The reaction is unusual in that the anhydride reacts with the related styrene to form a copolymer. The presence of the alpha-phenyl group activates the styryl group for a Diels–Alder reaction even at the expense of its aromaticity.
^The Condensation of 1,1-Diarylethylenes with Maleic Anhydride Felix Bergmann, Jacob Szmuszkowicz, and George Fawaz J. Am. Chem. Soc.; 1947; 69(7) pp 1773 - 1777; doi:10.1021/ja01199a055