Tailoring Polymers, One Electron at a Time

Recently, studies involving the use of external stimuli to control polymerizations have attracted considerable interest among researchers. Amongst the various available external stimuli such as mechanical force, chemical reagents and applied voltages, light has become ever more popular. It has enabled the effective manipulation of various reactions like in atom transfer radical polymerization where photo-active molecules are utilized to control the initiation, propagation and termination stages of polymerization.

Despite providing impressive polymerization control, unfortunately, these methods have been incapable of directing the resultant polymer’s microstructure. To address this limitation, Jordan M. Kaiser, W. Curtis Anderson, and Professor Brian K. Long at the University of Tennessee (Department of Chemistry) have recently discovered that visible light may be used in conjunction with a redox-active olefin polymerization catalyst and a photoreductant to control the microstructure of polyethylene; one of the world’s most widely produced and utilized polymers. Their work was recently published in the research journal, Polymer Chemistry (DOI: 10.1016/j.ccr.2018.06.007).

Their research team employed visible light as an external stimulus that could alter the electronic nature of the polymerization catalyst and thereby modulate polyethylene branching density and microstructure during the polymerization. This was accomplished via the use of an iridium-based photoreductant and a redox-active nickel α-diimine olefin polymerization catalyst. They supported their findings through quantitative ¹³C NMR spectroscopy, gel permeation chromatography (GPC) and ¹H NMR spectroscopy, which were used to quantify the changes in the branching densities of the polymers.

This study is the first report to successfully demonstrate the use of visible light as an external stimulus for polyolefin microstructure modulation. Long and Kaiser believe that this work further confirms the vast potential of employing redox-active olefin polymerization catalysts coupled with external stimuli to predictably alter catalytic reactivity. The authors are optimistic that this study will help advance future polymerization research efforts, and that it may one-day be extended to other polymeric materials, resulting in the ability to tailor polymers with desired properties for various applications.