He pursued his research with a passion he never tired of, driven by pure intellectual curiosity.

tired 是动词 tire 的过去分词形式，tire 的意思是"感到疲倦、厌倦"。 of 是介词，表示"关于、对于"的关联关系。 tired of 构成固定搭配，意思是"对...感到厌倦"，这里用 never tired of 表示"从不厌倦"。