Phosphasalen Indium Complexes Showing High Rates and Isoselectivities in rac‐Lactide Polymerizations

Polylactide (PLA) is the leading bioderived polymer produced commercially by the metal‐catalyzed ring‐opening polymerization of lactide. Control over tacticity to produce stereoblock PLA, from rac ‐lactide improves thermal properties but is an outstanding challenge. Here, phosphasalen indium catalysts feature high rates (30±3 m ⁻¹ min⁻¹, THF, 298 K), high control, low loadings (0.2 mol %), and isoselectivity (P _i=0.92, THF, 258 K). Furthermore, the phosphasalen indium catalysts do not require any chiral additives.     

三(2,4,6-三甲基苯氧基)稀土配合物引发DL-丙交酯开环聚合特征与机理的研究

Tris(2,4,6-trimethylphenolate) lanthartide [Ln(OTMP)3] has been prepared and employed for ring-opening polymerization of D,L-lactide (LA) as single-component catalysts. The characteristics, kinetics and mechanism were examined. The polymerization is first-order with respect to monomer and initiator concentration, and the overall activation energy amounts to 62.9 kJ/mol. DSC curve disclosed the random structure of PLA. ^1H NMR spectrum analysis demonstrates that the polymerization of LA proceeded through acyl-oxygen bond cleavage.     

Alkaline Earth Metal‐Mediated Highly Iso‐selective Ring‐Opening Polymerization of rac‐Lactide

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso‐selective catalysts for the ring‐opening polymerization (ROP) of rac‐lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac‐LA catalyzed by all Ae metal complexes had a first‐order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex‐mediated ROP of rac‐LA is discussed, based on controlled kinetic experiments and molecular chain mobility.     

Improving the Ring‐opening Polymerization of l‐Lactide Using n‐Butyl Lithium with Various Bidentate Ligands

A series of bidentate ligands were examined to improve the catalytic activity of l ‐lactide (LA) polymerization by using n‐butyl lithium and BnOH. For LA polymerization, n‐butyl lithium with tetraisopropyl methylenebis(phosphonate) (L⁸ ) showed the greatest catalytic activity but with poor controllability of the polymer molecular weight. The polydispersity indices (PDIs) could be improved without BnOH addition, but Mn _GPC was much different from the Mn _Cal . ¹H NMR spectra confirmed that the cyclized PLA was obtained, thus implying that active chain‐end mechanisms were operative in LA polymerization.     

Mechanistic Insight into the Stereochemical Control of Lactide Polymerization by Salan–Aluminum Catalysts

Alkyl aluminum complexes of chiral salan ligands assembled around the 2,2′‐bipyrrolidine core form as single diastereomers that have identical configurations of the N donors. Active catalysts for the polymerization of lactide were formed upon the addition of benzyl alcohol. Polymeryl exchange between enantiomorphous aluminum species had a dramatic effect on the tacticity of the poly(lactic acid) (PLA) in the polymerization of racemic lactide (rac‐LA): The enantiomerically pure catalyst of the nonsubstituted salan ligand led to isotactic PLA, and the racemic catalyst exhibited lower stereocontrol. The enantiomerically pure catalyst of the chloro‐substituted salan ligand led to PLA with a slight tendency toward heterotacticity, whereas the racemic catalyst led to PLA of almost perfect heterotacticity following an insertion/auto‐inhibition/exchange mechanism.     

Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis,Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

New polynuclear zinc complexes containing tridentate Schiff base ligands were successfully synthesized and fully characterized. The solid‐state structure of the complexes was determined using single crystal X‐ray diffraction. The complexes display a tetranuclear cubane‐like core structure [Zn₄O₄] and sowed good catalytic activity towards the ring‐opening polymerization (ROP ) of rac‐lactide (rac‐LA ) and ε‐caprolactone (ε‐CL ) under solvent‐free conditions. The polylactic acid (PLA ) obtained from rac‐LA showed isotactic enrichment, as proved by homonuclear decoupled ¹H‐NMR analysis. These complexes also showed good activity and superior control towards the ROP of rac‐LA and ε‐CL in the presence of benzyl alcohol as a co‐initiator. Furthermore, kinetic studies demonstrated that the ROP of rac‐LA and ε‐CL has a first order dependence on both monomer (rac‐LA and ε‐CL ) and catalyst concentration.     

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(L 6 ){N(SiHMe₂)₂}(THF)] ( 17 ) polymerized rac‐lactide to heterotactic PLA (P_r = 0.90 at 20 °C) and meso‐lactide to syndiotactic PLA (P_r = 0.75 at 20 °C). The in situ formation of [Y(L 6 )(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.     

Metal‐Size Influence in Iso‐Selective Lactide Polymerization

Iso‐selective initiators for the ring‐opening polymerization (ROP) of rac‐lactide are rare outside of Group 13. We describe the first examples of highly iso‐selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso‐selectivity, with a P_i value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (P_s=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.     

Isoselective Ring‐Opening Polymerization of rac‐Lactide Catalyzed by Sodium/potassium Tetradentate Aminobisphenolate Ion‐paired Complexes

Two sodium/potassium tetradentate aminobisphenolate ion‐paired complexes were synthesized and structurally characterized. These ion‐paired complexes are efficient catalysts for the ring‐opening polymerization of rac‐lactide (rac‐LA) in the presence of 5 equivalents BnOH as an initiator and the side reaction of epimerization can be suppressed well at low temperatures. The polymerizations are controllable, affording polylactides with desirable molecular weights and narrow molecular weight distributions; the highest molecular weight can reach 50.1 kg mol^?1 in this system, and a best isoselectivity of P_m=0.82 was achieved. Such polymerizations have rarely been reported for isoselective sodium/potassium complexes without crown ether as an auxiliary ligand. The solid structures suggest that BnOH can be activated by an interaction with the anion of sodium/potassium complex via a hydrogen bond and that the monomer is activated by coordination to sodium/potassium ion.     

Synthesis of Isotactic–Heterotactic Stereoblock (Hard–Soft) Poly(lactide) with Tacticity Control through Immortal Coordination Polymerization

A one‐pot method for the preparation of a new family of PLA materials is reported that combines heterotactic (soft) and isotactic stereoblocks (hard). The ring‐opening polymerization of rac‐lactide with a salan–rare‐earth‐metal–alkyl complex in the presence of excess triethanolamine was performed in an immortal mode to give three‐armed heterotactic poly(lactide) (soft) with excellent end‐hydroxy fidelity. The in situ addition of a salen–aluminum–alkyl precursor to the above polymerization system under any monomer‐conversion conditions activated the “dormant” hydroxy‐ended PLA chains to propagate through the incorporation of the remaining rac‐lactide monomer, but with isospecific selectivity (hard). The resultant PLA had a three‐armed architecture with controlled molecular weight and extremely narrow molecular‐weight distribution (PDI<1.08). More strikingly, each side‐arm simultaneously possessed highly heterotactic (soft) and highly isotactic (hard) segments and the ratio of these two stereoregular sequences could be swiftly adjusted by tuning the addition time of the salen–aluminum–alkyl precursor to the polymerization system. Therefore, star‐shaped hard–soft stereoblock poly(lactide)s with various P_m values and crystallinity were achieved in a single reactor for the first time. This strategy should be applicable to the synthesis of a series of new types of stereoblock polyesters by using an immortal‐polymerization process and a proper choice of specific, selective metal‐based catalysts.     

A Tetrameric Titanium Alkoxide as a Lactide Polymerization Catalyst

The tetrameric titanium alkoxide (MeC(CH₂‐μ₃‐O)(CH₂‐μ‐O)₂)₂Ti₄(O‐i‐Pr)₁₀ ( 1 ) catalyzes the ring‐opening polymerization (ROP) of lactide (LA) in toluene solution at various polymerization temperatures, and its bulk ROP at 130°C. Compound 1 facilitated reasonably controlled polymerization characteristics via a coordination/insertion mechanism in solution, whereas the bulk polymerization products displayed broad molecular‐weight distributions. The stereochemical microstructure of PLA was determined from homonuclear decoupled ¹H NMR spectroscopic studies.     

Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization

Synthetic routes to aluminium ethyl complexes supported by chiral tetradentate phenoxyamine (salan‐type) ligands [Al(OC₆H₂(R‐6‐R‐4)CH₂)₂{CH₃N(C₆H₁₀)NCH₃}‐C₂H₅] ( 4 , 7 : R=H; 5 , 8 : R=Cl; 6 , 9 : R=CH₃) are reported. Enantiomerically pure salan ligands 1–3 with (R,R) configurations at their cyclohexane rings afforded the complexes 4 , 5 , and 6 as mixtures of two diastereoisomers ( a and b ). Each diastereoisomer a was, as determined by X‐ray analysis, monomeric with a five‐coordinated aluminium central core in the solid state, adopting a cis‐(O,O) and cis‐(Me,Me) ligand geometry. From the results of variable‐temperature (VT) ¹H NMR in the temperature range of 220–335 K, ¹H–¹H NOESY at 220 K, and diffusion‐ordered spectroscopy (DOSY), it is concluded that each diastereoisomer b is also monomeric with a five‐coordinated aluminium central core. The geometry is intermediate between square pyramidal with a cis‐(O,O), trans‐(Me,Me) ligand disposition and trigonal bipyramidal with a trans‐(O,O) and trans‐(Me,Me) disposition. A slow exchange between these two geometries at 220 K was indicated by ¹H–¹H NOESY NMR. In the presence of propan‐2‐ol as an initiator, enantiomerically pure (R,R) complexes 4 – 6 and their racemic mixtures 7 – 9 were efficient catalysts in the ring‐opening polymerization of lactide (LA). Polylactide materials ranging from isotactically biased (P_m up to 0.66) to medium heterotactic (P_r up to 0.73) were obtained from rac‐lactide, and syndiotactically biased polylactide (P_r up to 0.70) from meso‐lactide. Kinetic studies revealed that the polymerization of (S,S)‐LA in the presence of 4 /propan‐2‐ol had a much higher polymerization rate than (R,R)‐LA polymerization (k_SS/k_RR=10.1).     

Synthetic and Mechanistic Aspects of the Immortal Ring‐Opening Polymerization of Lactide and Trimethylene Carbonate with New Homo‐ and Heteroleptic Tin(II)‐Phenolate Catalysts

Several new heteroleptic Sn^II complexes supported by amino‐ether phenolate ligands [Sn{LOⁿ}(Nu)] (LO¹=2‐[(1,4,7,10‐tetraoxa‐13‐azacyclopentadecan‐13‐yl)methyl]‐4,6‐di‐tert‐butylphenolate, Nu=NMe₂ ( 1 ), N(SiMe₃)₂ ( 3 ), OSiPh₃ ( 6 ); LO²=2,4‐di‐tert‐butyl‐6‐(morpholinomethyl)phenolate, Nu=N(SiMe₃)₂ ( 7 ), OSiPh₃ ( 8 )) and the homoleptic Sn{LO¹}₂ ( 2 ) have been synthesized. The alkoxy derivatives [Sn{LO¹}(OR)] (OR=OiPr ( 4 ), (S)‐OCH(CH₃)CO₂iPr ( 5 )), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO¹}]⁺[H₂N{B(C₆F₅)₃}₂]^? ( 9 ), a rare well‐defined, solvent‐free tin cation, was prepared in high yield. The X‐ray crystal structures of compounds 3 , 6 , and 8 were elucidated, and compounds 3 , 6 , 8 , and 9 were further characterized by ¹¹⁹Sn Mössbauer spectroscopy. In the presence of iPrOH, compounds 1 – 5 , 7 , and 9 catalyzed the well‐controlled, immortal ring‐opening polymerization (iROP) of L ‐lactide (L ‐LA) with high activities (ca. 150–550 mol_L?LA mol_Sn^?1 h^?1) for tin(II) complexes. The cationic compound 9 required a higher temperature (100 °C) than the neutral species (60 °C); monodisperse poly(L ‐LA)s were obtained in all cases. The activities of the heteroleptic pre‐catalysts 1 , 3 , and 7 were virtually independent of the nature of the ancillary ligand, and, most strikingly, the homoleptic complex 2 was equally competent as a pre‐catalyst. Polymerization of trimethylene carbonate (TMC) occurs much more slowly, and not at all in the presence of LA; therefore, the generation of PLA‐PTMC copolymers is only possible if TMC is polymerized first. Mechanistic studies based on ¹H and ¹¹⁹Sn{¹H} NMR spectroscopy showed that the addition of an excess of iPrOH to compound 3 yielded a mixture of compound 4 , compound [Sn(OiPr)₂]_n 10 , and free {LO¹}H in a dynamic temperature‐dependent and concentration‐dependent equilibrium. Upon further addition of L ‐LA, two active species were detected, [Sn{LO¹}(OPLLA)] ( 12 ) and [Sn(OPLLA)₂] ( 14 ), which were also in fast equilibrium. Based on assignment of the ¹¹⁹Sn{¹H} NMR spectrum, all of the species present in the ROP reaction were identified; starting from either the heteroleptic ( 1 , 3 , 7 ) or homoleptic ( 2 ) pre‐catalysts, both types of pre‐catalysts yielded the same active species. The catalytic inactivity of the siloxy derivative 6 confirmed that ROP catalysts of the type 1 – 5 could not operate according to an activated‐monomer mechanism. These mechanistic studies removed a number of ambiguities regarding the mechanism of the (i)ROPs of L ‐LA and TMC promoted by industrially relevant homoleptic or heteroleptic Sn^II species.     

Microwave‐assisted synthesis and characterization of biodegradable block copolyesters based on poly(3‐hydroxyalkanoate)s and poly(D,L‐lactide)

Microwave (MW)‐assisted ring‐opening polymerization (ROP) provides a rapid and straightforward method for engineering a wide array of well‐defined poly(3‐hydroxyalkanoate)‐b‐poly(D,L ‐lactide) (PHA‐b‐PLA) diblock copolymers. On MW irradiation, the bulk ROP of D,L ‐lactide (LA) could be efficiently triggered by a series of monohydroxylated PHA‐based macroinitiators previously produced via acid‐catalyzed methanolysis of corresponding native PHAs, thus affording diblock copolyesters with tunable compositions. The dependence of LA polymerization on temperature, macroinitiator structure, irradiation time, and [LA]₀/[PHA]₀ molar ratio was carefully investigated. It turned out that initiator efficiency values close to 1 associated with conversions ranging from 50 to 85% were obtained only after 5 min at 115 °C. A kinetic investigation of the MW‐assisted ROP of LA gave evidence of its “living”/controlled character under the experimental conditions selected. Structural analyses and thermal properties of biodegradable diblock copolyesters were also performed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Catalytic Gas‐Phase Production of Lactide from Renewable Alkyl Lactates

A new route to lactide, which is a key building block of the bioplastic polylactic acid, is proposed involving a continuous catalytic gas‐phase transesterification of renewable alkyl lactates in a scalable fixed‐bed setup. Supported TiO₂/SiO₂ catalysts are highly selective to lactide, with only minimal lactide racemization. The solvent‐free process allows for easy product separation and recycling of unconverted alkyl lactates and recyclable lactyl intermediates. The catalytic activity of TiO₂/SiO₂ catalysts was strongly correlated to their optical properties by DR UV/Vis spectroscopy. Catalysts with high band‐gap energy of the supported TiO₂ phase, indicative of a high surface spreading of isolated Ti centers, show the highest turnover frequency per Ti site.     

Effect of blending ratio and oligomer structure on the thermal transitions of stereocomplexes consisting of a D‐lactic acid oligomer and poly(L‐lactide)

Poly(lactic acid) (PLA) stereocomplexes have high potential as renewable materials for advanced polymer applications, mainly due to their high melting temperature (T_m, typically 230–240°C). The properties of PLA stereocomplexes consisting of linear high molar mass homopolymers have been studied extensively in the past, but the available information about the possibilities to affect the thermal properties of the stereocomplex by varying the structure of the blend components has not been sufficient. Novel stereocomplexes containing linear or star‐shaped D ‐lactic acid (D ‐LA) oligomers and high molar mass poly(L ‐lactide) (L‐ PLA) were thus prepared. The T_m and melting enthalpy (ΔH_m) of the racemic crystallites were found to depend strongly on both the blending ratio and the arm‐length of the D ‐lactic acid oligomer. The preparation method of the oligomers, i.e. step‐growth polymerization or ring‐opening polymerization (ROP), did not affect the T_m or ΔH_m of the blends significantly. Slightly higher ΔH_m values were, however, obtained, when linear oligomers were used. The results thus indicated that the T_m and ΔH_m of PLA stereocomplexes could be optimized, simply by selecting a D ‐LA oligomer having a suitable arm‐length and structure as the other blend component. The possibility to adjust the melting behavior of the stereocomplex blend is a significant advantage and could make PLA suitable for a wider range of products used at elevated temperatures. Copyright © 2010 John Wiley & Sons, Ltd.     

Ring‐opening Polymerization of L‐Lactide by an Anionic Cobalt(II) Aryloxide

The reaction of anhydrous CoCl₂ with NaOAr (ArO=2,4,6‐tri‐tert‐butylphenoxo) in THF at room temperature in 1:3 molar ratio afforded anionic cobalt aryloxide [Na(THF)₆][Co(OAr)₃] ( 1 ). The definite structure of this complex was characterized by X‐ray single crystal diffraction. It was found that this anionic aryloxo cobalt(II) complex could effectively initiate the ring‐opening polymerization of L‐lactide both in solution and in bulk, leading to high molecular weight poly(L‐lactide).     

The influence of organosuperbases on the structure and activity of dialkylgallium alkoxides in the polymerization of rac‐lactide: the road to stereo diblock PLA copolymers

Stereoselective polymerization of rac‐lactide is one of the most important issues as the properties of polylactide (PLA) depend strongly on its tacticity. There is, however, a paucity of catalysts that allow for easy switching between heteroselectivity and isoselectivity, which limits the synthesis of stereo copolymers of PLA and modification of polylactide properties. Dialkylgallium alkoxides activated by organosuperbases have been used as catalysts in the ring‐opening polymerization of racemic lactide (rac‐LA). The reaction of (S,S)‐[Me₂Ga(μ‐OCH(Me)CO₂Me)]₂ ( 1 ) with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) or 7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (MTBD) resulted in the formation of isoselective gallium species, highly active in the polymerization of rac‐LA. DOSY (diffusion‐ordered spectroscopy) NMR was indicative for the presence of dimeric gallium species. However, the structure of model monomeric gallium alkoxide Me₂Ga(ON) (where ON is monoanionic bidentate ligand possessing organosuperbase functionality) shows that the presence of an organosuperbase may substantially weaken Ga?O_alkoxide?Ga bridges. The facile switch of stereoselectivity upon addition of organosuperbase to nonselective/heteroselective 1 allowed for the first time the synthesis of diblock polylactide comprised of isotactically and heterotactically enriched blocks. Copyright © 2013 John Wiley & Sons, Ltd.     

Isotactic rac‐Lactide Polymerization with Copper Complexes: The Influence of Complex Nuclearity

Diiminopyrrolide copper alkoxide complexes, LCuOR (OR¹=N,N‐dimethylamino ethoxide, OR²=2‐pyridyl methoxide), are active for the polymerization of rac‐lactide at ambient temperature in benzene to yield polymers with M_w/M_n=1.0–1.2. X‐ray diffraction studies showed bridged dinuclear complexes in the solid state for both complexes. While LCuOR¹ provided only atactic polylactide, LCuOR² produced partially isotactic polylactide (P_m=0.7). The difference in stereocontrol is attributed to a dinuclear active species for LCuOR² in contrast to a mononuclear species for LCuOR¹.     

Block and random copolymerization of ε‐caprolactone,L‐, and rac‐lactide using titanium complex derived from aminodiol ligand

A series of di‐ and triblock copolymers [poly(L ‐lactide‐b‐ε‐caprolactone), poly(D,L ‐lactide‐b‐ε‐caprolactone), poly(ε‐caprolactone‐b‐L ‐lactide), and poly(ε‐caprolactone‐b‐L ‐lactide‐b‐ε‐caprolactone)] have been synthesized successfully by sequential ring‐opening polymerization of ε‐caprolactone (ε‐CL) and lactide (LA) either by initiating PCL block growth with living PLA chain end or vice versa using titanium complexes supported by aminodiol ligands as initiators. Poly(trimethylene carbonate‐b‐ε‐caprolactone) was also prepared. A series of random copolymers with different comonomer composition were also synthesized in solution and bulk of ε‐CL and D,L ‐lactide. The chemical composition and microstructure of the copolymers suggest a random distribution with short average sequence length of both the LA and ε‐CL. Transesterification reactions played a key role in the redistribution of monomer sequence and the chain microstructures. Differential scanning calorimetry analysis of the copolymer also evidenced the random structure of the copolymer with a unique T_g. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012