Polymerization of optically active β-substituted β-propiolactones. IV. β-1,1-dichloroalkyl β-propiolactones polymerized with aluminum triisopropoxide

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.