# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 9 Conclusions and Outlook  
Enzymatic polymerization is proven to be a powerful and versatile approach for the production of biobased polyesters and polyamides with different chemical compositions (aliphatic and semi-aromatic polymers), varied architectures (linear, branched and hyperbranched polymers), and diverse functionalities (pendant hydroxyl groups, carbon-carbon double bonds, epoxy groups, and so on). Among the enzymes studied for biobased polyester and polyamide synthesis, CALB, especially its immobilized form N435, shows broad monomer adaptability, stable and excellent catalytic performance, and great tolerance of various conditions. Moreover, with the mild synthetic conditions, non-toxic and renewable enzyme catalysts, and sustainable starting materials, synthesis of biobased polymers via enzymatic polymerizations provides an opportunity for achieving green polymers and a future sustainable polymer industry, which will eventually play an essential role for realizing and maintaining a green and sustainable society. However, this approach also possesses some limitations and disadvantages:the atom efficiency is low when ester derivatives rather than acids are used;
2 non-ecofriendly solvents including diphenyl ether and toluene are commonly used;
3 long polymerization times are required for achieving high molecular weights;
4 high reaction temperatures at around 100-140 \({}^{\circ}\)C were applied for enzymatic synthesis of polymers having a high \(T_{\rm m}\) and low solubility; and the catalytic reactivity of enzymes decreases significantly at such elevated temperatures;
5 the price of enzyme catalysts is still quite high;
6 enzymatic polymerizations involving monomers with short chain length like 1,3-propanediol, monomers with secondary hydroxyl groups such as isosorbide and 2,3-butanediol, and polyols, generally result in low molecular weight products;
7 the purify and price of biobased monomers remain a concern;
8 last but certainly not least, only limited variety of biobased monomers are currently commercially available. Therefore, more efforts are required to address these problems. For example, acids can be used to improve the atom efficiency, green solvents such as ionic liquids and supercritical CO\({}_{2}\) can be employed as the reaction media, more robust and thermal stable enzymes should be developed for enzymatic polymerizations, improved and optimal processes should be explored for the production of diverse biobased monomers with high purity and low price, and so on. Although numerous polyesters and polyamides are readily produced by using free lipases and immobilized lipases as the catalysts, the explanations for the different polymerization results are not clear yet. This could be an interest topic for the future research. Noteworthy is that many experimental results reveal that lipase-catalyzed polymerizations involving structurally similar monomers afford polymers with different compositions and varied molecular weights. This could be attributed to the synergistic effect caused by many reasons, for example, the specificity and selectivity of lipases towards different monomers, the physical properties of the starting materials (purity, melting temperature, and miscibility and solubility in the reaction media), the physical properties of the resulting intermediates and the final products (glass transition temperature, melting temperature, crystallization ability, and miscibility and solubility in the reaction media), the enzymatic polymerization conditions, and so on. However, such synergistic effect has not been fully understood yet, which requires systematic studies in the future. Besides, it would be of great interest to employ computer simulations to study the specificity and selectivity of lipases for the monomers in the enzymatic polymerization, as well as, the enzymatic polymerization mechanism. At present, enzymatic polymerizations have already been poised for use in commercial process to prepare polymers targeted for cosmetic and medical applications.