= Solution
At low <temperature> the four terminal <methyl groups> contain 12 equivalent protons and the two bridging methyls contain six, giving the \b[$2:1$ pair of singlets]. On warming, terminal and bridging positions interchange rapidly on the <NMR timescale>. A <methyl group> then samples both environments during the observation, producing a single population-weighted resonance, $\delta_{\rm av}=(2\delta_{\rm term}+\delta_{\rm bridge})/3$. This <Chemical exchange in NMR> is not merely faster rotation of each methyl about its own Al–C direction, which cannot turn a terminal site into a bridge.
The <concentration> independence argues against a rate-limiting collision between two intact dimers. An intramolecular bridge-opening and reclosing rearrangement can exchange which methyls are bridges; a unimolecular dissociation followed by rapid reassociation is another mechanism whose observed rate could be <concentration> independent. \b[The spectra establish rapid site exchange, but not a unique elementary exchange pathway.] A variable-temperature line-shape analysis or exchange experiment would be needed to distinguish detailed mechanisms. Primary organoaluminium studies describe the same low-temperature resolution and warm-temperature averaging, while considering dimer dissociation.<Organoaluminium compounds. XIII. Association through aryl and phenylethynyl bridges by E. A. Jeffery and others>[1968, printed p. 137, introduction: separate methyl environments and temperature-induced averaging]{ref}
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