Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization

We have investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization (ROMP). Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative 13C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, we also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, we observed results consistent with chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) when reaction times were extended.

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