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4 DESIGN AND SYNTHESIS OF DIFFERENT TYPES OF POLY(LACTIC ACID)/POLYLACTIDE COPOLYMERS
Ann‐Christine Albertsson, Indra Kumari Varma, Bimlesh Lochab, Anna Finne‐Wistrand, Sangeeta Sahu and Kamlesh Kumar
4.1 INTRODUCTION
High molar mass poly(lactic acid) (PLA) is obtained by either the polycondensation of lactic acid or ring‐opening polymerization (ROP) of the cyclic dimer 2,6‐dimethyl‐1,4‐dioxane‐2,5‐dione, commonly referred to as dilactide or lactide (LA). The stereochemistry of LA plays a crucial role in dictating the properties of the polymers which have already been described in earlier chapters (Chapters 1 and 3).
L‐Lactide (LLA) is prepared with relatively high enantiopurity from corn starch fermentation, which polymerized to form poly(L‐lactide) (PLLA). PLLA is a versatile, semicrystalline, degradable polymer with a relatively high melting (T m) and glass transition temperature (T g). PLLA has mechanical properties which makes it interesting for many applications such as degradable plastic for disposable consumer products [1–3]. It is also of interest in medical applications [4], due to its favorable interactions with the cells. PLLA is also explored as a degradable scaffold where the transplanted cells could remold their intrinsic tissue superstructural organization, and thereby led to the desirable three‐dimensional structure and physiological functionality of a regenerated organ [5]. However, certain shortcomings of PLLA may need to be overcome to extend its applications. In particular, high crystallinity [6], brittle behavior with a very‐low elongation at break value, and the hydrophobic nature of the polymer demands a long degradation time. The properties of PLLA are tailored by copolymerization (random, block, and graft), change in molecular architecture (hyperbranched polymers, star shaped, or dendrimers), introduction of polar groups such as carboxyl, amino, or thiol‐based via end group or main‐chain functionalization, or blending with other polymers. Physical properties, such as T g, T m, crystallinity, hydrophobicity, and mechanical properties are significantly affected by such modifications. Furthermore, functionalization of PLLA can provide specific bio‐interactions with cells, which is specifically needed in tissue engineering. Several reviews have summarized how functionalization of lactide and copolymerization with other hydroxyl‐acid‐based monomers influence the properties and applications of the resultant copolymers [5,7–15]. In this chapter, preparation of polymers and copolymers of LAs with different structures, using polycondensation and ROP, is described. The influence of macromolecular structure and composition on the properties of structurally modified polymers is also discussed. The stereocopolymers of LA prepared by the polymerization of various stereoisomers are discussed in a subsequent section in this book and will not be discussed here.
Typical comonomers and polymers which are used for lactic acid or LA copolymerization include glycolic acid or glycolide (GA) [16–22], poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO) [17, 18, 21,23–42], poly(propylene oxide) (PPO) [43, 44], β‐butyrolactone (BL) [45–48], δ‐valerolactone (VL) [49, 50], ε‐caprolactone (CL) [22,51–58], 1,5‐dioxepan‐2‐one (DXO) [59–64], p‐dioxanone [65–70], trimethylene carbonate (TMC) [58, 71], and N‐isopropylacrylamide (NIPAAm) [41,72–74]. The structures of some of these comonomers are given in Figure 4.1 [22, 42]. Monomer distribution (random or block) in the copolymers depends on the reactivity of monomer pairs, nature of the catalysts, and polymerization conditions.
FIGURE 4.1 Structures explored as comonomers with lactide monomer. (a) GA, (b) flourine substituted lactide monomer, (c) CL, (d) hydroxyl functionalized CL, (e) p‐dioxanone, (f) DXO, (g) BL, (h) VL, (i) TMC, (j) hydroxyl functionalized cyclic carbonate derivatives.
FIGURE 4.2 Structures of catalysts used in lactide polymerization.
Various catalysts and initiators used for ROP of LA monomers/comonomers are shown in Figure 4.2. Organometallic compounds such as stannous octoate Sn(Oct)2, aluminum isopropoxide [75], and zinc salt [76] as oxides, carboxylates, and alkoxides are reported as effective catalysts/initiators for polymerization reaction. Among all catalysts, Sn(Oct)2 is usually preferred as it mediates ROP reaction at a faster rate. However, resultant polymer showed a lower molar mass than when reaction was catalyzed by zinc metal or zinc lactate [13, 77]. Sn(Oct)2 allows formation of atactic PLAs with well controlled architecture, linear and star shape, and that too with high efficiency. Despite the usage of high temperature (usually 140°C) and/or high pressure, Sn(Oct)2 allows low percentage racemization [78]. Zn‐catalyzed PLA exhibited higher hydrophilicity and degradation susceptibility than the Sn‐mediated polymerization [19]. A partial esterification of alcohol chain ends in PLA by an octanoyl group is reported in Sn(Oct)2 catalyzed polymerization reaction [79]. While in case of Zn, occurrence of an extra lactoyl group is likely observed [80].
Metallic salts can either be used alone or along with a co‐initiator (water or alcohol) to affect