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The formation of sc‐PLA usually occurs by blending PLLA/PDLA, which requires synthesis of the individual polymers by the ring‐opening polymerization (ROP) of L‐ and D‐lactides prior to blending the respective enantiomeric PLA chains. The sc‐PLA thus formed exhibits improved thermal and mechanical properties; however, the process requires the formation of enantiopure polymers from the respective enantiopure monomers, which restricts its practical applications. In this regard, the single step preparation of sc‐PLA has been explored from the readily available racemic lactide (rac‐LA, or DL‐lactide, 1 : 1 mixture of L‐ and D‐lactides), which is inexpensive (Figure 5.1). The ROP of rac‐LA usually yields amorphous polymers (atactic or heterotactic) having lower T _g and T _m than those of the isotactic PLA, i.e., PLLA and PDLA. The random arrangement of L‐ and D‐lactide units in the backbone chain is usually observed upon polymerizing rac‐LA with a limited scope of application. To extend the utilization of rac‐LA precursor, significant efforts have been made in achieving the stereoselective polymerization of LA that causes the resulting stereochemistry in the product [41–43). The direct formation of sc‐PLA from rac‐LA has been achieved by Radano et al. using triethylaluminum‐based catalysts [44]. The formation of sc‐PLA is supported by XRD analysis; however, the lower degree of isotacticity reduces the T _m of the resulting polymer to ~191°C as compared with ~230°C of the usual sc. The degree of crystallinity of sc‐PLA is ~42% as determined from the enthalpy of fusion (ΔH _fus). In another study, semicrystalline stereoblock copolymers with HMW (~461 kg/mol) have been produced from rac‐LA using chiral oxazolinyl aminophenolate magnesium complexes. The isoselective control of the magnesium complexes is affected by the chirality of the ligand, which may be modified to develop more efficient catalysts [45]. Furthermore, the ROP of rac‐LA by a series of achiral iron complexes has been performed by Marin et al. to yield HMW stereoblock copolymers [46]. The stereoselective catalysts permit the formation of stereoblock copolymers under mild conditions. The catalyst complex also influences the tacticity of the resulting polymers and the stereochemistry. The thermal degradation temperature of the isotactic stereoblock PLA is increased up to ~350°C, which is reasonably dependent on its molecular weight [46]. The degradation temperature correlates more strongly with the molecular weight than with the stereoregularity. The developed stereoblock PLA having high molecular weight may be used for industrial applications.

TABLE 5.1 Unit Cell Parameters Reported for the sc Crystals

	Okihara et al. [32]	Brizzolara et al. [34]	Cartier et al. [33]	Sawai et al. [35]	Tashiro et al. [36]
Crystal system	Triclinic	Triclinic	Trigonal	Trigonal	Trigonal
Chain conformation	31	31	31 and 32	31 and 32	31
Unit cell parameter
a (nm)	0.916	0.912	1.498	1.50	1.494
b (nm)	0.916	0.913	1.498	1.50	1.494
c (nm)	0.870	0.930	0.870	0.823	0.862
α (°)	109.2	110	90	90	90
β (°)	109.2	110	90	90	90
γ (°)	109.8	109	120	120	120
ρ calc (g/cm3)	1.27	1.21	1.27	1.342	—

FIGURE 5.1 Stereoblock PLA formation from rac‐LA in a single step.