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In particular, organosodium and organopotassium compounds show different reactivities from organolithium compounds, albeit the underlying molecular reasons are largely unexplored due to the absence of relevant intermediates. We present the first\u2010step organometallic intermediates of an anionic styrene polymerization with sodium and potassium as counterions, and the first intermediate of an isoprene polymerization with potassium. The structures were crystallized and characterized by X\u2010ray diffraction, providing insights into structural characteristics of heavier alkali\u2010metal organyls. As the compounds were synthetically accessible, they do not only represent organometallic model compounds of the propagating chain ends but also new monometallic alkoxide\u2010free initiators. This enables, for the first time, a fundamental understanding of structure\u2010reactivity relationships in anionic polymerizations using organosodium or organopotassium reagents. The newly synthesized initiators were applied for a real\u2010time in situ near\u2010infrared spectroscopy kinetic investigation of the styrene\/isoprene copolymerization with heavier alkali metals in cyclohexane. Combined with DFT studies, these results elucidate structural effects of the isolated intermediates, showing that charge stabilization in the transition state mainly influences the reactivity. With this combination of molecular methods for anionic polymerizations, molecular insights into the behavior of heavier alkali\u2010metal organyls are obtained.<\/jats:p>","DOI":"10.1002\/smll.75765","type":"journal-article","created":{"date-parts":[[2026,9,23]],"date-time":"2026-09-23T10:38:02Z","timestamp":1790159882000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Isolated Organosodium and \u2010Potassium Intermediates: Understanding Metal Effects on Anionic Styrene\u2010Isoprene Copolymerization"],"prefix":"10.1002","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7189-0800","authenticated-orcid":false,"given":"Annika","family":"Schmidt","sequence":"first","affiliation":[{"name":"Inorganic Chemistry TU Dortmund University  Dortmund Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fr\u00e9d\u00e9ric S.","family":"Petrov","sequence":"additional","affiliation":[{"name":"Department of Chemistry Johannes Gutenberg University Mainz Mainz Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-6752-2902","authenticated-orcid":false,"given":"Yurdag\u00fcl","family":"Herg\u00fcl","sequence":"additional","affiliation":[{"name":"Inorganic Chemistry TU Dortmund University  Dortmund Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9423-9829","authenticated-orcid":false,"given":"Axel H. 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