Kinetic Analysis of the Living Ring‐Opening Polymerisation of L ‐Lactide with Tin(II) Initiators

Heteroallene Insertions into Tin(II) Alkoxide Bonds

A series of iso-carbamate complexes have been synthesized by the reaction of [SnII(OiPr)2] or [SnII(OtBu)2] with either aryl or alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), isopropyl (iPr), cyclohexyl (Cy) and tert-butyl (tBu)). In the case of aryl isocyanates, mono-insertion occurs to form structurally characterized complexes [Sn{κ2-N,O-R-NC(OiPr)O}(μ-OiPr)]2 (1: R = Mes, 2: R = Dipp) and [Sn{κ2-N,O-R-NC(OtBu)O}(μ-OtBu)]2 (3: R = Mes, 4: R = Dipp). The complicated solution-state chemistry of these species has been explored using 1H DOSY experiments. In contrast, reactions of tin(II) alkoxides with alkyl isocyanates result in the formation of bis-insertion products [Sn{κ2-N,O-R-NC(OiPr)O}2] (5: R = iPr, and 6: R = Cy) and [Sn{κ2-N,O-R-NC(OtBu)O}2] (7: R = iPr, 8: R = Cy), of which complexes 6-8 have also been structurally characterized. 1H NMR studies show that the reaction of tBu-NCO with either [Sn(OiPr)2] or [Sn(OtBu)2] results in a reversible mono-insertion. Variable-temperature 2D 1H-1H exchange spectroscopy (VT-2D-EXSY) was used to determine the rate of exchange between free tBu-NCO and the coordinated tBu-iso-carbamate ligand for the {OiPr} alkoxide complex, as well as the activation energy (Ea = 92.2 ± 0.8 kJ mol-1), enthalpy (ΔH‡ = 89.4 ± 0.8 kJ mol-1), and entropy (ΔS‡ = 12.6 ± 2.9 J mol-1 K-1) for the process [Sn(OiPr)2] + tBu-NCO ↔ [Sn{κ2-N,O-tBu-NC(OiPr)O}(OiPr)]. Attempts to form Sn(II) alkyl carbonates by the insertion of CO2 into either [Sn(OiPr)2] or [Sn(OtBu)2] proved unsuccessful. However, 119Sn{1H} NMR spectroscopy of the reaction of excess CO2 with [Sn(OiPr)2] reveals the presence of a new Sn(II) species, i.e., [(iPrO)Sn(O2COiPr)], VT-2D-EXSY (1H) of which confirms the reversible alkyl carbonate formation (Ea = 70.3 ± 13.0 kJ mol-1; ΔH‡ = 68.0 ± 1.3 kJ mol-1 and ΔS‡ = -8.07 ± 2.8 J mol-1 K-1).

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

Understanding catalytic synergy in dinuclear polymerization catalysts for sustainable polymers

Understanding the chemistry underpinning intermetallic synergy and the discovery of generally applicable structure-performances relationships are major challenges in catalysis. Additionally, high-performance catalysts using earth-abundant, non-toxic and inexpensive elements must be prioritised. Here, a series of heterodinuclear catalysts of the form Co(III)M(I/II), where M(I/II) = Na(I), K(I), Ca(II), Sr(II), Ba(II) are evaluated for three different polymerizations, by assessment of rate constants, turn over frequencies, polymer selectivity and control. This allows for comparisons of performances both within and between catalysts containing Group I and II metals for CO2/propene oxide ring-opening copolymerization (ROCOP), propene oxide/phthalic anhydride ROCOP and lactide ring-opening polymerization (ROP). The data reveal new structure-performance correlations that apply across all the different polymerizations: catalysts featuring s-block metals of lower Lewis acidity show higher rates and selectivity. The epoxide/heterocumulene ROCOPs both show exponential activity increases (vs. Lewis acidity, measured by the pKa of [M(OH2)m]n+), whilst the lactide ROP activity and CO2/epoxide selectivity show linear increases. Such clear structure-activity/selectivity correlations are very unusual, yet are fully rationalised by the polymerization mechanisms and the chemistry of the catalytic intermediates. The general applicability across three different polymerizations is significant for future exploitation of catalytic synergy and provides a framework to improve other catalysts.

A Highly Active and Selective Zirconium-Based Catalyst System for the Industrial Production of Poly(lactic acid)

The biodegradable, aliphatic polyester poly(lactic acid), PLA, is a leading bio-based alternative to petrochemical-derived plastic materials across a range of applications. Widely reported in the available literature as a benchmark for PLA production via the bulk ring-opening polymerization of lactides is the use of divalent tin catalysts, and particularly tin(II) bis(2-ethylhexanoate). We present an alternative zirconium-based system that combines an inexpensive Group IV metal with the robustness, high activity, control, and designed compatibility with existing facilities and processes, that are required for industrial use. We have carried out a comprehensive kinetic study and applied a combined experimental and theoretical approach to understanding the mechanism by which the polymerization of lactide proceeds in the presence of this system. In the laboratory-scale (20 g) polymerization of recrystallized racemic d,l-lactide (rac-lactide), we have measured catalyst turnover frequencies up to at least 56,000 h^-1, and confirmed the reported protocols' resistance toward undesirable epimerization, transesterification, and chain scission processes, deleterious to the properties of the polymer product. Further optimization and scale-up under industrial conditions have confirmed the relevance of the catalytic protocol to the commercial production of melt-polymerized PLA. We were able to undertake the efficient preparation of high-molecular-weight PLA on the 500-2000 g scale, via the selective and well-controlled polymerization of commercial polymer-grade l-lactide under challenging, industrially relevant conditions, and at metal concentrations as low as 8-12 ppm Zr by weight ([Zr] = 1.3 × 10^-3 to 1.9 × 10^-3 mol %). Under those conditions, a catalyst turnover number of at least 60,000 was attained, and the activity of the catalyst was comparable to that of tin(II) bis(2-ethylhexanoate).

Syntheses of polylactides by means of tin catalysts

Reaction mechanisms and synthetic methods used for the preparation of homo- and copolylactides based on tin(ii) and tin(iv) catalysts are reviewed.

Aerosol-assisted CVD of SnO from stannous alkoxide precursors

The stannous alkoxides [Sn(OR)₂] [R = i-Pr, t-Bu, C(Et)Me₂, CHPh₂, CPh₃] have been synthesised by reaction of Sn(NR'₂)₂ with two equivalents of HOR [R' = Me, R = i-Pr; R' = SiMe₃, R = t-Bu, C(Et)Me₂, CHPh₂, CPh₃]. Single crystal X-ray diffraction analysis of the bis(diphenylmethoxide) (4) and bis(triphenylmethoxide) (5) species have shown them to comprise three-coordinate Sn(ii) centres through dimerisation in the solid state with the alkoxide units adopting transoid and cisoid configurations across the {Sn₂O₂} cores respectively. Thermogravimetric analysis indicates clean decomposition and some evidence of volatility at temperatures >200 °C for all three aliphatic alkoxides, whereas both the diphenyl- and triphenylmethoxide compounds provide higher decomposition temperatures and, for the triphenylmethoxide derivative, a residual mass consistent with the formation of a carbon-containing residue. The previously reported iso-propoxide (1) and tert-butoxide (2) derivatives have been utilised in toluene solution to deposit SnO thin films by aerosol-assisted chemical vapour deposition (AACVD) on glass at temperatures between 300 and 450 °C. While SnO is deposited under hot wall conditions as the only identifiable phase by p-XRD and Raman spectroscopy for both precursors, morphological analysis by SEM reveals inferior substrate coverage in comparison to previously reported ureide-based precursor systems.

Macrocyclic Dizinc(II) Alkyl and Alkoxide Complexes: Reversible CO2 Uptake and Polymerization Catalysis Testing

The synthesis of three new dizinc(II) complexes bearing a macrocyclic [2 + 2] Schiff base ligand is reported. The bis(anilido)tetraimine macrocycle reacts with diethylzinc to form a bis(ethyl)dizinc(II) complex, [L(Et)Zn2Et2] (1). The reaction of complex 1 with isopropyl alcohol is reported, forming a bis(isopropyl alkoxide)dizinc complex, [L(Et)Zn2((i)PrO)2] (2). Furthermore, complex 1, with 2 equiv of alcohol, is applied as an initiator for racemic lactide ring-opening polymerization. It shows moderately high activity, resulting in a pseudo-first-order rate coefficient of 9.8 × 10(-3) min(-1), with [LA] = 1 M and [initiator] = 5 mM at 25 °C and in a tetrahydrofuran solvent. Polymerization occurs with good control, as evidenced by the linear fit to a plot of molecular weight versus conversion, the narrow dispersities, and the limited transesterification. The same initiating system is inactive for the ring-opening copolymerization of carbon dioxide (CO2) and cyclohexene oxide at 80 °C and 1 bar of CO2 pressure. However, stoichiometric reactions between complex 2 and CO2, at 1 bar pressure, result in the reversible formation of new dizinc carbonate species, [L(Et)Zn2((i)PrO)((i)PrOCO2)] (3a) and [L(Et)Zn2((i)PrOCO2)2] (3b), and the reaction was studied using density functional theory calculations. All of the new complexes, 1-3b, are fully characterized, including NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.

On the coordination chemistry of organochalcogenolates RNMe2^E− and RNMe2^E^O− (E = S, Se) onto lead(ii) and lighter divalent tetrel elements

The coordination chemistry of organochalcogenolato ligands containing hard (N, O) and soft (S, Se) atoms onto divalent Ge, Sn and Pb is explored.