Ring-Opening Polymerization of ε-Caprolactone with Anhydrous Iron (III) Chloride as Catalyst

Reactions of the tetraphosphine ligand N,N,N',N'-tetra(diphenylphosphanylmethyl)ethylene diamine) (dppeda) or 1,4-N,N,N',N'-tetra(diphenylphosphanylmethyl)benzene diamine (dpppda) with NiCl2, ancillary ligand (C2H5)2NCS2Na (Na(dtc)) and NH4PF6 afforded two binuclear complexes [(dtc)Ni(dppeda)Ni(dtc)](PF6)2 (1) and [(dtc)Ni(dpppda)Ni(dtc)](PF6)2 (2). Compounds 1 and 2 were characterized by elemental analyses, IR spectra, (1)H, (13)C{(1)H} NMR and (31)P{(1)H} NMR, electrospray ionization (ESI) mass spectra, thermogravimetric (TGA) experiments and single crystal X-ray diffraction. The Ni(ii) atoms in both compounds form a square planar coordination geometry while the ligands hold a similar end-to-end coordination mode. Both products can catalyze the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) while 1 exhibited a higher catalytic activity than 2. Influencing aspects such as solvent, temperature, the ratio of ε-CL to catalyst, the presence of benzyl alcohol, concentration of ε-CL and reaction intervals were investigated.

Synthesis of mono- and tetra-nuclear β-pyridylenolate zinc complexes and their catalysis toward the ring-opening copolymerization of ε-caprolactone and lactide

The Mn2+ and Mo6+ based metal-mercaptoacetate (metal-MA) hybrids were prepared by a simple acid–base titration method. The prepared metal-MA hybrids were analysed by FT-IR spectroscopy, UV–visible spectroscopy, SEM, particle size analyser (PSA), XRD and EDX. The prepared metal-MA hybrids were investigated to assess their catalytic activity for ring-opening polymerization (ROP) of ε-caprolactone (CL) in the absence of an initiator at 160 °C in N2 atmosphere by varying the catalyst concentration and temperature. The catalytic activity of the metal-MA hybrid towards the ROP of CL was evidenced by the FTIR-RI method. The % yield, degradation temperature (Td), melting temperature (Tm) and molecular weight (Mw) of poly(ε-caprolactone) (PCL) were also investigated. The structure of PCL was comprehended by analysing its NMR spectrum. The activation energy (Ea) for the ROP of CL was determined as 10,363 kcal/mol. A linear increase in Mw of PCL with the increasing [M0/I0] ratio was also noticed.Graphical abstract

Ring-opening polymerization of ε-caprolactone by Zr(IV) tris(β-diketonates): Electronic character of complexes in initiation and propagation steps

Binuclear aluminum alkyl complexes 2a–4g supported by linked bis(β-diketiminate) ligands were synthesized via the reaction of AlEt3 or AlMe3 and the corresponding proligand in a 2:1 molar ratio with moderate yields. The isolated complexes were well-characterized by 1H-NMR, 13C-NMR and elemental analysis. The binuclear nature of aluminum complex 2b was further confirmed by an X-ray diffraction study. All complexes 2a–4g could efficiently initiate the ring-opening polymerization (ROP) of e-caprolactone in toluene. The substituents at the aromatic rings and the linker unit in the auxiliary ligands exerted significant influence on the catalytic behavior of the investigated aluminum complexes. Complex 4g (R1 = R2 = Cl) containing propylenyl bridging unit exhibited the highest catalytic activity among these complexes, which might be attributed to the increased electrophilicity of the metal center as well as more opened coordination sphere. The molecular weights of obtained poly(e-caprolactone)s deviating considerably from the theoretical values indicated that the ROP of e-caprolactone by complexes 2a–4g was not well-controlled, which was also judged from the broad molecular weight distributions (MWD = 1.47–2.47) of produced poly(e-caprolactone)s. These complexes proved to be inactive toward the polymerization of rac-lactide alone. In the presence of alcohol the polymerization occurred, which was actually initiated by the decomposition species of the aluminum complex upon the treatment with isopropanol.

Five aluminum-based amine-bis(phenolate) complexes, three of them novel, with variation of the pendant donor arm were synthesized in excellent yields, and characterized by NMR spectroscopy and X-ray crystallogra-phy. The quantitative conversion of the aluminum alkyl species to the corresponding benzyl alkoxide was achieved by the addition of 1 mol eq. of benzyl alcohol, and was confirmed by 1H NMR spectroscopy. The aluminum alkox-ides were excellent mediators for the ring-opening polymerization (ROP) of rac-lactide, yielding atactic poly(lactic acid), having excellent correlation between theoretical and calculated molecular weights accompanied by narrow molecular weight distributions. ROP of ε-caprolactone by the aluminum alkoxides showed modest con-trol at 50°C in toluene, but much greater control was achieved when polymerizations were conducted at 25°C, with nar-rower molecular weight distributions observed in some cases. A relationship between the complex pendant donor arm and the resulting activity in the ROP of both rac-lactide and ε-caprolactone is discussed. Supplementary information is available at https://www.icevirtuallibrary.com/doi/suppl/10.1680/gmat.12.00006

The catalytic activities of metal complexes of 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole 1 for epoxide–CO2 coupling and ring opening polymerization (ROP) of e-caprolactone (CL) were explored. The Zn(II) 2, Cd(II) 3,4 and Cu(II) 5 complexes of 1, in the presence of tetrabutylammonium bromide (TBAB) as co-catalyst, effectively catalyzed the epoxide–CO2 coupling reaction at 1 atm of CO2 to produce cyclic carbonates. In addition, the catalytic activities of these complexes (2–5) were investigated for the ROP of CL in the presence and absence of benzyl alcohol (BnOH). All these complexes were active for ROP of CL at mild temperature. In particular, the complexes 2 and 5 exhibited remarkable catalytic activities for ROP of CL at 25 °C in toluene in the presence of BnOH. The ROP of CL catalyzed by 2 and 5 complexes, not only proceeded through living polymerization but also immortal.

Zirconium and hafnium amide, alkoxide, and pyrrolyl complexes manifesting with pyrrolyl-linked N-donor ligands: Syntheses, characterization, and ring opening polymerization of ε-caprolactone

A series of N-(2-(diphenylphosphino)ethyl)-2-alkyl-5,6,7,8-tetrahydroquinolin-8-amines was prepared and used in individually reacting with iron chloride under nitrogen atmosphere to form their iron(ii) complexes Fe1–Fe6. All compounds were characterized using FT-IR spectroscopy and elemental analyses, the organic compounds were confirmed with NMR measurements, and the iron complexes were submitted to single-crystal X-ray diffraction, revealing Fe1, Fe2, Fe4, Fe5, and Fe6 as either mono- or di-nuclear forms. Forming a binary system in situ with two equivalents of LiCH2SiMe3, all iron complexes Fe1–Fe6 efficiently initiated the ring opening polymerization of ε-caprolactone, achieving the TOF up to 8.8 × 103 h−1. More importantly, the resultant polycaprolactone (PCL) possessed high molecular weights with the Mn range of 9.21–24.3 × 104 g mol−1, being a rare case of the iron(ii) catalyst in producing PCL with such high molecular weight. The 1H NMR and MALDI-TOF investigations demonstrated that the PCLs were linear features capped with a methoxy group or CH2SiMe3 or cyclic structure that varied with the molar ratio of [ε-CL]/Fe.

The two-dimensional coordination polymers [Co(DMF)2L2]∙0.65H2O and [Mn(DMF)2L2]∙0.4H2O (L = N,N′-bis(glycinyl)pyromellitic diimide) have been prepared and structurally characterized by a variety of techniques including FTIR spectroscopy, powder and single crystal X-ray diffraction. Results for their use as catalysts in the ring opening polymerization of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) are reported. High conversions were achievable at 130 °C, resulting in the formation of mostly cyclic polycaprolactone (PCL) and polyvalerolactone (PVL), respectively, with only a small number of linear polymers formed. Kinetic studies revealed the presence of two distinct rate steps, the second of which is assigned to the formation of an active decomposition product.

Heterogeneous catalysts comprised of titanium immobilized on silica imidazolium-based systems have been synthesized for the ring opening polymerization of ε-caprolactone.

Anhydrous LnCl3 reads with 2 equiv. of the sodium salt of the bidentate Schiff base N‐(2, 6‐diisopropylphenyl) salicylaldimine in THF to form the monomeric lanthanide chlorides [2‐OC6H4CH = N(2,6‐i‐Pr2C6H3)]2LnCl(THF) [Ln=Yb (1), Er (2)]. Complex 1 crystallizes in P1 space group with a = 0.9215(2) nm, b = 1.36612(4) nm, c = 1.6899(2) nm, a = 74.83(3)°, β = 77.43(2)°, γ = 81.04(1)°, Z = 2, V = 1.9929(4) nm3, Dc = 1.402 g/cm3. The two complexes exhibited fairly good catalytic activity in the ring‐opening polymerization of ϵ‐caprolactone.

ABSTRACTPolycaprolactone (PCL) has been widely applied based on the biocompatibility and biodegradability. It is important to investigate the safe and efficient catalyst system for ring-opening polymerization of monomer ε-caprolactone (ε-CL). In our work, Halloysite/Graphene Oxide (HNTs/GO) composite is successfully prepared by electrostatic self-assembly and used as an effective catalyst for the ring-opening polymerization of ε-CL. The structures, morphologies and thermal properties of the products are subsequently characterized. The results show that the synthesized PCL are semi-crystalline and observed to be lamellar, which depend on the template effects of HNTs/GO composite. The HNTs/GO composite serves as an effective catalyst and a reinforcing filler simultaneously to improve the thermal stability of the prepared PCL.

The compounds LMgBun(THF) and L′MgBun(THF) where L = 1,5,9-trimesityldipyrromethene and L′ = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene have been prepared from reactions between MgBun2 and the protonated ligands LH and L′H, respectively. Single crystal X-ray crystallographic studies reveal that in each compound the Mg2+ ion is in a distorted tetrahedral environment and further that the ligand L is more sterically demanding with respect to access to the Mg–Bun group. Related alkyl complexes (R = Me, Et, Prn, Pri, n-hexyl and CH2CH2Ph) were prepared from reactions involving LLi(THF) or L′Li(THF) and the appropriate RMgX (X = Cl or Br) and characterized by 1H NMR spectroscopy. Those where R = CH2CH2X (X = alkyl or phenyl) have characteristic α-CH2 proton resonances which are the part of an AA′XX′ pattern. Reactions with alcohols ROH (1 equiv.) give the kinetic products LMg(OR)(THF) and L′Mg(OR)(THF) which are isolable and kinetically persistent when R = a bulky group such as But and the structure of LMg(OBut)(THF) is reported and compared with that of the known compound L′Mg(OBut)(THF). With less bulky groups the compounds are labile toward ligand scrambling and the compound L′MgBun(THF) reacts with alcohols (2 equiv.) to give L′H and Mg(OR)2. Reactions with amines and carbon dioxide are described which indicate the greater reactivity of the Mg–Bun group relative to both the L′ and L ligand. With PhCHO, Ph2CO and cyclohexanone both LMgBun(THF) and L′MgBun(THF) react via β-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by β-H transfer to give poly-L-lactide (P-L-LA) and 1-butene while rac-lactide yields atactic polylactide in toluene–dichloromethane solutions but in the presence of ≥10 equiv. of THF, heterotactic PLA is formed. The ring-opening polymerization of e-caprolactone also is initiated by β-H transfer and is about ten times faster than for lactide; kp(LA) = 10.7 M−1 s−1vs. kp(CL) = 110 M−1 s−1. Interestingly, the presence of lactide completely suppresses the polymerization of e-caprolactone.

An unsymmetrical N-heterocyclic carbene, namely 1-isopropyl-3-benzylimidazol-2-ylidene, is a highly active catalyst for ring-opening polymerization of ϵ-caprolactone (CL) to give polycaprolactone (PCL) with number average molecular weight (Mn) as high as 2.66 × 104 at 0°C in 100 min in tetrahydrofuran (THF). The effects of monomer/initiator molar ratio ([M]/[I]), catalyst/initiator molar ratio ([C]/[I]), monomer concentration, as well as polymerization temperature and time have been investigated. The kinetic studies of CL polymerization have indicated that the polymerization rate is first-order with respect to both monomer and catalyst concentrations. The apparent activation energy amounts to 56.04 kJ/mol. The proposed mechanism is a monomer-activated process.