“Coordination‐insertion” ring‐opening polymerization of 1,4‐dioxan‐2‐one and controlled synthesis of diblock copolymers with ε‐caprolactone

This communication deals with the coordination‐insertion ring‐opening polymerization of 1,4‐dioxan‐2‐one (DX) as initiated by aluminium triisopropoxide (Al(OⁱPr)₃) either in bulk or in solution. First, polymerization of DX has been carried out in bulk at 100°C and compared to the ring‐opening polymerization promoted by tin(II)octoate. Block copolymers of ε‐caprolactone (CL) and DX have been then selectively obtained by first initiating CL polymerization with Al(OⁱPr)₃ in toluene and then adding DX to the living PCL macroinitiator solution at room temperature. In spite of the inherent poor solubility of poly(1,4‐dioxan‐2‐one) in most organic solvents, DX polymerization has proven to proceed through a “living” mechanism. Interestingly enough, the semi‐crystalline P[CL‐b‐DX] block copolymers displayed two well separated melting endotherms at ca. 55 and 102°C for PCL and PDX sequences, respectively.     

Remarkable Lewis acid effects on polymerization of functionalized alkenes by metallocene and lithium ester enolates

Drastic effects of Lewis acids E(C₆F₅)₃ (E = Al, B) on polymerization of functionalized alkenes such as methyl methacrylate (MMA) and N,N-dimethyl acrylamide (DMAA) mediated by metallocene and lithium ester enolates, Cp₂Zr[OC(OⁱPr)CMe₂]₂ (1) and Me₂CC(OⁱPr)OLi, are documented as well as elucidated. In the case of metallocene bis(ester enolate) 1, when combined with 2 equiv. of Al(C₆F₅)₃, it effects highly active ion-pairing polymerization of MMA and DMAA; the living nature of this polymerization system allows for the synthesis of well-defined diblock and triblock copolymers of MMA with longer-chain alkyl methacrylates. In sharp contrast, the 1/2B(C₆F₅)₃ combination exhibits low to negligible polymerization activity due to the formation of ineffective adduct Cp₂Zr[OC(OⁱPr)CMe₂]⁺[OC(OⁱPr)CMe₂B(C₆F₅)₃]⁻ (2). Such a profound Al vs. B Lewis acid effect has also been observed for the lithium ester enolate; while the Me₂CC(OⁱPr)OLi/2Al(C₆F₅)₃ system is highly active for MMA polymerization, the seemingly analogous Me₂CC(OⁱPr)OLi/2B(C₆F₅)₃ system is inactive. Structure analyses of the resulting lithium enolaluminate and enolborate adducts, Li⁺[Me₂CC(OⁱPr)OAl(C₆F₅)₃]⁻ (3) and Li⁺[Me₂CC(OⁱPr)OB(C₆F₅)₃]⁻ (4), coupled with polymerization studies, show that the remarkable differences observed for Al vs. B are due to the inability of the lithium enolborate/borane pair to effect the bimolecular, activated-monomer anionic polymerization as does the lithium enolaluminate/alane pair.     

Ring-opening copolymerization of α-chloromethyl-α-methyl-β-propionolactone with ε-caprolactone

α-Chloromethyl-α-methyl-β-propionolactone (CMMPL) has been copolymerized with ε-caprolactone (CL) using a wide range of feed compositions and aluminium triisopropoxide [Al(OⁱPr)₃] as an initiator. Random copolymers of CMMPL with CL were obtained. The pendant chloromethyl groups of the copolymer were converted to quaternary ammonium salts by reaction with pyridine, resulting in an increased hydrophilicity of the copolymers.     

Kinetics of Anionic Polymerization of ε-Caprolactone (εCL). Propagation of Poly-ε=CL-K+ Ion Pairs

Kinetics of the anionic polymerization of ε-caprolactone (εCL) initiated with (CH₃)₃SiO^?K⁺ and carried out in THF solution has been studied in the temperature range from 0 to 20°C by using a calorimetric method. From the kinetic results and from conductometric measurements of the dissociation constant of the living Poly-εCL^?K⁺ ion pairs (K_D ²⁰ ? (4 ± 2) × 10^?10 mol/L), we concluded that at the conditions indicated above and for concentrations of active centers ranging from 10^?3 to 3.7 × 10^?2 mol/L, propagation proceeds on the ion pairs and is disturbed neither by dissociation nor by the formation of higher aggregates. For the polymerization of εCL proceeding on the poly-εCL^?K⁺     

Anionic polymerization of MMA and renewable methylene butyrolactones by resorbable potassium salts

Three resorbable potassium salts of hydride (K[H]), enolate Me₂C?C(OⁱPr)OK (K[E]), and allyl K[1,3‐(SiMe₃)₂C₃H₃] (K[A]) have been investigated for controlled anionic polymerization of methyl methacrylate (MMA) and its cyclic analogs, naturally renewable methylene butyrolactones including α‐methylene‐γ‐butyrolactone (MBL) and γ‐methyl‐α‐methylene‐γ‐butyrolactone (MMBL). When used alone at ambient temperature in toluene, these salts exhibit no (K[H]) to low (K[A]) to modest (K[E]) polymerization activity. Mixing of K[H] and Al(C₆F₅)₃ leads to the formation of an “ate” complex, K⁺[HAl(C₆F₅)₃]^?, which has been structurally characterized by X‐ray diffraction; this complex has a high polymerization activity producing atactic PMMA, but addition of another equiv of Al(C₆F₅)₃ further enhances both the rate and the efficiency of the polymerization, now producing syndiotactic PMMA with a narrow molecular weight (MW) distribution of 1.04. The K[H]/2Al(C₆F₅)₃ system also exhibits high activity for polymerization of (M)MBL. In sharp contrast, addition of Al(C₆F₅)₃ to K[A] shuts down the polymerization at various temperatures. The most active, controlled, and syndioselective polymerization system in this series is K[E]/2Al(C₆F₅)₃. Accordingly, the polymerization control and kinetics of this most effective system have been examined in more detail. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Latent cationic palladium(II) phosphine carboxylate complexes for norbornene polymerization

The catalytic efficacy of trans‐[(R₃P)₂Pd(O₂CR′)(LB)][B(C₆F₅)₄] ( 1 ) (LB = Lewis base) and [(R₃P)₂Pd(κ²‐O,O‐O₂CR′)][B(C₆F₅)₄] ( 2 ) for mass polymerization of 5‐n‐butyl‐2‐norbornene (Butyl‐NB) was investigated. The nature of PR₃ and LB in 1 and 2 are the most critical components influencing catalytic activity/latency for the mass polymerization of Butyl‐NB. Further, it was shown that 1 is in general more latent than 2 in mass polymerization of Butyl‐NB. 5‐n‐Decyl‐2‐norbornene (Decyl‐NB) was subjected to solution polymerization in toluene at 63(±3) °C in the presence of several of the aforementioned palladium complexes as catalysts and the polymers obtained were characterized by gel permeation chromatography. Cationic trans‐[(R₃P)₂PdMe(MeCN)][B(C₆F₅)₄] [R = Cy ( 3a ), and ⁱPr ( 3b )] and trans‐[(R₃P)₂PdH (MeCN)][B(C₆F₅)₄] [R = Cy ( 4a ), and ⁱPr ( 4b )], possible products from thermolysis of trans‐[(R₃P)₂Pd(O₂CMe)(MeCN)][B(C₆F₅)₄] [R = Cy ( 1a ) and ⁱPr ( 1g )], as well as trans‐[(R₃P)₂Pd(η³‐C₃H₅)][B(C₆F₅)₄] [R = Cy ( 5a ), and ⁱPr ( 5b )], were also examined as catalysts for solution polymerization of Decyl‐NB. A maximum activity of 5360 kg/(molPd h) of 2a was achieved at a Decyl‐NB/Pd: 26,700 ratio which is slightly better than that achieved with 5a [activity: 5030 kg/(molPd h)] but far less compared with 4a [activity: 6110 kg/(molPd h)]. Polydispersity values indicate a single highly homogeneous character of the active catalyst species. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 103–110, 2009     

Polymerization of ε-Caprolactone and its Copolymerization with γ-Butyrolactone using Metal Complexes

Solution polymerization of ε-caprolactone (ε-CL) was performed using four different initiators namely: tin(II) octanoate (Sn(Oct)₂)/ethanolamine, aluminium Schiff's base complex-HAPENAlOⁱPr, lithium diisopropyl amide (LDA) and aluminium isopropoxide. The reaction conditions varied with the initiator used. LDA gave rise to the most rapid polymerization with the highest amount of cyclic species as detected by ¹³C NMR. However, no cyclic species were detected when HAPENAlOⁱPr was used as initiator. The tin(II) octanoate/ethanolamine system lead to an α,ω-dihydroxy-polycaprolactone (PCL). The copolymerization of ε-CL was then performed with the hard to oligomerize γ-butyrolactone using the four initiators. GPC (Gel Permeation Chromatography) analyses showed the formation of copolymers. The highest incorporation of polybutyrolactone (PBL) in the copolymer was obtained using HAPENAlOⁱPr as evidenced by ¹H NMR. ¹³C NMR indicated the presence of pseudoperiodic random copolymers with short blocks of PCL whose block length varied with initiator used. The longest and shortest block length were obtained using Sn(Oct)₂ and HAPENAlOⁱPr respectively as calculated from ¹³C NMR. The reactivity ratios were determined using the Finemann-Ross method at low conversion with HAPENAlOⁱPr as initiator. The values obtained, r_CL = 19.4 and r_BL = 0.11, confirmed the presence of long blocks of CL units in the copolymer.     

Amphiphilic poly(N,N-dimethylamino-2-ethyl methacrylate)-g-poly(ε-caprolactone) graft copolymers: synthesis and characterisation

Amphiphilic poly(N,N-dimethylamino-2-ethyl methacrylate)-g-poly(ε-caprolactone) graft copolymers (PDMAEMA-g-PCL) with various compositions and molecular weights were synthesised via a fully controlled three-step strategy. First, poly(ε-caprolactone) macromonomers (PCLMA) were prepared by ring-opening polymerization (ROP) of ε-caprolactone (CL) initiated by aluminum triisopropoxide (Al(OⁱPr)₃), followed in a second step by quantitative esterification of PCL hydroxy end-groups with a methacrylic acid derivative. Finally, the controlled copolymerization of PCLMA and N,N-dimethylamino-2-ethyl methacrylate (DMAEMA) was carried out by atom transfer radical polymerisation (ATRP) in THF at 60 °C using CuBr ligated with 1,1,4,7,10,10, hexamethyl triethylenetetramine and ethyl 2-bromoisobutyrate as catalyst and initiator, respectively. Furthermore, PDMAEMA-g-PCL graft copolymers were reacted with methyl iodide to convert the pendant tertiary amines into quaternary ammonium iodides increasing accordingly their water solubility. Some preliminary experiments was further carried out by tensiometry and dynamic light scattering in order to shed so light on the tensioactive behaviour of these amphiphilic graft copolymers (with protonated amines or quaternary ammonium cations).     

Comparison study of ε-caprolactone,L-lactide,and ε-decalactone polymerizations using aluminum complexes bearing pyrazole derivatives,and synthesis of polylactide-gradual-poly-ε-caprolactone copolymer

This study compared ε-caprolactone (CL), L-lactide (LA), and ε-decalactone (DL) polymerizations, where aluminum complexes bridged by two pyrazole ligands was used as catalysts. The reactivites of these Al complexes between CL, LA, and DL polymerization were different that L ^Bu ₂ Al ₂ Me ₄ , with the distort boat form, exhibits the greatest catalytic activity for LA and DL polymerization at 60°C but the lowest catalytic activity for CL polymerization at room temperature. This may be because dinuclear L ^Bu ₂ Al ₂ Me ₄ cannot react with BnOH to form aluminum benzyl oxide at room temperature, making it unable to reduce catalytic activity. Because these aluminum complexes had different reactivities for LA and CL polymerizations, the selectivity of polylactide-gradual-poly-ε-caprolactones (PLA(10–80%)-gradual-PCL(59–79%)) (PLA-g-PCLs) was observed.     

Cyclometalated 2‐phenylpyridine complex [RuII(o‐C6H4‐py)(MeCN)4]PF6 as a tunable catalyst for living radical polymerization

The cyclometalated complex [Ru^II(o‐C₆H₄‐py)(MeCN)₄]PF₆ ( 1 ) with a σ‐Ru? C bond and four substitutionally labile acetonitrile ligands mediates radical polymerization of different vinyl monomers, viz. n‐butyl acrylate, methyl methacrylate, and styrene, initiated by three alkyl bromides: ethyl 2‐bromoisobutyrate, methyl 2‐bromopropionate, and 1‐phenylethyl bromide. The polymerization requires the presence of Al(OiPr)₃ and occurs uncontrollably as a conventional radical process. The variation of the molar ratio of the components of the reaction mixture, such as initiator, Al(OiPr)₃ and catalyst, affected the polymerization rates and the molecular weights but did not improve the control. A certain level of control has been achieved by adding 0.5 eq of SnCl₂ as a reducing agent. Tin(II) chloride decreased the rate of polymerization and simultaneously the molecular weights became conversion‐dependent and the polydispersities were also narrowed. Remarkably, the level of control was radically improved in the presence of excess of the poorly soluble catalyst ( 1 ), when the added amount of ( 1 ) was not soluble any more, i.e., under heterogeneous conditions, the system became adjustable and the living polymerization of all three monomers was finally achieved. Possible mechanisms of the ( 1 )‐catalyzed polymerization are discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4193–4204, 2008     

Rapid ring‐opening polymerization of 1,4‐dioxan‐2‐one initiated by titanium alkoxides

Ring‐opening polymerization of 1,4‐dioxan‐2‐one in bulk was initiated by three titanium alkoxides, titanium dichlorodiisopropoxide (TiCl₂(OiPr)₂), titanium chlorotriisopropoxide (TiCl(OiPr)₃), and titanium tetraisopropoxide (Ti(OiPr)₄). The results indicate that the polymerization rate increased with number of OiPr groups in the initiator. High conversion of monomer (90%) and high molecular weight (11.9 × 10⁴ g/mol) of resulting polymer can be achieved in only 5 min at 60 °C with Ti(OiPr)₄ as an initiator. Analysis on nuclear magnetic resonance (NMR) spectra suggests the initiating sites for TiCl₂(OiPr)₂, TiCl(OiPr)₃, and Ti(OiPr)₄ to be 1.9, 2.6, and 3.8, respectively. Coordination‐insertion mechanism for the polymerization via cleavage of the acyl–oxygen bonds of the monomer was proved by NMR investigation. Kinetic studies indicate that polymerization initiated by Ti(OiPr)₄ followed a first‐order kinetics, with an apparent activation energy of 33.7 kJ/mol. It is noteworthy that this value is significantly lower than earlier reported values with other catalysts, namely La(OiPr)₃ (50.5 kJ/mol) and Sn(Oct)₂ (71.8 kJ/mol), which makes it an attractive catalyst for reactive extrusion polymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

DSC Kinetics Analysis for the Synthesis of Three‐Arms Poly(ε‐caprolactone) Using Aluminum Tri‐sec‐Butoxide as Initiator

The kinetics of the ring‐opening polymerization (ROP) of ε‐caprolactone (ε‐CL) initiated by soluble aluminum tri‐sec‐butoxide (Al(OsecBu)₃) has been investigated by the differential scanning calorimetry (DSC). The DSC polymerizations were carried out under nonisothermal and isothermal conditions to obtain three‐arms poly(ε‐caprolactone) (PCL). From nonisothermal DSC, the polymerization rate (dα/dt) increased with increasing heating rates. The values of E_a were determined from Kissinger ( kJ mol^?1), Friedman (31.0 – 63.0 kJ mol^?1), and Starink (64.0 – 71.0 kJ mol^?1) methods. From isothermal DSC, the dα/dt and the apparent rate constant (k_app) increased with increasing polymerization temperatures. The ROP of ε‐CL initiated by Al(OsecBu)₃ occurred via the coordination insertion mechanism. The number average molecular weight () and percent yield of the synthesized PCL was enhanced by increasing polymerization temperature. The synthesized PCL with of 2.4 × 10⁴ was obtained using a molar ratio of monomer to Al‐O active center ([M]/[Al‐O]) of 400 at 150ºC for 24 h. Al(OsecBu)₃ is one of the promising initiator due to its solubility, low transesterification reaction, and high efficiency in ε‐CL polymerization.     

“Living” radical polymerization of styrene catalyzed by cyclometalated ruthenium(II) complexes bearing nonlabile ligands

Cationic substitutionally inert cyclometalated ruthenium (II) and osmium (II) complexes, ([Mt(o‐C₆H₄‐2‐py)(LL)₂]PF₆), where LL‐1,10‐phenanthroline (phen) or 2,2′‐bipyridine (bipy), were used for radical polymerization of styrene. Gradual modification of the complexes within the series allowed comparison of the catalytic activity and the redox properties. There was no correlation between the reducing powers of the complexes and their catalytic activities. The osmium compound of the lowest reduction potential was not active. All the ruthenium complexes catalyzed the polymerization of styrene in a controlled manner; but the level of control and the catalytic activity were different under the same polymerization conditions. [Ru(o‐C₆H₄‐2‐py)(phen)₂]PF₆ demonstrated the best catalytic performance though its redox potential was the highest. It catalyzed the “living” polymerization with a reasonable rate at a catalyst‐to‐initiator ratio of 0.1. 1 equiv. of Al(OiPr)₃ accelerated the polymerization and improved the control, but higher amount of Al(OiPr)₃ did not speed up the polymerization and moved the process into the uncontrollable regime. Under the most optimal conditions, the controlled polymerization occurs fast without any additive and the catalyst degradation. Added free ligands inhibited the polymerization suggesting that the catalytically active ruthenium intermediates are generated via the reversible dechelation of bidentate phen or bipy ligands. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3814–3828, 2009     

Synthesis, characterization, and norbornene polymerization of η-benzylnickel(II) complexes of N-heterocyclic carbenes

Neutral η¹-benzylnickel carbene complexes, [Ni(η¹-CH₂C₆H₅)(IiPr)(PMe₃)(Cl)] (3) (IiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) and [Ni(η¹-CH₂C₆H₅)(SIiPr)(PMe₃)(Cl)] (4) (SIiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene), were prepared by the reaction between [Ni(η³-CH₂C₆H₅)(PMe₃)(Cl)] and an equivalent amount of the corresponding free N-heterocyclic carbene. The preparation of η³-benzylnickel carbene complexes, [Ni(η³-CH₂C₆H₅)(IiPr)(Cl)] (5) and [Ni(η³-CH₂C₆H₅)(SIiPr)(Cl)] (6) were carried out by the abstraction of PMe₃ from 3 and 4 by the treatment of B(C₆F₅)₃. The treatment of AgX on 5 and 6 produced the anion-exchanged complexes, [Ni(η³-CH₂C₆H₅)(NHC)(X)] (7, NHC = IiPr, X = O₂CCF₃; 8, NHC = IiPr, X = O₃SCF₃; 9, NHC = SIiPr, X = O₂CCF₃; 10, NHC = SIiPr, X = O₃SCF₃). The solid state structures of 3 and 10 were determined by X-ray crystallography. The η³-benzyl complexes of IiPr (5, 7, and 8) alone, in the absence of any activators such as borate and MAO, showed good catalytic activity towards the vinyl-type norbornene polymerization. The catalyst was thermally robust and the activity increases as the temperature rises to 130 °C.     

Hypervalent Organoselenium(II) Compounds with Organophosphorus Ligands. Crystal and Molecular Structure of [2‐(iPr2NCH2)C6H4]Se[S2PR′2] (R′ = Ph,OiPr)

Redistribution reactions between diorganodiselenides of type [2‐(R₂NCH₂)C₆H₄]₂Se₂ [R = Et, iPr] and bis(diorganophosphinothioyl disulfanes of type [R′₂P(S)S]₂ (R = Ph, OiPr) resulted in the hypervalent [2‐(R₂NCH₂)C₆H₄]SeSP(S)R′₂ [R = Et, R′ = Ph ( 1 ), OiPr ( 2 ); R = iPr, R′ = Ph ( 3 ), OiPr ( 4 )] species. All new compounds were characterized by solution multinuclear NMR spectroscopy (¹H, ¹³C, ³¹P, ⁷⁷Se) and the solid compounds 1 , 3 , and 4 also by FT‐IR spectroscopy. The crystal and molecular structures of 3 and 4 were determined by single‐crystal X‐ray diffraction. In both compounds the N(1) atom is intramolecularly coordinated to the selenium atom, resulting in T‐shaped coordination arrangements of type (C,N)SeS. The dithio organophosphorus ligands act monodentate in both complexes, which can be described as essentially monomeric species. Weak intermolecular S ··· H contacts could be considered in the crystal of 3 , thus resulting in polymeric zig‐zag chains of R and S isomers, respectively.     

Homo- and hetero-bimetallic glycolates and triethanolaminates of niobium

[Nb(OⁱPr)₅] reacts with 2,5-dimethylhexane-2,5-diol (LH₂), 2,3-dimethylbutane-2,3-diol (L¹H₂) and triethanolamine (teaH₃) in different stoichiometric ratios to yield complexes of the types: [Nb(OⁱPr)₃(L)] (1), [Nb(OⁱPr)(L)₂] (2), [Nb(L)₂(LH)] (3), [Nb(L¹)₂(L¹H)] (4) and [Nb(tea)(teaH)] (5). Equimolar reactions of (3), (4) and (5) with Al(OⁱPr)₃, Ti(OⁱPr)₄ and [Ta(OⁱPr)₅] yield novel heterobimetallic isopropoxide-glycolate (6)–(9) and -triethanolaminate (10)–(12) derivatives. Reactions in appropriate molar ratios of (1), (2) and (10) with alkoxyethanols [ROC₂H₄OH; R = Me, Et] and acetylacetone [acacH] give derivatives [(MeOC₂H₄O)₃Nb(L)] (13), [(acac)Nb(L)₂] (14), [Nb(tea)₂{Al(OC₂H₄OMe)₂}] (15), [Nb(tea)₂{Al(OC₂H₄OEt)₂}] (16) and [Nb(tea)₂{Al(acac)₂}] (17). The complexes (6), (8) and (10) on reaction with an excess of t-BuOH give the tert-butoxo analogues (18), (19) and (20), respectively. These new derivatives have been characterized by elemental analyses, spectroscopic studies and molecular weight measurements.     

Controlled and highly effective ring-opening polymerization of α-chloro-ε-caprolactone using Zn- and Al-based catalysts

The present study details the highly effective and controlled ring-opening polymerization (ROP) of α-chloro-ε-caprolactone ( 1 , αClεCL), a cyclic ester that has been little explored thus far in ROP catalysis, using Zn- and Al-based catalysts [Zn(C₆F₅)₂(toluene)] ( 4 ), [N,N′-bis(3,5-di-tert-butylsalicylidene)1,3-diaminopropanato]aluminium(III)benzyloxide ( 5 ) and [N,N′-bis(3,5-di-tert-butylsalicylidene)1,3-diamino-2,2′-dimethylpropanato]aluminium(III)benzyloxide] ( 6 ). Chain-length-controlled PαClεCL material is produced under solution ROP conditions, as deduced from GPC, NMR, MALDI-TOF, and kinetic data. In contrast, the ROP of 1 is ill-defined under bulk ROP conditions due to partial thermal degradation of the polymer chain (presumably through C–Cl cleavage), reflecting the limited stability of PαClεCL. The Al Catalysts 5 and 6 are highly active ROP catalysts of αClεCL at room temperature (TOF up to 2,400 hr⁻¹) to afford well-defined P(αClεCL). In the case of Catalyst 6 , carrying out the ROP of αClεCL under immortal conditions (with BnOH as chain transfer agent) is clearly beneficial to ROP activity and control, with no apparent side-reaction of chloro-functionalized PCL chains as the ROP proceeds. The controlled character of these ROPs was further exploited for the production of chain-length-controlled PLLA-b-PαClεCL diblocks through sequential ROP of l -lactide and αClεCL, affording copolymers with improved thermal and biodegradable properties.     

RING-OPENING POLYMERIZATION OF ε-CAPROLACTONE BY LANTHANOCENE CATALYSIS*

Ring-opening polymerization of ε-caprolactone (CL) catalyzed bylanthanocenes, O(C_2H_4C_5H_3CH_3)_2YCl (Cat-YCl) and Me_2Si[(CH_3)_3SiC_5H_3]_2NdCl(Cat-NdCl) has been carried out for the first time. It has been found that both yttroceneand neodymocene are very efficient to catalyze the polymerization of CL, giving high molec-ular weight poly (ε-caprolactone) (PCL ). The effects of [cat] / [ε- CL] molar ratio, polymeriza-tion temperature and time, as well as solvents were investigated and polymerization tem-perature is found to be the most important factor affecting the polymerization. The bulkpolymerization gives higher molecular weight PCL and higher conversion than that in solu-tion polymerization. NaBPh_4 was found to promote the polymerization of ε-caprolactone,and thus to increase both the polymerization conversion and MW of poly (ε- caprolactone ).     

Modification of aluminum alkoxides with β-ketoesters: new insights into formation,structure and stability

[Al(OⁱPr)₂(β-ketoesterate)]₂ and Al(β-ketoesterate)₃ (β-ketoesterate = methyl, ethyl, iso -propyl, tert-butyl, allyl and 2-(methacryloyloxy)ethyl acetoacetate) were prepared by reaction of [Al(OⁱPr)₃]₄ with the corresponding β-ketoesters. Al(β-ketoesterate)₃ derivatives were exclusively formed at room temperature, whereas elevated reaction temperatures, causing thermal de-oligomerization of [Al(OⁱPr)₃]₄, were necessary for the formation of [Al(OⁱPr)₂(β-ketoesterate)]₂. All compounds were characterized by NMR spectroscopy, and [Al(OⁱPr)₂(tert-butyl acetoacetate)]₂ by a single crystal structure analysis. The [Al(OⁱPr)₂(β-ketoesterate)]₂ derivatives are asymmetrically substituted dimers with one octahedrally and one tetrahedrally substituted aluminum atom, bridged by two iso -propoxo groups, whereas the Al(β-ketoesterate)₃ derivatives are monomers with octahedrally coordinated aluminum. Transesterification as a possible side reaction was only observed at elevated temperatures for Al(tert-butyl acetoacetate)₃ in the presence of liberated iso -propanol. Dedicated to David Avnir on the occasion of his 60th birthday.     

Synthesis,Crystal Structures,and Thermolysis Studies of Heteronuclear Transition Metal Aluminum Alcoholates

Heteronuclear alcoholate complexes [M{Al(OiPr)₄}₂(bipy)] ( 2-M , M = Fe, Co, Ni, Cu, Zn) and [M{Al(OcHex)₄}₂(bipy)] ( 3-M , M = Fe, Co, Ni, Zn) are formed by adduct formation of [M{Al(OiPr)₄}₂] ( 1-M , M = Fe, Co, Ni, Cu, Zn) with 2,2'-bipyridine and transesterification reaction with cHexOAc. According to crystal structure analyses, in 2-M and 3-M the central transition metal ion M²⁺ is coordinated by two chelating Al(OR)₄^– moieties and one bipyridine ligand in an octahedral arrangement. Treating 1-Cu with 2,2'-bipyridine leads to a reduction process, whereat the intermediate [Cu{Al(OiPr)₄}(bipy)₂][Al(OiPr)₄] ( 4 ) could be structurally characterized. During conversion of the iso-propanolate ligands in 1-Cu to cyclohexanolate ligands, Cu²⁺ is reduced to Cu⁺ forming [Cu{Al(O^cHex)₄}(py)₂] ( 5 ). UV/Vis-spectra and results of thermolysis studies by TG/DTA-MS are reported.