Photoinduced Initiation of Olefin Polymerizations

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Brian K. Long and his team at the University of Tennessee-Knoxville are developing mechanistic insight and understanding of a new polymerization methodology called photoinitiated olefin polymerization. Olefins are small organic molecules composed of carbon and hydrogen in which each carbon is bonded to another carbon through a conventional chemical bond, and another unique bond called a pi-bond.

When this pi-bond is broken, a reactive molecule is created that can add to another olefin. This process can be repeated many times resulting in the formation of long chain macromolecules, commonly referred to as polymers or plastics. Plastics have benefited our society in numerous ways.

In fact, plastics has helped aeronautics technology take giant steps forward over the past 70-years, including advancements in satellites, shuttles, aircraft, and missiles. In addition, the building and construction, electronics, packaging, and transportation industries have all benefited greatly from the use of plastics. In this research, olefin polymerizations will be initiated by light, in combination with acid and a metal species.

The use of light in these polymerizations is advantageous because it causes cleavage of chemical bonds in non-invasive ways and is environmentally benign. Sophisticated studies will be conducted in order to mechanistically understand how the combination of these three components can be used to control the polymerization of olefins. These investigations are expected to provide insight into how plastics are produced with controlled length of polymer chains and architectures.

If successful, the fundamental knowledge gained will enable the chemistry required for many advanced applications such as, for example, olefin-based 3D printable polymers. The research spans multiple disciplines, including polymer chemistry, polymer science, organometallic chemistry and engineering. Consequently, it will provide a cross-cutting educational opportunity for the students involved.

The research team will also strive to recruit undergraduate students from underrepresented groups through the University of Tennessee-Knoxville SMaRT internship program, as well as Appalachian Students Promoting the Integration of Research and Education (ASPIRE) initiative.

This research will focus on the use of photoacid generators to activate homogeneous metallocene and post-metallocene catalysts for the controlled polymerization of ethylene, propylene and higher α-olefins. To realize this important goal, the first objective will focus on detailed mechanistic and kinetic studies of photoinduced activation of dialkyl substituted zirconium and hafnium pre-catalysts.

The designed strategy will intercept the precatalyst activation pathway by replacing a commonly used Bronsted acid activator with a photoacid generator (PAG). This PAG/ pre-catalyst mixture will be dormant in the absence of light, but upon irradiation will generate the open coordination site needed for the coordination-insertion polymerization of olefins.

The second objective will apply the generated knowledge and principles to enable control over polyolefin tacticity and molecular weight distribution. Finally, in the last objective, heterogenous photoinduced initiation of olefin polymerization will be developed with a strong focus on controlling polyolefin film formation on surfaces using a gaseous feed of ethylene or propylene.

If successful, the fundamental studies associated with this research will expand the library of light-based, 3D printable polymer types to include polyolefins. The knowledge gained from the mechanistic studies will be impactful to organometallic chemistry and catalysis field in general.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.