Iron guanidine complexes as polymerization catalysts for ROP and ATRP

  • Eisen-Guanidin-Komplexe als Polymerisationskatalysatoren für ROP und ATRP

Rittinghaus, Ruth Dorina; Herres-Pawlis, Sonja (Thesis advisor); Okuda, Jun (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

The massive environmental pollution from plastic waste leads to a need for new solutions applying the principles of a circular economy. A prominent plastic, which exhibits several options for a circular material flow is polylactide (PLA). It is made from biomass and can either biodegrade or alternatively be chemically or mechanically recycled. The production of this polymer relies to date unfortunately on a toxic tin catalyst which should be replaced applying the principles of green chemistry. Several biocompatible catalysts are known, however, most only show the required activity under non-industrially applicable conditions including the use of highly purified monomers or solvents. An industrially applicable catalyst should be robust towards impurities in technical grade monomers and active in solvent-free polymerizations. In this work, iron as a non-toxic and abundantly available metal is combined with neutral guanidine ligands to form robust complexes as potential substitute for the toxic tin compound as catalyst in the industrial production of PLA. Different iron guanidine complexes are therefore synthesized and characterized regarding their molecular structure. Polymerization experiments with different catalyst loadings are performed with the promising complexes to determine their reaction rate constants. The results deliver a major breakthrough: For the first time, a robust, biocompatible catalyst polymerizes lactide faster than the industrially used tin compound under industrially relevant conditions. Since a high-potential catalyst candidate was successfully identified, further experiments were performed including other monomers. PLA can already replace traditional plastics in a number of applications, but limitations regarding its properties allowing the utilization in even more areas have to be overcome by the introduction of comonomers. The copolymerization of lactide, ε-caprolactone and glycolide was therefore investigated and it was found that iron guanidine catalysts are capable of producing block as well as random microstructures in dependance of the polymerization set-up. The versatility of the combination of iron with guanidine ligands was even more proven by showing that one catalyst can perform the polymerization of lactide simultaneously with the atom transfer radical polymerization of styrene. This orthogonal copolymerization catalysis allows the effortless synthesis of block copolymers of two monomers not sharing a common functional group. The herein presented complexes show the great importance catalysts have on the way towards a sustainable polymer production. Having one catalyst with minor modifications being highly active in the homopolymerization of lactide, showing versatile properties, allowing the synthesis of different microstructures in copolymerizations and performing two polymerization mechanisms simultaneously gives an idea of how many polymerization processes can be simplified by the right choice of catalyst. Further research on catalytic systems will enable the replacement of energy and resource intensive polymer production routes and enable not only materials suited for a circular economy but also sustainable production pathways.

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