Cooperative polymerization catalysis of O-heterocyclic monomers

Abstract

The ring-opening polymerization (ROP) of (hetero)cyclic monomers provides access to a multitude of industrially important polymers such as poly(ester)s or poly(ether)s. In this context, meticulously designed catalysts with imposing ligand systems often feature the best performance of the abundantly available catalytic systems in literature. However, their arduous synthesis represents a major drawback and often precludes application in industrial processes. Within this framework, Naumann et al. have developed an interesting concept to mitigate this issue: via a cooperative polymerization setup consisting of simple Lewis Acids (LA) and Lewis Bases (LB), a selective monomer activation can be achieved, significantly enhancing overall polymerization control and reducing the complexity of the setup. During the Ph.D.-thesis presented herein, this approach was further investigated in order to unravel its potential and understand the underlying peculiarities which render this approach so versatile. Hereby, a novel class of organocatalysts was used as Lewis Bases; the so-called N-Heterocyclic Olefins (NHOs) feature an electron-rich, exocyclic double bond that renders them highly basic and, depending on the structure, also nucleophilic. Together with simple, readily available Lewis Acids (MgCl2, MgI2, LiCl, YCl3, …), it was first envisioned to broaden the scope of Lactones addressable via this setup. This ranged from the five-membered g-butyrolactone (GBL), which until recently was commonly referred to as “non-polymerizable”, to a 16-membered macrolactone (w-pentadecalactone, PDL), and included the six- and seven-membered d-Valerolactone (VL) and e-Caprolactone (CL). Depending on the combination of LA and LB, it was possible to directly influence the copolymer composition and thus the resulting material properties. Furthermore, incorporating up to 20 % of GBL into a copolymer with PDL was successful, a feat hitherto unknown in literature. This initial success instigated us to further investigate the underlying mechanism of the dual catalysis and explore the possibility to homopolymerize GBL. This resulted in intriguing findings: not only was it possible to generate poly(GBL), modifying the catalytic system also evoked a switch in polymerization mechanism. Combining MALDI-ToF MS and NMR analyses, three modes of operation present in homo-GBL polymerization were elicited. Finally, following the work of Naumann et al. on the polymerization of propylene oxide, this setup was employed in a dual catalytic setup to target a zwitterionic polymerization mechanism. This resulted in an unprecedented activity of the polymerization, leading to ultra-high molecular weight poly(propylene oxide) in short reaction times, again demonstrating the simplicity and versatility of the dual catalytic approach to polymerization catalysis.