Abstract The cationic polymerization of acrolein with boron trifluoride etherate at various temperatures has been studied. (1) The polymerization rate may be given as follows: RP=K[C] [M]2, on the basis of the polymerization in an ethyl ether solution, and the activation energy of reaction is 4.5 kcal./mol. (2) The structure of polymers depends very much on the polymerization temperature; the polymerization at the lower temperature gives a polymer containing the larger amount of the end vinyl group as a side chain, and the smaller amount of the aldehyde group.