Diffusion　behavior　of　catalyst　pores　plays　a　crucial　influence　in　polymer　heterogeneous　hydrogenation　reactions.　However,　experimental　methods　on　how　to　accurately　measure　the　diffusion　coefficient　of　polymer　molecules　are　still　lacking.　Herein,　we　develop　an　in-situ　infrared-based　method　combined　with　mathematical　models　to　accurately　obtain　the　diffusion　coefficients　of　polymers　and　investigate　the　effect　of　pore　size　on　the　diffusion　and　hydrogenation　performance.　Taking　dicyclopentadiene　resin　as　the　model　polymer,　the　results　show　that　the　selected　Pd/Al2O3　catalyst　with　a　pore　size　of　14.7　nm　possesses　an　apparent　diffusion　coefficient　of　3.83　×　10-15　m2/s　and　exhibits　the　best　catalytic　hydrogenation　activity　with　a　hydrogenation　degree　of　98.4%.　This　study　provides　a　universal　method　for　measuring　the　diffusion　coefficients　of　polymer　molecules,　which　holds　significant　guiding　importance　for　the　further　development　of　more　efficient　polymer　hydrogenation　catalysts.　©　2025　Elsevier　Ltd