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.     

Macromolecular Architecture-Nature as a Model for Degradable Polymers

Abstract

Nature usually combines polymers with short degradation times with polymers having long degradation times in an energy and material optimized process involving hierarchical systems. Sometimes a natural system of polymers has evolved to degrade in a month, sometimes in many years. The building blocks of the plant and animal kingdom are biopolymers which are either oxidizable or hydrolyzable. In natural composites, combinations of the two are common, e.g., in wood. Current trends in polymer research and marketing of plastics indicate an increasing demand for the development of a diversity of degradable polymer products with a predetermined service-life. We identify four main routes to design degradable polymers. The goal is to tailor-make a material which is more susceptible to environmental degradation factors (e.g., hydrolysis, biodegradation, photooxidation). The most convenient route is to use cheap synthetic bulk polymers and add a biodegradable or photooxidizable component. A more expensive solution is to change the chemical structure by introducing hydrolyzable or oxidizable groups in the repetitive chain of a synthetic polymer. The third route to degradable polymers is to use biopolymers or derivatives of these where the bacterial polyhydroxyalkanoates are perhaps the most studied material of them all. The fourth route is to tailor-make new hydrolyzable structures e.g., polyesters, polyanhydrides, and polycarbonates.     

Water‐soluble,biocompatible polyphosphazenes with controllable and pH‐promoted degradation behavior

The synthesis of a series of novel, water‐soluble poly(organophosphazenes) prepared via living cationic polymerization is presented. The degradation profiles of the polyphosphazenes prepared are analyzed by GPC, ³¹P NMR spectroscopy, and UV–Vis spectroscopy in aqueous media and show tunable degradation rates ranging from days to months, adjusted by subtle changes to the chemical structure of the polyphosphazene. Furthermore, it is observed that these polymers demonstrate a pH‐promoted hydrolytic degradation behavior, with a remarkably faster rate of degradation at lower pH values. These degradable, water soluble polymers with controlled molecular weights and structures could be of significant interest for use in aqueous biomedical applications, such as polymer therapeutics, in which biological clearance is a requirement and in this context cell viability tests are described which show the non‐toxic nature of the polymers as well as their degradation intermediates and products. © 2013 The Authors Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 287–294     

Photodegradable plastics: end-of-life design principles

Abstract

Photochemically degradable polymers and plastics are reviewed with an emphasis on the environmental and molecular factors that control the onset of degradation and the rate of degradation. A number of principles are beginning to emerge for the design of viable photochemically degradable plastics. Among the principles discussed are those relating to the effects of chromophores, initiators, antioxidants, temperature, oxygen diffusion into the plastic, polymer crystallinity, tensile and compressive stress, and the absorbed light intensity on the plastic. To obtain a plastic with a controlled lifetime and a specific rate of degradation, many of these parameters can be controlled or adjusted in the design stage of the plastic.     

Synthesis of degradable and chemically recyclable polymers using 4,4-disubstituted five-membered cyclic ketene hemiacetal ester (CKHE) monomers

Novel degradable and chemically recyclable polymers were synthesized using five-membered cyclic ketene hemiacetal ester (CKHE) monomers. The studied monomers were 4,4-dimethyl-2-methylene-1,3-dioxolan-5-one (DMDL) and 5-methyl-2-methylene-5-phenyl-1,3-dioxolan-4-one (PhDL). The two monomers were synthesized in high yields (80–90%), which is an attractive feature. DMDL afforded its homopolymer with a relatively high molecular weight (M_n >100 000, where M_n is the number-average molecular weight). DMDL and PhDL were copolymerized with various families of vinyl monomers, i.e., methacrylates, acrylates, styrene, acrylonitrile, vinyl pyrrolidinone, and acrylamide, and various functional methacrylates and acrylate. Such a wide scope of the accessible polymers is highly useful for material design. The obtained homopolymers and random copolymers of DMDL degraded in basic conditions (in the presence of a hydroxide or an amine) at relatively mild temperatures (room temperature to 65 °C). The degradation of the DMDL homopolymer generated 2-hydroxyisobutyric acid (HIBA). The generated HIBA was recovered and used as an ingredient to re-synthesize DMDL monomer, and this monomer was further used to re-synthesize the DMDL polymer, demonstrating the chemical recycling of the DMDL polymer. Such degradability and chemical recyclability of the DMDL polymer may contribute to the circular materials economy.

Novel degradable and chemically recyclable polymers were synthesized using five-membered cyclic ketene hemiacetal ester (CKHE) monomers.     

In vitro and in vivo degradation behaviors of thermosensitive poly(organophosphazene) hydrogels

Biodegradable and thermosensitive poly(organophosphazenes) with various substituents were synthesized and their hydrolytic degradation properties were investigated in vitro and in vivo. The aqueous solutions of all polymers showed a sol-gel phase transition behavior depending on temperature changes. The side groups of polymers significantly affected the polymer degradation and accelerated hydrolysis of polymers in the order of carboxylic acid > depsipeptide > without carboxylic acid and depsipeptide. The increased gel strength led to the decreased hydrolysis rate. The polymer hydrogels with 750 Da of α-amino-ω-methoxy poly(ethylene glycol) were rapidly decreased by dissolution. The polymer degradation was also influenced by pH and temperature. The in vivo behaviors of mass decrease of the polymer hydrogels were similar with the in vitro results. These results suggest that the biodegradable and thermosensitive poly(organophosphazenes) hold great potentials as an injectable and biodegradable hydrogel for biomedical applications with controllable degradation rate.     

Is a polymer semiconductor having a “perfect” regular structure desirable for organic thin film transistors?

This study utilized high temperature NMR and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry to reveal that appreciable amounts of structural defects are present in the diketopyrrolopyrrole (DPP)–quaterthiophene copolymers (PDQT) synthesized by the Stille coupling polymerization with Pd(PPh₃)₂Cl₂, Pd₂(dba)₃/P(o-tol)₃, and Pd(PPh₃)₄ catalyst systems. It was proposed that these structural defects were produced via homocoupling side reactions of the C–Br bonds and the organostannane species. Model Stille coupling reactions further substantiated that the amount of structural defects are catalyst-dependent following the order of Pd(PPh₃)₂Cl₂ > Pd₂(dba)₃/P(o-tol)₃ > Pd(PPh₃)₄. To verify the structural assignments, “perfect” structurally regular PDQT polymers were prepared using Yamamoto coupling polymerization. When compared to the structurally regular polymers, the polymers containing defects exhibited notable redshifts in their absorption spectra. Surprisingly, the “perfect” structurally regular polymers showed poor molecular ordering in thin films and very low charge transport performance as channel semiconductors in organic thin film transistors (OTFTs). On the contrary, all the “defected” polymers exhibited much improved molecular ordering and significantly higher charge carrier mobility.     

Mechanochemical generation of acid-degradable poly(enol ether)s

In an effort to develop polymers that can undergo extensive backbone degradation in response to mechanical stress, we report a polymer system that is hydrolytically stable but unmasks easily hydrolysable enol ether backbone linkages when force is applied. These polymers were synthesized by ring-opening metathesis polymerization (ROMP) of a novel mechanophore monomer consisting of cyclic ether fused bicyclohexene. Hydrogenation of the resulting polymers led to significantly enhanced thermal stability (T_d > 400 °C) and excellent resistance toward acidic or basic conditions. Solution ultrasonication of the polymers resulted in up to 65% activation of the mechanophore units and conversion to backbone enol ether linkages, which then allowed facile degradation of the polymers to generate small molecule or oligomeric species under mildly acidic conditions. We also achieved solid-state mechano-activation and polymer degradation via grinding the solid polymer. Force-induced hydrolytic polymer degradability can enable materials that are stable under force-free conditions but readily degrade under stress. Facile degradation of mechanically activated polymechanophores also facilitates the analysis of mechanochemical products.

A mechanically responsive polymer system that is hydrolytically stable without stress, but unmasks enol ether backbone linkages under force to allow facile hydrolytic degradation.     

Gel formation in cationic polymerization of divinyl ethers. II. 2,2-Bis{4-[(2-vinyloxy)ethoxy]phenyl}propane

Cationic polymerization of 2,2-bis{4-[(2-vinyloxy)ethoxy]phenyl}propane [CH₂CH O CH₂CH₂O C₆H₄ C(CH₃)₂ C₆H₄ OCH₂CH₂ O CHCH₂; 2], a divinyl ether with oxyethylene units adjacent to the polymerizable vinyl ether groups and a bulky central spacer, was investigated in CH₂Cl₂ at 0°C with the diphenyl phosphate [(C₆H₅O)₂P(O)OH]/zinc chloride (ZnCl₂) initiating system. The polymerization proceeded quantitatively and gave soluble polymers up to 85% monomer conversion. In the same fashion as the polymerization of 1,4-bis[2-vinyloxy(ethoxy)]benzene (CH₂CH O CH₂CH₂O C₆H₄ OCH₂CH₂ O CHCH₂; 1) that we already studied, the content of the unreacted pendant vinyl ether groups of the produced soluble polymers decreased with monomer conversion, and almost all the pendant vinyl ether groups were consumed in the soluble products prior to gelation. Alternatively, endo-type double bonds were gradually formed in the polymer main chains by chain transfer reactions and other side reactions as the polymerization proceeded. The polymerization behavior of isobutyl vinyl ether (3), a monofunctional vinyl ether, under the same conditions, showed that the endo-type olefins in the polymer backbones are of no polymerization ability with the growing active species involved in the present polymerization systems. These results indicate that the intermolecular crosslinking reactions occurred primarily by the pendant vinyl ether groups, and the final stage of crosslinking process leading to gelation also may occur by the small amount of the residual pendant vinyl ether groups (supposedly less than 2%). The formation of the soluble polymers that almost lack the unreacted pendant vinyl ether groups is most likely due to the frequent occurrence of intramolecular crosslinking reactions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1931–1941, 1999     

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (g_abs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (g_PL) of the order of 10⁻³ were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of g_abs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (g_EL) of 1.09 × 10⁻⁴ and −1.02 × 10⁻⁴ at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications.     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 3. Poly(hydroxyethyl methacrylate) with chemical derivatization

Molecular depth profiling of polymers by secondary ion mass spectrometry (SIMS) has focused on the use of polyatomic primary ions due to their low penetration depth and high damage removal rates in some polymers. This study is the third in a series of systematic characterizations of the effect of polymer chemistry on degradation under polyatomic primary ion bombardment. In this study, time‐of‐flight SIMS (ToF‐SIMS) was used to assess 5 keV SF₅⁺‐induced damage of ～90 nm thick spin‐cast poly(2‐hydroxyethyl methacrylate) (PHEMA) and ～130 nm thick trifluoroacetic anhydride‐derivatized PHEMA (TFAA‐PHEMA) films. The degradation of these polymers under extended SF₅⁺ bombardment (～2 × 10¹⁴ ions cm^?2) was compared to determine the effect of the pendant group chemistry on their degradation. The sputter rate and ion‐induced damage accumulation rate of PHEMA were similar to a poly(n‐alkyl methacrylate) of similar pendant group length, suggesting that the addition of a terminal hydroxyl group to the alkyl pendant group does not markedly change the stability of poly(n‐alkyl methacrylates) under SF₅⁺ bombardment. The sputter rate and ion‐induced damage accumulation rate of TFAA‐PHEMA were much higher than a poly(n‐alkyl methacrylate) of similar pendant group length, suggesting that derivatization of the terminal hydroxyl group can significantly reduce degradation of the polymer under SF₅⁺ bombardment. This result is in good agreement with the literature on the thermal and radiation‐induced degradation of fluorinated poly(alkyl methacrylates), which suggests that the electron‐withdrawing fluorinated pendant group increases the probability of depolymerization. Copyright © 2004 John Wiley & Sons, Ltd.     

Polymères porteurs de greffons peptidiques—III. Étude de l'hydrolyse des polymères issus de la polymérisation de monomères styréniques para-substitués par des oligopeptides

Hydrolysis of polymers I and II with a peptide group in the pendant chains has been studied. Their hydrolysed products can be characterised by GPC in THF.A preliminary study, with N-benzoylglycine and poly[N-(-4 vinylbenzoyl) ethylamine], allows comparison of two hydrolysis methods: by concentrated HCl in dioxane and by Na₂O₂. Generalization of acid hydrolysis to structures I and II shows that amide hydrolysis is difficult and that the length of peptide

pendant chains has an influence. Only polymers I are, after hydrolysis, completely soluble in tetrahydrofuran and can be characterised by GPC. Values of DP_n are around 80.     

Aconityl-derived polymers for biomedical applications. Modeling study of cis–trans isomerisation

 Soluble polymers have been prepared that are designed to undergo enhanced rates of hydrolysis at pH values less than that observed in blood circulation. The degradable element in the polymer mainchain is derived from cis-aconityl acid and is defined by a carboxylic acid pendent functionality (C-4) that is cis across a double bond to an amide at C-1 in the polymer mainchain. While degradation studies in vitro have confirmed these polymers do undergo enhanced rates of degradation at acidic pH values, there is also increasing evidence that during the degradation process the double bond isomerises to the trans configuration and thus prevents the full degradation of a polymer. From a molecular modelling perspective we are seeking to understand the propensity for this cis–trans isomerisation and the mechanism of this cis–trans isomerisation is discussed. Received: 29 April 2002 / Accepted: 6 September 2002 / Published online: 14 February 2003     

Investigation of the degradation mechanism of cross-linked polyethyleneimine by NMR spectroscopy

The degradation pathway of a cross-linked polyethyleneimine (c-PEI) suitable as a gene carrier was studied at different pH values by real-time ¹H NMR and diffusion-weighted ¹H NMR spectroscopy. The c-PEI was synthesized by cross-linking PEI segments with 2,4-pentanediol diacrylate (PDDA). The experimental results show that under basic, neutral and acidic conditions, the degradation of c-PEI occurs through hydrolysis of the ester moieties of the PDDA linkers. The degradation half-life of the polymer is 22, 48 and more than 720 h at pH 10.2, 7.4 and 4.6, respectively, showing that the degradation of c-PEI is highly pH sensitive. By using a modified version of diffusion-weighted ¹H NMR experiment, the variation of the apparent molecular weight of c-PEI in the degradation process was monitored. Furthermore, a Monte Carlo simulation was applied to simulate the relation between the average molecular weight and the number of chain ends during the degradation process of a model system of c-PEI. By comparing the NMR results with those obtained from simulation, the mechanism of degradation under various pH conditions is discussed. The present work demonstrates that the combination of real-time ¹H NMR, diffusion-weighted ¹H NMR spectroscopy and Monte Carlo simulation is a useful strategy for characterizing the degradation process of degradable polymers.     

Design and recognition of cucurbituril-secured platinum-bound oligopeptides

Platinum terpyridyl complexes, stacked on top of one another and secured as dimers with cucurbit[8]uril (CB[8]) in aqueous medium, were functionalized quantitatively and in situ with a pair of pentapeptides Phe-(Gly)₃-Cys by grafting their cysteine residues to the Pt centers. The resulting CB[8]·(Pt·peptide)₂ assemblies were used to target secondary hosts CB[7] and CB[8] via their pair of phenylalanine residues, again in situ. A series of well-defined architectures, including a supramolecular “pendant necklace” with hybrid head-to-head and head-to-tail arrangements inside CB[8], were obtained during the self-sorting process after combining only 3 or 4 simple building units.

A platinum terpyridyl complex, pentapeptide Phe-(Gly)₃-Cys and cucurbit[8]uril assemble into a “pendant necklace” with hybrid head-to-head and head-to-tail arrangements in aqueous medium.     

Synthesis and biodegradation of poly(2-pyrrolidone-co-ε-caprolactone)s

Copolyesteramides of 2-pyrrolidone with ε-caprolactone were synthesized by ring-opening copolymerization. The copolymers were random-like and their melting temperature and heat of fusion were dependent on the polymer composition. Biodegradation by a polyamide 4 (PA4) degrading microorganism showed rapid degradation in the region of amide-rich polymer composition. On the contrary, enzymatic hydrolysis using a lipase resulted in a different tendency, that is, ester-rich copolymers hydrolyzed rapidly. Activated sludge makes copolymers degrade to CO₂ in wide polymer composition ratio. Copolyesteramides are expected to be applied as an environmentally-friendly plastics or bioabsorbable polymers in medical fields.     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 1. Effect of main chain and pendant methyl groups

Polyatomic primary ions have been applied recently to the depth profiling of organic materials by secondary ion mass spectrometry (SIMS). Polyatomic primary ions offer low penetration depth and high damage removal rates in some polymers, but the relationship between polymer chemistry and degradation under polyatomic primary ion bombardment has not been studied systematically. In this study, positive and negative ion time‐of‐flight SIMS (ToF‐SIMS) was used to measure the damage of ～100 nm thick spin‐cast poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA) and poly(methacrylic acid) (PMAA), films under extended (～2 × 10¹⁴ ions cm^?2) 5 keV SF₅⁺ bombardment. These polymers were compared to determine the effect of the main chain and pendant methyl groups on their degradation under SF₅⁺ bombardment. The sputter rate of PMMA was approximately twice that of PMA or PMAA and the rate of damage accumulation was higher for PMA and PMAA than PMMA, suggesting that the main chain and pendant methyl groups played an important role in the degradation of these polymers under SF₅⁺ bombardment. These results are consistent with the literature on the thermal and radiation‐induced degradation of these polymers, which show that removal of the main chain or pendant methyl groups reduces the rate of depolymerization and increases the rate of intra‐ or intermolecular cross‐linking. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis and properties of polyether nitrile copolymers with pendant methyl groups

Polyether nitrile and polyether nitrile copolymers with pendant methyl groups were prepared by the nucleophilic substitution reaction of 2,6^′-dichlorobenzonitrile with hydroquinone (HQ) and with varying mole proportions of HQ and methyl hydroquinone (MeHQ) using N-methyl pyrrolidone solvent in the presence of anhydrous K₂CO₃. The polymers were characterised by different physico-chemical techniques. The crystallinity of the polymers was found to decrease with increase in concentration of the MeHQ units in the polymer. Thermogravimetric studies showed that all the polymers were stable up to 450 °C with a char yield above 50% at 900 °C in N₂ atmosphere. The glass transition temperature and activation energy of the polymers was found to increase with increase in concentration of the MeHQ units in the polymer.     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 2. Poly(n‐alkyl methacrylates)

Polyatomic primary ions offer low penetration depth and high damage removal rates in some polymers, facilitating their use in the molecular depth profiling of these polymers by secondary ion mass spectrometry (SIMS). This study is the second in a series of systematic characterizations of the effect of polymer chemistry on degradation under polyatomic primary ion bombardment. In this study, time‐of‐flight SIMS (ToF‐SIMS) was used to measure the damage of ～90 nm thick spin‐cast poly(methyl methacrylate), poly(n‐butyl methacrylate), poly(n‐octyl methacrylate) and poly(n‐dodecyl methacrylate) films under extended (～2 × 10¹⁴ ions cm^?2) 5 keV SF₅⁺ bombardment. The degradation of the poly(n‐alkyl methacrylates) were compared to determine the effect of the length of the alkyl pendant group on their degradation under SF₅⁺ bombardment. The sputter rate and stability of the characteristic secondary ion intensities of these polymers decreased linearly with alkyl pendant group length, suggesting that lengthening the n‐alkyl pendant group resulted in increased loss of the alkyl pendant groups and intra‐ or intermolecular cross‐linking under SF₅⁺ bombardment. These results are partially at variance with the literature on the thermal degradation of these polymers, which suggested that these polymers degrade primarily via depolymerization with minimal intra‐ or intermolecular cross‐linking. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigation of the degradation behaviour of poly(ethylene glycol-co-d,l-lactide) copolymer

The aim was to investigate the degradation behaviour of poly(ethylene glycol-co-d,l-lactide) (PEG-d,l-PLA) multiblock copolymer, in bulk and as microspheres, in aqueous medium. The degradation behaviour of PLA homopolymers in bulk and microspheres was evaluated as comparison.Microsphere preparation was performed by the double emulsion solvent evaporation method. Physical-chemical characterization of the raw polymers and the microspheres was performed by nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry (MDSC). Polymer molecular weight, before and after incubation in aqueous environment, was evaluated by GPC; water uptake and mass loss were determined gravimetrically.The presence of PEG segments inside PLA chains gave a characteristic spongy structure to the microspheres. A significant increase in polymer T_g values was found for the microsphere formulations compared to polymer in bulk. After 63 days of incubation in the aqueous environment, the PEG-d,l-PLA microspheres achieved an average M_w reduction of 47% compared to 20% for PLA microspheres. The corresponding M_w decrease of the polymers in bulk was significantly higher: 72% and 41% for PEG-d,l-PLA and PLA, respectively.The data show how the degradation behaviour of polymer in bulk in an aqueous environment is significantly different from the behaviour of the corresponding microspheres. These results highlight the importance of performing a thorough physical-chemical characterization on microsphere formulations.