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A poly(methyl methacrylate)-supported Pd 0 nanocatalyst was successfully prepared from solution reaction of Pd(CH 3 COO) 2 with a copolymer acid, poly(methyl methacrylate- ran -methacrylic acid) (MMA-MAA). The reaction was carried out in a benzene/methanol mixed solvent in the dark at room temperature (∼25 °C) in the absence of a typical chemical reductant. There was coordination between the Pd 0 nanoclusters and MMA-MAA, resulting in Pd 0 nanoclusters being stably and uniformly dispersed in the MMA-MAA matrix, with an average particle size of ∼2.5 ± 0.5 nm. Mechanistically, it can tentatively be proposed that PMMA-ionomerization of the Pd 2+ ions produces intramolecular -2COO - -Pd 2+ aggregate cross-links in the solution. On swelling of the chain-segments that are covalently bound via multiple C-C bonds, the resultant elastic forces cause instantaneous dissociation at the O-Pd coordination bonds to give transient bare ( i.e. , uncoordinated), highly-oxidative Pd 2+ ions and H + -associative carboxylate groups, both of which rapidly scavenge electrons and protons, respectively, of the active α-H atoms abstracted from the methanol molecules of the solvent to make Pd 0 nanoclusters supported by the re-formed MMA-MAA. The MMA-MAA acid copolymer, without itself undergoing any permanent chemical change, serves as a mechanical activator or catalyst for the mechanochemical reduction of Pd(CH 3 COO) 2 under mild conditions. Compared with traditional Pd/C catalysts, this Pd 0 nanocatalyst exhibited more excellent catalytic efficiency and reusability in the Heck reaction between iodobenzene and styrene, and it could be easily separated. The supported Pd 0 nanocatalyst prepared using this novel and simple preparation method may display high-efficiency catalytic properties for other cross coupling reactions., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)