Polymerization Of Lactide Research Articles

Understanding structure–activity relationships: iron(ii) complexes of “Legacy Guanidines” as catalysts for the synthesis of polylactide

Eight new iron(ii) complexes based on simple guanidine ligands were studied comprehensively as catalysts for the ring-opening polymerization of lactide demonstrating their potential under industrially-relevant conditions.

Simple magnesium alkoxides: synthesis, molecular structure, and catalytic behaviour in the ring-opening polymerization of lactide and macrolactones and in the copolymerization of maleic anhydride and propylene oxide.

The synthesis of two chiral bulky alkoxide pro-ligands, 1-adamantyl-tert-butylphenylmethanol HOCAdtBuPh and 1-adamantylmethylphenylmethanol HOCAdMePh, is reported and their coordination chemistry with magnesium(II) is described and compared with the coordination chemistry of the previously reported achiral bulky alkoxide pro-ligand HOCtBu2Ph. Treatment of n-butyl-sec-butylmagnesium with two equivalents of the racemic mixture of HOCAdtBuPh led selectively to the formation of the mononuclear bis(alkoxide) complex Mg(OCAdtBuPh)2(THF)2. 1H NMR spectroscopy and X-ray crystallography suggested the selective formation of the C2-symmetric homochiral diastereomer Mg(OCRAdtBuPh)2(THF)2/Mg(OCSAdtBuPh)2(THF)2. In contrast, the less sterically encumbered HOCAdMePh led to the formation of dinuclear products indicating only partial alkyl group substitution. The mononuclear Mg(OCAdtBuPh)2(THF)2 complex was tested as a catalyst in different reactions for the synthesis of polyesters. In the ROP of lactide, Mg(OCAdtBuPh)2(THF)2 demonstrated very high activity, higher than that shown by Mg(OCtBu2Ph)2(THF)2, although with moderate control degrees. Both Mg(OCAdtBuPh)2(THF)2 and Mg(OCtBu2Ph)2(THF)2 were found to be very effective in the polymerization of macrolactones such as ω-pentadecalactone (PDL) and ω-6-hexadecenlactone (HDL) also under mild reaction conditions that are generally prohibitive for these substrates. The same catalysts demonstrated efficient ring-opening copolymerization (ROCOP) of propylene oxide (PO) and maleic anhydride (MA) to produce poly(propylene maleate).

(Bipyridine bisphenolate)-aluminum/onium salt pair: a highly active binary catalyst for ring-opening polymerization of lactide with improved thermostability and protic tolerance

A series of bipyridine bisphenolate aluminum complexes with outstanding thermostability and tolerance to protic agents were synthesized and used to catalyze the ROP of LA with assist of onium salt as cocatalyst in epoxide.

Novel epoxide-promoted polymerization of lactides mediated by a zinc guanidine complex: a potential strategy for the tin-free PLA industry

An epoxy-promoted zinc guanidine complex system with extremely high efficiency (106 500 h−1) is reported towards the controlled ROP of lactides.

Guaiacolate derivatives‐containing sodium complexes as catalysts for l‐lactide polymerization

In this study, guaiacolate derivatives‐containing tetranuclear sodium catalysts were synthesized, and their catalytic activity in l‐lactide (LA) polymerization was evaluated. Using LBF‐Na (sodium complex bearing 2,3‐dihydro‐2,2‐dimethyl‐7‐benzofuranolate) as a catalyst, the polymerization of LA was studied in various solvents. LBF‐Na polymerized LA in dichloromethane, tetrahydrofuran, and toluene, and the catalytic activity of LBF‐Na was similar in these solvents. Among the sodium complexes, LOH‐Na (sodium complex bearing guaiacolate) exhibited higher catalytic activity (5 min, conversion = 93%; MnGPC = 7200) than the other tetranuclear sodium complexes. Furthermore, LOH‐Na exhibited an “immortal” property after 20 equivalents of benzyl alcohol loading during LA polymerization and good controllability in polylactide production. Thus, guaiacolate ligands increase the catalytic activity of sodium catalysts in LA polymerization in both coordinating and halogen‐containing solvents and under the condition of extra alcohol loading.

Heteroleptic Aluminum Allyls as Initiators for Lactide Polymerization

Organoaluminum complexes with the formula [(NHC)AlCl3-nA′n] (NHC = IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) or IDipp (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); A′ = [1,3-(SiMe3)2C3H3]−; n = 0–2) were synthesized either through solution or mechanochemical methods. Although an NHC adduct with [AlA′3] did not form, [(IDipp)AlCl2A′] and [(IMes)AlClA′2] were obtained and X-ray crystallography confirmed their construction around Al in a distorted tetrahedral environment. The [(NHC)AlCl3-nA′n] complexes, [AlA′3], and [Al(OiPr)3] (as a reference) were examined for their ability to polymerize l-lactide via ring-opening polymerization to produce polylactide. All of them did so with varying degrees of effectiveness. Of the organoaluminum species, the [(NHC)AlCl3] complexes were very weak initiators, and [AlA′3] the most effective, producing polymer molecular weights up to 49 kDa (Mw), with the mixed chloro/allyl complexes in between. All initiators were slower than [Al(OiPr)3].

Synergistic effect of star‐shaped architecture and poly(ethylene glycol) moieties on poly(lactic acid) stereocomplex crystallization behaviors

AbstractStereocomplex (sc) crystal formation represents an important strategy to improve the physical properties of poly(lactic acid) (PLA), including enhanced thermal resistance, anti‐hydrolysis and mechanical performance. Therefore, understanding and controlling the sc crystallization behaviors is an important and widely studied research topic. Herein, we investigate the cooperative effect of star‐shaped PLA chain topology and poly(ethylene glycol) incorporation on sc formation. We synthesize a series of poly(lactides), including poly(l‐lactic acid) (PLLA), poly(d‐lactic acid), star‐shaped PLLA and poly(d‐lactic acid)‐poly(ethylene glycol)‐poly(d‐lactic acid) (DED) triblock copolymer, via ring‐opening polymerization of lactide, followed by solution blending to form PLA stereocomplexes (scPLAs). The chemical structure of the prepared scPLAs is characterized by Fourier transform infrared and nuclear magnetic resonance, whereas the crystallization behavior and spherulite morphology are investigated by wide‐angle X‐ray diffraction, DSC and polarized optical microscopy. Linear PLLA/star‐shaped poly(d‐lactic acid) presents rapid crystallization behaviors and high crystallinity due to the excellent nucleation ability of star‐shaped chain topology. For isothermal crystallization, star‐shaped PLLA/DED exhibits the fastest crystallization kinetics, the highest crystallinity and the lowest crystallization half‐time of all samples. It is found that addition of poly(ethylene glycol) segments can increase chain mobility and backbone flexibility, further facilitating the crystallization of scPLA containing star‐shaped chain architecture. These results demonstrate an effective approach to cooperatively promote scPLA formation, with potential use for practical applications. © 2022 Society of Industrial Chemistry.

In Situ Ring-Opening Polymerization of L-lactide on the Surface of Pristine and Aminated Silica: Synthesis and Metal Ions Extraction.

The development of functional materials from food waste sources and minerals is currently of high importance. In the present work, polylactic acid (PLA)/silica composites were prepared by in situ ring-opening polymerizations of L-lactide onto the surface of pristine (Silochrom) and amine-functionalized (Silochrom-NH2) silica. The characteristics of the ring-opening polymerization onto the surface of modified and unmodified silica were identified and discussed. Fourier transform infrared spectroscopy was used to confirm the polymerization of lactide onto the silica surface, and thermogravimetric analysis determined that PLA constituted 5.9% and 7.5% of the composite mass for Silochrom/PLA and Silochrom-NH2/PLA, respectively. The sorption properties of the composites with respect to Pb(II), Co(II), and Cu(II) ions were investigated, and the effect of contact time, initial metal ion concentration, and initial pH were evaluated. Silochrom-NH2/PLA composites were found to have a higher adsorption capacity than Silochrom/PLA for all chosen ions, with the highest adsorption value occurring for Pb2+ at 1.5 mmol/g (90% removal efficiency). The composites showed the highest performance in the neutral or near-neutral pH (created by distilled water or buffer pH 6.86) during the first 15 min of phase contact. The equilibrium characteristics of adsorption were found to follow the Langmuir isotherm model rather than the Freundlich and Temkin models. Perspective applications for these PLA/silicas include remediation of industrial wastewater or leaching solutions from spent lead-acid and Li-ion batteries.

Ring opening polymerization of lactide promoted by Zinc and Magnesium complexes with a N-heterocyclic carbene-phenoxy-imine hybrid non-innocent ligand

Zinc and magnesium complexes of N-heterocyclic carbene (NHC)-phenoxy-imine ligands derived from chiral (S,S)-trans-1,2-diaminocyclohexane have been synthetized. Either monomeric or dimeric species have been obtained depending on the reaction conditions. All complexes were tested as catalysts for the ring opening polymerization (ROP) of lactide showing moderate activity and control degree. MALDI-ToF-MS spectra of PLA highlighted that magnesium complex promoted the formation of cyclic PLA, whereas zinc analogous and its monomeric species gave linear PLA, suggesting that different polymerization mechanisms occur depending on the metal center. Moreover, according to MALDI-ToF-MS spectra, in the absence of additional initiators, polymers arising from zinc catalysts would bear the NHC-phenoxy-imine hybrid ligand as terminal unit. Computational studies were performed in order to shed light on catalyst behaviors and on the mechanisms involved in the ROP of lactide. Free energy profiles of the initiation step suggest that microstructural differences lie in the ligand role. Indeed, in the absence of additional initiators, the ligand participates to the initiation involving its NHC moiety when wrapped around magnesium and its phenoxy-imine moiety when bound to zinc.

Taking the Next Step—Model‐Based Analysis of Robust and Non‐Toxic Zn Catalysts for the Ring Opening Polymerization of Lactide in the Polymer Melt

AbstractIn the field of bioplastics, complete kinetic models are rare but needed for industrial scale‐up. Herein, a complete kinetic model is developed to describe the behavior of robust and nontoxic guanidine carboxy Zn complexes in the lab‐scale ring opening polymerization (ROP) of l‐lactide, mimicking industrial melt polymerization in the presence of a co‐initiator. The model includes inter‐ and intramolecular transesterification and random chain scission, considering the single site activity of the studied catalyst class, based upon the “asme”‐ligand already established in lactide ROP. This allows for the description of reaction rate constants and the prediction of conversion as well as molar mass and dispersity of the obtained polymer. It is further shown that the model can be applied to describe the same characteristics for the faster, next generation catalysts based on the archetype “asme” catalyst. This lays the foundation for a faster catalyst design, targeting the needs of industry in developing fast and nontoxic alternatives for the industrially used toxic ROP catalyst Sn octanoate, bridging the gap between academia and industry.

N-alkyl-β-ketoiminate zinc complexes: Synthesis, structure, and reactivity in ring-opening polymerization of lactide

A series of N-alkyl-β-ketoiminate monozinc complexes [{tBuCN(R)CHC(O)CtBu}2Zn] (R = Me, 2a; Et, 2b; Pr, 2c; iPr, 2d; Cy, 2e) were synthesized from the reaction of diethyl zinc with the corresponding β-ketimines. All these zinc complexes were characterized by NMR, elemental analysis and HRMS spectroscopy. The molecular structures of complexes 2b-2e were further confirmed by single-crystal X-ray diffraction. The structures exhibit that each zinc center is four-coordinate, and surrounded by two bidentate β-ketoiminate ligands, forming a distorted tetrahedral geometry. These homoleptic bis(β-ketoiminate) zinc complexes are active towards the ring-opening polymerization of rac-lactide in the absence of alcohol, yielding over 90 % conversions within 1 h at 70 °C in toluene with the ratio of monomer to catalyst being 100.

Defining Stereochemistry in the Polymerization of Lactide by Aluminum Catalysts: Insights into the Dual-Stereocontrol Mechanism.

Aspects of the proposed pathway combining chain-end and enantiomorphic site control for the stereospecific polymerization of lactide (LA) were investigated through studies of aluminum complexes supported by enantiopure and racemic bipyrrolidine-based salan ligands, Lig1AlOBn and Lig2AlOBn. Spectroscopic analysis of stoichiometric initiation reactions and the definition of the stereochemistry of the selective formation of the "match" single-insertion products by X-ray crystallography led to key conclusions about the observed stereocontrol. Notably, it was determined to rely heavily on the preference for the trio of stereocenters around the metal to have a "match" formation (RR-ligand + S-polymer), which works synergistically with the enantiomorphic site preference of the catalyst to ring-open next to a stereocenter of a monomer of the same chirality as that of the ligand, resulting in highly heterotactic or syndiotactic PLA from rac- or meso-LA, respectively.

Redispersible Polymer Powders with High Bio‐Based Content from Core–Shell Nanoparticles

AbstractRedispersible polymer powders (RDPPs), i.e., additives obtained from core–shell nanoparticles and commercialized in the form of a dry powder, find intensive application in the concrete industry. However, they are mainly produced from fossil resources. Therefore, the development of bio‐based RDPPs is important to reduce the carbon footprint of these additives. In this work, two types of core–shell nanoparticles with a high percentage of bio‐based content are synthesized and show to be good candidates as RDPPs. In the first case, up to 75% of bio‐based content is obtained by combining lauryl acrylate, derived from coconut and palm kernel oil, as main core material, with isobornyl methacrylate, coming from pine resin, exploited to create the outer harder shell. In the second case, a degradable macromonomer obtained by the ring opening polymerization of lactide using 2‐hydroxyethyl methacrylate as initiator is used as the core‐forming monomer to obtain degradable RDPPs. In both cases, the particles are synthesized with a two‐step emulsion polymerization process conducted in one pot and then spray‐dried to obtain the RDPPs of interest. The morphology and redispersibility of the powders are characterized. Finally, their use as concrete additives is preliminarily assessed by evaluating their effect on changes in the surface morphologies of concrete specimens.

N‐Heterocyclic Carbene‐Based Zinc Complexes: Same Precursors for Different Lactide Ring‐Opening Polymerization Mechanisms.

AbstractTwo zinc complexes bearing NHC ligands were synthesized and fully characterized by NMR spectroscopy. Their behavior in the Ring‐Opening Polymerization (ROP) of lactides was investigated under several conditions of reaction. They showed to be efficient initiators, producing polylactides with molecular weights depending on the cocatalysts. Microstructural analysis of poly(rac‐LA) by1H NMR spectroscopy revealed a slightly enriched‐heterotactic PLAs withPrvalues up to 0.66. Inspection of the kinetic parameters for L‐LA andrac‐LA allowed to compare thekobsof the studied complexes1and2under different conditions, and showed that propagations present the usual pseudo‐first‐order dependence on monomer concentration. Hence, some reactivity studies were carried out, addressed to unravel the nature of the active species in the polymerization processes conducted in the presence of different catalytic systems, namely zinc complexes alone and in combination with neutral or ionic cocatalysts.

Lactide Polymerization Using Zinc Dichloride Complexes Containing a Neutral Bidentate Ligand with a Diacylated Cyclic Guanidine

Lactide Polymerization Using Zinc Dichloride Complexes Containing a Neutral Bidentate Ligand with a Diacylated Cyclic Guanidine

Highly Active Organocatalysts for Stereoselective Ring-Opening Polymerization of Racemic Lactide at Room Temperature.

Although significant advances have been achieved, highly stereocontrolled polymerization using organocatalysts is still a great challenge, such as ring-opening polymerization of racemic lactide (rac-LA) for the synthesis of stereoregular polylactide (PLA). In this context, a series of binary organocatalysts consisting of different phosphazenes (CTPB, C3N3-Me-P3, C3N3-Py-P3, t-BuP2, and t-BuP4) and achiral ureas (U1-U6) were applied for the stereocontrolled ROP of rac-LA under mild conditions. It is remarkable that C3N3-Py-P3/U4 with the compatible basicity/acidity showed both high activity (92% conversion within 10 min) and great stereoselectivity (Pm = 0.92) at room temperature (20 °C), and the resultant stereoblock PLA had predictable molar mass, narrow distribution (Đ = 1.07), and high melting temperature (Tm = 190 °C). Interactions involved among phosphazene, urea, and initiator were investigated by an in situ NMR technique. It was found that C3N3-Py-P3 reacted with benzyl alcohol (BnOH) to form a relatively loose ion pair in the presence of U4, which accounted for both high activity and stereoselectivity. Kinetics studies for different LA monomers at 20 °C showed kobs-1 (rac-LA) = 0.212 min-1, kobs-1 (D-LA) = 0.311 min-1, and kobs-1 (L-LA) = 0.317 min-1, indicative of the chain end control mechanism for stereocontrolled ROP.

Reaching High Stereoselectivity and Activity in Organocatalyzed Ring-Opening Polymerization of Racemic Lactide by the Combined Use of a Chiral (Thio)Urea and a N-Heterocyclic Carbene.

Stereochemical control during polymerization is a key strategy of polymer chemistry to achieve semicrystalline engineered plastics. The stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA), which can lead to highly isotactic polylactide (PLA), is one of the emblematic examples in this area. Surprisingly, stereoselective ROP of rac-LA employing chiral organocatalysts has been under-leveraged. Here we show that a commercially available chiral thiourea (TU1), or its urea homologue (U1), can be used in conjunction with an appropriately selected N-heterocyclic carbene (NHC) to trigger the stereoselective ROP of rac-LA at room temperature in toluene. Both a high organic catalysis activity (>90% monomer conversion in 5-9 h) and a high stereoselectivity (probability of formation of meso dyads, Pm, in the range 0.82-0.93) can be achieved by thus pairing a NHC and a chiral amino(thio)urea. The less sterically hindered and the more basic NHC, that is, a NHC bearing tert-butyl substituents (NHCtBu), provides the highest stereoselectivity when employed in conjunction with the chiral TU1 or U1. This asymmetric organic catalysis strategy, as applied here in polymerization chemistry, further expands the field of possibilities to achieve bioplastics with adapted thermomechanical properties.

Effective Ligand Design: Zinc Complexes with Guanidine Hydroquinoline Ligands for Fast Lactide Polymerization and Chemical Recycling

Invited for this month's cover is the group of Sonja Herres-Pawlis at RWTH Aachen University. The image shows how the zinc guanidine complexes help a circular economy of bioplastics by mediating the ring-opening polymerization of lactide and the depolymerization of polylactide (PLA). The Research Article itself is available at 10.1002/cssc.202201075.

Biocompatible and Photostable Photoacoustic Contrast Agents as Nanoparticles Based on Bodipy Scaffold and Polylactide Polymers: Synthesis, Formulation, and In Vivo Evaluation.

We have designed a new Bodipy scaffold for efficient in vivo photoacoustic (PA) imaging of nanoparticles commonly used as drug nanovectors. The new dye has an optimized absorption band in the near-infrared window in biological tissue and a low fluorescence quantum yield that leads to a good photoacoustic generation efficiency. After Bodipy-initiated ring-opening polymerization of lactide, the polylactide-Bodipy was formulated into PEGylated nanoparticles (NPs) by mixing with PLA-PEG at different concentrations. Formulated NPs around 100 nm exhibit excellent PA properties: an absorption band at 760 nm and a molar absorption coefficient in between that of molecular PA absorbers and gold NPs. Highly improved photostability compared to cyanine-labeled PLA NPs as well as innocuity in cultured macrophages were demonstrated. After intravenous injection in healthy animals, NPs were easily detected using a commercial PA imaging system and spectral unmixing, opening the way to their use as theranostic agents.

Front Cover: Effective Ligand Design: Zinc Complexes with Guanidine Hydroquinoline Ligands for Fast Lactide Polymerization and Chemical Recycling (ChemSusChem 18/2022)

Front Cover: Effective Ligand Design: Zinc Complexes with Guanidine Hydroquinoline Ligands for Fast Lactide Polymerization and Chemical Recycling (ChemSusChem 18/2022)