Ring-opening polymerization of ε-caprolactone and cyclohexene oxide initiated by aluminum β-ketoamino complexes: steric and electronic effect of 3-position substituents of the ligands

Abstract

A series of aluminum complexes supported by β-ketoamino, ligand-bearing, 3-position substituents LAlEt 2 (L=CH 3 C(O)C(Cl)=C(CH 3 )NAr (L 1 ), L=CH 3 C(O)C(H)=C(CH 3 )NAr (L 2 ), L=CH 3were synthesized in situ and employed in the ringopening polymerization (ROP) of ε-caprolactone (ε-CL) and cyclohexene oxide (CHO). The 3-position substituents on the β-ketoamino ligand backbone of the aluminum complexes influenced the catalyst activity remarkably for both ROP of ε-CL and CHO. Aluminum β-ketoamino complexes displayed different catalytic behavior in ROP of ε-CL and CHO. The order of the catalytic activity of LAlEt for ROP of ε-CL, being opposite to the electron-donating ability of the 3-position substituents on the β-ketoamino ligand, while the order of the catalytic activity for ROP of CHO was L . The effects of reaction temperature and time on the ROP were also investigated for both ε-CL and CHO. IntrodutionMono-anionic β-ketoamino ligand has emerged as one of the most versatile ligands in coordination chemistry for their strong metal-ligand bonds 1-11 and for their relatively facile tunablility to access derivatives containing a range of substituents around the ligands' skeleton. In the past, most work paid their attentions to the steric and electronic effects of substituents (R) on the nitrogen donor atom and side groups (R 1 and/or R 2 ) of these ligands backbone (see the Scheme I) of metal complexes on the catalyst behavior. 4-9 However, few investigations were focused on the effect of the substituents (X) on the 3-position of the β-ketoamino ligands skeleton. 10 The aluminum complexes draw considerable attention for both organic synthesis 12 and polymerization, 13 due to their strong Lewis acidity, relatively low toxicity, and ready availability. Up to date, many kinds of ligands with sterically bulky have been used in organo-aluminum chemistry to form aluminum complexes. However, few studies have focused on the aluminum complexes with mono-anionic bidentate β-ketoamino ligand as the catalyst for the application in polymerization and small molecule activation. 11 The polymerization activity of metal complexes can be influenced by the steric and electronic characteristics of the ancillary ligand framework. The effect of electronic perturbations of the supporting ligand (particularly in systems that Scheme I. A general structure of β-ketoamino ligand.

Chromium Amino-bis(phenolate) Complexes as Catalysts for Ring-Opening Polymerization of Cyclohexene Oxide

“…These values are consistent with those previously reported for varying molecular weights of PCHO. 5,16 Kinetic Studies. Initial reaction rates for the ROP of CHO by 1 were monitored by following conversion of CHO into PCHO using 1 H NMR spectroscopy at 5 min intervals for reactions run at 22 and 40 °C.…”

“…5−8,12−14 Heterogeneous solid acid catalysts such as montmorillite have also been used giving PCHC with T g of 75 °C. 15 The mechanical properties of PCHO differ from those of PEG and PPO in terms of T g , which for PCHO are found in the range of 58−75 °C, 5,7,8,15,16 resulting in different applications to the more ubiquitous PEG, for example. CHO has been used as a comonomer alongside lactide or anhydrides to give copolymers, 17−23 and the literature concerning CHO copolymeriza-tion with CO 2 is abundant and contains many recent reviews on the topic.…”

“…Less commonly used polyethers include poly­(cyclohexene oxide) (PCHO), which can be prepared from ring-opening polymerizations (ROPs) of cychlohexene oxide (CHO) with a suitable catalyst. − Findings for metal-catalyzed CHO ROP include the production of high-molecular weight polymers, sometimes with broad molecular weight dispersities. − ,− Heterogeneous solid acid catalysts such as montmorillite have also been used giving PCHC with T g of 75 °C . The mechanical properties of PCHO differ from those of PEG and PPO in terms of T g , which for PCHO are found in the range of 58–75 °C, ,,,, resulting in different applications to the more ubiquitous PEG, for example. CHO has been used as a comonomer alongside lactide or anhydrides to give copolymers, − and the literature concerning CHO copolymerization with CO 2 is abundant and contains many recent reviews on the topic. − …”

Coupling reaction of carbon dioxide and epoxides efficiently catalyzed by one-component aluminum–salen complex under solvent-free conditions

“…A possible interpretation is that steric hindrance of phenyl at the R1 and R2 positions and concomitant deformation of the planar ring structure occurred, inhibiting resonance in the chelate ring and playing a predominant role in contributing factor with respect to inductive effect of phenyl group. As a matter of fact, no general agreement exists as to whether phenyl groups are electron repelling or electron withdrawing, since it depends somewhat on the ligand type and the group's substituted position in the ligand [34,[49][50][51][52][53][54]. Significantly, this kind of novel salenAl(III)Cl bifunctional catalyst is extremely stable to moisture and robust to impurities.…”

Review – recent development of ring-opening polymerization of cyclic esters using aluminum complexes

“…[98][99][100][101][102][103][104][105][106][107] Nomura [98] studied the aluminum complexes containing phenoxyimine ligand for initiating polymerization of ɛ-caprolactone and found that the aluminum complexes 71 exhibited living polymerization characteristics when low monomer conversion, increasing conversion resulted in widening the PDI of PCL. Redshaw [99] also investigated the aluminum complexes 72 and 73 for ROP of ɛ-caprolactone, these catalyst system showed produced polymers with 44640 g/mol molecular weight, but, 46% monomer conversion.…”