kinetics have been investigated via time-resolved electron spin resonance spectroscopy, the polymer generated on the surfaces was cleaved and analyzed, for example, from nanoparticles and cellulose surfaces, as well as the polymer generated in solution via size-exclusion chromatography (SEC) . As the amount of detachable polymer is, in general, relatively low, its characterization is challenging.…”

Surface-Immobilized Photoinitiators for Light Induced Polymerization and Coupling Reactions

“…Photolysis of this grafted initiator generated a benzoyl-type radical covalently coupled to the surface and a free dimethoxybenzyl radical. The second one is not supposed to initiate polymerization and can be further cleaved into methyl benzoate and a methyl radical which is postulated to act as a terminating species due to its high reactivity and diffusivity [ 39 ]. An example of a macromolecular photoinitiator, which includes a benzoin (type I photoinitiator) and an anhydride moiety was given by Ohar et al This molecule is a surface modifier for filler materials, such as TiO 2 and hydroxyapatide.…”