AbstractPoly(methyl methacrylate) (PMMA) normally undergoes random main‐chain fracture when exposed to ionizing radiation. In the presence of radical traps, partial protection against degradation may occur, but no crosslinking reactions have hitherto been observed. In this work, diallyl sebacate has been incorporated into PMMA prior to exposure to ionizing radiation (2 m.e.v. electrons). The plasticized PMMA had a low softening point, which was increased by irradiation due to polymerization of the diallyl ester. At the same time, the PMMA was rendered insoluble. With methylene chloride as solvent, the gel fraction greatly exceeded the fraction of diallyl ester present, proving that the PMMA had become crosslinked or incorporated into the allyl network. Beyond a certain dose, however, which varied with the diallyl sebacate weight fraction, continued irradiation led to increased sol fractions and reduced transition temperatures. Spectroscopic measurements showed the disappearance of allyl unsaturation per unit radiation dose to be identical, within experimental error, in the presence of PMMA as in the pure diallyl sebacate. A semiquantitative examination of the gel/dose curve and dynamic energy absorption curves for a sample containing 16 diallyl sebacate has pointed to radicals produced by radiation on the PMMA chains to be the principal initiating species, these becoming junction points to the allyl network. When the gel is depleted of allyl monomer, further radiation causes degradation of the PMMA segment
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