Catalysis as an Enabling Science for Sustainable Polymers

In the paper

“This is in contrast to the carbon cycle for petroleum feedstocks, in which the rate and time scale of CO2 sequestration (millions of years) is much slower than the use and release (1-10 year time frame).”

In the paper

“That means, LCA results are not available before the production of a product, much less so during the design of a product as a result of inadequate data on environmental impacts.”

In the paper

“Such bio-derived and biodegradable polymers are therefore emerging * DGp° = standard free energy of polymerization as attractive alternatives to conventional petroleum-based polymers.”

In the paper

“In order for this process to be energetically efficient, plastic materials should optimally be derived from polymerization reactions that are not strongly exergonic to ensure that, once generated, the materials can be catalytically depolymerized or chemically recycled with minimal energy inputs”

In the paper

“This requires engineering of either the polymer composition/formulation or its degradation pathway so that it can be broken down by microorganisms to primarily CO2 and H2O in a reasonable time frame.”

In the paper

“Third, quantitative metrics of environmental sustainability [7][8][9][10] are not nearly as well advanced or universally accepted as metrics of economic sustainability.”

In the paper

“This is a systems-level challenge, as the economic incentives and infrastructure for recovery and re-use are not nearly as well developed as those for production of plastics.”

In the paper

“In the United States in 2014, over 75% of plastics wastes were landfilled, 39 with the remainder being either recycled (primarily HDPE and PET) or incinerated.”

In the paper

“The majority focus on the use of renewable resources for feedstocks and monomers for plastics, particularly biomass-derived monomers and processes for the generation of polymers derived from these monomers.”

In the paper

“Although LCA ideally should take into account all stages of the product life cycle, i.e. cradle-to-cradle, the scope of LCA is often limited to cradle-to-gate due to the lack of a complete understanding and inadequate data on the environmental fate and impact of a polymer.”

In the paper

“In one example, they showed that isomerizing alkoxycarbonylation of methyl oleate catalyzed by a palladium catalyst bearing phosphorus ligands can convert the internal double bond deep in the hydrocarbon chain to a terminal ester group with high yield and selectivity.100”

What the sources say

• Not checked yetcites [100] source

In the paper

“and the latter is the key to build a circular economy that increases resource utilization and reduces waste generation.”

In the paper

“Oilseed crops such as soybean, rapeseed, and oil palm are the major supply of oils at present, but algae has turned up as a cheaper alternative due to its non-food nature, fast production and recycle rate, and low use of production inputs.82”

What the sources say

• Not checked yetcites [82] source

In the paper

“Carbohydrates are the most exploited and abundant bio-feedstock for producing building blocks for these polymers.”

In the paper

“Therefore, one important message from this study is that switching from petroleum polymers to those derived from renewable resources does not necessarily reduce environmental impacts.”

In the paper

“A variety of long-chain a,w-diacids can be produced through oxidative cleavage of the alkene group of fatty acids.33”

What the sources say

• Not checked yetcites [33] source

In the paper

“These fatty-acid-derived α,ω-difunctional monomers provide access to a class of long-chain aliphatic polyesters with unique properties that bridge the gap between semicrystalline polyolefins and the traditional polycondensates.”

In the paper

“Furan-2,5-dicarboxylic acid (FDCA) is a useful bio-based feedstock for the production of aromatic polyesters.103”

What the sources say

• Not checked yetcites [103] source

In the paper

“However, it should be noted that plastics derived from renewable feedstocks are not necessarily sustainable.”

In the paper

“The most well-established approach is based on a dehydration process to convert C6 sugars to hydroxymethylfurfural (HMF), which is then oxidized to FDCA.106”

What the sources say

• Not checked yetcites [106] source

In the paper

“Due to the oxygen-rich character of polyesters and polycarbonates, they can be readily sourced from a wide variety of renewable feedstocks.”

In the paper

“Its physical properties allow it to replace petroleum-based plastics in some types of packaging and fiber applications.”

In the paper

“Due to the relatively high costs and limited material properties, renewable plastics currently make up only a small share of the global plastics market.”

In the paper

“Currently, most of the renewable monomers are produced from sugar-or starch-rich crops such as corn and sugar cane.”

In the paper

“For example, a MnCo2O4 supported Ru catalyst was found to afford excellent yield of FDCA (99%) from base-free air-oxidation of HMF in water.110”

What the sources say

• Not checked yetcites [110] source

In the paper

“In addition to these two major classes of biomass, terpenes and CO2 are also useful feedstocks for polyesters and polycarbonates.16,83”

What the sources say

• Not checked yetcites [16] source
• Not checked yetcites [83] source

In the paper

“1,4-diacids, including succinic, fumaric, and itaconic acids, are some of the most successful platform chemicals obtained from carbohydrate biorefinery.91”

What the sources say

• Not checked yetcites [91] source

In the paper

“One disadvantage of this transformation is that only part (about half) of the fatty acid chain is utilized in the α,ω-difunctional product and the rest is lost as a coupling product of less value.”

In the paper

“The conversion typically involves a mechanical/physical pretreatment (milling, extraction, steam reforming), followed by multiple biological, chemical, or thermochemical processes.77”

What the sources say

• Not checked yetcites [77] source

In the paper

“However, none of these routes are economically feasible yet.”

In the paper

“First, the monomers of many polyesters and polycarbonates can be derived from biomass or waste gases via biological or chemical transformations.”

In the paper

“Vanadyl phosphate catalysts can convert fructose to FDCA in 97% selectivity at 84% conversion; 112”

What the sources say

• Not checked yetcites [112] source

In the paper

“Given the multiple-step nature of the synthesis from not readily available platform chemicals, these monomers are not ideal candidates for sustainable polymers.”

In the paper

“ROP of these macrolactones generates long-chain polyesters with higher and more predicable molecular weights than the ones prepared from the polycondensation of fatty acids.”

What the sources say

• Not checked yetcites [181] source

In the paper

“Au/TiO2 catalysts with addition of small amount of Pt can convert succinic acid to g-BL with 97% selectivity at 97% conversion.173”

What the sources say

• Not checked yetcites [173] source

In the paper

“However, the overall yields of all these routes are below 10%.”

In the paper

“These rigid diols have proven particularly useful for the synthesis of polyester and polycarbonates with high glass transition temperatures.88,154”

What the sources say

• Not checked yetcites [88] source
• Not checked yetcites [154] source

In the paper

“The reaction of furoic acid with molten Cs2CO3 under a CO2 flow or 8 bar CO2 can give FDCA 2-in 70-89% yield within 12 h.”

What the sources say

• Not checked yetcites [116] source

In the paper

“It has been industrially produced on a large scale (over 50,000 tons annually) by aerobic fermentation of glucose from corn syrup by a genetically modified strain of E. coli, developed by DuPont Tate & Lyle BioProducts. 137”

What the sources say

• Not checked yetcites [137] DuPont Tate & Lyle BioProducts. http://www.duponttateandlyle.com/susterra(accessed May 2, 2017).

In the paper

“Lactic acid is a versatile precursor to a variety of monomers, including lactide, propylene glycol, propylene oxide, and acrylic acid.33”

What the sources say

• Not checked yetcites [33] source

In the paper

“Dimethyl carbonate (DMC) can be produced by this method in yields of 40-60% based on acetal.185”

What the sources say

• Not checked yetcites [185] source

In the paper

“Co(acac)3 in a silica sol-gel system gives 99% selectivity to FDCA from fructose at 72% conversion.113”

What the sources say

• Not checked yetcites [113] source

In the paper

“Zhang found that the coupling of 3-chloro-1-propanol with CO2 in the presence of 1.1 equiv. of Cs2CO3 proceeded efficiently under mild conditions (40 °C, 1 atm CO2) to provide trimethylene carbonate in 95% yield.200”

What the sources say

• Not checked yetcites [200] source

In the paper

“A leading technology for the synthesis of BDO from renewable resources is through reduction of bio-based succinic acid.142”

What the sources say

• Not checked yetcites [142] bioamber. https://www.bio-amber.com/bioamber/en/products(accessed May 2, 2017).

In the paper

“Although the step-growth method has been known since the discovery of synthetic polymers in early 20 th century, 212 it is still the most frequently used method for the synthesis of polyesters and polycarbonates in industry.”

What the sources say

• Not checked yetcites [212] source

In the paper

“These catalytic polycondensations are typically conducted at very high temperature (>200 °C) and high vacuum to reach high conversion.”

In the paper

“For example, aliphatic polyesters with high glass transition temperatures (Tg > 60 °C) can be prepared by using a rigid diol such as isosorbide.214”

What the sources say

• Not checked yetcites [214] source

In the paper

“This two-stage procedure has enabled the synthesis of polyesters and polycarbonates with molecular weights over 50 kDa.215,223”

What the sources say

• Not checked yetcites [215] source
• Not checked yetcites [223] source

In the paper

“Metal salts such as titanium alkoxides, tin alkoxides, zinc or magnesium carboxylates are commonly used catalysts for the step-growth synthesis of polyesters and polycarbonates.[224][225][226][227]228,229”

What the sources say

• Not checked yetcites [224] source
• Not checked yetcites [225] source
• Not checked yetcites [226] source

In the paper

“For example, SGP of plant oils derived diols and diacids with > 20 methylene sequences generates polyesters with structures and thermal properties reminiscent of PE. 213”

What the sources say

• Not checked yetcites [213] source

In the paper

“The crystallinity and biodegradation rate of poly(alkylene carbonates) can be adjusted by employing diols with different chain lengths.215,216”

What the sources say

• Not checked yetcites [215] source
• Not checked yetcites [216] source

In the paper

“Methyl 9,10-epoxystearate (MES) derived from oleic acid has been used to prepare polyesters with extremely low Tg (-44 °C).207”

What the sources say

• Not checked yetcites [207] source

In the paper

“Moreover, the use of multifunctional monomers such as glycerol and citric acid enables a facile synthesis of biodegradable thermosets.217”

What the sources say

• Not checked yetcites [217] source

In the paper

“For example, Ti(O i Pr)4 can mediate the polycondensation of several bio-derived diesters (succinate, adipate, sebacate) with 1,4-butanediol under high temperature (up to 240 °C) and in vacuo to generate aliphatic polyesters with Mn > 20 kDa.225,230,231”

What the sources say

• Not checked yetcites [225] source
• Not checked yetcites [230] source
• Not checked yetcites [231] source

In the paper

“Given that many diols and diacids can be readily obtained from biomass (Section 2.1 and 2.2), SGP provides a straightforward route to renewable polyesters and polycarbonates.”

In the paper

“Organic carbonates such as dimethyl carbonate (DMC) are currently employed as a safer and greener substitute for phosgene.”

In the paper

“Melt condensation of ω-hydroxyl fatty acids, e.g.OH(CH2)13COOH, catalyzed by Ti(O i Pr)4 via a two-stage polymerization (200 °C under N2 then 220 °C under 0.1 mmHg) generate polyesters with molecular weights ranging from 50 to 110 kDa.232”

What the sources say

• Not checked yetcites [232] source

In the paper

“Kobayashi developed a unique catalytic system [250][251][252] using a surfactant-like acid, such as dodecylbenzenesulfonic acid (DBSA), to mediate the emulsion polycondensation of aliphatic carboxylic acids and alcohols in water.”

What the sources say

• Not checked yetcites [250] source
• Not checked yetcites [251] source
• Not checked yetcites [252] source

In the paper

“Recently, several studies have demonstrated the potential of using CO2 as a renewable and safe carbonyl reagent for the synthesis of polycarbonates (Scheme 5).”

In the paper

“This was the first time that a high-molecular-weight linear polyester was produced from itaconic acid without isomerization or cross-linking of the vinylic double bond.”

What the sources say

• Not checked yetcites [233] source

In the paper

“A few studies using Bi(OTf)3 to polymerize plant oil-237 and suberin-derived 238 long-chain fatty acids generated highly crystalline polyesters with high melting and degradation temperatures as attractive alternatives to petroleum-based polyolefins.”

What the sources say

• Not checked yetcites [237] source
• Not checked yetcites [238] source

In the paper

“This mild and environmentally benign system has been successfully applied to several fatty-acid type monomers.253,254”

What the sources say

• Not checked yetcites [253] source
• Not checked yetcites [254] source

In the paper

“Because of their strong Lewis acidity, they can catalyze polycondensation reactions at much milder temperatures (<100 °C).”

In the paper

“Metal triflates have become a class of popular polycondensation catalysts for this reason.”

In the paper

“Meier recently used Sn(Oct)2 to polymerize dimethyl itaconate and long chain diols in the presence of 0.5 wt% of 4-methoxyphenol as a radical inhibitor to make linear unsaturated polyesters with Mn up to 11500 Da. 233”

What the sources say

• Not checked yetcites [233] source

In the paper

“However, the high costs and long reaction times of enzyme catalysts largely limit their utility in large-scale manufacture.257,260”

What the sources say

• Not checked yetcites [257] source
• Not checked yetcites [260] source

In the paper

“Takasu found that scandium triflate (Sc(OTf)3) selectively esterifies primary hydroxyl groups over the secondary or tertiary alcohol.241”

What the sources say

• Not checked yetcites [241] source

In the paper

“Kadokawa reported a PR3/CBr4/base system (Scheme 5a) to facilitate the direct polycondensation of CO2 with alkane diols at room temperature and ambient pressure.261,262”

What the sources say

• Not checked yetcites [261] source
• Not checked yetcites [262] source

In the paper

“They developed a chemoselective polycondensation of tartaric acid or malic acid with polyols such as glycerol and sorbitol under mild conditions (60 °C, 0.3-3.0mmHg, 0.5 mol% Sc(OTf)3) to afford linear polyesters (Mn between 4 and 26 kDa) with negligible branching or crosslinking.”

What the sources say

• Not checked yetcites [241] source

In the paper

“SGP mediated by mild and selective enzyme catalysis also provides a metal-free and eco-friendly route to polyesters and polycarbonates.[255][256][257][258][259]”

What the sources say

• Not checked yetcites [255] source
• Not checked yetcites [256] source
• Not checked yetcites [257] source

In the paper

“Several classes of organocatalysts have been used to catalyze the SGP of diols with diacids or carbonates, including strong Brønsted acids such as H2SO4, triflic acid, p-toluenesulfonic acid (PTSA), bis(trifluoromethanesulfonyl)-imide (Tf2NH), 243,244 and organic bases such as TBD, 221,245-247 MTBD, 248 1-n-butyl-3-methylimidazol-2-carboxylate.249”

What the sources say

• Not checked yetcites [243] source
• Not checked yetcites [244] source
• Not checked yetcites [221] source

In the paper

“Using Bi(OTf)3 as the catalyst, the polycondensation of sebacic acid and various a,w-alkanediols can be performed at 80 °C under vacuum for 48 h to afford polyesters with Mn up to 30 kDa. 235,236”

What the sources say

• Not checked yetcites [235] source
• Not checked yetcites [236] source

In the paper

“For this reason, ROP has been the method of choice for the synthesis of tailor-made drug delivery systems where complex architectures and precise molecular weights are essential to their performance.269,270”

What the sources say

• Not checked yetcites [269] source
• Not checked yetcites [270] source

In the paper

“Because the thermodynamic driving force for ROP is highly dependent on the ring size and substitution pattern of the monomer, 273 only a limited set of cyclic esters and carbonates can be polymerized under practical conditions.”

What the sources say

• Not checked yetcites [273] Duda A., 2009

In the paper

“Recently a class of strained 8-memebered cyclic carbonates derived from diethanolamine were prepared and polymerized to well-defined N-substituted polycarbonates.283,284”

What the sources say

• Not checked yetcites [283] source
• Not checked yetcites [284] source

In the paper

“The first catalytic approach for the polycondensation between CO2 and diols was reported by Tomishige in 2016 using CeO2 as the catalyst and 2-cyanopyridine as the dehydrating agent (Scheme 5b).263”

What the sources say

• Not checked yetcites [263] source

In the paper

“As opposed to step-growth polymerizations, ROP allows for a fine degree of control over several important macromolecular parameters including molecular weight, polydispersity, end-group fidelity, regio-and stereoselectivity, and architecture (block, cyclic, star, etc). 267,268”

What the sources say

• Not checked yetcites [267] source
• Not checked yetcites [268] source

In the paper

“Ring-opening polymerization of cyclic esters and carbonates is an efficient method for the synthesis of aliphatic polyester and polycarbonates.”

In the paper

“This method was recently revisited by Feng and Gnanou, who improved the reaction efficiency by using Cs2CO3 as the base and 10 bar CO2 (Scheme 5d).265”

What the sources say

• Not checked yetcites [265] source

In the paper

“However, Chen recently found that at low temperatures (e.g.-40 °C), the ROP of γ-BL mediated by La[N(SiMe3)2]3 or tert-Bu-P4 can proceed to high conversions (60 ~ 90%).278”

What the sources say

• Not checked yetcites [278] source

In the paper

“ROP of a few macrolactones (ring size = 14, 16) have been successfully applied to prepare longchain polyesters with similar structures but higher and more predictable molecular weights than the ones obtained from the polycondensation of fatty acids.[279][280][281]”

What the sources say

• Not checked yetcites [279] source
• Not checked yetcites [280] source
• Not checked yetcites [281] source

In the paper

“Characterizations of the resulting polymers suggested an alternating incorporation of diols and dihalides into the polycarbonates.”

What the sources say

• Not checked yetcites [264] source

In the paper

“In this case, alternating ether and carbonate linkages were observed in the polymer.”

What the sources say

• Not checked yetcites [266] source

In the paper

“The polymerization also generated quantitative wastes including CHBr3, R3P=O, protonated bases.”

What the sources say

• Not checked yetcites [261] source
• Not checked yetcites [262] source

In the paper

“For small-and medium-size rings (ring size < 8), ring-opening is driven by enthalpy as a result of release of ring strain.”

In the paper

“In comparison to the direct coupling of CO2 and diols, the use of alkyl dihalides as a third condensation component has been more successful to generate polycarbonates with higher molecular weights.[264][265][266]”

What the sources say

• Not checked yetcites [264] source
• Not checked yetcites [265] source
• Not checked yetcites [266] source

In the paper

“Because of the strong nucleophilic and/or basic character of the anionic initiator/chain ends, transesterification and epimerization are commonly observed for these anionic ROPs.”

In the paper

“The relative rates of these steps are mainly controlled by the catalytic processes.”

In the paper

“Common to all chain-growth polymerizations, chain transfer can compete with chain propagation to undermine the control on the structure and molecular weight distribution of the resulting polymers.273”

What the sources say

• Not checked yetcites [273] Duda A., 2009

In the paper

“As the monomer and alkoxide chain end are both covalently bound to the metal center, the catalyst can exert a great level of control on the rate and selectivity of the polymerization.”

In the paper

“Strong acids and acylating agents can polymerize cyclic esters and carbonates through a cationic pathway.273”

What the sources say

• Not checked yetcites [273] Duda A., 2009

In the paper

“Therefore, the development of selective catalysts that favor the chain propagation over any chain transfer and termination events is critical for the synthesis of polyesters and polycarbonates of well-defined structures and architectures.”

In the paper

“Alkali metal complexes such as butyllithium 285 and potassium methoxide 286 can mediate anionic ROPs via a nucleophilic or enolate-type initiating mechanism.”

What the sources say

• Not checked yetcites [285] source
• Not checked yetcites [286] source

In the paper

“A variety of initiator/catalyst systems have been developed for ROP of cyclic esters and carbonates.268,270”

What the sources say

• Not checked yetcites [268] source
• Not checked yetcites [270] source

In the paper

“For example, a zinc catalyst with bdiiminate amido ligand ((BDI i Pr)Zn[N(SiMe3)2] exhibits excellent reactivity and selectivity for the ROP of a variety of lactones and cyclic carbonates.296”

What the sources say

• Not checked yetcites [296] source

In the paper

“The synthesis of high molecular weight polymers is rather difficult with these catalysts due to the strong tendency of intramolecular transesterification (cyclization) and proton transfer reactions of the cationic propagating species.288”

What the sources say

• Not checked yetcites [288] source

In the paper

“In the similar vein, Byers developed a chemoselective copolymerization of LA and epoxides using a redox-switchable iron catalyst (Scheme 8B).320”

What the sources say

• Not checked yetcites [320] source

In the paper

“In addition, this catalyst showed exceptionally high regioselectivity (< 1% regio-defects) for the ROP of methyl-substituted cyclic carbonates.298”

What the sources say

• Not checked yetcites [298] source

In the paper

“Recently, the use of switchable metal catalysts to strategically modulate the rate and monomer selectivity of ROP is emerging.267”

What the sources say

• Not checked yetcites [267] source

In the paper

“Studies of the resulting polymers have showed a strong correlation between the thermal and mechanical properties of PLAs and their stereosequences.300”

What the sources say

• Not checked yetcites [300] source

In the paper

“This distinct substrate selectivity allowed the one-pot copolymerization of the two monomers by in situ switching of the redox state of the catalyst to synthesize poly[(LA-minor-CL)-block-(CL-minor-LA)].”

What the sources say

• Not checked yetcites [319] source

In the paper

“Diaconescu reported several redox-switchable group 4 metal complexes supported by ferrocenyl-based ligand (Scheme 8A).319”

What the sources say

• Not checked yetcites [319] source

In the paper

“This innovative catalytic strategy has enabled the facile one-pot synthesis of block copolymers of polyesters and polycarbonates.”

In the paper

“Other metal catalysts such as zirconium complexes with bispyrrolidine-salan ligand 312 and zinc complexes with chiral or achiral auxiliary ligands [313][314][315][316] (Figure 7) have also been used to generate iso-enriched stereoblock PLAs from rac-LA.”

What the sources say

• Not checked yetcites [312] source
• Not checked yetcites [313] source
• Not checked yetcites [314] source

In the paper

“Syndiotactic poly(3-hydroxyalkanoate)s (PHAs) have been synthesized by stereospecific ROP of racemic b-lactones by several rare-earth metal catalysts.318”

What the sources say

• Not checked yetcites [318] source

In the paper

“Similar copolymerization of LA and epoxides were obtained using a redox-switchable zirconium complex.321”

What the sources say

• Not checked yetcites [321] source

In the paper

“With only 20 ppm of this catalyst, poly(trimethylene carbonate) with Mn as high as 237 kDa and Ð = 1.68 can be synthesized in quantitative yield within 2 h.297”

What the sources say

• Not checked yetcites [297] source

In the paper

“This allowed the formation of clean diblock copolymers.”

What the sources say

• Not checked yetcites [320] source

In the paper

“Recently, this approach was extended to a monomer mixture of epoxide, lactone, anhydride, and CO2 to synthesize a variety of block copolymers with great sequence control and predictable compositions in the polymer chains.323”

What the sources say

• Not checked yetcites [323] source

In the paper

“Coulembier developed a zwitterionic ammonium betaine catalyst that exhibited high selectivity for the ROP of lactide.333”

What the sources say

• Not checked yetcites [333] source

In the paper

“The synthesis of well-defined linear polyesters or polycarbonates with molecular weights over 100 kDa has been difficult using organocatalysts due to the competitive catalyst-initiated process at low alcohol initiator concentrations.”

In the paper

“PLA of Mn = 97 kDa (DP = 750) with molecular weight conformed to the monomer-to-alcohol ratios and Mw/Mn = 1.18 can be synthesized by this catalyst.”

What the sources say

• Not checked yetcites [333] source

In the paper

“Alternatively, they can activate the monomer through nucleophilic addition.”

What the sources say

• Not checked yetcites [326] source

In the paper

“Thus, repetitive introduction and removal of CO2 resulted in the formation of multiblock poly(carbonateblock-ester).”

What the sources say

• Not checked yetcites [322] source

In the paper

“Despite the weak basicity of aryloxide, these catalysts showed good reactivity toward L-LA owing to the supplementary ionic activation.”

What the sources say

• Not checked yetcites [333] source

In the paper

“Williams developed a strategy using CO2 as a chemical switch as well as a co-monomer for the one-pot chemoselective polymerization of epoxide and CL (Scheme 8C).322”

What the sources say

• Not checked yetcites [322] source

In the paper

“Catalytic systems combining both H-bond donor and H-bond acceptor moieties, such as thiourea/amine, can simultaneously activate the monomer and alcohol initiator/chain end.”

In the paper

“The living polymerizations of VL and CL mediated by NHO-1/Al(C6F5)3 generated linear polyesters with high molecular weights up to 855 kDa (DP = 3200) and narrow molecular weight distributions (Ð = 1.03 -1.63).”

What the sources say

• Not checked yetcites [335] source

In the paper

“They proposed a charged-assisted hydrogen-bonding mechanism for this catalyst where it activates the alcohol initiator/chain-end through a combination of hydrogen bonding between the aryloxide group and the H(d+) of ROH and the Coulomb attraction between the ammonium group and the O(d-) of ROH (Scheme 10).”

What the sources say

• Not checked yetcites [333] source

In the paper

“Organic bases such as N-heterocyclic carbenes (NHCs) and amidines can activate the alcohol initiator/chain ends through hydrogen bonding interactions.”

What the sources say

• Not checked yetcites [326] source

In the paper

“The replacement of metal catalysts with organic catalysts has emerged for the synthesis of plastics, especially for those used in biomedical and microelectronic applications.”

In the paper

“Recently a class of (thio)urea anions catalysts were disclosed that exhibit both high activity and selectivity for the ROP of a variety of cyclic esters and carbonates (Scheme 12).321,336”

What the sources say

• Not checked yetcites [321] source
• Not checked yetcites [336] source

In the paper

“But as soon as CO2 was removed, the catalyst turned on the ROP of CL with no incorporation of epoxide.”

What the sources say

• Not checked yetcites [322] source

In the paper

“Several attributes of organic catalysts, including their low cost and wide availability, easy removal from polymers, low toxicity, and versatile catalytic mechanisms, make possible a greener and more versatile synthesis of polyesters and polycarbonates.”

In the paper

“This has led to the development of zwitterionic ring-opening polymerization for the synthesis of high molecular weight polyesters 327 and polycarbonates 284 with cyclic topology.”

What the sources say

• Not checked yetcites [327] source
• Not checked yetcites [284] source

In the paper

“Depending on the nature of the catalyst, organocatalysts can operate through a variety of mechanisms (Scheme 9).326”

What the sources say

• Not checked yetcites [326] source

In the paper

“Their catalytic behavior largely depends on the ring size, (un)saturation of the backbone, and sterics on the exocyclic carbon.”

In the paper

“Recently, N-heterocyclic olefins (NHOs) have emerged as a class of active and versatile catalysts for the ROP of cyclic esters and carbonates.334,335”

What the sources say

• Not checked yetcites [334] source
• Not checked yetcites [335] source

In the paper

“Chen recently combined NHOs with a Lewis acid Al(C6F5)3 to develop a highly efficient catalytic system.335”

What the sources say

• Not checked yetcites [335] source

In the paper

“This catalyst exerts double stereo-differentiation for the ROP of rac-LA with kL/kD up to 53 (for TU-ICD, Figure 9).”

What the sources say

• Not checked yetcites [341] source

In the paper

“The rate of the polymerization can be tuned over several orders of magnitude by modifying the urea or thiourea substituents.”

What the sources say

• Not checked yetcites [336] source

In the paper

“DFT calculations suggested a bifunctional mechanism analogous to that of TBD, 321 where the (thio)imidiates behave as a proton shuttle to pick up the proton from the initiator/chain end, to stabilize the tetrahedral intermediate via double H-bonds, and to deliver the proton back to the chain end after ring-opening.”

What the sources say

• Not checked yetcites [321] source

In the paper

“The operational simplicity of these urea or thiourea anions coupled with their high activity, selectivity, and versatility illustrate the potential of these catalysts for the sustainable synthesis of polyesters and polycarbonates.”

In the paper

“The ROP of rac-LA mediated by TU-ICD generated isotactic PLAs (Pm = 0.88) at 25 °C and 50% conversion.”

What the sources say

• Not checked yetcites [341] source

In the paper

“Mecerreyes and Cossío found that a cocatalytic system consisted of a densely substituted proline (PRO, Figure 9) and DBU was able to kinetically resolve racemic lactide.”

What the sources say

• Not checked yetcites [342] source

In the paper

“The success of ROCP is largely driven by the development of efficient and selective catalysts.”

In the paper

“Early studies focused on heterogeneous catalysts such as zinc glutarate 353,354 and double metal cyanides 355,356 .”

What the sources say

• Not checked yetcites [353] source
• Not checked yetcites [354] source
• Not checked yetcites [355] source

In the paper

“On the other hand, it is often the only route to polycarbonates whose carbonates are 2 carbons apart since these polymers are otherwise difficult to obtain from the ROP or stepgrowth method for the reasons mentioned above.”

In the paper

“An alternative chain-growth route to aliphatic polyesters and polycarbonates is the alternating copolymerization of epoxides and cyclic anhydrides (for polyesters) or epoxides and CO2 (for polycarbonates).”

In the paper

“Because of the large economic and environmental benefits of this process, it has stimulated a number of commercialization, such as QPAC® by Empower Materials 70 and Converge® by Saudi Aramco.71”

What the sources say

• Not checked yetcites [70] Coca-Cola.http://www.coca-colacompany.com/plantbottle-technology(accessed February 10, 2017).
• Not checked yetcites [71] Empower Materials.http://www.empowermaterials.com/(accessed February 10, 2017).

In the paper

“Because of the limitation on the ring size of epoxides (3,4membered) and cyclic anhydrides (mainly 5,6-membered) that can be incorporated, ROCP is restricted to the synthesis of polyesters and polycarbonates whose ester/carbonate moieties are 2 or 3 carbons apart.”

In the paper

“Because of the mechanistic similarities between epoxide/CO2 copolymerization and epoxide/cyclic anhydride copolymerization, the two can be mediated by a similar array of catalysts.343”

What the sources say

• Not checked yetcites [343] source

In the paper

“The development of single-site metal complexes with ancillary ligands has largely boosted the understanding and the scope of ROCP.343”

What the sources say

• Not checked yetcites [343] source

In the paper

“This leads to a metal bound alkoxide to which CO2 (or cyclic anhydride) readily inserts to form a metal carbonate (or carboxylate) species.”

What the sources say

• Not checked yetcites [344] source

In the paper

“This can cause the molecular weight of the resulting polymers to be lower than that expected.[357][358][359]”

What the sources say

• Not checked yetcites [357][358][359]

In the paper

“Although the exact mechanism for each catalytic system may vary, ROCP of epoxide with CO2 or cyclic anhydride generally follows the coordination-insertion mechanism shown in Scheme 14. 344”

What the sources say

• Not checked yetcites [344] source

In the paper

“ROCP of cyclohexene oxide/CO2 is less susceptible to backbiting than that of propylene oxide/CO2 due to the unfavorable ring strain of cyclohexene carbonate.”

In the paper

“One detrimental side reaction for the ROCP of epoxide/CO2 is the formation of fivemembered cyclic carbonates (5CCs) as a result of intramolecular cyclization (Scheme 14, yellow box).352”

What the sources say

• Not checked yetcites [352] source

In the paper

“The extent of this reaction depends on the nature of the epoxide, the catalyst, and the reaction conditions.83,361”

What the sources say

• Not checked yetcites [83] source
• Not checked yetcites [361] source

In the paper

“Recently, a series of multi-nuclear catalysts have been shown to display superior performance than their mono-nuclear analogues.”

In the paper

“For example, the introduction of positively charged moieties (e.g.ammonium) to the ligand can suppress the 5CC formation.[362][363][364]”

What the sources say

• Not checked yetcites [362][363][364]

In the paper

“The polymerization is initiated by the nucleophilic attack of a ligand (X) or an exogenous nucleophile onto a coordinated epoxide.”

What the sources say

• Not checked yetcites [344] source

In the paper

“Among the different metal catalysts, b-diiminate (BDI) zinc complexes and metal complexes (Al, Cr, Co) with Schiff-base ligands (salen and their derivatives) are the most successful ones (Figure 10).”

In the paper

“This selectivity is attributed to the faster rate of anhydride insertion than CO2 insertion into the metal-alkoxide intermediate.367”

What the sources say

• Not checked yetcites [367] source

In the paper

“Nevertheless, it is the most suitable way for dealing with highly mixed plastics.388”

What the sources say

• Not checked yetcites [388] source

In the paper

“Copolymerization of different epoxides or anhydrides provides even more possibilities of structures and properties that can be tuned by the ratios of different monomers.83”

What the sources say

• Not checked yetcites [83] source

In the paper

“However, for polymers derived from less exergonic polymerization reactions, chemical recycling can provide an attractive option to recover value, provided that efficient processes can be developed and implemented.”

In the paper

“The large variety of structures yields a diverse set of polymer properties that can be easily controlled by the choice of monomer.”

In the paper

“When the two blocks are miscible, only one intermediate Tg higher than that of the pure polyester and lower than the polycarbonate is obtained.368”

What the sources say

• Not checked yetcites [368] source

In the paper

“For certain bacteria under optimized growth condition, the yield of PHA obtained from the intracellular inclusions can be as high as 89% of the organism's dry weight.380”

What the sources say

• Not checked yetcites [380] source

In the paper

“One important advantage of this biosynthetic route is that the feedstock can be completely sourced from biomass (sugars, oils) or methane waste.”

In the paper

“By using a different combination of epoxide and cyclic anhydride, Tg of polyesters can be tuned to almost any value from -44 to 184 °C.211”

What the sources say

• Not checked yetcites [211] source

In the paper

“only PET and HDPE in the form of relatively pure packaging materials are currently recycled routinely, 388”

What the sources say

• Not checked yetcites [388] source

In the paper

“and the majority of the recyclates is repurposed for lower grade materials.389”

What the sources say

• Not checked yetcites [389] source

In the paper

“This provides an opportunity for transforming waste gas to valuable plastic materials.383”

What the sources say

• Not checked yetcites [383] The use of methane waste as a feedstock for PHAs has been tested by Mango Materials.https://www.biobasedworldnews.com/ask-the-industry-dr.molly-morse-ceo-and-co-founder-mango-materials(accessed May 8, 2017).

In the paper

“This approach is considered uneconomical for petroleum plastics due to the low price of petrochemical feedstock compared with the process costs incurred to depolymerize and recover the monomer.388”

What the sources say

• Not checked yetcites [388] source

In the paper

“High level of recycling can potentially allow for lower energy and material inputs for a given level of products and thus improve eco-efficiency.387”

What the sources say

• Not checked yetcites [387] The term “Eco-efficiency” was coined by the World Business Council for Sustainable Development (WBCSD) in its 1992, publication “Changing Course”.

In the paper

“Therefore, the addition of CO2 to an epoxide/cyclic anhydride copolymerization is a simple way to improve the thermal properties of polyesters for many applications that do not require pure polyesters.”

In the paper

“For example, switching the epoxide from PO to CHO significantly increases the Tg of polycarbonates from 30-40 °C to 80-120 °C.351”

What the sources say

• Not checked yetcites [351] source

In the paper

“The microbial synthesis by naturally occurring or genetically modified microorganisms has been practiced industrially for the production of bioplastics.369,370”

What the sources say

• Not checked yetcites [369] source
• Not checked yetcites [370] Chen G. G.-Q., 2010 source

In the paper

“A number of companies have implemented PET depolymerization at pilot or commercial level, including Loop Industries, 398 Carbios, 399 Ioniqa, 400 and Gr3n Project.401”

What the sources say

• Not checked yetcites [398] loop Industries: Leading the sustainable plastic revolution. http://www.loopindustries.com/en/(accessed August 5, 2017).
• Not checked yetcites [399] CARBIOS: Reinvent Polymers Lifecycle. https://carbios.fr/en/(accessed August 5, 2017).
• Not checked yetcites [400] Ioniqa Technologies. http://www.ioniqa.com/(accessed August 5, 2017).

In the paper

“This is due to a common downgrading problem of mechanical recycling, that is, the recovered plastics tend to lose some quality attributes of the virgin plastics, such as color, clarity, or mechanical properties.395”

What the sources say

• Not checked yetcites [395] source