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FIGURE 5.3 Two‐step ROP for the synthesis of tri‐sb PLA followed by chain extension to form multi‐sb‐PLA.

      5.4.3 Chain Coupling Method

       5.4.3.1 Chain Extension

The formation of stereoblock copolymers by chain extension is another strategy well explored by researchers for tailoring the properties of PLA. In a study reported by our group, the ROP of D‐ and L‐lactides was performed to yield mono‐maleimide‐terminated PDLA (M‐PDLA) and mono‐anthracene‐terminated PLLA (A‐PLLA), respectively [52]. This was followed by the reaction of A‐PLLA with hexamethylene diisocyanate (HMDI) to dimerize and form di‐anthracene‐terminated two‐armed PLLA (A‐PLLA‐A). The stereo di‐ and tri‐block copolymers were readily formed by the terminal Diels‐Alder coupling reaction between A‐PLLA/M‐PDLA and A‐PLLA‐A/M‐PDLA, respectively. This process resulted in the development of copolymers with improved thermomechanical and thermal properties due to the easy sc crystal formation. Extending the possibilities of applying Diels–Alder reactions to the sb‐PLA formation, the isocyanate coupling of maleimide‐terminated PLLA (M‐PLLA) and furan‐terminated PDLA (F‐PDLA) was conducted to yield bis‐maleimide‐terminated PLLA (M‐PLLA‐M) and bis‐furan‐terminated PDLA (F‐PDLA‐F), which were mixed in 1 : 1 ratio in solution. The resulting solution was electrospun where the sc‐PLA was formed by the terminal Diels–Alder coupling between the respective enantiomeric polymers [53]. The chain extension reaction was ascertained by the molecular weight of the electrospun fibers, which was found to increase from 10 to 45 kDa after the electrospinning and annealing. The fibers (as spun) were converted from the amorphous or semicrystalline state to the fully crystalline state by thermal annealing with the formation of sc crystals [53]. Chain extension by HMDI has also been used to develop multiblock copolymers with significantly improved mechanical properties. In line with this, tri‐sb copolymers synthesized by two‐step ROP have been subjected to chain extension by using HMDI to develop multiblock sb‐PLA (multi‐sb) copolymers. For example, tri‐sb‐PLA copolymers having equivalent composition of PLLA and PDLA enantiomers were reacted with HMDI to form multi‐sb‐PLA copolymers with controlled block sequences, which were reported to exclusively form sc crystallites by suppressing the hc crystallization [54]. The multi‐sb copolymers having longer PLLA/PDLA blocks, upon annealing, showed improved thermo‐mechanical properties along with higher sc crystallinity. It was remarked that the thermal properties of multi‐sb copolymers can be tailored by controlling the block lengths of tri‐sb‐PLA [54]. The sc crystallization in multiblock copolymers by dynamic Monte Carlo simulations has been reported by Qiu et al. who identified the effect of block numbers and the crystallization temperature on the sc formation [55]. Several systems with alternating sequences of A and B were used, namely A/B blend, A–B diblock, tetrablock, octablock, and sixteen‐block copolymers. In multiblock copolymers with low block numbers, sc formation was found to increase with increasing crystallization temperature. The effect of crystallization temperature was not detected for the multiblock copolymers with relatively high block numbers. The miscibility between the different blocks, the block length, and the size of the crystal thickness were reported to be the governing parameters for sc formation in the multiblock copolymers.

       5.4.3.2 Click Chemistry

      Click chemistry [56] has evolved as an efficient and simple approach to develop HMW sb‐PLA with controlled block lengths and chain architecture. The combination of ROP and click chemistry has been shown by Han et al. to prepare HMW sb‐PLA (PLLA–PDLA) with controlled composition and block length [57]. The sc‐crystallinity was found to increase when the PLLA/PDLA block ratio approached 50/50, which also increased the storage modulus of HMW sb‐PLA. In another study, Isono et al. applied the combination of sb and star polymer approaches and succeeded in synthesizing stereo‐miktoarm star‐shaped PLAs by a click coupling method [58]. This approach led to the preferential sc formation in the solution cast samples, without any homo‐crystallite formation. The variation in the arm number also led to the variation in T _m of the stereo‐miktoarm star polymers, thereby providing a mechanism to tailor the physical properties of sc‐type PLAs. Besides these approaches, the synthesis of cyclic stereoblock copolymers of PLLA and PDLA with head‐to‐head and head‐to‐tail configurations has been carried out by implementing the click chemistry and ring closing metathesis approaches [59]. The orientation of PLLA and PDLA segments in the cyclic structure may be controlled in a facile way by using this approach, along with modulated thermal properties.

       5.4.3.3 Polycondensation

The melt polycondensation of low‐molecular‐weight PLLA/PDLA blends has been adopted to synthesize stereo multiblock PLAs having different block lengths. In contrast to the chain extension reaction, the stereo multiblock PLAs synthesized by melt‐polycondensation have higher crystallizability due to the connecting ester groups [60]. Further, stereo multiblock copolymers were synthesized by melt‐polycondensation as reported by Rahaman et al. with a wide range of block lengths [61], which led to the preferential formation of stereocomplex crystallites irrespective of the crystallization temperature and block length. In a study reported by our group, the PLLA and PDLA prepolymers were made by the melt‐polycondensation technique and mixed to develop sb‐PLA consisting of short sequences of D‐ and L‐lactate units where the molecular weight was as high as 100 kDa at the reaction temperature of 180°C [62]. The molecular weight of the homopolymer PLLA or PDLA obtained by SSP was much higher than sc‐PLA, even at a considerably lower reaction temperature. This fact was attributed to the partial chain racemization and difficulty of the elongated chains to crystallize out of the amorphous domain into the solid state. To enhance the crystallization of the elongated chains, it was speculated that the presence of homo‐crystallites may be essential. To further substantiate the hypothesis, non‐equivalent mixtures of PLLA and PDLA were used for SSP [63]. The melt blending of PLLA and PDLA (medium molecular weight) obtained by melt‐polycondensation of L‐ and D‐lactic acids was performed to obtain sb‐copolymers with different compositions and block sequences. The SSP reaction under mild conditions resulted in products with higher yield and larger molecular weight. The molecular weight and the composition of the block sequences were found to govern the thermal properties. The elongated chains of the homo‐sequences were able to crystallize out from the reaction system to allow for the chain extension reaction. Thus, the necessity of the hc domain for developing sb‐PLA with HMW by SSP was determined. Based on this evidence, a new design was proposed where the prepolymers PLLA and PDLA were mixed in the powder state (1 : 1) and subjected to heat treatment to form partial sc‐crystallization at the boundaries, followed