DEVELOPMENT OF NOVEL (THIO)UREA ORGANOCATALYSTS FOR RING OPENING POLYMERIZATION OF CYCLIC LACTONES

Abstract

Amidst multiple catalysts for ROP, H-bonding organocatalysts stand out in the precise level of reaction control they are able to render during ROP. The H-bonding class of organocatalysts are thought to effect ROP via dual activation of both monomer and chain end. The only drawback of these class of catalysts is the low activity shown despite their high selectivity towards ROP. A new class of H-bond donating ureas was developed for the ring-opening polymerization (ROP) of lactone monomers to address this problem of low activity. The most active of these new catalysts, a tris-urea H-bond donor, is among the most active organocatalysts known for ROP, yet it retains the high selectivity of H-bond mediated organocatalysts. The urea cocatalyst, along with an H-bond accepting base, exhibits characteristics of a “living” ROP, is highly active, in one case, accelerating a reaction from days to minutes, and remains active at low catalyst loadings. To understand the structure function relationship of the multi H-bonding (thio)ureas, a series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to the ring-opening polymerization of lactones. The rate of the ROP displays a strong dependence upon the length and identity of the tether, where a circa five methyleneunit long tether exhibits the fastest ROP. The developing urea class of H-bond donors and the discovery of Triclocarban as a commericially available efficient catalysts for ROP, facilitates the solvent-free ROP of lactones at ambient and elevated temperatures, displaying enhanced rates and control versus other known organocatalysts for ROP under solvent-free conditions. One-pot block copolymerizations of lactide and valerolactone, which had previously been inaccessible in solution phase organocatalytic ROP, can be achieved under these reaction conditions, and one-pot triblock copolymers are also synthesized. For the ROP of lactide, however, thioureas remain the more effective H-bond donating class. For all (thio)urea catalysts under solvent-free conditions and in solution, the more active catalysts are generally more controlled. With the discovery of multi H-bonding (thio)urea that is highly efficient and effective towards ROP, a novel bis-(thio)urea catalysts was developed as an enantio-selective chiral catalysts. This catalysts not only shows high stereoselectivity at mild conditions, but also show faster rates for the polymerization of rac-LA forming stereoblock PLA with precise control in molecular weight and enhanced thermal properties.