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The well‐known sequential polymerization has largely been employed to develop stereoblock PLA with high molecular weight and crystallinity [47]. The formation of diblock copolymers of PLLA and PDLA was first reported by Yui et al. by adopting the sequential ROP strategy using aluminum tris(2‐propanolate) as a catalyst [48]. The interaction between the enantiomeric block sequences of di‐stereoblock (di‐sb) copolymers resulted in the formation of sc crystals with little homo‐crystallization. The molecular weight of the diblock copolymers was reported to be ~20 kDa; however, it is often essential to obtain the polymers with HMW (more than 100 kDa) in order to render them processable and dimensionally stable for possible applications. The two‐step ROP of L‐ and D‐lactides has been conducted to form di‐sb copolymers (PLLA–PDLA) where stannous octoate (Sn(Oct)₂) was employed as a catalyst (Figure 5.2). The prepolymer (PLLA or PDLA) having a molecular weight of <50 kDa was prepared in the first step and then purified to remove the residual lactide prior to synthesizing di‐sb PLA having different block ratios of PLLA/PDLA in the second stage of ROP. The di‐sb‐PLA had a M _w > 150 kDa, which, when solution‐cast into films, showed the formation of exclusive sc crystals. However, the heat deflection temperature of the di‐sb PLA having a non‐equivalent PLLA/PDLA ratio was somewhat lower, and the di‐sb‐PLAs having complementary PLLA/PDLA ratios were blended to adjust the whole PLLA/PDLA ratio to 50/50 and to increase the sc crystallinity [49]. In another study, tri‐stereoblock (tri‐sb) PLAs (ABA) having non‐equivalent block compositions were prepared by a two‐step ROP in the presence of 1,12‐dodecanediol as an initiator. The bis‐hydroxyl terminated PDLA was prepared in the first stage of ROP and purified to remove the residual D‐lactide. In the second stage of ROP, L‐lactide was used as a monomer to develop tri‐sb copolymers (PLLA‐PDLA‐PLLA) with different compositions of PLLA and PDLA blocks (Figure 5.3). The molecular weight of tri‐sb copolymers was higher than 100 kDa, and exclusive formation of sc crystals was found out without hc crystallization [50]. Another study applied a living polymerization system using magnesium‐based catalysts for synthesizing sb copolymers by the sequential ROP of L‐ and D‐lactides [51]. The first monomer L‐lactide was consumed rapidly (in minutes) in the presence of a magnesium complex to yield PLLA with a narrow molecular weight distribution. This process was followed by the addition of D‐lactide monomer for formation of a PLLA‐PDLA diblock copolymer with 500 repeating units within 30 min in a one‐pot manner. The resultant diblock copolymer having a regulated molecular weight resulted in the exclusive formation of sc crystals.

FIGURE 5.2 Two‐step ROP for the synthesis of di‐sb‐PLA.

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