Poly(lactic Acid): A Versatile Biobased Polymer for the Future with Multifunctional Properties-From Monomer Synthesis, Polymerization Techniques and Molecular Weight Increase to PLA Applications

Abstract

Environmental problems, such as global warming and plastic pollution have forced researchers to investigate alternatives for conventional plastics. Poly(lactic acid) (PLA), one of the well-known eco-friendly biodegradables and biobased polyesters, has been studied extensively and is considered to be a promising substitute to petroleum-based polymers. This review gives an inclusive overview of the current research of lactic acid and lactide dimer techniques along with the production of PLA from its monomers. Melt polycondensation as well as ring opening polymerization techniques are discussed, and the effect of various catalysts and polymerization conditions is thoroughly presented. Reaction mechanisms are also reviewed. However, due to the competitive decomposition reactions, in the most cases low or medium molecular weight (MW) of PLA, not exceeding 20,000-50,000 g/mol, are prepared. For this reason, additional procedures such as solid state polycondensation (SSP) and chain extension (CE) reaching MW ranging from 80,000 up to 250,000 g/mol are extensively investigated here. Lastly, numerous practical applications of PLA in various fields of industry, technical challenges and limitations of PLA use as well as its future perspectives are also reported in this review.

Keywords: applications; catalysts; chain extension; melt polycondensation; poly(lactic acid); ring opening polymerization; solid state polymerization; synthesis.

Figure 1

(a) Global production of bioplastics 2020 (by material type); (b) global production capacities of bioplastics 2020 (by market segment) (https://www.european-bioplastics.org/market, accessed on 27 April 2021) [8].

Scheme 1

Lactic acid production by fermentation procedure [29].

Scheme 2

Reactions taking place during poly (lactic acid) synthesis [22].

Scheme 3

Coordination– insertion mechanism for ROP of lactide Reprinted with permission from ref. [55]. Copyright 2017 Springer International Publishing AG.

Scheme 4

Activated monomer mechanism for ROP of lactide Reprinted with permission from ref. [55]. Copyright 2017 Springer International Publishing AG.

Scheme 5

Ring-opening polymerization of lactide by a mechanism involving nucleophilic activation of the polymer chain end in the presence of an N-heterocyclic carbene catalyst Reprinted with permission from ref. [55]. Copyright 2017 Springer International Publishing AG.

Scheme 6

Cationic ROP of LA for PLA synthesis Reprinted with permission from ref. [71]. Copyright 2000 Springer International Publishing AG.

Figure 2

Plot of Mn vs. time at for L-lactide polymerization at different temperatures: o—150 °C; ■—160 °C; ▲—180 °C [74].

Scheme 7

Reaction mechanism for PLA synthesis.

Scheme 8

Anionic ROP of LA for PLA synthesis Reprinted with permission from ref. [77]. Copyright 1993 Hüthig & Wepf Verlag, Basel.

Scheme 9

Ring-opening of monomer by (1) acyl- oxygen bond cleavage and (2) alkyl-oxygen bond cleavage Reprinted with permission from ref. [78]. Copyright 2002 Hüthig & Wepf Verlag, Basel. Springer-Verlag Berlin Heidelberg.

Scheme 10

Mechanism of lipase ROP.

Figure 3

MW growth during SSP at constant and step-raised temperatures for pcPLLA70/60 and pcPLLA110/30, respectively: (○) 70 °C/60 min + 150 °C/20 h; (□) 110 °C/30 min + 150 °C/40 h; (●) 70 °C/60 min + 150 °C/5 h + 155 °C/5 h + 160 °C/10 h; (■) 110 °C/30 min + 150 °C/5 h + 155 °C/5 h + 160 °C/5 h + 165 °C/5 h Reprinted with permission from ref. [89]. Copyright 2012 American Chemical Society.

Figure 4

The molecular weight of PLLA prepolymers crystallized for various times as a function of SSP time (a) crystallized for 15 min, (b) crystallized for 30 min, (c) crystallized for 45 min, (d) crystallized for 60 min, (e) crystallized for 75 min, and (f) crystallized for 90 min Reprinted with permission from ref. [90]. Copyright 2006 Taylor&Francis.

Scheme 11

Chemical structure of chain extenders: (a) Joncryl^® ADR 4368 copolymer produced by BASF Co, (b) 1,3-phenylene-bis-2-oxazoline, (c) pyromellitic dianhydride, (d) 1,10-carbonyl bis caprolactam, (e) diisocyanate and (f) diepoxide.

Figure 5

Weight average molecular mass and the polydispersity of PLA with CESA [101].

Figure 6

PLA applications in end-use industries and its’ global market consumption.

Figure 7

Appearance of packaged bread stored in (a) PP and (b) PLA and PBAT blend films with different concentrations of transcinnamaldehyde (2%, 5% and 10%) for 21 days at 25 °C. Circles indicate mold growth on bread Reprinted with permission from ref. [134]. Copyright. 2020 Elsevier Ltd.

Figure 8

SEM micrographs of 3D hierarchical domains on the surface of PLA films at different magnifications: (a) black silicon nanostructures on spherical micropillars [136]; (b) rectangular hierarchical nanostructured PLA nanocomposite surfaces [137].

Figure 9

(a) SEM analysis, the reference strain SEM observations of “rod-shaped” E. coli and “coccoid-shaped” S. aureus on flat unpatterned, black silicon, and hierarchical domains on the surface of the PLA films [136]. (b) Number of CFU formed on the surface of the films prepared in this work [137].

Figure 10

Array of organic field-effect transistors fabricated on PLA Reprinted with permission from ref. [165]. Copyright 2014 Elsevier B.V.

Figure 11

Components and designs of a wall structure with PLA and date pit powder Reprinted with permission from ref. [168]. Copyright 2020 Elsevier Ltd.

Figure 12

Schematic formation of the CS/Col/PLA/nHAP hybrid scaffold Reprinted with permission from ref. [194]. 2019 Elsevier B.V.

Figure 13

(I) SEM micrographs of risperidone microspheres prepared by neat PLA (a), PLA/PPAd 80/20 w/w (b), PLA/PPAd 60/40 w/w (c), PLA/PPAd 40/60 w/w (d), PLA/PPAd 20/80 w/w (e) and neat PPAd (f). (II) In vitro release profiles of risperidone drug loaded microspheres prepared using PLA, PPAd and PLA/PPAd blends [229].

Figure 14

The Global PLA Plant Capacity vs. the Market Demand Predicted by Jem’s Law [9].