Recent advances in the synthesis of biodegradable polyesters by sustainable polymerization: lipase-catalyzed polymerization

Over the past three decades, enzymatic polymerization has dramatically developed and gradually broadened as a creative methodology in the construction of polymeric materials with tailor-made structures and properties. Compared with transition metal catalyst polymerizations, enzymatic polymerization is more attractive in the biomedicine field due to the metal-free residue, good biocompatibility, and few by-products. Meanwhile, enzymatic polymerization has far more activity towards macrolides. In this review, the synthesis of lipase-catalyzed polymer materials is systematically summarized, focusing on the synthesis of the complex and well-defined polymers. The enzymatic polyester synthesis was then discussed concerning the different reaction types, including ring-opening polymerization, polycondensation, a combination of ring-opening polymerization with polycondensation, and chemoenzymatic polymerization. Besides, exploration of novel biocatalysts and reaction media was also described, with particular emphasis on the enzymes obtained via immobilization or protein engineering strategies, green solvents, and reactors. Finally, recent developments in catalytic kinetics and mechanistic studies through the use of spectroscopy, mathematics, and computer techniques have been introduced. Besides, we addressed the remaining central issues in enzymatic polymerization and discussed current studies aimed at providing answers.

Enantio-, Regio-, and Chemoselective Lipase-Catalyzed Polymer Synthesis

In contrast to chemical routes, enzymatic polymerization possesses favorable characteristics of mild reaction conditions, few by-products, and high activity toward cyclic lactones which make it a promising technique for constructing polymeric materials. Meanwhile, it can avoid the trace residue of metallic catalysts and potential toxicity, and thus exhibits great potential in the biomedical fields. More importantly, lipase-catalyzed polymer synthesis usually shows favorable enantio-, regio-, and chemoselectivity. Here, the history and recent developments in lipase-catalyzed selective polymerization for constructing polymers with unique structures and properties are highlighted. In particular, the synthesis of polymeric materials which are difficult to prepare in a chemical route and the construction of polymers through the combination of selective enzymatic and chemical methods are focused. In addition, the future direction is proposed especially based on the rapid developments in computational chemistry and protein engineering techniques.

Lipase-catalyzed synthesis of chiral poly(ester amide)s with an alternating sequence of hydroxy acid and l/d-aspartate units

Alternating poly(hydroxyhexanoic acid-alt-l/d-β-aspartate)s with α-benzyl or α-methyl ester side groups were prepared via the enzymatic polycondensation of N-(6-hydroxyhexanoyl) l/d-aspartate diesters.

Polymers made of bile acids: from soft to hard biomaterials

Bile acids are gaining increasing importance as building blocks in the development of novel polymeric materials. This is evidenced by the growing number of publications advocating the advantages of their incorporation in the design and construction of materials. Composed of a rigid steroid backbone, functional groups with potential towards diverse reactions, and a biocompatible framework, there are various ways in which these molecules can be utilized to afford biomaterials via distinct architectures. Soft materials utilize the intrinsic capacity of bile acids to self-assemble and have seen a range of applications, most notably in the field of drug delivery. On the other hand, there is also the possibility of including bile acids in the polymer backbone, which has been used in the preparation of elastomers. This review discusses a selection of materials that can be prepared using bile acids and the advantages afforded by these molecules. Focus will be on the development of soft and hard materials, where soft materials are described as being held by weak intermolecular interactions, whereas hard materials are mechanically stronger with bile acids covalently incorporated in the polymer network.

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.

The use of polymer-supported Candida antarctica lipase B to achieve the entropically-driven ring-opening polymerization of macrocyclic bile acid derivatives via transesterification: selectivity of the reactions and the structures of the polymers produced

ED-ROPs of macrocyclic lactones by transesterifications catalyzed by CALB are further explored and predictions of the product structures made.

Entropically-driven ring-opening metathesis polymerization (ED-ROMP) of macrocyclic olefins prepared from deoxycholic acid to give functionalized polymers

The macrocycles shown were prepared, then polymerized and copolymerized by ROMP. This gave polymers with free OH or free CO₂R groups. Treatment of the polymers having R =t-Bu with trifluoroacetic acid gave polymers with free CO₂H groups.

Two Enzyme Cooperatively Catalyzed Tandem Polymerization for the Synthesis of Polyester Containing Chiral (R)- or (S)-Ibuprofen Pendants

An interesting cooperation between Candida antarctica Lipase B (CAL-B) and alkaline protease from Bacillus subtilis (BSP) in the copolymerization of bulky ibuprofen-containing hydroxyacid methyl ester (HAEP) and ε-caprolactone (ε-CL) is observed. This cooperation improved the M¯n of the polymers from 3130 (CAL-B) to 9200 g mol-1 (CAL-B/BSP). Experimental results clearly indicate that CAL-B mainly catalyzes the ring-opening polymerization (ROP) of ε-CL under the initiation of HAEP to form the homopolymer of ε-CL, while BSP catalyzes the subsequent polycondensation of the ROP product to yield the copolymer with increased molecular weight. Furthermore, using suitable chemo-enzymatic methods, valuable polyesters with chiral (R)- or (S)-ibuprofen pendants can be tailor-made.