Ring-opening Polymerization of Benzo-fused Caprolactone and its Copolymerization with Epoxides

doi:10.6023/A25060226

Abstract

As an important type of polymer material, polyester is widely used due to its diverse properties and plays a significant role in the plastic economy, accounting for about 10% of the global plastic market. Facing the environmental problems caused by discarded plastics, the development of new types of polyesters with closed-loop chemical recycling properties has become the focus of attention in both academic and industrial research. In this work, benzo-fused caprolactone 4,5-dihydrobenzo[b]oxepin-2(3H)-one (DHBO) was synthesized from α-tetrahydronaphthone via Baeyer-Villiger oxidation reaction, and structurally characterized via NMR spectroscopy and single crystal X-ray diffraction. The ring-opening polymerization of DHBO under varied conditions was investigated using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst, and the molecular weights of the resultant poly(DHBO) obtained via gel permeation chromatography are lower than theoretical values due to transesterification. The polymerization kinetics of DHBO in toluene with TBD/4-MeBnOH shows a first-order dependence on DHBO concentration. The poly(DHBO) is amorphous with a glass transition temperature T_g of 4.8 ℃, and a thermal decomposition temperature T_d (5% weight loss) of 194 ℃, which can be enhanced to 252 ℃ by capped the chain-end OH group with acetyl. In addition, the polyester can be rapidly thermally degraded into the initial monomer DHBO without any catalyst at 110 ℃ under reduced pressure, with a recovery rate of 97% and a monomer purity of 99%. Moreover, using bis(triphenylphosphine) iminium chloride (PPNCl) as the catalyst, DHBO can undergo ring-opening copolymerization with commercial epoxides, such as phenyl glycidyl ether (PGE), allyl glycidyl ether (AGE), 1,2-butylene oxide (BO), (R)-(BO), (S)-(BO), 1,2-hexene oxide (HO) and cyclohexene oxide (CHO) in bulk, producing poly(ether-ester) with highly alternating sequences. Representative copolymers were characterized with NMR spectroscopy (¹H, ¹³C, DOSY) and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). These poly(ether-ester)s showed good thermal stability with T_d over 290 ℃, where the poly((S)-BO-alt-DHBO) exhibited the highest T_d of 353 ℃. The structure of epoxides effected the T_gs of corresponding poly(ether-ester), and wide range of T_g from －19 ℃ to 43 ℃ can be regulated. Finally, the mechanism of the alternating poly(ether-ester) formation was investigated.

Key words: aromatic polyester, closed-loop chemical recycling, ring-opening polymerization, alternating polymer, poly(ether-ester)

Cite this article

Lishuang Shi, Yanan Dai, Weihao Wu, Shihui Li, Zehuai Mou. Ring-opening Polymerization of Benzo-fused Caprolactone and its Copolymerization with Epoxides[J]. Acta Chimica Sinica, 2025, 83(12): 1488-1497.

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Fig. & Tab. 13

Figure 1. Representative aromatic lactone

Scheme 1. Synthesis of DHBO

Figure 2. X-ray crystal structure of DHBO Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (nm) and angles (°): C(1)—O(1) 0.1364(2), C(1)—O(2) 0.11993(19), C(10)—O(1) 0.14052(18), O(1)—C(1)—O(2) 117.57(14), C(2)—C(1)—O(2) 117.29(13), C(1)—O(1)—C(10) 120.60(11)

Entry	[M]₀/[Cat.]/[ROH]	Solvent	[M]₀/(mol•L^－1)	t/h	T/℃	Conv.^b/%	M_n,_GPC^c/(kg•mol^－1)	Ð^c
1	100/1/1	toluene	1	5	r.t.	73	4.79	1.88
2	100/1/1	toluene	2	5	r.t.	85	7.17	1.86
3	100/1/1	toluene	4	5	r.t.	93	6.06	1.62
4	100/1/1	THF	2	5	r.t.	77	6.03	1.62
5	100/1/1	DMF	2	5	r.t.	36	1.36	1.89
6	100/1/1	DCM	2	5	r.t.	48	3.00	1.24
7	100/1/1	DCM	2	24	r.t.	55	8.75	2.15
8	100/1/1	—	—	10 min	90	90	7.49	2.09
9	100/1/1	—	—	10 min	110	92	6.21	1.93
10	300/1/1	—	—	0.5	90	84	9.00	1.89
11	500/1/1	—	—	0.5	90	80	10.20	2.00

Table 1. Homopolymerization of DHBO mediated with TBD

Figure 3. Semilogarithmic plots of DHBO conversion versus the reaction time Conditions: [DHBO]/[TBD]/[4-MeBnOH]=100/1/1, [DHBO]0=1.0 mol/L in toluene

Figure 4. TGA curves of Poly(DHBO)s with different chain-ends

Figure 5. Thermal depolymerization of Poly(DHBO) under reduced pressure and the stacked 1H NMR spectra of starting DHBO, poly(DHBO) and recycled DHBO (CDCl3, 400 MHz)

Entry	[Epoxide]₀/[M]₀/[PPNCl]₀	Epoxide	T/℃	Conv.^b/%	Alternating^c/%	Regioselectivity^c/%	M_n^d/kDa	Ð^d
1	—/100/1	—	110	0			—	—
2	20/—/1	PGE	110	16			—	—
3	200/100/1	PGE	110	>99	98	99	14.4	1.45
4	200/100/1	PGE	90	95	98	98	15.0	1.47
5	200/100/1	PGE	60	75	96	94	4.75	1.31
6	200/100/1	AGE	110	>99	96	97	16.1	1.38
7	200/100/1	BO	110	>99	97	97	13.0	1.37
8^e	200/100/1	R-BO	110	>99	98	97	8.9	1.67
9^e	200/100/1	S-BO	110	>99	98	97	6.6	1.62
10	200/100/1	HO	110	>99	96	98	8.8	1.34
11	200/100/1	CHO	110	>99	95	—	11.3	1.39

Table 2. Copolymerization of DHBO with epoxides using PPNCl

Figure 6. 1H NMR spectrum (CDCl3, 400 MHz) of ester group region and 13C NMR spectrum (CDCl3, 100 MHz) of carbonyl region

Figure 7. 1H NMR spectrum of obtained Poly(PGE-alt-DHBO) in CDCl3, only phenol ether and ester were shown for regioselective analyses (CDCl3, 400 MHz)

Figure 8. TGA curves of Poly(epoxide-alt-DHBO)

Figure 9. DSC second heating curves of poly(epoxide-alt-DHBO) (10 ℃/min)

Scheme 2. Proposed mechanism of the alternating poly(ether-ester) formation

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