Ketene Dimer: Ketene Left to Polymerise, the 126-127 °C Liquid (Thorpe 1912)

Kept for a while, ketene turns into a dark liquid. Its chief part is a colourless, pungent liquid boiling at 126-127 °C, C4H4O2: two ketene molecules joined. Source: Thorpe, Dictionary of Applied Chemistry vol. 3 (1912), Ketenes. Unknown: how long and at what temperature, Thorpe gives neither, so they are linked to unknown heads. The separation is ours, after Organic Syntheses Coll. Vol. 3, p. 508 (1955). Metal chlorides or tertiary bases speed the change; that route is another blueprint. Picture drawn by Youblob.