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

• 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
• 2a T _m = 162 °C: Yui, N. In Polymeric Materials Encyclopedia, Vol. 10. Salamone, J. C., CRC Press; Boca Raton: 1996: 7947
• 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
• 4a Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Macromolecules 1987; 20: 904
• 4b Tsuji H, Horii F, Hyon SH, Ikada Y. Macromolecules 1991; 24: 2719

For some representative studies, see:
• 5a Le Borgne A, Vincens V, Jouglard M, Spassky N. Makromol. Chem., Macromol. Symp. 1993; 73: 37
• 5b Spassky N, Wisniewski M, Pluta C, Le Borgne A. Macromol. Chem. Phys. 1996; 197: 2627
• 5c Radano CP, Baker GL, Smith MR. III. J. Am. Chem. Soc. 2000; 122: 1552
• 5d Ovitt TM, Coates GW. J. Polym. Sci., Part A: Polym. Chem. 2000; 38: 4686
• 5e Zhang L, Nederberg F, Messman JM, Pratt RC, Hedrick JL, Wade CG. J. Am. Chem. Soc. 2007; 129: 12610
• 5f Bakewell C, Cao T.-P.-A, Long N, Le Goff XF, Auffrant A, Williams CK. J. Am. Chem. Soc. 2012; 134: 20577
• 5g Bhattacharjee J, Peters M, Bockfeld D, Tamm M. Chem. Eur. J. 2021; 27: 5913
• 6a Nomura N, Ishii R, Akakura M, Aoi K. J. Am. Chem. Soc. 2002; 124: 5938
• 6b Ishii R, Nomura N, Kondo T. Polym. J. (Tokyo, Jpn.) 2004; 36: 261
• 6c Nomura N, Ishii R, Yamamoto Y, Kondo T. Chem. Eur. J. 2007; 13: 4433
• 6d Nomura N, Hasegawa J, Kishida H. Kobunshi Ronbunshu 2013; 70: 744
• 7a Kishida H, Hasegawa T, Moriya T, Ohara H, Nomura N. Energy Procedia 2014; 56: 187
• 7b Hasegawa T, Nomura N, Moriya T, Nishikawa H, Yamaguchi S, Kishida H. Energy Procedia 2014; 56: 195
• 7c Hasegawa T, Kishida H, Nomura N, Moriya T. Chem. Lett. 2015; 44: 375
• 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
• 9 Syndiotactic P(rac-LAA) is semicrystalline, see: Ovitt TM, Coates GW. J. Am. Chem. Soc. 1999; 121: 4072
• 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

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
• 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.