Highly Polarized Olefins: Enabling Polymerization Catalysts for Homo- and Copolymers Based on “Non-Polymerizable” gamma-Butyrolactone

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This project aims for the development of practicable, efficient (co)polymerization processes to utilize gamma-butyrolactone (GBL) for materials science. This five-membered cyclic ester has historically been largely ignored for application as monomer on account of adverse thermodynamics; until recently, only oligomers would result from the harshest of conditions. However, new research has revealed the compelling properties of poly(GBL) as a fully recyclable, tunable polyester, whereby crucially the feedstock, GBL, is abundantly available, cheap and potentially bio-sourced. To fully unlock this potential for polymer chemistry, the corresponding polymerization catalysis must be developed beyond its current rudimentary state regarding this specific monomer. To this end, it is proposed to employ a special class of olefins – so-called N-heterocyclic olefins (NHOs) – as a novel tool to advance GBL homo- and copolymerization. NHOs inherently eliminate some of the issues complicating the preparation of defined poly(GBL) to date, and additionally enable alternative polymerization pathways which are considered crucial for further improvement, namely zwitterionic polymerization and anionic polymerization by cooperative Lewis pairs.While a better understanding of GBL homopolymerization will be rewarding, thermodynamic constraints will always severely limit the range of applicable reaction conditions, imposing a challenge for economic realization. Copolymerization, where some of these limitations can be circumvented, is therefore a highly promising alternative to achieve efficient utilization of GBL and will be a focus of this project. Two complementing strategies will be followed to prepare high GBL-content copolymers. For one, this will be alternating copolymerization, using 3,4-dihydrocoumarin as comonomer. The second strategy concentrates on copolymerization with omega-pentadecalactone or with 1,2-butylene oxide, resulting in a GBL-containing copolyester or copoly(ester-ether). A novel concept, monomer-selective activation of lactones by NHO/metal halide Lewis pairs, will be employed and systematically investigated to preferentially activate GBL over its comonomers resulting in a direct manipulation of the copolymerization parameters.The NHO-mediated catalysis as proposed in this project shall enable access to polymers which are currently not available, yet highly desired. This includes (degradable) GBL-derived poly(ester-ether)s; also, the preparation of poly(GBL) itself will profit, since presently this interesting material is limited in its applicability by a lack of control over end groups, molecular weight (distribution) and topology.