Reverse orders of reactivities in the polymerization of cyclic esters using N₂O₂aluminium alkoxide complexes

Abstract

Three aluminium alkoxide complexes containing N(2)O(2) bis(phenoxy)-amine ligands were synthesized from reactions of the corresponding N(2)O(2) ligands with Al(O(i)Pr)(3) in toluene. Different amine side chains of the ligands included pyridine (1), CH(2)NMe(2) (2), and CH(2)NEt(2) (3). The related chloro aluminium analog (4) was prepared from a reaction between AlCl(3) and the potassium salt of the N(2)O(2) ligand having CH(2)NMe(2) side chain. X-Ray crystallography reveals that complexes 3 and 4 have a monomeric five-coordinate aluminium center. Complexes 1-3 catalyzed the polymerization of epsilon-caprolactone (epsilon-CL) at 70 degrees C in toluene with the relative reactivities of 1<2< 3. In contrary, only complex 1 was active for the polymerization of lactide under the same polymerization conditions. (1)H NMR spectroscopy shows that treatment of 2 with 1 equivalent of lactide afforded the ring-opened product L(2)Al-OCH(Me)C(O)OCH(Me)C(O)O(i)Pr. Electronic effects are believed to be responsible for the observed trend in the epsilon-CL polymerization rates. On the other hand, steric hindrance at the amine side chain is the main contributor to the observed rates of lactide polymerization.

Random copolymerization of l‐lactide and ε‐caprolactone by aluminum alkoxide complexes supported by N2O2 bis(phenolate)‐amine ligands

“…In this work, crystals of Complexes 1 and 2 suitable for single‐crystal X‐ray crystallographic studies were grown by placing a mixture of dichloromethane and hexane in a freezer. X‐ray crystal structure of Complex 3 was reported earlier …”

“…All reagents such as aluminum isopropoxide and trimethyl aluminum were purchased from commercial suppliers and used as received. N , N ‐bis[methyl(2‐hydroxy‐3‐ tert ‐butyl‐5‐methylphenyl)] aminomethylpyridine (L 1 H 2 ), N , N ‐bis[methyl(2‐hydroxy‐3‐ tert ‐butyl‐5‐methylphenyl)]‐ N ′, N′ ‐dimethylethylenediamine (L 2 H 2 ) and N,N ‐bis[methyl(2‐hydroxy‐3‐ tert ‐butyl‐5‐methylphenyl)]‐ N′ , N′ ‐diethylethylenediamine (L 3 H 2 ), L 1 AlO i Pr ( 1 ), L 2 AlO i Pr ( 2 ), L 3 AlO i Pr ( 3 ), and isopropyl ( s )‐lactate were synthesized according to literature with minor modification. ε‐CL were dried over CaH 2 overnight at room temperature and distilled under vacuum prior to use, then stored at −30 °C under N 2 atmosphere.…”

“…Complexes 1 – 3 were synthesized according to our earlier work . In this work, crystals of Complexes 1 and 2 suitable for single‐crystal X‐ray crystallographic studies were grown by placing a mixture of dichloromethane and hexane in a freezer.…”

“…From our previous work, we have been fascinated by the N 2 O 2 ligand framework where the N 2 O 2 bis(phenolate)‐amine ligand can create a bowl‐shaped structure around the metal center. In the aluminum complexes supported by N 2 O 2 bis(phenolate)‐amine ligands having pyridine, dimethylamine and diethylamine as a side arm, the rates of the homopolymerization of LA were faster for the N 2 O 2 complex containing pyridine while the rates of homopolymerization of ε‐CL were faster when the side arm was diethylamine . These results suggested that, through some variations of the amine side arms, the catalytic activities toward LA and ε‐CL could be adjusted that could lead to random copolymerization of LA and ε‐CL.…”

Aluminium coordination complexes in copolymerization reactions of carbon dioxide and epoxides

“…Historically however, of the main group metals, aluminium complexes have played an important role in the development of copolymerization reactions of carbon dioxide and should be investigated further. Importantly, aluminium complexes are known to catalyse a wide range of other polymerization reactions including ring-opening polymerization (ROP) of cyclic esters [14][15][16][17][18][19][20][21][22][23][24][25][26] and trimethylene carbonate, 18,27,28 and cationic polymerization of alkenes (incl. isoprene, benzofuran and styrenes).…”