This article presents an experimental study of the spontaneous thermal homopolymerization of methyl acrylate (MA) and n-butyl acrylate (nBA) in the absence of any known added initiators at 120 and 140 degrees C in a batch reactor. The effects of the solvent type, oxygen level, and reaction temperature on the monomer conversion and polymer average molecular weights were investigated. Three solvents, dimethyl sulfoxide (DMSO; polar, aprotic), cyclohexanone (polar, aprotic), and xylene (nonpolar) were used. The spontaneous thermal polymerization of MA and nBA in DMSO resulted in a lower conversion and higher average molecular weights in comparison to polymerization in cyclohexanone and xylene under the same conditions. The highest final conversion of both monomers was obtained in cyclohexanone. The high polymerization rate in cyclohexanone was most likely due to an additional initiation mechanism where cyclohexanone complexed with the monomer to generate free radicals. Bubbling air through the mixture led to a higher monomer conversion during the early stage of the polymerization and a lower polymer average molecular weight in xylene and cyclohexanone; this indicated the existence of a distinct behavior between the air- and nitrogen-purged systems. Matrix-assisted laser desorption/ionization time-of-flight analysis of the polymer samples taken from nitrogen-bubbled batches did not reveal fragments from initiating impurities. On the basis of the identified families of peaks, monomer self-initiation is suggested as the principal mode of initiation in the spontaneous thermal polymerization of MA and nBA at temperatures above 100 degrees C. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1898-1909, 2010