Lewis acid-driven accelerated living polymerization of lactones initiated with aluminum porphyrins. Chemoselective activation of ester groups by Lewis acid

Ring-opening polymerization of delta-valerolactone (delta-VL) initiated with (5,10,15,20-tetraphenylporphinato)aluminum methoxide ((TPP)AlOMe, 1; X = OMe) was dramatically accelerated upon addition of methylaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (2a) and bis(2,4,6-tri-tert-butylphenolate) (2b) via coordinative activation of monomer, but was accompanied by transesterification, resulting in broadening of the polymer molecular weight distribution (MWD) at the later stage of polymerization. On the contrary, when methylaluminum bis(2,6-di-X-phenolate)s (X = Cl (2c), Br (2d), I (2e), and Ph (2f)) were used as Lewis acids, the polymerization proceeded also rapidly, retaining the narrow MWD. C-13 NMR studies on the Lewis acid (2b or 2f)-delta-VL and -poly(delta-VL) mixtures showed that the coordination abilities of 2b and 2f toward the monomer ester group are comparable to each other, while 2f shows a lower affinity to the polymer ester group than 2b. 2a was also able to accelerate the polymerization of beta-butyrolactone (beta-BL) with (TPP)AlOMe or (5,10,15,20-tetraphenylporphinato)aluminum chloride ((TPP)AlCl, 1; X = Cl) to different extents depending on the mode of ring scission, where the MWD of the polymer remained narrow throughout the accelerated polymerization (M(w)/M(n) = 1.1-1.2).