Ring Opening Polymerization

The goal of the research discussed in this paper is to demonstrate that N-heterocyclic carbenes can be bonded to a transition metal center resulting in a complex that is able to effectively catalyze the ring opening polymerization of caprolactone. While N-Heterocyclic carbene ligands have been documented as effective catalysts for the ring opening polymerization (ROP) of lactones and lactides, less is known about the catalytic abilities of these ligands when bound to a labile metal center. This article focuses on examining the synthesis and reactivity of different transition metal complexes containing tin, zinc, aluminium and titanium metal centers. In addition to comparing metal centers, the performance of a uni-functional alcohol initiator, benzyl alcohol, was compared to a bifunctional alcohol initiator 1,4-butanediol. The aluminium based complex combined with the uni-functional initiator resulted in some of the highest degrees of polymerization and increased control over the polymer chain length. Lastly, a series of studies were performed to better understand the kinetics of this optimized system. The findings detailed in this paper indicate that transition metal complexes containing N-Heterocyclic carbene ligands are capable of polymerizing caprolactone monomers resulting in good product yields and the ability to grow chains ranging from 10-30 units in length by varying feed ratios. One of the most pertinent uses of these short chained polyesters is their ability to be grafted into other polymers providing an efficient method of manufacturing specialty polyurethanes