Organoaluminium Complexes Derived from Anilines or Schiff Bases for the Ring-Opening Polymerization of ε-Caprolactone, δ-Valerolactone and rac-Lactide

Reaction of (RRCHN)-R-1-C-2=CH(3,5-tBu(2)C(6)H(2)-OH-2) (R-1 = R-2 = Me (LH)-H-1; R-1 = Me, R-2 = Ph (LH)-H-2; R-1 = R-2 = Ph (LH)-H-3) with slightly greater than one equivalent of R-3 Al-3 (R-3 = Me, Et) afforded the complexes [(L1-3) AlR23] (L-1, R-3 = Me 1, R-3 = Et 2; L-2, R-3 = Me 3, R-3 = Et 4; L-3 R-3 = Me 5, R-3 = Et 6); complex 1 has been previously reported. Use of the N,O-ligand derived from 2,2'-diphenylglycine afforded either 5 or the byproduct [Ph2NCH2(3,5-tBu(2)C(6)H(2)-O-2)AlMe2] (7). The known Schiff base complex [2-Ph2PC6H4CH2(3,5-tBu(2)C(6)H(2)-O-2)AlMe2] (8) and the product of the reaction of 2-diphenylphosphinoaniline 1-NH2,2-PPh2C6H4 with Me3Al, namely {Ph2PC6H4N[(Me2Al)(2)mu-Me](mu-Me2Al)} (9), were also isolated. For structural and catalytic comparisons, complexes resulting from the interaction of Me3Al with diphenylamine (or benzhydrylamine), namely {Ph2N[(Me2Al)(2)mu-Me]} (10) and [Ph2CHNH(mu-Me2Al)](2)center dot MeCN (11), were prepared. The molecular structures of the Schiff proligands derived from Ph2CHNH2 and 2,2'-Ph2C(CO2H)(NH2), together with those of complexes 5, 7 and 9-11 center dot MeCN were determined; 5 contains a chelating imino/phenoxide ligand, whereas 7 contains the imino function outside of the metallocyclic ring. Complex 9 contains three nitrogen-bound Al centres, two of which are linked by a methyl bridge, whilst the third bridges the N and P centres. In 10, the structure resembles 9 with a bridging methyl group, whereas the introduction of the extra carbon in 11 results in the formation of a dimer. All complexes have been screened for their ability to promote the ring-opening polymerization (ROP) epsilon-caprolactone, delta-valerolactone or rac-lactide, in the presence of benzyl alcohol, with or without solvent present. Reasonable conversions were achievable at room temperature for epsilon-caprolactone when using complexes 7, 9 and 12, whilst at higher temperatures (80-110 degrees C), all complexes produced good (> 65 %) to quantitative conversions over periods as short as 3 min, albeit with poor control. In the absence of solvent, conversions were nearly quantitative at 80 degrees C in 5 min with better agreement between observed and calculated molecular weight (M-n). For rac-lactide, conversions were typically in the range 71-86 % at 110 degrees C in 12 h, with poor control affording atactic polylactide (PLA), whilst for delta-valerolactone more forcing conditions (12-24 h at 110 degrees C) were required for high conversion. Co-polymerization of epsilon-caprolactone with rac-lactide afforded co-polymers with appreciable lactide content (35-62.5 %); the reverse addition was ineffective, affording only (polycaprolactone) PCL.