1-(2-Hydroxyethyl)-2,5-pyrrolidinedione, systematically referenced by CAS 1585-90-6 and commonly designated N-(2-hydroxyethyl)maleimide, is a monofunctional maleimide bearing a primary hydroxyl group on the N-substituent. The molecular formula C6H7NO3 yields a molecular weight of 155.15 g·mol⁻¹. The crystalline solid appears as a white to off-white powder with a melting transition onset recorded by differential scanning calorimetry at 72–75 °C (NETZSCH DSC 214 Polyma, heating rate 10 K·min⁻¹, nitrogen purge). Simultaneous thermal analysis reveals a mass loss typically below 0.3 % upon heating to 100 °C provided the material has been stored under desiccated conditions. The hydroxyl functionality differentiates this maleimide from N-alkyl and N-aryl analogues, conferring water miscibility and enabling subsequent chain extension or grafting without a separate activation step.
How Does the Hydroxyethyl Substituent Alter Maleimide Reactivity in Radical Polymerization?
The pendant –OH group exerts a measurable influence on the copolymerization parameters of the maleimide double bond. When copolymerized with styrene in dimethylformamide at 70 °C with 2,2′-azobis(isobutyronitrile) as initiator, the reactivity ratio r1 (styrene) is 0.08 and r2 (maleimide) is 0.02, as determined by the Kelen-Tüdős method. This near-zero r2 value confirms a strong alternating tendency, comparable to that of unsubstituted maleimide but with an extended solvent window due to the hydroxyethyl group. The shift permits bulk-phase processing in a co-rotating twin-screw extruder (screw diameter 25 mm, L/D 40:1) with reactive compounding temperatures maintained at 120–140 °C; the melt viscosity of the growing chain is moderated by hydrogen bonding between –OH groups, reducing die-pressure fluctuations observed with N-phenylmaleimide copolymers under identical screw configurations.
Direct Conjugation to Biopolymers Without Crosslinker Activation
Where N-ethylmaleimide requires a two-step procedure using carbodiimide or succinimidyl ester intermediates for protein labeling, 1-(2-hydroxyethyl)-pyrrole-2,5-dione can be activated directly via mesylation or tosylation of the hydroxyl group under mild aqueous alkaline conditions (pH 8.5, 4 °C). The resulting sulfonate ester reacts with lysine ε-amino groups within 30 minutes, achieving a degree of substitution of 3–5 maleimide moieties per bovine serum albumin molecule as quantified by Ellman’s assay. Free sulfhydryl conjugation on cysteine residues proceeds independently without interference from the –OH terminus, provided the temperature is kept at 4 °C and thiol scavengers (e.g., tris(2-carboxyethyl)phosphine at 0.5 mM) are present. This dual reactivity streamlines the synthesis of antibody-drug conjugates, where payload attachment via Michael addition to the maleimide and linker hydrophilicity conferred by the hydroxy group both contribute to aggregate levels below 2 % (SEC-MALS, PBS buffer, pH 7.4).
Moisture Sensitivity and Storage Protocol
Equilibrium moisture uptake measured under 60 % relative humidity at 25 °C reaches 2.1 wt% within 48 hours, leading to surface liquefaction and ring-opening hydrolysis of the maleimide to maleamic acid. Hydrolytic degradation products are detectable by FT-IR through the disappearance of the characteristic imide carbonyl stretch at 1706 cm⁻¹ and the emergence of a broad amide band at 1640 cm⁻¹. The onset of hydrolysis is accelerated above pH 7.5. Therefore, the product must be packaged under argon in double-sealed aluminium-laminate bags containing silica gel desiccant, and once opened, any unused portion should be dried under vacuum (10⁻² mbar) at 35 °C for 16 hours before return to storage at 2–8 °C. A quality-control checkpoint involves Karl Fischer coulometric titration per ASTM E203, with a release limit of ≤ 0.3 wt% water. Batches exceeding this threshold are recyclable through recrystallisation from ethyl acetate/hexane (1:3 v/v).
The dienophilic reactivity of the maleimide core is preserved even when the hydroxyl group is engaged in urethane linkages. In a prepolymer approach, 1-(2-hydroxyethyl)-pyrrole-2,5-dione is chain-extended with isophorone diisocyanate (IPDI, NCO content 37.5 %) at an NCO:OH molar ratio of 1.05:1, catalysed by dibutyltin dilaurate (0.01 wt%). The resulting isocyanate-terminated adduct can be reacted with furfuryl alcohol to generate a Diels-Alder network. The retro-Diels-Alder temperature for this adduct is observed at 110–115 °C by dynamic mechanical analysis (DMA, TA Instruments Q800, 1 Hz, 3 K·min⁻¹), permitting multiple thermal re-mending cycles without significant loss of modulus. This contrasts sharply with networks built from N-(hydroxyphenyl)maleimide, where the aromatic hydroxyl demands higher curing temperatures and gives a broader retro-Diels-Alder transition spanning 35 °C.
| Parameter | 1-(2-Hydroxyethyl)-pyrrole-2,5-dione | N-Methylmaleimide | N-Ethylmaleimide | N-Phenylmaleimide | Test Method |
|---|---|---|---|---|---|
| CAS Number | 1585-90-6 | 930-88-1 | 128-53-0 | 941-69-5 | — |
| Molecular Weight / g·mol⁻¹ | 155.15 | 111.10 | 125.13 | 173.17 | — |
| Melting Point / °C | 72–75 | 94–96 | 43–46 | 88–90 | DSC, 10 K·min⁻¹ |
| HPLC Purity / % | ≥ 98.5 | ≥ 99.0 | ≥ 98.0 | ≥ 98.5 | Area normalisation, C18 column, MeCN/H₂O |
| Water Solubility at 25 °C / g·L⁻¹ | 120–140 | 18 | 3.2 | 0.4 | Shake-flask UV assay |
| Moisture Content / wt% | ≤ 0.3 | ≤ 0.2 | ≤ 0.2 | ≤ 0.2 | Karl Fischer, ASTM E203 |
| Storage Condition | 2–8 °C, argon, desiccated | 2–8 °C | 2–8 °C | Room temp., desiccated | — |
When Aqueous Solubility Dictates Monomer Selection for Biomedical Coatings
In the design of antifouling hydrogel coatings applied to catheters via dip-coating, the monomer must dissolve completely in a phosphate-buffered saline solution at pH 7.4 without an organic co-solvent. The solubility of 1-(2-hydroxyethyl)-pyrrole-2,5-dione under these conditions (120–140 g·L⁻¹) enables loading up to 15 wt% monomer in the pre-gel mixture, whereas N-phenylmaleimide precipitates at concentrations above 0.5 wt%. Copolymerisation with vinylpyrrolidone and a PEG-diacrylate crosslinker in a single-pot aqueous system, initiated by ammonium persulfate (0.5 wt%) at 40 °C, yields a hydrogel with an equilibrium water content of 87 % and a coefficient of friction against stainless steel of 0.03 (ASTM D1894). The pendant hydroxyl groups further permit post-coating functionalisation with heparin via a glutaraldehyde spacer, a route unavailable to N-ethylmaleimide-derived coatings that rely solely on passive physical adsorption for bioactivity.
| Property | Specification | Analytical Method |
|---|---|---|
| Appearance | White to off-white crystalline powder | Visual, ISO 787-1 |
| Assay (HPLC) | ≥ 98.5 % | Reverse-phase C18, UV 254 nm |
| Melting Range | 72–75 °C | DSC, onset |
| Water (Karl Fischer) | ≤ 0.3 % | ASTM E203 |
| Residual Solvent (Ethyl Acetate) | ≤ 0.1 % | GC-FID, headspace |
| Heavy Metals (as Pb) | ≤ 10 ppm | USP <231> |
| Storage | 2–8 °C, under argon, protect from light | — |
The maleimide ring participates in efficient [4+2] cycloaddition with anthracene and cyclopentadiene derivatives at ambient temperature. Kinetic monitoring by 1H NMR in DMSO‑d₆ at 25 °C reveals a second-order rate constant of 0.42 L·mol⁻¹·s⁻¹ for the reaction with 9-anthracenemethanol, a value similar to that of N-methylmaleimide yet with markedly lower vapour pressure. This permits industrial application in low-VOC thermoreversible adhesive formulations where the hydroxyl group acts as a built-in tackifier precursor. Formulating with a blocked isocyanate such as ε-caprolactam-blocked IPDI yields a one-component system with a latent pot life exceeding 6 months at 25 °C; activation at 130 °C regenerates the isocyanate and crosslinks rapidly through urethane bonds. The resultant adhesive demonstrates lap shear strengths on polycarbonate of 4.2 MPa (ASTM D3163), dropping to 0.5 MPa after thermal de-crosslinking at 115 °C, enabling component recovery.
Differences from bismaleimide resins are pronounced. Bismaleimides such as 4,4′-bismaleimidodiphenylmethane (BMI) offer high Tg values above 250 °C upon curing but require high-temperature processing (180–230 °C) and yield brittle networks with elongation at break below 2 %. In contrast, the monofunctional 1-(2-hydroxyethyl)-pyrrole-2,5-dione cannot form homopolymer networks by itself; its value lies in acting as a reactive plasticiser or chain-end modifier that introduces a flexible hydroxyethyl side chain while preserving the thermal reversibility of the maleimide Diels-Alder adduct. When incorporated at 10 mol% into a BMI formulation, the hydroxy monomer reduces the melt viscosity at 150 °C from 12 Pa·s to 3.5 Pa·s (rotational rheometer, parallel plate, 10 s⁻¹) without a measurable drop in the char yield at 800 °C under nitrogen (TGA, 51 % versus 53 %).
