Copolymers to Stabilize Morphology in Conjugated Polymer-Fullerene Blends and Understanding Alkene Spacing for Repurposing Polyethylene via Alkane Metathesis and Cyclodepolymerization

Abstract

It is indisputable that human activity has warmed Earth’s oceans, land, and atmosphere, posing challenges for human and non-human life. To mitigate the effects of climate change, more sustainable technologies and systems are needed. This thesis describes research in two areas of study that each relate to sustainability: 1) conjugated polymer synthesis for application in organic photovoltaics and 2) open-loop chemical recycling of polyethylene. The first part of the thesis is motivated by the need for efficient renewable energy generation and storage technology. Conjugated polymers are promising materials for these applications because they enable devices that are flexible, lightweight, and potentially inexpensive to manufacture. Although polymer properties like molar mass and sequence can affect device performance, the scope of monomers that can be polymerized via catalyst-transfer polymerization (CTP) to control these properties remains narrow. We highlight the current state and future outlook of CTP and also demonstrate how polymers synthesized via CTP can be used to stabilize morphology and performance in organic photovoltaics (OPVs).

In Chapter 2, we outline the current limitations of CTP for synthesizing polymers for state-of-the-art devices and suggest palladium precatalysts to expand the scope of CTP. Inspired by recent examples of Pd-catalyzed CTP and by difunctionalization reactions in small-molecule cross-coupling literature, we suggest Buchwald and N-heterocyclic carbene ligated Pd precatalysts to expand the scope of monomers that can be polymerized via CTP.

In Chapter 3, we evaluate the effect of sequence, composition, and concentration for a series of conjugated copolymer additives to stabilize morphology in blends for OPVs. We used CTP to synthesize these fullerene-functionalized poly(3-hexylthiophene) (P3HT) copolymers and found that a random copolymer with 20 mol% fullerene-functionalized side chains at 8 wt% in the blend best stabilized morphology. P3HT/fullerene OPV devices with this copolymer demonstrated improved efficiencies over time with thermal annealing to mimic aging.

In Chapter 4, we evaluated our optimized copolymer additive for stabilizing higher-performing donor polymer/fullerene blends for OPVs. We found that this copolymer could stabilize morphology for multiple blends, suggesting that it could be used as a general stabilizing additive. We tested this copolymer in OPV devices for one of the blends and found that although it stabilizes morphology in devices, other factors ultimately limit performance for devices with the copolymer.

The second part of this thesis is motivated by the need for more sustainable end-of-use options for commodity plastics. While plastic production has grown exponentially over the last century, this growth has not been matched by effective waste and has resulted in thousands of tons of plastic pollution. We discuss current limitations for recycling polyethylene (PE), which is produced on the largest scale of any polymer world-wide, and outline our aims to repurpose PE waste via alkane metathesis and depolymerization to macrocycles by taking advantage of ring-chain equilibria.

In Chapter 5 we hypothesized that the quantity of alkenes formed along the PE backbone during alkane metathesis would affect cyclodepolymerization efficiency. We therefore study each step of alkane metathesis to evaluate 1) PE dehydrogenation efficiency and 2) how spacing between alkene backbones for unsaturated polyolefins affects cyclodepolymerization. We find that lower concentrations of backbone alkenes reduce cyclodepolymerization efficiency and that dehydrogenation yields for PE are low. We describe our aims to improve PE dehydrogenation via catalyst design and also discuss the outlook for macrocyclic products that we may obtain.