Hi Me and my friends have been cracking our heads over this question: Why does PhenylEthene solidify when exposed to air? Me and my friends have come up with an explanation: The oxygen in the air oxidises the phenylethene to a diol. The hydrogen bonds existing between the molecules bring them closer, thus it solidifies. My teacher's explanation was that PhenylEthene undergoes an addition polymerisation reaction and the oxygen in the air acts as an initiator/catalyst for the reaction/. My question is, why is the first explanation unacceptable and secondly, is oxygen really an initiator or catalyst for addition polymerisation reactions?