Kinetic Investigation on the Catalytic Ring-Opening (Co)Polymerization of (Macro)Lactones Using Aluminum Salen Catalysts

The kinetic behavior of the catalytic ring-opening polymerization (cROP) of a range of macrolactones, including omega-pentadecalaconte (PDL), ambrettolide (Amb), and butylene adipate (BA), and small-ring lactones, including L-lactide (LLA), epsilon-caprolactone (epsilon-CL), epsilon-decalactone (epsilon-DL), and beta-butyrolactone (B-BL), using various aluminum salen complexes was investigated. The cROP rates were shown to be first order both in catalyst and in monomer. The activation energies of the polymerization of PDL and LLA in combination with aluminum salen complexes, with and without tert-butyl groups, were determined, showing that the increase in steric hindrance is negatively affecting the polymerization rate of LLA more than of PDL. Interestingly, an increase of the salen diimine bridge from ethylene to 2,2-dimethyl propylene leads to a dramatic increase in rate for the polymerization of small ring lactones, while it leaves the rate of polymerization of macrolactones practically unchanged. In order to exploit this difference in reactivity, the synthesis of block copolymers of epsilon-CL and PDL was attempted using kinetic resolution. However, all the polymers obtained over time were found to be fully random, which appeared to be the result of fast transesterification. Poly(PDL-b-CL) block copolymers were successfully synthesized applying the high reactivity of epsilon-CL in a sequential feed strategy. However, these block copolymers rapidly transform into fully random copolymers as a result of transesterification, which was shown to have a similar rate constant as the rate constant of the polymerization of PDL. By carefully tuning the reaction time polymers with block, gradient or random topology can be obtained.