Chemical upcycling of poly(lactide) plastic waste to lactate ester,lactide and new poly(lactide) under Mg-catalysis condition

Chemical upcycling of end-of-life poly(lactide) plastics to lactide, lactate ester and new poly(lactide) has been achieved by using magnesium bis[bis(trimethylsilyl)amide] [Mg(HMDS)₂] as promoter. Mg(HMDS)₂ showed high efficiency in l-lactide polymerization and poly(lactide) depolymerization. Mg(HMDS)₂/Ph₂CHOH catalytic system displayed high ring-opening selectivity and the characteristic of immortal polymerization. Taking advantage of transesterification, depolymerizations of end-of-life poly(lactide) plastics to lactate ester (polymer to value-added chemicals) and lactide (polymer to monomer) were achieved with high yields. Besides, a new “depolymerization-repolymerization” strategy was proposed to directly transform poly(lactide) into new poly(lactide). This work provides a theoretical basis for the design of polymerization and depolymerization catalysts and promotes the development of degradable polymers.     

Resorbable initiators for polymerizations of lactones

Numerous nontoxic (resorbable) salts were prepared from cations and anions belonging to the human metabolism, such as Na, K, Mg, Ca, Zn and Fe in combination with chloride, iodide, hydroxide, carbonate, acetate, stearate, glycolate, L‐lactate, D‐mandelate and various N‐substituted α‐amino acids. All these salts were used as initiators for polymerizations of L‐lactide in bulk at 100–180°C. Furthermore, Grignard reagents, hemin and hematin were included in this study. Zn L‐lactate was found to be the most useful initiator in terms of reactivity, maximum molecular weight of the isolated poly(L‐lactide) and its optical purity. Zn L‐lactate initiated copolymerizations of L‐lactide and glycolid or L‐lactide and ϵ‐caprolactone were also studied. Finally, first studies of the polymerization mechanism of zinc stearate and zin 2‐ethylhexanoate were performed. They suggest that zinc carboxylates combined with an alcohol as coinitiator form reactive zinc alkoxides which are the true initiators at temperatures < 150°C.     

低毒锌类催化剂制备聚乳酸的研究

采用低毒锌类催化剂制备了一系列具有高分子量、不同光学纯度及热力学性质的聚乳酸材料. 以金属锌作催化剂制备丙交酯, 研究了不同裂解温度对产物光学纯度的影响. 随后在低毒催化剂乳酸锌的作用下使丙交酯开环聚合, 进一步研究了单体的光学纯度对聚乳酸立体规整性的影响, 以及聚合过程中的结晶对聚合物分子量和热力学行为的影响. 并用旋光仪、核磁共振氢谱(1H NMR)、凝胶渗透色谱(GPC)、差示扫描量热分析(DSC)等方法对产物进行表征. 结果表明, 合适的裂解温度有利于合成高光学纯度的丙交酯; 在低毒乳酸锌的催化作用下, 高光学纯度的单体以及聚合过程中的结晶都有利于制备高分子量聚乳酸.     

Bioresorbable Hydrogels Prepared Through Stereocomplexation between Poly(L‐lactide) and Poly(D‐lactide) Blocks Attached to Poly(ethylene glycol)

Block copolymers were synthesized by ring‐opening polymerization of L ‐lactide or D ‐lactide in the presence of mono‐ or dihydroxyl poly(ethylene glycol), using zinc metal as catalyst. The resulting copolymers were characterized by various techniques, namely ¹H NMR spectroscopy, differential scanning calorimetry (DSC), X‐ray diffractometry, and Raman spectrometry. The composition of the copolymers was designed such that they were water soluble. Bioresorbable hydrogels were prepared from aqueous solutions containing both poly(L ‐lactide)/poly(ethylene glycol) and poly(D ‐lactide)/poly(ethylene glycol) block copolymers. Rheological studies confirmed the formation of hydrogels resulting from stereocomplexation between poly(L ‐lactide) and poly(D ‐lactide) blocks.

Ring‐opening polymerization of L (D )‐lactide in the presence of dihydroxyl PEG using zinc powder as catalyst.     

Synthesis and cationic ring-opening polymerization of mono- and bifunctional spiro orthoesters containing ester groups and depolymerization of the obtained polymers: An approach to chemical recycling for polyesters as a model system

A spiro orthoester having an ester moiety, 2-acetoxymethyl-1,4,6-trioxaspiro[4.6]undecane (4) was synthesized, and its cationic polymerization and depolymerization of the obtained polymer (5) were carried out. The monomer 4 underwent cationic polymerization with a cationic catalyst to afford the corresponding poly(cyclic orthoester) 5. The obtained polymer 5 could be depolymerized with a cationic catalyst to regenerate the monomer 4 in an excellent yield. Further, bifunctional spiro orthoesters (6, 8, 9) having diester moieties were synthesized from terephthalic acid, succinic acid, and 1,4-cyclohexanedicarboxylic acid, and their acid-catalyzed reversible crosslinking–decrosslinking was examined. The bifunctional monomer 6 derived from terephthalic acid underwent cationic crosslinking to afford the corresponding network polymer (7), which could be also depolymerized to regenerate the original bifunctional monomer 6. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2551–2558, 1999     

Photoinitiation. IV. The effect of lewis acid on the vinyl monomer polymerization photoinitiated by aromatic hydrocarbons

Polymerization of acrylonitrile photoinitiated by naphthalene, anthracene, phenanthrene, and pyrene is accelerated by an admixture of zinc (II) chloride, acetate, or nitrate. The effect of zinc (II) salts on the rate of pyrene-photoinitiated polymerization of acrylonitrile leads to an increase in this rate in the order Zn/OCOCH_3/2 < ZnCl₂ < Zn/NO_3/2. The maximum polymerization rate is achieved at the molar ratio [ZnCl₂]/([ZnCl₂] + [pyrene]) approximately 0.7. In contrast to the photoinitiated polymerization of acrylonitrile, the methyl methacrylate admixture of zinc (II) chloride exerts a smaller effect on the polymerization rate. In the pyrene-photoinitiated polymerization of styrene an admixture of zinc (II) chloride retards the polymerization rate. Fluorescence of aromatic hydrocarbon in the system acrylonitrile–aromatic hydrocarbon is efficiently quenched by zinc (II) chloride. Stern–Volmer constants determined for pyrene (80 dm³ mole^?1), phenanthrene (66 dm³ mole^?1), and naphthalene (49 dm³ mole^?1) are higher by about 2–3 orders of the Stern–Volmer constants for fluorescence quenching of aromatic hydrocarbons by acrylonitrile in the absence of ZnCl₂. The fluorescence of anthracene in acrylonitrile is not quenched by ZnCl₂. The acceleration effect of Zn (II) salts on the polymerization of acrylonitrile photoinitiated by aromatic hydrocarbons depends on two factors: an increase in the ratio of the rate constant of the growth and termination reactions, k_p/k_t, and an increase in the quenching constant of fluorescence of aromatic hydrocarbon, k_q, by the complex {acrylonitrile…ZnCl₂}. ZnCl₂ thus influences both the growth and initiation reactions of the polymerization process.     

Acyclic diene metathesis (ADMET) depolymerization of functionalized furan-based polymers

Acyclic dience metathesis (ADMET) depolymerization of functionalized furan-based polymers prepared via aqueous ring-opening polymerization of 7-oxanobornenes has been investigated. Results indicate that while very high molecular weight poly [exo-N-methyl-7-oxabicyclo [2.2.1] hept-2,5-diene-2,3-dicarboximide] can be depolymerized to oligomers with ease, poly [2,3-dicarbomethoxy-7-oxabicyclo [2.2.1] hept-2,5-diene] is more resistant to depolymerization under similar conditions. This difference may be due to differential interaction of the carbonyls in the side chains with the metal atom of the catalyst in the proposed metallacyclobutane intermediate. ADMET depolymerization of poly [2,3-bis (trifluoromethyl)-7-oxabicyclo [2.2.1] hept-2,5-diene] was feasible, however, the extent of depolymerization was decreased due to the use of a coordinating solvent (THF) used during the depolymerization process. © 1994 John Wiley & Sons, Inc.     

About formation of cycles in Sn(II) octanoate‐catalyzed polymerizations of lactides

At first, formation of cycles in commercial poly(l ‐lactide)s is discussed and compared with benzyl alcohol‐initiated polymerizations performed in this work. This comparison was extended to polymerizations initiated with 4‐cyanophenol and pentafluorothiophenol which yielded cyclic polylactides via end‐biting. The initiator/catalyst ratio and the acidity of the initiator were found to be decisive for the extent of cyclization. Further polymerizations of l ‐lactide were performed with various diphenols as initiators/co‐catalysts. With most diphenols, cyclic polylactides were the main reaction products. Yet, only catechols yielded even‐numbered cycles as main reaction products, a result which proves that their combination with SnOct₂ catalyzed a ring‐expansion polymerization (REP). The influence of temperature, time, co‐catalyst, and catalyst concentrations was studied. Four different transesterification reactions yielding cycles were identified. For the cyclic poly(l ‐lactide)s weight average molecular weights (M_w's) up to 120,000 were obtained, but ¹H NMR end group analyses indicated that the extent of cyclization was slightly below 100%. The influence of various parameters like structure of initiator and catalyst and temperature on the formation of cyclic poly(l ‐lactide)s has been investigated. Depending on the chosen conditions, the course of the polymerization can be varied from a process yielding exclusively linear polylactides to mainly cyclic polylactides. Three different reaction pathways for cyclization reactions have been identified. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1915–1925     

Synthesis and characterization of novel poly(ester carbonate)s based on pentaerythritol

Novel poly(ester carbonate)s were synthesized by the ring‐opening polymerization of L ‐lactide and functionalized carbonate monomer 9‐phenyl‐2,4,8,10‐tetraoxaspiro[5,5]undecan‐3‐one derived from pentaerythritol with diethyl zinc as an initiator. ¹H NMR analysis revealed that the carbonate content in the copolymer was almost equal to that in the feed. DSC results indicated that T_g of the copolymer increased with increasing carbonate content in the copolymer. Moreover, the protecting benzylidene groups in the copolymer poly(L ‐lactide‐co‐9‐phenyl‐2,4,8,10‐tetraoxaspiro[5,5]undecan‐3‐one) were removed by hydrogenation with palladium hydroxide on activated charcoal as a catalyst to give a functional copolymer, poly(L ‐lactide‐co‐2,2‐dihydroxylmethyl‐propylene carbonate), containing pendant primary hydroxyl groups. Complete deprotection was confirmed by ¹H NMR and FTIR spectroscopy. The in vitro degradation rate of the deprotected copolymers was faster than that of the protected copolymers in the presence of proteinase K. The cell morphology and viability on a copolymer film evaluated with ECV‐304 cells showed that poly(ester carbonate)s derived from pentaerythritol are good biocompatible materials suitable for biomedical applications. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45:1737 –1745, 2007     

Synthesis and characterization of poly(L‐lactide)s and poly(D‐lactide)s of controlled molecular weight

High molecular weight poly(L ‐lactide)s (PLLAs) and poly(D ‐lactide)s (PDLAs) were synthesized in toluene at 70 °C by ring‐opening polymerization of optically pure L ‐lactide and D ‐lactide, using tin(II) 2‐ethylhexanoate (SnOct₂) and 2‐(2‐methoxyethoxy)ethanol as initiator and coinitiator, respectively. Under these conditions, polarimetry as well as ¹³C NMR spectroscopy indicated that the synthesized poly(lactide)s (PLAs) are more than 99% isotactic. The molecular weight was successfully controlled by adjusting the monomer‐to‐initiator molar ratio. Gel permeation chromatography and MALDI‐TOF mass spectrometry analyses showed that the polydispersity index of the PLAs is below 1.1. Moreover, MALDI‐TOF spectra showed two different chain distributions, one characterized by an even number of lactic acid repeat units and the other by an odd number of lactic acid repeat units. The second distribution, indicative of the presence of intermolecular transesterification reactions, appears at the very beginning of the polymerization and its intensity increases with the polymerization time. Finally, a reversible reaction kinetic model was used to determine the monomer equilibrium concentration ([M]_eq = 1.4 ± 0.5%) and the propagation rate constant (k_p = 14.4 ± 0.5 L mol^?1 h^?1) of the polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1944–1955, 2007     

Aspects of Living Radical Polymerization Mediated by Cobalt Porphyrin Complexes

Living radical polymerization (LRP) of methyl acrylate (MA), acrylic acid (AA), and vinyl acetate (VAc) mediated by cobalt(II) porphyrin complexes ((TMP)Co^II·, (TMPS)Co^II·) are reported. The polymeric products with relatively low polydispersity and controlled number average molecular weight (M_n) based on one polymer chain per cobalt complex demonstrate the living characters of the polymerization process. The formation of block copolymers of poly(methyl acrylate)‐b‐poly(vinyl acetate) (PMA‐b‐PVAc) and poly(methyl acrylate)‐b‐poly(vinyl pyrrolidone) (PMA‐b‐PVP) demonstrate another important feature of LRP and extend the application of cobalt porphyrin mediated radical polymerization to a wider array of functionalized monomers. Kinetic studies using ¹H NMR to follow the formation of orGano‐cobalt complexes reveal that two mechanisms, reversible termination (RT) and degenerative transfer (DT), occur during the polymerization process. MA and VAc polymerization mediated by cobalt porphyrin complexes are used to illustrate the properties of these two LRP pathways and evaluate the kinetic and thermodynamic properties for several of the central reactions.     

Neodymium Catalyst for the Polymerization of Dienes and Polar Vinyl Monomers

Ziegler–Natta catalysts have played a major role in industry for the polymerization of dienes and vinyl monomers. However, due to the deactivation of the catalyst, this system fails to polymerize polar vinyl monomers such as vinyl acetate, methyl methacrylate, and methyl acrylate. Herein, a catalytic system composed of NdCl₃⋅3TEP/TIBA is reported, which promotes a quasi‐living polymerization of dienes and is also active for the homopolymerization of polar vinyl monomers. Additionally, this catalytic system generates polymyrcene‐b‐polyisoprene and poly(myrcene)‐b‐poly(methyl methacrylate) diblock copolymers by sequential monomer addition. To encourage the replacement of petroleum‐based polymers by environmentally benign biobased polymers, polymerization of β‐myrcene is demonstrated with a catalytic activity of ≈106 kg polymer mol Nd⁻¹ h⁻¹.     

Synthesis and characterization of multiblock copolymers composed of poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one) outer blocks and poly(L‐lactide) inner blocks

Ethylene glycol (EG) initiated, hydroxyl‐telechelic poly(L ‐lactide) (PLLA) was employed as a macroinitiator in the presence of a stannous octoate catalyst in the ring‐opening polymerization of 5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one (MBC) with the goal of creating A–B–A‐type block copolymers having polycarbonate outer blocks and a polyester center block. Because of transesterification reactions involving the PLLA block, multiblock copolymers of the A–(B–A)_n–B–A type were actually obtained, where A is poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one), B is PLLA, and n is greater than 0. ¹H and ¹³C NMR spectroscopy of the product copolymers yielded evidence of the multiblock structure and provided the lactide sequence length. For a PLLA macroinitiator with a number‐average molecular weight of 2500 g/mol, the product block copolymer had an n value of 0.8 and an average lactide sequence length (consecutive C₆H₈O₄ units uninterrupted by either an EG or MBC unit) of 6.1. For a PLLA macroinitiator with a number‐average molecular weight of 14,400 g/mol, n was 18, and the average lactide sequence length was 5.0. Additional evidence of the block copolymer architecture was revealed through the retention of PLLA crystallinity as measured by differential scanning calorimetry and wide‐angle X‐ray diffraction. Multiblock copolymers with PLLA crystallinity could be achieved only with isolated PLLA macroinitiators; sequential addition of MBC to high‐conversion L ‐lactide polymerizations resulted in excessive randomization, presumably because of residual L ‐lactide monomer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6817–6835, 2006     

Synthesis and characterization of novel biodegradable block copolymer poly(ethylene glycol)‐block‐poly(L‐lactide‐co‐2‐methyl‐2‐carboxyl‐propylene carbonate)

An amphiphilic block copolymer, poly(ethylene glycol)‐block‐poly(L ‐lactide‐co‐2‐methyl‐2‐benzoxycarbonyl‐propylene carbonate) [PEG‐b‐P(LA‐co‐MBC)], was synthesized in bulk by the ring‐opening polymerization of L ‐lactide with 2‐methyl‐2‐benzoxycarbonyl‐propylene carbonate (MBC) in the presence of poly(ethylene glycol) as a macroinitiator with diethyl zinc as a catalyst. The subsequent catalytic hydrogenation of PEG‐b‐P(LA‐co‐MBC) with palladium hydroxide on activated charcoal (20%) as a catalyst was carried out to obtain the corresponding linear copolymer poly(ethyleneglycol)‐block‐poly(L ‐lactide‐co‐2‐methyl‐2‐carboxyl‐propylenecarbonate) [PEG‐b‐P(LA‐co‐MCC)] with pendant carboxyl groups. DSC analysis indicated that the glass‐transition temperature (T_g) of PEG‐b‐P(LA‐co‐MBC) decreased with increasing MBC content in the copolymer, and T_g of PEG‐b‐P(LA‐co‐MCC) was higher than that of the corresponding PEG‐b‐P(LA‐co‐MBC). The in vitro degradation rate of PEG‐b‐P(LA‐co‐MCC) in the presence of proteinase K was faster than that of PEG‐b‐P(LA‐co‐MBC), and the cytotoxicity of PEG‐b‐P(LA‐co‐MCC) to chondrocytes from human fetal arthrosis was lower than that of poly(L ‐lactide). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4771–4780, 2005     

Dual zinc catalysts for L‐lactide polymerization and reaction of CO2 with cyclohexene oxide

A series of zinc complexes, [ L ^X ZnEt] ( 1–5 ) and [ L ^X Zn ₂ (OAc) ₃ ] (6–9) , associated with NNO‐tridentate Schiff base ligands (2‐(((2‐((cyclohexyl[methyl]amino)methyl)phenyl)imino)methyl)phenolate (CAP) derivatives), were synthesized, and their activity toward ring‐opening polymerization (ROP) of L‐lactide (LA) and the reaction of CO₂ with cyclohexene oxide were also investigated. All of [ L ^X ZnEt] revealed excellent catalytic activity to ring‐opening polymerization (ROP) of LA in the presence of benzyl alcohol. Among them, [ L ^H ZnEt] (1) showed the highest activity with 82% conversation within 45 s. In contrast, [L ^X Zn ₂ (OAc) ₃ ] (6–9) were inactive in ROP of L‐lactide. In addition, all of these Zn complexes demonstrated moderate activity in the reaction of CO₂ with cyclohexene oxide in the presence of Bu₄NCl.     

Well-defined polyisocyanates via organotitanium(IV) catalyzed living polymerizations of isocyanates

The development of living organotitanium(IV) catalyzed polymerizations is presented. A living polymerization of alkyl isocyanates was developed using TiCl₃OCH₂CF₃, I, as a catalyst, through which large quantities of well-defined (in terms of molecular weight and polydispersity) polyisocyanates are obtainable under ambient conditions. η⁵-CpTiCl₂OCH₂CF₃ (Cp = cyclopentadienyl), II, is also an excellent catalyst for the polymerization of isocyanates and, unlike I, will polymerize monomers containing Lewis basic functional groups in the side chain. Compound II was used to synthesize poly(allyl isocyanate) and poly(2-isocyanatoethyl methacrylate). Both polymers were found to be soluble, in contrast to previous reports, and spectroscopic characterization showed that no crosslinking of the side chains had occurred.     

The selective recycling of mixed plastic waste of polylactic acid and polyethylene terephthalate by control of process conditions

The glycolysis of postconsumer polyethylene terephthalate (PET) waste was evaluated with catalysts of zinc acetate, zinc stearate and zinc sulfate, showing that zinc acetate was the most soluble and effective. The chemical recycling by solvolysis of polylactic acid (PLA) and PET waste in either methanol or ethanol was investigated. Zinc acetate as a catalyst was found to be necessary to yield an effective depolymerization of waste PLA giving lactate esters, while with the same reaction conditions PET remains as an unconverted solid. This provides a strategy to selectively recycle mixed plastic waste by converting one plastic to a liquid and recovering the unreacted solid plastic by filtration.     

Ring-opening polymerization of L-lactide by means of different iron compounds

L-lactide was bulk-polymerized in the presence of various commercially available iron compounds. The polymerization temperature was in the range of 140 and 230 °C, the monomer to initiator/catalyst ratio varied between 100 and 10 000, and the polymerization time between 30 minutes and 24 hours. Iron oxides, iron chlorides and sulfuric iron compounds were low efficient and are not suitable for melt polymerization of lactide. The oxidation state was noticed not to affect the efficiency. Ferrocene required long polymerization times and a high concentration of the compound before a high molar mass polymer was received. Organic iron salts were also found to be efficient initiators/catalysts, except for the hydrated iron(III)citrate. Especially iron(II)acetate caused a rapid polymerization with a high conversion and molar mass.     

Highly active polymerization of methyl methacrylate catalyzed by Pd-based β-ketoiminato complexes/MAO systems

The substituted β-ketoiminato palladium(II) complexes, Pd[CH₃C(O)CHC(NAr)CH₃](Pph₃)(Me) (1 Ar = α-napthyl, 2 Ar = fluorenyl), when was activated by methylaluminoxane (MAO), were used as highly active catalysts for methyl methacrylate (MMA) polymerization. The effects of temperature, co-catalyst to catalyst molar ratio and polymerization time on catalyst activities were reported. Structural analyses of the polymers by ¹H NMR spectra indicated that polymerization yielded poly(methyl methacrylate) (PMMA) with moderate syndiotactic microstructures. The polymerization results showed that PMMAs obtained using these complexes possibly arose from a coordination-insertion mechanism rather than a radical mechanism.     

Aluminum salen and salan complexes in the ring‐opening polymerization of cyclic esters: Controlled immortal and copolymerization of rac‐β‐butyrolactone and rac‐lactide

Aluminum‐based salen and salan complexes mediate the ring‐opening polymerization (ROP) of rac‐β‐butyrolactone (β‐BL), rac‐lactide, and ε‐caprolactone. Al‐salen and Al‐salan complexes exhibit excellent control over the ROP of rac‐β‐butyrolactone, yielding atactic poly(3‐hydroxybutyrate) (PHB) with narrow PDIs of <1.15 for Al‐salen and <1.05 for Al‐salan. Kinetic studies reveal pseudo‐first‐order polymerization kinetics and a linear relationship between molecular weight and percent conversion. These complexes also mediate the immortal ROP of rac‐β‐BL and rac‐lactide, through the addition of excess benzyl alcohol of up to 50 mol eq., with excellent control observed. A novel methyl/adamantyl‐substituted Al‐salen system further improves control over the ROP of rac‐lactide and rac‐β‐BL, yielding atactic PHB and highly isotactic poly(lactic acid) (P_m = 0.88). Control over the copolymerization of rac‐lactide and rac‐β‐BL was also achieved, yielding poly(lactic acid)‐co‐poly(3‐hydroxybutyrate) with narrow PDIs of <1.10. ¹H NMR spectra of the copolymers indicate a strong bias for the insertion of rac‐lactide over rac‐β‐BL. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013