When macroscopic crystalline monomers were polymerized by a free-radical solid-state reaction in the presence of supercritical CO2 (scCO(2)), the resultant products were found to be composed of unexpected nanoparticle morphologies. In particular, the solid-state polymerization (SSP) of amino acid based monomers, acryloyl-p-alanine (ABA) and methacryloyl-beta-alanine (MBA), initiated by azobisisobutyronitrile in scCO(2) (at 65 degrees C and 34.5 MPa), produced corresponding polymers having aggregated spherical architectures. The average diameters of the PABA and PMBA particles were measured to be 94 and 102 nm, respectively. In addition, high molecular weight polymers (PMBA, M-w = 3.8 X 10(5) g/mol) with a high yield (similar to 96%) were obtained. The microscopic investigation revealed that a unique particle formation mechanism was involved in the SSP in which large sized crystalline monomers were chipped into small pieces during the initial stage of polymerization and subsequently converted into nanoscale objects after 24 h. (C) 2009 Elsevier B.V. All rights reserved.