Chiral Diboranes as Metal-free, Cooperative Catalyst Setups for the Stereoselective Polymerization of Propylene Oxide and other Substituted Epoxides

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Aliphatic polyethers, typically derived from the ring-opening polymerization (ROP) of epoxides, are among the most important products of chemical industry and de facto ubiquitous in our daily lives. Yet in spite of this success, without exception, polymerizations on relevant scale do not address control of stereochemistry (tacticity) as an obvious tuning site for a further optimization of polyether-based products (i.e., poly(propylene oxide), PPO). This is largely down to insufficiencies of the available organometallic catalysts. This project aims for the development of a metal-free (organocatalytic) polymerization setup for the preparation of isotactic and isotactic-enriched (it) polyethers, crucially targeting the incorporation of the thus tailored polyether moieties into more complex polymer architectures (block copolymers). The overall motivation is to identify and understand a highly selective, scalable and user-friendly catalyst system with pronounced functional group tolerance to circumvent the key issues which to date prevent utilization of polyethers to their full extent. These aims are to be achieved by building on the cooperative polymerization mechanism observed for borane-supported ROP of substituted epoxides: by connecting two borane functionalities via a chiral backbone, a stereoselective, kinetically favored environment will be generated. The specific work packages will include a detailed investigation of the catalyst tuning sites (chiral backbones, linker length between borane and backbone, electron deficiency at the boron centers). In conjunction with systematic screening regarding the polymerization of PO, a well understood structure-performance correlation should result, focusing on stereoselectivity (as measured by isotactic triad placement, % mm), TON, TOF and a narrow molar mass distribution, aiming for high selectivity to result in semi-crystalline polyether (> 90% mm). In a subsequent phase of the project, this principle is to be extended to other substituted epoxides (such as 1-butylene oxide, allyl glycidyl ether, styrene oxide) and different types of macroinitiators. Importantly, for the latter, sensitive polymers (i.e., polyesters, polycarbonates) will be selected to create polyester-block-(it)-polyether (multi)block copolymers, thereby addressing structures which cannot be prepared with established catalytic setups and thus filling a gap which to date notoriously aggravates the manufacture of polyether-based additives.