Expandable styrene polymer particles which are photodegradable are produced by suspending styrene polymer particles in an aqueous medium and impregnating a blowing agent into the particles in the presence of 0.5 - 5.0 parts of photosensitizer per 100 parts of polymer particles. The photosensitizer is selected from the steroids having a 3-keto-group combined with an ethylenic unsaturation in either the 1-, 4-, or 1,4-positions.
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