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DOI: 10.1055/s-0042-1751472
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)
This work was supported by JSPS KAKENHI Grant Number JP19K05579.
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, isotacticSupporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0042-1751472.
- Supporting Information
Publication History
Received: 26 April 2023
Accepted after revision: 22 May 2023
Article published online:
04 July 2023
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References and Notes
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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 [Mn = 122.17 + 72.02 × conversion (%)]/0.58. The correlation factor (0.58) was used for the values of Mn 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 Pm = 0.90 is 10.
For some representative studies, see:
For our preliminary results, see: