Recent Advances in the Enzymatic Synthesis of Polyester

Polyester is a kind of polymer composed of ester bond-linked polybasic acids and polyol. This type of polymer has a wide range of applications in various industries, such as automotive, furniture, coatings, packaging, and biomedical. The traditional process of synthesizing polyester mainly uses metal catalyst polymerization under high-temperature. This condition may have problems with metal residue and undesired side reactions. As an alternative, enzyme-catalyzed polymerization is evolving rapidly due to the metal-free residue, satisfactory biocompatibility, and mild reaction conditions. This article presented the reaction modes of enzyme-catalyzed ring-opening polymerization and enzyme-catalyzed polycondensation and their combinations, respectively. In addition, the article also summarized how lipase-catalyzed the polymerization of polyester, which includes (i) the distinctive features of lipase, (ii) the lipase-catalyzed polymerization and its mechanism, and (iii) the lipase stability under organic solvent and high-temperature conditions. In addition, this article also focused on the advantages and disadvantages of enzyme-catalyzed polyester synthesis under different solvent systems, including organic solvent systems, solvent-free systems, and green solvent systems. The challenges of enzyme optimization and process equipment innovation for further industrialization of enzyme-catalyzed polyester synthesis were also discussed in this article.

Enzymatic polymerization to polyesters in nonaqueous solvents

Aliphatic polyesters are thermoplastic and biodegradable polymers with promising potentials to substitute synthetic polymers derived from petrochemicals. In particular, polylactides (PLAs) and other polylactones can be renewable and biocompatible. A more benign approach for polyester synthesis is the enzymatic polycondensation or ring-opening polymerization (ROP) reactions, whose outcomes largely depend on the reaction conditions including solvents, water content and temperature. This chapter illustrates several examples of enzymatic polymerization to polyesters using various solvents (i.e., organic solvents, supercritical fluids, ionic liquids, and aqueous biphasic systems). Hydrophobic solvents containing little water tend to promote the enzymatic polymerization and lead to high molecular masses of polyesters. Since some enzymatic polymerization reactions are performed at high temperatures (such as ring-opening polymerization of lactide at >100°C), these processes demand solvents with high boiling points (such as many ionic liquids). Supercritical fluids (such as supercritical CO₂) can be "green" solvents, but their compatibility with enzymes and their practicability of scaling up remain as challenges. On the other hand, ionic liquids can be tailored to be compatible with enzymes and to have high thermal stability although the studies of their uses in enzymatic polycondensation and ROP reactions are still at an early stage.

Enzymatic Ring-Opening Polymerization (ROP) of Polylactones: Roles of Non-Aqueous Solvents

Aliphatic polyesters such as polylactides (PLAs) and other polylactones are thermoplastic, renewable and biocompatible polymers with high potentials to replace petro-chemical-based synthetic polymers. A benign route for synthesizing these polyesters is through the enzyme-catalyzed ring-opening polymerization (ROP) reaction; this type of enzymatic process is very sensitive to reaction conditions such as solvents, water content and temperature. This review systematically discusses the crucial roles of different solvents (such as solvent-free or in bulk, organic solvents, supercritical fluids, ionic liquids, and aqueous biphasic systems) on the degree of polymerization and polydispersity. In general, many studies suggest that hydrophobic organic solvents with minimum water contents lead to efficient enzymatic polymerization and subsequently high molecular weights of polyesters; the selection of solvents is also limited by the reaction temperature, e.g. the ROP of lactide is often conducted at above 100 °C, therefore, the solvent typically needs to have its boiling point above this temperature. The use of supercritical fluids could be limited by its scaling-up potential, while ionic liquids have exhibited many advantages include their low-volatility, high thermal stability, controllable enzyme-compatibility, and a wide range of choices. However, the fundamental and mechanistic understanding of the specific roles of ionic liquids in enzymatic ROP reactions is still lacking. Furthermore, the lipase specificity towards l- and d-lactide is also surveyed, followed by the discussion of engineered lipases with improved enantioselectivity and thermal stability. In addition, the preparation of polyester-derived materials such as polyester-grafted cellulose by the enzymatic ROP method is briefly reviewed.

Polyesters from Macrolactones Using Commercial Lipase NS 88011 and Novozym 435 as Biocatalysts

The demand for environmentally friendly products allied with the depletion of natural resources has increased the search for sustainable materials in chemical and pharmaceutical industries. Polyesters are among the most widely used biodegradable polymers in biomedical applications. In this work, aliphatic polyesters (from globalide and ω-pentadecalactone) were synthesized using a new commercial biocatalyst, the low-cost immobilized NS 88011 lipase (lipase B from Candida antarctica immobilized on a hydrophobic support). Results were compared with those obtained under the same conditions using a traditional, but more expensive, commercial biocatalyst, Novozym 435 (lipase B from C. antarctica immobilized on Lewatit VP OC). When NS 88011 was used in the polymerization of globalide, longer reaction times (240 min)-when compared to Novozym 435-were required to obtain high yields (80-90 wt%). However, higher molecular weights were achieved. When poly(ω-pentadecalactone) was synthesized, high yields and molecular weights (130,000 g mol^-1) were obtained and the enzyme concentration showed strong influence on the polyester properties. This is the first report describing NS 88011 in polymer synthesis. The use of this cheaper enzymatic preparation can provide an alternative for polyester synthesis via enzymatic ring-opening polymerization.

CaLB Catalyzed Conversion of ε-Caprolactone in Aqueous Medium. Part 1: Immobilization of CaLB to Microgels

The enzymatic ring-opening polymerization of lactones is a method of increasing interest for the synthesis of biodegradable and biocompatible polymers. In the past it was shown that immobilization of Candida antarctica lipase B (CaLB) and the reaction medium play an important role in the polymerization ability especially of medium ring size lactones like ε-caprolactone (ε-CL). We investigated a route for the preparation of compartmentalized microgels based on poly(glycidol) in which CaLB was immobilized to increase its esterification ability. To find the ideal environment for CaLB, we investigated the acceptable water concentration and the accessibility for the monomer in model polymerizations in toluene and analyzed the obtained oligomers/polymers by NMR and SEC. We observed a sufficient accessibility for ε-CL to a toluene like hydrophobic phase imitating a hydrophobic microgel. Comparing free CaLB and Novozym® 435 we found that not the monomer concentration but rather the solubility of the enzyme, as well as the water concentration, strongly influences the equilibrium of esterification and hydrolysis. On the basis of these investigations, microgels of different polarity were prepared and successfully loaded with CaLB by physical entrapment. By comparison of immobilized and free CaLB, we demonstrated an effect of the hydrophobicity of the microenvironment of CaLB on its enzymatic activity.

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.

Lipase-catalyzed polyester synthesis--a green polymer chemistry

This article is a short comprehensive review describing in vitro polyester synthesis catalyzed by a hydrolysis enzyme of lipase, most of which has been developed for these two decades. Polyesters are prepared by repeated ester bond-formation reactions; they include two major modes, ring-opening polymerization (ROP) of cyclic monomers such as cyclic esters (lactones) and condensation polymerization via the reaction between a carboxylic acid or its ester group and an alcohol group. Polyester synthesis is, therefore, a reaction in reverse way of in vivo lipase catalysis of ester bond-cleavage with hydrolysis. The lipase-catalyzed polymerizations show very high chemo-, regio-, and enantio-selectivities and involve various advantageous characteristics. Lipase is robust and compatible with other chemical catalysts, which allows novel chemoenzymatic processes. New syntheses of a variety of functional polyesters and a plausible reaction mechanism of lipase catalysis are mentioned. The polymerization characteristics are of green nature currently demanded for sustainable society, and hence, desirable for conducting 'green polymer chemistry'.

Miniemulsion polymerization and the structure of polymer and hybrid nanoparticles

The miniemulsion process allows the formation of complex structured polymeric nanoparticles and the encapsulation of a solid or liquid, an inorganic or organic, or a hydrophobic or hydrophilic material into a polymer shell. Many different materials, ranging from organic and inorganic pigments, magnetite, or other solid nanoparticles, to hydrophobic and hydrophilic liquids, such as fragrances, drugs, or photoinitators, can be encapsulated. Functionalization of the nanoparticles can also be easily obtained. Compared to polymerization processes in organic solvents, polymerization to obtain polymeric nanoparticles can be performed in environmentally friendly solvents, usually water.

Polymersome nanoreactors for enzymatic ring-opening polymerization

Polystyrene-polyisocyanopeptide (PS-PIAT) polymersomes containing CALB in two different locations, one in the aqueous inner compartment and one in the bilayer, were investigated for enzymatic ring-opening polymerization of lactones in water. It is shown that the monomers 8-octanolactone and dodecalactone yield oligomers with this polymersome system. It is also observed that the polymerization activity is dependent on the position of the enzyme in the polymersome. SEM investigations show that the polymersome structures were destabilized during the polymerization. Further investigations show that the vesicular morphology of the polymersomes was destabilized only in the case of polymer product formation.

In vitro biosynthesis of polyesters

In vitro synthesis of polyesters using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. In most cases, lipase was used as catalyst and various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives/glycols, and lactones, afforded the polyesters. The enzymatic polymerization often proceeded under mild reaction conditions in comparison with chemical processes. By utilizing characteristic properties of lipases, regio- and enantioselective polymerizations proceeded to give functional polymers, most of which are difficult to synthesize by conventional methodologies.