<jats:title>Abstract</jats:title><jats:p>Anionic ring‐opening polymerization of propylene oxide in the presence of a potassium alkoxide initiator was accelerated by the addition of the bulky phosphonium salts tetrakis[cyclohexyl(methyl)amino]phosphonium‐, tetrakis[propyl(methyl)amino]phosphonium‐, and tetrakis[octyl(methyl)amino]phosphonium‐tetrafluoroborate. Dipropylene glycol (DPG) was partially deprotonated (5%) and used as initiator. The delocalization of the positive charge over five atoms promoted the formation of a separated ion‐pair, thus enhancing nucleophilicity and reactivity. The polymerization behavior of the counterions at varied temperatures was studied. Characterization of poly(propylene oxide)s by means of <jats:sup>1</jats:sup>H NMR spectroscopy, size exclusion chromatography (SEC), and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS) showed low polydispersities and the absence of by‐products and impurities. The degree of polymerization (<jats:styled-content><jats:italic>DP</jats:italic></jats:styled-content><jats:sub>n</jats:sub>) for the polymers was in the range of 8–60 (<jats:styled-content><jats:italic>M</jats:italic></jats:styled-content><jats:sub>n</jats:sub> = 630–3 620 g · mol<jats:sup>−1</jats:sup>) and <jats:styled-content><jats:italic>M</jats:italic></jats:styled-content><jats:sub>w</jats:sub>/<jats:styled-content><jats:italic>M</jats:italic></jats:styled-content><jats:sub>n</jats:sub> obtained was 1.03–1.35 and 1.11–1.32 for MALDI‐TOF‐MS and SEC, respectively. Values calculated from the titration of hydroxyl groups (OHV) showed good agreement. Determination of the total degree of unsaturation in the range 13–60 mmol · kg<jats:sup>−1</jats:sup> indicated larger amounts with decreasing polymerization rates and increasing polymerization temperatures.</jats:p><jats:p><jats:boxed-text content-type="graphic" position="anchor"><jats:caption><jats:p>The phosphonium salts Cy<jats:sub>4</jats:sub>P<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-1.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-1" />BF<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-2.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-2" />, Pr<jats:sub>4</jats:sub>P<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-3.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-3" />BF<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-4.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-4" />, and Oc<jats:sub>4</jats:sub>P<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-5.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-5" />BF<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-6.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-6" />.</jats:p></jats:caption><jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" mimetype="image/gif" position="anchor" specific-use="enlarged-web-image" xlink:href="graphic/mgra001.gif"><jats:alt-text>magnified image</jats:alt-text><jats:caption><jats:p>The phosphonium salts Cy<jats:sub>4</jats:sub>P<jats:inline-graphic xlink:href="graphic/tex2gif-stack-1.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-1" />BF<jats:inline-graphic xlink:href="graphic/tex2gif-stack-2.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-2" />, Pr<jats:sub>4</jats:sub>P<jats:inline-graphic xlink:href="graphic/tex2gif-stack-3.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-3" />BF<jats:inline-graphic xlink:href="graphic/tex2gif-stack-4.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-4" />, and Oc<jats:sub>4</jats:sub>P<jats:inline-graphic xlink:href="graphic/tex2gif-stack-5.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-5" />BF<jats:inline-graphic xlink:href="graphic/tex2gif-stack-6.gif" xlink:title="urn:x-wiley:10221352:media:MACP200390086:tex2gif-stack-6" />.</jats:p></jats:caption></jats:graphic></jats:boxed-text> </jats:p>