Synthesis and Ring-Opening Polymerization/Copolymerization of Serine-Derived Cyclic Esters

doi:10.6023/A26040141

Abstract

Serine is a naturally occurring amino acid; polyesters derived from serine exhibit inherent renewability and biodegradability, rendering them a promising class of eco-friendly polymers. In this work, two kinds of novel serine-derived cyclic ester monomers were synthesized, and their ring-opening polymerization (ROP) was systematically investigated. The four-membered cyclic lactones L- and D-3-(dibenzylamino)oxetan-2-one (L-M1, D-M1) were synthesized via three steps from serine methyl ester hydrochloride, benzyl bromide and 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) in an overall yield of ca. 40%. The structure of M1 was further confirmed by NMR and X-ray single crystal diffraction studies. The cyclic carbonate monomer 5-(dibenzylamino)-1,3-dioxan-2-one (M2) was prepared from serinol, benzyl bromide and N,N'-carbonyldiimidazole (CDI) via a two-step procedure in an overall yield of ca. 53%. The ring-opening polymerization (ROP) of M1 was catalyzed by lanthanum complex La1, affording poly(M1)s (P(M1)s) with narrow polydispersities (PDI=1.02~1.08). The experimentally determined number-average molecular weights of the resulting polymers closely match the corresponding theoretical values, with the highest molecular weight reaching 1.28×10⁵ g/mol. The polymer P(D-M1) has a melting point of 210 ℃ and the melting process is accompanied by partial decomposition; moreover, two thermal decomposition stages are observed. The polymerization of racemic M1 (rac-M1) yielded a polymer predominantly with isotactic sequences. Using N,N-dibenzylserinol (IV) as the initiator, the ROP of M2 could be initiated by sodium complex Na1 efficiently to afford poly(M2)s (P(M2)s) with number-average molecular weights close to the theoretical values and relatively broad polydispersities (PDI=1.35~1.69). P(M2) possesses one thermal decomposition stage and a melting point of T_m=170.1 ℃. The copolymerization of M2 with 1,3-dioxan-2-one (TMC) was also realized successfully by using sodium complexes Na1~Na3 as catalysts, wherein TMC exhibited slightly faster initiation kinetics than M2. Regardless of the variations in polymerization time, monomer feed ratio, or catalyst structures, the number-average molecular weights of the resulting polymers consistently approached the theoretical values. Analysis of the average number of units per chain segment and assessment of sequence distribution characteristics revealed that the resulting copolymers are predominantly random. The typical copolymer possesses three thermal decomposition stages and a melting temperature of T_m=168.6 ℃.

Key words: serine derivative, cyclic lactone, cyclic carbonate, ring-opening polymerization, copolymerization, polyester

Cite this article

Yang Huang, Haiyan Ma. Synthesis and Ring-Opening Polymerization/Copolymerization of Serine-Derived Cyclic Esters[J]. Acta Chimica Sinica, 2026, 84(7): 1077-1086.

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Fig. & Tab. 18

Figure 1. The structures of L- and D-3-(dibenzylamino)oxetan-2-one (L-M1, D-M1) and 5-(dibenzylamino)-1,3-dioxan-2-one (M2)

Scheme 1. Synthetic route of 3-(dibenzylamino)oxetan-2-one (M1)

Figure 2. The molecular structure of D-M1 Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (nm) and angles (°): O(1)—C(1), 0.1188(2); O(2)—C(1), 0.1355(2); O(2)—C(2), 0.1477(2); C(1)—C(3), 0.1515(2); C(2)—C(3), 0.1549(3); N(1)—C(3), 0.1438(2); C(1)-O(2)-C(2), 91.82(13); O(2)-C(1)-C(3), 95.34(13); O(2)-C(2)-C(3), 89.19(13); C(1)-C(3)-C(2), 83.30(13)

Scheme 2. Synthetic route of 5-(dibenzylamino)-1,3-dioxan-2-one (M2)

Run	Monomer	[M1]₀/[La]₀	Time /min	Conv.^b	M_n,cacld^c /(×10⁴)	M_n^d /(×10⁴)	PDI^d
Run	Monomer	[M1]₀/[La]₀	Time /min	/%	M_n,cacld^c /(×10⁴)	M_n^d /(×10⁴)	PDI^d
1	D-M1	50/1	20	>99	1.34	1.16	1.08
2	D-M1	500/1	55	>99	13.3	12.8	1.02
3	L-M1	500/1	53	>99	13.3	12.4	1.03
4	rac-M1	500/1	60	>99	13.3	9.40	1.03
5	rac-M1	1000/1	240	38	10.2	6.39	1.08
6^e	L-M1+ D-M1	500/1	32+31	>99	13.3	8.33	1.03
7^f	D-M1	50/1	4	>99	1.34	8.90	2.43
8^f	D-M1	500/1	20	>99	13.3	20.5	2.06

Table 1. ROP of M1 catalyzed by complex La1a

Figure 3. The DSC and TG curves of polymer P(D-M1): [D-M1]0/[La1]0=500/1, Conv.>99%

Figure 4. The DTG curve of polymer P(D-M1): [D-M1]0/[La1]0=500/1, Conv.>99%

Figure 5. The 1H NMR spectra of P(rac-M1) and P(L-M1) (Partial)

Run	[M2]₀/[Na1]₀	Time	Conv.^b	M_n,cacld^c	M_n^d	PDI^d
Run	/[IV]₀	/s	/%	/(×10⁴)	/(×10⁴)
1	250/1/2	90	>99	3.71	3.40	1.61
2^e	250/1/2	90	96	3.56	1.45	1.35
3	500/1/2	40	59	4.38	3.17	1.45
4	500/1/2	61	>99	7.42	6.69	1.69
5^f	500/1/2	55	95	7.05	4.45	1.59

Table 2. ROP of M2 catalyzed by complex Na1 a

Figure 6. The DSC curve of polymer P(M2): [M2]0/[Na1]0/[IV]0= 500/1/2, Conv.>99%

Figure 7. The TG curve of polymer P(M2): [M2]0/[Na1]0/[IV]0= 500/1/2, Conv.>99%

Figure 8. The DTG curve of polymer P(M2): [M2]0/[Na1]0/[IV]0= 500/1/2, Conv.>99%

Scheme 3. Copolymerization of M2 with TMC catalyzed by sodium complexes

Run	Cat.	[M2]₀/[TMC]₀	Time	Conv./%^b	M_n,cacld^c	M_n^d	PDI^d
Run		/[Cat.]₀/[IV]₀	/s	M2, TMC	/(×10⁴)	/(×10⁴)
1	Na1	250/250/1/2	10	45, 67	2.52	3.37	1.41
2	Na1	250/250/1/2	25	95, >99	4.79	3.43	1.74
3	Na1	250/250/1/2	61	>99, >99	4.99	3.61	1.76
4	Na1	375/125/1/2	30	83, >99	5.25	4.55	1.47
5	Na1	125/375/1/2	20	96, 98	3.66	3.37	1.40
6	Na2	250/250/1/2	26	95, >99	4.79	3.88	1.49
7	Na3	250/250/1/2	21	78, 87	3.82	2.18	1.11

Table 3. Copolymerization of M2 with TMC catalyzed by complexes Na1~Na3a

Figure 9. The carbonyl region of quantitative 13C NMR spectrum of copolymer of M2 and TMC (CDCl3, 125 MHz): [M2]0/[TMC]0/[Na1]0/ [IV]0=250/250/1/2, Conv.M2=95%, Conv.TMC>99%

Figure 10. The DSC curve of copolymer of M2 and TMC: [M2]0/[TMC]0/[Na1]0/[IV]0=250/250/1/2, Conv.M2=95%, Conv.TMC>99%

Figure 11. The TG curve of copolymer of M2 and TMC: [M2]0/[TMC]0/[Na1]0/[IV]0=250/250/1/2, Conv.M2=95%, Conv.TMC>99%

Figure 12. The DTG curve of copolymer of M2 and TMC: [M2]0/[TMC]0/[Na1]0/[IV]0=250/250/1/2, Conv.M2=95%, Conv.TMC>99%

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