Abstract
Given the fossil fuel crisis and the steady consumption of finite resources, the use of green polymers is becoming necessary. However, the term “green” describes materials that present green properties (such as biological origin and/or biodegradability) and are produced via sustainable processes conducted under mild conditions and not requiring the use of chemical catalysts, toxic solvents or reagents. Truly green materials must combine these characteristics; consequently, enzymatically synthesized bio-based and/or biodegradable polymers can be characterized as truly green. The present review focuses on the most promising, commercially available aliphatic and alipharomatic polyesters that can be synthesized enzymatically. In particular, the recent developments in the enzymatic polymerization of PLA and PBS and alipharomatic furan-based polyesters (e.g., PBF) are herein analyzed. Based on this analysis, it can be concluded that important steps have been taken toward synthesizing sustainably green polymers. Still, it is necessary to evaluate the applied methods regarding their capability to be used on an industrial scale.
Highlights
Even though most plastics are still fossil-based, the fossil fuel crisis has become more and more evident, especially since the late 1990s [1]
Bio-based polymers include a. natural polymers obtained from biomass, b. polymers synthesized from microorganisms (e.g., PHAs) and c. polymers originated from bio-based monomers and synthesized in laboratories or by industry (e.g., PLA, poly(butylene succinate) (PBS), Poly(ethylene furanoate) (PEF)) [3]
The used enzyme was Novozyme 435 (N435), but PDLA was synthesized in toluene at 80 ◦C, while PLLA was synthesized in a homogenous media composed of the ionic liquid [C4MIM] [PF6] in combination with miscible compressed 1,1,1,2-tetrafluoroethane (R134a). This is very promising, given that the PDIs of the polymers were in the range of 1.1–1.7, the percentages of crystallinity of the PLLAs were in the range 43–50%, the Tg of the polymers were in the range of 48.8–62 ◦C, the melting temperatures ranged between 109 and 138 ◦C and the degradation temperatures of the polymers were between 155 and 122 ◦C
Summary
Even though most plastics are still fossil-based, the fossil fuel crisis has become more and more evident, especially since the late 1990s [1]. The use of SCFs may, influence enzyme activity by a direct pressure effect, which may lead to denaturation, and by the indirect effects of pressure on enzymatic activity and selectivity; when supercritical CO2 is used, lower catalytic activities may be observed due to the formation of carbonic acid [66] Another approach to developing a green process for the enzymatic synthesis of PLA is the combination of ILs and SCFs. According to Mena et al [43], the use of a homogenous medium composed of the ionic liquid [C4MIM] [PF6] with miscible compressed 1,1,1,2-tetrafluoroethane (R134a) above the critical points of the mixture led to the successful enzymatic syntheses of linear and hyperbranched PLLA. Zhao et al [41] noticed that a temperature of 130 ◦C led to higher molecular weights and conversions than lower temperatures (80–110 ◦C) in [BMIM] [PF6], so 130 ◦C was the selected reaction for the polymerization process that was conducted for 7 days
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