Stereoselective Ring-Opening Deacetonation Polymerization of Racemic 2,2,5-Trimethyl-1,3-dioxolan-4-one by Using Homosalen–Aluminum Complexes: A Novel Approach to Isotactic ­Poly(rac-lactic acid)

Dedicated to Professor Hisashi Yamamoto on the occasion of his 80th birthday.

Abstract

A novel approach is reported for the synthesis of the highly isotactic poly(racemic lactic acid) [P(rac-LAA)] from racemic 2,2,5-trimethyl-1,3-dioxolan-4-one by using achiral homosalen–Al catalysts. The polymerization proceeded through two steps: stereoselective ring-opening and deacetonation. The effects of substituents on the homosalen–Al catalyst were clarified regarding the polymerization rates and polydispersity of the obtained P(rac-LAA), but were still unclear regarding the iso-stereoselectivity. The P(rac-LAA) obtained by using an isopropyl-substituted homosalen–Al catalyst showed the highest isotacticity (P _m = 0.96), a moderate number-average molecular weight (M _n = 6,200), and broad polydispersity (PDI = 2.2₀). Differential scanning calorimetric measurements indicated that the polymer was semicrystalline with a melting temperature of 156 °C.

Poly(lactic acid) [P(LAA)] is a biobased biodegradable polymer. Consequently, it is generally considered to be an environmentally benign material, and has gained great attention due to recent environmental concerns.[1] Commercially available P(LAA) is, in reality, isotactic poly(l-lactide) [P(l-LA)] produced via the ring-opening polymerization of l-lactide (l-LA) with high optical purity (Scheme [1a]). The melting temperature (T _m) of P(l-LA) is ~170 °C,[2] and is highly dependent on the optical purity of the l-LA. It is reported that polymerization of l-LA with ~15% of the diastereomeric meso-lactide (meso-LA), corresponding to a mixture of the ~92% l-lactic unit and ~8% d-lactic unit in the system, afforded amorphous P(LAA), due to its stereoirregularity.[3] The formation of a stereocomplex between P(l-LA) and poly(d-lactide) [P(d-LA)] is known, and the stereocomplex P(LAA) has a higher T _m (230 °C)[4] than that of the optically pure P(l-LA) or P(d-LA) (~170 °C). The iso-selective polymerization of racemic lactide (rac-LA) has extensively been studied over the last three decades.[5]

There are two patterns of stereoregularity of P(rac-LA): isotactic (semicrystalline) and heterotactic (amorphous) (Scheme [1b]). In highly isotactic P(rac-LA) a stereocomplex is formed between the P(l-LA) blocks and the P(d-LA) blocks. We have reported a highly iso-selective polymerization of rac-LA using homosalen–Al catalysts,[6] and have applied this technique for the utilization of racemic lactic acid (rac-LAA) derived from a glycerol byproduct.[7] However, the synthetic efficiency from rac-LAA to rac-LA could not be high due to the formation of meso-LA in addition to the desired rac-LA in a ~1:1 ratio. This issue arises from the presence of two lactic units in a lactide molecule. This prompted us to develop a polymerization process using a racemic monomer containing one lactic unit.[8] Among the various candidates, the framework of 5-methyl-1,3-dioxolan-4-one (1, R = H; Scheme [1c]) seemed to show promise for this purpose. The expected polymerization includes the two repeating processes of stereoselective ring-opening and deketonation [– R¹C(O)R²], and two patterns of the stereoregular P(LAA) can be expected: isotactic and syndiotactic. This polymerization system is intriguing because either stereoselectivity affords the semicrystalline poly(rac-LAA) [P(rac-LAA)] from the racemic monomer.[9] In addition, it could be a new route to synthesize syndiotactic P(LAA), which is inaccessible from rac-LA.

We synthesized five derivatives of 1 (R _² = H₂; Me₂; Et₂; Ph, H; O) in yields of 45, 68, 37, 15, and 30%, respectively. Racemic 2,2,5-trimethyl-1,3-dioxolan-4-one (1, R = Me) (rac-TDO) was found to be the most accessible due to its easy synthesis and simple purification with a reasonable boiling point (86 °C at 68 Torr), good thermal stability, and the suitable boiling point of acetone (56 °C) for deacetonation from the reaction system at the polymerization temperature (100 °C). To the best of our knowledge, the stereoselective polymerization of rac-TDO has never previously been reported.[10]

We then evaluated the bulk polymerization of rac-TDO under N₂ at 100 °C using a series of homosalen–Al catalysts 2, previously found to be effective for the highly iso-selective ring-opening polymerization of rac-LA (Table [1]).[11] We used 2.5 mol% of the precatalysts 2 [12] in the presence of 1 mol% of 2-phenylethyl alcohol, and we successfully obtained P(rac-LAA).[13] The polymerization was generally slower when R¹ was bulkier (Table [1], entries 1–3 vs entries 4 and 5). The number-average molecular weights (M _n) of the polymers were lower than expected,[14] probably due to traces of H₂O in the system and in the other starting compounds, such as the ligand of the catalyst (see below). The polydispersity index number (PDI) was high when R¹ was small (entries 1–3), probably as a result of transesterification of the polymer main chains due to the small R¹. These substituent effects of the homosalen–Al catalyst were also observed during the polymerization of rac-LA. In sharp contrast, the substituent effects on the stereoselectivity [probability of meso-linkage (P _m)] were much smaller, and the P _m values of the resulting P(rac-LAA) were between 0.94 and 0.96.

Table 1 Substituents Effects of Homosalen–Al Catalysts in the Iso-Selective Polymerization of rac-TDO in the Presence of R³OH^a

Entry	R1	R2	Time (h)	Conv.b (%)	M n c	PDId	P m e	T m f (°C)
1	H	H	3	81	4,900	2.21	0.95	154
2	Me	Me	6	86	6,300	2.15	0.94	155
3	i-Pr	H	6	93	6,100	2.20	0.96	156
4	t-Bu	t-Bu	23	57	2,800	1.23	–g	–g
5	PhC(Me2)	H	23	50	4,200	1.12	0.95	136

^a Reaction conditions: rac-TDO (2 μL, 2.00 mmol), precatalyst (2.5 mol %), PhCH₂CH₂OH (1.0 mol %), 100 °C, under N₂ (1 atm; natural-rubber gas-sampling bag).

^b Conversion of rac-TDO.

^c Number-average molecular weight of the crude P(rac-LAA) as determined by size-exclusion chromatography (SEC; CHCl₃, 40 °C, polystyrene standards).

^d Polydispersity index number (M _w/M _n) of the crude P(rac-LAA), as determined by SEC.

^e Stereoregularity of the purified P(rac-LAA) (probability of the meso-linkage). The P _m values of crude P(rac-LAA) were the same as those of purified samples, except for entry 3 (P _m = 0.95 before purification).

^f Melting temperature.

^g Not measured.

The crude samples of P(rac-LAA) obtained in entries 1–3 were analyzed by size-exclusion chromatography (SEC) to determine their M _n, and an unexpectedly broad UV absorption (λ = 254 nm) was detected. The crude P(rac-LAA) obtained in entries 4 and 5 did not show a corresponding absorption. The contrasting SEC traces of the samples of P(rac-LAA) in entries 3 and 4 are shown in Figure [1]. We assumed that a proportion of the phenoxide groups of catalyst ligands that had small substituents (R¹ = H, Me, i-Pr) initiated the polymerization of rac-TDO. The initiation reaction rate of the phenoxide group should be much slower than that of the alkoxide group; therefore, a broad UV absorption was observed. When the crude P(rac-LAA) was stirred in CHCl₃ as purchased, which contained 0.3–1% of EtOH as a stabilizer, the UV absorption of P(rac-LAA) became weak, and the values of M _n and PDI determined by using the refractive-index (RI) detector were practically maintained. It is suggested that the reactive phenyl ester group at the initiation terminus was converted into an ethyl ester group by EtOH and/or a carboxylic acid group by H₂O. Despite of our examination of various conditions (for example, the use of 0.50 mol% of R³OH or a longer polymerization time), attempts to synthesize isotactic P(rac-LAA) with an M _n of over 10,000 were not successful.

Stereoirregular P(rac-LAA) should give multiplet peaks in the methine region at δ = 5.15 ppm in the ¹H NMR spectrum, whereas a quartet peak should be observed in isotactic or syndiotactic P(rac-LAA).[5] [6] [9] The ¹H NMR spectrum of the obtained P(rac-LAA) showed a major quartet peak (Figure [2a]), and the homonuclear-decoupled ¹H NMR showed a sharp singlet peak with two minor peaks (Figure [2b]). We examined the ¹H NMR spectrum of a mixture of the P(rac-LLA) obtained here and the authentic isotactic P(rac-LA) obtained from rac-LA (1:2 weight ratio) to identify the tacticity and we excluded the possibility of formation of syndiotactic P(rac-LLA).

The isotactic P(rac-LAA) (M _n = 3,400 and PDI = 1.4₇ by SEC) synthesized from rac-TDO using homosalen–Al catalyst 2 (R₂ = Ph, H, not listed in Table 1) was characterized by MALDI-TOF mass spectrometry. Figure [3] shows a part of the MALDI-TOF mass spectrum. The major peaks marked with asterisks (*) appeared at 72 m/z intervals, corresponding to the molecular weight of a repeating lactic unit (monoisotopic mass: 72.021), and their molecular weights were consistent with a 2-phenylethyl ester group from the initiation reaction as the α-terminus and a hydroxy group as the ω-terminus. The other minor peaks marked with daggers (^†) also appeared at 72 m/z intervals, and their molecular weights were consistent with a carboxylic acid group as the α-terminus and a hydroxy group as the ω-terminus. The carboxylic acid seemed to form by an initiation reaction with trace amounts of H₂O. Both the ¹H NMR and MALDI-TOF mass analyses confirmed that deacetonation proceeded effectively during the polymerization.

The stereoregularity of P(LAA) affects its crystallinity and T _m, and we have reported a linear relationship between P _m and the melting temperature of P(LAA).[15] We studied the thermal properties of the samples of isotactic P(rac-LAA) obtained in Table [1] by differential scanning calorimetry (DSC). The T _m value and the enthalpy of fusion (ΔH _f) were, respectively, 154 °C and 41.4 J/g for Table [1], entry 1, 155 °C and 46.9 J/g for entry 2, and 156 °C and 45.0 J/g for entry 3 (Figure [4]). Although the stereoregularity was high, the T _m values were much lower than those of isotactic P(rac-LA). This is reasonable because LA contains two lactic units with the same chirality, whereas TDO contains only one lactic unit. Therefore, P(rac-LAA) with P _m = 0.95 should have stereoblock sequences identical to those of P(rac-LA) with a P _m = 0.90.[16] It should also be acknowledged that the T _m values are dependent on the molecular weights until M _n typically reaches values of tens of thousands.

The examined homosalen–Al catalysts do not have any chiral auxiliaries, and their conformation is flexible at room temperature.[6c] Therefore, the iso-selectivity of P(rac-LAA) is attributed to the chain-controlled mechanism, as reported for the polymerization of rac-LA. In the first step, a ring-opening reaction occurs through the reaction of achiral L₂Al–OR with rac-TDO; this is followed by deacetonation to afford (S)- and (R)-A (Scheme [2]). These species were also detected in the polymerization of rac-LA. The presence of the asymmetric alkoxide close to the Al center leads to a chiral environment around the Al center. (S)- and (R)-A can then differentiate the chiral sense of TDO and selectively react with TDO bearing the same sense of chirality to afford the isotactic P(rac-LAA) with multistereoblock sequences.

In conclusion, a highly iso-selective ring-opening deacetonation polymerization of rac-TDO was achieved for the first time. Homosalen–Al catalysts were effective for this stereoselective polymerization, and isotactic P(rac-LAA) (P _m = 0.96) with M _n = 6,100, PDI = 2.2₀, and T _m = 156 °C (ΔH _f = 45.0 J/g) was obtained using the isopropyl-substituted catalyst. Although there are still issues to overcome, such as a broad polydispersity, low molecular weight, and low T _m, this study showed that rac-TDO is a suitable monomer for the synthesis of highly isotactic P(rac-LAA).

Conflict of Interest

The authors declare no conflict of interest.

• References and Notes

• 1 Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications. Auras R, Lim L.-T, Selke SE. M, Tsuji H. Wiley; Hoboken: 2010
  Search in Google Scholar
• 2a T _m = 162 °C: Yui, N. In Polymeric Materials Encyclopedia, Vol. 10. Salamone, J. C., CRC Press; Boca Raton: 1996: 7947
  Search in Google Scholar
• 2b T _m = 164 °C: P(l-LA) (M _n = 12,400 by SEC) synthesized using Al(O-i-Pr)₃ in our laboratory.
• 2c T _m = 174 °C: P(l-LA) (M _n = 59,300 by SEC) synthesized by using a homosalen–Al complex with t-Bu-substituents in our laboratory.
• 3 Drumright RE, Gruber PR, Henton DE. Adv. Mater. (Weinheim, Ger.) 2000; 12: 1841
  CrossrefPubMedSearch in Google Scholar
• 4a Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Macromolecules 1987; 20: 904
  CrossrefSearch in Google Scholar
• 4b Tsuji H, Horii F, Hyon SH, Ikada Y. Macromolecules 1991; 24: 2719
  CrossrefSearch in Google Scholar

For some representative studies, see:
• 5a Le Borgne A, Vincens V, Jouglard M, Spassky N. Makromol. Chem., Macromol. Symp. 1993; 73: 37
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• 5b Spassky N, Wisniewski M, Pluta C, Le Borgne A. Macromol. Chem. Phys. 1996; 197: 2627
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• 5c Radano CP, Baker GL, Smith MR. III. J. Am. Chem. Soc. 2000; 122: 1552
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• 5d Ovitt TM, Coates GW. J. Polym. Sci., Part A: Polym. Chem. 2000; 38: 4686
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• 5e Zhang L, Nederberg F, Messman JM, Pratt RC, Hedrick JL, Wade CG. J. Am. Chem. Soc. 2007; 129: 12610
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• 5f Bakewell C, Cao T.-P.-A, Long N, Le Goff XF, Auffrant A, Williams CK. J. Am. Chem. Soc. 2012; 134: 20577
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• 6a Nomura N, Ishii R, Akakura M, Aoi K. J. Am. Chem. Soc. 2002; 124: 5938
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• 6c Nomura N, Ishii R, Yamamoto Y, Kondo T. Chem. Eur. J. 2007; 13: 4433
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• 6d Nomura N, Hasegawa J, Kishida H. Kobunshi Ronbunshu 2013; 70: 744
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• 7a Kishida H, Hasegawa T, Moriya T, Ohara H, Nomura N. Energy Procedia 2014; 56: 187
  CrossrefSearch in Google Scholar
• 7b Hasegawa T, Nomura N, Moriya T, Nishikawa H, Yamaguchi S, Kishida H. Energy Procedia 2014; 56: 195
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• 7c Hasegawa T, Kishida H, Nomura N, Moriya T. Chem. Lett. 2015; 44: 375
  CrossrefSearch in Google Scholar
• 8 Another approach to the stereoselective polymerization of a racemic monomer containing one lactic unit using O-carboxy anhydrides has been reported for the synthesis of highly controlled isotactic P(rac-LAA) (M _n = 12,400; PDI = 1.0₄; P _m = 0.92; T _m = 162 °C), see: Feng Q, Yang L, Zhong Y, Guo D, Liu G, Xie L, Huang W, Tong R. Nat. Commun. 2018; 9: 1559
  CrossrefPubMedSearch in Google Scholar
• 9 Syndiotactic P(rac-LAA) is semicrystalline, see: Ovitt TM, Coates GW. J. Am. Chem. Soc. 1999; 121: 4072
  CrossrefSearch in Google Scholar
• 10 A ring-opening deacetonation polymerization of optically active (S)-TDO using TsOH as a catalyst to give isotactic P(l-LAA) has been independently reported, see: Gazzotti S, Ortenzi MA, Farina H, Silvani A. Polymers (Basel, Switz.) 2020; 12: 2396
  CrossrefPubMedSearch in Google Scholar

For our preliminary results, see:
• 11a Abstracts of Papers, 68th SPSJ Annual Meeting, Osaka 2019; Society of Polymer Science, Japan: Tokyo; Sakai, K.; Nomura, N. 2Pb018.
• 11b Abstracts of Papers, 68th Symposium on Macromolecules, Fukui, 2019; Society of Polymer Science, Japan: Tokyo; Sakai, K.; Nomura, N. 1Pc003.
• 12 When we used 1 mol% of precatalyst (homosalen–AlEt), we sometimes failed to achieve a polymerization. We assume that the air- and acid-sensitive catalyst was deactivated by adventitious O₂ or LAA from decomposed TDO. However, we obtained reproducible results by using 2.5 mol% of the precatalyst.
• 13 P(rac-LAA); Typical Procedure The appropriate homosalen–Al precatalyst (L₂AlEt) was prepared under N₂ according to the procedure previously reported.⁶ To L₂AlEt (0.050 mmol) in toluene (0.40 mL) was added a 0.20 M solution of 2-phenylethanol in toluene (100 μL, 0.020 mmol) at 70 °C, and the stirring was continued for 30 min. The toluene was completely removed in vacuo at rt, and the reaction vessel was equipped with a natural-rubber gas-sampling bag filled with N₂. rac-TDO (250 μL, 2.00 mmol) was added and the resulting mixture was heated at 100 °C. The polymerization was terminated by cooling the reaction mixture to rt, and the mixture was diluted with 1–2 mL of CHCl₃ under air.
• 14 The expected values of M _n in Table 1, considering the amount of PhCH₂CH₂OH and the conversion of rac-TDO, were 10300, 10900, 11800, 7300, and 6400, respectively [M_n = 122.17 + 72.02 × conversion (%)]/0.58. The correlation factor (0.58) was used for the values of M_n estimated by SEC using polystyrene standards.
• 15 Nomura N, Hasegawa J, Ishii R. Macromolecules 2009; 42: 4907
  CrossrefPubMedSearch in Google Scholar
• 16 The average unit number of the iso-lactyl sequence in P(rac-LAA) with P _m = 0.95 is 20, and that of the iso-dilactyl sequence in P(rac-LA) with P_m = 0.90 is 10.

Corresponding Author

Publication History

Received: 26 April 2023

Accepted after revision: 22 May 2023

Article published online:
04 July 2023

© 2023. Thieme. All rights reserved

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• References and Notes

• 1 Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications. Auras R, Lim L.-T, Selke SE. M, Tsuji H. Wiley; Hoboken: 2010
  Search in Google Scholar
• 2a T _m = 162 °C: Yui, N. In Polymeric Materials Encyclopedia, Vol. 10. Salamone, J. C., CRC Press; Boca Raton: 1996: 7947
  Search in Google Scholar
• 2b T _m = 164 °C: P(l-LA) (M _n = 12,400 by SEC) synthesized using Al(O-i-Pr)₃ in our laboratory.
• 2c T _m = 174 °C: P(l-LA) (M _n = 59,300 by SEC) synthesized by using a homosalen–Al complex with t-Bu-substituents in our laboratory.
• 3 Drumright RE, Gruber PR, Henton DE. Adv. Mater. (Weinheim, Ger.) 2000; 12: 1841
  CrossrefPubMedSearch in Google Scholar
• 4a Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Macromolecules 1987; 20: 904
  CrossrefSearch in Google Scholar
• 4b Tsuji H, Horii F, Hyon SH, Ikada Y. Macromolecules 1991; 24: 2719
  CrossrefSearch in Google Scholar

For some representative studies, see:
• 5a Le Borgne A, Vincens V, Jouglard M, Spassky N. Makromol. Chem., Macromol. Symp. 1993; 73: 37
  CrossrefSearch in Google Scholar
• 5b Spassky N, Wisniewski M, Pluta C, Le Borgne A. Macromol. Chem. Phys. 1996; 197: 2627
  CrossrefSearch in Google Scholar
• 5c Radano CP, Baker GL, Smith MR. III. J. Am. Chem. Soc. 2000; 122: 1552
  CrossrefSearch in Google Scholar
• 5d Ovitt TM, Coates GW. J. Polym. Sci., Part A: Polym. Chem. 2000; 38: 4686
  CrossrefSearch in Google Scholar
• 5e Zhang L, Nederberg F, Messman JM, Pratt RC, Hedrick JL, Wade CG. J. Am. Chem. Soc. 2007; 129: 12610
  CrossrefPubMedSearch in Google Scholar
• 5f Bakewell C, Cao T.-P.-A, Long N, Le Goff XF, Auffrant A, Williams CK. J. Am. Chem. Soc. 2012; 134: 20577
  CrossrefPubMedSearch in Google Scholar
• 5g Bhattacharjee J, Peters M, Bockfeld D, Tamm M. Chem. Eur. J. 2021; 27: 5913
  CrossrefPubMedSearch in Google Scholar
• 6a Nomura N, Ishii R, Akakura M, Aoi K. J. Am. Chem. Soc. 2002; 124: 5938
  CrossrefPubMedSearch in Google Scholar
• 6b Ishii R, Nomura N, Kondo T. Polym. J. (Tokyo, Jpn.) 2004; 36: 261
  CrossrefSearch in Google Scholar
• 6c Nomura N, Ishii R, Yamamoto Y, Kondo T. Chem. Eur. J. 2007; 13: 4433
  CrossrefPubMedSearch in Google Scholar
• 6d Nomura N, Hasegawa J, Kishida H. Kobunshi Ronbunshu 2013; 70: 744
  CrossrefSearch in Google Scholar
• 7a Kishida H, Hasegawa T, Moriya T, Ohara H, Nomura N. Energy Procedia 2014; 56: 187
  CrossrefSearch in Google Scholar
• 7b Hasegawa T, Nomura N, Moriya T, Nishikawa H, Yamaguchi S, Kishida H. Energy Procedia 2014; 56: 195
  CrossrefSearch in Google Scholar
• 7c Hasegawa T, Kishida H, Nomura N, Moriya T. Chem. Lett. 2015; 44: 375
  CrossrefSearch in Google Scholar
• 8 Another approach to the stereoselective polymerization of a racemic monomer containing one lactic unit using O-carboxy anhydrides has been reported for the synthesis of highly controlled isotactic P(rac-LAA) (M _n = 12,400; PDI = 1.0₄; P _m = 0.92; T _m = 162 °C), see: Feng Q, Yang L, Zhong Y, Guo D, Liu G, Xie L, Huang W, Tong R. Nat. Commun. 2018; 9: 1559
  CrossrefPubMedSearch in Google Scholar
• 9 Syndiotactic P(rac-LAA) is semicrystalline, see: Ovitt TM, Coates GW. J. Am. Chem. Soc. 1999; 121: 4072
  CrossrefSearch in Google Scholar
• 10 A ring-opening deacetonation polymerization of optically active (S)-TDO using TsOH as a catalyst to give isotactic P(l-LAA) has been independently reported, see: Gazzotti S, Ortenzi MA, Farina H, Silvani A. Polymers (Basel, Switz.) 2020; 12: 2396
  CrossrefPubMedSearch in Google Scholar

For our preliminary results, see:
• 11a Abstracts of Papers, 68th SPSJ Annual Meeting, Osaka 2019; Society of Polymer Science, Japan: Tokyo; Sakai, K.; Nomura, N. 2Pb018.
• 11b Abstracts of Papers, 68th Symposium on Macromolecules, Fukui, 2019; Society of Polymer Science, Japan: Tokyo; Sakai, K.; Nomura, N. 1Pc003.
• 12 When we used 1 mol% of precatalyst (homosalen–AlEt), we sometimes failed to achieve a polymerization. We assume that the air- and acid-sensitive catalyst was deactivated by adventitious O₂ or LAA from decomposed TDO. However, we obtained reproducible results by using 2.5 mol% of the precatalyst.
• 13 P(rac-LAA); Typical Procedure The appropriate homosalen–Al precatalyst (L₂AlEt) was prepared under N₂ according to the procedure previously reported.⁶ To L₂AlEt (0.050 mmol) in toluene (0.40 mL) was added a 0.20 M solution of 2-phenylethanol in toluene (100 μL, 0.020 mmol) at 70 °C, and the stirring was continued for 30 min. The toluene was completely removed in vacuo at rt, and the reaction vessel was equipped with a natural-rubber gas-sampling bag filled with N₂. rac-TDO (250 μL, 2.00 mmol) was added and the resulting mixture was heated at 100 °C. The polymerization was terminated by cooling the reaction mixture to rt, and the mixture was diluted with 1–2 mL of CHCl₃ under air.
• 14 The expected values of M _n in Table 1, considering the amount of PhCH₂CH₂OH and the conversion of rac-TDO, were 10300, 10900, 11800, 7300, and 6400, respectively [M_n = 122.17 + 72.02 × conversion (%)]/0.58. The correlation factor (0.58) was used for the values of M_n estimated by SEC using polystyrene standards.
• 15 Nomura N, Hasegawa J, Ishii R. Macromolecules 2009; 42: 4907
  CrossrefPubMedSearch in Google Scholar
• 16 The average unit number of the iso-lactyl sequence in P(rac-LAA) with P _m = 0.95 is 20, and that of the iso-dilactyl sequence in P(rac-LA) with P_m = 0.90 is 10.

• Supplementary Material