Coordination-insertion Mechanism Research Articles

Unprecedented polymerization of epsilon-Caprolactone initiated by a single-site lanthanide borohydride complex, [Sm(eta-C5Me5)2(BH4)(thf)]: mechanistic insights.

The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, epsilon-CL opening (through cleavage of the cyclic ester oxygen-acyl bond) and insertion into the Sm--HBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2[O(CH2)6O(BH2)]] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)5C(O)[O(CH2)5C(O)]nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.

Ring‐Opening Polymerization of the Cyclic Ester Amide Derived from Adipic Anhydride and 1‐Amino‐6‐hexanol in Melt and in Solution

AbstractThe ring‐opening polymerization (ROP) of the cyclic ester amide (cEA) 5 (systematic name, 1‐oxa‐8‐aza‐cyclotetradecane‐9,14‐dione) ‐ prepared from adipic anhydride and 1‐amino‐6‐hexanol ‐ in the melt at 165 °C and in solution at 100 °C and 120 °C with Bu2Sn(OMe)2 or Ti(OBu)4 as initiator yields the alternating poly(ester amide) (PEA) 4 (systematic name, poly(5‐(6‐oxyhexylcarbamoyl)‐pentanoate) with regular microstructure. Kinetic studies for different monomer‐to‐initiator ratios, different reaction media, initiators and temperatures reveal that the ROP is a first‐order reaction with respect to the monomer. Under suitable polymerization conditions termination and transfer reactions are suppressed. The elementary chain growth reaction proceeds by a coordination insertion mechanism in analogy to the polymerization of lactones. By using monohydroxy‐ and bishydroxy‐functional telechelic poly(ethylene oxide) and Sn(octoate)2 as the initiating system poly(ethylene oxide)‐block‐poly(ester amide)s and poly(ester amide)‐block‐poly(ethylene oxide)‐block‐poly(ester amide)s are obtained. The poly(ester amide) 4 is a semicrystalline material with a melting point of 140 °C, the block copolymers are phase separated systems showing two melting points characteristic for the respective homopolymers.

Kinetic modeling studies of ethylene polymerization reactions using supported chromium catalysts

AbstractKinetic models for ethylene polymerization based on a general coordination–insertion mechanism, in which either a monocoordinated species or a bicoordinated species could lead to migratory insertion, were constructed. These models were implemented through the solution of a set of differential equations resulting from the material balances for all the species involved. The application of these kinetic models to monomer consumption for different supported catalysts produced very good fittings and allowed the estimation of the kinetic rate constants of each elementary step. Although the same kinetic scheme was used to describe all the observations, the results of the fitting showed that the supported chromium species behaved very differently according to the support. Only in the case of the silica‐supported catalysts was mechanical fragmentation of the particles observed during the course of the reaction, and this implied the inclusion of a new term in the model. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3464–3472, 2004

Kinetic and Mechanistic Studies of l-Lactide Polymerization in Supercritical Chlorodifluoromethane

The ring-opening polymerization of L-lactide (L-LA) initiated by 1-dodecanol/tin(II) octoate was carried out in supercritical chlorodifluoromethane at 110 °C under a pressure of 200 bar. MALDI- TOF mass spectroscopy and 1 H NMR studies suggested that polymerization proceeded via a traditional coordination-insertion mechanism involving a tin alkoxide bond and the cleavage of the acyl-oxygen bond of the monomer. The polymerization was controlled using the linear relation between ln((LA)0/ (LA)) on the polymerization time and the molecular weight on conversion. The relative polymerization rate constants increased with increasing tin(II) octoate and 1-dodecanol concentrations. Gel permeation chromatography analysis indicated that the molecular weight distribution was bimodal, at least for certain monomer conversion. The bimodal distribution may be a result of specific intermolecular interactions between chlorodifluoromethane and hydroxyl groups on 1-dodecanol or on dormant macromolecules. The mechanisms of the initiation and chain transfer steps of L-lactide polymerization in supercritical chlorodifluoromethane are proposed.

Ring‐opening polymerization of ε‐caprolactone by lanthanide tris (2, 6‐dimethylphenolate) s

AbstractLanthanide tris (2,6‐dimethylphenolate)s [Ln(ODMP)3] were used as initiators for ring‐opening polymerization of s‐caprolactone (CL) for the first time. The influence of different rare earth elements and solvents was investigated. 1H NMR spectral data of polycaprolactone (PCL) obtained showed that the polymerization mechanism is in agreement with the coordination‐insertion mechanism and the selective cleavage of the acyl‐oxygen bond of CL.

Characteristics, Linetics and Mechanism of ?-Caprolactone Polymerization by Lanthanide Tris(2.6-dimethylphenolate)s

PolycaproIactone(PCL) was synthesized via the ring-opening polymerization of e-caprolactone(e-CL) with lanthanide tris(2.6 -dimethylphenolate)s[Ln(ODMP)3] as initiator under mild conditions for the first time. The effects of reaction conditions on the e-CL polymerization are discussed. Kinetic studies indicate that the polymerization rate is first order with respect to both monomer concentration and catalyst concentration, respectively, and the overall reaction activation energy amounts to 62.3kJ/mol. 1H NMR end group analysis on the polymer demonstrates the ring-opening polymerization of e-CL proceeds according to coordination-insertion mechanism with acyl-oxygen bond cleavage of the monomer.

Biodegradable compositions by reactive processing of aliphatic polyester/polysaccharide blends

AbstractCommercially available biodegradable aliphatic polyesters, i.e., high molecular weight poly(ϵ‐caprolactone) (PCL) and polylactide (PLA), were melt blended with a well‐known natural and biodegradable polysaccharide: starch either as corn starch granules or as thermoplastic corn starch after plasticization with glycerol. Conventional melt blending yielded compositions with poor mechanical performances as a result of lack of interfacial adhesion between the rather hydrophobic polyester matrix and the highly hydrophilic and moisture sensitive starch phase. Interface compatibilization was achieved via two different strategies depending on the nature of the polyester chains. In case of PLA/starch compositions, PLA chains were grafted with maleic anhydride through a free radical reaction conducted by reactive extrusion. The maleic anhydride‐grafted PLA chains (MAG‐PLA) allowed for reinforcing the interfacial adhesion with granular starch as attested by TEM of cryofracture surface. As far as PCL/starch blends were concerned, the compatibilization was achieved via the interfacial localization of amphiphilic graft copolymers formed by grafting of PCL chains onto a polysaccharide backbone such as dextran. The PCL‐grafted polysaccharide copolymers were synthesized by controlled ring‐opening polymerization of ϵ‐caprolactone proceeding via a coordination‐insertion mechanism. These compatibilized PCL/starch compositions displayed much improved mechanical properties as determined by tensile testing as well as a much more rapid biodegradation as measured by composting testing.

Ring-Opening Polymerization of the Cyclic Ester Amide Derived from Adipic Anhydride and 1-Amino-5-pentanol

The ring-opening polymerization of the cyclic ester amide (cEA, 2) (systematic name, 1-oxa-7-aza-cyclotridecane-8,13-dione)prepared from adipic anhydride and 1-amino-5-pentanolin the melt at temperatures above 145 °C with Bu2Sn(OMe)2, Ti(OBu)4, Al(O-sec-Bu)3, or Sn(octoate)2 as initiator yields the poly(ester amide) (PEA, 3) (systematic name, poly(5-(5-oxypentylcarbamoyl)pentanoate) with regular microstructure. This poly(ester amide) is a semicrystalline material with a melting point of 108 °C. The elementary chain growth reaction proceeds by a coordination insertion mechanism in analogy to the polymerization of lactones. The monomer-to-initiator ratio and the conversion determine the number-average molecular weight. By using hydroxy telechelic poly(ethylene oxide) with Sn(octoate)2 as initiator poly(ester amide)-block-poly(ethylene oxide) and poly(ester amide)-block-poly(ethylene oxide)-block-poly(ester amide) were obtained. Kinetic studies for different monomer-to-initiator ratios, different temperatures and initiators reveal that the ring-opening polymerization is a first-order reaction with respect to the monomer and shows no termination and transfer reactions.

Ring-opening polymerization of ε-caprolactone and l-lactide using organic amino calcium catalyst

Poly (ε-caprolactone) (PCL) and poly ( l-lactide) (PLA) were prepared by ring-opening polymerization catalyzed by organic amino calcium catalysts (Ca/PO and Ca/EO) which were prepared by reacting calcium ammoniate Ca(NH 3) 6 with propylene oxide and ethylene oxide, respectively. The catalysts exhibited high activity and the ring-opening polymerization behaved a quasi-living characteristic. Based on the FT-IR spectra and the calcium contents of the catalysts, and based on the 1H NMR end-group analysis of the low molecular weight PCL prepared using catalysts Ca/PO and Ca/EO, it was proposed that the catalysts have the structure of NH 2–Ca–O–CH(CH 3) 2 and NH 2–Ca–O–CH 2CH 3 for Ca/PO and Ca/EO, respectively. The ring-opening polymerization of CL and LA follows a coordination-insertion mechanism and the active site is the Ca–O bond.

Synthesis, characterization, and mechanism studies on novel rare-earth calixarene complexes initiating ring-opening polymerization of 2,2-dimethyltrimethylene carbonate

Rare-earth (Nd, Y) p-tert-butylcalix[n]arene (n = 4, 6, and 8) complexes without coligands were synthesized from rare-earth isopropoxides in toluene. The products were characterized as the following structures: [C4(OH)O3 · CH3C6H5]Nd (4), [C6(OH)2O4 · CH3C6H5]3Ln4 [Ln = Nd (5), Y (6)], and [C8(OH)2O6 · CH3C6H5]Nd2 (7). 2,2-Dimethyl trimethylene carbonate (DTC) can be polymerized with complexes 4–7 alone as the initiator. PolyDTC (weight-average molecular weight: 5700, polydispersity index: 1.11, measured by gel permeation chromatography) initiated by complex 5 was obtained with a conversion of 69.1% within 6 h in toluene at 80 °C. The thermal behavior of polyDTC has been compared with the published data. The DTC ring is opened via acyl-oxygen bond cleavage with end-group examination. NMR analyses of the polymerization reaction mixture indicated that the polymerization proceeds via a coordination-insertion mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1390–1399, 2003

Novel unsaturated ε-caprolactone polymerizable by ringopening and ring-opening metathesis mechanisms

AbstractRing-opening polymerization (ROP) and ring-opening metathesis polymerization (ROMP) of an unsaturated ε-caprolactone, 6,7-dihydro-2(3H)-oxepinone (DHO2), are alternative routes to produce unsaturated aliphatic polyesters with the same molecular structure. Polymerization of DHO2 initiated by Al isopropoxide in toluene at room temperature or at 0°C proceeds by a coordination-insertion mechanism, although intramolecular transesterification takes place beyond complete monomer conversion. The molecular weight distribution is narrow as long as monomer conversion does not exceed 90%. Ring-opening metathesis polymerization of DHO2 initiated by Schrock’s Mo-based catalyst, 1, at 60°C allows higher molecular weight unsaturated polyester to be prepared, even though an intramolecular side reaction also operates. The structure of poly(DHO2) synthesized by ROP and ROMP is the same, as confirmed by 1H, 13C NMR, and FT-IR spectra. Copolymers of DHO2 with norbornene, cis-cyclooctene, and 1,5-cyclooctadiene have been successfully prepared.

Single-site beta-diiminate zinc catalysts for the ring-opening polymerization of beta-butyrolactone and beta-valerolactone to poly(3-hydroxyalkanoates).

Polymerization of beta-butyrolactone (BBL) and beta-valerolactone (BVL) using the zinc alkoxide initiator (BDI-1)ZnO(i)()Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] proceeds very rapidly under mild conditions to produce poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV), respectively. For the monomer-to-initiator ratio 200:1, PHB number-average molecular weights (M(n)) are proportional to conversion throughout the reaction and polydispersity indices (PDIs) are narrow, consistent with a living polymerization. Higher monomer-to-initiator ratios can be used to achieve high molecular weight PHB (M(n) > 100 000). End-group analysis verifies that the polymerization of BBL follows a coordination-insertion mechanism, where complexes of the form (BDI-1)ZnOCH(Me)CH(2)CO(2)R are the active species. Variable temperature NMR experiments show that (BDI-1)ZnO(i)()Pr is monomeric in benzene-d(6) solution. In contrast, (BDI-2)ZnO(i)()Pr [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] is a poor initiator at room temperature because it prefers to form a bis-mu-isopropoxide dimer in solution. According to kinetic studies, propagation by (BDI-1)ZnO(i)()Pr is first order in both monomer as well as (BDI-1)ZnO(i)()Pr concentration. These results lead us to propose a monometallic active species. Several results suggest that elimination side reactions are slowly catalyzed by zinc alkoxides, leading to degradation of the polymer.

Layered silicate/polyester nanohybrids by controlled ring-opening polymerization

In this study, layered silicate/aliphatic polyester nanohybrids were synthesized by ring-opening polymerization of ϵ-caprolactone as promoted by the so-called coordination-insertion mechanism. These nanocomposites were formed in presence of montmorillonite surface-modified by ammonium cations bearing hydroxyl group(s), such as bis(2-hydroxyethyl)methyl (hydrogenated tallow alkyl) ammonium. The lactone polymerization could be initiated by all the hydroxyl functions available at the clay surface, after activation into either tin(II) or Al(III) alkoxide active species. Hybrid nanocomposites were accordingly generated through the covalent grafting of every polyester chain onto the filler surface. Surface-grafted polycaprolactone (PCL) chains were untied and isolated by ionic exchange reaction with LiCl in THF solution and molar masses were measured by size exclusion chromatography. The PCL molar masses could be controlled and readily tuned by the content of hydroxyl groups available at the clay surface. Interestingly, initiation reaction by aluminum trialkoxide active species yielded grafted PCL chains characterized by very narrow molecular weight distribution (Mw/Mn∼1.2). These polyester-grafted layered silicate nanohybrids displayed complete exfoliation of silicate sheets as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM).

Polyester layered silicate nanohybrids by controlled grafting polymerization

Poly(e-caprolactone) (PCL) layered silicate nanohybrids were synthesized by ring opening polymerization of e-caprolactone according to a well-controlled coordination–insertion mechanism. Montmorillonites were surface-modified by non functional (trimethylhexadecylammonium) and hydroxy functional alkylammonium cations, i.e., (2-hydroxyethyl)dimethylhexadecylammonium. The hydroxy functions available at the clay surface were activated into tin(II or IV) or Al(III) alkoxide initiators for lactone polymerization, thus yielding surface-grafted PCL chains. The surface-grafted PCL chains were recovered by an ionic exchange reaction with lithium chloride and they were analyzed by size exclusion chromatography. The PCL molar mass was measured as a function of the hydroxy content of the clay that was modulated by exchanging the Na cations with mixtures of non-functional and hydroxy functional ammonium cations of different compositions. Nanohybrids were also characterized by small-angle X-ray diffraction, transmission electron microscopy and thermogravimetry. The PCL molar mass and the nanocomposite morphology (i.e., exfoliation and/or intercalation) were readily tuned by the content of the hydroxy groups available at the clay surface. Surface-grafted aluminium trialkoxide species proved highly efficient in initiating polymerization that leads to PCL chains of controlled molar mass and narrow molecular weight distribution with polydispersity indices as low as 1.2.

Transition metal acetylide catalysts for polymerization of alkynes: 3. Polymerization mechanism

A coordination insertion mechanism is proposed for the polymerization of alkynes initiated by transition metal acetylides on the basis of some experimental results. A transition metal acetylide coordinates with an alkyne resulting in the formation of a penta-coordinated complex, which is a preliminary activation and rate-determining step. The polymeric chain begins by insertion of the π-coordinated monomer molecule into the metal–carbon σ-bond. For large-bulk monomers such as p-diethynylbenzene and its derivatives, the insertion is almost trans-type to give rich trans polymers. On the other hand, for small-bulk monomers such as phenylacetylene, cis insertion easily occurs to yield cyclotrimers. The propagation of the polymer chain can be interrupted when the π-bonded monomer transfers its acidic hydrogen to the growing chain.

Syntheses and application of polylactides

Polylactides can be prepared by direct polycondensation of lactic acid, or better, by ring-opening polymerization of cyclic dilactides (usually called lactides). These lactides exist in the form of four stereoisomers two of which ( l, l- and rac. - d, l-) are technically produced in large quantities. These lactides can by polymerized via four different classes of initiators and reaction mechanisms. The characteristics mechanistic and preparative features of the cationic polymerization, anionic polymerization and of the coordination–insertion mechanism are described. Furthermore, copolymerizations of lactides with glycolide or ε-caprolactone and their analytical problems are discussed. Finally, a new type of wound dressing based on transparent are resorbable films of copolylactides is mentioned.

Living polymerization ofD,L-3-methylglycolide initiated with bimetallic (Al/Zn) ?-oxo alkoxide and copolymers thereof

D,L-3-Methylglycolide (MG) was successfully polymerized with bimetallic (Al/Zn) μ-oxo alkoxide as an initiator in toluene at 90 °C. The effect of the initiator concentration and monomer conversion on the molecular weight was studied. It is shown that the polymerization of MG follows a living process. A kinetic study indicated that the polymerization approximates the first order in the monomer, and no induction period was observed. 1H NMR spectroscopy showed that the ring-opening polymerization proceeds through a coordination–insertion mechanism with selective cleavage of the acyl–oxygen bond of the monomer. On the basis of 1H NMR and 13C NMR analyses, the selective cleavage of the acyl–oxygen bond of the monomer mainly occurs at the least hindered carbonyl groups (P1 = 0.84, P2 = 0.16). Therefore, the main chain of poly(D,L-lactic acid-co-glycolic acid) (50/50 molar ratio) obtained from the homopolymerization of MG was primarily composed of alternating lactyl and glycolyl units. The diblock copolymers poly(ϵ-caprolactone)-b-poly(D,L-lactic acid-alt-glycolic acid) and poly(L-lactide)-b-poly(D,L-lactic acid-alt-glycolic acid) were successfully synthesized by the sequential living polymerization of related lactones (ϵ-caprolactone or L-lactide). 13C NMR spectra of diblock copolymers clearly show their pure diblock structures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 357–367, 2001

Aliphatic polyester-based biodegradable materials: new amphiphilic graft copolymers

Biodegradable brush-like amphiphilic graft copolymers were synthesized by covalently grafting poly(ε-caprolactone) PCL sequences onto a natural and mainly linear α(1–6) exopolysaccharide backbone, i.e. dextran. A three-step procedure is proposed to control the synthesis which consists in the reversible protection of the hydroxyl groups of the polysaccharide backbone by silylation, followed by the ring-opening polymerization of ε-caprolactone initiated by the remaining free hydroxyl groups of the partially silylated dextran after adequate activation into Al alkoxide active species. The third and final step involves the deprotection of the polysaccharide hydroxyl groups under very mild conditions. The high efficiency of grafting and the control over the graft molecular weight and molecular weight distribution rely upon the well-known “living” character of the coordination-insertion mechanism of the ring-opening polymerization that is initiated by aluminum alkoxides. Poly(ε-caprolactone)-grafted dextran copolymers with precise composition and well controlled number and length of PCL grafts were incorporated into PCL/granular corn starch composites by melt kneading at 130 °C. When located at the filler/matrix interface, the copolymers proved to be very efficient compatibilizers, enhancing the interfacial adhesion, and accordingly the mechanical properties of the composite materials as evidenced by tensile testing. To ensure the migration of the amphiphilic graft copolymer at the starch/PCL interface, it is better to first precipitate it onto the filler surface or to blend it with the starch granules before melt blending with the polyester matrix.

?-Caprolactone/ZnCl2 complex formation: Characterization and ring-opening polymerization mechanism

ε-Caprolactone (ε-CL) has been mixed with ZnCl2 at different mol ratios. The resulting complex was characterized through 1H and 13C NMR spectroscopy in bulk and in solutions, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and optical microscopy. Ring-opening polymerization of ε-caprolactone [M] using ZnCl2 as an initiator [I] at different monomer/initiator ratios has been successfully performed in xylene. The molecular weight of poly(ε-caprolactone) (PCL) as measured by gel permeation chromatografy (GPC) was found to depend linearly on the [M]/[I] ratio. Theoretical calculations were carried out to understand the geometry of the complex and the operating ring-opening mechanism. Both experimental and computational results and the presence of methylene–chloride end group, confirmed by NMR, are in agreement with a coordination–insertion mechanism for the ring-opening polymerization proposed in this article. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1355–1365, 2000

Polylactones 49: Bu 4Sn-initiated polymerizations of ε-caprolactone

1H NMR spectra of mixtures of Bu 4Sn and benzyl alcohol indicate the formation of a weak complex. Furthermore, Bu 4Sn catalyzed polymerizations of ε-caprolactone at 100°C are accelerated by addition of benzyl alcohol, but a coordination–insertion mechanism based on a reaction between two ligands of the Sn-catalyst was not definitely proven. In contrast it was found that Bu 4Sn is sensitive to oxidation by oxygen (air), a process which yields mainly Bu 2SnO and only a trace of Bu 3SnOBu. Hence, the Bu 2SnO dissolved in Bu 4Sn is the main initiator, but added benyzl alcohol acts as coinitiator yielding benzyl ester chain ends.