|The mechanism and kinetics of metal-catalyzed radical polymerization were investigated by spectroscopic means. A particular focus was set on Fe-mediated atom-transfer radical polymerization (ATRP) as there is a growing interest for an economic alternative to the extensively used Cu-mediated ATRP.
Experiments were started with an iron bromide-based catalyst, which simply operates without any external ligands. FT-nearIR and Mössbauer spectroscopy were used to determine the structures of [Fe(II)Bru(Solv)v] and [Fe(III)Brw(Solv)x] complexes in a variety of solvents. It was found that the tetrahedral species [Fe(II)Br3(Solv)]− and [Fe(III)Br4]− essentially govern the activation−deactivation equilibrium of ATRP. The structure of these complexes is correlated with the measured ATRP activation rate coefficients, kact, and with the equilibrium constants, KATRP, for monomer-free model systems. In weakly polar solvents such as esters, ketones, and substituted benzenes, kact and also KATRP are up to two orders of magnitude higher than with strongly polar solvents, such as N-methylpyrrolidin-2-one (NMP), acetonitrile, and dimethylform-amide, where the [Fe(II)Br3(Solv)]− complex is more stabilized.
Since further tuning of catalyst activity is important to access a wide range of monomers for ATRP, several types of Fe−ligand systems were tested for a potential enhancement of KATRP. The NIR spectroscopic analysis indicated that tetrahedral [Fe(II)BruLv]u+v=4 complexes also play a role with external ligands, L, such as N-heterocyclic carbenes and phosphines. However, these compounds do not significantly improve KATRP compared with solvent molecules being the ligands. Nevertheless, the studies were helpful to clarify the role of phosphines in ATRP. The highly Lewis basic tris(2,4,6-trimethoxy-phenyl)phosphine (TTMPP) may coordinate to Fe(II), but primarily acts as a reducing agent for [Fe(III)Br4]−, thus transforming TTMPP to TTMPP-Br+. Triphenylphosphine (TPP) is a less effective reducing agent.
An enhanced KATRP was found for amine–bis(phenolate) iron complexes. A combined Mössbauer, EPR, NMR, and online VIS/NIR spectroscopic analysis was carried out to determine the relevant Fe species. An interplay between ATRP and organometallic-mediated radical polymerization (OMRP), which is based on the reaction of propagating radicals with Fe(II), may occur depending on the monomer under investigation. Styrene polymerization operates via ATRP, whereas an interplay between ATRP and OMRP occurs for MMA polymerization.
The kinetics of ATRP and OMRP were quantitatively measured by highly time-resolved EPR spectroscopy in conjunction with pulsed-laser application for radical production, i.e., the so-called SP–PLP–EPR method. ATRP deactivation of methacrylate-type radicals by an amine–bis(phenolate)iron catalyst was monitored without interference by organometallic reactions. Toward higher temperatures, the ratio of deactivation to propagation rate increases, which is beneficial for ATRP control.
SP–PLP–EPR was also applied to quantify the catalytic termination (CRT) of two propagating radicals by Fe(II) via an organometallic intermediate. In case of the [Fe(II)Br3(Solv)]− catalyst, the organometallic reaction plays a role for acrylate rather than for methacrylate polymerization, where CRT is by about three orders of magnitude slower. As a consequence, ATRP of acrylates should be carried out with low levels of the Fe(II) catalyst to avoid CRT and thus improve the living character of ATRP.
The investigations into metal-catalyzed radical polymerization were expanded up to pressures of 6000 bar. Applying pressure results in a redistribution of iron bromides in favor of the charged species [FeBr4]2− and [Fe(Solv)6]2+, which is particularly pronounced in polar solvents such as NMP or acetonitrile. As a consequence, the reaction volume, ΔrV(KATRP), is positive for [Fe(II)Xu(Solv)v] catalysts (up to 18 cm3 mol−1). The studies demonstrated the advantage of the well-defined amine–bis(phenolate)iron system: ΔrV(KATRP) is negative, (−17 ± 2) cm3 mol−1, which is associated with a favorable shift of the ATRP equilibrium toward the side of the activated radical. Along with the increase in propagation rate, ATRP rate is thus enhanced by more than two orders of magnitude between 1 and 6000 bar.
ATRP also benefits from an improved living character under high pressure, which is due to the lowering of diffusion-controlled termination. This facilitates the synthesis of polystyrenes and polyacrylates with molar masses above 100,000 g mol−1 and dispersities below 1.29 under either Fe or Cu catalysis. These advantages were not compromised by an increase in the rate of intramolecular transfer, i.e., the backbiting reaction during acrylate polymerization under high pressure, which was deduced from modeling the ATRP experiments.
This thesis has improved the understanding of the mechanism and kinetics of Fe-mediated ATRP, in particular, of the potential interplay with OMRP. Moreover, the studies provide guidance for the selection of suitable reaction conditions that yield predominantly ATRP-mediated polymerizations with improved control.