Kazunari Masutani

The ABCBA pentablock copolymers (p‐d‐l‐PPS) comprising poly(d‐lactide) (PDLA: A), poly(l‐lactide) (PLLA: B) and poly(propylene succinate) (PPS: C) were successfully synthesized by two‐step ring‐opening polymerization (ROP) of d‐ and l‐lactide using a dihydroxy‐terminated PPS as a macro‐initiator. The pentablock copolymers revealed the high stereocomplex (sc) crystallinity, thermal stability and elastomeric property in their solution‐cast films. It was found that the T_g was found to be proportional to the PPS content, whereas the T_m was proportional to their average block length. The thermal resistivity of the copolymer films was found to be as high as 202°C owing to their sc formation. The copolymers also showed improved stereocomplexibility compared to the enantiomeric mixtures of triblock copolymers (PLLA‐PPS‐PLLA and PDLA‐PPS‐PDLA) having similar PLLA and PDLA chain lengths. These pentablock copolymers can afford thermoplastic elastomers or flexible plastic materials having a 100% bio‐based content, showing high heat‐resistive property.

Bis‐hydroxyl‐terminated poly(1,2‐propylene succinate) (PPS‐diols) with high molecular weight (10–40 kDa) are prepared by two‐step melt polycondensation of succinic acid and 1,2‐propanediol with Ti(BuO)₄ as the catalyst. By using these PPS‐diols as macroinitiators, the ring‐opening polymerization of d‐ and l‐lactides is readily conducted to obtain enantiomeric ABA triblock copolymers consisting of poly(l‐lactide) and PPS (B) (t‐l‐PPS) as well as those of poly(d‐lactide) and PPS (B) (t‐d‐PPS) which have higher PPS compositions (20–70 wt%) in addition to high molecular weight (20–80 kD). The T_g, T_m, and ΔH_m values of the t‐l‐PPS copolymers as well as the stereo mixtures of t‐l‐PPS/t‐d‐PPS are controlled to linearly decrease with increasing the PPS content. The copolymers also exhibit higher elastomeric properties with increasing the PPS content. The tensile properties of the copolymer films having higher PPS contents (both the single block copolymers and stereo mixtures) are comparable to those of the oil‐based thermoplastic elastomers. It is therefore concluded that these block copolymers can afford thermoplastic elastomers or flexible plastic materials having a 100% biobased content.

Poly(1,2‐propylene succinate) (PPS) having high molecular weight can be synthesized by multi‐step melt‐polycondensation of succinic acid (SA) and 1,2‐propylene glycol (PG) with various catalysts. The first step is noncatalytic esterification/oligomerization of the two monomers, followed by the second step of catalytic melt‐polycondensation. In this step, co‐catalyst systems of Zn(AcO)₂/Ge(OBu)₄ and Zn(AcO)₂/Ti(BuO)₄ are effective for obtaining PPS having middle molecular weights (>10.0 kDa). This middle‐molecular‐weight PPS is chain‐elongated in the third‐step polycondensation with Zn(AcO)₂ as the catalyst to obtain a molecular weight reaching 120 kDa. As verified by ¹H‐ and ¹³C‐NMR spectra combined with two‐dimensional experiments, PPS has a ω‐bis‐hydroxy structure where the PG units leave the secondary hydroxyl terminals in larger ratio than the primary hydroxyl terminals. The PPS polymers are amorphous in nature, showing T_g around −4 °C. PPS can be solution‐ and melt‐blended with poly(l‐lactide) (PLLA). By melt‐blending a high‐molecular‐weight PPS in an amount of 7.5–15 wt %, the modulus of the PLLA films decreases below 2000 MPa and the tear strength increases twice, supporting the effectiveness of PPS polymer in imparting flexible nature to PLLA. PPS polymers can therefore be applicable as elastomeric or flexible plastic modifiers having a 100 % biobased content. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1795–1805

The ability to control the micelle size of poly(lactic acid) and poly(ethylene glycol) (PLA–PEG) block copolymers is important for controlling their circulation in blood cell recognition, drug release and therapeutic effects. We successfully controlled the micelle size by changing the block number of copolymers (multiblock index). PLA–PEG multiblock copolymers with multiblock indexes ranging from 1.35 to 2.78 were synthesized by direct polycondensation with tin chloride/p-toluenesulfonic acid binary catalysts, using PEG with a molecular weight (M_w) of 3200 Da. The M_w of PLA–PEG copolymers increased with an increase in the multiblock index, while micelle size, measured by dynamic light scattering, decreased greatly from 349 to 28 nm. In addition, the X-ray diffraction peak of the PLA crystal disappeared when the multiblock index was increased. These results indicate that a multiblock structure is useful for controlling micelle size without changing the PLA/PEG composition or PEG molecular weight, which strongly influences other micelle features.