UC Santa Barbara

Linear Low Density Polyethylene (LLDPE) is one of the largest volume consumer plastics with production in excess of 100 million tons annually. This form of polyethylene, which is achieved through the copolymerization of ethylene with an α olefin, has been extensively studied for its role in the ‘comonomer effect’, where the introduction of a relatively small amount of an α olefin increases productivity by an order of magnitude. The work presented here explores two single site organometallic complexes active towards olefin polymerization to gleam insight into the ‘comonomer effect’. When a catalyst that builds high molecular weight polyethylene is used in a solution saturated in ethylene the polymer grows to high molecular weight and large volume of solvent is trapped that prematurely arrests productivity. However, with the introduction of 1-hexene the system produces an order of magnitude more polymer of lower molecular weight polymer instead without trapping as much solvent. These results were able to be replicated without introduction of 1-hexene by controlling the catalyst/ethylene ration through a more gradual introduction of ethylene to the system or pre-activation of the catalyst, which is able to explain the comonomer effect in terms of monomer access to active sites and without any chemical effect of 1-hexene on the catalyst. In studying a second catalyst system for copolymerization under sufficient hexene incorporation (25-80%) that polymers remain fully soluble throughout the reaction allows monomers to have full access to active sites. Therefore we construct a kinetic model to extract rate constants by analyzing molecular weight data, monomer consumption, and end groups. It is found that higher the 1-hexene concentration preserves active polymerization sites for longer throughout our reactions. Additionally, through analysis of 13C NMR of these copolymers units of three monomers, triads, are quantified which allows the model to consider the effect of the previous monomer insertion on the next insertion. It was discovered that the following insertion will proceed twice as fast, regardless of identity, if the previous insertion was 1-hexene relative to ethylene. This can also lead to the ‘comonomer effect’ being observed in homogenous polymerization. Finally, through the Wittig Reaction α olefins labeled with deuterium on the α and β carbons were synthesized in good yields for polymerization into copolymers with site specific isotopic labeling suitable for kinetic isotope effect experiments to support mechanisms into the pyrolysis of LLDPE to develop more efficient polymer upcycling reactions.