Synthesis and characterization of poly(lactic acid) based graft copolymers

Abstract

This review summarizes recent developments in the preparation and characterization of grafting of poly(lactic acid) or polylactide (PLA). PLA is the most expansively researched and utilized biodegradable, biocompatible, compostable, recyclable and renewable thermoplastic polyester. The graft copolymers of PLA have been synthesized and characterized by different spectroscopic techniques, including FTIR spectra and NMR data. The graft copolymers of PLA have been analyzed critically by taking different monomers/polymers; such as chitosan, cellulose, starch, polyethylene glycol, vinyl based polymers, lignin, dextran, methyl methacrylate, maleic anhydride and graphene oxide. In the first part of this review, the grafting of PLA and applications of grafted PLA has been discussed briefly. The second part, the major objective of this paper, focuses on the synthesis and characterization of different PLA based graft copolymers. For few cases, where useful properties, such as high molecular weight, narrow PDI, or stereocontrol, have been observed, a more detailed examination of the graft copolymers is provided.

Introduction

Since the beginning of the concept of macromolecular chemistry, it has been the constant challenge for polymer scientists to search and find new monomer-to-polymer systems that may lead to the polymers with accurately prepared architectures, controlled molecular weights and promising properties. Suitably customized polymers are capable of spontaneous or stimuli-induced supramolecular self-assembly in aqueous media or at interfaces to yield micro or nano-structured entities with a virtually unlimited number of potential applications in pharmaceutics [1], medical diagnostics [2], personal care [3], and biotechnology [4].

Common practice of organic polymer chemists is to adapt the known organic reactions in such a way that compounds of low molecular weights are converted to polymers of high molecular weights. The same approach has been taken in living polymerization, which was first discovered by Szwarc for the polymerization of vinyl and diene monomers and published in Nature [5] and Journal of American Chemical Society [6]. Later on, in 1968, he published the monograph on living polymerization, which gives the most difficult fragments of kinetics and thermodynamics of polymerization [7]. This discovery led to develop the controlled cationic [8] and radical [9] vinyl polymerizations, closely related to the living anionic polymerizations as given by Szwarc [5], [6], [7]. Further, surface modification of polymers is of great interest in many fields, ranging, for example, from the protection of surfaces from corrosion to improving their biocompatibility [10].

Graft copolymerization is a well-known technique to impart a new property or enhance the existing properties in the parent polymer with minimum degradation of the original properties. The nature of the changes in the properties depends on the type of monomer being grafted, percentage of grafting, method of grafting, and distribution of the grafted chain throughout the parent polymer [11], [12]. McDowall and co-workers reported [12] that the high energy radiation induced pre-irradiation method showed higher elongation, recovery, and more uniform graft copolymer distribution throughout the cross section than the ceric ion-initiation method. Graft copolymers are finding their applications in development of selective permeable membranes [13] and outstanding sorption agents [14]. Graft copolymers have also been used as stabilizers between grafted and ungrafted polymers [15], [16].

Various extracellular matrix (ECM) proteins, such as collagen, fibronectin, and laminin, have been used as substrates for cell culture [17], [18], [19], [20]. However, they cannot be easily or reproducibly processed into three-dimensional scaffolds of good stability and mechanical properties. Thus, the use of polymeric materials as substrates has increased substantially in recent years because of their excellent mechanical properties and processability [21], [22]. Although bulk properties dictate the mechanical performance of biomaterials, cell and tissue-biomaterials interactions are surface phenomena and are governed by surface properties. Therefore, surface modification of polymer substrates is often required to improve their biocompatibility and also to allow the subsequent surface functionalization, such as enzyme and protein immobilization via covalent bonding [22], [23]. Thus, grafting provides a significant route to alter the physical and chemical properties of the polymer for specific end uses.