1,1'-(1,3-Phenylene)Bis-1H-Pyrrole-5-Dione

Designated 1,1'-(1,3-Phenylene)Bis-1H-Pyrrole-5-Dione (CAS 3006-93-7), also referenced as N,N′-(m-phenylene)bismaleimide, constitutes a crystalline bismaleimide monomer in which two maleimide heterocycles are coupled through a 1,3-disubstituted benzene ring. The meta-oriented aromatic bridge departs from the more widely employed 4,4′-linkage geometries, lowering molecular symmetry and yielding a melting point typically clustered between 198 °C and 203 °C (DSC, 10 °C/min under nitrogen, ASTM D3418). This thermal characteristic, together with a molecular weight of 268.23 g·mol⁻¹ and an HPLC purity normally exceeding 97.0%, positions the monomer as a reactive building block for high-temperature thermosets where tailored melt processing and crosslink density control are primary design variables. The meta-substituted phenylene unit imparts a kinked backbone that, after thermal or free-radical-initiated addition polymerisation, generates networks with a fundamentally different free-volume distribution than their para- or diphenylmethane-linked counterparts. Commercial lots are supplied as a pale-yellow to tan powder, pre-dried to a moisture content below 0.3 wt% (Karl Fischer, ASTM D6869) when packed under inert atmosphere, and are predominantly utilised in resin transfer moulding (RTM) formulations, high-performance composite prepregs, and as a co-agent in peroxide-cured elastomer systems.

Analytical Specifications and Lot-to-Lot Consistency

Routine quality-control release for 1,1'-(1,3-Phenylene)Bis-1H-Pyrrole-5-Dione is benchmarked against the set of criteria tabulated below, derived from multiple supplier certificates of analysis and verified by differential scanning calorimetry, high-performance liquid chromatography, and Fourier-transform infrared spectroscopy. The melting endotherm onset, recorded with a sealed aluminium pan and a heating rate of 10 °C/min, shall lie within the indicated range; any deviation exceeding ±2 °C is correlated with isomeric impurities or incomplete imidisation detectable via the carbonyl stretching region (1710 cm⁻¹ and 1775 cm⁻¹) in FTIR spectra. Moisture uptake during handling is reversible, but prolonged exposure to ambient relative humidity above 60% necessitates vacuum drying at 50 °C under  −0.09 MPa for a minimum of 4 h before the material is incorporated into anhydrous reactive blends; failure to do so initiates hydrolytic ring-opening of the maleimide moiety, producing maleamic acid impurities that act as oligomerisation catalysts and reduce the pot life of formulated resins by up to 40%. The solubility profile is dominated by polar aprotic solvents: at 25 °C the monomer dissolves to ≥20 wt% in N,N-dimethylformamide and ≥15 wt% in N-methyl-2-pyrrolidone, enabling homogeneous mixing with diallyl bisphenol A or other reactive diluents without cosolvent assistance.

Property	Specification / Typical Range	Test Method
Appearance	Pale-yellow to light-tan crystalline powder	Visual (ISO 787-25)
Purity (HPLC, area%)	≥ 97.0 (typical 98.2–99.1)	In-house HPLC-UV at 254 nm
Melting point (peak)	198–203 °C	ASTM D3418 / ISO 11357-3
Moisture (Karl Fischer)	≤ 0.3 wt% (as‑packed)	ASTM D6869
Ash content	≤ 0.05 wt%	ISO 3451-1
Solubility in DMF at 25 °C	≥ 20 g/100 g	Gravimetric after 0.45 µm filtration
Residual maleic anhydride	≤ 0.15 wt%	GC–MS after derivatisation

Why Does the 1,3‑Substitution Pattern Alter Cure Shrinkage and Fracture Toughness?

The presence of the meta-phenylene connector introduces a 120° kink that disrupts the rigid-rod packing characteristic of para-linked or triphenylmethane-type bismaleimides. During non‑isothermal cure of a stoichiometric blend with o,o′‑diallyl bisphenol A (DABA), dynamic mechanical analysis (ISO 6721-10, single‑cantilever bending at 1 Hz) shows that the glass‑transition temperature of the fully post‑cured network reaches 285–305 °C, comparable to that of 4,4′‑bismaleimidodiphenylmethane‑based analogues, while the linear cure shrinkage measured by pycnometry (ISO 3521) is reduced by approximately 0.8–1.2 % absolute. This lower shrinkage translates directly into decreased residual interlaminar stress in carbon‑fibre laminates; according to published data, ±45° tensile fatigue life (ASTM D3479) at a stress ratio of 0.1 improves by a factor of 1.5–2.0 when the meta‑substituted monomer constitutes 15–25 wt% of the resin matrix. The origin of this toughening lies in the increased nanoscale free volume, as inferred from positron annihilation lifetime spectroscopy, which facilitates segmental mobility ahead of the crack tip and promotes a more distributed micro‑plastic zone. Consequently, the critical strain energy release rate G_Ic (ASTM D5528) for a unidirectional T‑700 carbon‑fibre composite post‑cured at 230 °C for 6 h ranges from 280 J·m⁻² to 340 J·m⁻², whereas the para‑linked counterpart typically yields 180–240 J·m⁻² under identical cure and fibre‑volume‑fraction conditions. An operational constraint emerges from the monomer’s intrinsic reactivity: the exothermic polymerisation onset recorded by DSC (8 °C/min) lies at 188 ± 3 °C, only 6–8 °C below the melting endotherm peak. This tight processing window demands precise temperature control during compounding; when a twin‑screw extruder with an L/D = 48 is run at a barrel set‑point of 165 °C and a screw speed of 120 min⁻¹, the residence‑time distribution must be kept below 55 s to limit the degree of advancement to ≤2.5%, otherwise gel‑particle formation causes nozzle‑blockage events at 3–5% frequency across a 500 kg batch. During injection moulding of glass‑fibre‑reinforced BMI‑DABA compounds formulated with 30 wt% of the meta‑bismaleimide as a viscosity modifier, a melt residence time exceeding 8 min at 130 °C produces a 12% reduction in spiral‑flow length as measured by ISO 1133‑1:2022 Procedure A, accompanied by a torque rise of 18–22% on the plastication screw. Tooling maintained at 200 °C with a polished Ra 0.05 µm surface finish yields a demoulded part with a 60° gloss value of 92 GU (ASTM D523), provided that the mould‑release agent is a semi‑permanent polysiloxane coating applied at 2–3 µm dry film thickness. In hot‑air vulcanisation of polychloroprene (CR) automotive synchronous belts, the addition of 1.8–2.4 phr of the bismaleimide as a co‑agent alongside zinc oxide and magnesium oxide enhances the modulus at 100% elongation by 0.8–1.2 MPa relative to a control compound cured solely with ethylene thiourea. The scorch time (t_s2 at 170 °C, moving‑die rheometer, ASTM D5289) is extended by 25–40 s, affording a wider processing safety margin during profile extrusion at a die temperature of 85 °C. Tensile strength retention after aging 70 h at 125 °C in air (ISO 188) reaches 91% for the bismaleimide‑modified stock, compared with 78% for the unmodified CR.

When the Monomer Serves as a Reactive Diluent in Autoclave‑Cured Carbon‑Fibre Prepregs

Prepreg manufacturing with unidirectional T800‑HB‑12K fibre plies and a resin content of 35 ± 2 wt% benefits from the meta‑bismaleimide’s ability to depress the mixed‑resin melting point. A ternary blend comprising 45 pbw 4,4′‑bismaleimidodiphenylmethane, 35 pbw DABA, and 20 pbw 1,1'-(1,3-Phenylene)Bis-1H-Pyrrole-5-Dione exhibits a melt‑viscosity minimum of 0.35 Pa·s at 120 °C measured with a parallel‑plate rheometer (ISO 6721-10, 1 mm gap, 10 rad·s⁻¹). The reduced viscosity allows prepregging at a line speed of 12 m·min⁻¹ with a resin‑film thickness of 45 µm, while maintaining a volatile‑content below 0.8%. Autoclave consolidation of an 8‑ply quasi‑isotropic laminate follows a cycle of ramp 1.5 °C/min to 180 °C, dwell 2 h under 0.6 MPa external pressure with full vacuum, and a free‑standing post‑cure at 230 °C for 4 h. Ultrasonic C‑scan inspection according to EN 1330-4 reveals a void content ≤0.5%, demonstrating the low‑volatile evolution characteristic of the meta‑substituted architecture. The cured laminate’s hot‑wet compressive strength after 72 h water immersion at 70 °C (ASTM D6641/D5229) retains 83% of the dry value at 150 °C, a result attributed to the lower equilibrium moisture uptake of 1.1 wt% versus 1.6 wt% for the all‑para reference.

Thermal and Mechanical Benchmarking Against 4,4′-Linked Bismaleimide Systems

Direct comparative data obtained with a standard cure schedule (180 °C/2 h + 230 °C/4 h) for neat resin castings are summarised below. The meta‑phenylene monomer produces a network with a moderately lower glass‑transition temperature but significantly higher strain‑at‑break and impact resistance, making it suitable for applications where thermal soakback tolerance coexists with a requirement for damage tolerance.

Property	1,1′-(1,3‑Phenylene)Bis‑1H‑Pyrrole‑5‑Dione / DABA	4,4′‑Bismaleimidodiphenylmethane / DABA	Test Standard
Monomer melting point (°C)	198–203	156–160	ASTM D3418
Cure exotherm onset (°C)	188 ± 3	192 ± 4	ISO 11357‑1 (dynamic, 8 °C/min)
Glass‑transition temperature, Tg (DMA, tan δ peak, °C)	290–305	310–325	ISO 6721‑10
Flexural strength at 25 °C (MPa)	142–158	135–150	ISO 178
Flexural modulus (GPa)	3.9–4.2	4.4–4.7	ISO 178
Elongation at break, flexure (%)	5.8–7.2	3.9–4.8	ISO 178
Charpy impact strength, unnotched (kJ·m⁻²)	24–31	14–19	ISO 179‑1/1eU
Saturated moisture uptake (70 °C/85 % RH, %)	1.1–1.3	1.6–1.9	ISO 62 (method 2)

When the meta‑substituted monomer is deployed as a co‑agent in peroxide‑cured EPDM dense profiles, the crosslink density determined by equilibrium swelling in toluene (Flory‑Rehner method) increases by 12–18% at a constant 2.0 phr loading of 1,1'-(1,3-Phenylene)Bis-1H-Pyrrole-5-Dione relative to the same peroxide level without co‑agent. The corresponding compression set after 22 h at 150 °C (ASTM D395 Method B) is reduced from 32% to 19%. Because the meta‑phenylene bismaleimide undergoes homopolymerisation and grafting at the EPDM backbone in the presence of radicals, the scorch safety (Mooney t₅ at 125 °C) is preserved within 6.5–8.0 min, compared with 5.0–6.0 min for trimethylolpropane trimethacrylate at the equimolar double‑bond content. This balance of scorch retention and crosslinking efficiency is exploited in continuous‑vulcanisation lines operating with a salt‑bath temperature of 230 °C and a linear throughput of 45 m·min⁻¹. In all mixing operations, contact with primary or secondary amines at temperatures above 10 °C must be avoided: the maleimide ring undergoes rapid Michael addition even with sterically hindered amines, elevating the blend viscosity uncontrollably within 2–5 min and forming insoluble aggregates that clog static mixers and metering‑pump channels. Amine‑containing mould‑release agents or amine‑cured epoxy tool coatings are therefore incompatible with the monomer unless an intermediate sealed‑barrier coating is applied.