N N M P Chain transfer suppressed Axial Blocking group Restricted bond rotation Polymer Architecture Design through Catalysis Christopher Levins, Christopher Popeney, Prof. Zhibin Guan, Department of Chemistry · · S ummer U ndergraduate 2 R esearch 0 F ellowship in 0 I nformation 6 T echnology Introduction The goal of this research has been to develop advanced transition metal catalysts for olefin polymerization. More specifically, we have been creating a family of cyclophane-based catalysts based off of an existing acyclic catalyst developed by Maurice Brookhart at UNC Chapel Hill. The cyclophane-based catalysts show excellent activity and high thermal stability for olefin polymerization compared to the acyclic catalyst at high temperatures. The cyclophane design helps eliminate chain transfer, which effectively stops the polymerization and yields shorter polymers, by blocking off 2 of the coordination sites on the metal. It also has increased stability because there is no rotation about the C-N bond, which reduces the likelihood of decomposition. “dendritic” linear hyperbranc h There are two competing reactions in these polymerizations. They are insertion of the olefin, and the “chain walking.” The insertion is what causes the polymers to increase in length, while the chain walking is what causes the polymers to branch out in different directions. Topology of a polymer determines most of the physical properties of the polymer. Linear polymers tend to be more rigid while more branched polymers tend to be more flexible. Recent research of late transition metal catalysts for polymerization of olefins has shown an enhanced ability to control polymer topology. The branched polymer structures produced by these catalysts are attributed to an isomerization mechanism, or “chain walking” of the catalyst along the polymer chain. By creating catalysts that have specific control over the rate of chain walking, we can make target polymers with specific topologies ranging from linear to hyperbranched to "dendritic” Research Acyclic Cyclophane xx Cyclophane catalyst (detailed benefits) This data is from substituted acyclic catalyst. It is clear that the functional groups have an effect on the topology of the polymer. More specifically, the more electron rich the metal center is, the less branched the polymer becomes. The research being done here is to successfully synthesize a whole family of different cyclophanes with different functional groups and analyze to what extent the differences in catalyst design will affect the polymers they produce. X-ray diffraction structure of cyclophane catalyst Linear polymers have a wide variety of uses. One main use is in hard plastics and other materials. Hyperbranched and dendritic polymers are mainly used for drug delivery. The cyclophane-based catalysts we have been focusing on making have different functional groups which modify the electron density around the metal center where the polymerization takes place. Changing the electron density around the metal center allows for control of the topology of the polymer by controlling the rates of chain walking and of olefin insertion. By increasing chain walking, more highly branched polymers are produced, whereas by decreasing the rate of chain walking, more linear polymers are produced Polymer Radius (nm) Molecular Weight (kg/mole) c O 5 x x x x x