Monomeric and dimeric Al(iii) complexes for the production of polylactide

Abstract

A series of monometallic and bimetallic Al(iii) complexes with substituted naphthyl based Schiff base ligands have been prepared and characterised. When 1-aminonaphthalene based ligands were reacted with AlMe3 monometallic complexes were isolated, however, with 1,5 and 1,8-diaminonaphthalene based ligands bimetallic complexes were formed. In all cases 4-coordinate tetrahedral Al(iii) centres were observed in the solid state and in solution. There was little difference in rate of polymerisation of rac-lactide between the monometallic and bimetallic complexes based on 1,5-diaminonaphthalene. However, for the 1,8-diaminonaphthalene the complex was an order of magnitude faster than the monometallic and the analogous 1,5-system. Moreover, this complex was active at room temperature, which is rare for aluminium initiators, and PLA with a high degree (Pm = 0.82) of isotacticity was observed.

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

“…Despite the apparent mismatch of stoichiometry, the use of one equivalent of BnOH (per aluminium) for R 2 Al‐containing pre‐catalysts is well established; the use of two equivalents (per aluminium) has been found to afford inferior results. [6a,c,e] In our systems, extending the reaction time (see Table S3) or varying the amount of BnOH was not found to be beneficial. Pre‐catalyst 2 was employed to ascertain the optimum conditions (see Table S4) and was found to be effective for the ROP of ε‐CL at temperatures of 80 to 110 °C affording conversions > 67 %.…”

“…Complex 1 was previously reported by Milione et al and used for halide anion binding through hydrogen‐bonding, whereas the debutylated version of complex 5 has recently been employed by Chiang, Chen and Chen and co‐workers for the ROP of ε‐caprolactone (ε‐CL) and l ‐lactide; however, the structure of debutylated 5 was not reported. [6v] Herein, crystals of 5 suitable for an X‐ray diffraction study were grown from acetonitrile on prolonged standing at ambient temperature. The molecular structure is shown in Figure with selected bond lengths and angles given in the caption; crystallographic data are presented in Table .…”

“…For complexes 1 – 6 , the trend is for the methyl derivatives to outperform the ethyl derivatives at both 80 and 100 °C, a trend that has been seen previously, the opposite trend has also been reported. [6u] Within the series 1 – 6 , on changing the sterics of the precursor aniline, there is little change in the conversion rates for either the methylaluminium or ethylaluminium derivatives.…”

“…For an overview of the phenoxyimine aluminium systems that have previously been employed in the ROP of cyclic esters, see Table S1 (in the Supporting Information). [4b], [4d], [4g], [4j], , Having employed this type of ligand set recently in vanadium‐based α‐olefin polymerization, we now, as part of our continued interest in designing new aluminium‐based initiators/catalysts for the ROP of cyclic esters,[6o] report our results on the series of complexes [R 1 R 2 CHN=CH(3,5‐ t Bu 2 C 6 H 2 ‐O‐2)AlR 3 2 ] (R 1 = R 2 = R 3 = Me 1 ; R 1 = R 2 = R 3 = Et 2 ; R 1 = R 3 = Me, R 2 = Ph 3 ; R 1 = Me, R 2 = Ph, R 3 = Et 4 ; R 1 = R 2 = Ph, R 3 = Me 5 ; R 1 = R 2 = Ph, R 3 = Et 6 ) and compare their behaviour against organoaluminium complexes derived from the amine component only (i.e., minus the phenoxy‐containing 3,5‐di‐ tert ‐butylsalicyl motif, see Schemes and ). We note that Nomura and colleagues have previously investigated the effect of the imino substituent on the ROP of ε‐caprolactone (CL) and observed greatly enhanced activity for aryl substituents (C 6 F 5 , 2,6‐ i Pr 2 C 6 H 3 ) versus aliphatic substituents (adamantyl, tert ‐butyl) .…”

Highly active Mg(ii) and Zn(ii) complexes for the ring opening polymerisation of lactide

“…[9][10][11][12][13] Aluminium has received considerable attention in the literature. [14][15][16][17][18][19][20][21][22][23] In particular, {ONNO}Al(III) complexes have been shown to be stereoselective in the ROP of rac-LA, albeit often with poor activity and subject to severe air/ moisture sensitivity. For the ROP of rac-LA, strong isotactic bias has often been demonstrated by such complexes, the most notable examples from Spassky, 24,25 Feijen 26,27 and Nomura.…”