Intrinsically ﬂ ame retardant polyamides: Research progress in the last 15 years

Polyamides are essential thermoplastics whose current worldwide annual production exceeds 10 million tons. They are ubiquitous and easily ignitable polymeric materials that require addition of ﬂ ame re-tardants to comply with ﬁ re safety requirements for various applications. Flame retardant additives can be incorporated into polymer matrix as ﬁ llers or at the molecular level, implying use of reactive additives. The latter approach is less developed, but usually offers several advantages over adding ﬂ ame-retardant ﬁ llers: lower additive loading used to achieve speci ﬁ c level of ﬁ re performance, no ﬂ ame-retardant migration with time, lower corrosiveness, better polymer stability etc. Rendering polyamides intrinsically ﬂ ame retardant is therefore highly desirable. In this review we survey progress in inherently ﬂ ame-resistant polyamides done over the period from 2004 to 2020. The polymers are grouped according to their chemical structures: aliphatic, semiaromatic and aromatic polyamides, polyamidoimides and hybrid siloxane-polyamides. Their monomer preparation, synthesis details, thermal properties and ﬁ re performance are discussed. The minimal inclusion criterion for this review was reported ﬁ re-resistance performance: either V-1/V-0 rating achieved in UL-94 burning tests or experimental or calculated LOI above 23%.


Introduction
Polyamides are polymers that incorporate repeating amide functionality.Based on their origin, polyamides can be divided into natural polyamides and synthetic ones that are obtained artificially.Naturally derived polyamides are better known as polypeptides, proteins and usually composed of natural aliphatic proteinogenic Laminoacids [1].Wool, silk, collagen, b-keratin, polyglutamic acid are just few examples of naturally occurring polypeptides.Nonetheless, solely wool dominates over other natural proteins in its use as fibers for consumer clothing and special textiles, e.g.train and aircraft upholstery.Wool is composed of a large variety of L-aminoacids, which renders it intrinsically flame-resistant properties.Specifically, low H-content (~6e7%) along with high amount of N (~16%) and S (~3e4%) enable increased moisture regain (up to 15%) [2,3] that leads to high ignition temperature (570 C-600 C), relatively low THR (20.5 kJ/g) and LOI values of 25e28% without use of FR finishes [3,4].Furthermore, compositional and structural complexity of wool macromolecules brings about multistep thermooxidation that occurs predominantly by charring with relatively low emission of toxic gases and smoking (for untreated wool) [4].In contrast to natural polypeptides, synthetic polyamides are usually homopolymers (polylactams, also referred to as A-B polyamides) or diacid-diamine-based polymers (or AA-BB polyamides), prepared from aliphatic and/or aromatic diamines and diacids.According to the chemical structure of a monomer used in polycondensation, the resulting synthetic polyamides are usually distinguished as aliphatic, partially aromatic, semiaromatic and aromatic, also known as aramids.
Aliphatic polyamides, or nylons, are predominantly linear polymers (PA6, PA46, PA66, PA12 etc.) derived from aliphatic monomers (ε-aminocaproic acid, u-aminolauric acid, hexamethylenediamine, adipic acid, etc.).Therefore, macromolecular backbones in conventional artificial aliphatic PA display higher Hcontent, lower water absorption and higher flammability hazard as compared to polypeptides.Aliphatic polyamides are easily ignitable materials that burn completely leaving nearly no solid residues (except for PA66) [5].Once ignited, bulk samples of aliphatic PAs can self-extinguish due to extensive sample shrinkage and dripping thanks to low melt viscosity of PAs.However, LOI of virgin PAs is relatively low (~20e26%) and polymer melt drips are inflamed, maintaining high fire hazard of PA materials.Combustion of PAs is accompanied by high pHRR and THR, emission of flammable small molecules and toxic gases that depend on polymer composition and temperature [5e8].Notably, burning PAs almost do not emit vision-obscuring smoke.
Despite unsatisfactory fire performance, aliphatic PAs still dominate on the market, covering about 90% of the global demand on polyamides and thus urging research to develop FR solutions with little compromise on physical properties of the polymer as well as no complications associated with polymer-additive compatibility, additive migration, phase separation, corrosiveness to processing machinery, coloring, toxicity etc.
Partially aromatic PAs and semi aromatic PAs incorporate aromatic monomers in macromolecular backbones; thus, these polymers contain amide links directly bonded to aromatic rings (e.g.PA6T, PA6I, PA 4T/6T etc.).The amount of aromatic component in semi aromatic PAs is normally around 50%, whereas its content is not strictly set and may vary in partially aromatic polyamides, reaching as high as 85%of an aromatic monomer [5].Replacement of aliphatic monomers with aromatic units usually results in a significant enhancement of mechanical, thermal properties and a reduction in moisture uptake, while also improving fire resistance of PA [6e9].This can be attributed to a combination of various factors as higher T g , reduced amount of aliphatic segments with easily oxidizable eCH 2 -NH-bonds, reduced amount of flammable volatiles and increased amount of char formed upon combustion [10].As a result, LOI of partially aliphatic polyamides are usually higher as compared to the aliphatic counterparts and correlate with the amount of aromatic monomers in the macromolecular backbone.Upon combustion, partially aromatic and semiaromatic polyamides form complex mixture of products, strongly dependent on temperature and monomers used for synthesis, and therefore combine molecules emitted upon thermooxidative decomposition of corresponding nylons and/or aramides [11].Commercial partially aromatic PA are high-performance materials that enjoy wide range of applications, requiring superior mechanical and thermal properties compared to the aliphatic PAs.Compared to nylons, there are substantially less studies on flame retardation of partially aromatic and semiaromatic PAs, despite their widespread and increasing use combined with apparent exposure of these plastics to harsh thermal conditions [6].
Wholly aromatic PAs, or aramides, contain >85% of amide groups directly connected to aromatic rings, ensuring paramount macromolecular chain stiffness, hydrolytic stability and inherent flame retardancy among PAs [5].Aramids are hardly ignitable, they display low THR, T g higher than their decomposition and melting temperatures and belong to highly charring polymers [12].Upon combustion, wholly aromatic PAs form ample char residues thanks to high density of aromatic segments in the structure, and release heavy C-rich aromatic molecules formed mostly through homoand hydrolytic cleavage of amide bonds [13,14].Subsequently, water content, presence of substituents grafted to monomers, as well as crystallinity may affect thermal and FR properties of aramids [14e16].Nonetheless, wholly aromatic PAs are indispensable high-performance heat-resistant polymers used extensively in fabrication of fire-resistant clothing, firefighting equipment and other fireproof civil and military components [17].Aramides usually do not require additional FR solutions, while many efforts are concentrated on improvement of polymer processability properties with no compromise on valuable mechanical and FR characteristics.Compared to the aliphatic polyamides, fully aromatic polyamides are relatively expensive and require special processing conditions (processing with inorganic acids) which are suitable for bulk applications.

Fire and flame-retardation
Burning of organic materials is a complex phenomenon that depends on many variables: temperature, atmosphere composition, chemical composition and physicochemical properties of a burning substrate, its specific geometrical shape and surface area, etc. [18] During the burning process the substrate undergoes few distinguished phases: heating, phase transitions, degradation, decomposition, oxidation, ignition, combustion and propagation.Therefore, an overall polymer's fire performance arises from its behavior on each of these stages (Scheme 1), while it also suggests means for flame retardation [19].
Combustion of hydrogen-and carbon-rich polymer materials (Scheme 2) with no inhibitors is accompanied with exothermic reactions happening in the gas zone of flame and at the condensed phase of a burning substrate (peroxidation, depolymerization, polymer chain scission, hydrolysis etc.), that supplies the gas zone with combustible small molecules and active radicals.Thus, firesuppression approaches usually address processes occurring in the gas phase of the flame zone and/or at condensed phase of a burning substrate.FR acting in the gas phase are devised to scavenge reactive species that account for fire propagation.It is usually attained through either thermally induced release of radical acceptors that can recombine $H, $OH reactive species and cause reaction termination; or emission of non-flammable gases (H 2 O, CO 2 , SO 2 , NH 3 etc.) and/or solid particles that dilute gasses in the flame zone, thus increasing a flash point of the gaseous "fuel mixture".
Old generation gas-phase active FRs usually relied on halogenated compounds that can easily emit $Hal and poison fire.An evident drawback of this flame retardation approach is abundant generation of HHal as the product of radical scavenging.In addition, halogenated FRs as well as products formed upon their incineration are recognized persistent pollutants that eventually led to phasing out of these additives [20].Modern FR solutions are mostly halogen-free and employ P-based compounds that are considered significantly less hazardous for both humans and environment [21].Phosphorus FRs, containing P with lower oxidation number (phosphine oxides, phosphinates etc.) usually act in the gas phase, releasing flame-inhibiting $PO, $P, $HPO and other radicals, and do not promote formation of ample char residues [21,22].These additives are often used in combination with other elements to gain cooperative effect for fire suppression.For example, N-, Si-rich compounds and compounds susceptible to dehydration decompose with the release of inert gases and/or facilitate formation of a layer of solid residues that prevent a burning substrate from further thermooxidative decomposition.
The prevention of fire spread through a modification of burning substrate is usually appreciated as condensed phase mode of flame Scheme 1. Radical reactions in the gas zone of flame.

N. Drigo and S. Gaan
Advanced Industrial and Engineering Polymer Research 6 (2023) 95e131 retardation.This is usually attained through formation of a mechanical barrier of solid glassy, rubbery or foamy residues with decreased flammability (P-enriched char, B 2 O 3 , SiO 2 , metal oxides etc.) that protect the underlying intact polymer substrate from reactive radicals, oxygen and heat transfer.Condensed-phase active FR additives include derivatives of polyols and polyphenols as charpromoters; B-, Si-or P-based (with high oxidation number: phosphonates, phosphates etc.) compounds that tend to decompose forming solid protective layer; metal salts catalyzing char formation and incombustible fillers that hinder heat transfer.
Particularly for PAs, the vast majority of FRs solutions are nonreactive additives that are mechanically mixed with polymers during processing [19,23e25].This popular method is inexpensive and relatively simple, enabling a widespread application in polymer industry.However, polymer-FR additive formulations usually require high FR loadings and often suffer from additive aggregation or phase segregation due to poor compatibility between small FR molecules and macromolecules, causing additive to leach over time; strong alteration of polymer properties and potentiallycomplications in recycling.An alternative approach to obtain FR polymers is to improve thermooxidative stability by suitable chemical modification of macromolecules themselves.Specifically, monomers with "FR substituents" can be utilized in polymerization to yield inherently flame retardant (co)polymers.This approach eliminates the mentioned issues intrinsic to formulations, except for alteration of polymer's properties e particularly important for PAs, whose properties are primarily defined by intermolecular hydrogen bonding that ensures its partial crystallinity.Nevertheless, the incorporation of FR monomers into molecular backbone was demonstrated to have a superior FR effect as compared to formulations with non-reactive additives of similar structures [26e29].Finally, the "reactive" approach for PAs is less reported and reviewed as compared to use of small molecule additives [30]; thus, this review summarizes recent progress in development of (co)polyamides with inherent heat-and flame-resistant properties to shed more light on research performed in this field as well as indicate prospects in this area.

Scope and structure of the review
This review aims to survey the progress in intrinsically heatresistant and flame-retardant (co)polyamides, in cases when this property originates from a chemical composition of macromolecules in bulk.In this review scientific literature published in English during the period since 2004 until 2020 is only considered with no emphasis of patents.The selection criteria for this review were PAs either attaining V-1/V-0 rating in UL-94 burning test or LOI values above 23%, obtained experimentally or derived from amount of char residues in TGA or MCC experiments using Van Krevelen-Hoftyzer equation: LOI ¼ 17.5 þ 0.4CR, where CR e char yield [31].Several polymers with LOI lower than 23% were still included as benchmarks or a part of a "structural family".Synthesis of monomers for intrinsically FR polyamides is discussed briefly.Molecular and thermal properties, as well as fire performance of FR polymers are reported in separate tables in the end of the respective section: aliphatic PAs (Table 1, Table 2), partially aromatic PAs (Table 3), aromatic PAs (Table 4), polyamidoimides (Table 5) and polyamidesiloxane copolymers (Table 6, Table 7).The latter group of polymers was considered separately due to their specific hybrid organicinorganic and block-copolymer nature, moreover these polymers were used for preparation of thermosetting materials.
Studies on FR additives for conventional polyamides are not considered here as they were reported in numerous reviews and book chapters [23e25,145e152].Finally, reactive extrusion, surface modification and FR finishing for PA and wool textiles beyond the scope of this review, and can be found elsewhere [3,153e160].

PA6 copolymers
PA6 is an important commercial thermoplastic obtained through ring-opening polymerization of ε-caprolactam [7].Few reported examples of reactive FR monomers for PA6 are mostly Pcontaining diacids and their salts with aliphatic diamines (see Fig. 1).They are commonly introduced as co-monomers during polycondensation and randomly distributed along macromolecular backbones.Thereby, substitutional pattern of FR co-monomers plays an important role: the presence of bulky pendants in the structure of reactive FR additives may disrupt close packing of PA6 chains, eventually deteriorating crystallinity of PA6 and its mechanical properties.Moreover, non-equal length of the aliphatic and FR co-monomers alters frequency of inter-chain hydrogen bonding, while steric hindrance and/or non-equivalence of reacting groups was reported to affect polymerization propagation, resulting in polymers with inferior crystallinity (vide infra).
Mourgas reported preparation and properties of FR copolyamides 1 and 2 synthesized by melt co-polycondensation of ε-caprolactam and two FR organophosphorus diacids: CEPPA and DOPO-ITA [32].The synthesized polymers contained P in the range of 0.1e0.3wt%.Importantly, higher P-content resulted in lowmolecular-weight co-polymers with lower viscosities unsuitable for processing.Co-polyamides 1 and 2 with 0.15% and 0.10 wt% of P respectively displayed properties (T m , T c , X c and viscosity) similar to the reference unmodified PA6 and were chosen for melt-spinning and knitting inherently FR fabrics.The bulky substituents of CEPPA and DOPO-ITA did not affect hydrogen bonding in PA at low wt% loading.Fibers spun using 1 and 2 realized mechanical properties almost identical to the reference PA6 filaments.In contrast to PA6, fabrics knitted from FR fibers 1 and 2 showed excellent FR properties, achieving high LOI of 35.6% and 35.8% respectively and V-0 category according UL-94 standard.FR PAs 1 and 2 were also used as polymeric FR additives for PA6 to prepare inherently FR fabrics [161].
Liu reported preparation and characterization of FR polyamides 3 obtained via melt co-polycondensation of ε-caprolactam with DOPO-ITA decane-1,10-diammonium salt at different wt% loading of the FR co-monomer.DOPO-ITA co-monomer content ranged  (continued on next page) from 2 to 5 wt% and resulted in polyamides with different properties [33].Specifically, increase of the FR-additive loading decreased molecular weight of FR polymers and broadened molecular weight distribution.The authors explained the reduction of M n with the bulkiness of DOPO-substituent that hindered polymerization, due to a decrease in reactivity of the vicinal carboxylic group that eventually hampered propagation of polymerization.As a result, mechanical properties of melt-spun fibers using FR PAs 3 were inferior to samples made of pristine PA6.On the contrary, FR PAs 3 with the highest P content (4 and 5 wt% of DOPO-ITA respectively) achieved V-0 rating according UL-94 standard and high LOI values of 31.6% and 33.7% respectively.In relation to pristine PA6, FR PAs 3 (5% of DOPO-ITA) showed lower FOT, decreased pHRR and THR by 13.93% and 24.44% correspondingly.Fabrics knitted using FR PA 3 with the highest P-content showed the best flame retardancy among other samples, attaining LOI of 28.4%.Melt dripping was not suppressed, but it did not ignite a cotton indicator.Replacement of the DOPO-ITA salt with DPPO-based analogue (DPPO-ITA) resulted in FR PAs 4 with properties rather similar to those reported for DOPO-ITA-based FR congeners 3 [34].Consistently, increase of P-content lead to lower molecular weight polymers, while FR properties were improved.PAs 4 containing 4 and 5 wt% of the DPPO-ITA achieved V-0 rating according in UL-94 test and the highest LOI values of 30.1% and 31.7%respectively.Finally, FR PA 4 (5 wt% of the DPPO-ITA) demonstrated decreased pHRR and THR, as compared to pure PA6.Nevertheless, comparison of MCC experiments' results for FR PAs 3 and 4 (5 wt% of the DOPO-ITA and DPPO-ITA respectively) suggested that DOPO-substituent had advantage over DPPO-based congener.Notably, DPPO-based FR co-monomers were reported to as act as gas-phase FRs, whilst DOPObased analogues were ascribed to perform in both gas-and condensed-phases [33,34].
Zhang reported that introduction P-free PTCDA above 1.5 wt% in polymerization of ε-caprolactam results in a family of PAs 5 with improved FR properties [35].Similarly to P-containing additives, increase of a feed fraction of PTCDA lead to a decrease in average molecular weight of a co-polyamide and disrupted regularity of hydrogen bonding, therefore leading to poorer mechanical properties as opposed to unmodified PA6.Nevertheless, addition of 2 wt % of PTCDA brought about a reduction of THR by 21.3% as compared to virgin PA6, yet none of the investigated co-polyamides could achieve V-0 rating in UL-94 vertical burning test.

PA66 copolymers
PA66 is another widely used aliphatic PA usually derived either through polycondensation of adipic acid or adipoyl chloride with 1,6-hexamethylenediamine or through direct melt condensation of preliminary prepared HMDAA (Scheme 3) [5,7].Analogous synthetic approaches are used to introduce FR co-monomers into macromolecular backbone of PA6: the reactive FR additives are either P-containing diacids or diamines used as salts with corresponding organic diamides or diacids suitable for the polymerization conditions (see Figs. 2e4).
Li described preparation of FR co-polyamides 66 6, employing 1,6-hexamethylenediammmonium salt of DOPO-ITA as a FR additive that was co-polymerized with HMDAA [36].The content of DOPO-ITA ranged from 2 to 5 wt%, resulting in a family of FR PA66 with different properties.Increase of DOPO-ITA fraction in the polymer lead to a gradual decrease in average molecular weight and higher PDI, thereby bringing about lower T m , T c , X c , and worse mechanical properties.The deterioration of the polymers properties was explained with the steric effect of DOPO-ITA on one hand and possible branching of the main PA chain on the other hand.However, the incorporation of the P-endowed diacid improved flame retardancy.The FR copolymers 6 containing 4 and 5 wt% of DOPO-ITA respectively exhibited the best FR properties, attaining high LOI values above 30% and V-0 rating in UL-94 burning test.In addition, pHRR and THR of PA 6 (5 wt% of DOPO-ITA) decreased by 19.27% and 24.66% respectively as compared to pristine PA66 samples.Notably, DOPO-ITA-endowed co-polyamides exhibited flame retardancy both in gas-and condensed phases, that correlates well results for PA6 [33].
Ge reported interfacial polymerization methodology to synthesize a similar family of FR PA66 6 incorporating DOPO-ITA unit [37].A mixture of adipoyl chloride and DOPO-ITA acyl dichloride (DOPO-ITA-Cl, 2.5e10 wt%) and 1,6-hexanediamine in a biphasic system was used for polymerization.Like 6, polymers derived through melt polycondensation, molecular weights, T c , T m and X c were lower when P-content increased.Conversely, FR properties of the final FR PAs were improved when more DOPO-ITA-Cl was added.However, none of the investigated samples attained V-0 rating in UL-94 burning test.The FR PA 6, containing 1.57% of P (NMR-based assessment), realized the highest LOI of 29.5%, while pHRR was reduced by 47.8% as compared to the virgin PA66.FR PA 6 also generated more char residues (7.6% at 800 C, at 20 C/min heating rate).
Fu revealed preparation FR PA66 7 that incorporate a "trinity" FR reactive additive.The P-containing monomer was derived in a consecutive reaction of pentaerythritol with phosphoryl chloride, followed by grafting p-aminobenzoic acid units [38] The FR diacid   was designed to increase char formation and emission of nonflammable gases upon combustion.In fact, FR PA 7 was obtained using as low as 3 wt% of the corresponding FR 1,6hexamethylenediamonium salt (TRFR salt) and was rated V-0 in UL-94 vertical burning test, owing to the formation of ample porous char that prevented polymer from dripping when burningю The PA 7 improved LOI by 5% in comparison to pure PA66.
Yang reported preparation and thermal properties of FR PA66 8 using BCPPO 1,6-hexanediammonium salt as a reactive FR additive [39].In the initial studies, the amount of BCPPO varied from 1.5% to 9 wt%.The increase of FR monomer content decreased the T m and T c of 8 due to interruption of intermolecular hydrogen bonding periodicity.Only FR PA 8 with the highest P-content (9 wt% of BCPPO) could attain reproducible V-0 rating in UL-94 burning test and improved LOI of 27.2% (22.3% for unmodified PA66) to.Remarkably, FR PA 8 with 9 wt% of BCPPO displayed FR performance similar to a formulation of PA66 with 10% MERP (80 wt% red phosphorus and 20 wt% melamine cyanurate).Further studies on thermal properties of BCPPO-based FR PAs revealed a three-stage decomposition mechanism: initial PeC bond cleavage (E a ¼ 75 kJ/mol) that generates prime thermolysis intermediates, followed by decomposition of PA segments and final carbonization [46,47].The former stage was unique for FR PA 8, while two latter stages were characteristic for both pure and FR PAs.Furthermore, the corresponding E a for FR polymers were 50 kJ/mol (middle thermolysis stage) and 46 kJ/mol (late thermolysis stage) higher than that for pristine PA66, suggesting better protective properties of a char layer for FR PAs with BCPPO units [47].
Lyu reported series of designed phosphinamide and phosphonamide-based reactive additives (diacids and diamines) used as co-monomers for inherently FR PA66 9e13 [41,42].All comonomers, except NENP, were derived from phenylphosphonic dichloride through one-step synthesis procedures followed by polycondensation with HMDAA.The first screening study disclosed that introduction of 5 wt% of NENP, BNPO, DPPD or MCPO additives resulted in inherently FR polymers 9e12 that passed UL-94 burning test achieving V-0 rating.Notably, melt dripping was observed only for burning 10 specimen, but it did not ignite a cotton indicator, while other FR PAs formed ample char, which prevented melt dripping.All specimens made of FR copolymers self-extinguished within 3e7 s after the first ignition and displayed LOI >26%.
Further study confirmed that 5 wt% loading of NENP prepolymer is essential to achieve V-0 classification in UL-94 burning test, while lower FR co-monomer feed fraction resulted in worse fire performance of FR PAs 9 [40].MCC experiments confirmed that PA 9 with the highest P-content displayed better fire performance: pHRR and THR of the FR polymer 9 decreased by 33.3% and 22.8% respectively as compared to unmodified PA66.However, increase in the amount of FR additive lead to a decrease of T m , T g , X c and mechanical properties of FR PAs 9.
Short (n ¼ 2) and longer (n ¼ 5) oligomeric FR diamines were obtained from phenylphosphonic dichloride and p-phenylene diamine and used as co-monomers to prepare FR PA66 12 and 13 (5 wt% and 4.5 wt% of the respective reactive FR additive) [41,42].Both polymers attained V-0 rating in UL-94 burning test and exhibited high LOI of 29%.Increase of PDPPD content in FR polymer 13 lead to a significant reduction in T 5% , T m and T g , while it was essential for improvement of fire performance [42].Thus, MCC experiments clearly demonstrated that increase of P-content in FR PAs reduces pHRR, THR and TSP.Importantly, synthetic protocols for FR monomers were not supported with unequivocal data on  identification and characterization of new compounds, while microphotographs of char residues of burnt PAs 9 and 13 looked similar [42,40].
Chen described the preparation of FR PAs 14, using CEPPA as a reactive FR additive [43].The content of CEPPA ranged from 2% to 6%, rendering all FR PAs 14 excellent FR properties.All FR PAs passed UL-94 burning test, attaining V-0 rating and with no melt dripping.LOI values incremented with amount of CEPPA added and were the highest (LOI ¼ 28.0%) for FR polymer 14, containing 6 wt% of the reactive FR additive.In addition, increase of CEPPA content in the macromolecules decreased the decomposition temperature and improved char formation with continuous dense surface.
Tan reported a study on thermolysis of a similar FR PA 14 incorporating 10 wt% of CEPPA [44].Thermolysis results were analyzed and rationalized using Flynn-Wall-Ozawa and Coast-Redfern methods.Like BCPPO-based co-polyamides, thermal degradation of FR PA 14 with 10 wt% of CEPPA displayed three distinct stages related to different decomposition rates and scenarios, while pure PA66 displayed only a two-stage thermal decomposition.The prime and the middle stages of FR PA 14 thermolysis were assigned to dissociation of PeC bonds with lower thermal stability as compared to CeC bonds.Therefore, PA 14 with 10 wt% of CEPPA demonstrated a decreased E a of decomposition: 148.3 kJ/mol and 139.1 kJ/mol for the respective two stages vs 142.8 kJ/mol and 262.3 kJ/mol for pure PA66.The third stage of thermolysis of FR PA 14 resulted in a noticeable increase of E a that attained a peak value of 454 kJ/mol followed by a fast E a decline.This behavior was accounted for char layer formation that gave rise to superior FR properties of FR PA 14 compared to pristine PA66.Interestingly, FR PA 14 with 10 wt% of CEPPA attained LOI of 27%, that is 1% lower compared to FR PA 22 containing 6 wt% of CEPPA [43].

PA11 copolymers
PA11 is an important biobased niche PA that is derived from castor oil, thereby enabling polymer synthesis in more sustainable manner.PA11 has a lower amide frequency which results in lower T g , T m , T c , Young and flexural modules as compared to PA6 and PA66.In contrast to PA6 and PA66, "more aliphatic" structure of PA11 accounts for lower water absorption and higher dimensional stability [7].Expectedly, similar approaches in development and use of FR reactive monomers can be foreseen, yet scarcely reported.
Negrell described preparation and characterization of FR PAs 15, synthesized from 11-aminoundecanoic acid, decane-1,10-diamine, decane-1,10-dioic acid and DOPO-ITA reactive FR additive that was copolymerized in the concentration range of 3.3 wt% to 11.1 wt% (equivalent to 0.3e1.0wt% of P) (see Fig. 5) [45].As a result, increase of a feed fraction of DOPO-ITA lead to a decrease in intrinsic viscosity, molecular weight, T g , T m and T c of the FR polymers, similarly to trends in FR PA66-type polymers.The deterioration of properties was rationalized with a competing imidization reaction: DOPO-ITA may form cycloimides in presence of amine end-groups at elevated temperatures, thus limiting polymer chain growth and therefore decreasing molecular weight of FR PAs 15 (see Scheme 4, Table 1).In addition, shorter polymerization times were suggested to avoid thermal degradation of DOPO-ITA.Noteworthy, presence of residual amount of phosphoric acid, used as polycondensation catalyst, was also reported to cause branching of PA11 [162].
Expectedly, higher phosphorus content in FR PAs 15 brought about superior FR properties as compared to pristine commercial and in-house synthesized reference PA11.FR PAs 15 containing more than 0.5 wt% of P were rated as V-0 materials in UL-94 burning test.Melt dripping was not suppressed, but drips were non-flaming.Similarly, LOI values were also improved for P-containing co-polymers.Unusually high LOI of 40e45% measured for FR polymer 15 with 1 wt% of P was admitted to be roughly assessed.Finally, MCC experiments demonstrated that the presence of 0.5 wt % of P in FR PA 15 allows to reduce pHRR and THR by only 8.3% and 12.1% respectively (as compared to commercial PA11 sample).

Araliphatic, semiaromatic and partially aromatic polyamides
As compared to conventional aliphatic nylons and their copolymers with reactive aromatic FR additives, PAs incorporating aromatic monomers (araliphatic, semiaromatic and partially aromatic) usually display higher intrinsic thermal robustness and flame retardancy, thanks to the presence of higher number of aromatic segments in the polymer structure.Fine-tuning of thermal, g Referred to the champion copolymer that attained V-0 UL-94 rating.mechanical, optical properties, solubility and fire performance is possible through length of aliphatic chains (amide frequency); balance between aliphatic and aromatic monomers or use of araliphatic units; introduction of heteroatoms, functional groups and/or side-substituents along macromolecular backbones.Thus, judicious chemical design often affords polymers with prominent thermal and fire properties that do not require addition of extra FR additives.
PAs containing aromatic monomers are vastly AABB-type polyamides and are derived from diacids, diamines or their analogues (see Fig. 6).
Various substituted aromatic diamines based on 2,4,6triphenylpyridine core were synthesized and used in direct polycondensation with aliphatic diacids of different length to deliver a family of related semiaromatic FR PAs: 16e20.Nazari and Shabanian reported the synthesis and properties FR PAs 16e18 composed of -Cl, -Br and -NMe 2 -endowed 4,4'-(4-phenylpyridine-2,6-diyl) dianilines and adipic acid segments [48].The diamine monomers were derived from cyclization of 4-nitroacetophenone with 4substituted benzaldehydes in presence of ammonium acetate followed by complete reduction of nitro-groups.Halogen-substituted polymers 16 and 17 exhibited similar T g of ~150 C, while N,Ndimethylamine-endowed analogue displayed T g ca.20 C lower, that was rationalized with the bulkiness and flexibility of the substituent.In contrast, thermal stability (T 5% , T 10% ) followed a different trend increasing in the row 16, 18, 17, while a reverse trend for char yield (800 C, N 2 -atmosphere) was observed, i.e. the highest for Br-modified PA 17. LOI values were assessed based on char yield and were above 34%.Therefore, the polymers 16e18 were classified as self-extinguishing polymers.In addition, in good agreement with char formation trend FR PAs 16e18 demonstrated low pHRR of 111.7; 66.6 and 92.5 W/g respectively, while THR for FR PAs 16e18 was 14.7; 11.6 and 15.7 kJ/g accordingly.
Amininasab reported thermally stable semiaromatic PAs 19 and 20 obtained through a direct polycondensation of adipic acid with two isomeric aromatic diamines based on 2,4,6-triphenylpyridine core endowed with a bulky xanthene pendant group [49].The fluorochrome synthon was prepared via a condensation reaction of two equivalents of 2-naphthol and one equivalent terephthaldehyde and used further to construct the photoactive diamine following a similar protocol as reported for 16e18 monomers.Direct polycondensation with adipic acid resulted in amorphous polyamides 19 and 20, displaying excellent thermal robustness with T g of 195 C and 201 C and T 10% of 388 C and 375 C respectively e the highest values among other related macromolecules.Finally, PAs 19 and 20 formed ample CR (measured at 700 C under N 2 atmosphere) and therefore displayed high calculated LOI above 35%.
In an earlier work, Amininasab reported the synthesis of thermally stable semiaromatic PA 21 and 22 with bulky xanthene pendant bonded to imidazole core [50].The designed diamines were obtained through a multi-step convergent protocol, involving preparation of the xanthene synthon as for 19 and 20, as well as derivatized benzil precursor with terminal nitro-groups.Thus, the imidazole core was constructed in Debus-Radziszewski reaction followed by the reduction of terminal nitro-groups.Direct polycondensation with adipic acid afforded target PAs 21 and 22 that were amorphous and well soluble in polar aprotic solvents, including pyridine.The polymers showed low water uptake less than 3.95% and good thermal properties with T g of 195 C and 184 C respectively, and no significant mass loss up to 410 C. PAs 21 and 22 formed 28e35% of CR and therefore demonstrated high calculated LOI.Finally, polymers 19e22 exhibited rather similar thermal behavior.
A number of structurally related thermally robust semiaromatic PAs 23e28 with 4,5-diphenylimidazole bulky pendants were reported in series of works [51e54].All aromatic polyamides featured alike diamine motif derived via SNAr reaction of 2-(2-chloro-5nitrophenyl)-4,5-diphenyl-1H-imidazole with various bisphenols, followed by reduction of nitro groups.The resulting designed aromatic diamines were directly polycondensed with adipic or sebacic acids affording PAs 23e28.All PAs were amorphous and wellsoluble in polar amide solvents, DMSO as well as pyridine, THF and m-cresol.Furthermore, PAs 23e28 were thermally robust polymers with high T g in the range of 180e234 C and T 10% beyond 330 C. Notably, cardo PA 25 with only CeC bonds at node C-atom displayed superior thermal properties as compared to PA 24, based on phenolphthalein containing three CeC and one CeO linkage.Better thermal properties were also observed for cardo PA 26 compared to the acyclic congeners 23, 27, 28 with similar acid monomers.All polymers formed ample CR and thus exhibited high calculated LOI values.
Ghaemy and Alizadeh reported thermally stable semiaromatic 29, 30 and aromatic (vide infra) PAs, containing a bulky carbazole substituent grafted to a diamine monomer [55].The designed diamine was obtained through a 4-step linear synthetic protocol, including SNAr reaction between 1-fluoro-4-nitrobenzene with carbazole, followed by complete reduction of eNO 2 -group and its acylation with 3,5-dinitrobenzoyl chloride.The resulting molecule was reduced further to provide the desired diamine that was used in direct polycondensation with adipic or sebacic acid, yielding thermally resistant and fully amorphous semiaromatic PAs 29 and 30.The polymers were well soluble in amide solvents, DMSO and pyridine.In addition, macromolecules 29 and 30 demonstrated high T g of 230 C and 195 C respectively, and T 10% above 385 C. Expectedly, PA 29 with higher amide frequency was superior in thermal properties, while both polymers formed similar amount of CR and therefore displayed comparable assessed LOI.
Mehdipour-Ataei and Ehsani described the preparation of thermally robust nicotinamide-based semiaromatic 31 and 32 and aromatic (vide infra) poly (etheramides) composed of pyridinebased diamine and adipic or sebacic acid respectively [56].The diamines with etheramide linkages were synthesized in two-step process: acylation of 2,6-diaminopyridine with 6-chloronicotinoyl chloride followed by SNAr with 5-aminonaphthalen-1-ol.The semiaromatic PAs were designed to combine high thermal stability with good solubility thanks to polar pyridines constituting the monomer.PAs 31 and 32 displayed T g of 195 C and 186 C and T 10% of 365 C and 353 C respectively.Elongation of the aliphatic acid chain lead to slight decrease of T g , T 10% , lower char yield (measured at 650 C under air atmosphere) and assessed LOI.Specifically, PA 31 yielded 50% of solid residues with LOI of 37.5%, while PA 32 formed 47% of char residues and exhibited LOI of 36.3% In a different work, Mehdipour-Ataei reported the synthesis and thermal properties of semiaromatic 33 and aromatic (vide infra) polyester-amides incorporating quinoline heterocycle [57].The hetaromatic diamines with internal ester linkage were derived via a 2-step protocol from 8-hydroxy-5-nitroquinoline and 4nitrobenzoyl chloride, followed by reduction of nitro groups and polycondensation with commercial diacyl chlorides.PA 33 displayed high T g of 184 C and T 10% of 320 C, forming ample CR and exhibiting high calculated LOI of 32.7%.
Quinoxaline-containing semiaromatic 34e36 and aromatic (vide infra) PAs were obtained via two different methodologies: either conventional polycondensation and/or Ullmann-coupling polymerization between commercial diamides and 2,3-bis-(4bromophenyl)-quinoxaline, synthesized via 1-step cyclisation of ophenylenediamine with 4,4 0 -dibromobenzil [58,59].Conventional polymerization protocol required the preparation of the designed diamine -4,4'-(quinoxaline-2,3-diyl)dianiline, derived from benzoin through 4-step linear protocol: consecutive condensation with urea; nitration affording 4,4 0 -dinitrobenzil; its condensation with o-phenylenediamine and reduction of terminal nitro-groups.PA 34 was prepared by exploiting both methods that yielded PAs of different thermal properties.Specifically, the product of conventional polycondensation displayed very low T 10% (not reported explicitly), while the product of Ullmann coupling showed T 10% of 335 C and T g of 196 C. Nevertheless, PAs 34 formed 65e67% of CR, regardless of the protocol used.Furthermore, Ullmann-type polymerization protocol enabled PA 36 with rigid multicyclic structure (see Fig. 7), obtained using 2,5-piperazinedione and inaccessible via conventional polycondensation process.The resulting polymer 36 was less soluble in polar aprotic solvents yet displayed superior thermal properties as compared to acyclic 34 and was rather similar to congener aramides, obtained similarly.Unfortunately, data reported for the semiaromatic PAs do not allow adequate comparison of the synthetic methods and polymers' properties.
Mondal and Das reported thermally resistant semiaromatic PAs 37e39 and aramids (vide infra) containing bulky 2,6diaminotriptycene [60].The designed aromatic diamine was synthesized via a 2-step protocol, including double nitration of triptycene core followed by reduction of terminal nitro-groups.Further direct polycondensation with commercial diacids with even number of C-atoms in aliphatic chain yielded semiaromatic PAs 37e39 that were well-soluble in polar amide solvents, DMSO and pyridine.In addition, the polymers 37e39 displayed high thermal robustness with no significant thermal degradation up to 372 C. Notably, the thermal stability of the macromolecules improved upon elongation of the aliphatic chain of the monomers and was the highest for 39.In addition, PAs 37e39 formed >40.2% of char residues (measured at 800 C under N 2 atmosphere) and exhibited high assessed LOI in the range of 33.6e39.3%.
Mehdipour-Ataei et al. investigated organometallic semiaromatic PAs 40 and 41, based on ferrocene [61].The ferrocene diamine monomer was synthesized via consecutive preparation of 1,1 0 -ferrocene dicarboxylic acid and its amidation with 4,4'-(pyridine-2,6-diylbis (oxy))dianiline.Combination of multiple ether bonds, bulky ferrocenyl unit, and polar heterocycles gave rise to good solubility in polar aprotic solvents and imparted high thermal stability.Specifically, polymers 40 and 41 exhibited T g of 181 C and 186 C accordingly and a high T 10% above 417 C.Moreover, both PAs realized 43e45% of solid residues (measured at 600 C under air atmosphere) and LOI in the range of 26.5e27.0%that were 5.5% higher as compared to ferrocene-free semiaromatic analogues.Related ferrocene-based PAs with either higher aliphatic content in macromolecules or expanded p-system of aromatic monomers are also known and were reported to exhibit LOI in the range of 25.5e28.5%,yet no details on LOI was provided for the specific polymers [163,164].
Mallakpour reported the preparation and thorough synthetic optimization of polymerization conditions that enable optically active thermally robust semiaromatic PAs 42e47 (see Fig. 8) and aramides (vide infra) [62e67].The polymers were obtained through polymerization between commercially available diisocyanates and designed structurally alike amide-endowed diacids.The diacids were prepared via multistep protocols, including consecutive imidization of L-aminoacids with phthalic or 1,8-naphthalic anhydride, followed by formation of a chloroanhydride that was further grafted to 5-aminoisophthalic acid either directly or via 4aminobenzoyl bridge.The authors reported how different polimerization conditions affect properties of resulting semiaromatic PAs 42e47.Specifically, an effect of ammonium or imidazolium ILs as a reaction medium, different catalysts, conventional and MWassisted heating was disclosed.Thermally robust PAs 42 and 43 contained 1,8-naphthalimide substituent and hexamethylene or isophorone diamine monomers respectively.These polymers displayed T g in the range of 120e135 C and T 5% above 289 C. Notably, polymerization performed in molten DIPIB provided 42 and 43 with superior thermal properties, compared to the same polymers isolated from a reaction in TBAB.In contrast to the solvent effect, variation in heating method almost did not affect thermal properties of the resulting PAs.As compared to 42 with linear aliphatic monomer, isophorone-based PA 43 displayed lower T g and higher T 5% , while both polymers formed ample CR in the range of 45e47%, leading to very high calculated LOI >35%.Difference in temperatures of thermal decomposition of 42 and 43 was rationalized in a study of kinetic parameters.Specifically, both Coast-Redfern and Dharwarkar-Kharkhanawala methods gave comparable results: E a was in range of 60.7e64.6 kJ/mol and 57.6e71.8kJ/mol for 42 and 43 respectively and corresponded to TGA results.PAs 44 and 46 contained hexamethylenediamine repeating units and side phthalimide-endowed substituents of different bulkiness that were grafted to the aromatic diacid.The polymers 44, 46 demonstrated inferior T g , T 5% and two times lower thermal decomposition E a of 30.5 kJ/mol (Coast-Redfern method) as compared to 42 with a compact 1,8-naphthalimide entity.Moreover, 44 and 46 formed less CR and therefore displayed lower calculated LOI.Unfortunately, thermal properties of other isophoronediamine-based polymers were not disclosed in detail.
Zhang reported the synthesis and study of series of semiaromatic polyetheramides 48e52 containing diamine segments with different even number of carbon atoms in the aliphatic chain (see Fig. 9) [68].The polymers were prepared using S N Ar polymerization between aryl fluorides with internal amide functionality and 1,1-bis(4-hydroxyphenyl)-1-phenylethane.Elongation of the aliphatic segments in the monomers lead to gradual decrease of T g and water absorption.On the contrary, thermal stability (T 5% , T max and E a of thermal decomposition) increased with increasing the number of methylene units in aliphatic chains.Specifically, T 5% grew from 405 C to 443 C and E a rose from 180.1 kJ/mol to 203.6 kJ/mol (estimated using Kissinger method).Notably, E a of thermal decomposition for 48e52 is significantly higher than values reported for aliphatic and other semiaromatic co-PAs.Semiaromatic polymers 48 and 49 showed superior flame retardancy and improved char formation (28.3% and 7.5% respectively) as compared to 50e52 and commercial PA9T.In addition, 48 and 49 demonstrated the highest LOI values of 34% and 30% and were rated as V-1 and V-2 materials respectively in UL-94 vertical burning test.Both polymers displayed no melt dripping, and the authors referred PAs 48 and 49 as intrinsically FR polymers.However, PAs 50e52 with longer aliphatic segments were fully combustible.

Aramides
Along with semiaromatic PAs 42e47, Mallakpour reported the preparation and properties of aramids 53e56 and 67, 70 obtained from the similar derivatives of 5-aminoisophthalic acid and commercial aromatic diisocyanates (see Fig. 10) [62e67].Expectedly, aramids displayed better thermal properties as compared to the semiaromatic counterparts, demonstrating T g and T 5% in the range of 148e238 C and 250e407 C respectively, that is on average higher in relation to 42e47.Notably, similar to the semairomatic PAs, aramids isolated from reactions performed in ILs showed superior thermal properties, compared to the products from polymerizations carried out in NMP.Moreover, E a of thermal decomposition of 53, 54 and 70 was higher, yet significantly discrepant values were reported.The elevated E a for PA 53 was attributed to either homolytic cleavage of Ph-CH 3 bond in 2,4diaminotoluene monomer resulting in highly cross-linked structures resistant to further decomposition, or enhanced intermolecular hydrogen bonding due to þ I effect of the methyl affecting electronic properties of proximal amide-groups.However, the significant discrepancy of ~110 kJ/mol for 53 was not discussed.In addition, char yields for PAs containing 2,4-diaminotoluene were in the same range as for other aramids, despite hypothesized extensive crosslinking.Lastly, PA 54 displayed overall E a of thermal decomposition ranging from 54.4 kJ/mol [62] to 131.8 kJ/mol [63], while aramid 57 had even higher E a of 153.6 kJ/mol [65].
Further works by Mallakpour, Dinari and Rafiee described the development of methodologies for synthesis of structurally related PAs 57e95, based on derivatives of 5-aminoisophthalic acid endowed with other bulky chiral imide-containing substituents, while the polyamides were obtained through direct polycondensation of the diacids with aromatic diamines [69e77].The chiral pendants were derived following a similar protocol as for the substituents of 42e47 followed by its grafting to the amino-group of 5-aminoisophthalic acid via an amide linkage.The designed diacid monomers were polycondensed with a number of aromatic diamines to deliver families of thermally stable aramids 57e95.The polymerization optimization included the use of ILs and MWassisted reactions studied in comparison with a conventional polycondensation method in aprotic solvents [70e72,76].Despite different polymerization protocols, resulting aramids showed minor differences in properties, suggesting an advantage of IL and/or MW-assisted heating to substitute hazardous solvents and shorten polymerization times.For example, aramids 65 and 72 showed similar thermal properties with a T g of 168 C and 155 C respectively and no significant thermal decomposition up to 348 C. Similarly, PAs 94 and 95 with ethanoanthracene entity also showed little differences in thermal properties with no evident dependence  of the polymers properties on polymerization method employed [76].The families of the reported PAs demonstrated high T g ranging from 155 C to 236 C and no significant decomposition up to 315e459 C.Moreover, the polymers formed >38% of char residues (measured at 800 C under N 2 atmosphere), thus displaying high calculated LOI above 32.7%.Thermal decomposition of PAs 65, 72, 82 and 84 was investigated in TGA experiments with different heating rates to derive kinetic parameters of thermal degradation using Coats-Redfern equation [70,74].The studied aramids displayed an overall E a of thermal degradation in the range of 52.1e78.5 kJ/mol.Furthermore, the authors rationalized the thermal oxidative degradation of 82 and 84 that was best described by random nucleation mechanism with one nucleus on individual particle.Lastly, all polymers were readily soluble in H 2 SO 4 , amide solvents and DMSO, thanks to bulky substituents that prevented close interaction and formation of hydrogen bonds among macromolecules.
Rafiee reported synthesis and properties of photoactive aramids 96e103 containing acetoxynaphthalene moiety, grafted to the amino-group of 5-aminoisophthalic acid via 4-aminobenzoyl bridge [78].The designed diacid monomer was synthesized via a 5-step procedure from 3-hydroxynaphthalene-2-carboxylic acid and introduced into MW-assisted polycondensation with a number of commercial and one bulky designed aromatic diamine (vide infra for synthesis) that furnished a family of thermally stable fluorescent polymers.Two selected aramids 101 and 103 were characterized in DSC and TGA experiments, exhibiting T g of 169 C and 176 C respectively and no significant thermal degradation up to 415 C.Moreover, the polymers formed 62e63% of char residues (measured at 800 C under N 2 atmosphere), and as a result demonstrated high calculated LOI above 42.3%.The selected representative results expectedly contrasted with the thermal stability of related macromolecules 104e117 that contained smaller isomeric m-and p-acetoxyphenyl-based substituents and no 4-aminobenzoyl bridge [79].The related designed diacylchlorides were obtained following a 4-step protocol, including acetylation of isomeric hydroxybenzoic acids, its grafting to the amino group of 5aminoisophthalic acid followed by conversion of carboxyls to carbonyl chlorides.Aramids 104e117 were synthesized as NP using commercial aromatic diamines in dioxane-water mixture under ultrasonic irradiation.Except for 105, polymers 104 and 106e117 formed noticeably less char in the range of 3.8e11.9%,therefore exhibiting LOI below 23.8% typical for readily combustible materials with no flame retardancy.Importantly, the aramids 104e117 were tested in the form of NP, therefore flame resistance of bulk samples may differ significantly.
G omez-Valdemoro reported synthesis and properties of triazole containing aramids 118e122 designed for Hg 2þ extraction from aqueous media, nevertheless displaying advantageous heat-resistant properties thanks to rigid N-rich hetaromatic backbone with aptitude for hydrogen-bond formation (see Figs. 10 and 11) [80].A 1,2,4triazole unit was either grafted as a side substituent or integrated into a macromolecular backbone.In the first case, diethyl-5aminoisophthalate was converted to diethyl-5-(3benzoylthioureido)isophthalate followed by reaction with hydrazine monohydrate to form 1,2,4-triazole heterocycle and subsequent ester hydrolysis.In the latter case, a similar synthetic concept was used to prepare an amino acid monomer: 4-(3-(4-nitrobenzoyl)thioureido)benzoate was reacted with hydrazine monohydrate to form the heterocycle followed by conversion of terminal ester-and nitrogroups into free carboxylic acid and amine respectively.Additionally, the excess amino acid was also reacted with isophthaloyl chloride to furnish a new symmetric elongated diacid with two internal amide bonds.The short diacid monomer with side triazole substituent was condensed with m-or p-phenylenediamine, yielding 118 and 119 respectively.The amino acid was either homo-or co-polymerized with isophthalic acid and m-phenylenediamine to form 120 and 121.Finally, the elongated diacid monomer was directly polycondensed with m-phenylenediamine, forming 122.Presence of polar N-rich heterocycles and amide bonds in the structure resulted in very high water uptake above 10%.Nevertheless, aramides 118e121 displayed excellent heat robustness with very high T g in the range of 293e375 C and exhibited no decomposition up to 365 C that was almost insensitive to atmosphere used.Moreover, the polyamides formed >46% of char residues (measured at 800 C under N 2 atmosphere), therefore exhibiting high LOI above 35% and thus classified as self-extinguishing materials.
Est evez reported synthesis and properties of thermally stable aramids 123e128 that incorporate chromophore and fluorophore fluorene-based substituents [81].The photoactive fluorene-based monomer was obtained conjugating the amino group of 5- aminoisophthalic acid with 9H-fluoren-2-yl isocyanate and introduced into copolymerization with m-phenylenediamide, substituting 10% of isophthalic acid.The resulting aramid 123 was converted to 124 via post-polymerization oxidation of fluorenes' methylene.Similarly, the preparation of other photoactive macromolecules 125e128 implied modification of fluorenone-endowed 124, exploiting reactivity of the cyclic carbonyl group.All PAs were readily soluble in amide solvents, DMSO and demonstrated high water uptake of 8.3e9.4%.Finally, aramids 123e128 demonstrated good thermal resistance up to 255 C regardless of the atmosphere used and formed >47% of char residues (measured at 700 C under N 2 atmosphere), resulting in high assessed LOI above 36.3%.
Thanks to affordability of cyanuric chloride and temperaturedependent stepwise substitution of Cl-atoms in it, S-triazine is a widely used heteroaromatic platform which allows to graft up to three reactive nucleophiles (-NH 2 , eNHR, eOH, eSH etc.) in metarelation.S-triazine core was used in synthesis of diacid monomers for preparation of series of heat resistant aramids 129e161 (see Fig. 12).The synthesis of the monomers included consecutive reactions of cyanuric chloride with one equivalent of a monofunctional nucleophile (morpholine, aniline or 1-naphthylamine) followed by reaction with two equivalents of amino-or hydroxyl-carboxylic acid [82e85].The resulting designed N-rich aromatic diacids were used in polycondensation with several aromatic diamides either directly, thus delivering PAs 129e133, or converted to the corresponding acyl chlorides in order to prepare polymer NP of 134e161, analogously to 104e117.Aramids 129e133 demonstrated high thermal robustness, displaying T g beyond 208 C and no significant thermal decomposition up to 370 C [82,83].Moreover, the aramids formed ample char residues ranging from 44% to 59% (measured at 800 C under N 2 atmosphere), thus resulting in high calculated LOI above 35.1%.Additionally, aramid 129 was studied in MCC experiments and revealed pHRR of 111 W/g and THR of 8.2 kJ/ g.Further reinforcement of 129 with CPN nanofiller improved thermal parameters and fire performance [82].
As for melamine-based 130e133, congener PAs 134e161 were well soluble in polar amide solvents, DMSO, and exhibited good thermal stability.However, char yield of these polymers strongly varied: from 3.2% to 36.8% (measured at 890 C under N 2 atmosphere), suggesting that TGA of polymers in form of NP may bring about strong scattering in results of char yield (similarly to 104e115).Thus, the calculated LOI of aramids 134e161 were in the range of 18.3e31.7%[84,85].
Pyridine is another heteroaromatic unit used to support diamine or di-acid functionalities and therefore widely employed in the synthesis of heat-resistant aramids.Unlike similar carbocyclic analogues, pyridine is a polar electron-deficient heterocycle capable of providing an additional H-bond accepting site which may strongly influence the thermal properties of a polymer.Oppositely, pyridine can also provide a lone pair of electrons for coordination of metal species, thereby suitable for application in synthesis of hybrid functional materials.Beside the simplest commercially available pyridine dicarboxylic acids and diamines, researchers have also reported the preparation of designed pyridine monomers.
Barrio-Manso described synthesis and properties of the simplest pyridine-containing aramid 163 and congener macromolecules 164 and 165 combining bipyridine with piperidine as building blocks for the synthesis of aromatic diacid monomer [86].The aromatic PAs were obtained through direct polymerization and studied in comparison with readily available carbocyclic aramid 162 -poly [N,N 0 -(1,3-phenylene)isophthalamide].In addition to good thermal stability, the designed hetaromatic polymers 163 and 164 displayed fluorescence and were used for sensing of Cr(VI), Fe (III) and Cu(II) thanks to the presence of coordinating N-atoms.The bulky rod-like monomers were derived from 4-hydroxypyridine-2,6-dicarboxylic acid via multistep protocol, including a Pd-catalyzed crosscoupling.Introduction of the bulky rod-shaped pendant substituent did not improve solubility of 163, 164 and slightly affected the thermal decomposition temperatures (increase of T 5% ) as compared to the substituent-less macromolecule 163, that demonstrated T 5% ca.50 C lower than for 162.All aramids formed char residues in the range of 53e65% (measured at 800 C under N 2 atmosphere) and demonstrated high assessed LOI of 39e43% and therefore rendering polymers as self-extinguishing materials.
Koomareh and Souri reported a family of thermally resistant bipyridine-based homo-and co-polyamides 166e172, where the heterocycles were integrated into the structure of bulky starshaped aromatic diamines [87].The designed hetaromatic monomers were obtained following 4-step protocol and using Kr€ ohnke reaction to construct bipyridine fragment from 1-(2-oxo-2-(pyridin-2-yl)ethyl)pyridin-1-ium and 1,3-bis(4-nitrophenyl)prop-2en-1-one, followed by the reduction of the terminal nitro-groups.Thereafter, the monomers were directly polycondensed with commercial diacids and p-phenylenediamine to yield (co)aramids exhibiting crystallinity, yet soluble in polar amide solvents, DMSO and H 2 SO 4 .Additionally, aramids 166e172 displayed high T g in the range of 160e180 C in conjunction with thermal stability up to 360 C. Notably, homoaramids 167 and 168 with the "elongated" and flexible diacid monomers displayed the highest T g .Lastly, the polymers formed 44e63% of char residues (measured at 800 C under N 2 atmosphere) resulting in high calculated LOI above 35.1%.
Zhang reported a family of heat resistant aramids 173e178 based on a related bulky 2,4,6-triphenylpyridine diamine monomers, additionally endowed with triphenylphosphine substituent [88].The designed diamines were obtained following Hantzsch pyridine synthesis methodology from 4-nitroacetophenone and 4-(diphenylphosphino)-benzaldehyde with subsequent reduction of nitro-groups to amino-groups.In addition, few aromatic diacids with internal ether groups were prepared from commercial phenols and 4-fluorobenzonitrile. Subsequent direct polycondensation of the diamine with various commercial and in-house made aromatic diacids delivered amorphous aramids 173e178 with good solubility in polar aprotic solvents including pyridine and less polar m-cresol and CHCl 3 .Moreover, the polymers demonstrated excellent thermal robustness with very high T g in the range of 316e332 C and no significant mass loss up to 496 C. Lastly, polyamides 173e178 formed 49e60% of char residues (measured at 800 C under N 2 atmosphere) resulting in high calculated LOI in the range of 39e43%.Notably, PA 186 containing a sulfone-group exhibited the lowest decomposition temperature and the highest char yield that was attributed to easier thermal degradation of less stable eSO 2 -bridge.
Along with semiaromatic polymers 19e22, Amininasab reported aramids 179e186, endowed with a photoactive xanthene substituent, grafted to heterocyclic star-shaped platforms (see Fig. 13) [49,50].PAs 179e182 were derived from isomeric designed diamides with 2,4,6-triphenylpyridine core and xanthene pendant through a direct polycondensation with terephthalic and dipicolinic acid [49].The obtained polymers 179e182 were soluble in fewer solvents, than 19 and 20, including polar amide solvents, DMSO and only partially soluble in pyridine and m-cresol.Yet, aramids 179e182 displayed high thermal robustness with T g in the range of 218e268 C, T 10% above 338 C, ability to form ample char residues >49% (measured at 700 C under N 2 atmosphere) and exhibit LOI >37%.Remarkably, PA 182 exhibited a superior char yield of 66.3%, however this phenomenon was not rationalized.In addition, to the individual PAs, the authors also reported the preparation of electrospun blends of PAN with 10 wt% and 30 wt% of 179, however the polymer formulations displayed inferior thermal stability compared to the unmodified PAN or 179.Aramids 183e186 with xanthene unit grafted to imidazole core displayed superior thermal stability compared to 179e182 [50].The T g of the polymers was in the range of 254e277 C and T 10% beyond 438 C.However, the char yield and LOI of 183e186 were almost in the same range as for congener 179e182: i.e. 41-61% (measured at 800 C under N 2 atmosphere) and 34e42% respectively.Comparison of thermal parameters of 183e186 do not suggest any evident dependence or correlation of thermal properties of the macromolecules with aromatic diacid used.
Abdolmaleki and Molavian reported the synthesis and properties of N-rich amorphous aramid 187 by incorporating a designed hetaromatic diamine based on 2,6-bis(2-benzimidazol-2-yl)pyridine [89].The monomer was synthesized via 3-step protocol from 1,2-phenylenediamine and 2 6-pyridinedicarboxylic acid followed by nitration and reduction of the terminal nitro groups.Subsequent direct polycondensation of the diamine with isophthalic acid in TBAB afforded the coordinating macromolecule 187.Aramid 187 displayed moderate thermal stability with T 5% of 174 C, while it formed ample char residues of 61% (measured at 800 C under N 2 atmosphere) and therefore exhibited high calculated LOI of 41.9%.The coordinating capacity of 187 was exploited to support and stabilize Co NP, used for catalysis.The nanofiller slightly improved char yield and LOI.
Hassan reported synthesis and properties of aramids 188e193 in their native and Cu(II)-doped forms, studying dielectric properties of resulting hybrid materials [90].The polymers were obtained as NP from commercial aromatic diamines and aromatic diacylchlorides, followed by doping of the macromolecules with Cu(OAc) 2 in DMSO.The authors investigated the kinetic parameters for thermal decomposition and calculated LOI of 25.2e43.5% for pristine aramids 188e193.Lower LOI of 20.3e29.5% was obtained for Cu-doped polymers.However, the latter values were derived from char yield obtained at 700 C and not 500 C as determined for native 188e193.
Similar to semiaromatic PA 29 and 30, aramids 194e196 contained diamine with a carbazole-pendant group [55].The resulting aramids derived from direct polycondensation with commercial aromatic diacids demonstrated good solubility in polar amide solvents, DMSO and pyridine.In addition, aromatic PAs 194e196 showed high T g > 275 C and no significant thermal decomposition up to 435 C.Moreover, the formation of ample char for 194e196 (measured at 700 C under N 2 atmosphere) ensured high assessed LOI in the range of 38.3e41.9%for the polymers.
FR aramids 197 and 198 incorporated similar 4,4'-(butane-1,4diylbis (oxy)dibenzoic acid and different designed aromatic diamides.Faridi et al. described aramid 197 with bulky rigid rod-like diamine containing 1,3,4-oxadiazole unit synthesized via a 5-step protocol and utilizing 3,5-dinitrobenzoyl chloride as the key precursor for the diamine [91].Direct polycondensation of the designed monomers afforded 197 with T g of 154 C and T 5% of 200 C.Moreover, the aramid 197 formed ample char residues of 42.3% (measured at 800 C under N 2 atmosphere) and therefore exhibited high assessed LOI of 34.4%.The aramid 197 was also reinforced with ZnO NP that slightly improved T g , T 5% , char yield and LOI of the polymer.
The triphenylphosphineoxide-based aramid 198 was obtained through direct polycondensation of the designed monomers [92].The engineered phosphorus-based diamine was prepared in a twostep protocol, including nitration of triphenylphosphine oxide followed by reduction of terminal nitro-groups.In relation to 197, aramid 198 demonstrated superior T g and T 5% of 165 C and 350 C respectively, while char yield and LOI were on par with the P-free polymer.Specifically, 198 formed 39.4% of char residues and exhibited high assessed LOI of 33%, that corresponded well to the experimental value of 32%.Reinforcement of 198 with an organomodified clay improved fire resistance and thermal parameters of the polymer.
Faghihi and Hagibeygi reported another triphenylphosphine oxide-based aramid 200 incorporating 4,4-azobisbenzoic acid monomer [93].The diacid with azo-bridge was obtained by a reductive coupling of 4-nitrobenzoic acid in alkaline solution of glucose followed by conversion of the product to the corresponding diacylchloride used for polycondensation with aromatic diamines.The P-containing polymer 200 was studied in comparison with a congener P-free aramid 199, which was synthesized from p-phenylenediamine and exhibited inferior thermal properties and fire performace.Specifically, 200 demonstrated T 5% of ~70 C higher than that for 199and exhibited LOI of 29%, while LOI of 199 was only 20%, typical for easily combustible materials.
Wei reported unconventional approach for the preparation of aramids 201e205 which incorporated triphenylphosphine oxide unit, cardo groups and ether linkages [94].The synthesis of aromatic polymers was realized in a heterogeneous Pd-catalyzed carbonylation-polycondensation, employing bis(4-(3-iodophenoxy)phenyl)phenylphosphine oxide, designed diamines with cardo groups and CO in the presence of PdCl 2 complexed on the surface of magnetic NP.Thus, the phosphine oxide formally constituted an aromatic diacid.The resulting aramids were well soluble in polar aprotic amide solvents, DMSO, pyridine and partially -in less polar THF.Moreover, the aramids 201e205 high thermal stability, displaying T g of 237e256 C and T 5% in the range of 475e499 C. In addition, all polymers formed ample char residues above 52.1% residues (measured at 800 C under N 2 atmosphere), resulting in high assessed LOI >39%.
In different works, Mehdipour-Ataei reported synthesis and thermal properties of aromatic polyester-amides 206, 207 by incorporating quinoline and 208, 209 containing multiple pyridine units [56,57].The hetaramides were obtained through similar polycondensation of commercial diacylchlorides with designed diamines (vide supra for synthesis).The synthesized PAs were well soluble in polar amide solvents, DMSO and m-cresol.Moreover, the aramids 206e209 displayed T g above 203 C and T 10% higher than 370 C, while char yield in the range of 43e56% brought about high assessed LOI beyond 34.7%.Notably, aramids 208 and 209 contained bigger pyridine monomers and displayed superior thermal robustness, as compared to 206 and 207 with compact quinolonebased diamines.Like semiaromatic PAs 33e36, quinoxaline-containing aramids 210e213 were obtained via two different methodologies, using commercial diacids and diamides respectively [58,59].The resulting aramids displayed significantly differing T 10% and char yield, depending on polymerization protocol.Specifically polycondensation products 211 and 212 demonstrated 10% mass loss at temperatures <100 C, while the macromolecules 210 and 212 obtained in cross-coupling polymerization exhibited high T 10% above 460 C.However, char residues for the synthesized aramids were less dependent on the preparation method and were in the range of 69.3e82.0%(measured at 600 C under N 2 atmosphere), and ensuring high calculated LOI >45%.
Synthesis of families of structurally related thermally robust aramids 214e225 with 4,5-diphenylimidazole bulky pendant groups were reported in series of works along with analogous semiaromatic PAs 23e28 (see Fig. 14) [51e54].The aramids were obtained through direct polycondensation method and were well soluble in polar amide solvents, DMSO, pyridine and, in some cases, in THF, m-cresol or CHCl 3 .All aramids displayed T g in the range of 258e330 C and no significant thermal degradation up to 295 C. Notably, except for 214, all aramids incorporating dipicolinic acid displayed the lowest T g and T 5% (or T 10% ) as compared to carbocyclic congeners.All aromatic PAs formed ample char residues >42% and therefore exhibited high assessed LOI >34%.
Trigo-L opez described the preparation of thermally stable intrinsically colored aramids 226e228, that incorporate azadipyrromethene chromophore [95].The designed diamine dye-core was prepared via three-step protocol and copolymerized at 0.1e10 wt% with m-phenylenediamine and isophthaloyl chloride to approach properties of poly-(m-phenylene isophthalamide).The resulting colored aramids were well soluble in polar amide solvents, DMSO, and displayed similar water uptake of 7% as the reference aramid 162, studied for comparison.In addition, the incorporation of the heteroaromatic chromophore did not have influence on thermal properties as compared to 162.The polymers 226e228 displayed T 5% beyond 416 C, high char yield above 56% and therefore assessed LOI >40%.
Miguel-Ortega described pyridine-containing aramids 229 and 230 used for synthesis of hybrid luminescent materials 231 and 232, bearing Ir(III) cyclometalated species [96].Copolyamides were structurally related to poly-(m-phenylene isophthalamide), but incorporated 10% or 1% of 2,2 0 -bipyridine-4,4 0 -dicarboxylic acid as a binding site for Ir.Pristine copolyamides displayed no significant thermal decomposition up to 425 C, regardless the atmosphere used, and formed >60% of char residues (measured at 800 C under N 2 atmosphere).Coordination of Ir (ppy) 2 Cl species led to a significant decrease in T 5% , char yield and as a result lower assessed LOI of 36.0e38.0%.Moreover, the incorporation of polar bipyridine entities and ionic Ir species lead to increased water uptake of 15%, 17% for pristine 229 and 230 and 21%, 18% for hybrid 231 and 232 respectively.
Two other organometallic aramids 233 and 234 containing ferrocene entities were reported together with semiaromatic counterparts 40 and 41 [61].Expectedly, the aromatic PAs displayed superior thermal properties as compared to 40 and 51 and more char residues.Both aramids 233 and 234 exhibited high experimental LOI of 27.5% Mirsamiei and Faghihi reported wholly aromatic PA 235 endowed with flexible ether linkages [97].The designed diamine with flexible ether bonds was obtained in S N Ar reaction of 2,7dihydroxynaphthalene with 1-fluoro-4-nitrobenzene followed by reduction of terminal nitro-groups.Afterwards, direct polymerization with isophthalic acid afforded thermally robust 235, displaying T 5% of 285 C and T 10% of 426 C.Moreover, aramid 235 formed ample char of 61% (measured at 700 C under N 2 atmosphere).andtherefore exhibited high calculated LOI of 41.9%.Addition of MWCNT filler strongly improved T 5% and had little positive effect on char yield formation and calculated LOI.
Triptycene-based aramids 236e238 were synthesized along with semiaromatic PAs 37e39 [60].The designed diamine was polycondenced directly with commercial aromatic carbocyclic diacids, yielding fully aromatic PAs soluble in polar amide solvents, DMSO and pyridine.In addition, the polymers 236e238 displayed high thermal robustness with no significant thermal degradation up to 428 C. Notably, 236 with isophthalic acid monomer exhibited the superior thermal stability among with all tryptycenediaminebased PAs.Moreover, polymers 236e238 formed >66.7% of char residues (measured at 800 C under N 2 atmosphere), thus exhibiting high assessed LOI in the range 44.1e46.9%.
Jessop reported synthesis and properties of oligoamides 239e241 incorporating designed thiophene-and diphenylsilanecontaining monomers [98].Aromatic 2,2'-(thiophene-2,5-diyl) dianiline was prepared using 1-step Suzuki-Miyaura cross coupling, while the Si-endowed dicarbonyl chlorides were obtained via 3-step procedure.The latter included lithiation of pbromotoluene followed by a reaction with different dichlorosilanes, oxidation of terminal Me-substituents and conversion of the corresponding carboxylic groups into canbonyl chlorides.Consecutive polycondensation afforded short oligomers of 6e11 repeating units that were well soluble in polar aprotic solvents CHCl 3 , and THF.Despite short chains, the compounds displayed promising thermal properties: T g in the range of 157e178 C, T 10% beyond 439 C and char residue above 38% (measured at >600 C under N 2 atmosphere), and LOI >32%.
Salunkhe reported the preparation and properties of other FR aramids 242e246 composed of thiophene-quinoxaline aromatic diamines and aromatic diacid monomers [99] The designed diamines were synthesized via 5-step procedure followed by direct polycondensation with commercial aromatic diacids.The introduction of polar heterocycles and bulky phenyl pendant groups into the macromolecular backbone lead to a good solubility of the PAs 242e246 in polar aprotic solvents, including DMAC, NMP, Py and less polar m-cresol.The polymers were amorphous and hydrophobic, attaining water contact angle up to 82 (for films made of 244).In addition, the polyamides demonstrated excellent thermal properties, typical for aramides.Specifically, thepolymers 242e246 displayed very high T 10% above 698 C and T g in the range of 251e274 C, depending on the rigidity of a diacid moiety.Furthermore, upon heating to 900 C in N 2 atmosphere all aramids decomposed to yield high amount of char residues in the range of 43e56% and therefore high calculated LOI >34.7%.Notably, aramids 245 and 246 containing more rigid diacids displayed superior thermal properties.

Polyamidoimides
Polyamidoimides represent an important class of hybrid polymers with high mechanical, thermal and chemical resistance.They combine properties of two parental polymers: polyamides and polyimides.The presence of amide linkers imparts improved processability, while retaining excellent polymer strength that is intrinsic to polyimides.A judicious choice of monomers affects polymer properties and may further improve solubility, processability, thermal properties and confer inherent flame-retardancy.
Polyamidoimides contain at least two different functional groups as polymer-forming bonds (amide and imide groups).Thus, the preparation of monomers implies introduction of either amide or imide bond into the structure beforehand, while the second remaining type of the bonding is formed during the subsequent polycondensation reaction.
Reaction between two equivalents of trimellitic anhydride and primary diamines is a common way to prepare diacid monomers containing bisimide functionality (see Fig. 15).Since PAI considered in this section are aramids, polymerization reaction is identical to aromatic PA, requiring activation of the functional groups.
Faghihi described the use of bis(3-aminophenyl)phenylphosphine oxide for preparation of FR PAI 247 and 248 [100].The phosphorus-containing aromatic diamine monomer (vide supra 198 for the monomer synthesis) was polymerized with a designed imide-based diacid derived from commercial 4,4 0 -oxydianiline.The polycondensation of the monomers afforded PAIs 247 and 248, that were well soluble in polar aprotic solvents while also resistant to thermal decomposition up to 270 C and 340 C respectively, and forming high amount of CR 41% and 69% of solid residues respectively (measured at 600 C under N 2 atmosphere).Moreover, LOI values were measured to be 21% for 247 and 29% for 248, suggesting that replacement of a short p-phenylenediamine unit with P-containing monomer might be beneficial for thermal stability and FR properties of the PAIs.
In other publications Faghihi reported synthesis and properties of series of related araliphatic polyetheramidoimides 249e264 containing linear aliphatic spacers of different length [101e103].Ether-endowed flexible diamine "bridges" were synthesized starting from 4-nitrophenol and desired a,u-dihaloalkanes, followed by reduction of terminal nitro-groups to amino-groups and their conversion into the corresponding designed imide-endowed diacids.Polycondensation of the resulting monomers with commercial aromatic diamines afforded hybrid FR polyetheramidoimides that were soluble or partly soluble in polar aprotic solvents.Some selected polymers were studied in TGA experiments and demonstrated high thermal stability, displaying T 5% in the range of 285e390 C.However, almost no correlation between thermal stability and number of carbon atoms in aliphatic linkers was observed for a series of polymers with isomeric aminophenylsulfones 251, 256, 261 and 251, 257, 262 reported systematically.Interestingly, T g values for 256, 261 were reported to be lower as compared to 257, 262 containing longer aliphatic bridges.All polymers formed ample amount of char residues above 38.9%(measured at 800 C under N 2 atmosphere), therefore ensuring high assessed LOI values in the range of 33.1e40.0%.
Fully aromatic polyetheramidoimides were reported in a series of publications and mostly varied with a structure of a bridging unit connecting two imide cycles.These bridges are usually prepared via S n Ar reaction between a phenol and an activated arylhalides, while one entity necessarily bears a masked amine functionality used further in imidization reaction.Expectedly, structure and functional groups embodied in the connecting unit can affect properties of the resulting macromolecules.
Shabanian and Hajibeygi reported aromatic polyetheramidoimide 265, containing bisphenol A entity as a flexible ether linker [104].The corresponding diacid monomer was derived from 4-fluoronitrobenzene and bisphenol A in S n Ar reaction, followed by reduction of terminal nitro groups and formation of the corresponding imide-endowed diacid monomer.Interestingly, the aromatic congener 265 exhibited rather similar thermal properties as compared to the polymers with aliphatic linkers, despite its fully aromatic backbone.Specifically, polyetheramidoimide 265 displayed T 5% of 344 C, T g of 180 C and formed 46.3% of solid residues (measured at 800 C under N 2 atmosphere).Reinforcement of 297 with Fe 3 O 4 NP affected these parameters, mainly increasing T g , char yield and the calculated LOI value.
Rafiee reported the preparation and properties of aromatic polyetheramidoimides 266e272 with bulky triptycene-based ether linker [105].Diacid monomer synthesis was realized following similar concept as for 265, whilst initial 1,4-dihydroxytriprycene scaffold was constructed via a Diels-Alder reaction between anthracene and p-benzoquinone.Direct polycondensation of commercial diamines and tailored triptycene diacid delivered amorphous polyetheramidoimides 266e272 with excellent solubility in polar organic solvents, which was ascribed to the presence of bulky triptycene group, preventing packing and hydrogen bonding among macromolecules.Moreover, all polyetheramidoimides displayed very high thermal stability without significant mass loss up to 500 C; T g in the range of 229e277 C and formed >39% of solid residues (measured at 800 C under N 2 atmosphere).Notably, 267 and 268 demonstrated the best thermal stability and the highest char yield.Finally, assessed LOI values were above 33.1%,rendering the polyetheramidoimides 266e272 as self-extinguishing materials.
Thiruvasagam and Vijayan reported the synthesis and properties of series of fully aromatic polyetheramidoimides 273e284 based on benzophenone or diphenylsulfone central cores [106,107].Synthesis of symmetric imide-containing diacid monomers was accomplished via S n Ar reaction between 4,4 0dichlorodiphenyl sulfone or 4,4 0 -difluorobenzophenone and a selected aminophenols, followed by condensation with trimellitic anhydride.Polycondensation of commercial diamines and synthesized multifunctional diacid furnished series of polyamidoimides 273e284 with improved solubility in polar aprotic solvents.Notably, polymers 273, 276, 279 and 282 incorporating 4,4 0 -oxydianiline unit were fully amorphous, exhibiting the best solubility among the investigated polyetheramidoimides.Moreover, all polymers demonstrated high thermal stability, without significant mass loss up to 370 C and T g in the range of 174e215 C. Notably, macromolecules 279e284 with diphnenylisopropylidene bridge showed superior properties compared to naphthalene-endowed 273e278.In addition, investigated polyetheramidoimides formed 15e30% of char residues (measured at 800 C under N 2 atmosphere) and therefore realized assessed LOI values up to 30%.
Mallakpour and Zeraatpisheh described environmentally benign synthesis of PAI 285e288 [108].The imide-containing diacid monomer was prepared using commercial 4,4 0 -methylenebis(3-chloro-2,6-diethylaniline) and trimellitic anhydride.The resulting diacid was polymerized with several commercial aromatic diamines, under MW-irradiation and using molten tetrabutylammonium bromide as a replacement of conventional solvents.Polymers 285e288 displayed good solubility in polar aprotic solvents and H 2 SO 4 .In addition, two PAI 287 and 288 were studied in TGA experiments and demonstrated good thermal stability up to 332 C.Moreover, formation of >38% of solid residues (measured at 800 C under N 2 atmosphere) ensured high assessed LOI values above 32%.
Faghihi reported synthesis and characterization of thermally stable PAI 289e296, containing dibenzylideneacetone repeating subunit [109].The imide-containing aromatic diacid monomer was synthesized following a three-step protocol: consecutive condensation of 4-nitrobenzaldehyde with acetone, reduction of terminal nitro-groups followed by cyclic imide formation using trimellitic anhydride.The resulting monomer was further used in the polycondesation reaction with commercial diamines, yielding PAI 289e296 soluble in polar aprotic solvents and H 2 SO 4 .Three polymers 289, 290 and 294 were also studied in TGA and DSC experiments and demonstrated good thermal stability.Specifically, these PAI showed high T g in the range of 177e212 C and no significant mass loss up to 365 C. In addition, the investigated dibenzylideneacetone-based polymers formed solid residues above 42% (measured at 800 C under N 2 atmosphere), thus manifesting high calculated LOI over 34%.
Parhami described the preparation and properties of PAIs 297e302 by incorporating polar 2,6-dibenzoylpyridine units [110].The heterocyclic diacid monomer was derived from 2,6pyridinedicarbonyl dichloride in double Friedel-Crafts reaction with benzene, followed by consecutive double nitration, reduction of nitro-groups and imidization with trimellitic anhydride.Polycondensation of commercial diamines with the tailored heterocyclic diacid resulted in a hybrid and mostly amorphous PAIs 297e302 that were well soluble in polar aprotic solvents, while also soluble in m-cresol and pyridine upon heating.Moreover, all polymers demonstrated excellent thermal stability with almost no Jalalian reported the preparation and properties of siliconendowed PAI 303e306 [111].Si-bridged symmetric imide-based diacid monomer was derived from dichlorodiphenylsilane reacted with 3-aminophenol.Polycondensation of the designed diacid with four commercial aromatic diamines delivered PAI 303e306.Polymers were well soluble in polar aprotic solvents and in m-cresol upon heating and showed some crystallinity.Furthermore, PAI 303e306 displayed high T g values in the range of 187e196 C and excellent thermal robustness with no significant thermal decomposition up to 375 C. In addition, the hybrid aromatic polymers formed ample solid residues above 39% (measured at 700 C under air atmosphere), thus manifesting high assessed LOI value and selfextinguishing behavior.Remarkably, PAI 304 containing 4,4 0 -oxydianiline monomers demonstrated the poor thermal robustness and formed the least char.
Trimellitic anhydride is a versatile molecular platform and is used to graft affordable a-aminoacids.This facile condensation provides asymmetric chiral imide-endowed diacid monomers, which allows the introduction of functionalized side-substituents along macromolecular backbone, break regularity of monomer connection and therefore increase solubility of PAI (see Fig. 16).This approach was investigated in a series of works by Mallakpour S. and co-authors.Chiral PAI 307e310 were obtained from trimellitic imide-based araliphatic diacids and L-DOPA-endowed designed aromatic diamine [112,113].The diamine was prepared via 2-step protocol, exploiting similar chemistry as for monomers for PAs 29, 30 and 194e196 (vide supra).The synthesis included amidation of L-DOPA with 3 5-dinitrobenzoyl chloride, followed by a reduction or nitrogroups.Further polycondensation of the designed monomers was performed using MW-heating and resulted in chiral PAI 307e310 that were soluble in H 2 SO 4 and polar aprotic solvents, including pyridine.Two PAI 307 and 310 were characterized in TGA experiments and demonstrated thermal stability up to 326 C. The investigated PAI formed 46.8% and 45.4% of char residues (measured at 800 C under N 2 atmosphere) respectively, and therefore demonstrated high calculated LOI >35%.
Structurally similar PAIs 311e314 contain more compact 2aminothiazole pendant group instead of L-DOPA [114].Polycondensation reaction of the designed monomers afforded a family of the related chiral PAIs that were soluble in H 2 SO 4 and polar aprotic solvents, including pyridine and m-cresol at room temperature.Thermal studies of 311 and 312 revealed no significant thermal decomposition up to 290 C. PAIs 311 and 312 formed 34% and 28% of solid residues (measured at 800 C under N 2 atmosphere), and therefore exhibited high calculated LOI above 28%.
Replacement of the amide-containing pendant groups with a directly connected benzimidazole group at aromatic diamine monomers resulted in PAI 315e318 with the least thermalrobustness [115,116].The designed diamine monomer was obtained through 2-step protocol, that included direct condensation of 1,2-phenylenediamine and 3,5-dinitrobenzoyl chloride with Eaton's reagent.Subsequently, direct polycondensation of several synthesized imide-based diacids with the aromatic diamine yielded the desired polymers 315e318.These PAI demonstrated T 5% in the range of 220e260 C, depending on the substituent of the parental amino acid used and polymerization protocol.Despite their moderate thermal stability, PAI 315e318 formed 30e46% of char residues (measured at 800 C under N 2 atmosphere), thus demonstrating high LOI >29%.The aramid 319 was obtained through polycondensation of Lleucine based imidodiacid and commercial 4,4 0 -diaminediphenylsulphone [117,118].Despite similar synthetic protocols used, 319 demonstrated slightly different decomposition temperatures: T 5% of 276 C or 260 C and T 10% of 360 C or 301 C.Moreover, scattering in char yield results was also demonstrated and had similar trend with the decomposition temperatures: 44.8% and 39.1%.Nonetheless, 352 formed ample char that ensures high assessed LOI values above 39%.The polymer 319 was reinforced with organo-modified ZrO 2 NP that slightly improved char yield and LOI.
Mallakpour and Barati described PAI 320 that was obtained in polycondensation of L-isoleucine based imidodiacid with a commercial 4,4 0 -oxydianiline [119].The polymer demonstrated no significant thermal decomposition up to 197 C and formed ample char residues of 52.9%, thus ensuring high assessed LOI of 38.6%.PAI 320 was also reinforced with organo-modified TiO 2 nanofiller that improved thermal resistance of the polymer.
Interestingly, structurally related PAI 321 containing L-methio- nine exhibited significantly higher thermal resistance [120].Specifically, 321 displayed T 5% of 326 C, while formed 52.0% of char residues, similarly to 320.Thus, high calculated LOI of 38.3% was achieved.Further reinforcement of 321 with organo-modified SiO 2 nanofiller improved thermal stability of the polymer and char formation.
Mallakpour and Khadem reported the preparation of PAI 322 via polycondensation of L-phenylalanine-based imidodiacid with commercial 1,5-diaminonaphthalene [121].PAI 322 showed high T 5% of 353 C and formed ample char residues of 49%, therefore giving rise to high calculated LOI of 37%.Introduction of organomodified a-Al 2 O 3 filler only moderately increased the T 5% and T 10% of 322 with little improvement in the char formation and LOI.
Hajibeygi reported the synthesis and properties of photosensitive intrinsically colored PAI 323e328, based on diamines with an expanded p-system [122].Several imide-containing diacid monomers were obtained from L-aminoacids with hydrophobic side chains, while the photosensitive amine monomer was synthesized in a two-step reaction including double condensation of 4nitrobenzaldehyde with cyclopentanone, followed by reduction of the terminal nitro-groups.Polycondensation of the designed monomers afforded a family of PAI that were soluble in H 2 SO 4 and polar aprotic solvents.Moreover, three selected polymers 323, 326 and 328 were characterized in TGA experiments and displayed excellent thermal stability: no significant thermal decomposition was observed up to 370 C. In addition, PAI 323, 326 and 328 demonstrated T g in the range of 166e194 C and formed ample char above 30% (measured at 800 C under N 2 atmosphere) and exhibited high assessed LOI >29.6%.
Tagle described the synthesis and properties of PAI 329e331 composed of Si-based diamines and imide-based diacid monomers extended with ester functionalities (see Fig. 17) [123].The designed diamine was obtained from 4-bromo-N,N-bis(trimethylsilyl)aniline via consecutive metalation, reaction with diphenyldiclorosilane and completed with deprotection of terminal amino groups.The designed ester-imide diacids were derived from selected L-aminoacids reacted with trimellitic anhydride, followed by the conversion of free carboxylic groups into carbonyl chlorides and their coupling with 4-hydroxybenzoic acid.Polycondensation of the designed monomers resulted in PAI 329e331 that were well soluble in polar aprotic organic solvents, including m-cresol, but displayed moderate thermal stability.Thus, PAI 329e331 exhibited T 5% in the range of 226e263 C, while elongation of the alkyl side substituents had negative effect on their thermal stability, including T g .Nonetheless, all PAI formed ample char residues above 28% (measured at 800 C under N 2 atmosphere), and exhibited high assessed LOI value > 28.7%.
Faghihi reported systematic preparation and thermal characterization of semiaromatic chiral FR PAI 332e345 synthesized from with L-amidoacids grafted to a bridged cycloaliphatic core via imide linkers and designed aromatic diamines: bis(3-aminophenyl)phenylphosphine oxide or 1,5-bis(4-aminophenyl)penta-1,4-dien-3one (see Fig. 18) [124e126].The diacid monomers with bis-imide functionalities were prepared from commercial bicyclooctenetetracarboxylic dianhydride and selected L-aminoacids.The respective diamines were synthesized like monomers for polymers 198 and 323e328 but using acetone as a starting enolizable ketone (vide supra).Further polycondensation of the designed monomers resulted in a thermally stable chiral PAI 332e345.All polymers were soluble in DMSO and H 2 SO 4 , while PAI 339e345 displayed better solubility in other polar aprotic solvents (amide solvents) as compared to the triphenylphosphine oxide-containing 332e338.In addition, selected PAI were investigated in TGA experiments and displayed T 5% above 250 C.Moreover, the studied polymers formed ample solid residues in the range of 60e65% (measured at 600 C under N 2 atmosphere) and exhibited high LOI in the range of 29e33%.
Like the rigid aliphatic bicyclooctenetetracarboxylic dianhydride, aromatic pyromellitic dianhydride was widely employed to synthesize bisimide-containing diacid monomers from affordable L-aminoacids.For example, families of chiral polyamides 346e351, 352e356 and 357e362 were derived from proteinogenic amino acids and tailored aromatic diamines: bis(3-aminophenyl)phenylphosphine oxide (vide supra PA 198 for the monomer synthesis), 3,5-diamino-N-(4-hydroxyphenyl)benzamide (vide supra PA 307e314 for similar monomers synthesis) and 5-(1H-benzo [d] imidazole-2-yl)benzene-1,3-diamine (vide supra PA 315e318 for the monomer synthesis) [127e129].Subsequent polycondensation between the designed monomers resulted in PAIs 346e362 that were well-soluble in DMSO.PAIs 352e362 displayed very low crystallinity and good solubility also in other polar aprotic and amide solvents.Moreover, polymers with bulky amide-and benzimidazole-pendants displayed superior thermal stability and no significant mass loss up to 315 C, as compared to triphenylphosphineoxide-based polymers 346e351 which displayed T 5% in the range of 250e305 C. Notably, thermal stability (T 5% and T 10% ) for alanine and phenylalanine-based polymers increased in rows: 346, 352, 357 and 351, 356, 361, suggesting that grafting a benzimidazole side-substituent may improve thermal stability of the pyromellitic-based PAIs.All PAIs formed ample solid residues over 38.9% and therefore displayed high assessed LOI >33.%.Additionally, water uptake for polyamides 352e356 was in the range 4.4e4.8%,higher than that for typical aramides due to the presence of hydroxyl group on the diamine monomer.
Ahmadi reported polymers 363 and 364 with catechol sidesubstituents derived from L-DOPA grafted to a pyromellitic core via imide linkage [130,131].Polycondensation of the designed diacid with commercial aromatic diamines afforded 363 and 364, showing no significant mass loss up to 257 C and 184 C respectively.In addition, PAI 363 and 364 formed ample char residues: 51.0% (measured at 600 C under N 2 atmosphere) and 55.0% (measured at 800 C under N 2 atmosphere) accordingly; thus ensuring high calculated LOI above 37%.Both polymers were further reinforced with inorganic fillers, that improved their resistance to thermolysis and increased char yield.
Mallakpour and Derakhshan reported preparation and properties of PAI 365 synthesized from L-leucine grafted to a pyromellitic core via amide linkage and commercial 4,4 0 -diaminodiphenylmethane [132].The polymer demonstrated no significant thermal decomposition up to 238 C and formed 41.0% of char residues (measured at 800 C under Ar atmosphere)., therefore providing high calculated LOI of 33.9%.Reinforcement of 365 with organomodified TiO 2 nanofiller slightly improved char yield.
Hajibeygi and Shabanian reported the preparation and properties of thermally robust PAI 366 [133].The diacid monomer was prepared from 3,3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride and glycine, and was polycondensed with 4,4 0 -diaminophenylsulfone to yield 366.The polymer exhibited T g of 142 C and no thermal decomposition up to 294 C. Furthermore, 366 formed 29.8% of char residues (measured at 800 C under N 2 atmosphere).and ensured high calculated LOI of 29.4%.Fire performance of 366 was evaluated in MCC experiments: pHRR of 252 W/g and THR of 15.8 kJ/g were obtained.Incorporation of organo-modified clay further improved T g , T 5% and boosted char formation, while had little effect on fire performance.
Structurally related to 366, PAI 367 was derived from phenylglycine grafted to 3,3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride and the diamine obtained as for PAI 249e264 (vide supra) [134].Polycondensation of the tailored monomers afforded PAI 367, that showed no significant thermal degradation up to 220 C, while formed more char residues by 21.6% as compared to 366.Thus, calculated LOI of 38.1% was obtained.PAI 367 was reinforced with organo-modified clay filler, that improved thermal stability of the polymer and promoted char formation.
Faghihi reported the preparation and properties of chiral photosensitive PAIs 368e373 composed of designed monomers [135].The family of imide-endowed diacid monomers was synthesized by grafting proteinogenic L-aminoacids to 3,3 0 ,4,4 0 -benzophenonetetracarboxylic dianhydride core.Polycondensation of the tailored diacids with photosensitive diamine (vide supra PA 323e328 for the monomer synthesis) resulted in a family of chiral polymers 368e373 that were well soluble in polar aprotic solvents.Thermal properties of two selected polymers 368 and 372 were studied and revealed T 5% of 330 C and 245 C accompanied with the formation of 34.3% and 32.3% of solid residues respectively (measured at 700 C under N 2 atmosphere).
Hajibeygi described synthesis, thermal properties and fire performance of PAI 374 [136].The diacid monomer was derived from flexible 4,4 0 -oxydiphthalic anhydride and phenylglycine, and subsequently polycondensed with 4,4 0 -diaminophenylsulfone to yield 374, that displayed T g of 127 C and no thermal decomposition up to 211 C. The polymer 374 formed 40.2% of char residues (measured at 800 C under N 2 atmosphere) and therefore ensured high calculated LOI of 33.5%.In addition, fire performance of 375 was studied in MCC experiments: pHRR of 250 W/g and THR of 13.5 kJ/g were achieved.
Thiruvagasam and Venkatesan reported the synthesis and properties of fully aromatic processable PAI 375e382 with good insulating properties and high thermal stability [137].The polymers were derived from designed imide-endowed diacids and commercial diamines.Tailored diacids were prepared from either 4,4 0 -oxydiphthalic anhydride and 4,4 0 -(hexafluoropropylidene) diphthalic anhydride condensed with 4-(4-aminophenoxy)benzoic acid.Subsequent polycondesation between the chosen monomers delivered a family of PAI 375e382 displaying good solubility in H 2 SO 4 and polar aprotic solvents.Notably, the polymers 379e382, containing hexafluoropropylidene bridges displayed lower crystallinity and better solubility in THF, m-cresol and pyridine in contrast to 375e378.All PAIs demonstrated high thermal stability with T 10% from 424 C and char yield in the range of 12e40% (measured at 800 C under N 2 atmosphere).Importantly, fluorinecontaining macromolecules 379e382 showed overall superior thermal properties and LOI of 22e34%.
To the best of our knowledge, the preparation of imide-endowed diamines is rarely reported despite apparent simplicity of the reaction and its similarity to the preparation of imide-containing diacid monomers.A family of PAI 383e386 is example of polymers where the imide unit was embodied into amine monomer (see Fig. 19) [138].
The designed multifunctional aromatic diamine monomer was synthesized from trimellitic anhydride chloride, that was reacted with KSCN followed by a treatment with 4-nitroaniline to simultaneously form thiourea and imide functionalities.The final step in the monomer preparation involved the reduction of two terminal nitro-groups.Direct polycondensation of the monomers yielded FR PAIs 383e386 displaying high T g in the range of 286e292 C and T 10% above 517 C. In addition, all PAIs formed ample char residues >44% of initial mass (measured at 650 C, N 2 atmosphere) and high assessed LOI of 44e54%.The authors attributed good FR properties and excellent thermal stability of PAI 383e386 to the introduction of the thiourea-based monomer.
An alternative way to synthesize PAI macromolecules involves the preparation of terminal diamines containing inner amide functionality.Lateral diamines are used in a two-step reaction with carboxylic acid dianhydrides, firstly forming polyamic acids via ring-opening polyaddition followed by cycloimidation (see Fig. 20).To the best of our knowledge, the following polymerization technique is used seldom as compared to the conventional polyamidation methods.
Mehdipour-Ataei described the preparation and FR properties of nicotinic-based poly (amide-ether-imide)s 387e389 [139].The multifunctional diamine monomer was prepared from 2,6diaminopyridine following two-step procedure, that included amidification with 6-chloronicotinoyl chloride followed by nucleophilic aromatic substitution with 5-amino-1-naphthol.Afterwards, polycondensation with three aromatic bis-anhydrides delivered aramides 387e389 of very low crystallinity.The solubility of polymers was about 1.3e1.7 g/dL in polar aprotic solvents and in m-cresol.In addition, the PAI displayed high T g in the range of 227e242 C and T 10% beyond 385 C.Moreover, the aramids 387e389 formed ample char residues (measured at 600 C under air atmosphere), therefore ensuring high assessed LOI values in the range of 40.3e41.1%.Besides excellent thermal resistance, the aramids demonstrated water absorption values below 1.2%.
Ghulam reported the preparation and properties of PAI 390e393 using a diamine monomer with diphenylsulfone core and embedded amide groups [140].The tailor-made diamine was synthesized through 2-step protocol starting from 4,4 0 -diaminophenylsulfone reacted with 4-nitrobenzoyl chloride followed by reduction of side nitro-groups into amines.Polycondensation of the crafted diamine with selected commercial carboxylic dianhydrides resulted in PAI 390 and 391, which exhibited a crystalline morphology, while polymers 392 and 393 were fully amorphous.Despite the morphological differences, all polyamides demonstrated good solubility in polar aprotic solvents and low water uptake in the range of 0.14e0.27%.Moreover, PAI 390e393 displayed T g of 220e261 C and high thermal stability without significant mass loss up to 520 C. In addition, the investigated polymers formed solid residues above 46% of the initial mass (measured at 800 C under N 2 atmosphere), thus exhibiting calculated LOI over 36%.

Polyamide-siloxane copolymers
Kumar reported environmentally benign enzyme-assisted preparation of hybrid organic-inorganic polymers by incorporating polydimethylsiloxane and polyamide fragments (see Fig. 21) [141].Lipase-assisted (lipase B from Candida antarctica) transamidation between aminopropyl-terminated polydimethylsiloxane and selected esters furnished hybrid polymers 394e397 under mild reaction conditions.The hybrid polyamides demonstrated good thermal stability and flame retardant properties.Specifically, PAs 394e396 with aliphatic diacids realized T onset in the range of 277e302 C, while polymer 397 incorporated 5-hydroxyisophthalic acid monomer and exhibited the highest T onset of 402 C. Notably, PAs 394e397 left less than 9% of solid residues when heated up to 1200 C. Low amount of solid residues was rationalized with the formation of cyclic siloxane compounds [SiO(CH 3 ) 2 ] n where n ¼ 3e6, rather than silica particles upon thermal decomposition in an inert atmosphere.In addition, MCC experiments revealed very low HRC in the range of 100 J g À1 K À1 for aliphatic PAs 394e396 and 200 J g À1 K À1 for 397, indicating relatively low flammability comparable with Kevlar or polyether-ether-ketone.
In further studies, Mosurkal explored crosslinking of a related hybrid PA 398 composed of 5-aminoisophthalic acid monomer and polydimethylsiloxane-based diamine (see Fig. 22) [142].Hardening of 398 was realized by reacting available amino-group with commercially available dianhydrides or diacylchlorides.The use of 20% of pyromellitic dianhydride crosslinkers was optimal for hardening the polymer and rendering it insoluble in common organic solvents, as well as achieving the lowest THR of 11.6 kJ/g and HRC of 100 J/(g$K).Importantly, a type of a crosslinker and its  amount had a pronounced effect on the thermal and flame retardant properties of the final material.Specifically, all cross-linked polyamide-siloxanes 399e401 decomposed at lower temperatures (decomposition temperatures at 20 wt% loss) and formed more residual char (measured at 800 C, under air atmosphere) as compared to the parental macromolecule 398.In addition, polymers 399 and 400 exhibited significant reduction in HRC, as opposed to 401 that performed even worse than virgin non-crosslinked PA 398.The use of isomeric benzenedicarboxylic and aliphatic diacid crosslinkers for 398 resulted in polymers 402e405 with improved FR properties that, however, could not surpass metrics of the champion FR PA 399 (20 wt% of the pyromellitic dianhydride crosslinker) [143].Similarly to 399e401, the choice of a cross-linking precursor affected thermal properties of the final thermoset too.For example, the use of terephthaloyl chloride instead of terephthalic acid resulted in lower char yield, THR and HRC for the thermoset polymer 403.Importantly, properties of the unmodified 398 in both studies, as well as the loading of crosslinkers were significantly different [142,143].
Song reported the preparation and properties of a hybrid organic-inorganic copolymer 406 composed of monomer-cast PA6 with 1e7% of polydimethylsiloxane (see Fig. 23) [165].The grafting of PA6 to siloxane chains was realized using reactive isocyanate terminal groups of the polydimethylsiloxane macromolecules.The resulting hybrid PAs 406 exhibited T m and T c similar to pure PA6 (T m of 219.7 and T c of 173.4), despite the incorporation of flexible hydrophobic inorganic part.Contrarily, the thermal stability of 406 and pHRR decreased, when the content of PDMS in the polymer increased.For example, the incorporation of 3% of inorganic chains led to pHRR reduction by 28.7% from 654 kW m À2 for pure PA6 to 466 kW m À2 for the hybrid.
Finally, all siloxane conjugated displayed superior hydrophobic properties and decreased crystallinity.
Fan reported preparation of hybrid FR polymers containing PA6 and polydiphenylsiloxane (PDPS) chains which are bonded together with ethylene glycol and terephthalate linkers [144].The synthesis included stepwise ring-opening polymerization of ε-caprolactam in the presence of terephthalic acid, followed by esterification with ethylene glycol.Afterwards, ethylene-glycol terminated esters were either self-condensed, furnishing a reference PAs with ester linkers 407, or copolymerized with various amount of PDPS in the presence of zinc acetate to yield hybrid FR PAs 408.The authors reported that the introduction of ester and siloxane into macromolecule chains did not have strong effect on crystallinity and T g of polymers, therefore ensuring values similar to pure PA6 (T g of 47 C).The increase in the amount of ethylene glycol decreased T m by ~20e50 C, T 5% by ~10e30 C and T max by ~35e60 C for PDPS-free polyamides 407 in comparison to the benchmark PA6.Contrarily, variation in polydiphenylsiloxane ratio in 408 with fixed amount of ethylene glycol linkers resulted in polymers with similar T m , T 5% and T max , that were slightly lower than that for PA6.The authors note that the incorporation of glycol units in the polymer brought some positive FR effect, however only the presence of PDPS was essential to attain V-0 rating in UL-94 test, implove LOI and to reduce pHRR, THR substantially, as compared to pure PA6 or 407.Specifically, the champion hybrid PA 408 attaining V-0 rating in UL-94 burning test demonstrated significant reduction in pHRR by 454 kW/m 2 and in THR by 21.2 MJ/m 2 as compared to the performance of unmodified PA6 in cone calorimeter tests.Notably, polydiphenylsiloxane in conjunction with ethylene glycol units were rationalized to act both in gas and condensed phase, forming ample Si-enriched char and nonflammable volatile cyclic siloxanes.

Conclusion and outlook
In this review we have surveyed developments in heat resistant and intrinsically flame-resistant polyamides that have been published since 2004 to 2020.PAs remain to be a very important class of thermoplastics that urge researchers to develop novel nonhazardous and efficient reactive FR additives as co-monomers that do not compromise mechanical properties of the polymers and render them fire-safe even at low loadings.Despite the advantage of the method and the abundance of Nylons, halogenfree FR co-monomers for aliphatic polyamides remain at a development stage, scarce and still relatively expensive for high-volume production.They are mainly represented by few P-containing monomers, based on CEPPA, DOPO-and DPPO-containing structures.Use of other P-containing monomers, their fire performance and effect on aliphatic PAs properties remains largely unknown.Moreover, significant effort must be undertaken to make FR monomers competitive compared to the non-reactive additives used in PAs nowadays: metal salts of alkylphosphinic acids, MCA, red phosphorus etc.Some engineering and heavy-duty PAs are partially aromatic or semiaromatic PAs, that are often exposed to elevated temperatures during their life cycle.In most of the cases, these polymers require improvement in their fire performance as well as processability.Use of external non-reactive FR fillers may not be desirable for melt processing that occurs at higher temperatures (by ~100 C) than for aliphatic PAs.Thus, incorporation of reactive FRs may be an optimal approach to address these challenges.Existing literature in this domain often describe novel macromolecules, but fails to address FR monomers Moreover, reported design of novel fire-resistant polymers focusses mainly on solubility of macromolecules, while detailed melt properties and fire performance is rarely investigated and reported.Published data are often incomplete, limited or indirect: fire-performance of developed polymers is rarely supported by common fire experiments (UL-94 tests, experimental LOI, MCC, cone calorimetry etc.).Thus, development of FR monomers for engineering PAs and processable aramids and exhaustive data generation for fire performance of new macromolecules remain critical.

Declaration of competing interest
The authors declare no conflict of interest.

Fig. 20 .
Fig. 20.FR PAI obtained in a two-step condensation of amines with acid dianhydrides.

Table 1
Preparation, selected molecular properties and DSC data of aliphatic FR PAs.
a Referred to the theoretical amount of an additive, unless otherwise indicated.b Referred to the theoretical amount of phosphorus.c Experimental amount of phosphorus from elemental analysis.d derived from DSC.

Table 2
TGA and UL-94 tests results for aliphatic FR PAs.

Table 2
TGA data, obtained applying 10 K min À1 heating rate under N 2 atmosphere unless stated otherwise.h temperature at the onset of the first stage of the thermal weight loss.
b a fabric sample.c commercial.d Referred to the theoretical amount of phosphorus.e Experimental amount of phosphorus from elemental analysis.f where availble, sample thicknes in cm is indicated in parentheses.g i under air atmosphere.j Obtained applying 15 K min À1 heating rate.k Obtained applying 20 K min À1 heating rate.l Obtained applying 5 K min À1 heating rate.m Obtained applying 40 K min À1 heating rate.

Table 3
Preparation, selected molecular and thermal properties of semiaromatic FR PAs.Unless otherwise indicated, assesed from the respective char yield, usng Van Krevelen-Hoftyzer equation: LOI ¼ 17.5 þ 0.4CR, where CR e char yield.
b Under air atmosphere.c d Experimental values.

Table 4
Preparation, selected molecular and thermal properties of aromatic FR PAs.

Table 4
(continued ) a TGA data, obtained applying 10 K min À1 heating rate under N 2 atmosphere unless stated otherwise.b Under O 2 atmosphere.c Temperature at the onset of the first stage of the thermal weight loss.d Under air atmosphere.e Unless otherwise indicated, assesed from the respective char yield, usng Van Krevelen-Hoftyzer equationLOI ¼ 17.5 þ 0.4CR, where CR e char yield.f Obtained applying 20 K min À1 heating rate.g Experimental values.

Table 5
Preparation, selected molecular and thermal properties of PAI.

Table 5
(continued ) a TGA data, obtained applying 10 K min À1 heating rate under N 2 atmosphere unless stated otherwise.b Obtained applying 20 K min À1 heating rate.c Under air atmosphere.d Under Ar atmosphere.e Data obtarined in MCC measurements.f Experimental values.g Assessed values do not correspond to the reported char yield.

Table 6
Preparation, selected molecular and thermal properties of polyamide-siloxane copolymers.
a TGA data, obtained applying 10 K min À1 heating rate under N 2 atmosphere unless stated otherwise.b temperature at the onset of the first stage of the thermal weight loss.c Heating rate was not indicated.d Under air atmosphere.e Obtained applying 20 K min À1 heating rate.f data obtained in MCC experiments.