Enzyme-catalyzed synthesis of malonate polyesters and their use as metal chelating materials.

Following the environmental problems caused by non-degradable plastics there is a need to synthesise greener and more sustainable polymers. In this work we describe, for the first time, the facile enzyme-catalysed synthesis of linear polyesters using dimethyl malonate as the diester. These polymers, containing a different aliphatic diol component (C4, C6 or C8), were synthesised in solventless conditions using immobilized Candida antarctica lipase B as the biocatalyst. The potential of enzymes for catalysing this reaction is compared with the unsuccessful antimony- and titanium-catalysed synthesis (T > 150 °C). The application of the synthesized polymers as effective metal chelators in biphasic, green solvent systems was also described, together with the characterisation of the synthesised materials.

Effects of Methyl Branching on the Properties and Performance of Furandioate-Adipate Copolyesters of Bio-Based Secondary Diols.

Furandioate-adipate copolyesters are an emerging class of bio-based biodegradable polymers with great potential to replace fossil-derived terephthalic acid-based copolyesters such as poly(butylene adipate-co-terephthalate) (PBAT). Furandioate-adipate polyesters have almost exclusively been prepared with conventional primary (1°) alcohol diols, while secondary (2°) alcohol diol monomers have largely been overlooked until now, despite preliminary observations that using methyl-branched diols increases the T g of the resultant polyesters. Little is known of what impact the use of 2° alcohol diols has on other properties such as material strength, hydrophobicity, and rate of enzymatic hydrolysis-all key parameters for performance and end-of-life. To ascertain the effects of using 2° diols on the properties of furandioate-adipate copolyesters, a series of polymers from diethyl adipate (DEA) and 2,5-furandicarboxylic acid diethyl ester (FDEE) using different 1° and 2° alcohol diols was prepared. Longer transesterification times and greater excesses of diol (diol/diester molar ratio of 2:1) were found to be necessary to achieve M ws > 20 kDa using 2° alcohol diols. All copolyesters from 2° diols were entirely amorphous and exhibited higher T gs than their linear equivalents from 1° diols. Compared to linear poly(1,4-butyleneadipate-co-1,4-butylenefurandioate), methyl-branched, poly(2,5-hexamethyleneadipate-co-2,5-hexamethylenefurandioate) (0:7:0.3 furandioate/adipate ratio) displayed both higher modulus (67.8 vs 19.1 MPa) and higher extension at break (89.7 vs 44.5 mm). All other methyl-branched copolyesters displayed lower modulus but retained higher extension at break compared with their linear analogues. Enzymatic hydrolysis studies using Humicola insolens cutinase revealed that copolyesters from 2° alcohol diols have significantly decreased rates of biodegradation than their linear equivalents synthesized using 1° alcohol diols, allowing for fine-tuning of polymer stability. Hydrophobicity, as revealed by water contact angles, was also found to generally increase through the introduction of methyl branching, demonstrating potential for these materials in coatings applications.

Thermal Upgrade of Enzymatically Synthesized Aliphatic and Aromatic Oligoesters.

The enzymatic synthesis of polyesters in solventless systems is an environmentally friendly and sustainable method for synthetizing bio-derived materials. Despite the greenness of the technique, in most cases only short oligoesters are obtained, with limited practical applications or requiring further chemical processing for their elongation. In this work, we present a catalyst-free thermal upgrade of enzymatically synthesized oligoesters. Different aliphatic and aromatic oligoesters were synthesized using immobilized Candida antarctica lipase B (iCaLB) as the catalyst (70 °C, 24 h) yielding poly(1,4-butylene adipate) (PBA, Mw = 2200), poly(1,4-butylene isophthalate) (PBI, Mw = 1000), poly(1,4-butylene 2,5-furandicarboxylate) (PBF, Mw = 600), and poly(1,4-butylene 2,4-pyridinedicarboxylate) (PBP, Mw = 1000). These polyesters were successfully thermally treated to obtain an increase in Mw of 8.5, 2.6, 3.3, and 2.7 folds, respectively. This investigation focused on the most successful upgrade, poly(1,4-butylene adipate), then discussed the possible effect of di-ester monomers as compared to di-acids in the thermally driven polycondensation. The herein-described two-step synthesis method represents a practical and cost-effective way to synthesize higher-molecular-weight polymers without the use of toxic metal catalysts such as titanium(IV) tert-butoxide, tin(II) 2-ethylhexanoate, and in particular, antimony(IV) oxide. At the same time, the method allows for the extension of the number of reuses of the biocatalyst by preventing its exposure to extreme denaturating conditions.

Improving the Post-polymerization Modification of Bio-Based Itaconate Unsaturated Polyesters: Catalyzing Aza-Michael Additions With Reusable Iodine on Acidic Alumina.

Bio-based platform molecules such as itaconic, fumaric, and muconic acid offer much promise in the formation of sustainable unsaturated polyester resins upon reaction with suitable diols and polyols. The C=C bonds present in these polyester chains allows for post-polymerization modification and such moieties are conventionally utilized in curing processes during the manufacture of coatings. The C=C modification sites can also act as points to add useful pendants which can alter the polymers final properties such as glass transition temperature, biodegradability, hardness, polarity, and strength. A commonly observed modification is the addition of secondary amines via an aza-Michael addition. Conventional procedures for the addition of amines onto itaconate polyesters require reaction times of several days as a result of undesired side reactions, in particular, the formation of the less reactive mesaconate regioisomer. The slow reversion of the mesaconate back to itaconate, followed by subsequent amine addition, is the primary reason for such extended reaction times. Herein we report our efforts toward finding a suitable catalyst for the aza-Michael addition of diethylamine onto a model substrate, dimethyl itaconate, with the aim of being able to add amine onto the itaconate units without excessive regioisomerization to the inactive mesaconate. A catalyst screen showed that iodine on acidic alumina results in an effective, heterogeneous, reusable catalyst for the investigated aza-Michael addition. Extending the study further, itaconate polyester was prepared by Candida Antartica Lipase B (CaL-B) via enzymatic polytranesterification and subsequently modified with diethylamine using the iodine on acidic alumina catalyst, dramatically reducing the required length of reaction (>70% addition after 4 h). The approach represents a multidisciplinary example whereby biocatalytic polymerization is combined with chemocatalytic modification of the resultant polyester for the formation of useful bio-based polyesters.

Safer bio-based solvents to replace toluene and tetrahydrofuran for the biocatalyzed synthesis of polyesters.

With increased awareness of environmental issues caused by traditional petrochemical processes, both academia and industry are making enormous efforts towards the development of sustainable practices using renewable biomass as a feedstock. In this work, the biocatalyzed synthesis of polyesters derived from renewable monomers was performed in safer, bio-derivable organic solvents. Candida antarctica lipase B (CaLB), an enzyme belonging to the Ser-hydrolase family (adsorbed on methacrylic resin, also known as Novozym 435) was tested for its performance in the synthesis of adipate- and furandicarboxylate-based polyesters. In addition, the traditional solvents toluene and tetrahydrofuran were compared with a series of green solvents, 2,2,5,5-tetramethyloxolane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran and pinacolone for the enzymatic polymerizations. We can conclude that the monomer conversions and molecular masses of the obtained polyesters in all the tested alternative solvents were suitable, and in some cases superior, with CaLB immobilized via physisorption on acrylic resin being the optimal biocatalyst for all reactions. Strikingly, it was found that for the majority of the new solvents, lower reaction temperatures gave comparable monomer conversions and polymers with similar molecular weights whilst pinacolone yielded better polymers with M n > 2000 Da and conversions of over 80%.

Enzymatic synthesis of lignin derivable pyridine based polyesters for the substitution of petroleum derived plastics.

Following concerns over increasing global plastic pollution, interest in the production and characterization of bio-based and biodegradable alternatives is rising. In the present work, the synthesis of a series of fully bio-based alternatives based on 2,4-, 2,5-, and 2,6-pyridinedicarboxylic acid-derived polymers produced via enzymatic catalysis are reported. A similar series of aromatic-aliphatic polyesters based on diethyl-2,5-furandicarboxylate and of the petroleum-based diethyl terephthalate and diethyl isophthalate were also synthesized. Here we show that the enzymatic synthesis starting from 2,4-diethyl pyridinedicarboxylate leads to the best polymers in terms of molecular weights (Mn = 14.3 and Mw of 32.1 kDa when combined with 1,8-octanediol) when polymerized in diphenyl ether. Polymerization in solventless conditions were also successful leading to the synthesis of bio-based oligoesters that can be further functionalized. DSC analysis show a clear similarity in the thermal behavior between 2,4-diethyl pyridinedicarboxylate and diethyl isophthalate (amorphous polymers) and between 2,5-diethyl pyridinedicarboxylate and diethyl terephthalate (crystalline polymers).

Laminaria digitata and Palmaria palmata Seaweeds as Natural Source of Catalysts for the Cycloaddition of CO2 to Epoxides.

Seaweed powder has been found to act as an effective catalyst for the fixation of CO2 into epoxides to generate cyclic carbonates under solvent free conditions. Model background reactions were performed using metal halides and amino acids typically found in common seaweeds which showed potassium iodide (KI) to be the most active. The efficacy of the seaweed catalysts kelp (Laminaria digitata) and dulse (Palmaria palmata) was probed based on particle size, showing that kelp possessed greater catalytic ability, achieving a maximum conversion and selectivity of 63.7% to styrene carbonate using a kelp loading of 80% by weight with respect to epoxide, 40 bar of CO2, 120 °C for 3 h. Maximizing selectivity was difficult due to the generation of diol side product from residual H2O found in kelp, along with a chlorinated by-product thought to form due to a high quantity of chloride salts in the seaweeds. Data showed there was loss of organic matter upon use of the kelp catalyst, likely due to the breakdown of organic compounds and their subsequent removal during product extraction. This was highlighted as the likely cause of loss of catalytic activity upon reuse of the Kelp catalyst.

New bio-based monomers: tuneable polyester properties using branched diols from biomass.

A family of monomers, including 2,5-hexandiol, 2,7-octandiol, 2,5-furandicarboxylic acid (FDCA), terephthalic acid (TA), and branched-chain adipic and pimelic acid derivatives, all find a common derivation in the biomass-derived platform molecule 5-(chloromethyl)furfural (CMF). The diol monomers, previously little known to polymer chemistry, have been combined with FDCA and TA derivatives to produce a range of novel polyesters. It is shown that the use of secondary diols leads to polymers with higher glass transition temperatures (Tg) than those prepared from their primary diol equivalents. Two methods of polymerisation were investigated, the first employing activation of the aromatic diacids via the corresponding diacid chlorides and the second using a transesterification procedure. Longer chain diols were found to be more reactive than the shorter chain alternatives, generally giving rise to higher molecular weight polymers, an effect shown to be most pronounced when using the transesterification route. Finally, novel diesters with high degrees of branching in their hydrocarbon chains are introduced as potential monomers for possible low surface energy materials applications.

New catalysts for carboxylation of propylene glycol to propylene carbonate via high-throughput screening.

High throughput methodologies screened 81 different metal salts and metal salt combinations as catalysts for the carboxylation of propylene glycol to propylene carbonate, as compared to a 5 mol% Zn(OAc)2/p-chlorobenzene sulfonic acid benchmark catalyst. The reactions were run with added acetonitrile (MeCN) as a chemical water trap. Two new catalysts were thereby discovered, zinc trifluoromethanesulfonate (Zn(OTf)2) and zinc p-toluenesulfonate. The optimal reaction parameters for the former catalyst were screened. Zn(OTf)2 gave an overall propylene carbonate yield of greater than 50% in 24 h, twice as large as the previous best literature yield with MeCN as a water trap, with 69% selectivity and 75% conversion of propylene glycol at 145 °C and 50 bar CO2 pressure.

Clean, reusable and low cost heterogeneous catalyst for amide synthesis.

We have developed a heterogeneous silica catalyst that can effectively catalyse amide synthesis from acid and amine, without production of toxic by-products and with the advantage of being readily available, low cost, environmentally benign and reusable.

Synthes tubular external fixation system for isolated subtalar arthrodesis.

The authors present the use of the AO/ASIF tubular system for isolated subtalar arthrodesis. Internal and external fixation are both acceptable means of promoting successful arthrodesis. Appropriate patient selection is vital when considering external fixation. Indications, operative technique, and a discussion are included as well as two case studies.