Synthesis of Cyclic Polymers by Ring Expansion and Ring Opening Metathesis Polymerization

Citation

Gan, Quan (2023) Synthesis of Cyclic Polymers by Ring Expansion and Ring Opening Metathesis Polymerization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/g9em-gt08. https://resolver.caltech.edu/CaltechTHESIS:03232023-190331423

Abstract

Cyclic polymers are topologically interesting and envisioned as a lubricant material. However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recover a used catalyst after the synthesis of cyclic polymers and reuse it. Unfortunately, this is demanding because of the catalyst’s vulnerability and inseparability from polymers, which reduce the practicality of the process. In Chapter 1 we describe a continuous circular process, where polymerization, polymer separation, and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. This process is enabled by introducing silica-supported ruthenium catalysts and newly designed glassware. Different depolymerization kinetics of the cyclic polymer from its linear analogue are also discussed. This process minimizes manual labour, maximizes the security of vulnerable catalysts and guarantees the purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased turnovers (≥415,000) of precious catalysts.

α-Oxygenated Z-olefins are ubiquitous in biologically active molecules and serve as versatile handles for organic synthesis, but their syntheses are often tedious and less selective. In Chapter 2 we report the efficient Z-selective metathesis of various terminal acrylates and allyl alcohols, which enables facile and selective construction of high value-added α-oxygenated Z-olefins from readily available feedstock chemicals. These challenging metathesis transformations are enabled by novel cyclometalated Ru-carbene-nitrate complexes bearing bulky-yet-flexible side arms, whose assembly was unlocked by new organometallic syntheses.

Efficient separation of macrocyclic polyolefins from reaction mixtures of ring-opening metathesis polymerization is crucial for their application in materials science, drug delivery, and for the mechanistic study of the reaction mechanism. In Chapter 3, we present a facile method for obtaining topologically pure macrocyclic fractions by modifying chain ends using enyne metathesis chemistry and introducing polar functional groups into linear polymer chains through the addition of polar monomers. Nonpolar cyclic polyolefins are then readily separated using silica gel chromatography. The purity of the cyclic fractions was verified using multiple techniques, including gel permeation chromatography, nuclear magnetic resonance spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. We investigate the reaction factors affecting the yield and molecular weight of macrocycles during ring-opening metathesis polymerization of cyclooctadiene and discuss macrocycle formation in the ring-opening metathesis polymerization of cyclooctene, cyclopentene, and norbornene. Our work offers crucial insights into the synthesis and separation of macrocycles.

Thesis Files