The polymerization of three optically active β‐1,1‐dichloroalkyl β‐propiolactones has been investigated in toluene, at 55°C, using aluminum triisopropoxide (Al(OiPr)3) as initiator in a range of monomer/initiator molar ratios smaller than 150. β‐1,1‐dichloroethyl β‐propiolactone polymerizes according to a living mechanism. However, the ability to polymerize decreases with an increase in the length of the alkyl substituent. For instance, β‐1,1‐dichloro‐n‐propyl β‐propiolactone is obtained only in low yields, whereas β‐1,1‐dichloro‐n‐butyl β‐propiolactone does not polymerize at all. Actually, each of the lactones investigated reacts with Al(OiPr)3 in an initiation step that obeys a coordination‐insertion mechanism. However, the size of the chloroalkyl substituent has a critical effect on the propagation: when the alkyl group contains more than two methylene units, the insertion of a second monomer becomes exceedingly slow.

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