Study on the enzymatic polymerization mechanism of lactone and the strategy for improving the degree of polymerization

The Effect of Solvent Hydrophilicity on the Enzymatic Ring-Opening Polymerization of L-Lactide by Candida rugosa Lipase

Contradictions have been reported on the effect of organic solvents, especially toluene, on enzymatic ring-opening polymerization (eROP) of L-lactide. Studies have shown that log P, a common measure of hydrophilicity, affects enzyme activity. This study examines the effect of solvents with various log P values on the eROP of L-lactide, performed using Candida rugosa lipase (CRL). N,N-dimethylacetamide (DMA), 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, diphenyl ether, and dodecane were used as the organic solvents. The eROP in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) was also conducted to compare its performance with the organic solvents. The results show that [BMIM][PF₆]-mediated eROP gave better conversion and molecular weight than the organic solvent-mediated eROP. In this study, the effects of solvents hydrophilicity are discussed, including the possibility of hexafluorophosphate ion ([PF₆]⁻) hydrolysis to occur.

Studying the catalytic activity of DBU and TBD upon water-initiated ROP of ε-caprolactone under different thermodynamic conditions

Studies performed revealed that a novel catalytic system for water-initiated ε-CL ROP based on DBU protonation showed enhanced performance under high-pressure/high-temperature conditions.

Lipase-mediated direct in situ ring-opening polymerization of ε-caprolactone formed by a chemo-enzymatic method

A novel method to synthesize poly(ε-caprolactone) (PCL) through a three-step, lipase-mediated chemo-enzymatic reaction from cyclohexanone using an immobilized lipase from Trichosporon laibacchii (T. laibacchii) CBS5791 was developed. The immobilized preparation with 1280 U· g^-1 used here was obtained by a method of purification and in situ immobilization where the crude intracellular lipase (cell homogenate) was subjected to partial purification by an aqueous two-phase system (ATPS) consisting of 12% (w/w) polyethylene glycol (PEG) 4000 and 13% (w/w) potassium phosphate (K₂HPO₄) and then in situ immobilization directly on diatomite from the top PEG-rich phase of ATPS. In this multi-step process, the ε-caprolactone (ε-CL) produced by lipase-mediated one-pot two-step chemo-enzymatic oxidation of cyclohexanone was directly subjected to in situ ring-opening polymerization (ROP) started by adding highly hydrophobic solvents. It is necessary to note that ε-CL synthesis and its subsequent ROP were catalyzed by the same lipase. The impact of various reaction parameters, e.g., solvent, cyclohexanone: hydrogen peroxide molar ratio, hydrogen peroxide forms and reaction temperature were investigated. Toluene was selected as a preferred solvent due to supporting the highest molecular weight (M_n = 2168) and moderate ε-CL conversion (65.42%). Through the optimization of reaction conditions, PCL was produced with a M_n of 2283 at 50 °C for 24 h. These results reveal that this lipase-mediated direct ring-opening polymerization of in situ formed ε-CL is an alternative route to the conventional synthesis of PCL.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Glutaraldehyde cross-linking of immobilized thermophilic esterase on hydrophobic macroporous resin for application in poly(ε-caprolactone) synthesis

The immobilized thermophilic esterase from Archaeoglobus fulgidus was successfully constructed through the glutaraldehyde-mediated covalent coupling after its physical adsorption on a hydrophobic macroporous resin, Sepabeads EC-OD. Through 0.05% glutaraldehyde treatment, the prevention of enzyme leaching and the maintenance of catalytic activity could be simultaneously realized. Using the enzymatic ring-opening polymerization of ε-caprolactone as a model, effects of organic solvents and reaction temperature on the monomer conversion and product molecular weight were systematically investigated. After the optimization of reaction conditions, products were obtained with 100% monomer conversion and M_n values lower than 1010 g/mol. Furthermore, the cross‑linked immobilized thermophilic esterase exhibited an excellent operational stability, with monomer conversion values exceeding 90% over the course of 12 batch reactions, still more than 80% after 16 batch reactions.

Lipases in polymer chemistry

Lipases are highly active in the polymerization of a range of monomers. Both ring-opening polymerization of cyclic monomers such as lactones and carbonates as well as polycondensation reactions have been investigated in great detail. Moreover, in combination with other (chemical) polymerization techniques, lipase-catalyzed polymerization has been employed to synthesize a variety of polymer materials. Major advantages of enzymatic catalysts are the often-observed excellent regio-, chemo- and enantioselectivity that allows for the direct preparation of functional materials. In particular, the application of techniques such as Dynamic Kinetic Resolution (DKR) in the lipase-catalyzed polymerization of racemic monomers is a new development in enzymatic polymerization. This paper reviews selected examples of the application of lipases in polymer chemistry covering the synthesis of linear polymers, chemoenzymatic polymerization and applications of enantioselective techniques for the synthesis and modification of polymers.

Lipase-catalyzed ring-opening copolymerization of ε-caprolactone and β-lactam

The enzymatic ring-opening copolymerization of ε-caprolactone (ε-CL) and β-lactam by using Candida antarctica lipase B (CAL-B) as catalyst was studied. Variation of the feed ratios of 25:75, 50:50, and 75:25 of ε-CL/β-lactam was performed. The products contain poly(ε-CL-co-β-lactam) and the homopolymers of poly(ε-CL) and poly(β-lactam). The structure of the copolymers was determined by MALDI-ToF MS. Poly(ε-CL-co-β-lactam) has an alternating and random structure consisting of alternating repeating units with oligo(ε-CL) or oligo(β-lactam). The highest fraction of the alternating copolymers resulted from the reaction with a feed ratio 50:50. The copolymer is a semicrystalline polymer with a Tm at 124 °C and Tgs at -15 and 50 °C. Interestingly, the copolymer also demonstrated cold crystallization at 29 and 74 °C, after quenching the sample from the melt in liquid nitrogen.

Biocatalytic synthesis of poly(δ-valerolactone) using a thermophilic esterase from archaeoglobus fulgidus as catalyst

The ring-opening polymerization of δ-valerolactone catalyzed by a thermophilic esterase from the archaeon Archaeoglobus fulgidus was successfully conducted in organic solvents. The effects of enzyme concentration, temperature, reaction time and reaction medium on monomer conversion and product molecular weight were systematically evaluated. Through the optimization of reaction conditions, poly(δ-valerolactone) was produced in 97% monomer conversion, with a number-average molecular weight of 2225 g/mol, in toluene at 70 °C for 72 h. This paper has produced a new biocatalyst for the synthesis of poly(δ-valerolactone), and also deeper insight has been gained into the mechanism of thermophilic esterase-catalyzed ring-opening polymerization.

Increasing Molecular Mass in Enzymatic Lactone Polymerizations

Using a model developed for the enzyme-catalyzed polymerization and degradation of poly(caprolactone), we illustrate a method and the kinetic mechanisms necessary to improve molecular mass by manipulating equilibrium reactions in the kinetic pathway. For these polymerization/degradation reactions, a water/linear chain equilibrium controls the number of chains in solution. Here, we control the equilibrium by adding water-trapping molecular sieves in the batch polymerization reactions of ε-caprolactone. While ring-opening rates were mostly unaffected, the molecular mass shifted to higher molecular masses after complete conversion was reached, and a good agreement between the experimental and modeling results was found. These results provide a framework to improve the molecular mass for enzyme-catalyzed ring-opening polymerization of lactone.

Modeling enzymatic kinetic pathways for ring-opening lactone polymerization

A unified kinetic pathway for the enzyme-catalyzed polymerization and degradation of poly(ε-caprolactone) was developed. This model tracks the complete distribution of individual chain lengths, both enzyme-bound and in solution, and successfully predicts monomer conversion and the molecular mass distribution as a function of reaction time. As compared to reported experimental data for polymerization reactions, modeled kinetics generate similar trends, with ring-opening rates and water concentration as key factors to controlling molecular mass distributions. Water is critically important by dictating the number of linear chains in solution, shifting the molecular mass distribution at which propagation and degradation equilibrate. For the enzymatic degradation of poly(ε-caprolactone), the final reaction product is also consistent with the equilibrium dictated by the propagation and degradation rates. When the modeling framework described here is used, further experiments can be designed to isolate key reaction steps and provide methods for improving the efficiency of enzyme polymerization.

Continuous flow enzyme-catalyzed polymerization in a microreactor

Enzymes immobilized on solid supports are increasingly used for greener, more sustainable chemical transformation processes. Here, we used microreactors to study enzyme-catalyzed ring-opening polymerization of ε-caprolactone to polycaprolactone. A novel microreactor design enabled us to perform these heterogeneous reactions in continuous mode, in organic media, and at elevated temperatures. Using microreactors, we achieved faster polymerization and higher molecular mass compared to using batch reactors. While this study focused on polymerization reactions, it is evident that similar microreactor based platforms can readily be extended to other enzyme-based systems, for example, high-throughput screening of new enzymes and to precision measurements of new processes where continuous flow mode is preferred. This is the first reported demonstration of a solid supported enzyme-catalyzed polymerization reaction in continuous mode.

Toward advanced ionic liquids. Polar, enzyme-friendly solvents for biocatalysis

Ionic liquids, also called molten salts, are mixtures of cations and anions that melt below 100 °C. Typical ionic liquids are dialkylimidazolium cations with weakly coordinating anions such as [MeOSO₃] or [PF₆]. Advanced ionic liquids such as choline citrate have biodegradable, less expensive and less toxic anions and cations. Deep eutectic solvents are also included in the advanced ionic liquids. Deep eutectic solvents are mixtures of salts such as choline chloride and uncharged hydrogen bond donors such as urea, oxalic acid, or glycerol. For example, a mixture of choline chloride and urea in 1:2 molar ratio liquifies to form a deep eutectic solvent. Their properties are similar to those of ionic liquids. Water-miscible ionic liquids as cosolvents with water enhance the solubility of substrates or products. Although traditional water-miscible organic solvents also enhance solubility, they often inactivate enzymes, while ionic liquids do not. The enhanced solubility of substrates can increase the rate of reaction and often increases the regio- or enantioselectivity. Ionic liquids can also be solvents for non-aqueous reactions. In these cases, they are especially suited to dissolve polar substrates. Polar organic solvent alternatives inactivate enzymes, but ionic liquids do not even when they have similar polarities. Besides their solubility properties, ionic liquids and deep eutectic solvents may be greener than organic solvents because ionic liquids are non-volatile and can be made from non-toxic components. This review covers selected examples of enzyme catalyzed reaction ionic liquids that demonstrate their advantages and unique properties and point out opportunities for new applications. Most examples involve hydrolases, but oxidoreductases and even whole cell reactions have been reported in ionic liquids.

Synthesis of polycaprolactone: a review

Polycaprolactone (PCL) is an important polymer due to its mechanical properties, miscibility with a large range of other polymers and biodegradability. Two main pathways to produce polycaprolactone have been described in the literature: the polycondensation of a hydroxycarboxylic acid: 6-hydroxyhexanoic acid, and the ring-opening polymerisation (ROP) of a lactone: epsilon-caprolactone (epsilon-CL). This critical review summarises the different conditions which have been described to synthesise PCL, and gives a broad overview of the different catalytic systems that were used (enzymatic, organic and metal catalyst systems). A surprising variety of catalytic systems have been studied, touching on virtually every section of the periodic table. A detailed list of reaction conditions and catalysts/initiators is given and reaction mechanisms are presented where known. Emphasis is put on the ROP pathway due to its prevalence in the literature and the superior polymer that is obtained. In addition, ineffective systems that have been tried to catalyse the production of PCL are included in the electronic supplementary information for completeness (141 references).