Tehshik Yoon

Effective Recycling Pathways of Commodity Polymers Enabled by Mechanoradical Capture Plastics pervade every aspect of modern life, yet effective mechanical recycling remains a major challenge. This is, in part, because of the mechanical forces that are involved in reprocessing, which break polymer chains and generate mechanoradicals, leading to a reduction in molecular weight and diminished material properties. This work introduces a robust strategy to capture and redirect these reactive intermediates, enabling value-preserving recycling pathways for widely used polymers polystyrene (PS) and poly(methyl methacrylate) (PMMA). By employing ball milling to induce chain scission, we demonstrate that mechanoradicals can be trapped by bis(butyl trithiocarbonate), yielding polymers with trithiocarbonate (TTC) end groups. Polymers degraded via ball milling showed significant reduction in molecular weight, ≈90% lower than the pristine polymers. These low molecular weight, TTC-functionalized polymers then served as macroinitiators for light-mediated controlled polymerization or, in the case of PMMA, as mediators for depolymerization under mild conditions. Chain extension of the degraded materials led to restored or increased molecular weight compared to the pristine polymers. Shear oscillatory rheology experiments revealed a recovery of entangled polymer properties, as evidenced by the reappearance of the rubbery plateau. We further showed that this “capture-and-repair” strategy is compatible with multiple cycles of degradation and chain extension, achieving repeated molecular weight recovery over three cycles. Additionally, we found that ball milling alone lowers the thermal depolymerization temperature of PMMA, enabling up to ≈44% depolymerization at 220 °C. Together, these findings highlight mechanoradical capture as a promising strategy to both enhance circularity and improve overall performance of mechanically recycled plastics.

Very excited to share our recent article in JACS where we showed that we could capture mechanoradicals formed during polymer degradation and use them to grow polymers back to high MWs or prime them for depolymerization! pubs.acs.org/doi/10.1021/...

Oxygen migration into carbon–carbon single bonds by photochemical oxidation - Nature Synthesis Heteroatom insertions into chemically inert carbon–carbon single bonds are rare compared to their unsaturated analogues. Now, ligand-to-metal charge transfer offers a promising entry point for oxygen ...

Now online and open access:

Article by Tehshik P. Yoon & co-workers @tehshik.bsky.social @uwyoongroup.bsky.social

Oxygen migration into carbon–carbon single bonds by photochemical oxidation

www.nature.com/articles/s44...
#Chemsky

A busy mentor taking time to reach out to a potential PI can be very impactful. It doesn't happen often. It's especially helpful if you can provide some texture that might not be evident from the applicant's resume and transcript.

Assistant Professor of Chemistry - Madison, Wisconsin, United States Current Employees: If you are currently employed at any of the Universities of Wisconsin, log in to Workday to apply through the internal application process.Job Category:FacultyEmployment Type:Regula...

Come be my newest colleague!

jobs.wisc.edu/jobs/assista...

The New York Times piece today about US science is terrible and wrong—in many ways.

I could write a whole article about this, but as one example:

“To close observers, the original crisis began well before any of this…”
No. I’m a close observer of science, and this is incorrect.

Stereo-reversed E2 unlocks Z-selective C–H functionalization The stereoselective functionalization of C–H bonds represents a central challenge in modern organic synthesis. Despite decades of innovation in C–H activation chemistry, methods for Z-selective functi...

In @science.org this week @zachwickens.bsky.social and collaborators find a way to do Z-selective elimination using thianthrenium chemistry in a versatile route to Z olefins.

chemsky 🧪

www.science.org/doi/10.1126/...

Really proud of Riley's preprint on a new class of chiral Lewis acid photocatalysts. This project benefitted from a terrific collab with @chemguyeli.bsky.social chemrxiv.org/engage/chemr...

Excited to share our new preprint, which was years in the making! chemrxiv.org/engage/chemr...
New reactions are typically developed by trial and error. How can we speed up this process? Read on to learn how we used DNA scaffolding to perform >500,000 parallel reactions on attomole scale.
1/n

I don't perceive this crisis in Chemistry - not as a reviewer, editor, or author. Sure I'd like it if I got a couple fewer requests each month and if my reviewers accepted a couple more invitations apiece - but neither constitutes a hashtag-level issue.

Last night, the 2025 Roger Adams Award recipient - Prof. Eric Jacobsen - delivered a fantastic lecture at the National Organic Symposium. Prof. John Wood provided a brilliant introduction.

The award is sponsored by @OrgReactions and @OrgSynth

Congratulations!

PhD scholarship in Organic Chemistry and Enantioselective Homogeneous Catalysis - DTU Chemistry A fully-funded 3-year PhD position in organic chemistry, C-H functionalization, and homogeneous enantioselective metal catalysis under the supervision of Associate Professor Søren Kramer is available ...

We have an open PhD position! The project involves enantioselective C-H functionalization with transition-metal catalysis +/- photocatalysis. Please share with relevant candidates or apply here: efzu.fa.em2.oraclecloud.com/hcmUI/Candid...
Deadline: July 6
Link to our recent work in comments.

PhD position in machine learning for photocatalysis

We are recruiting a PhD student in machine learning for photocatalysis! In this project, we will collaborate with the group of Frank Glorius @gloriusgroup.bsky.social to develop predictive tools for energy-transfer-catalyzed photocycloadditions.

Reposts appreciated!
jobs.ethz.ch/job/view/JOP...

Spring 25' saw the kick-off of our first #MerckChemistry Symposium at UW-Madison. We were blessed with beautiful weather and an amazing day of science. Special thanks to our hosts @tehshik.bsky.social & Shannon Stahl, Scott and Emma as Merck organizers, and the entire UW Chem and ChemEng community!

Like shh man.

Some good news for a change–Queen's has set aside funds to assist any students who have lost their visa status in the US and are looking to continue their graduate studies. Please share widely.

www.queensu.ca/grad-postdoc...

U.S. citizen interested in grad studies at the University of British Columbia in Vancouver (Canada)?

Grad programs are re-opening applications of US programs for one week. With expedited decisions.

U.S. Applicant Week:

www.grad.ubc.ca/us-applicant...

#AcademicChatter #Canada #GradSchool

#teamroslyn

Intramolecular ortho Photocycloaddition of 4-Substituted 7-(4′-Alkenyloxy)-1-indanones and Ensuing Reaction Cascades 4-Substituted 7-(4′-alkenyloxy)-1-indanones were prepared from the respective substituted aryl propanoic acids and subjected to UV-A irradiation (λ = 350 or 366 nm). While the 4-chloro compound was directly converted at λ = 366 nm into a pentacyclic product (47% yield) by a three-photon cascade process, the oxygenated substrates reacted in trifluoroethanol at λ = 350 nm by a two-photon cascade, involving an ortho photocycloaddition, a thermal disrotatory ring opening, and a [4π] photocyclization (six examples, 67–82% yield). An ensuing photochemical di-π-methane rearrangement of the latter products was achieved by irradiation at λ = 350 nm in toluene (five examples, 36–70% yield). The diastereoselectivity of the reaction was probed employing a chiral 1-indanone with a stereogenic center at carbon atom C3. 1-Indanones with a 4-hexenyloxy side chain [(E)- or (Z)-configured] at carbon atom C7 served to interrogate the stereospecifity of the reaction.

Check out Audrey's work on ortho photocycloaddition reaction cascades published in JOC! #Photochemistry #WomeninSTEM 💡👩‍🔬
pubs.acs.org/doi/full/10....

Dani!!! That is so awesome!!!

"No no, nobody calls me Dr Freeze, Dr Freeze is my dad. Call me Todd."

Researchers overlooked airborne diseases for centuries — then COVID-19 changed everything A fascinating exploration of microbes that can travel through the air reveals how the pandemic marked a turning point for a crucial research field.

Insightful review @nature.com by @linseymarr.bsky.social
www.nature.com/articles/d41...

Stereochemical Editing at sp3-Hybridized Carbon Centers by Reversible, Photochemically Triggered Hydrogen Atom Transfer ConspectusMillions of chiral compounds contain a stereogenic sp3-hybridized carbon center with a hydrogen atom as one of the four different substituents. The stereogenic center can be edited in an increasing number of cases by selective hydrogen atom transfer (HAT) to and from a photocatalyst. This Account describes the development of photochemical deracemization reactions using chiral oxazole-annulated benzophenones with a bonding motif that allows them to recognize chiral lactam substrates by two-point hydrogen bonding. The backbone of the catalysts consists of a chiral azabicyclo[3.3.1]nonan-2-one with a U-shaped geometry, which enables substrate recognition to occur parallel to the benzoxazole part of the aromatic ketones. The photocatalysts facilitate a catalytic photochemical deracemization of several compound classes including hydantoins, N-carboxyanhydrides, oxindoles, 2,5-diketopiperazines, and 4,7-diaza-1-isoindolinones. In addition, if more than one stereogenic center is present, the editing delivers a distinct diastereoisomer upon the appropriate selection of the respective photocatalyst enantiomer. The chiral photocatalysts operate via the benzophenone triplet that selectively abstracts a properly positioned hydrogen atom in exclusively one of the two substrate enantiomers. The photochemical step creates a planar carbon-centered radical and erases the absolute configuration at this position. While returning HAT to the same position would likely recreate the stereogenic center with the same absolute configuration, spectroscopic and quantum chemical studies suggest that the hydrogen atom is delivered from the photocatalyst to a heteroatom that is in conjugation to the radical center. Two scenarios can be distinguished for the hydrogen atom shuttling process. For hydantoins, N-carboxyanhydrides, and 4,7-diaza-1-isoindolinones, the back HAT occurs to a carbonyl oxygen atom or an imine-type nitrogen atom which is not involved in binding to the catalyst. For oxindoles and 2,5-diketopiperazines, a single lactam carbonyl group in the substrate is available to accept the hydrogen atom. It is currently assumed that back HAT occurs to this group, although the carbonyl oxygen atom is involved in hydrogen bonding to the catalyst. In comparison to the former reaction pathway, the latter process appears to be less efficient and more prone to side reactions. For both cases, an achiral enol or enamine is formed, which delivers upon dissociation from the catalyst statistically either one of the two stereoisomers of the substrate. Since only one substrate enantiomer (or diastereoisomer) is processed, a high enantioselectivity (or diastereoselectivity) results. Even though the editing is a contra-thermodynamic process, the described decoupling of a photochemical and a thermal step allows the usage of a single catalyst in loadings that vary between 2.5 and 10 mol % depending on the specific mode of action.

Interested in stereochemical editing by photochemically triggered hydrogen atom transfer? Check out our latest review by Maxi 💡🧪
pubs.acs.org/doi/10.1021/...

🚨 NEWS 🚨 Super excited to share that our first paper has been published in @angewandtechemie.bsky.social, describing our development of Vitamin B6-inspired amino acid photodecarboxylation! doi.org/10.1002/anie...

Newton International Fellowships | Royal Society This fellowship is for non-UK scientists who are at an early stage of their research career and wish to conduct research in the UK.

📢 Royal Society Newton International Fellowship applications open now, to host international postdocs in the UK! Are you interested in a fellowship in photochemistry, heterocycles and/or strained molecules? Email me with your CV & details of your research interests 👍 royalsociety.org/grants/newto...

Second!

Really excited to see this amazing work out in ChemRxiv from our #ChemCollab between the @uwyoongroup.bsky.social @tehshik.bsky.social and #PfizerChemistry
Photochem-mediated oxygen migration into sp3 bonds delivering small ring ethers as well as an OH - methylene transposition
#ChemSky 1/

oxygen and nitrogen insertion reactions into saturated carbocycles

Friends, I'm pleased to draw your attention to two new preprints today describing complementary heteroatom insertion reactions into saturated carbocycles, one from our group and another from the Knowles laboratory.

chemrxiv.org/engage/chemr...
chemrxiv.org/engage/chemr...

Oxygen Migration into Carbon–Carbon Single Bonds by Photochemical Oxidation The editing of organic molecules through single-atom modifications is an enabling capability for medicinal chemistry. While several examples of single-atom insertions into the carbon–carbon double bon...

Up today on the Rxiv! Cade, Caitlin, Matt, Felix, Desiree, and Scott utilize photochemical oxidation for oxygen migration into C–C single bonds chemrxiv.org/engage/chemr...

What will really annoy chemists is the Texas carbon….

Hey Organic Asst Profs coming up for tenure (and your senior colleagues) — it is nomination season for the ACS DOC Young Academic Investigators Symposium. Deadline is Feb. 6, 2025. The website has last year’s date 😳 but the other info is right. #ChemSky www.organicdivision.org/meetingsuppo...