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Ring-Opening Polymerizations of Cyclic Ester Amides

Haipeng Bian , Yan Yan , Xin Hu , Ning Zhu

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (5) : 888 -903.

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Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (5) : 888-903 DOI: 10.7536/PC20250720
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Ring-Opening Polymerizations of Cyclic Ester Amides
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Abstract

Poly（ester amide）s （PEAs） are a class of functional polymers， which contain both ester and amide groups in their backbone. These structures provide PEAs with good biodegradability of polyesters and excellent mechanical properties of polyamides， leading to promising applications in biomedical and sustainable materials. As one of synthetic methods for preparing PEAs， ring-opening polymerizations （ROP） of cyclic ester amides offer well-controlled feature， which have attracted growing research interest in polymer synthesis and materials chemistry. In recent years， a serials of cyclic ester amides with different ring sizes and substituents have been designed and synthesized as the monomers for PEAs. By developing metal catalysts， organo-catalysts， and enzymes， ROP of cyclic ester amides have been achieved， and the properties and applications of the resultant PEAs have been evaluated. This review focuses on the advances in ROP of cyclic ester amides. According to the monomer structure， cyclic ester amides without substituents， cyclic ester amides with aliphatic substituents， and cyclic ester amides with aromatic substituents are highlighted from the aspects of monomer design， catalytic ROP， and the properties and applications of the corresponding polymers. Moreover， challenges and opportunities are discussed to guide further development of ROP of cyclic ester amides.

Contents

1 Introduction

2 ROP of cyclic ester amides without substituents

2.1 ROP of 6-membered cyclic ester amides

2.2 ROP of macrocyclic ester amides

3 ROP of cyclic ester amides with aliphatic substituents

3.1 ROP of 3-substituted-6-membered cyclic ester amides with aliphatic substituents

3.2 ROP of 6-substituted-6-membered cyclic ester amides with aliphatic substituents

3.3 ROP of 3，6-substituted-6-membered cyclic ester amides with aliphatic substituents

3.4 ROP of macrocyclic ester amides with aliphatic substituents

4 ROP of cyclic ester amides with aromatic substituents

4.1 ROP of 3-substituted-6-membered cyclic ester amides with aromatic substituents

4.2 ROP of 6-substituted-6-membered cyclic ester amides with aromatic substituents

4.3 ROP of 3，6-disubstituted-6-membered cyclic ester amides with aromatic substituents

4.4 ROP of macrocyclic ester amides with aromatic substituents

5 Conclusion and outlook

Graphical abstract

Keywords

cyclic ester amide / ring-opening polymerization / aliphatic-substituted / aromatic-substituted / poly（ester amide）

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Haipeng Bian, Yan Yan, Xin Hu, Ning Zhu. Ring-Opening Polymerizations of Cyclic Ester Amides. Progress in Chemistry, 2026, 38(5): 888-903 DOI:10.7536/PC20250720

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1 Introduction

Poly(ester amide) (PEA) is an important class of heterochain polymers containing both ester bonds (—COO—) and amide bonds (—CONH—) in the main chain[1-4], combining the good degradability of polyesters with the excellent mechanical strength of polyamides[5-9], thermal stability[10-13], and broad application prospects in biomedical[14-18], sustainable materials[19-23], and other fields. The synthesis strategies for poly(ester amide)s mainly include condensation polymerization[24-29], multicomponent polymerization[30-31], and ring-opening polymerization[32-37]. Condensation polymerization of diacids (esters, acid chlorides) with diols and diamines is the earliest method used for synthesizing poly(ester amide)s[27,38-40]. Multicomponent polymerization (MCP) of three or more compounds (e.g., isocyanides, aldehydes/ketones, amines, carboxylic acids, etc.) can efficiently prepare poly(ester amide)s[41-45], with concise steps[46], enabling the convenient introduction of functional groups[47-50]. Ring-opening polymerization (ROP) of lactones/lactams and cyclic ester-amides offers advantages such as good controllability and mild conditions[51-54]; among these, cyclic ester-amides as monomers can simultaneously introduce ester and amide bonds into the repeating units, efficiently obtaining poly(ester amide) materials with diverse structures and properties[55-56], and has become a research hotspot in fields such as polymer chemistry and materials chemistry.
Monomer structure design, catalytic systems, material properties, and applications are key issues in the research on ring-opening polymerization of lactam-lactones[57-61]. In recent years, starting from different substrates[62-66], particularly using bio-based compounds such as amino acids as raw materials[67-71], a series of novel lactam-lactone monomers with different ring strains and substituents have been designed and synthesized. Catalytic systems including metal complexes, small organic molecules, and enzymes have been developed[72-74], and fundamental principles such as polymerization mechanisms, reaction kinetics, and polymerization controllability have been investigated. The basic properties of polyesteramide materials, including thermal properties, crystallization behavior, and degradability, have been preliminarily evaluated, and their application potential in biomedical and sustainable materials has been explored[75-76].
This article reviews the research progress on the ring-opening polymerization of lactams, focusing on monomer design, catalytic systems (Table 1), properties, and applications, categorized by unsubstituted lactams, aliphatic-substituted lactams, and aromatic-substituted lactams (Figure 1), summarizing the research achievements in the ring-opening polymerization of lactams, while also discussing and prospecting the opportunities and challenges facing this field.

2 Ring-opening polymerization of unsubstituted cyclic ester amides

2.1 Ring-opening polymerization of unsubstituted six-membered cyclic ester amides

Nissen et al.[77]during the thermal polymerization experiment of L-acryloyl-L-lactate pentachlorophenyl ester hydrochloride, the cyclic byproduct morpholine-2,5-dione (MD) was discovered for the first time. Li et al.[78]studied the anionic ring-opening polymerization of MD at room temperature using potassium ethoxide as an initiator in N,N-dimethylformamide (DMF) to prepare poly(ester amide) (PMD), with a yield of 32% and a molar mass of 1430 g/mol. Further addition of lactide (LA) yielded the block copolymer PMD-b-PLA (Figure 2). Ohya et al.[79]used dihydroxyl-terminated polyethylene glycol (PEG) as a difunctional macromolecular initiator and stannous octoate [Sn(Oct)2] as a catalyst to prepare an ABA triblock copolymer (PMD-b-PEG-b-PMD) (Figure 2), where the number of repeating units of MD was 3~26, and the dispersity Ðwas 1.3~1.5.

2.2 Ring-opening polymerization of unsubstituted macrolactams

Fey et al.[80-82]Using adipic anhydride and α,ω-amino alcohols as starting materials, a series of semi-crystalline polyesteramides were obtained via solution polycondensation and bulk polycondensation. Under conditions of 170 °C and negative pressure, these polyesteramides can be converted into macrocyclic esteramides a~e (Figure 3). Macrocyclic esteramides can also be obtained by sublimating the acid produced from the reaction between adipic anhydride and α,ω-amino alcohols at high temperatures. In the presence of dibutyl dimethoxytin [Bu2Sn(OMe)2] or tetrabutyl titanate [Ti(OBu)4] as catalysts, the aforementioned macrocyclic esteramides can undergo ring-opening polymerization to regenerate polyesteramides.
Robertz et al.[83]Based on the ring-expansion strategy, using N-(acryloyl)-ε-caprolactam as the raw material, the eleven-membered cyclic ester-amide monomer 5-aza-1-oxacycloundecane-4,11-dione [c(3HP-AC)] was synthesized. The Bu2Sn(OMe)2-catalyzed ring-opening polymerization yielded a polyesteramide with a molar mass of 14,000 g/mol and a dispersity of 1.39. Lian get al.[84]utilized a sequential ring-expansion strategy to synthesize the macrocyclic ester-amide monomer 1-oxa-4-azacycloundecane-3,11-dione (4A11-D). Well-defined, crystalline polyesteramides were prepared via ring-opening polymerization catalyzed by the organic catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (Figure 4), with a molar mass of 13,000 g/mol and a dispersity of 2.2. Using NaOH as a catalyst, the polyesteramide completely degraded within 3 h at 37 °C, exhibiting excellent chemical degradability. The aforementioned sequential ring-expansion strategy holds promise for designing and synthesizing new macrocyclic ester-amide monomers with various specific structures.
The structure of the aforementioned unsubstituted cyclic ester amides is relatively simple. Although relevant research started early, no significant advantages have been demonstrated in aspects such as monomer design and polymerization controllability, resulting in low attention and few research reports. There is an urgent need to develop new strategies for controlled polymerization to increase molar mass and improve polymerization controllability, laying the foundation for performance evaluation and application research.

3 Ring-opening polymerization of aliphatic substituted cyclic ester amides

3.1 Ring-opening polymerization of six-membered cyclic ester amides with three aliphatic substituents

Helder et al.[85]studied the synthesis and ring-opening polymerization of morpholine-2,5-dione derivatives. Among them, using Sn(Oct)2as a catalyst, 3(S)-methylmorpholine-2,5-dione (3M-MD) substituted with a methyl group at the 3-position can yield a semi-crystalline polymer with a molar mass of up to 45,000 g/mol within 90 min at a conversion rate of 95%S)[77]; 3(S)-isopropylmorpholine-2,5-dione (3S)iPr-MD) achieved a conversion rate of 70% after 48 h of reaction, yielding an amorphous polymer with a molar mass of 74,000 g/mol; the glass transition temperature (Tg) decreased slightly with the increase in substituent size (103 °C vs 99 °C)[78].
Feng et al.[86]catalyzed the copolymerization of 3M-MD with L-lactide (L-LA) and glycolide (GA) using Sn(Oct)2 to synthesize the ternary random copolymer P[LA-co-GA-co-(3M-MD)] (Figure 5). Nanofiber scaffolds were prepared using electrospinning technology. The in vitro degradation weight loss rate increased significantly with the increase in 3M-MD content, which is mainly attributed to the faster hydrolysis rate of ester bonds. Meanwhile, the scaffolds can promote endothelial cell proliferation, showing promise for application in vascular tissue engineering.
Lian get al.[87]Using Bu2Sn(OEtS)2 as a catalyst, 3sBu-MD underwent ring-opening copolymerization with p-dioxanone (PDX) to obtain disulfide-linked oligomers with molar masses of 2000~3400 g/mol (Figure 6). Based on Bu2Sn(OEtS)2 catalysis, the ring-opening copolymerization of 3sBu-MD, 3iBu-MD, and 3M-MD can also be achieved to prepare P[(3sBu-MD)-co-(3iBu-MD)-co-(3M-MD)], with molar masses of 4000~8600 g/mol and dispersities of 1.4~2.1[88].
Franz et al.[89]introduced unsaturated aliphatic substituents containing carbon-carbon double bonds at the 3-position and prepared a series of 3-position functionalized poly(ester amide)s via click chemistry post-modification strategies. Using Sn(Oct)2as a catalyst, 3(S)-(4-((tert-butoxycarbonyl)amino)butyl)morpholine-2,5-dione (3NHBoc-MD) was polymerized at 110 °C for 20 h, yielding a polymer with a molar mass of 21,700 g/mol and a dispersity of 2.0 at 77.3% yield. Furthermore, for the 3-position carbon-carbon double bond aliphatic substituted 3(S)-allylmorpholine-2,5-dione (3P-MD), reaction under identical conditions yielded a poly(ester amide) with a molar mass of 13,300 g/mol (Ð=1.8); utilizing the double bonds introduced in the side chains, radical addition with various thiols was conducted under AIBN initiation (Figure 7), achieving quantitative conversion of the allyl groups while maintaining stable backbone molar mass, successfully preparing poly(ester amide)s with different side-chain functionalities such as carboxyl, amino, hydroxyl, thioether, and perfluorohexyl groups.
Feng et al.[90-92]studied the ring-opening polymerization of 3iPr-MD, 3sBu-MD, and 3iBu-MD initiated by polyethylene oxide (PEO) as a macromolecular initiator to prepare polyesteramide-polyether block copolymers. Using calcium alkoxide generated from the reaction of dihydroxy-terminated PEO (Mₙ=6000) with CaH2as an initiator, the ring-opening polymerization of 3iPr-MD was initiated at 140 °C to prepare P[(3iPr-MD)-b-PEO-b-P(3iPr-MD)] triblock copolymer. During the polymerization, significant racemization of the 3iPr-MD units occurred (approximately 12% at 24 h and about 40% at 96 h). Using Sn(Oct)2as a catalyst, the ring-opening polymerization of 3sBu-MD initiated by dihydroxy-terminated PEO was catalyzed, yielding approximately 90% without inducing racemization. By changing the structure of the PEO macromolecular initiator and using Sn(Oct)2to catalyze the ring-opening polymerization of 3iBu-MD, block copolymers with four structures such as AB, ABA, (A)Figure 8were prepared as shown in 2B, and (A)2B(A)2 . Hydrophilicity increased with increasing PEO content, while the crystallinity of the PEO segment decreased with increasing ester-amide unit content. Affected by the copolymer structure, the order of PEO crystallinity from high to low is: AB > (A)2B > ABA > (A)2B(A)2.
Dirauf et al.[93]investigated the organocatalytic ring-opening polymerization of 3iPr-MD, 3iBu-MD, and 3(S)-isobutylmorpholine-2,5-dione (3sBu-MD). Using TBD as catalyst, tetrahydrofuran (THF) as solvent, and benzyl alcohol (BnOH) as initiator, poly(ester amide)s with a molar mass of 15,000 g/mol and dispersity below 1.2 were synthesized at room temperature. After monomer conversion exceeded 50%, transesterification reactions led to a slight increase in dispersity (Ð<1.34).
Burfon et al.[94]introduced a binary catalytic system of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/thiourea (TU), achieving rapid and controlled ring-opening polymerization of 3sBu-MD. Conversion reached 99% within 5–10 min, with molar masses and dispersities of 8100–25,200 g/mol and 1.13–3.03, respectively; dispersity decreased with increasing TU content. Further copolymerization with LA yielded gradient copolymers, whose glass transition temperatures increased with the rigid segment content in the copolymersof Bu-MD (47.1–51.8 °C).
Ball milling mechanochemistry is an efficient and environmentally friendly reaction method that requires no large amount of solvent and can proceed rapidly at room temperature. Feng et al.[90]utilized a DBU\TU binary catalytic system to achieve the mechanochemical ring-opening polymerization of 3iBu-MD (Figure 9). Under liquid-assisted grinding, the conversion of 3iBu-MD reached up to 97%, with a molar mass of 5100 g/mol and a dispersity of 1.39. The ring-opening copolymerization of 3iBu-MD with LA was further investigated, preparing random copolymers P[(3iBu-MD)-co-LA] with monomer conversions of 78%~86%, molar masses of 6500~8500 g/mol, and dispersities of 1.40~1.42. The glass transition temperature of the copolymers increased with the content of 3iBu-MD (43.9~51.1 ℃), consistent with predictions from the Fox equation.
Schubert et al.[95]using (1R,8S,9S)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) as an initiator and DBU/TU as a catalytic system, studied the ring-opening polymerization of 3iPr-MD and 3sBu-MD. The polymerization of 3iPr-MD followed first-order kinetics, with conversion exceeding 80% and a molar mass of 35,800 g/mol (Ð=1.19); in comparison, the polymerization rate of 3sBu-MD decreased significantly early in the reaction due to chain termination and chain transfer, but the molar mass still increased linearly with conversion, reaching 25,800 g/mol (Ðoxazoline) was coupled with cyclooctyne-modified polyesteramide, successfully constructing well-defined block copolymers (Figure 10).
Enzyme catalysis features green and environmentally friendly characteristics, offering advantages in the preparation of biomedical materials. Feng et al.[96-97]explored various lipases (PPL, PC, PS, MJ, CR, ES) (Table 1) to catalyze the bulk ring-opening polymerization of 3iPr-MD and 3(S)-sec-butylmorpholine-2,5-dione (3sBu-MD). The conversion rate of 3iPr-MD ranged from 3.2% to 73.8%, with polyesteramide molar masses between 3500 and 17,500 g/mol and dispersities from 1.05 to 3.33[96]; the polymerization followed an activated monomer mechanism (Figure 11), where 3iPr-MD interacted with the lipase to form an enzyme-activated monomer (EAM); water in the system attacked the EAM to formN-(α-hydroxyacetyl)-S-valine (Initiation).N-(α-hydroxyacetyl)-S-valine's hydroxyl group attacked the EAM, leading to a continuous increase in repeating units (Propagation) until high molar mass products were formed. For 3sBu-MD, the conversion rate reached up to 92%, the molar mass peaked at 19,900 g/mol, and the dispersity dropped to as low as 1.02[97]. During polymerization catalyzed by porcine pancreatic lipase (PPL), racemization of substituents occurred, accompanied by depolymerization or other side reactions, resulting in a yield of high molar mass polymers lower than the conversion rate. The polymerization rate increased with higher PPL content and water content; the molar mass decreased with increasing water content but increased with rising polymerization temperature. Furthermore, random copolymerization of 3iPr-MD with D,L-LA was used to regulate the glass transition temperature[98](Figure 12); as the content of rigid 3iPr-MD units increased, intermolecular forces were enhanced, raising the glass transition temperature from 40 °C to 74 °C.

3.2 Ring-opening polymerization of six-membered cyclic ester amides with aliphatic substituents at the 6-position

in 't Veld et al.[99]investigated Sn(Oct)2-catalyzed bulk ring-opening polymerization of 6(R,S)-methylmorpholine-2,5-dione (6M-MD). A poly(ester amide) with a molar mass of 23,000 g/mol was obtained at 70% conversion. Compared with the 3-methyl-substituted cyclic ester amide, both the conversion and molar mass were relatively lower, and the reaction time was longer. The glass transition temperature of the polymer increased compared to the 3-methyl substitution (117 °C vs. 103 °C).
Du et al.[100]evaluated the fundamental properties of polymers obtained from the ring-opening copolymerization of 6M-MD and GA catalyzed by Sn(Oct)2. As the content of ester-amide units in the copolymer increased, intermolecular interactions strengthened and crystallinity decreased, resulting in an increase in the glass transition temperature from 42.5 °C to 90.5 °C. The in vitro degradation rate of the copolymer was faster in PBS buffer (pH=7.4) than in distilled water (pH=6.8). During degradation, crystallinity first increased and then decreased, indicating that degradation initially occurred primarily in the amorphous regions rich in 6M-MD, with the crystalline regions subsequently participating in the degradation process.
Guo et al.[101]and Shi et al.[102]designed ten types of 6-substituted morpholine-2,5-dione derivatives and achieved controlled polymerization and closed-loop recycling using a TBD/TU or DBU/TU two-component catalytic system (Figure 13). When used alone, TBD or DBU exhibited poor controllability and were accompanied by cyclic by-products; after adding TU, TBD or DBU preferentially deprotonated it to form TU anions rather than deprotonating the monomer to form cyclic amide anions, thereby achieving the controlled synthesis of linear poly(ester-amide)s (Figure 14). The poly(ester-amide)s exhibited excellent thermal stability (267 °C <Td,5%< 295 °C), with glass transition temperatures tunable between 24.5 and 141.8 °C depending on the substituents. Mechanical properties transitioned from brittle (elongation at break ~4%) to tough (elongation at break >300%) as the substituent length increased. Furthermore, these poly(ester-amide)s possess excellent chemical recycling potential: monomer precursors or monomers can be quantitatively recovered via TBD-catalyzed alcoholysis or acid-catalyzed depolymerization in solution; in the bulk state, via Sn(Oct)2-catalyzed vacuum sublimation, the monomer recovery rate exceeds 90%, and they can be repolymerized without purification, achieving a closed-loop cycle.
Feng et al.[103]investigated the ring-opening polymerization of 6M-MD catalyzed by various commercial lipases. It was found that PPL, CR, and PC exhibited high activity at 100 °C. With PPL catalysis, the monomer conversion reached 88% within 6 days, with a molar mass of 9,400–15,100 g/mol and a dispersity of less than 1.1; the polymerization followed an activated monomer mechanism. With CR and PC catalysis, the monomer conversion exceeded 70%, but the resulting molar masses were lower (8,200–9,800 g/mol).

3.3 Ring-opening polymerization of six-membered cyclic ester amides with dual aliphatic substituents at positions 3 and 6

in 't Veld et al.[99]studied the ring-opening polymerization of disubstituted cyclic ester amides catalyzed by Sn(Oct)2. The disubstituted monomers (3S, 6R,S)-3-isopropyl-6-methylmorpholine-2,5-dione (3iPr,6M-MD) and (3S, 6R,S)-3-methyl-6-methylmorpholine-2,5-dione (3M,6M-MD) exhibited lower conversion rates in ring-opening polymerization compared to monosubstituted monomers (70% vs 95%), and the resulting molar masses were lower (9000 g/mol). This difference is attributed to the steric hindrance of the disubstituent groups, which reduces monomer reactivity and limits chain growth.
Shirahama et al.[104]investigated the copolymerization of 3M,6M-MD with 2catalyzed by Sn(Oct)ε-caprolactone (ε-CL) and L-LA, evaluating the thermal, mechanical, and biodegradation properties of the copolymers. The yield of 3M,6M-MD/L-LA binary random copolymers was 30%~90%. The glass transition temperature increased with the content of rigid ester-amide units (59.6~88 °C), while the melting point and crystallinity showed the opposite trend. Further introduction of ε-caprolactone (ε-CL) can improve mechanical properties. When the ratio of 3M,6M-MD/ε-CL/L-LA in the copolymer is 4:17:79, the copolymer exhibits both high elongation at break (671.2%) and high tensile strength (44 MPa). Regarding biodegradation performance, copolymers containing 11%~13% ester-amide units completely degraded within 50 h and 37 d under Proteinase K and composting conditions, respectively. Among them, the higher the content of ester-amide units, the faster the degradation rate. The degradation mechanism involves enzymes preferentially hydrolyzing the ester-amide units, thereby triggering the hydrolysis of L-LA segments.
Castro et al.[105]investigated amino-alkoxy-bisphenolate yttrium complexes as catalysts for the ring-opening polymerization of 3M and 6M-MD under relatively mild conditions (60~100 °C). Compared with traditional tin metal catalysts, the yttrium complexes achieved a monomer conversion of 85% at 60 °C, yielding poly(ester amide)s with molar masses up to 7400 g/mol. For the complex [L]Y-OiPr, polymerization is initiated by nucleophilic attack of the isopropoxy group on the ester group, whereas for the complex [L]Y-N(SiHMe2)2, polymerization is initiated by the amino group generated in situ via aminolysis of the monomer (Figure 15). No racemization occurs during the polymerization process of this catalytic system; however, intramolecular transamidation side reactions occur at high temperatures, leading to changes in the active end groups and a reduction in molar mass.
Jörres et al.[106]studied the bulk ring-opening polymerization of 3iPr,6M-MD using Sn(Oct)2and tin(II) acetylacetonate (Sn(acac)2) as catalyst and initiator. When Sn(Oct)2was used as the catalyst, the monomer conversion reached 80%, and the resulting polymer had a number-average molar mass of 29,500 g/mol with a dispersity of 1.24. The molar mass first increased and then decreased with reaction time, because Sn(Oct)2causes chain scission through transesterification (Figure 16), leading to a decrease in molar mass. Sn(acac)2acted as an initiator directly participating in chain initiation, resulting in a faster polymerization rate and a molar mass reaching 44,500 g/mol, while the dispersity increased slowly with polymerization time. The group further investigated the enzymatic bulk ring-opening polymerization of 3iPr,6M-MD[107], where the conversion after 72 h was only 9%, with a molar mass and dispersity of 6,900 g/mol and 1.16, respectively. In comparison, monomers with only an isopropyl substitution at position 3 or a methyl substitution at position 6 achieved conversions of 73.8% and 88%, respectively, under enzymatic catalysis[78,100].
Wang et al.[108]reported six novel morpholine-2,5-dione monomers with dual aliphatic substitutions at the 3 and 6 positions, preparing narrow-distribution polyesteramides via two-component organocatalytic ring-opening polymerization (Figure 17). The polymerization process followed first-order kinetics, achieving 90% conversion at room temperature, with molar masses up to 123,900 g/mol (Đ=1.2) for the polyesteramides. The glass transition temperatures were tunable within the range of 31–84 °C. Under Sn(Oct)2catalysis, the polymers could be efficiently depolymerized back to the initial monomers via the terminal depolymerization mechanism shown in Figure 18.
Lian et al.[109]introduced thioethers with multiple reactive sites into disubstituted morpholine-2,5-dione monomers to construct a functionalized poly(ester amide) platform. Using a DBU/TU catalytic system at room temperature, they achieved controlled ring-opening polymerization of 3-(2-(methylthio)ethyl)morpholine-2,5-dione (3TE-MD), 6-methyl-3-(2-(methylthio)methyl)morpholine-2,5-dione (3TE,6M-MD), and 6-methyl-3-(2-(methylthio)ethyl)morpholine-2,5-dione (3TE,6Et-MD)[109]. The excessive steric hindrance of the substituent at the 6-position of 6-butyl-3-(2-(methylthio)methyl)morpholine-2,5-dione (3TE,6nBu-MD) prevented its polymerization. After polymerizing the remaining monomers for 3–12 h, poly(ester amides) with molar masses ranging from 8,100 to 28,200 g/mol and dispersities of 1.07–1.12 were obtained. Through a "thioether click" post-polymerization modification strategy, sulfone, alkyne, or hydroxyl groups could be efficiently introduced onto the polymer side chains (Figure 19), simultaneously improving the water solubility of the materials while maintaining backbone integrity without degradation.

3.4 Ring-opening polymerization of aliphatic substituted macrocyclic lactam

Keul et al.[110]and Robertz et al.[111]employedN-hydroxyacyl lactam cyclization andN,N,N-bis(β-hydroxyacyl)-2,5-diketopiperazine ring-expansion strategies to prepare eleven-membered cyclic ester-amides (c(3HB-AC), c(3HPv-AC)) and fourteen-membered diester-diamides (3,10-diMe-DDCT-tetraone, 4,11-diMe-DDCT-tetraone, diMe-DDCT-tetraone). Using Bu2Sn(OMe)2as a catalyst for the ring-opening polymerization of the eleven-membered cyclic ester-amides (Figure 20), the reaction of c(3HPv-AC) yielded a polymer with a molar mass of 20,000 g/mol, whereas c(3HB-AC) afforded only oligomers due to steric hindrance and transannular interactions from the dimethyl groups, and the fourteen-membered cyclic ester-amides failed to polymerize due to their higher thermodynamic stability.
Introducing aliphatic substituents onto lactide-amide monomers allows for the regulation of ring strain, steric hindrance, and reactivity of the monomers. Generally, monomers substituted at the 3-position exhibit higher reactivity, while those substituted at the 6-position show lower reactivity; disubstitution further increases steric hindrance and polymerization difficulty, with some macrocyclic monomers being unable to polymerize. Metal catalysts are prone to side reactions such as transesterification at high temperatures, and enzymatic ring-opening polymerization proceeds at a relatively low rate. In contrast, two-component organocatalysis enables rapid and controlled ring-opening polymerization under mild conditions. The thermal properties, degradation performance, and recyclability of aliphatic-substituted poly(ester amide)s were preliminarily evaluated, demonstrating potential application prospects in the biomedical field.

4 Ring-opening polymerization of aromatic substituted cyclic ester amides

4.1 Ring-opening polymerization of six-membered cyclic ester amides with three aromatic substituents

Göppert et al.[112]utilized a DBU/TU two-component catalytic system to achieve the ring-opening polymerization of (Soxazoline) (PEtOx) as macroinitiators, amorphous block copolymers with a molar mass of approximately 18,000 g/mol (Ð<1.3) were prepared. The copolymers exhibited a single glass transition temperature, indicating good compatibility between the two blocks.
Naolou et al.[113]Based on the Hard and Soft Acids and Bases (HSAB) theory, the effects of metal catalysts with different hardness/softness, including Sn(Oct)2, Fe(OAc)2, In(OEt)3, Mg(OEt)3, Al(OEt)3, Fe(OEt)3on the ring-opening polymerization of cyclic ester-amides were investigated. The polymerization follows a coordination-insertion mechanism (Figure 21). During polymerization, metal catalysts undergo irreversible coordination with amide groups to form stable chelates, leading to catalyst deactivation; however, softer metal centers can reduce coordination with amide groups and promote alkoxy transfer. For 3BnOM-MD with small substituents, Fe(II) catalysis offers advantages such as high monomer conversion and low dispersity of the polymer product. For 3(S)-3-(4-(benzyloxy)benzyl)morpholine-2,5-dione (BnMOBn-MD) with bulky substituents, the steric effect of the substituents hinders coordination and insertion during chain growth. However, if the metal center has a large radius, it can provide more space to mitigate steric effects; therefore, Sn(Oct)2, with its larger ionic radius, performs better in the polymerization of monomers with bulky substituents.
By introducing protecting groups such as benzyl into cyclic ester-amide monomers, followed by ring-opening polymerization and deprotection, polyesteramides with functional groups like hydroxyl and carboxyl in the side chains can be obtained. Wang et al.[114], Ouchi et al.[115]and Mansfield et al.[116]used glycolic acid, acetic anhydride, L-lysine, L-aspartic acid, and L-glutamic acid as raw materials to synthesize 3-(benzyloxycarbonyl)methylmorpholine-2,5-dione (3CbzM-MD), 3-[4-(benzyloxycarbonylamino)butyl]morpholine-2,5-dione (3NHZ-MD), and 3-(benzyloxycarbonyl)ethylmorpholine-2,5-dione (3CbzEt-MD). Sn(Oct)2catalyzed the ring-opening polymerization of the above monomers to obtain polyesteramides with a degree of polymerization of 9–13. After deprotecting the benzyl groups, water-soluble polyesteramides containing carboxyl and amino groups were prepared. The degradation rate is related to the specificity of enzyme recognition of the substituents; polyesteramides containing amino units can be specifically degraded by trypsin, while polymers containing carboxyl units are sensitive to V8 protease. Furthermore, 3-(((4-methoxybenzyl)thio)methyl)morpholine-2,5-dione (3BnTeM-MD), 3NHZ-MD, and 3CbzM-MD were copolymerized with L-LA orε-CL to prepare random copolymers P[LA-co-(3BnTeM-MD)], P[LA-co-(3NHZ-MD)], P[CL-co-(3CbzM-MD)][116-118].
Using functionalized polyesteramides after benzyl deprotection as macroinitiators, topological copolymers can be designed and synthesized. Tasaka et al.[119-120] and John et al.[121]via Sn(Oct)2-catalyzed ring-opening polymerization of 3-((benzyloxy)methyl)morpholine-2,5-dione (3BnOM-MD) with L-LA, prepared a random copolymer P[(3BnOM-MD)-co-LA)] with a molar mass of 50,600 g/mol. Using the resulting hydroxyl-containing polymer after deprotection as a macroinitiator, they synthesized a comb-type polymer Comb-Type PLA (Figure 22), which exhibited an amorphous state when the mole fraction of esteramide units exceeded 15%. In vitro degradation tests indicated that the degradation rate of the comb copolymer accelerated, primarily attributed to the increased number of nonlinear structural end groups, which promoted the hydrolysis reaction.
Introducing unsaturated double bonds into functionalized polyesteramides after benzyl deprotection enables the preparation of crosslinked copolymers. John et al.[122-123]Using Sn(Oct)2 as a catalyst, ring-opening copolymerization of ε-CL/L-LA with 3BnOM-MD was achieved at 130 °C (Figure 23). The hydroxyl groups after benzyl deprotection underwent acylation with acryloyl chloride; the introduced double bonds were crosslinked under UV light (in solution, bulk, and suspension polymerization) to prepare microspheres with sizes of 25~170 μm. After swelling in DMSO and CHCl3, they appeared transparent, whereas in water, they formed opaque gels.

4.2 Ring-opening polymerization of six-membered cyclic ester-amides with 6-position aromatic substituents

Shi et al.[102]Using glycine and phenylacetaldehyde as raw materials, 6-benzyl-morpholine-2,5-dione (6BnM-MD) was synthesized, and its ring-opening polymerization in dichloromethane catalyzed by TBD/TU or DBU/TU was investigated. The polymerization process follows first-order kinetics, with conversion exceeding 90% and a molar mass of 35,800 g/mol (Ð=1.19); the glass transition temperature is higher than that of long alkyl substituents (81.4 °C vs. 24.5 °C), but lower than that of the more rigid 3-position benzyl substituent (100.4 °C), while also exhibiting a high thermal decomposition temperature (Td, 5%=294 °C). Under acidic conditions, the polymer can be completely depolymerized, quantitatively converting back to high-purity monomers; when catalyzed by TBD, the polymer undergoes rapid alcoholysis at room temperature, quantitatively converting to monomer precursors within minutes (Figure 24).

4.3 Ring-opening polymerization of six-membered cyclic ester amides with dual aromatic substituents at positions 3 and 6

Feng et al.[124]investigated the copolymerization of Sn(Oct)2-catalyzed 3-(benzyloxycarbonyl)methyl-6-methylmorpholine-2,5-dione (3CbzM, 6M-MD) with D,L-LA (Figure 25), preparing random copolymers with molar masses of 4100–55,000 g/mol and dispersities of 1.35–3.05. After deprotection, the copolymers yielded products with carboxyl groups on the side chains, exhibiting good water solubility; the glass transition temperature increased significantly (33.7 °C vs. 87.2 °C), possibly because removing the protecting groups altered the intermolecular packing mode, thereby increasing intermolecular forces.
Wang et al.[108]investigated the ring-opening polymerization of (3S, 6S)-3-benzyl-6-methylmorpholine-2,5-dione (3Bn,6M-MD) under mild conditions using organocatalysis. The conversion reached 90% within 24 h, with a molar mass of 22,400 g/mol (Đ=1.15); the steric hindrance of the benzyl group resulted in a high glass transition temperature (84 °C). Similar to aliphatic substitutions, this polyesteramide can efficiently recover monomers following an end-group depolymerization mechanism under Sn(Oct)2 catalysis.

4.4 Ring-opening polymerization of aromatic substituted macrocyclic lactam

Guo et al.[125]synthesized five eight-membered cyclic ester-amide monomers from readily available phthalic anhydride and β-amino alcohols, achieving controlled ring-opening polymerization using the organic catalysts TBD or DBU/TU (Figure 26), obtaining poly(ester amide)s with molar masses of 4600–40,000 g/mol and dispersities of 1.02–1.18. Influenced by tertiary amide substituents in the backbone, two conformations exist: cis (cis) and trans (trans); the highest proportion of trans isomers was observed with rigid pyrrole ring substitution (trans∶cis≈3∶1). The polymers are amorphous with excellent thermal stability (Td,5%>260 °C); the glass transition temperature decreases as the flexible alkyl side chains increase in size (101 °C vs 61.9 °C). The poly(ester amide) containing rigid pyrrole rings exhibits the highest glass transition temperature (138 °C) and thermal decomposition temperature (Td,5%= 301 °C). Furthermore, copolymerizing M1with rac-LA, during the initial stage of polymerization, rac-LA preferentially polymerizes to form PLA segments, subsequently generating random copolymers via transesterification reactions; the glass transition temperature increases with the increasing proportion of M1 (51.9–95.7 °C). Under NaOH catalysis, the ester bonds in the polymer backbone are selectively hydrolyzed, allowing complete degradation into monomer precursors within 24 h at 30 °C.
Aromatic substituents can not only provide rigidity but also serve as protecting groups; after deprotection, functional groups such as hydroxyl, carboxyl, amino, and mercapto groups can be introduced into the side chains of polyesteramides, providing a platform for further post-modification. Catalysts for the ring-opening polymerization of aromatic substituted lactams are similar to those for aliphatic substituted monomers. In addition to thermal, degradation, and recycling properties, other properties such as mechanical performance remain to be investigated, laying the foundation for expanding application areas.

5 Conclusion and Outlook

This article focuses on the ring-opening polymerization of lactams, summarizing relevant research progress from three aspects: monomer structure, catalytic systems, and properties and applications. It reviews the design and synthesis of a series of unsubstituted, aliphatic-substituted, and aromatic-substituted lactam monomers, as well as the development of three types of catalytic systems for ring-opening polymerization: metal catalysis, organocatalysis, and enzymatic catalysis. The properties of poly(ester amide)s and the phased achievements obtained are preliminarily evaluated. Looking to the future, research on the ring-opening polymerization of lactams is suggested to focus on the following directions: (1) Innovate lactam structural design, develop efficient monomer synthesis methods, and improve monomer yield and purity to lay the foundation for polymerization reactions and property studies; (2) Develop novel catalytic systems with high activity, high selectivity, and low loading, combine them with external force fields and flow chemistry to enhance the controllability of ring-opening polymerization, and elucidate polymerization mechanisms and kinetics; (3) Comprehensively evaluate various indicators of poly(ester amide)s, reveal the structure-property relationships of the materials, and promote related research from fundamentals to applications.

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Funding

National Natural Science Foundation of China(22278223)

National Natural Science Foundation of China(22278205)

National Natural Science Foundation of China(22478188)

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