Reactions of 2,2′-(2-methoxybenzylidene)bis(4-methyl-6-tert-butylphenol) with trimethylaluminum:: Novel efficient catalysts for "living" and "immortal" polymerization of ε-caprolactone

A sterically hindered biphenol 2,2'-(2-methoxybenzylidene)bis(4-methyl-6-tert-butylphenol) (MEBBP-H-2) (1) has been prepared by the reaction of o-anisaldehyde with 2-tert-butyl-4-methylphenol in the presence of a catalytic amount of benzenesulfonic acid. Further reaction of compound 1 with a stoichiometric amount of Me3Al in tetrahydrofuran produces a four-coordinated monomeric aluminum complex [(MEBBP)AlMe(THF)] (2). [(MEBBP)Al(mu-OBn)](2) (3) can then be synthesized by the reaction of 2 with I mol equiv of benzyl alcohol at ambient temperature. Compound 3 has demonstrated highly efficient activities toward ring-opening polymerization of E-caprolactone. The "living" and the "immortal" character of 3 has paved a way to synthesize as much as 256-fold polymer chains of poly(epsilon-caprolactone) with a very narrow polydispersity index in the presence of a small amount of initiator. In addition, the polystyrene-b-poly(epsilon-caprolactone) copolymer has also been prepared using polystyrene containing a hydroxy chain end as an initiator in the presence of 2.