Coordination-insertion Mechanism Research Articles

Ring-opening polymerization of L-lactide with rare earth aryloxides substituted by various alkyl groups

Rare earth aryloxides substituted by various alkyl groups [Ln(OAr)3] such as methyl, isopropyl and tertbutyl, were used as single component catalysts to affect ring-opening polymerization of L-lactide (LLA). The catalytic activity, polymerization characteristics, polymerization kinetics and the mechanism were studied. It was found that the catalytic activity of rare earth aryloxides is influenced by both the structure and the number of alkyl groups on the phenyl ring. The stronger the electron-donation ability of the alkyl group, the higher the catalytic activity will be. An increase in the number of the substitute group will result in a higher catalytic activity. Lanthanum tris(2,4,6-tri-tert-butylphenolate) [La(OTTBP)3] exhibits the highest activity among all lanthanum aryloxides. According to the 1H-NMR data, it was proposed that the LLA polymerization proceeded via a coordination-insertion mechanism involving cleavage of acyl-oxygen bond of the lactide.

Cationic organozinc complexes of a bis(phosphinimine) pincer ligand: synthesis, structural and polymerization studies

Cationic organozinc complexes of a neutral bis(phosphinimine) pincer ligand (L) have been prepared and structurally characterized. This recently introduced ligand was constructed from a dibenzofuran (dbf) framework with symmetric attachment of phosphinimine groups at the 4 and 6 positions. Starting from protonated derivatives [LH][B(C(6)F(5))(4)] (1a), [LH][BPh(4)] (1b), or [LH(2)][BPh(4)](2) (1c), the complexes [LZnCH(3)][B(C(6)F(5))(4)] (2a), [LZnCH(3)][BPh(4)] (2b), and [LZnOAc][BPh(4)] (3), were prepared via protonolysis of an appropriate alkylzinc precursor. The complex [LZnPh][BPh(4)] (4) is generated as a side-product in the synthesis of 2b. Solid-state structural studies have revealed the compounds to be charge separated cationic zinc species with 3-coordinate trigonal planar geometry. Preliminary studies have shown these complexes to be inactive for the polymerization of lactide. Upon modification of the initiating group to a methyl-(S)-lactate, however, complex [LZnOCH(Me)CO(2)Me][B(C(6)F(5))(4)] (5) demonstrated significant polymerization activity at 60 degrees C. Additionally, NMR and mass spectrometry data confirmed a coordination-insertion mechanism was operative for this catalyst.

Ring‐opening polymerization of ε‐caprolactone by a new yttrium complex: Y[2,2′‐ethylidene‐bis(4,6‐di‐tert‐butylphenoxy)]2(ethylene glycol dimethyl ether)Na(ethylene glycol dimethyl ether)3

AbstractThe main aims of the work reported here were to synthesize and characterize a new 2,2′‐ethylidene‐bis(4,6‐di‐tert‐butylphenol) (EDBPH2)‐based bimetal yttrium complex, Y(EDBP)2(DME)Na(DME)3 (1c; where DME is ethylene glycol dimethyl ether), which was employed as an efficient initiator for the ring‐opening polymerization of ε‐caprolactone (ε‐CL). From single‐crystal X‐ray diffraction, the solid structure of this new bimetal initiator was well established. Experimental results show that 1c initiates the ring‐opening polymerization of ε‐CL to afford poly(ε‐CL) with a narrow molecular weight distribution (Mw/Mn = 1.09–1.36, 65 °C). Based on an in situ NMR study, a plausible coordination–insertion mechanism is then proposed. The bimetal complex 1c can be used as an initiator for the ring‐opening polymerization of ε‐CL with some living characteristics. A study of the mechanism reveals that DME displacement in 1c by ε‐CL is involved in the initiation process and the propagation may proceed through three pathways by NaO insertion or YO insertion. Copyright © 2009 Society of Chemical Industry

Ring opening polymerization of ε‐caprolactone initiated by decamolybdate anion: Determination of kinetic and thermodynamic parameters by DSC and 1H‐NMR

AbstractThe aim of this work is the kinetic and thermodynamic study (by differential scanning calorimetry (DSC) and proton nuclear magnetic resonance (1H‐NMR)) of the polymerization of ε‐caprolactone initiated by ammonium decamolybdate. By means of isothermal kinetics, enthalpies of reaction in the range 150–160°C, as well as constant rates of polymerization (using an nth‐order kinetics function model), were determined. From an Arrhenius plot, activation energy (Ea = 85.3 kJ/mol) and preexponential factor (A = 1.78 × 108 min−1) were estimated. Using dynamic methods, crystallization and melting temperatures for the polymer obtained in situ were derived. Kinetic data for polymerization (obtained by 1H‐NMR) were fitted to 13 different model reaction functions. It was found that power law equations represent better the conversion versus time plots for this system. On the basis of experimental facts, a coordination‐insertion mechanism involving molybdenum(V) species is proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Highly active polymerization of methyl methacrylate catalyzed by Pd-based β-ketoiminato complexes/MAO systems

The substituted β-ketoiminato palladium(II) complexes, Pd[CH 3C(O)CHC(NAr)CH 3](Pph 3)(Me) ( 1 Ar = α-napthyl, 2 Ar = fluorenyl) , when was activated by methylaluminoxane (MAO), were used as highly active catalysts for methyl methacrylate (MMA) polymerization. The effects of temperature, co-catalyst to catalyst molar ratio and polymerization time on catalyst activities were reported. Structural analyses of the polymers by 1H NMR spectra indicated that polymerization yielded poly(methyl methacrylate) (PMMA) with moderate syndiotactic microstructures. The polymerization results showed that PMMAs obtained using these complexes possibly arose from a coordination-insertion mechanism rather than a radical mechanism.

High Molar Mass Poly(trimethylene carbonate) by Ph2BiOEt and Ph2BiBr-Initiated Ring-Opening Polymerizations

Trimethylene carbonate, (TMC), was polymerized in bulk with Ph2BiOEt or Ph2BiBr as initiators. The dependence of the molecular weight on the monomer-initiator ratio (M/I) and the MALDI-TOF (MT) mass spectra suggest that these polymerizations obey a coordination-insertion mechanism. Due to “back-biting” cyclic oligomers were also formed. With Ph2BiBr particularly high molecular weights were achieved (number averages, Mns, up to 340 × 103 Da), but the Mns were significantly higher than the M/Is. However, a satisfactory control of Mn was achieved by addition of tetraethylene glycol as coinitiator. In the case of Ph2BiOEt the Mns fell below the M/Is and only a crude control of the Mns was achieved. Both Bi-compounds were more reactive than Sn(II)2-ethylhexanoate (SnOct2) activated with ethanol. This difference was particularly conspicuous at temperatures below 100°C.

Heterogeneous ring‐opening polymerization of lactones for biomedical applications

AbstractCatalysis of the ring‐opening reaction of L‐lactide and ε‐caprolactone to form the corresponding biodegradable polyesters is frequently performed using organometallic compounds such as tin(II) 2‐ethylhexanoate, which operates by a living, coordination insertion mechanism. For biomedical applications, non‐toxic calcium salts have been researched but these rarely approach the same efficiency. The use of heterogeneous immobilized catalysts offers an alternative approach, but competing reactions such as transesterification affect the final molecular weight and polydispersity. The formation of hybrid materials by using silica as the initiation site offers the prospect of biomaterials with novel properties. Copyright © 2009 Society of Chemical Industry

Synthesis of poly(ɛ-caprolactone)-block-poly(n-butyl acrylate) by combining ring-opening polymerization and atom transfer radical polymerization with Ti[OCH2CCl3]4 as difunctional initiator: I. Kinetic study of Ti[OCH2CCl3]4 initiated ring-opening polymerization of ɛ-caprolactone

A new titanium alkoxide, Ti[OCH2CCl3]4, designed to combine the ring-opening polymerization (ROP) of ɛ-caprolactone and atom transfer radical polymerization (ATRP) of n-butyl acrylate, was synthesized through the ester-exchange reaction of titanium n-propoxide and 2,2,2-trichloroethanol. The mechanism and kinetics of Ti[OCH2CCl3]4 initiated bulk polymerization of ɛ-caprolactone were studied. The results demonstrate that the polymerization proceeds through the coordination–insertion mechanism and all the four alkoxide groups in Ti[OCH2CCl3]4 share a similar activity in the initiation. The determined polymerization activation energy is 70kJ/mol. The polymerization process can be well predicted by the obtained kinetic parameters. Furthermore, PCL synthesized with Ti[OCH2CCl3]4 can be used as the macroinitiator in ATRP of n-butyl acrylate to synthesize poly(ɛ-caprolactone)-block-poly(n-butyl acrylate) (PCL-b-PBA) copolymer.

LnNa8[OC(CH3)3]10(OH) alkoxide clusters: Highly active single‐component catalysts for the homopolymerization and copolymerization of ε‐caprolactone and trimethylene carbonate

AbstractHeterometal alkoxide clusters of lanthanide and sodium, LnNa8[OC(CH3)3]10(OH) [where Ln = Nd (1) or Yb (2)], which were synthesized by the metathesis reaction of LnCl3 with NaOC(CH3)3 and NaOH in a 1 : 10 : 1 molar ratio in high yields and fully characterized including X‐ray analysis for 2, were found to be highly active catalysts in the ring‐opening polymerization of ε‐caprolactone and trimethylene carbonate and their copolymerization. All of the polymers obtained showed a unimodal molecular weight distribution, indicating that 1 and 2 could really be used as single‐component catalysts. The dependence of catalytic activity on the lanthanide metals was observed: Yb < Nd. A coordination–insertion mechanism for the ring‐opening polymerization is proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Monomer insertion mechanism of ring-opening polymerization of ɛ-caprolactone with yttrium alkoxide intermediate: A DFT study

The mechanism of ɛ-caprolactone (CL) insertion into a Y–OCH 3 bond was investigated using density functional theory (DFT) calculations. The optimized geometries and corresponding Gibbs-free energies of the intermediates were obtained, which confirmed a four-step coordination-insertion mechanism. The coordination of CL onto yttrium center led to a nucleophilic addition of the carbonyl group of CL, followed by an intramolecular alkoxide ligand exchange. A monomer insertion was completed by the CL ring opening via acyl–oxygen bond cleavage. The formation of the five-coordinated yttrium intermediate, 3, was found to be the rate-determining step. This study could be applicable to ring-opening polymerisation (ROP) of CL initiated by lanthanide metal complexes.

High Molar Mass Poly(ε-caprolactone) by Means of Diphenyl Bismuth Ethoxide, a Highly Reactive Single Site Initiator

Diphenylbismuth ethoxide, Ph2BiOEt, initiates the ring-opening polymerization of e-caprolactone via a coordination-insertion mechanism. The matrix assisted laser induced desorption and ionization-time-of-flight (MALDI−TOF) mass spectra proved the formation of one ethyl ester and one CH2OH endgroup. The reactivity of this initiator is high enough to allow even for polymerizations at 20 °C. At 120 °C the reactivity of Ph2BiOEt is lower than that of tin(II) 2-ethylhexanoate (SnOct2) + ethanol, whereas below 90 °C, the reactivity of Ph2BiOEt is higher. Small amounts of cyclic oligoesters are even formed at 20 °C. At 120 °C the number average molecular weights (Mns) parallel the monomer-initiator ratios, and Mns up to 115 000 Da were obtained. The time−conversion curves and the crystal structure indicate association of Ph2BiOEt via O → Bi donor–acceptor interactions.

Synthesis and molecular structure of new heterometal alkoxide clusters Ln 2Na 8(OCH 2CF 3) 14(THF) 6 (Ln = Sm, Y, Yb): Highly active catalysts for polymerization of ε-caprolactone and trimethylene carbonate

The new polynuclear heterometal alkoxide clusters Ln 2Na 8(OCH 2CF 3) 14(THF) 6 (Ln = Sm 1, Y 2, Yb 3) have been synthesized by the reaction of anhydrous LnCl 3 with 7 equiv. of NaOCH 2CF 3 in 68–75% yields. Crystal structural analysis revealed clusters 1– 3 are isomorphous composed of two cubanes and a double open cubane, with one face of an Ln 1Na 2O 4 open cubane capped by an additional Ln 1O 2 layer. Clusters 1– 3 show extremely high activity for the polymerization of ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). The reactivity is much higher than those found for the monometallic alkoxides lanthanide complexes previously reported. The dependence of catalytic activity on lanthanide metals is observed: Yb ≈ Y < Sm for ε-CL and Yb < Y < Sm for TMC. The polymers obtained with these clusters all show a unimodal molecular weight distribution with moderate molecular weight distributions ( M w/ M n = 1.4–1.7), indicating that clusters 1– 3 can really be used as single-component catalysts. The bimetallic cooperation and the coordination–insertion mechanism were proposed.

Kinetics and mechanism of the ring opening polymerization of ( R, S)-β-butyrolactone initiated with dibutylmagnesium

Ring opening polymerization (ROP) of ( R, S)-β-butyrolactone (BL) using dibutylmagnesium (Bu 2Mg) as initiator was investigated both in bulk and in solution. The synthetic poly-3-hydroxybutyrates (P3HB) were characterized by 1H NMR, 13C NMR, FT-IR and GPC. Effects of molar ratio of initiator to monomer, reaction temperature and time on the monomer conversion and the polymer molecular weight and its distribution were discussed. The kinetics of the solution polymerization of BL was examined and showed a first order both in monomer concentration and initiator concentration. The end groups analysis suggested that the monomer inserted into the growing chain proceeding through the coordination-insertion mechanism based on the acyl–oxygen bond scission rather than the alkyl–oxygen bond cleavage of the BL ring. Furthermore, a possible mechanism for the initiation and propagation procedures of P3HB synthesized from BL with Bu 2Mg was proposed.

Unprecedented Polymerization of Trimethylene Carbonate Initiated by a Samarium Borohydride Complex: Mechanistic Insights and Copolymerization with ε‐Caprolactone

Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, [Sm(BH(4))(3)(thf)(3)]. This initiator shows a high activity to give high-molar-mass PTMCs with molar-mass distributions ranging from 1.2 to 1.4, and with a regular structure void of ether linkages. The polymers were characterized by (1)H and (13)C NMR spectroscopy, (1)H-(1)H COSY, (1)H-(13)C HMQC NMR spectroscopy, size-exclusion chromatography (SEC), viscosimetry, and MALDI-TOF MS analyses. A coordination-insertion mechanism was established based on detailed NMR characterizations, especially of the polymer chain end-functions. The monomer initially coordinates the samarium to give [Sm(BH(4))(3)(tmc)(3)], 1. The monomer then opens up through cleavage of the cyclic ester oxygen--acyl bond and inserts into the Sm--HBH(3) bond resulting in an alkoxide complex, [Sm{O(CH(2))(3)OC(O)HBH(3)}(3)], 2, or [Sm{O(CH(2))(3)OC(O)H}(3)], 2', which then propagates the polymerization of TMC to give the active polymer [Sm({O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)HBH(3))(3)], 3 or [Sm(O(CH(2))(3)OC(O){O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)H)(3)], 3'. Finally, acidic hydrolysis of 3 or 3' gives HO(CH(2))(3)OC(O)[O(CH(2))(3)OC(O)](n)O(CH(2))(3)OC(O)H, 4. This novel alpha-hydroxy,omega-formatetelechelic PTMC represents the first example of a formate-terminated polycarbonate. TMC and epsilon-caprolactone (CL) were copolymerized to afford both random PTMC-co-PCL and block PTMC-b-PCL copolymers that were characterized by (1)H NMR spectroscopy, SEC, and differential scanning calorimetry (DSC). The structure of the block copolymers depends on the order of addition of monomers: if CL is introduced first, dihydroxytelechelic HO-PTMC-b-PCL-OH polymers are formed, whereas introduction of TMC first or simultaneous addition of comonomers leads to hydroxyformatetelechelic HC(O)O-PTMC-b-PCL-OH analogues.

Homopolymerization of cyclic esters initiated by lanthanide isopropoxides supported by 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenolate) ligands

AbstractLanthanide isopropoxides supported by carbon‐bridged bisphenolate ligands of 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenoxo) {[(EDBP)Ln(μ‐OPri)(THF)2]2, where Ln is Nd (1), Sm (2), or Yb (3) and THF is tetrahydrofuran} were synthesized by protic exchange reactions in high yields with Cp3Ln compounds as raw materials, and complex 1 was structurally characterized. Complexes 1–3 were shown to be efficient initiators for the ring‐opening polymerization of ε‐caprolactone (ε‐CL) and 2,2‐dimethyltrimethylene carbonate (DTC). Complexes 1–3 could initiate the controlled polymerization of ε‐CL, and the polymerization rate was first‐order with respect to the monomer. The influence of the reaction conditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. End‐group analyses of the oligomers of ε‐CL and DTC showed that the polymerization underwent a coordination–insertion mechanism. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4409–4419, 2006

Ring‐opening polymerization of lactones using RuCl2(PPh3)3 as initiator: Effect of hydroxylic transfer agents

Abstractε‐Caprolactone and δ‐valerolactone were polymerized in bulk at 150°C using the ruthenium(II) complex RuCl2(PPh3)3 as initiator in the presence of 1,3‐propanediol (PD) with a series of alcohols as coinitiators. Polymerization of lactones proceeds via ruthenium(II) alkoxide active centers. 1H‐NMR analysis revealed that the ruthenium complex reacted with the alcohol, generating in situ a ruthenium alkoxide. This species became a more active initiator of ring‐opening polymerization than was RuCl2(PPh3)3. The obtained polylactones were characterized by 1H‐ and 13C‐NMR and matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF). The results showed the formation had occurred of α,ω‐telechelic PCL and PVL diols, in which PD had been incorporated into the polymer backbone. Depending on the nature of the alcohol used as coinitiator, PCLs with different end groups could be synthesized. Insertion of an alcohol as an end group (benzyl alcohol, n‐octanol, or isopropanol) or into the polymeric backbone (propanediol) provided support for the conclusion that a classical coordination–insertion mechanism was operating during lactone polymerization. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

Ring‐opening polymerization of ε‐caprolactone with a divalent samarium bis(phosphido) complex

AbstractThe ring‐opening polymerization of ε‐caprolactone initiated with a divalent samarium bis(phosphido) complex [Sm(PPh2)2] is reported. The polymerization proceeded under mild reaction conditions and resulted in polyesters with number‐average molecular weights of 8.2 × 103 to 12.5 × 103. The yield and molecular weight of poly(ε‐caprolactone)s were dependent on the experimental parameters, such as the monomer/initiator molar ratio, the monomer concentration, the reaction temperature, and the polymerization time. The obtained polymers were characterized with Fourier transform infrared, NMR, gel permeation chromatography, and differential scanning calorimetry. On the basis of an end‐group analysis of low‐molecular‐weight polymers by NMR spectroscopy, a coordination–insertion mechanism is proposed for the polymerization. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1558–1564, 2005

Oligomerization of ɛ-caprolactone and δ-valerolactone using heteropolyacid initiators and vanadium or molybdenum complexes

Lactones are efficiently oligomerized by heteropolyacids H 3[PM 12O 40]·aq with M = Mo or W, “H 3+ n [PMo 12− n V n O 40]·aq” (HPA- n) and molybdenum(VI) acetylacetonate, vanadium(V) sulfate initiators in the presence of dioxygen. The resulting linear oligomers display relatively low polydispersity (≤1.7) and high monomer conversion (≥99%), and are obtained with relatively short reaction times (1–24 h, 20–60 °C). The products were analysed by various physicochemical methods: size-exclusion chromatography (SEC) (to determine the molecular weight properties); IR; 1H and 13C NMR; matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOFMS). Lactone oligomerization is proposed to proceed by a coordination-insertion mechanism via cationic species (VO 2 + and/or MO 2 2+; M = Mo or W) with chain growth involving acyl-oxygen bond cleavage and an activated monomer. Brönsted acids catalyse the reaction. These reactions are the first examples of the ring-opening oligomerization of lactones catalysed by heteropolyacids and the inexpensive “VOSO 4· xH 2O/THF–H 2O/O 2” system.

Living Polymerization of Lactide Using Titanium Alkoxide Catalysts

AbstractSeveral titanium isopropoxides have facilitated the ring opening polymerization of l‐lactide (LA) and rac‐lactide in toluene solution at various polymerization temperatures via a coordination insertion mechanism. Depending on catalysts, the controlled/living poly(l‐lactide), or the heterotactic‐biased poly(rac‐lactide) were obtained. The stereochemical microstructure of polylactide (PLA) was determined from homonuclear decoupled 1H NMR spectral studies. Such spectra of PLA derived from rac‐LA featured the characteristic five‐methine resonance pattern, whereas corresponding spectra derived from l‐LA exhibited only one methine peak.

Mechanistic study on Sn(Oct)2-catalyzed, ring-opening polymerization of p-dioxanone by surface-initiated polymerization and x-ray photoelectron spectroscopy

Two mechanisms on Sn(Oct)2-catalyzed, ring-opening polymerization of p-dioxanone (PDX) were mainly proposed: activated monomer mechanism and coordination-insertion mechanism. The activated monomer mechanism assumes that alcohol complexed with Sn(Oct)2 reacts with monomer and forms a ternary complex. The initiation and chain growth then proceeds within the complex with liberation of the intact Sn(Oct)2. In contrast, the coordination-insertion mechanism hypothesizes that an actual initiator is the -Sn(OR) species and the polymerization proceeds on the -Sn-O-polymer. In this paper, to investigate the presence/absence of the Sn species in the PPDX chain, we used X-ray photoelectron spectroscopy (XPS) of poly(p-dioxanone) (PPDX) grown from a gold surface using the “grafting-from” approach, and the XPS data supports the coordination-insertion mechanism. After the polymerization of PPDX from the surface, the peaks from Sn were observed at 153, 486, and 495 eV in XPS spectrum, and the peaks disappeared after quenching with 1N HCl.