Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators.

A series of aluminum salen-type complexes [where salen is N,N′-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor

Electrical Conductivity Response of Poly(Phenylene-vinylene)/Zeolite Composites Exposed to Ammonium Nitrate

Poly(p-phenylenevinylene) (PPV) was chemically synthesized via the polymerization of p-xylene-bis(tetrahydrothiophenium chloride) monomer and doped with H2SO4. To improve the electrical conductivity sensitivity of the conductive polymer, Zeolites Y (Si/Al = 5.1, 30, 60, 80) were added into the conductive polymer matrix. All composite samples show definite positive responses towards NH4NO3. The electrical conductivity sensitivities of the composite sensors increase linearly with increasing Si/Al ratio: with values of 0.201, 1.37, 2.80 and 3.18, respectively. The interactions between NH4NO3 molecules and the PPV/zeolite composites with respect to the electrical conductivity sensitivity were investigated through the infrared spectroscopy

A broad scope of aliphatic polyesters prepared by elimination of small molecules from sustainable 1,3-dioxolan-4-ones

Biodegradable aliphatic polyesters built from sustainable feedstocks are one of the most promising solutions to address the pollution and oil-dependence challenges of modern plastics, but remain limited in monomer scope and thus the accessible polymer properties. We report a family of monomers that are built from renewable resources and use the elimination of small molecules to access aliphatic polyesters, circumventing challenging monomer syntheses to make these functionalised polymers. The driving force for ring opening polymerisation is the elimination of formaldehyde or acetone from easy-tosynthesise 1,3-dioxolan-4-ones to produce an array of structurally divergent polyesters. The polymers are prepared with high retention of stereochemistry, meaning isotactic polymers are easily prepared from natural enantiopure feedstocks. Reaction kinetics, structure/property relationships, copolymers of traditional cyclic esters, and direct recycling of waste paraformaldehyde showcase the scope of this new reaction in polymer chemistry

Salalens and Salans derived from 3‐Aminopyrrolidine: Aluminium Complexation and Lactide Polymerisation

In this paper a series of 7 salalen ligands based on an aminopyrrolidine backbone have been prepared and characterised. Several systems have been reduced to the salan ONNO type-ligand. All ligands have been complexed to Al III with Al(1–7)Me, Al(2a)(OiPr) and Al(7a)Me being characterised by single-crystal X-ray diffraction. In general the Al III centres are best described as being in a trigonal bipyramidal geometry. The solution and solid-state structures are discussed. All complexes have all been trialled for the production of PLA from rac-lactide, the salalen complexes had a preference for heterotactic PLA (P r = 0.71), whereas the salan had a more isotactic bias (P m = 0.72). In all cases PLA with low dispersities and predictable molecular weights were prepared. The activity of the two classes of ligands is compared with the salan complexes appearing to be significantly more active than the salalen systems. </p

Aluminum salen and salan complexes in the ring-opening polymerization of cyclic esters: Controlled immortal and copolymerization of <em>rac</em>-β-butyrolactone and <em>rac</em>-lactide

Aluminum-based salen and salan complexes mediate the ring-opening polymerization (ROP) of rac-β-butyrolactone (β-BL), rac-lactide, and ε-caprolactone. Al-salen and Al-salan complexes exhibit excellent control over the ROP of rac-β-butyrolactone, yielding atactic poly(3-hydroxybutyrate) (PHB) with narrow PDIs of <1.15 for Al-salen and <1.05 for Al-salan. Kinetic studies reveal pseudo-first-order polymerization kinetics and a linear relationship between molecular weight and percent conversion. These complexes also mediate the immortal ROP of rac-β-BL and rac-lactide, through the addition of excess benzyl alcohol of up to 50 mol eq., with excellent control observed. A novel methyl/adamantyl-substituted Al-salen system further improves control over the ROP of rac-lactide and rac-β-BL, yielding atactic PHB and highly isotactic poly(lactic acid) (Pm = 0.88). Control over the copolymerization of rac-lactide and rac-β-BL was also achieved, yielding poly(lactic acid)-co-poly(3-hydroxybutyrate) with narrow PDIs of <1.10. 1H NMR spectra of the copolymers indicate a strong bias for the insertion of rac-lactide over rac-β-BL

Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency

Aluminium-alkyl complexes are well known as initiators for lactide ring-opening polymerisation, yet aluminium-chloride complexes remain underexplored despite benefits such as ease of synthesis and improved air-stability. While aluminium-chloride complexes are typically poor initiators, recent studies have shown that electron rich amino-substituted (salen)AlCl complexes can efficiently initiate lactide polymerisation in the presence of an epoxide. Herein, we report eight ether-substituted complexes as efficient initiators for lactide polymerisation, where exchanging strongly electron-donating amino groups for weaker electron-donating methoxy substituents maintains efficient initiation and also improves the propagation rate by a factor of four. Investigation of ortho-, meta-, para- and meta’-methoxy-substituted regioisomers established that the ortho-substituted complex was twice as active as the other regioisomers. Kinetic and spectroscopic studies suggest that the initiation efficiency is influenced by the electronics (ortho and para &gt; meta and meta’), with substituents closer to Al giving improved initiation (ortho &gt; para and meta’ &gt; meta). While electron-donating ortho-substituents often decrease catalyst activity in lactone polymerisation, here we show that ether groups can act as σ-electron-withdrawing groups and π-electron donors, to deliver improved propagation rates, initiation and tacticity control

Iron (III) Salalen Complexes for the Polymerisation of Lactide

Herein, we report the preparation and characterisation of iron (III) salalen complexes, with variation of ligand substituents and backbone investigated. Six new complexes were prepared and characterised by elemental analysis, mass spectrometry and X-ray crystallography. These complexes have been applied for the ring opening polymerisation (ROP) of rac-lactide in propylene oxide. Fe(1)Cl was found to have a moderate isotactic preference (Pm = 0.75 – 0.80) and demonstrated good molecular weight control in solution (Đ = 1.02 – 1.18). Fe(2-7)Cl were also active for ROP and activities could be related to ligand structure

Accelerated syntheses of amine-bis(phenol) ligands in polyethylene glycol or “on water” under microwave irradiation

Pure amine-bis(phenol) ligands are readily accessible in high yield, often >90%, when the Mannich condensation reactions are performed “on water” or in poly(ethyleneglycol) (PEG). Microwave-assisted synthesis dramatically reduces the time and energy required to prepare these molecules, typically from 24 h to 5 min. The approach seems to be widely applicable (7 amines and 5 phenols were tested to yield a diverse set of bis(phenol) ligands). Significant improvements in yield were observed for ligands derived from di-tert-amyl and di-tert-butyl phenols, possibly resulting from a hydrophobic effect. Single crystal X-ray diffraction data for the ligand derived from p-cresol and N,N′-dimethylethylenediamine is reported