Phosphasalen yttrium complexes: highly active and stereoselective initiators for lactide polymerization

Preparation and characterization of three yttrium alkoxide complexes with new phosphasalen ligands are reported. The phosphasalens are analogues of the well-known salen ligands but with iminophosphorane donors replacing the imine functionality. The three yttrium alkoxide complexes show mono- and dinuclear structures in the solid state, depending on the substituents on the ligand. The new ligands and complexes are characterized using multinuclear NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction experiments. The complexes are all rapid initiators for lactide polymerization; they show excellent polymerization control on addition of exogeneous alcohol. The mononuclear complex shows extremely rapid rates and a high degree of stereocontrol in rac-lactide polymerization, yielding heterotactic PLA (P(s) of 0.9). The phosphasalens are, therefore, excellent ligands for lactide ring-opening polymerization catalysis showing superior rates and stereocontrol versus salen ligands, which may be related to their excellent donating ability and the high degrees of steric protection they can confer.     

A series of bis(phosphinic)diamido yttrium complexes as initiators for lactide polymerization

A series of new bis(phosphinic)diamido yttrium complexes have been synthesized and fully characterized. The complexes adopt dimeric structures, both in solution and in the solid state, where one phosphinic group bonds to one yttrium center and the other bonds to two yttrium centers. The complexes have all been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is controlled by the diamine backbone substituent with the rate depending on the backbone flexibility. The order of decreasing rates were 2,2-dimethyl-1,3-propylene > trans-1,2-cyclohexylene > 1,2-ethylene > 1,2-phenylene. The polymerization kinetics showed, in most cases, an initiation period, during which the percentage conversion and the rate of polymerization were much lower than during propagation. This was attributed to relatively slow initiation by the bulky amido group. The initiator structure was probed using (31)P{ (1)H} NMR spectroscopy, which showed that the dimeric structure was maintained throughout the polymerization. The initiators give rise to controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight.     

Ring-opening polymerization of 3,6-dimethyl-2,5-morpholinedione with discrete amino-alkoxy-bis(phenolate) yttrium initiators: mechanistic insights

Alkoxy-amino-bis(phenolate) yttrium amide and alkoxide complexes promote the ring-opening polymerization of (3S,6S)-dimethyl-2,5-morpholinedione at 60-100 degrees C via a coordination-insertion polymerization mechanism.     

Unprecedented reversible migration of amide to schiff base ligands attached to tin: latent single-site initiators for lactide polymerization

In an unprecedented transformation, amide ligands are found to attack the imine carbon centers of tridentate Schiff base ligands attached to tin. The process is reversible, and the resultant (masked) amide species can be exploited as latent single-site initiators for the controlled polymerization of rac-lactide.     

Stereochemistry of lactide polymerization with chiral catalysts: new opportunities for stereocontrol using polymer exchange mechanisms

The synthesis of chiral aluminum and yttrium alkoxides and their application for lactide polymerization are reported. The complexes (SalBinap)MOR [4, M = Al, R = (i)Pr; 5, M = Y, R = (CH(2))(2)NMe(2)] are synthesized by reacting the ligand (SalBinap)H(2) [2,2'-[(1,1'-binaphthalene)-2,2'-diylbis(nitrilomethylidyne)]bisphenol] with the appropriate metal trisalkoxide. While enantiomerically pure yttrium complex 5 did not effect stereocontrol in the polymerization of either meso- or rac-lactide, homochiral 4 was found to exhibit excellent stereocontrol in a range of lactide polymerizations. Enantiomerically pure 4 polymerizes meso-lactide to syndiotactic poly(lactic acid) (PLA), while rac-4 polymerizes meso- and rac-lactide to heterotactic and isotactic stereoblock PLA, respectively. On the basis of the absolute stereochemistry of ring-opening of meso-lactide using (R)-4, a polymer exchange mechanism is proposed to account for the PLA microstructures resulting from rac-4.     

Synthetic, structural, mechanistic, and computational studies on single-site beta-diketiminate tin(II) initiators for the polymerization of rac-lactide

A family of tin(II) complexes supported by beta-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of M(n) versus conversion and [M](o)/[I](o) versus conversion, along with narrow molecular weight distributions (typically 1.07-1.10), are indicative of well-controlled, "living" polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s(2) lone pair of electrons rather than the steric or electronic properties of the beta-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, ((Me)BDI(DIPP))SnOCH(Me)COO(i)Pr (14), has been synthesized as a model compound for the propagating species by treatment of ((Me)BDI(DIPP))Sn(NMe(2)) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.     

Rare-earth metal complexes supported by 1,omega-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, structure, and heteroselective ring-opening polymerization of rac-lactide

Monomeric yttrium and lutetium bis(phenolato) complexes [Ln(OSSO){N(SiHMe 2) 2}(THF)] (Ln = Y, Lu) were prepared from the reaction of silylamido complexes [Ln{N(SiHMe 2) 2} 3(THF) 2] with 1 equiv of tetradentate 1,omega-dithiaalkanediyl-bridged bis(phenol) (OSSO)H 2 1- 9 in moderate to high yields. In contrast to the rigid configuration of scandium analogues, the yttrium complexes 2b and 3b and the lutetium complex 3c that contain a C 2 bridge between the two sulfur donors of the ligand are symmetric in solution. The monomeric nature of these complexes was indicated by an X-ray diffraction study of the yttrium complex 6b. The yttrium center in 6b is coordinated to the tetradentate [OSSO]-type ligand, one silylamido group and one THF ligand with the two oxygen donors of the [OSSO]-type ligand located trans. Corresponding bis(phenolato) silylamido complexes of larger rare-earth metals could not be obtained from similar reactions: Reaction of [La{N(SiHMe 2) 2} 3(THF) 2] with 1,2-xylylene-linked bis(phenol) gave a dinuclear lanthanum complex 6d of the formula [La 2(OSSO) 3] with two inequivalent eight-coordinate metal centers. The yttrium and lutetium complexes efficiently initiated the ring-opening polymerization (ROP) of lactides in THF. The heteroselectivity during the ROP of rac-lactide was enhanced when the steric demand of the bis(phenolato) ligand was increased, either by extending the bridge length or by introducing bulky ortho-substituents in the phenoxy units. A C 3 bridge within the ligand backbone is essential to allow configurational interconversion of the active site between Lambda and Delta configuration during polymerization, allowing accommodation of both enantiomers of the monomer in an alternating fashion.     

Ring-opening polymerization of lactide with group 3 metal complexes supported by dianionic alkoxy-amino-bisphenolate ligands: combining high activity, productivity, and selectivity

A series of new alkoxy-amino-bis(phenols) (H2L 1-6) has been synthesized by Mannich condensations of substituted phenols, formaldehyde, and amino ethers or diamines. The coordination properties of these dianionic ligands towards yttrium, lanthanum, and neodymium have been studied. The resulting Group 3 metal complexes have been used as initiators for the ring-opening polymerization of rac-lactide to provide poly(lactic acid)s (PLAs). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers, together with the linear natures of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. Complex [Y(L6){N(SiHMe2)2}(THF)] (17) polymerized rac-lactide to heterotactic PLA (Pr = 0.90 at 20 degrees C) and meso-lactide to syndiotactic PLA (Pr = 0.75 at 20 degrees C). The in situ formation of [Y(L6)(OiPr)(THF)] (18) from 17 and 2-propanol resulted in narrower molecular weight distributions (PDI = 1.06). With complex 18, highly heterotactic PLAs with narrow molecular weight distributions were obtained with high activities and productivities at room temperature. The natures of the ligand substituents were shown to have a significant influence on the degree of control of the polymerizations, and in particular on the tacticity of the polymer.     

Redox control of a ring-opening polymerization catalyst

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and M?ssbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.     

Comparative study of the coordination chemistry and lactide polymerization of alkoxide and amide complexes of zinc and magnesium with a beta-diiminato ligand bearing ether substituents

A series of beta-diiminato complexes of the form (BDI-3)MX where (BDI-3) = [CH(CMeNC(6)H(4)-2-OMe)(2)]; M = Zn, Mg; X = O(i)()Pr, O(t)()Bu, or N(SiMe(3))(2) has been synthesized. The (BDI-3) ligand is bidentate in (BDI-3)ZnN(SiMe(3))(2) and tetradentate in (BDI-3)MgN(SiMe(3))(2). The alkoxide complexes are shown to be active for lactide polymerization. Polymerization of rac-lactide with (BDI-3)ZnO(i)Pr gives a moderate preference for heterotactic PLA. Polymerization of rac-lactide with [(BDI-3)MgO(t)Bu](2) shows a slight preference for heterotactic PLA in CH(2)Cl(2) but is highly stereoselective in THF in the production of heterotactic PLA.     

Aluminium alkyl complexes supported by [OSSO] type bisphenolato ligands: synthesis, characterization and living polymerization of rac-lactide

Aluminium alkyl complexes [(OSSO)AlR](1-3: R = Me, Et) were isolated in good yields from the protonolysis reaction of AlR3 with the corresponding tetradentate 1,omega-dithiaalkanediyl-bridged bisphenols (1,4-dithiabutanediyl-bis(6-tert-butyl-4-methylphenol), etbmpH2; ortho-xylylenedithio-bis(6-tert-butyl-4-methylphenol), xytbmpH2). The monomeric structures of all three complexes were confirmed by X-ray diffraction studies. Complexes 1 and 2 have an isotypic packing arrangement. The aluminium center is coordinated by the etbmp ligand and one alkyl group with distorted trigonal bipyramidal geometry. Complex 3 shows Cs symmetry with square pyramidal geometry around the metal center. Substitution reaction of complex 1 with trityl alcohol gave the monomeric alkoxide complex [(etbmp)Al(OCPh3)] 4, which has a similar trigonal bipyramidal geometry around the aluminium atom as complex 1. In the presence of isopropanol, complexes 1-3 initiated the living ring-opening polymerization of rac-lactide (PDI = 1.03-1.06, Mw/Mn). The ligand structure influenced the tacticity of the obtained polymer, with complex 3 giving heterotactic-enriched polylactides.     

Cationic and charge-neutral calcium tetrahydroborate complexes and their use in the controlled ring-opening polymerisation of rac-lactide

The first cationic main group tetrahydroborate complexes are reported. [Ca(BH(4))(THF)(5)][BPh(4)] and the charge neutral (Tp((t)Bu,Me))Ca(BH(4))(THF) are initiators for the living ring opening polymerization of rac-lactide, the latter producing PLA with high levels of heterotactic enrichment. These represent a new class of ROP initiators for main group metals.     

Magnesium and zinc complexes of a potentially tridentate beta-diketiminate ligand

The synthesis of the unsymmetrically substituted beta-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N'-bis(2,6-diisopropylphenyl) beta-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex, (BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds, and reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).     

Zinc and magnesium complexes incorporated by bis(amine) benzotriazole phenoxide ligand: synthesis, characterization, photoluminescent properties and catalysis for ring-opening polymerization of lactide

A new bis(amine) benzotriazole phenoxide ligand, (C8NN)BTP-H (1) was prepared from the Mannich condensation of 2-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol with the mixtures of excess paraformaldehyde and N,N,N-trimethylethylenediamine under reflux conditions. Zinc and magnesium complexes bearing the N,N,O-tridentate (C8NN)BTP ligand were synthesized and fully characterized. The reaction of (C8NN)BTP-H with ZnEt(2) (1.2 molar equiv.) in toluene gives the monomeric tetra-coordinated zinc complex [((C8NN)BTP)ZnEt] (2). The homoleptic and monomeric complexes [((C8NN)BTP)(2)M] (M = Zn (3) and M = Mg (4)) result from treatment of ZnEt(2) or Mg(n)Bu(2) with (C8NN)BTP-H (two equiv.), in which the metal center is hexa-coordinated by two tridentate (C8NN)BTP ligands. Luminescent properties and catalysis for lactide (LA) polymerizations of complexes 2-4 are studied. Complexes 2-4 produce bright green fluorescence with emission maxima ranging from 510 to 520 nm in the solid state. Experimental results indicate that complex 2 catalyzes the ring-opening polymerization of LA in the presence of 9-anthracenemethanol (9-AnOH) with efficient catalytic activities in a controlled fashion, yielding poly(rac-lactide) with a slight isotactic predominance (P(m) = 0.59).     

Preparation of supported yttrium alkoxides as catalysts for the polymerization of lactones and oxirane

Two methods have been reported that allow yttrium alkoxides to be supported on porous silica and to be used afterward as heterogeneous catalysts in the ring‐opening polymerization of oxirane and ?‐caprolactone. In the two methods, [tris(hexamethyldisilyl)‐amide]yttrium {Y[N(SiMe₃)₂]₃} is the metal alkoxide precursor. It is directly reacted with the silanol groups of the support, in the first method, and this is followed by alcoholysis of the unreacted amide groups. The flexibility of this method seems to be limited because the grafting density and the structure of the grafted Y alkoxide (less than one alkoxide by metal) are independent of the experimental conditions. In the second method, Y[N(SiMe₃)₂]₃ is first reacted with 1 or 2 equiv of alcohol with the formation of the mixed Y alkoxide/amide. The amide functions are used to attach Y to the support. This method is free from side reactions, quite reproducible, and well suited to support one type of active species (monoalkoxide or dialkoxide). Preliminary experiments with ?‐caprolactone polymerization have confirmed the activity of the supported Y alkoxide, whatever preparation method is used. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 569–578, 2003     

Synthesis, magnetic behaviour, and X-ray structures of dinuclear copper complexes with multiple bridges. Efficient and selective catalysts for polymerization of 2,6-dimethylphenol

The use of a potentially tridentate mono-anionic bridging ligand, 1,3-bis(3,5-dimethylpyrazol-1-yl)-propan-2-ol (bdmpp-H), in assembling new dimeric copper complexes with interesting magnetic properties has been investigated. The reaction of copper hydroxide or copper acetate with phenyl phosphinic acid or diphenyl phosphinic acid in the presence of bdmpp-H produces the dinuclear complexes [Cu(bdmpp)(ppi)]2 (1) and [Cu(bdmpp)(dppi-H)]2(dppi)2 (2) (ppi-H = phenylphosphinic acid: dppi-H = diphenylphosphinic acid), respectively. The products have been characterized with the help of analytical, thermal, and spectroscopic (IR, UV-vis, and EPR) techniques. Single crystal X-ray diffraction studies of 1 and 2 reveal that the two bdmpp ligands hold together the dimeric copper unit in each complex through mu-O alkoxide and the pyrazolyl nitrogen ligating centers. Two phenyl phosphinate ligands additionally bridge the dicopper core in 1 to result in octahedral coordination geometry around each metal ion. The diphenyl phosphinic acid acts as a terminal ligand in 2, and thus completes a square pyramidal geometry around each copper ion. Both complexes show a very short Cu...Cu separation (3.001 and 3.065 angstroms for 1 and 2, respectively). The investigation of the magnetic properties reveals the efficiency of the double alkoxide bridge between the two paramagnetic copper ions to mediate strong antiferromagnetic interactions [J/k(B) = -620(5) K (-431(4) cm(-1)) and -685(5) K (-476(4) cm(-1)) for 1 and 2, respectively]. Compounds 1 and 2, along with a few other copper phosphate complexes, were tested as catalysts for the oxidative polymerization of 2,6-dimethylphenol; 1 and 2 were found to be efficient catalysts with an increased selectivity for the formation of the polyphenylene ether. However a related mononuclear octahedral copper complex [Cu(imz)4(dtbp)2] (dtbp-H = di-tert-butylphosphate) was found to be more efficient.     

Controlled and stereoselective polymerization of lactide: kinetics, selectivity, and microstructures

Chiral aluminum isopropoxides based on enantiopure or racemic cyclohexylsalen ligand (Jacobsen ligand) have been prepared and employed for stereoelective/stereoselective ring-opening polymerization of lactide in toluene at 70 degrees C. The kinetics, selectivity of the catalysts, and microstructure of the obtained polylactides, using different combinations of lactide enantiomers and catalysts, were determined. In all cases, polylactides of controlled molecular weight, low polydispersity, and defined end groups were obtained. The polymerizations are first-order in both monomer(s) and catalyst. (R,R)-CyclohexylsalenAlO(i)()Pr [(R,R)-1] polymerizes l-lactide significantly faster than d-lactide with a rate constant ratio k(l)/k(d) of approximately 14. The polymerization of rac-lactide using (R,R)-1 yields crystalline polymers, for which a selectivity factor of approximately 5.5 could be calculated up to 50% conversion based on the optical purity of the isolated polymers. The polymerization of a l-lactide/d-lactide (molar ratio: 80/20) mixture by (R,R)-1 furnishes an isotactic-atactic block copolylactide, which is highly crystalline with a T(m) = approximately 155 degrees C. Polymerization of rac-lactide applying rac-cyclohexylsalenAlO(i)()Pr [rac-1] yields isotactic stereoblock polylactides with a high T(m) = approximately 185 degrees C and a high degree of crystallinity.     

Ytterbium amides of linked bis(amidinate): synthesis, molecular structures, and reactivity for the polymerization of L-lactide

The steric effect of an amide group on the synthesis, molecular structures and reactivity of ytterbium amides supported by linked bis(amidinate) L (L = [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]) is reported. Reaction of LYbCl(THF)2 with equimolar NaNHAr' and NaNHAr (Ar' = 2,6-Me2C6H3; Ar = 2,6-iPr2C6H3), respectively, gave the corresponding monometallic amide complexes LYb(NHAr')(DME) 1 and LYb(NHAr)(DME) 2, in which the linked bis(amidinate) is coordinated to the metal center as a chelating ligand. The similar reaction with NaN(SiMe3)2 afforded a bimetallic amide complex (TMS)2NYb(L)2YbN(TMS)2 3 formed through the rearrangement reaction of L induced by the bulky N(SiMe3)2 group. In complex 3 the two linked bis(amidinate)s act as bridging ancillary ligands to link two YbN(TMS)2 species in one molecule. The definite molecular structures of 1-3 were provided by single-crystal X-ray analysis. Complexes 1-3 are efficient initiators for the polymerization of L-lactide, and their catalytic performance is highly dependent on the amido groups and molecular structures. The polymerizations initiated by complexes 1 and 2 proceeded in a living fashion as evidenced by the narrow polydispersities of the resulting polymers, together with the linear natures of the number average molecular weight versus conversion plots, while the polymerization system with complex 3 provided polymers with rather broad molecular weight distributions.     

Highly active and stereoselective zirconium and hafnium alkoxide initiators for solvent-free ring-opening polymerization of rac-lactide

Under solvent-free conditions (at 130 degrees C), zirconium and hafnium amine tris(phenolate) alkoxides are extremely active, well-controlled, single-site initiators for the ring-opening polymerization of rac-lactide, yielding highly heterotactic polylactide.     

Robust chiral zirconium alkoxide initiators for the room-temperature stereoselective ring-opening polymerisation of rac-lactide

Chiral Schiff bases (1H to 4H) and a series of their Group 4 metal alkoxide complexes [(R-1)2Ti(O(i)Pr)2, (R-2)2Ti(O(i)Pr)2, (R-1)(2)Zr(O(i)Pr)2, (R-2)2Zr(O(i)Pr)2, (R-3)2Zr(O(i)Pr)2, (R-4)2Zr(O(i)Pr)2, (S-1)2Zr(O(i)Pr)2 and (rac-1)2Zr(O(i)Pr)2] have been prepared and characterised by 1H, and 13C NMR spectroscopy. In solution, both Lambda and Delta isomers were observed, suggesting a low degree of chiral induction from the ligand. One ligand (R-4H) and three complexes [Delta-(R,R-2)2Ti(O(i)Pr)2, Lambda-(R,R-1)2Zr(O(i)Pr)2 and Delta-(R,R-3)2Zr(O(i)Pr)2] have also been characterised by single crystal X-ray diffraction. All complexes were found to have a pseudo-octahedral alpha-cis geometry. The complexes were tested as initiators for the ring-opening polymerisation of rac-lactide in solution and in the melt. The titanium complexes are inactive in solution and afford atactic polylactide in the melt. Zr(iv) complexes afford heterotactically enriched polylactide both in toluene solution (at 20 degrees C and 80 degrees C) and in the melt. Polymerisations were generally found to be well-controlled, giving predictable molecular weights and low molecular weight distributions. Ligand variation (substituents and/or chirality) has little effect on either the activity or selectivity of initiators. Zirconium initiators were found to be unusually robust as they were able to maintain well-controlled polymerisation following addition of water to reactions in solution and when using unpurified monomer for reactions in the melt.