Advancing Polymerization Catalysis by Cooperativity: Dual Catalysis as a Tool to Achieve High Performance at Maximum Simplicity

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Dual catalytic pairs consisting of an organocatalyst and a non-modified metal salt will be employed for ring-opening polymerization of O-heterocyclic monomers. The synergistic effects of such combinations will be investigated in a comprehensive study, in an effort to unify and rationalize a field where so far only scattered, yet highly promising reports exist. To achieve this, a finely tuned array of organic components is to be used, ranging from less reactive N-based catalysts (pyridine, 4-dimethylaminopyridine (DMAP)) to typical organocatalysts such as 1,8-diazabicyclo-undec-7-ene (DBU) and including also the more powerful N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs), the latter partially anionic in character. Those are to be applied in a catalytic setup with readily available Lewis acids, such as MgCl2, MgI2, CaCl2, ZnCl2, FeCl3, BiCl3 or YCl3, to polymerize different lactones (i.e. the macrolactone pentadecalactone, the medium size delta-valerolactone and the highly strained beta-butyrolactone). The data derived from such a screening will not only identify suitable combinations for competitive lactone homopolymerization (where dual catalysis can result in massively increased polymerization rates, notably without synthetic efforts), but also for convenient manipulation of the corresponding copolymer composition, as very recent results have revealed. This effect is based on the monomer specific activation by a given Lewis acid and will also be used to generate block-copolyesters by selective activation of inactive/dormant organocatalysts with the aim to realize challenging or unusual block sequences. The overall goal is to establish parameters which allow for an operationally simple synthesis of the desired lactone-based (co)polyester at will, conveniently by employing commercially available Lewis acids and well accessible organic compounds. With this collection of organocatalysts (ca. 20 compounds) and Lewis acids (ca. 10 different metal halides) at hand, the study will also be extended to include other heterocyclic monomers, namely epoxides and carbonates, targeting specifically challenging monomers. On the one hand this will be substituted epoxides (propylene oxide, cyclohexene oxide, styrene oxide) for which organocatalysts alone still suffer from relatively low turnover and slow reactions; a situation where dual catalysis is well positioned to resolve the issue. Secondly, ethylene carbonate will be focused on. This monomer notoriously requires harsh reaction conditions and is prone to CO2-loss, resulting in a mixed carbonate/ethylene oxide backbone. Lewis acid activation in combination with strong nucleophiles is therefore an attractive option to generate polymer with variable carbonate/ethylene oxide content.