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.

Key words

ring-opening polymerization - deacetonation - lactic acid - polylactic acid - polymerization catalyst - homosalen, isotactic

Publication History

Received: 26 April 2023

Accepted after revision: 22 May 2023

Article published online:
04 July 2023

• References and Notes

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• 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
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• 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.
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• 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.