Recent Developments in Organophosphorus Flame Retardants Containing P-C Bond and Their Applications

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Review Recent Developments in Organophosphorus Flame Retardants Containing P-C Bond and Their Applications

Sophie Wendels †, Thiebault Chavez †, Martin Bonnet, Khalifah A. Salmeia * and Sabyasachi Gaan *

Additives and Chemistry Group, Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland; [email protected] (S.W.); [email protected] (T.C.); [email protected] (M.B.) * Correspondence: [email protected] (K.A.S.); [email protected] (S.G.); Tel.: +41-587-657-038 (K.A.S.); +41-587-657-611 (S.G.) † These two authors contributed equally to this work.

Received: 13 May 2017; Accepted: 4 July 2017; Published: 11 July 2017

Abstract: Organophosphorus compounds containing P-C bonds are increasingly developed as flame retardant additives due to their excellent thermal and hydrolytic stability and ease of synthesis. The latest development (since 2010) in organophosphorus flame retardants containing P-C bonds summarized in this review. In this review, we have broadly classified such phosphorus compounds based on the carbon unit linked to the phosphorus atom i.e., could be a part of either an aliphatic or an aromatic unit. We have only considered those published literature where a P-C bond was created as a part of synthetic strategy to make either an intermediate or a final organophosphorus compound with an aim to use it as a flame retardant. General synthetic strategies to create P-C bonds are briefly discussed. Most popular synthetic strategies used for developing P-C containing phosphorus based flame retardants include Michael addition, Michaelis–Arbuzov, Friedels–Crafts and Grignard reactions. In general, most flame retardant derivatives discussed in this review have been prepared via a one- to two-step synthetic strategy with relatively high yields greater than 80%. Specific examples of P-C containing flame retardants synthesized via suitable synthetic strategy and their applications on various polymer systems are described in detail. Aliphatic phosphorus compounds being liquids or low melting solids are generally applied in polymers via coatings (cellulose) or are incorporated in the bulk of the polymers (epoxy, polyurethanes) during their polymerization as reactive or non-reactive additives. Substituents on the P atoms and the chemistry of the polymer matrix greatly influence the flame retardant behavior of these compounds (condensed phase vs. the gas phase). Recently, aromatic DOPO based phosphinate flame retardants have been developed with relatively higher thermal stabilities (>250 ◦C). Such compounds have potential as flame retardants for high temperature processable polymers such as polyesters and polyamides. A vast variety of P-C bond containing efficient flame retardants are being developed; however, further work in terms of their economical synthetic methods, detailed impact on mechanical properties and processability, long term durability and their toxicity and environmental impact is much needed for their potential commercial exploitations.

Keywords: P-C bond; substitution; addition; Michaelis-Arbuzov reaction; Michael addition; ﬂame retardant; organophosphorus compounds; transformation; TGA; Cone calorimetry; UL 94

Materials 2017, 10, 784; doi:10.3390/ma10070784 www.mdpi.com/journal/materials Materials 2017, 10, 784 2 of 32

1. Introduction P-C containing organophosphorus compounds find application as metal complex catalysts [1], reagents in organic chemistry [2,3], pesticides, insecticides, herbicides, medicine and biological systems [4], additives for polymers [5], surfactants, lubricants [6], surface treatments on metals [7], etc. In the past few decades, many review articles and book chapters regarding the creation of P-C bond and their potential applications have been published [6,8–13]. Organophosphorus compounds with P-C bonds are increasingly investigated as flame retardants due their high thermal and hydrolytic stability (P-C bond), ease of synthesis and suitability of processing at high temperature. One should pay attention to other properties like miscibility in the polymer matrix, reactivity with polymer or monomer, moisture absorption, tendency for blooming, toxicological properties, etc., which are very important and need to be considered before any commercial exploitation of these compounds can be realized. Unfortunately most published literature are academic and limited to aspects like synthesis, processability, flame retardancy and thermal properties of these compounds, thus it becomes very difficult to judge if any of these compounds discussed in this review can have any real commercial impact. The intention of this review is to summarize the work done in development flame retardants containing P-C bond without being critical of the shortcomings of the published literature. P-C bonds are thermally quite stable and exhibit typical bond energies of ~272 kJ/mol [14]. Though the bond energies are lower than P-O (360 kJ/mol) and P-N (290 kJ/mol), the carbon substituents are not good leaving groups and thus resistant to possible nucleophilic attacks, which becomes relevant in high temperature polymer processing [15]. However, hydrolysis of P-C bonds can help in their biodegradation and reduce their possible harmful impact to the environment. Recent studies have provided insights into possible enzymatic hydrolysis of suitable P-C bonds [16–18]. Organophosphorus-based flame retardants are quite versatile in their flame retardant action and often exhibit both condensed and gas phase activity Depending on the substrate and phosphorus chemistry, there could be chemical interactions in the condensed phase at elevated temperatures, which lead to changes in the decomposition pathway of the polymer and possible formation of carbonaceous char residues on the surface of decomposing polymer, hence preventing its further oxidation. The char thus formed acts as a protective thermal barrier. In other instances, the phosphorus compounds and some of their decomposed products preferably volatilize from the polymer substrate when heated. These phosphorus species further decompose to release reactive phosphorus species, which then interact with the combustion intermediates in the gas phase as flame inhibitors [19,20]. In most cases, such interactions lead to recombination of the H and OH radicals and prevent their oxidation P-C bond containing phosphorus compounds like any organophosphorus flame retardants can exhibit either condensed phase or gas phase flame inhibition and is very specific to a polymer/flame retardant combination. For polymers (cellulose, wool), with relatively high concentration of hydroxyl or amino groups the phosphorus compounds primarily work in the condensed phase. In case of synthetic polymers containing oxygen and nitrogen atoms, catalytic hydrolysis of the ester or amide groups by phosphorus acids promotes an enhanced melt dripping and fast shrinkage from the flame. As far as olefin-based polymers are considered, the phosphorus compounds mainly act in the gas phase by recombining the key fuel species such as H and OH radicals and preventing their oxidation. Some minor physical effects due to volatilization of phosphorus compounds and dilution of the fuel can also occur [21]. In the following section, several analytical methods and techniques used by researchers to evaluate the thermal and fire properties of the new materials developed are mentioned. The choice of specific analytical technique depends mostly on the intended end use of the materials. TGA is generally employed to determine the thermal stability of new flame retardants and materials made from them. The degradation temperature of the flame retardant gives possible indication of its processing temperature. Higher char residue formed for the flame retardant polymer/formulation compared to the basis polymer indicates its possible condensed phase action. Similarly, higher char formation observed in other techniques such as PCFC and Cone calorimeter also indicate possible Materials 2017, 10, 784 3 of 32 condensed based flame retardant action. Cone calorimeter and PCFC instrument is generally used to measure heat release of materials. Flame retardant action in condensed and gas phase leads in normally reduction in heat release rates. In cone calorimeter, one can also measure smoke and its composition. MaterialsMaterials 2017 2017, ,10 10, ,784 784 33 of of 31 31 Flame retardants working in condensed phase usually leads to reduction in smoke toxicity. The ratio of CO/COmeasuremeasure2 ratioheatheat release asrelease measured ofof materials.materials. in the cone FlameFlame calorimeter retardantretardant action indicatesaction inin condensedcondensed the activity andand of flamegasgas phasephase retardant leadsleads in inin gas phase.normallynormally Higher reductionreduction ratio for inin flame heatheat release retardantrelease rates.rates. materials InIn conecone indicatescalorimeter,calorimeter, possible oneone cancan gas alsoalso phase measuremeasure flame smokesmoke inhibition andand itsits [ 22 ]. LOIcomposition.composition. is a common FlameFlame technique, retardantsretardants which workingworking was used inin condensedcondensed for evaluating phasephase the usuallyusually flame leads retardancyleads toto reductionreduction of textile inin smoke materialssmoke andtoxicity.toxicity. films. MaterialsThe The ratio ratio of of exhibiting CO/CO CO/CO2 2ratio ratio LOI as as values measured measured of 25% in in the the and cone cone greater calorimeter calorimeter are considered indicates indicates the the to activity self-extinguishing.activity of of flame flame Usually,retardantretardant inherently in in gas gas phase. phase. flame Higher retardantHigher ratio ratio materials for for flame flame exhibit retardant retardant LOI materials materials values greaterindicates indicates than possible possible 30%. gas gas phase phase flame flame inhibitioninhibitionSeveral small[22]. [22]. LOI LOI case is is firea a common common tests like te technique,chnique, UL 94 which andwhich vertical was was used used burning for for evaluating evaluating tests are the the also flame flame commonly retardancy retardancy used of of to evaluatetextiletextile the materials materials flame retardancyand and films. films. Materials Materials of materials. exhibiting exhibiting In these LOI LOI tests, values values the of of reaction 25% 25% and and of greater greater materials are are considered toconsidered small fire to to eitherself- self- in extinguishing. Usually, inherently flame retardant materials exhibit LOI values greater than 30%. verticalextinguishing. and horizontal Usually, orientation inherently measured. flame retardant materials exhibit LOI values greater than 30%. SeveralSeveral smallsmall casecase firefire teststests likelike ULUL 9494 andand veverticalrtical burningburning teststests areare alsoalso commonlycommonly usedused toto 2. Generalevaluateevaluate Principlesthe the flame flame retardancy retardancy of P-C Bond of of materials. materials. Formation In In these these te tests,sts, the the reaction reaction of of materials materials to to small small fire fire either either inin vertical vertical and and horizontal horizontal orientation orientation measured. measured. The P-C bond formation has been reviewed in the past by various researchers [6,8–10,12,13,23–30]. Conventional2.2. General General syntheticPrinciples Principles strategies of of P-C P-C Bond Bond like Formation MichaelFormation addition, Arbuzov rearrangement reactions, Friedel-Crafts reactions, radical addition reactions, Pudovik reactions are the most common strategy for P-C bond TheThe P-C P-C bond bond formation formation has has been been reviewed reviewed in in th thee past past by by various various researchers researchers [6,8–10,12,13,23–30]. [6,8–10,12,13,23–30]. creation. In the last decade, there has been an increased focus on development of new synthetic ConventionalConventional syntheticsynthetic strategistrategieses likelike MichaelMichael addition,addition, ArbuzovArbuzov rearrangementrearrangement reactions,reactions, methodologiesFriedel–CraftsFriedel–Crafts for reactions, reactions, P-C bond radical radical formation. addition addition As reacti reacti a result,ons,ons, newPudovik Pudovik novel reactions reactions and green are are the methodologiesthe most most common common[24 strategy strategy,28] in the organicforfor P-C P-C synthesis bond bond creation. creation. gave impetus In In the the last tolastreplace decade, decade,toxic there there and has has harmfulbeen been an an increased phosphorusincreased focus focus starting on on development development materials withof of new new more benignsyntheticsynthetic alternatives methodologies methodologies [31]. for for P-C P-C bond bond formation. formation. As As a a result, result, new new novel novel and and green green methodologies methodologies [24,28][24,28]Due to inin the thethe electronic organicorganic synthesissynthesis nature of gavegave the impetus phosphorusimpetus toto replacereplace atom, toxictoxic the organophosphorus andand harmfuharmful l phosphorusphosphorus compounds startingstarting can be classifiedmaterialsmaterials with with into more more different benign benign chemical alternatives alternatives classes [31]. [31]. based on the coordination number and the oxidation state of theDueDue phosphorus to to the the electronic electronic atom nature nature [23,32 of of]. the the Accordingly, phosphorus phosphorus theatom, atom, chemical the the organophosphorus organophosphorus reactivity of the compounds compounds organophosphorus can can be be compoundsclassifiedclassified caninto into bedifferent different influenced chemical chemical by theirclasses classes structural based based on on geometrythe the coordination coordination and valency number number of and and the the the phosphorus oxidation oxidation state atom.state of of For example,thethe phosphorusphosphorusH-phosphonates atomatom [23,32]. with[23,32]. P(O)H Accordingly,Accordingly, bonds are thethe commonly chemicalchemical usedreactivityreactivity as starting ofof thethe materials organophosphorusorganophosphorus for P-C bond compounds can be influenced by their structural geometry and valency of the phosphorus atom. For formationcompounds and thuscan be their influenced chemical by reactionstheir structural have beengeometry investigated and valency in detail of the[ phosphorus6,9,12,24,25 ,atom.29–31 For,33, 34]. example,example, H H-phosphonates-phosphonates with with P(O)H P(O)H bonds bonds are are commonly commonly used used as as starting starting materials materials for for P-C P-C bond bond It isexample, reported H that-phosphonates presence of with the P=OP(O)H bond bonds is theare drivingcommonly force used for as their starting reactions materials [23]. for In P-C general, bond the formationformation andand thusthus theirtheir chemicalchemical reactionsreactions havehave beenbeen investigatedinvestigated inin detaildetail [6,9,12,24,25,29–[6,9,12,24,25,29– phosphorus center of the H-phosphonates is electrophilic and can be modified to be nucleophilic [35]. 31,33,34].31,33,34]. It It is is reported reported that that presence presence of of the the P=O P=O bond bond is is the the driving driving force force for for their their reactions reactions [23]. [23]. In In Moreover, the P-H bond of H-Phosphonates is considered acidic and can go through acid base general,general, thethe phosphorusphosphorus centercenter ofof thethe HH-phosphonates-phosphonates isis electrophilicelectrophilic andand cancan bebe modifiedmodified toto bebe reactions [36]. On the other hand, the H-phosphonates are quinquevalent, tetra-coordinated nucleophilicnucleophilic [35].[35]. Moreover,Moreover, thethe P-HP-H bondbond ofof HH-Phosphonates-Phosphonates isis consideredconsidered acidicacidic andand cancan gogo phosphorusthroughthrough acid acid compounds, base base reactions reactions which [36]. [36]. exist On On inthe the equilibrium other other hand, hand, inthe the solution H H-phosphonates-phosphonates with their are are corresponding quinquevalent, quinquevalent,tervalent, tetra- tetra- tri-coordinatedcoordinatedcoordinated phosphorus phosphorus species (phosphite compounds, compounds, ester). which which The exist exist phosphite in in equilibrium equilibrium esters in in solution solution have a with with lone their their electron corresponding corresponding pair on the phosphorustervalent,tervalent, centertri-coordinated tri-coordinated and thus species canspecies react (phosphite (phosphite with various ester). ester). electrophiles The The phosphite phosphite [23 esters esters]. When have have these a a lone lone phosphite electron electron pair pair esters on on are protonatedthethe phosphorus phosphorus or bear center electroncenter and and withdrawing thus thus can can react reactgroups, with with vari vari theyousous electrophiles reactelectrophiles with nucleophiles. [23]. [23]. When When these these Based phosphite phosphite on the esters chemicalesters reactivityareare protonatedprotonated of the organophosphorus oror bearbear electronelectron withdrawingwithdrawing compounds, grougrou diverseps,ps, theythey synthesis reactreact with approacheswith nucleophiles.nucleophiles. of P-C Based bondBased formationonon thethe havechemicalchemical been explored reactivity reactivity and of of the canthe organophosphorus organophosphorus be categorized as comp illustratedcompounds,ounds, in diverse diverse Tables synthesis 1synthesis–3. approaches approaches of of P-C P-C bond bond formationformation have have been been explored explored and and can can be be categorized categorized as as illustrated illustrated in in Table Table 1–3. 1–3. Table 1. P-H transformation into P-C bond. TableTable 1. 1. P-H P-H transformation transformation into into P-C P-C bond. bond.

ReactionReactionReaction Type Type General General General Examples Examples Selected Selected Selected Ref. Ref.

PudovikPudovikPudovik reaction reactionreaction [6[6,30,37–39][6,30,37–39],30,37–39]

Kabachnik–FieldsKabachnik–FieldsKabachnik–Fields [[6,30]6[6,30],30] reactionreactionreaction

Michaelis–BeckerMichaelis–BeckerMichaelis–Becker [[26]26[26]] reactionreactionreaction

NucleophilicNucleophilic ring ring [6][6] openingopening of of epoxides epoxides

Materials 2017, 10, 784 3 of 31

measure heat release of materials. Flame retardant action in condensed and gas phase leads in normally reduction in heat release rates. In cone calorimeter, one can also measure smoke and its composition. Flame retardants working in condensed phase usually leads to reduction in smoke toxicity. The ratio of CO/CO2 ratio as measured in the cone calorimeter indicates the activity of flame retardant in gas phase. Higher ratio for flame retardant materials indicates possible gas phase flame inhibition [22]. LOI is a common technique, which was used for evaluating the flame retardancy of textile materials and films. Materials exhibiting LOI values of 25% and greater are considered to self- extinguishing. Usually, inherently flame retardant materials exhibit LOI values greater than 30%. Several small case fire tests like UL 94 and vertical burning tests are also commonly used to evaluate the flame retardancy of materials. In these tests, the reaction of materials to small fire either in vertical and horizontal orientation measured.

2. General Principles of P-C Bond Formation The P-C bond formation has been reviewed in the past by various researchers [6,8–10,12,13,23–30]. Conventional synthetic strategies like Michael addition, Arbuzov rearrangement reactions, Friedel–Crafts reactions, radical addition reactions, Pudovik reactions are the most common strategy for P-C bond creation. In the last decade, there has been an increased focus on development of new synthetic methodologies for P-C bond formation. As a result, new novel and green methodologies [24,28] in the organic synthesis gave impetus to replace toxic and harmful phosphorus starting materials with more benign alternatives [31]. Due to the electronic nature of the phosphorus atom, the organophosphorus compounds can be classified into different chemical classes based on the coordination number and the oxidation state of the phosphorus atom [23,32]. Accordingly, the chemical reactivity of the organophosphorus compounds can be influenced by their structural geometry and valency of the phosphorus atom. For example, H-phosphonates with P(O)H bonds are commonly used as starting materials for P-C bond formation and thus their chemical reactions have been investigated in detail [6,9,12,24,25,29– 31,33,34]. It is reported that presence of the P=O bond is the driving force for their reactions [23]. In general, the phosphorus center of the H-phosphonates is electrophilic and can be modified to be nucleophilic [35]. Moreover, the P-H bond of H-Phosphonates is considered acidic and can go through acid base reactions [36]. On the other hand, the H-phosphonates are quinquevalent, tetra- coordinated phosphorus compounds, which exist in equilibrium in solution with their corresponding tervalent, tri-coordinated species (phosphite ester). The phosphite esters have a lone electron pair on the phosphorus center and thus can react with various electrophiles [23]. When these phosphite esters are protonated or bear electron withdrawing groups, they react with nucleophiles. Based on the chemical reactivity of the organophosphorus compounds, diverse synthesis approaches of P-C bond formation have been explored and can be categorized as illustrated in Table 1–3.

Table 1. P-H transformation into P-C bond.

Reaction Type General Examples Selected Ref.

Pudovik reaction [6,30,37–39]

Materials 2017, 10, 784 4 of 32 Kabachnik–Fields [6,30] reaction

Table 1. Cont. Michaelis–Becker [26] reaction Reaction Type General Examples Selected Ref.

NucleophilicNucleophilic ring ring Materials 2017, 10, 784 [6][6] 4 of 31 Materialsopeningopening 2017 of,, 10 ofepoxides,, epoxides784784 4 of 31 MaterialsMaterials 2017 2017, 10, 10, 784, 784 4 of4 of 31 31 MaterialsMaterials 2017 2017, 10, ,10 784, 784 4 of4 31of 31 C-O/P-H cross C-O/P-HC-O/P-H cross cross [40] C-O/P-HcouplingC-O/P-H crossreaction cross [40[40]] couplingcoupling reaction reaction [40] [40][40] couplingcouplingC-O/P-HC-O/P-H reaction reaction cross cross [40][40] couplingDehydrogenativecoupling reaction reaction DehydrogenativeDehydrogenative DehydrogenativeDehydrogenativecoupling with [6,12,41] couplingcoupling with with [6,[6,12,41]12,41] DehydrogenativecouplingterminalcouplingDehydrogenative withalkynes with [6,12,41][6,12,41] terminalterminal alkynes alkynes terminalterminalcouplingcoupling alkynes alkynes with with [6,12,41][6,12,41] terminalterminal alkynes alkynes P-H insertion of α- P-HP-H insertion insertion of α- of [42] P-HP-HImino-carbenes insertion insertion of of α -α - [42[42]] Imino-carbenesα-Imino-carbenes [42][42] P-HImino-carbenesImino-carbenesP-H insertion insertion of αof - α- [42][42] Imino-carbenesImino-carbenes

[43] [43[43]] O2-induced radical [43] O2-induced radical [43][43] OO-induced2-induced radical radical Ooxyphosphorylation2O-induced2-induced radical radical oxyphosphorylationoxyphosphorylation [43][43] oxyphosphorylationoxyphosphorylation2 2 reaction O2-inducedO -inducedreaction radical radical reaction [44] oxyphosphorylationoxyphosphorylationreactionreaction [44[44]] [44] reactionreaction [44][44] reaction [44][44] Phospha–Michael Phospha–MichaelPhospha–Michael [5,15,29] Phospha–Michael [5,[5,15,29]15,29] Phospha–MichaelPhospha–Michaeladditionaddition reaction reaction addition reaction [5,15,29][5,15,29] additionPhospha–MichaeladditionPhospha–Michael reaction reaction [5,15,29][5,15,29] additionaddition reaction reaction

TableTable 2. 2.Rearrangement/transformation Rearrangement/transformation Reactions. Reactions. TableTable 2. 2. Rearrangement/transformation Rearrangement/transformation Reactions. Reactions. Reaction Type General Examples Selected Ref. Reaction Type TableTable 2. Rearrangement/transformation2. Rearrangement/transformation General Examples Reactions. Reactions. Selected Ref. ReactionReaction Type Type Type General General General Examples Examples Selected Selected Selected Ref. Ref. Michaelis– ReactionMichaelis–Reaction Type Type General General Examples Examples Selected Selected Ref. Ref. [5,27,28,45,46] Michaelis–ArbuzovArbuzovMichaelis–Michaelis– reaction [5,27,28,45,46] Arbuzov reaction [5,27,28,45,46][5[5,27,28,45,46],27,28,45,46 ] ArbuzovArbuzovreactionMichaelis–Michaelis– reaction reaction [5,27,28,45,46][5,27,28,45,46] Arbuzov reaction ArbuzovSandmeyer-typeArbuzov reaction reaction Sandmeyer-type [47,48] Sandmeyer-typeSandmeyer-typeSandmeyer-typereaction [47,48] reaction [47,48][[47,48]47,48 ] Sandmeyer-typereactionSandmeyer-typereaction [47,48][47,48] reactionreaction Decarbonylation Decarbonylation [49] DecarbonylationDecarbonylationofDecarbonylation Acylphosphines of [49] of Acylphosphines [49] [49][[49]49 ] ofAcylphosphinesof AcylphosphinesDecarbonylation AcylphosphinesDecarbonylation [49][49] of Acylphosphinesof Acylphosphines Abramov reaction [30,35,50] Abramov reaction [30,35,50] AbramovAbramovAbramov reaction reaction reaction [30,35,50][30[30,35,50],35,50 ]

AbramovAbramov reaction reaction [30,35,50][30,35,50]

Table 3. C-H to C-P transformation. TableTableTable 3. 3. 3. C-HC-H C-H to to C-P C-P transformation. transformation. Reaction Type General Examples Selected Ref. Reaction Type TableTable 3. C-H3. C-H General to toC-P C-P Examplestransformation. transformation. Selected Ref. ReactionReaction Type Type General General Examples Examples Selected Selected Ref. Ref. ReactionFriedel–CraftsReactionReaction Type Type Type General General General Examples Examples Selected Selected SelectedRef. Ref. Ref. Friedel–Crafts [5,8] Friedel–CraftsFriedel–Craftsreaction [5,8] reaction [5,8][5,8] Friedel–CraftsreactionFriedel–Craftsreaction Friedel–Crafts reaction [5,8] [5,8][5,8] reactionreaction

Via Aryne Via Aryne [51] ViaViaformation Aryne Aryne [51] formation [51][51] formationViaformationVia Aryne Aryne Via Aryne formation [51] [51][51] formationformation Nucleophilic Nucleophilic [8,52] Nucleophilic [8,52] NucleophilicNucleophilicsubstitution substitution [8,52][8,52] substitutionNucleophilicsubstitutionNucleophilic [8,52][8,52] Nucleophilicsubstitutionsubstitution substitution [8,52]

As seen in the table, there are numerous ways P-C bonds can be created and its potential AsAs seen seen in in the the table, table, there there are are numerous numerous ways ways P-C P-C bonds bonds can can be be created created and and its its potential potential academic research or industrial exploitations needs careful consideration. Flame retardant academicacademicAsAs seen researchseenresearch in inthe the or table,or table, industrialindustrial there there are exploitationsareexploitations numerous numerous ways needs waysneeds P-C carefulP-Ccareful bonds bonds consideration. canconsideration. can be be created created Flame andFlame and its retardantits retardantpotential potential development is an applied research with a great potential for commercial exploitation and thus developmentacademicdevelopmentacademic research isresearch is an an applied appliedor orindustrial industrialresearch research exploitationswith withexploitations a agrea great tpotential needs potentialneeds careful forcareful for commercial commercialconsideration. consideration. exploitation exploitation Flame Flame and retardantand retardant thus thus attention must be paid to the synthetic strategies for upscaling. In most cases, commercial availability attentiondevelopmentattentiondevelopment must must beis be ispaidan paid an applied toapplied to the the synthetic researchsynthetic research strategieswith strategies with a greaa fogrea for tupscaling.r potentialupscaling.t potential Infor In formost mostcommercial commercial cases, cases, commercial commercial exploitation exploitation availability availability and and thus thus of the starting chemicals and their cost determine their potential commercial exploitation. However, ofattentionof the attentionthe starting starting must must chemicals chemicalsbe be paid paid to and toandthe the their synthetic their synthetic cost cost determin strategies determin strategiese foetheir theirrfo upscaling.r potentialupscaling. potential In commercial Incommercialmost most cases, cases, exploitation. commercialexploitation. commercial However,availability However, availability

of ofthe the starting starting chemicals chemicals and and their their cost cost determin determine theire their potential potential commercial commercial exploitation. exploitation. However, However,

Materials 2017, 10, 784 5 of 32

As seen in the table, there are numerous ways P-C bonds can be created and its potential academic research or industrial exploitations needs careful consideration. Flame retardant development is an applied research with a great potential for commercial exploitation and thus attention must be paid to the synthetic strategies for upscaling. In most cases, commercial availability of the starting chemicals and their cost determine their potential commercial exploitation. However, many other likely challenges can be encountered in industry, which hinders their industrial upscaling and utilities such as:

(1) Expensive synthesis approaches, (2) Toxicity of the starting materials, (3) Non-desirable wastes or by-product, (4) Multiple puriﬁcation steps, (5) Difﬁculty handling of the chemical materials.

Currently only a few chemical reactions listed in Tables1–3 are industrially exploited to manufacture ﬂame retardants.

3. Flame Retardants with P-Caliphatic Bonds

In the following sections, we have summarized examples of P-Caliphatic bond containing organophosphorus compounds, which are classiﬁed based on the synthetic strategy for creation of P-C bond. For clarity, the speciﬁc P-C bonds that were created in the structure have been marked in bold.

3.1. Substitution Reactions Substitution reactions are one of the most commonly used reactions used in organic chemistry and can be suitably exploited to create new bonds and eventually final products. Such reactions are commonly exploited commercially [53] and in research to create P-C bond containing useful flame retardants. In this section, we summarize development of organophosphorus compounds obtained by substitution reactions other than Grignard type reaction. The Grignard type reaction is described in detail later in the section. The reaction of but-3-1-yl 4-methylbenzenesulfonate and butyl 4-methylbenzenesulfonate with triphenylphosphine afforded the phosphonium salts 1a and 1b respectively (Figure1)[ 54]. The phosphonium salts 1a and 1b were obtained by precipitation in petroleum ether in 73% and 86% yield, respectively. Their thermal properties were studied using thermogravimetric analysis (TGA) ◦ ◦ under N2 atmosphere and the temperatures for 5% weight loss (Td5%) of 335.8 C and 208.9 C were observed for compounds 1a and 1b, respectively. They were then incorporated in polycarbonate (PC) composites at a concentration of 5% together with an anti-dripping agent. TGA was used to study the thermal stability of these composites. Cone calorimeter, UL94 and limiting oxygen index (LOI) tests were used to evaluate the flame retardant performance of the composites. It was found that the composite containing flame retardant with the alkene group (1a) exhibited a significant increase of LOI value (32.8%) compared to composite with compound 1b (30.8%) and a lower peak heat release rate (PHRR). The LOI value of the neat PC is 25%. The char yield at 700 ◦C was found to be higher for both compounds (17 wt % and 15.6 wt % for compounds 1a and 1b, respectively) than for the virgin PC (13.9 wt %), which means that the two compounds improved the flame retardancy of the PC and act in the condensed phase. It was also noted for both compounds that 5% in the composite was enough to obtain UL 94 V-0 rating. The conjugated dienes 2 and 3 (Figure2) were synthesized by reaction of mesityl oxide with phosphorus acid and hypophosphorus acid, respectively [55,56]. It was described in the patent that these compounds can be used as flame retardants, potentially in the plastic industry or in textile coatings. The same researchers have also studied the synthesis of acrylamide derivative (Compound 4, Materials 2017, 10, 784 5 of 31

many other likely challenges can be encountered in industry, which hinders their industrial upscaling and utilities such as: (1) Expensive synthesis approaches, (2) Toxicity of the starting materials, (3) Non-desirable wastes or by-product, (4) Multiple purification steps, (5) Difficulty handling of the chemical materials. Currently only a few chemical reactions listed in Table 1–3 are industrially exploited to manufacture flame retardants.

3. Flame Retardants with P-Caliphatic Bonds

In the following sections, we have summarized examples of P-Caliphatic bond containing organophosphorus compounds, which are classified based on the synthetic strategy for creation of P-C bond. For clarity, the specific P-C bonds that were created in the structure have been marked in bold.

3.1. Substitution Reactions Substitution reactions are one of the most commonly used reactions used in organic chemistry and can be suitably exploited to create new bonds and eventually final products. Such reactions are commonly exploited commercially [53] and in research to create P-C bond containing useful flame retardants. In this section, we summarize development of organophosphorus compounds obtained by substitution reactions other than Grignard type reaction. The Grignard type reaction is described in detail later in the section. The reaction of but-3-1-yl 4-methylbenzenesulfonate and butyl 4-methylbenzenesulfonate with triphenylphosphine afforded the phosphonium salts 1a and 1b respectively (Figure 1) [54]. The phosphonium salts 1a and 1b were obtained by precipitation in petroleum ether in 73% and 86% yield, respectively. Their thermal properties were studied using thermogravimetric analysis (TGA) under N2 atmosphere and the temperatures for 5% weight loss (Td5%) of 335.8 °C and 208.9 °C were observed for compounds 1a and 1b, respectively. They were then incorporated in polycarbonate (PC) composites at a concentration of 5% together with an anti-dripping agent. TGA was used to study the thermal stability of these composites. Cone calorimeter, UL94 and limiting oxygen index (LOI) tests were used to evaluate the flame retardant performance of the composites. It was found that the composite containing flame retardant with the alkene group (1a) exhibited a significant increase of LOI value (32.8%) compared to composite with compound 1b (30.8%) and a lower peak heat release Materialsrate (PHRR).2017, 10, 784The LOI value of the neat PC is 25%. The char yield at 700 °C was found to be higher6 of 32 for both compounds (17 wt % and 15.6 wt % for compounds 1a and 1b, respectively) than for the virgin PC (13.9 wt %), which means that the two compounds improved the flame retardancy of the FigurePC and2) from act in phosphorus the condensed acid, phase. mesityl It was oxide, also acetic noted acid, for aceticboth compounds anhydride andthat phenothiazine5% in the composite and no additionalwas enough ﬁre to performance obtain UL 94 of V-0 these rating. compounds were reported [57].

MaterialsMaterials 2017 2017, 10, ,10 784, 784 6 of6 31of 31

FigureFigure 2) 2)from from phosphorus phosphorus acid, acid, mesityl mesityl oxide,oxide, acetic acid, aceticacetic anhydride anhydride and and phenothiazine phenothiazine and and no noadditional additional fire fire performanceFigure performanceFigure 1. 1.Chemical Chemical of of these these structures structures compounds compounds ofof 1a1a andand were 1b and reportedreported their their counter counter[57]. [57]. ion ion −OTs−OTs. .

The conjugated dienes 2 and 3 (Figure 2) were synthesized by reaction of mesityl oxide with phosphorus acid and hypophosphorus acid, respectively [55,56]. It was described in the patent that these compounds can be used as flame retardants, potentially in the plastic industry or in textile coatings. The same researchers have also studied the synthesis of acrylamide derivative (Compound 4,

FigureFigure 2. 2. Chemical Chemical structuresstructures ofof 22, ,3 3 and and 4 4. .

Dimethylpropyl phosphonate (5a, Figure 3) and dimethylallyl phosphonate (5b, Figure 3) were Dimethylpropyl phosphonate ( 5a,, Figure Figure 33)) andand dimethylallyldimethylallyl phosphonatephosphonate ((5b,, Figure Figure 33)) werewere synthesized by the reaction of dimethyl phosphite in presence of inorganic base with propylbromide synthesized by the reaction of dimethyl phosphite in presence presence of inorganic inorganic base with with propylbromide propylbromide and allylbromide, respectively. They were obtained as liquids in 92% (Compound 5a) and 93% and allylbromide, respectively. They They were were obtained obtained as as liquids liquids in in 92% 92% (Compound (Compound 5a5a)) and and 93% 93% (Compound 5b) yield. Both compounds were characterized using their boiling point: 105 °C at (Compound 5b)) yield.yield. BothBoth compoundscompounds werewere characterized characterized using using their their boiling boiling point: point: 105 105◦C °C at 23at 23 mbar for 5a and 104 °C at 24 mbar for 5b [14]. They were incorporated in flexible polyurethane 23 mbar for 5a and 104◦ °C at 24 mbar for 5b [14]. They were incorporated in flexible polyurethane mbar(PU) for foams5a and at concentrations 104 C at 24 mbar of 1, 2, for 5 5band[ 1410]. wt They % based were on incorporated polyol content in flexibleand their polyurethane flame retardant (PU) (PU)foamsperformance foams at concentrations at concentrations were evaluated. of 1, of 2,The 1, 5 2, andthermal 5 and 10 10 wtstability wt % % based based and onflame on polyol polyol retardancy content content were and and investigated theirtheir flameflame by retardant TGA performanceand PCFC, whichwere evaluated. showed that The the thermal flame retardant stability additives and and flame flame act retardancy predominantly were in investigated the gas phase. by The TGA andflammability PCFC, PCFC, which which of PUshowed showed foams that were that the thefurther flame flame investigatedretardant retardant additives by additives LOI, actUL act predominantly94 HB predominantly and BKZ-VB in the inSwiss gas the standardphase. gas phase. The flammabilityThefire flammability test. It wasof PU offound PUfoams foams at werea 10 were wtfurther % further (based investigated investigated on weight by byof LOI, polyol) LOI, UL UL 94flame 94 HB HB retardantand and BKZ-VB BKZ-VB loading, Swiss Swiss all standard standardflame firefireretarded test. It It PUwas was passed found found the at at BKZ-VBa a 10 wt %compared (based on to theweight virgin of PU polyol) foam. flame flameMoreover, retardant the flame loading, retarded all flame flamePU retardedwith compound PU passed 5a the showed BKZ-VB a HF2 compared rating in to the the UL virgin 94 HB PU test foam. while Moreover, Moreover, the treated the PU flame flamewith compoundretarded PU with5b compoundshowed HF1 5a rating. showed LOI a HF2 values rating of the in PU the foam UL 94s increased HB test while with increasing the treated the PU flame with retardant compound 5b concentrations.showed HF1 rating.The LOI LOI LOI values values values of treatedof of the the PU PU foam foamsfoamss increased with the flame with retardantincreasing additives the flame flame 5a retardant and 5b concentrations.were 22.8% and TheThe 23.9%, LOILOI values valuesrespectively. of of treated treated PCFC PU PU measur foams foams withements with the ofthe flame the flame flame retardant retardant retarded additives additivesPU foams5a and 5adid 5band notwere 5b were22.8%show 22.8% and a char 23.9%, and residue 23.9%, respectively. which respectively. confirms PCFC theirPCFC measurements gas measur phaseements mode of the of of flameaction. the flame retarded retarded PU foams PU foams did not did show not showa char a residue char residue which which confirms confirms their gas their phase gas phase mode mode of action. of action.

Figure 3. Chemical structures aliphatic phosphonates 5a and 5b.

Compounds 5aFigure and 5b3. ChemicalChemical(Figure 3)structures structures were also aliphatic compared phosphonates to a commonly 5a and 5b used. flame retardant tris(1-chloro-2-propyl) phosphate (TCPP) [58]. It was concluded from both studies that compounds Compounds 5a and 5b (Figure 3) were also compared to a commonly used flame retardant withCompounds an allyl group5a areand more5b efficient(Figure flame3) were retardan also comparedts for PU foams to a than commonly compounds used with flame alkyl retardantgroups. tris(1-chloro-2-propyl)Allyltriphenylphosphonium phosphate phosphate (TCPP) (TCPP)bromide [58]. [58 (Compound]. It It was was concluded 6, Figure from from 4) both was studiessynthesized that compoundscompoundsfrom the withreaction an allyl of group triphenylphosphine are more more efficient efficient and flame flame allyl retardanbromide retardants tsin for fordry PU PU toluene foams foams [59].than than Subsequently,comp compoundsounds with with bentonite alkyl alkyl groups. groups.clay wasAllyltriphenylphosphonium modified with this phosphoniu bromidebromidem salt (Compound[60]. (Compound The surface6, Figure6 modified, Figure4) was bentonite4) synthesized was claysynthesized was from then the usedfrom reaction asthe reactionofa triphenylphosphine flame of retardanttriphenylphosphine additive and allyl(1, and2 bromideand allyl 3 wtbromide in%) dry for in toluenepossible dry toluene [59improvement]. Subsequently,[59]. Subsequently, in flame bentonite resistance bentonite clay and clay was wasmodifiedmechanical modified with propertieswith this this phosphonium phosphoniuof polyester salt acrylatem salt [60 ].[60]. base The The UV surface -curablesurface modified modifiedcoatings. bentonite Tensile bentonite properties clay clay was was of then thenfilms used usedwere as as a aflame flameevaluated retardant retardant from additive theadditive stress–strain (1, 2 (1, and 2 3and wtcurve. %)3 forwtThe possible%) Young’ for possible improvements modulus, improvement tensile in flame strength resistancein flame and andelongationresistance mechanical atand mechanicalpropertiesbreak of the of properties polyester films were of acrylate determined.polyester base acrylate UV-curableIt was basefound UV coatings.that-curable the addition Tensile coatings. of properties the Tensile modified properties of films clay wereincreased of films evaluated the were evaluatedfromYoung’s the stress–strain frommodulus the butstress–strain curve. decreased The Young’s curve.the tensile modulus,The strength.Young’ tensiles The modulus, strengthelongation tensile and at elongation break strength at first atand break decreased elongation of the but films at werebreakthen determined.of increased the films with were It was the determined. found increasing that theItclay was addition content. found of thatThe the thecoating modified addition became clay of theincreasedmore modified brittle the withclay Young’s increasedaddition modulus of the Young’smodified modulus clay. The but thermal decreased properties the tensile and flammabi strength.lity The properties elongation were at then break investigated at first decreased using TGA but thenand increased LOI. The with Td10% theof the increasing neat coating clay wascontent. 373 °C The while coating addition became of clay more first brittle increased with thisaddition value of modified(the formulation clay. The thermalwith 1% propertiesmodified clay and showed flammabi a Tlityd10% properties of 387 °C), werebut then then decreased investigated to 361 using °C forTGA the formulation with modified clay content of 3 wt %. The char yield increased significantly from no and LOI. The Td10% of the neat coating was 373 °C while addition of clay first increased this value char for the neat coating to 12% for the 3 wt % modified clay/coatings. The same trend was observed (the formulation with 1% modified clay showed a Td10% of 387 °C), but then decreased to 361 °C for thein formulation the LOI measurements with modified 17% clay and content22% for ofunmodified 3 wt %. The and char 3 wt yield % clay/coatings. increased significantly It was concluded from no from these results that the addition of flame retardant additive 6 to the clay had a significant impact char for the neat coating to 12% for the 3 wt % modified clay/coatings. The same trend was observed in the LOI measurements 17% and 22% for unmodified and 3 wt % clay/coatings. It was concluded from these results that the addition of flame retardant additive 6 to the clay had a significant impact

Materials 2017, 10, 784 7 of 32 but decreased the tensile strength. The elongation at break at first decreased but then increased with the increasing clay content. The coating became more brittle with addition of modified clay. The thermal properties and flammability properties were then investigated using TGA and LOI. The Td10% of the neat coating was 373 ◦C while addition of clay first increased this value (the formulation with ◦ ◦ 1% modified clay showed a Td10% of 387 C), but then decreased to 361 C for the formulation with modified clay content of 3 wt %. The char yield increased significantly from no char for the neat coating to 12% for the 3 wt % modified clay/coatings. The same trend was observed in the LOI measurements 17%Materials and 2017 22%, 10 for, 784unmodified and 3 wt % clay/coatings. It was concluded from these results that7 of the 31 addition of flame retardant additive 6 to the clay had a significant impact on the mechanical as well asonMaterials thethe flamemechanical 2017 retardancy, 10, 784 as well of as the the polyester flame retardancy acrylate base of the UV-curable polyester coatingsacrylate base acting UV-curable dominantly coatings7 of in 31 the condensedacting dominantly phase. in All the results condensed were attributedphase. All toresults the crosslinking were attributed density to the of thecrosslinking allyl modified density clay of containingtheon allyl the modifiedmechanical UV-cured clay as coating. wellcont ainingas the flame UV-cured retardancy coating. of the polyester acrylate base UV-curable coatings acting dominantly in the condensed phase. All results were attributed to the crosslinking density of the allyl modified clay containing UV-cured coating.

Figure 4. Chemical structures of phosphonium derivatives: 6, 6a and 6b. Figure 4. Chemical structures of phosphonium derivatives: 6, 6a and 6b. Ionic liquids are known to be nonflammablenonflammable and have recently generatedgenerated a lot of attention as environment friendly reagents. Compounds 6a and 6b (Figure 4) are ionic liquids and were environmentIonic liquids friendly are reagents. known Compoundsto be nonflammable6a and 6band(Figure have 4recently) are ionic generated liquids anda lot were of attention synthesized as fromsynthesizedenvironment trimethylphosphine, from friendly trimethylphosphine, reagents. methoxyacetic Compounds me acidthoxyacetic and6a and allyl acid6b chloride (Figure and allyl ( 6a4)) chloride orare chloromethyl ionic (6aliquids) or chloromethyl methyland were ether (methyl6bsynthesized)[61 ].ether Associated (from6b) [61].trimethylphosphine, with Associated methoxyacetate with me methoxyacetatethoxyacetic as counter anion,acid asand thesecounter allyl compounds chloride anion, (these6a are) or described compoundschloromethyl as theare optimaldescribedmethyl ionic etheras the liquids ( 6boptimal) [61]. for the ionicAssociated dissolution liquids with for of cellulose;themethoxyacetate dissolution however, ofas cellulose;counter no real applicationanion, however, these datano compounds real was application presented are indatadescribed the was paper. presented as the optimal in the paper.ionic liquids for the dissolution of cellulose; however, no real application dataRecently, was presented synthesissynthesis in andtheand paper. application application of of different different phosphinic phosphinic acid acid derivatives derivatives has has been been reported reported [7]. [7]. DibenzylphosphinicRecently, synthesis acid and ( 7aapplication, Figure 5) of was different synthesized phosphinic from acid reaction derivatives of H2PO has2NH been4 with reported benzyl Dibenzylphosphinic acid (7a, Figure5) was synthesized from reaction of H 2PO2NH4 with benzyl bromide;[7]. Dibenzylphosphinic diallylphosphinic acid acid (7a (7b, Figure, Figure 5) was5) was synthesized synthesized from from reaction reaction of H of2PO H22NHPO24NH with4 with benzyl allyl bromide; diallylphosphinic acid (7b, Figure5) was synthesized from reaction of H 2PO2NH4 with allylbromide.bromide; bromide. These diallylphosphinic These compounds compounds wereacid ( were7bdeveloped, Figure developed 5) for was app forsynthesizedlication application in from epoxy in reaction epoxy resinsresins of for H 2flamePO for2NH flame retardant4 with retardant allyl and bromide. These compounds were developed for application in epoxy resins for flame retardant and andmechanical mechanical properties properties improvement. improvement. They Theywere werethen thentreated treated with withaluminium aluminium hydroxide hydroxide to give to mechanical properties improvement. They were then treated with aluminium hydroxide to give givealuminium–organophosphorus aluminium–organophosphorus hybrids hybrids (AOPH) (AOPH) as asfollows: follows: compound compound 7a7a gavegave AOPH-NR AOPH-NR and aluminium–organophosphorus hybrids (AOPH) as follows: compound 7a gave AOPH-NR and compound 7b gave AOPH-C2. AOPH AOPH derivatives derivatives were were then then applied applied in in epoxy resins to produce compound 7b gave AOPH-C2. AOPH derivatives were then applied in epoxy resins to produce composites. LOI LOI and and TGA were performed for these composites and the analytical results showed thatcomposites. the AOPH-NR LOI and exhibits TGA werethe best performed flame retardantfor these compositesproperties: and LOI the for analytical AOPH-NR results was showed 28% and thatthat the the AOPH-NR AOPH-NR exhibits exhibits the the bestbest flameflame retardantretardant properties: LOI LOI for for AOPH-NR AOPH-NR was was 28% 28% and and 23.6% for AOPH-C2. Moreover, the TGA analysis show that only AOPH-NR had an impact on the decomposition23.6% for AOPH-C2. of the epoxy Moreover, composites: the TGA the analysis 5% an dshow 50% that decomposition only AOPH-NR temperatures had an impact decreased on the for decompositiondecomposition of of the the epoxy epoxy composites: composites: thethe 5% an andd 50% 50% decomposition decomposition temperatures temperatures decreased decreased for for AOPH-NR, whereas the other AOPH composites showed no real difference in thermal performance AOPH-NR,AOPH-NR, whereas whereas the the other other AOPH AOPH compositescomposites showedshowed no no real real difference difference in in thermal thermal performance performance compared to the neat resin. TGA data indicated a slight increase in the char residue at 500 °C for◦ both comparedcompared to to the the neat neat resin.resin. TGA TGA data data indicated indicated a slight a slight increase increase in the in char the residue char residue at 500 at°C 500for bothC for composites compared to the neat epoxy resin, showing a flame retardant mode of action in the bothcomposites composites compared compared to the to theneat neat epoxy epoxy resin, resin, showing showing a flame a flame retardant retardant mode mode of ofaction action in inthe the condensed phase. condensedcondensed phase. phase.

FigureFigure 5. 5. Chemical Chemical structure ofof 7a7a and and 7b 7b. .

3.2.3.2. Addition Addition Reactions Reactions AdditionAddition reactions reactions are are also also commonly commonly usedused to create P-CP-C bond bond containing containing flame flame retardants. retardants. The The followingfollowing section section summarizes summarizes organophosphorus organophosphorus compounds obtained obtained via via addition addition reactions reactions other other thanthan Michaelis–Arbuzov Michaelis–Arbuzov type type reaction, reaction, whichwhich willwill be describeddescribed in in detail detail in in the the subsequent subsequent section. section. PhosphinicPhosphinic acid acid derivatives derivatives bis( bis(2-cyanoethyl)phosphinic2-cyanoethyl)phosphinic acid acid ( 7c(7c, ,Figure Figure 6) 6) was was synthesized synthesized from from reaction of H2PO2NH4 with allyl cyanide; and bis(3-methoxy-3-oxopropyl)phosphinic acid (7d, Figure 6) reaction of H2PO2NH4 with allyl cyanide; and bis(3-methoxy-3-oxopropyl)phosphinic acid (7d, Figure 6) was synthesized from reaction of H2PO2NH4 with methyl acrylate [7]. These products were was synthesized from reaction of H2PO2NH4 with methyl acrylate [7]. These products were subsequently treated with aluminium hydroxide to give aluminium–organophosphorous hybrids subsequently treated with aluminium hydroxide to give aluminium–organophosphorous hybrids (AOPH). The produced AOPH derivatives were also applied in epoxy resins. (AOPH). The produced AOPH derivatives were also applied in epoxy resins.

Materials 2017, 10, 784 8 of 32

3.2. Addition Reactions Addition reactions are also commonly used to create P-C bond containing ﬂame retardants. The following section summarizes organophosphorus compounds obtained via addition reactions other than Michaelis–Arbuzov type reaction, which will be described in detail in the subsequent section. Phosphinic acid derivatives bis(2-cyanoethyl)phosphinic acid (7c, Figure6) was synthesized from reaction of H2PO2NH4 with allyl cyanide; and bis(3-methoxy-3-oxopropyl)phosphinic acid (7d, Figure6) was synthesized from reaction of H 2PO2NH4 with methyl acrylate [7]. These products were subsequently treated with aluminium hydroxide to give aluminium–organophosphorous hybrids Materials(AOPH). 2017 The, 10 produced, 784 AOPH derivatives were also applied in epoxy resins. 8 of 31 Materials 2017, 10, 784 8 of 31

Figure 6. PhosphinicPhosphinic acid derivatives 7c and 7d.. Figure 6. Phosphinic acid derivatives 7c and 7d. Diethylphosphinic acid (8, Figure 7) was obtained from a gas-liquid free-radical addition of Diethylphosphinic acid acid ( (88, ,Figure Figure 7)7) was was obtained obtained from from a a ga gas-liquids-liquid free-radical free-radical addition addition of sodium hypophosphite monohydrate and ethylene in acetic acid using benzoyl peroxide as initiator ofsodium sodium hypophosphite hypophosphite monohydrate monohydrate and ethylene and ethylene in acetic in aceticacid using acid benzoyl using benzoyl peroxide peroxide as initiator as [62]. The thermal stability of compound 8 was investigated by TGA and the results showed two initiator[62]. The [ 62thermal]. The thermalstability stability of compound of compound 8 was investigated8 was investigated by TGA by and TGA the and results the results showed showed two degradation peaks at 396 °C (small)◦ and 424 °C (large),◦ corresponding to the P-C bond breakage and twodegradation degradation peaks peaks at 396 at °C 396 (small)C (small) and 424 and °C 424 (large),C (large), corresponding corresponding to the to P-C the bond P-C bond breakage breakage and combustion gas release, respectively. This compound was reported as a potential flame retardant; andcombustion combustion gas gasrelease, release, respectively. respectively. This This compou compoundnd was was reported reported as as a apotential potential flame ﬂame retardant; retardant; however, no further fire performance of this compound was reported. however, nono furtherfurther ﬁrefire performanceperformance ofof thisthis compoundcompound waswas reported.reported.

Figure 7. Chemical structure of 8. Figure 7. Chemical structure of 8.. The synthesis of a cyclic tert-butyl phosphonate (Compound 9, Figure 8) is patented. Compound 9 The synthesis of a a cyclic terttert-butyl-butyl phosphonate phosphonate (Compound (Compound 99, ,Figure Figure 8)8) is is patented. patented. Compound Compound 9 was obtained by reaction of tert-butyl phosphonic dichloride and neopenthyl glycol [63]. This 9waswas obtained obtained by by reaction reaction of of terttert-butyl-butyl phosphonic phosphonic dichloride dichloride an andd neopenthyl glycolglycol [[63].63]. This compound was claimed as a flame retardant due to the presence of phosphorus atom and can be used compound was was claimed claimed as as a aflame ﬂame retardant retardant due due to th toe the presence presence of phosphorus of phosphorus atom atom and can and be can used be as an additive for polymer resin like polyphenylene ether. The Izod impact strength as well as the usedas an asadditive an additive for polymer for polymer resin resinlike polyphenylen like polyphenylenee ether. ether. The TheIzod Izod impact impact strength strength as well as well as the as melt viscosity index at 200 °C were measured for a formulation with a composition of 85 wt % themelt melt viscosity viscosity index index at 200 at 200 °C ◦wereC were measured measured for for a aformulation formulation with with a acomposition composition of of 85 85 wt wt % aromatic vinyl resin, 15 wt % polyphenylene ether resin and 20 wt % compound 9 of the final resin. aromatic vinyl resin, 15 wt % polyphenylene ether ether resin resin and and 20 20 wt wt % % compound compound 99 ofof the the final ﬁnal resin. resin. The results showed 8.8 kgf.cm/cm Izod impact strength and a melt viscosity index of 7.9. In addition, The results showedshowed 8.88.8 kgf.cm/cmkgf.cm/cm Izod Izod impact impact strength strength and a melt melt viscosity viscosity index of 7.9. In In addition, addition, this formulation also exhibited UL 94 V-0 rating. this formulation alsoalso exhibitedexhibited ULUL 9494 V-0V-0 rating.rating.

Figure 8. Cyclic tert-butyl phosphonate 9. Figure 8. Cyclic tert-butyl phosphonate 9.. The triazine-based DOPO compound (10a, Figure 9) was synthesized using Michael addition The triazine-based DOPO compound (10a, Figure 9) was synthesized using Michael addition reactionThe of triazine-based DOPO with 2-vinyl-4,6-diamino-1,3,5-triazine DOPO compound (10a, Figure9) using was synthesized DBU as a catalyst using Michael[15]. The addition product reaction of DOPO with 2-vinyl-4,6-diamino-1,3,5-triazine using DBU as a catalyst [15]. The product wasreaction collected of DOPO as a white with 2-vinyl-4,6-diamino-1,3,5-triazinesolid power in 93% yield with a m.p. using of DBU~257 °C. as aIt catalyst was dried [15 ].overnight The product and was collected as a white solid power in 93% yield with a m.p. of ~257 °C. ◦It was dried overnight and thenwas collectedused as a as PA6 a white additive solid in power co-extrusion. in 93% yieldIts impact with on a m.p. flammability, of ~257 C.thermal, It was driedmechanical overnight and then used as a PA6 additive in co-extrusion. Its impact on flammability, thermal, mechanical and rheologicalrheological propertiesproperties forfor PA6PA6 werewere studiedstudied andand comparedcompared toto aa commerciallycommercially availableavailable flameflame rheological properties® for PA6 were studied and compared to a commercially available flame retardant Exolit ® OP 1230. PA6 formulations containing 17.1 wt % of compound 10a and 16.8 wt % of retardant Exolit® OP 1230. PA6 formulations containing 17.1 wt % of compound 10a and 16.8 wt % of retardant® Exolit OP 1230. PA6 formulations containing 17.1 wt % of compound 10a and 16.8 wt % of Exolit ® OP 1230 were prepared. Detailed thermal, mechanical and morphological characterizations Exolit® OP 1230 were prepared. Detailed thermal, mechanical and morphological characterizations using TGA, DSC, Scanning Electron Microscope (SEM), tensile tests, hardness and Charpy impact using TGA, DSC, Scanning Electron Microscope (SEM), tensile tests, hardness and Charpy impact strength measurements were performed for these formulations. Flammability measurements and strength measurements were performed for these formulations. Flammability measurements and mechanistic studies were performed using PCFC, UL 94, vertical burning test and direct insertion mechanistic studies were performed using PCFC, UL 94, vertical burning test and direct insertion probe mass spectroscopy (DIP-MS). It was found that in N2 environment, neat PA6 showed a Td5% of 2 d5% probe mass spectroscopy (DIP-MS). It was found that in N2 environment, neat PA6 showed a Td5% of 388 °C and exhibited a one-step degradation process. In case of PA6 formulations, the Td5% of the d5% 388 °C and exhibited a one-step degradation process. In case of PA6 formulations, the Td5% of the formulation decreased to 386 °C and 372 °C for Exolit® OP 1230 and compound 10a respectively. In ® formulation decreased to 386 °C and 372 °C for Exolit® OP 1230 and compound 10a respectively. In air, the decomposition took place in three steps for all formulations and the Td5% followed the same d5% air, the decomposition took place in three steps for all formulations and the Td5% followed the same trend as in N2 environment. Thus, the addition of both flame retardants lowered the thermal stability 2 trend as in N2 environment. Thus, the addition of both flame retardants lowered the thermal stability of the PA6 polymer. Mechanical tests showed that the incorporation of compound 10a in the PA6 of the PA6 polymer. Mechanical tests showed that the incorporation of compound 10a in the PA6

Materials 2017, 10, 784 9 of 32 and then used as a PA6 additive in co-extrusion. Its impact on flammability, thermal, mechanical and rheological properties for PA6 were studied and compared to a commercially available flame retardant Exolit® OP 1230. PA6 formulations containing 17.1 wt % of compound 10a and 16.8 wt % of Exolit® OP 1230 were prepared. Detailed thermal, mechanical and morphological characterizations using TGA, DSC, Scanning Electron Microscope (SEM), tensile tests, hardness and Charpy impact strength measurements were performed for these formulations. Flammability measurements and mechanistic studies were performed using PCFC, UL 94, vertical burning test and direct insertion probe mass spectroscopy (DIP-MS). It was found that in N2 environment, neat PA6 showed a Td5% ◦ of 388 C and exhibited a one-step degradation process. In case of PA6 formulations, the Td5% of the formulation decreased to 386 ◦C and 372 ◦C for Exolit® OP 1230 and compound 10a respectively. In air, the decomposition took place in three steps for all formulations and the Td5% followed the Materialssame trend 2017, as10, in784 N 2 environment. Thus, the addition of both flame retardants lowered the thermal9 of 31 stability of the PA6 polymer. Mechanical tests showed that the incorporation of compound 10a in the formulationPA6 formulation resulted resulted in a inmore a more rigid rigid and and brittle brittle material material (higher (higher Young’s Young’s modulus modulus but but an an impact strength of 4.53 kJ/mkJ/m22).). Neat Neat PA6 PA6 did did not not break break in in the the Charpy Charpy test. test. PCFC PCFC measurements measurements registered a PHRR of 738738 W/gW/g for PA6/PA6/10a10a formulation, followed by PA6PA6 (590(590 W/g)W/g) andand PA6/ExolitPA6/Exolit® OP 1230 formulation (538 W/g). This This high high PHRR PHRR for for PA6/ PA6/10a10a formulationformulation may may be be attributed attributed to to a catalytic decomposition of PA6 formed from the decompositio decompositionn of the DOPO-derivative at at high high temperature. temperature. However, based onon the THR, compound 10a10a showedshowed aa similarsimilar behaviorbehavior comparedcompared toto ExolitExolit®® OP 1230 (~27 kJ/g for for both) both) and and decreased decreased the the THR THR of of the the PA6 PA6 (28.5 kJ/g). The The best best UL UL 94 94 results results were were obtained obtained for PA6 containingcontaining compoundcompound 10a10a withwith aa V-0V-0 ratingrating andand afterflame afterflame times times t 1t1/t/t2 (time(time the flame flame stays after the first/the first/the second second flame) flame) of of 0 0s/0 s/0 s respectively s respectively (2 (2s/3 s/3 s for s for Exolit Exolit® OP® OP 1230 1230 and and 2 s/0 2 s/0s for s the for ◦ neatthe neat PA6). PA6). The The char char yield yield under under N2 N at2 at700 700 °C Cwas was found found significantly significantly higher higher for the PA6/PA6/10a formulation (2.4(2.4 wtwt %) compared toto thethe PA6/ExolitPA6/Exolit® OPOP 1230 1230 formulation formulation (0.9 (0.9 wt wt %) %) and and the the neat neat PA6 PA6 (0.28 wt %). Based on all analytical data, it was concluded that compound 10a exhibit both gas and condensed phase active flame flame retardancy for PA6, and can be considered as a suitable alternative to halogen and halogen free flameflame retardants commerciallycommercially availableavailable forfor PA6PA6 application.application.

Figure 9. DOPO derivatives of 10a and 10b structures.

The same group also synthesized DOPO-derivative 10b (Figure 9) and compared its thermal and The same group also synthesized DOPO-derivative 10b (Figure9) and compared its thermal and fire performances with compound 10a [64]. Like in case of 10a, 10b was obtained using the synthetic fire performances with compound 10a [64]. Like in case of 10a, 10b was obtained using the synthetic procedure i.e., Michael addition of DOPO with acrylamide using 1,8-diazabicyclo(5.4.0)undec-7-ene procedure i.e., Michael addition of DOPO with acrylamide using 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) as a catalyst. It was obtained in a 75% yield and with m.p. ~ 170 °C. Both flame retardants were (DBU) as a catalyst. It was obtained in a 75% yield and with m.p. ~170 ◦C. Both flame retardants incorporated in rigid polyurethane foams (RPUF) with 1 wt % and 2 wt % phosphorus content and were incorporated in rigid polyurethane foams (RPUF) with 1 wt % and 2 wt % phosphorus content analyzed using UL 94 HB test, LOI, cone calorimetry, PCFC, TGA, DSC, SEM and Pyrolysis-Gas and analyzed using UL 94 HB test, LOI, cone calorimetry, PCFC, TGA, DSC, SEM and Pyrolysis-Gas Chromatography Mass Spectroscopy (Py-GC-MS). Results showed that both flame retardants had Chromatography Mass Spectroscopy (Py-GC-MS). Results showed that both flame retardants had HF-1 UL 94 HB rating while neat RPUF had a HBF rate. LOI values increased from 19.3% for the neat HF-1 UL 94 HB rating while neat RPUF had a HBF rate. LOI values increased from 19.3% for the neat foam to 20.5% for PU foams containing compound 10b and 20.9% for compound 10a. Such increase foam to 20.5% for PU foams containing compound 10b and 20.9% for compound 10a. Such increase in LOI are considered rather low compared to self-extinguishing systems which usually exhibit LOI in LOI are considered rather low compared to self-extinguishing systems which usually exhibit > 25%. The cone calorimetry tests showed a significant decrease of the THR with the addition of flame LOI > 25%. The cone calorimetry tests showed a significant decrease of the THR with the addition of retardant 10a (23.1 MJ/m2) and compound 10b (19.3 MJ/m2) compared to the neat PU foam flame retardant 10a (23.1 MJ/m2) and compound 10b (19.3 MJ/m2) compared to the neat PU foam (24.9 MJ/m2). TGA showed a slight decrease of the Td10% for RPUF containing flame retardants 10a and 10b (300 °C for neat RPUF, 298 °C and 296 °C for RPUF with 10a and 10b, respectively). Evolved gas analysis, cone calorimetry and TGA experiments proved that DOPO derivatives-containing RPUF exhibit lower smoke and toxicant production and increased char residue, indicating the condensed phase mode of action.

Materials 2017, 10, 784 10 of 32

2 (24.9 MJ/m ). TGA showed a slight decrease of the Td10% for RPUF containing ﬂame retardants 10a and 10b (300 ◦C for neat RPUF, 298 ◦C and 296 ◦C for RPUF with 10a and 10b, respectively). Evolved gas analysis, cone calorimetry and TGA experiments proved that DOPO derivatives-containing RPUF exhibit lower smoke and toxicant production and increased char residue, indicating the condensed phase mode of action.

3.3. Michaelis–Arbuzov Type Reactions and Rearrangement Michaelis–Arbuzov reactions are one of the most common reactions used for the creation of phosphonate compounds. For example, Michaelis–Arbuzov reaction of 2-methoxy-5,5-dimethyl- 1,3,2-dioxaphosphinane (11, Scheme1) with allyl bromide produced 2-oxo-2-allyl-5,5-dimethyl-1,3,2- dioxaphosphinane (11a, Scheme1). Epoxidation reaction of the later compound using m-CBPA, afforded the epoxy containing cyclic phosphonate 11b [65]. Compound 11b was reported as a white solid with 80% yield and a m.p. ~57–59 ◦C. It was then mixed with polyurethane (PU) at a 10 molar % Materialsin two different2017, 10, 784 methods. The Blend method involved incorporation of the flame retardant as additive10 of 31 afterMaterials the 2017 polymerization, 10, 784 of the PU whereas the Prep method involved copolymerizing compound10 of11b 31 withcompound PU. Pyrolysis 11b with combustion PU. Pyrolysis flow combustion calorimetry flow (PCFC) calorime test wastry (PCFC) used to test evaluate was used the THRto evaluate of the the PU compound 11b with PU. Pyrolysis combustion flow calorimetry (PCFC) test was used to evaluate the THRsamples. of the It was PU foundsamples. that It the was neat found and thethatPrep thePU neat had and nearly the thePrep same PU THRhad nearly (~22 kJ/g), the same which THR was THR of the PU samples. It was found that the neat and the Prep PU had nearly the same THR (~22lower kJ/g), than which that for was the lowerBlend thanmethod that (~25 for kJ/g).the Blend The method PCFCresults (~25 kJ/g). confirmed The PCFC that PUresults manufactured confirmed (~22 kJ/g), which was lower than that for the Blend method (~25 kJ/g). The PCFC results confirmed thatby both PU manufactured methods had noby effectboth methods on THR ofhad the no PU. effe Act possibleon THR explanationof the PU. A forpossible this observation explanation was for that PU manufactured by both methods had no effect on THR of the PU. A possible explanation for thisattributed observation to the was loss attributed of compound to the11b lossduring of compound the final washing11b during step the in final PU Prep washingmethod step as in well PU asPrep to this observation was attributed to the loss of compound 11b during the final washing step in PU Prep methodits weak as incorporation well as to its in weak PU inincorporation the Blend method. in PU in the Blend method. method as well as to its weak incorporation in PU in the Blend method.

Scheme 1. Chemical synthesis of compound 11b. Scheme 1. Chemical synthesis of compound 11b. Compound 14 (Scheme 2) was synthesized from reaction of compound 12 (Scheme 2) with Compound 1414(Scheme (Scheme2) 2) was was synthesized synthesized from from reaction reaction of compound of compound12 (Scheme 12 (Scheme2) with 2) DOPO with DOPO derivative 13 (Scheme 2), via Michaelis–Arbuzov rearrangement [66]. The synthesized derivativeDOPO derivative13 (Scheme 13 2(Scheme), via Michaelis–Arbuzov 2), via Michaelis–Arbuzov rearrangement rearrangement [ 66]. The synthesized [66]. The phosphoroussynthesized phosphorous compounds were then used as flame retardants additives in epoxy resins. compoundsphosphorous were compounds then used were as ﬂame then used retardants as flame additives retardants in epoxy additives resins. in epoxy resins.

Scheme 2. Synthetic route of compound 14. Scheme 2. Synthetic route of compound 14. Diethyl allylphosphonate (15, Figure 10) has been synthesized from triethyl phosphite and allyl Diethyl allylphosphonate (15, Figure 10) has been synthesized from triethyl phosphite and allyl bromideDiethyl and allylphosphonate used in free radical (15, Figureprecipitation 10) has polymerization been synthesized in fromaqueous triethyl phase phosphite to produce and bromide and used in free radical precipitation polymerization in aqueous phase to produce poly(urethane-co-ester)allyl bromide and used (PMMD) in free radical copolymers precipitation [67]. polymerization in aqueous phase to produce poly(urethane-co-ester) (PMMD) copolymers [67]. poly(urethane-co-ester) (PMMD) copolymers [67].

Figure 10. Flame retardant compounds 15, 16 and 17. Figure 10. Flame retardant compounds 15, 16 and 17. The PMMD copolymers were synthesized from poly(ethylene glycol), methacrylate, methyl The PMMD copolymers were synthesized from poly(ethylene glycol), methacrylate, methyl methacrylate and monomer 15 in different molar ratios 1:6:2 (PMMD-162) and 1:6:3 (PMMD-163). methacrylate and monomer 15 in different molar ratios 1:6:2 (PMMD-162) and 1:6:3 (PMMD-163). BMMD copolymers were blended with ABS to improve its flame retardancy. Improvements in flame BMMD copolymers were blended with ABS to improve its flame retardancy. Improvements in flame retardancy have been observed using LOI and UL 94 tests. LOI values were 20.8% for 20 wt % retardancy have been observed using LOI and UL 94 tests. LOI values were 20.8% for 20 wt % PMMD-162 and 20.3% for 20 wt % PMMD-163, while neat ABS showed an LOI value of 17.8%. UL 94 PMMD-162 and 20.3% for 20 wt % PMMD-163, while neat ABS showed an LOI value of 17.8%. UL 94 tests also showed better fire results with increased amount of FR. tests also showed better fire results with increased amount of FR. Diethyl bromoethylphosphonate (16, Figure 10) has been synthesized from 1,2-dibromoethane Diethyl bromoethylphosphonate (16, Figure 10) has been synthesized from 1,2-dibromoethane and triethyl phosphite to improve flame resistance of several polymers [68]. Compound 16 was used and triethyl phosphite to improve flame resistance of several polymers [68]. Compound 16 was used to synthesize other monomers for ROMP homopolymerization. Dimethyl benzylphosphonate (17) to synthesize other monomers for ROMP homopolymerization. Dimethyl benzylphosphonate (17)

Materials 2017, 10, 784 10 of 31

compound 11b with PU. Pyrolysis combustion flow calorimetry (PCFC) test was used to evaluate the THR of the PU samples. It was found that the neat and the Prep PU had nearly the same THR (~22 kJ/g), which was lower than that for the Blend method (~25 kJ/g). The PCFC results confirmed that PU manufactured by both methods had no effect on THR of the PU. A possible explanation for this observation was attributed to the loss of compound 11b during the final washing step in PU Prep method as well as to its weak incorporation in PU in the Blend method.

Scheme 1. Chemical synthesis of compound 11b.

Compound 14 (Scheme 2) was synthesized from reaction of compound 12 (Scheme 2) with DOPO derivative 13 (Scheme 2), via Michaelis–Arbuzov rearrangement [66]. The synthesized phosphorous compounds were then used as flame retardants additives in epoxy resins.

Scheme 2. Synthetic route of compound 14.

Diethyl allylphosphonate (15, Figure 10) has been synthesized from triethyl phosphite and allyl Materials bromide2017, 10, 784and used in free radical precipitation polymerization in aqueous phase to produce 11 of 32 poly(urethane-co-ester) (PMMD) copolymers [67].

FigureFigure 10. 10.Flame Flame retardant retardant compounds 15,15 16, and16 and 17. 17.

The PMMD copolymers were synthesized from poly(ethylene glycol), methacrylate, methyl Themethacrylate PMMD copolymers and monomer were 15 in synthesizeddifferent molar from ratios poly(ethylene 1:6:2 (PMMD-162) glycol), and 1:6:3 methacrylate, (PMMD-163). methyl methacrylateBMMD andcopolymers monomer were 15blendedin different with ABS molar to impr ratiosove its flame 1:6:2 retardancy. (PMMD-162) Improvements and 1:6:3 in (PMMD-163). flame BMMDretardancy copolymers have were been blended observed with using ABS LOIto and improve UL 94 tests. its flame LOI values retardancy. were 20.8% Improvements for 20 wt % in flame retardancyPMMD-162 have beenand 20.3% observed for 20 wt using % PMMD-163, LOI and while UL ne 94at tests.ABS showed LOI valuesan LOI value were of 20.8%17.8%. UL for 94 20 wt % tests also showed better fire results with increased amount of FR. PMMD-162Diethyl and 20.3% bromoethylphosphonate for 20 wt % PMMD-163, (16, Figure while 10) has neat been ABS synthesized showed anfrom LOI 1,2-dibromoethane value of 17.8%. UL 94 tests alsoand showed triethyl phosphite better fire to results improve with flame increased resistance amountof several ofpolymers FR. [68]. Compound 16 was used Diethylto synthesize bromoethylphosphonate other monomers for ROMP (16, Figure homopolymerization. 10) has been synthesizedDimethyl benzylphosphonate from 1,2-dibromoethane (17) and triethyl phosphite to improve flame resistance of several polymers [68]. Compound 16 was used to synthesize other monomers for ROMP homopolymerization. Dimethyl benzylphosphonate (17) was synthesized from trimethyl phosphite and benzyl bromide and was applied in PU foam to improve the flame retardancy of PU foams. The structural influence of the organophosphorous compounds on its flame retardancy was also studied [14]. It was reported that applying compound 17 in PU foams improves the flame retardancy of PU foams as confirmed by LOI, BKZ-VB and UL 94 HB tests. The concentrations of compound 17 used in these experiments were 1, 2, 5 and 10%. LOI values for the formulations ranged from 21.2% for the lowest to 22.5% for the highest concentrate respectively. Burning velocity decreased from 5.9 mm/s for the least concentration of additive to 4.1 mm/s for the highest concentration. UL 94 HB tests confirmed HBF rating for the PU foam with 1% to 5% concentration of the flame retardant additive 17 and it improved to an HF2 rating for 10% concentration. Different DOPO-based derivatives (Figure 11) have been synthesized using the Michaelis–Arbuzov reaction with the aim of studying their flame retardant mechanism in PU foams [69]. For example, methyl-DOPO (18) was synthesized in 97% yield and incorporated in polyol to obtain the PU foam. Three different concentrations were tested: 5, 7.5 and 10 phpp (parts per hundred parts polyol). Burning test FMVSS 302 has been performed on the produced foams. With increase in the concentration of 18, the total heat release (THR) of the foam as measured in cone calorimeter decreased, but the total smoke release (TSR) increased, indicating a strong gas flame inhibiting effect of compound 18. Methyl-DOPO (3.18) and compounds 19 to 28 (Figure 11) can also be synthesized using Michaelis–Arbuzov rearrangement [70]. For example, a new synthetic route involves using the transesterification of phosphonous acid diesters with the corresponding aliphatic alcohols and subsequent catalyzed Michaelis–Arbuzov rearrangement reaction, which leads to the formation of DOPO derivatives 19 to 28. These structures were developed as halogen-free flame retardant alternatives for application in various polymers. The melting points were measured for compounds 18 (131 ◦C), 21 (106 ◦C), 23 (59–61 ◦C), 27 (108 ◦C) and 28 (105 ◦C). No further analytical studies were performed on these compounds. Such compounds can find potential applications in polymers with processing temperature below 200 ◦C. The synthesis of DOPO derivatives 18 and 28 were also investigated in other studies [71]. Compound 30 (Scheme3) was synthesized via Michaelis–Arbuzov rearrangement reaction with a yield of 82% and applied as flame retardant additive in epoxy resin [72]. TGA and UL 94 test were performed on the resin samples. It was observed in TGA measurements that there was only a slight ◦ ◦ difference in Td5% between the neat epoxy resin (320 C) and resin with the compound 30 (340 C), which indicates a possible gas phase action for this flame retardant. V-0 rating was obtained for epoxy formulation containing compound 30. It was inferred in this work that the lower the oxidation state of the phosphorus compound, the better is its flame retardancy. Materials 2017, 10, 784 11 of 31

was synthesized from trimethyl phosphite and benzyl bromide and was applied in PU foam to improve the flame retardancy of PU foams. The structural influence of the organophosphorous compounds on its flame retardancy was also studied [14]. It was reported that applying compound 17 in PU foams improves the flame retardancy of PU foams as confirmed by LOI, BKZ-VB and UL 94 HB tests. The concentrations of compound 17 used in these experiments were 1, 2, 5 and 10%. LOI values for the formulations ranged from 21.2% for the lowest to 22.5% for the highest concentrate respectively. Burning velocity decreased from 5.9 mm/s for the least concentration of additive to 4.1 mm/s for the highest concentration. UL 94 HB tests confirmed HBF rating for the PU foam with 1% to 5% concentration of the flame retardant additive 17 and it improved to an HF2 rating for 10% concentration. Different DOPO-based derivatives (Figure 11) have been synthesized using the Michaelis–Arbuzov reaction with the aim of studying their flame retardant mechanism in PU foams [69]. For example, methyl-DOPO (18) was synthesized in 97% yield and incorporated in polyol to obtain the PU foam. Three different concentrations were tested: 5, 7.5 and 10 phpp (parts per hundred parts polyol). Burning test FMVSS 302 has been performed on the produced foams. With increase in the concentration of 18, the total heat release (THR) of the foam as measured in cone calorimeter Materials decreased,2017, 10, 784 but the total smoke release (TSR) increased, indicating a strong gas flame inhibiting effect 12 of 32 of compound 18.

Materials 2017, 10, 784 12 of 31

18 (131 °C), 21 (106 °C), 23 (59–61 °C), 27 (108 °C) and 28 (105 °C). No further analytical studies were performed on these compounds. Such compounds can find potential applications in polymers with processing temperature below 200 °C. The synthesis of DOPO derivatives 18 and 28 were also investigated in other studies [71]. Compound 30 (Scheme 3) was synthesized via Michaelis–Arbuzov rearrangement reaction with a yield of 82% and applied as flame retardant additive in epoxy resin [72]. TGA and UL 94 test were performed on the resin samples. It was observed in TGA measurements that there was only a slight difference in Td5% between the neat epoxy resin (320 °C) and resin with the compound 30 (340 °C), which indicates a possible gas phase action for this flame retardant. V-0 rating was obtained for epoxy formulation containing compoundFigure 30. It 11. was Various inferred DOPO in derivatives.this work that the lower the oxidation state of the phosphorus compound,Figure the better 11. isVarious its flame DOPO retardancy. derivatives. Methyl-DOPO (3.18) and compounds 19 to 28 (Figure 11) can also be synthesized using Michaelis–Arbuzov rearrangement [70]. For example, a new synthetic route involves using the transesterification of phosphonous acid diesters with the corresponding aliphatic alcohols and subsequent catalyzed Michaelis–Arbuzov rearrangement reaction, which leads to the formation of DOPO derivatives 19 to 28. These structures were developed as halogen-free flame retardant alternatives for application in various polymers. The melting points were measured for compounds

Scheme 3. Synthetic route for oligomer 30. Scheme 3. Synthetic route for oligomer 30. The spiro-cyclic phosphorus-containing flame retardant 32b was synthesized from reaction of The5-hydroxymethyl-5-methyl-2-phenyl-1,3,2-dioxaphosphorinane-2-oxide spiro-cyclic phosphorus-containing flame retardant 32b withwas compound synthesized 32a (Scheme from 4), reaction of 5-hydroxymethyl-5-methyl-2-phenyl-1,3,2-dioxaphosphorinane-2-oxidein 70% yield [73]. P-C bond in compound 32a was formed via Michaelis–Arbuzov reaction withcompound using the 32a starting material 31 followed by chlorination reaction using thionyl chloride. The final product 32b (Scheme4), in 70% yield [ 73]. P-C bond in compound 32a was formed via Michaelis–Arbuzov was then blended with ABS in 20 wt % and 30 wt % loading and the thermal decomposition and reactionflammability using the startingof the formulations material were31 followed investigated by using chlorination TGA and reaction UL 94 tests. using A 20 thionylwt % loading chloride. of The final product32b in ABS32b didn’twas show then any blended improvement with ABSin flame in retardancy. 20 wt % and However, 30 wt for % a loadinghigher concentration and the thermal decompositionof 32b (30and wt %), flammability the temperature of theof 10% formulations weight loss (T wered10%) was investigated 402 °C and 384 using °C under TGA N and2 and UL air, 94 tests. A 20 wtrespectively. % loading For of the32b neatin ABSABS, a didn’t Td10% of show 326 °C any and improvement306 °C in N2 and inair flamewere observed, retardancy. respectively. However, for For the 30 wt % 32b formulation, a PHRR of 268.4 W/g and a THR of 26.9 kJ/g (the neat ABS having ◦ a higher concentration of 32b (30 wt %), the temperature of 10% weight loss (Td10%) was 402 C and ◦ showed a PHRR of 505 W/g and a THR of 32.7 kJ/g) was observed. It was suggested◦ that ◦this flame 384 C underretardant N acts2 and in a air, condensed respectively. phase when For theincorporat neat ABS,ed in aPC T andd10% inof a gas 326 phaseC and when 306 incorporatedC in N2 and air were observed,in ABS. respectively. For the 30 wt % 32b formulation, a PHRR of 268.4 W/g and a THR of 26.9 kJ/g (the neat ABS having showed a PHRR of 505 W/g and a THR of 32.7 kJ/g) was observed.

Scheme 4. Schematic route for compound 32b synthesis.

Materials 2017, 10, 784 12 of 31

Scheme 3. Synthetic route for oligomer 30.

The spiro-cyclic phosphorus-containing flame retardant 32b was synthesized from reaction of 5-hydroxymethyl-5-methyl-2-phenyl-1,3,2-dioxaphosphorinane-2-oxide with compound 32a (Scheme 4), in 70% yield [73]. P-C bond in compound 32a was formed via Michaelis–Arbuzov reaction using the starting material 31 followed by chlorination reaction using thionyl chloride. The final product 32b was then blended with ABS in 20 wt % and 30 wt % loading and the thermal decomposition and flammability of the formulations were investigated using TGA and UL 94 tests. A 20 wt % loading of 32b in ABS didn’t show any improvement in flame retardancy. However, for a higher concentration of 32b (30 wt %), the temperature of 10% weight loss (Td10%) was 402 °C and 384 °C under N2 and air, Materialsrespectively.2017, 10, 784 For the neat ABS, a Td10% of 326 °C and 306 °C in N2 and air were observed, respectively.13 of 32 For the 30 wt % 32b formulation, a PHRR of 268.4 W/g and a THR of 26.9 kJ/g (the neat ABS having showed a PHRR of 505 W/g and a THR of 32.7 kJ/g) was observed. It was suggested that this flame It wasretardant suggested acts that in a thiscondensed ﬂame phase retardant when acts incorporat in a condenseded in PC and phase in a whengas phase incorporated when incorporated in PC and in a gasin phase ABS. when incorporated in ABS.

SchemeScheme 4. 4.Schematic Schematic routeroute for compound 32b32b synthesis.synthesis.

For a formulation of 30 wt % of compound 32b in ABS resin, one could achieve a V-0 rating. V-0 classification is the highest classification in terms of fire safety in UL 94 test. However, for the PC blend, aMaterials UL 94 2017 V-0, 10 rating, 784 was obtained with a much lower concentration (4 wt %) of compound13 of 31 32b. The norbornene-basedFor a formulation of phosphorus 30 wt % of compound containing 32b compoundin ABS resin, 33aone –could33e (Figure achieve a12 V-0) were rating. synthesized V-0 using ringclassification opening is the metathesis highest classification (ROMP) in and terms investigated of fire safety in as UL flame 94 test. retardants However, for [68 the]. PC However, blend, the main blocka UL monomer94 V-0 rating33 washas obtained been synthesizedwith a much lower in multistep concentration reactions (4 wt %) includingof compound Michaelis–Arbuzov 32b. rearrangementThe norbornene-based in 68% yield. The phosphorus polymers containing33a–33e compoundwere then 33a applied–33e (Figure by coating12) were synthesized to foldedprinted paper andusing subsequent ring opening burning metathesis tests (ROMP) were and performed. investigated Theas flame TGA retardants results [68]. of theseHowever, polymers the main showed block monomer 33 has been synthesized in multistep reactions including Michaelis–Arbuzov that mostrearrangement of them (33a in ,68%33b yield., 33d ,The33e polymers) work in 33a condensed–33e were then phase: applied the by residual coating weightsto folded wereprinted 31 wt %, 34 wt %,paper 27 wt and %, subsequent 38 wt %, burning respectively, tests were whereas, performed. for The33c ,TGA a residual results of weight these polymers of 21 wt showed % was that observed. These polymersmost of them were (33a also, 33b, subjected33d, 33e) work to photolithographyin condensed phase: the using residual “thiol-ene weights were click” 31 chemistry,wt %, 34 wt which allowed%, the 27 phosphonatedwt %, 38 wt %, respectively, polymer towhereas, get a promisingfor 33c, a residual negative weight photoresist. of 21 wt % was TGA observed. was performed These on polymers were also subjected to photolithography using “thiol-ene click” chemistry, which allowed the polymer samples and homopolymer of 33a exhibited a two-step thermal decomposition. A Td10% of the phosphonated polymer to get a promising negative photoresist. TGA was performed on the 331 ◦C was observed, which was 50 ◦C lower than for other polymers studied. However, the obtained polymer samples and homopolymer of 33a exhibited a two-step thermal decomposition. A Td10% of coating331 from °C was compound observed,33a whichshowed was 50 °C high lower flame than resistance.for other polymers This studied. was attributed However, tothe higher obtained residual weight (37coating wt %)from for compound the treated 33a papershowed as high observed flame resistance. in the TGA This experiments.was attributed to LOI higher value residual of the blank paper wasweight 21% (37 with wt %) a flamefor the speed treated of paper 1 cm/s. as observed With 40 in wt the % TGA of 33a experiments.homopolymer LOI value coating of the onblank paper, the LOI valuepaper was was increased 21% with toa flame 25% withspeed a of burning 1 cm/s. With speed 40 <0.1wt % cm/s.of 33a homopolymer At a oxygen concentrationcoating on paper, 35%, the homopolymerthe LOI value33a coated was increased paper to showed 25% with a burninga burning speedspeed <0.1 <0.5 cm/s. cm/s, At a which oxygen was concentration half of that 35%, obtained the homopolymer 33a coated paper showed a burning speed <0.5 cm/s, which was half of that for the blank paper. The norbornene derived phosphonated polymer was tested for the first time as obtained for the blank paper. The norbornene derived phosphonated polymer was tested for the first flame retardanttime as flame and retardant the study and confirmed the study confirmed its self-extinguishing its self-extinguishing ability ability when when applied applied on on paper.paper.

FigureFigure 12. 12.Monomer Monomer33 33and and its corresponding corresponding polymers. polymers. Methylphenylphosphinic acid (34a, Figure 13) was obtained from hydrolysis of methyl Methylphenylphosphinicmethylphenylphosphinate (MMPP) acid ( 34ain hydrochloric, Figure 13 acid) wasin 90% obtained yield. MMPP from was hydrolysis synthesized in of a methyl methylphenylphosphinatetwo-step synthetic strategy. (MMPP) It involved in hydrochloric reactions of dichlorophenylphosphine acid in 90% yield. MMPP with methanol was synthesized to form in a dimethyl phenylphosphonite and subsequent rearrangement reaction with methyl iodide to yield MMPP [74], in 65% overall yield. It was observed that the thermal decomposition of compound 34a occurred in one step compared to other structurally analogues flame retardants studied (34b, 34c and 34d, Figure 13). In addition, the compound 34a showed a Td10% < 300 °C in the TGA experiment. The flame retardant additives were also copolymerized with acrylonitrile-butadiene-styrene (ABS) and ethylene-vinyl acetate (EVA) separately with a 20 wt % loading (~4 wt % phosphorus). Based on the residue obtained in TGA, it was reported that all these flame retardants act in the gas phase in EVA formulation, compounds 34a and 34b act in gas phase in ABS formulations and compounds 34c and 34d act in both gas and condensed phases in ABS formulations. A UL 94 V-0 rating for formulation

Materials 2017, 10, 784 14 of 32 two-step synthetic strategy. It involved reactions of dichlorophenylphosphine with methanol to form dimethyl phenylphosphonite and subsequent rearrangement reaction with methyl iodide to yield MMPP [74], in 65% overall yield. It was observed that the thermal decomposition of compound 34a occurred in one step compared to other structurally analogues flame retardants studied (34b, 34c and ◦ 34d, Figure 13). In addition, the compound 34a showed a Td10% < 300 C in the TGA experiment. The flame retardant additives were also copolymerized with acrylonitrile-butadiene-styrene (ABS) and ethylene-vinyl acetate (EVA) separately with a 20 wt % loading (~4 wt % phosphorus). Based on the residue obtained in TGA, it was reported that all these flame retardants act in the gas phase in EVA formulation, compounds 34a and 34b act in gas phase in ABS formulations and compounds 34c and 34d act in both gas and condensed phases in ABS formulations. A UL 94 V-0 rating for formulation containingMaterials 20 wt2017 %, 10, of 784 compound 3.34a in ABS was observed. Compounds 34a, 34b, 34c and14 of 34d31 were comparedcontaining for their 20 wt flame % of retardantcompound performance.3.34a in ABS wasIt observed. was observed Compounds that 34a compound, 34b, 34c and34b 34dexhibited were the best flamecomparedMaterials resistance; 2017 for, 10 , their784 however, flame retardant it was perfor consideredmance. It not was suitable observed as that real compound flame retardant 34b exhibited because14 of the 31 of its high solubilitybest flame in resistance; water. Compound however, it was34a consideredexhibited not the suitable second as best real flame result retardant in the UL because 94 test, of its was also containing 20 wt % of compound 3.34a in ABS was observed. Compounds 34a, 34b, 34c and 34d were the mosthigh volatile, solubility and in followed water. Compound by 34c and 34a 34dexhibited. A general the second conclusion best result on in thethe influenceUL 94 test, ofwas the also structure thecompared most volatile, for their and flame followed retardant by 34c perfor and mance.34d. A generalIt was observed conclusion that on compoundthe influence 34b of exhibitedthe structure the on the flame retardancy was made i.e., flame retardant having a methyl group attached directly to onbest the flame flame resistance; retardancy however, was made it was i.e., considered flame retard notant suitable having as a realmethyl flame group retardant attached because directly of itsto the phosphorusthehigh phosphorus solubility atom in atom showedwater. showed Compound greater greater flame34a flame exhibited retardancyretardancy the second compared compared best resultto compounds to in compounds the UL having 94 test, having a was phenyl, also a phenyl, hydrogenhydrogenthe ormost a hydroxyl volatile, or a hydroxyl and group. followed group. by 34c and 34d. A general conclusion on the influence of the structure on the flame retardancy was made i.e., flame retardant having a methyl group attached directly to the phosphorus atom showed greater flame retardancy compared to compounds having a phenyl, hydrogen or a hydroxyl group.

FigureFigure 13. 13.Flame Flame retardant retardant 34a34a–34d–34d structures.structures.

3.4. Grignard Type Reaction 3.4. Grignard Type Reaction Figure 13. Flame retardant 34a–34d structures. Grignard reactions can also be used for the creation of new P-C bonds [75]. Grignard reactions can also be used for the creation of new P-C bonds [75]. Hexaallyltriphosphazene Hexaallyltriphosphazene3.4. Grignard Type Reaction (HAPP) (35, Figure 14) was prepared from the reaction of allylmagnesium (HAPP)bromide (35, Figurewith hexachlorocyclotriphosphazene 14) was prepared from (in the 94% reaction yield) for ofapplication allylmagnesium in epoxy system bromide to with hexachlorocyclotriphosphazeneimproveGrignard its flamereactions retardancy. can (in also 94%HAPP be yield)wasused furt forforher applicationthe reacted creation in intwoof epoxy newsteps systemP-Cto givebonds tohexa(3- improve[75]. its flame retardancy.triglycidyloxysilylpropyl)triphosphazeneHexaallyltriphosphazene HAPP was (HAPP) further (35 reacted, Figure (36 in14)) intwo was 92% stepsprepared yield, to from givewhich the hexa(3-triglycidyloxysilylpropyl) wasreaction then of allylmagnesiumused in epoxy bromide with hexachlorocyclotriphosphazene (in 94% yield) for application in epoxy system to triphosphazeneformulations. (36 )LOI in tests 92% were yield, performed which wasfor the then epoxy used system in epoxywith and formulations. without flame retardant LOI tests were improve its flame retardancy. HAPP was further reacted in two steps to give hexa(3- performedadditive for the36. epoxyThe results system showed with that and compound withoutflame 36 played retardant a major additive role in 36improving. The results flame showed retardancy.triglycidyloxysilylpropyl)triphosphazene For example, the LOI value of the(36 )ne inat resin92% wasyield, 22.5%, which whereas, was withthen 15 used wt % inof 36,epoxy the that compoundLOIformulations. value36 increasedplayed LOI tests to a 32.1%. majorwere Moreover,performed role in improving thefor TGA the epoxydata flame revealed system retardancy. thatwith the and higher For without example, the concentrationflame theretardant LOI of value of the neatcompoundadditive resin was 36 36. 22.5%,The in the results epoxy whereas, showed system, with thethat 15higher compound wt %the of Td5% 3636 and, theplayed Td50% LOI temperatures.a valuemajor increasedrole Thein improving TGA to 32.1%.results flame also Moreover, the TGAshowedretardancy. data revealedthat, For for example, the that most the the concentrated higherLOI value the resinof concentrationthe (15 ne atwt resin % of was compound of 22.5%, compound whereas, 36), the 36char within residue 15 the wt epoxy % under of 36, system, was the the LOI value increased to 32.1%. Moreover, the TGA data revealed that the higher the concentration of higher the10.2%. Td5% Therefore,and Td50% it wastemperatures. reported that this The flame TGA retardant results acts also predominantly showed that, in for the the gas most phase. concentrated compound 36 in the epoxy system, the higher the Td5% and Td50% temperatures. The TGA results also resin (15 wt % of compound 36), the char residue under was 10.2%. Therefore, it was reported that this showed that, for the most concentrated resin (15 wt %R of compound 36), the char residue under was flame retardant acts predominantly in the gas phase.R Si 10.2%. Therefore, it was reported that this flame retardant acts predominantlyR in the gas phase. R i R R S R P N P R N N R R Si R= Si P N RR R O O P P NP Si N R R i R SR R R P Si N P R N N R R R= Si P N Si R R O O P P R R Si N R NP R R R R 35 R Si 36 R Si R R Figure 14. Chemical structures of 35R and 36.

35 36 Diallylphenylphosphine oxide (38, Figure 15) was prepared from reaction of allylmagnesium bromide with phenylphosphonicFigureFigure dichloride 14. 14.Chemical Chemical (37, Figure structures 15), of in of 35 about35 andand 36 55%. 36. yield [76]. The light yellow product obtained was applied in UV-cured organic–inorganic hybrid coatings on corona treated PlexiglassDiallylphenylphosphine® panels. The formulations oxide contained(38, Figure tetraethyl 15) was orthosilicateprepared from (TEOS), reaction diallylphenylphosphine of allylmagnesium oxidebromide (5 towith 20 wtphenylphosphonic %), an aliphatic urethanedichloride diacrylate (37, Figure resin 15), and in abouta silane-coupling 55% yield [76]. agent. The Free light films yellow on Teflon™product obtainedwells were was also applied prepared in andUV-cured cured withorganic–inorganic UV-light. Mechanical hybrid testscoatings were on performed corona treated on the curedPlexiglass samples® panels. with The and formulations without addition contained of compound tetraethyl orthosilicate 38 and evaluated (TEOS), for di tensileallylphenylphosphine properties like theoxide Young’s (5 to 20 modulus wt %), an and aliphatic the elongation urethane at diacrylate break, the resin contact and angle, a silane-coupling the pendulum agent. hardness Free films and the on cross-cutTeflon™ wellsadhesion. were Italso was prepared found thatand curedthe addition with UV-light. of diallylphenylphosphine Mechanical tests were oxide performed increased on the Young’scured samples modulus with but and decreased without bothaddition the tensileof compound strength 38 and and the evaluated elongation for tensileat break properties compared like to the Young’s modulus and the elongation at break, the contact angle, the pendulum hardness and the cross-cut adhesion. It was found that the addition of diallylphenylphosphine oxide increased the Young’s modulus but decreased both the tensile strength and the elongation at break compared to

Materials 2017, 10, 784 15 of 32

Diallylphenylphosphine oxide (38, Figure 15) was prepared from reaction of allylmagnesium bromide with phenylphosphonic dichloride (37, Figure 15), in about 55% yield [76]. The light yellow product obtained was applied in UV-cured organic–inorganic hybrid coatings on corona treated Plexiglass® panels. The formulations contained tetraethyl orthosilicate (TEOS), diallylphenylphosphine oxide (5 to 20 wt %), an aliphatic urethane diacrylate resin and a silane-coupling agent. Free films on Teflon™ wells were also prepared and cured with UV-light. Mechanical tests were performed on the cured samples with and without addition of compound 38 and evaluated for tensile properties Materialslike the 2017 Young’s, 10, 784 modulus and the elongation at break, the contact angle, the pendulum hardness15 of and 31 the cross-cut adhesion. It was found that the addition of diallylphenylphosphine oxide increased the Young’sthe formulation modulus without but decreased the compound both the 38 tensile. The contact strength angle and thealso elongation decreased atwith break the compared addition of to compoundthe formulation 38 to withoutthe formulation the compound due to 38its. high The contactpolarity. angle The alsopendulum decreased hardness with theincreased addition with of additioncompound of 38theto flame the formulation retardant due due to to a its higher high polarity.cross-linking The pendulumdensity and hardness the cross-cut increased adhesion with (whichaddition ranged of the flamefrom 0 retardant for the best due and to a 5 higher the poorest cross-linking adhesion) density showed and a the result cross-cut of 0 for adhesion a 15 wt (which% and 20ranged wt % from flame 0 forretardant the best content. and 5 the The poorest thermal adhesion) stability showedof the samples a result was of 0 foralso a investigated 15 wt % and by20 TGA wt % andflame it retardantwas found content. that addition The thermal of 20 wt stability % of compound of the samples 38 lowered was also the investigated Td5% by 40 °C by compared TGA and to it ◦ wasthe sample found thatwithout addition the offlame 20 wt retardant. % of compound The char38 contentlowered as the measured Td5% by 40in TGAC compared for the tomost the concentratedsample without resin the (20 flame wt % retardant. of 38) is lesser The char for lower content (10% as measured of 38) concentration in TGA for formulation. the most concentrated Normally forresin a (20flame wt retardant % of 38) is working lesser for in lower the condensed (10% of 38 phase,) concentration the char formulation.content in TGA Normally analysis for increases a flame withretardant increase working in its inconcentration the condensed in the phase, polymer. the char Thus, content one incan TGA infer analysis from the increases TGA data with that increase flame retardantin its concentration 38 work mostly in the polymer.in the gas Thus, phase. one The can photo- infer fromDSC thetest TGAwas dataalso performed, that flame retardant and it was38 workconcluded mostly that in thethe gas enthalpy phase. Theof polymerization photo-DSC test wasincreased also performed, with the andincrease it was in concluded concentration that theof compoundenthalpy of 38 polymerization. increased with the increase in concentration of compound 38.

Figure 15. Phosphine oxide derivative 38 and its precursor 37.

4. Flame Retardants with P-C aromatic BondsBonds

In thethe followingfollowing sections,sections, wewe havehave summarizedsummarized specificspecific examples examples of of P-C P-Caromaticaromatic bond containing organophosphorus compounds, which are classifiedclassified basedbased on the synthetic strategy for creation of P-C bond. For For clarity, clarity, the the specific specific P-C P-C bonds bonds that that were were created created in in the the structure structure have have been been marked marked in in bold. bold.

4.1. Friedel–Crafts Type Reactions One of of the the main main routes routes to phosphorus-aromatic to phosphorus-aromatic carbon carbon bonds bonds formation formation is a modified is a modiﬁed Friedel– CraftsFriedel–Crafts reaction usually reaction involving usually phosphorus involving phosphorus trichloride and trichloride a Lewis acid. and DOP-Cl a Lewis (Compound acid. DOP-Cl 39, (CompoundFigure 16) has39, Figurereportedly 16) hasbeen reportedly synthesized been starting synthesized from startingo-phenylphenol from o-phenylphenol using phosphorus using phosphorustrichloride in trichloridepresence of inzinc presence chloride of catalyst zinc chloride in 95% yield catalyst [66,77]. in 95%DOP-Cl yield is an [66 important,77]. DOP-Cl precursor is an forimportant DOPO precursorderivatives for due DOPO to its derivatives high chem dueical toreactivity its high towards chemical nucleophiles reactivity towards [5]. As nucleophiles an example, [5 it]. Asis shown an example, in the it literature is shown that in the DOP-OMe literature thatreacts DOP-OMe with hydroxymethyl reacts with hydroxymethyl substituted phenols substituted via a phenolsMichaelis–Arbuzov via a Michaelis–Arbuzov type rearrangement type rearrangement to give compound to give 14 compound (Scheme 2)14 in(Scheme 32% yield.2) in Compound 32% yield. 14Compound has a glass14 transitionhas a glass temperature transition temperature of 108.5 °C. of In 108.5 the ◦publishedC. In the publishedpatent, it patent,was claimed it was that claimed this familythat this of familycompounds of compounds can be used can as beflame used retardan as ﬂamet for retardant a wide range for a of wide polymers range such of polymers as PU, epoxy such andas PU, other epoxy thermosetting and other thermosetting resins or thermoplastics resins or thermoplastics in general. in It general. is further It is furtherclaimed claimed that these that compoundsthese compounds are particularly are particularly adapted adapted to protective to protective coating coating formulations, formulations, electrical electrical laminates laminates and molded thermoplastic products [[77].77].

Figure 16. Structure of DOP-Cl 39.

The same reagents as described earlier were also used in another patent to prepare compound 39 (Figure 16) and was subsequently oxidized [78]. Next, esterification with phenols or polyhydroxybenzenes was performed to develop several flame retardants and one such example is

Materials 2017, 10, 784 15 of 31 the formulation without the compound 38. The contact angle also decreased with the addition of compound 38 to the formulation due to its high polarity. The pendulum hardness increased with addition of the flame retardant due to a higher cross-linking density and the cross-cut adhesion (which ranged from 0 for the best and 5 the poorest adhesion) showed a result of 0 for a 15 wt % and 20 wt % flame retardant content. The thermal stability of the samples was also investigated by TGA and it was found that addition of 20 wt % of compound 38 lowered the Td5% by 40 °C compared to the sample without the flame retardant. The char content as measured in TGA for the most concentrated resin (20 wt % of 38) is lesser for lower (10% of 38) concentration formulation. Normally for a flame retardant working in the condensed phase, the char content in TGA analysis increases with increase in its concentration in the polymer. Thus, one can infer from the TGA data that flame retardant 38 work mostly in the gas phase. The photo-DSC test was also performed, and it was concluded that the enthalpy of polymerization increased with the increase in concentration of compound 38.

Figure 15. Phosphine oxide derivative 38 and its precursor 37.

4. Flame Retardants with P-Caromatic Bonds

In the following sections, we have summarized specific examples of P-Caromatic bond containing organophosphorus compounds, which are classified based on the synthetic strategy for creation of P-C bond. For clarity, the specific P-C bonds that were created in the structure have been marked in bold.

4.1. Friedel–Crafts Type Reactions One of the main routes to phosphorus-aromatic carbon bonds formation is a modified Friedel– Crafts reaction usually involving phosphorus trichloride and a Lewis acid. DOP-Cl (Compound 39, Figure 16) has reportedly been synthesized starting from o-phenylphenol using phosphorus trichloride in presence of zinc chloride catalyst in 95% yield [66,77]. DOP-Cl is an important precursor for DOPO derivatives due to its high chemical reactivity towards nucleophiles [5]. As an example, it is shown in the literature that DOP-OMe reacts with hydroxymethyl substituted phenols via a Michaelis–Arbuzov type rearrangement to give compound 14 (Scheme 2) in 32% yield. Compound 14 has a glass transition temperature of 108.5 °C. In the published patent, it was claimed that this family of compounds can be used as flame retardant for a wide range of polymers such as PU, epoxy and other thermosetting resins or thermoplastics in general. It is further claimed that these

Materialscompounds2017, 10are, 784 particularly adapted to protective coating formulations, electrical laminates16 ofand 32 molded thermoplastic products [77].

Figure 16. Structure of DOP-Cl 39.

The same same reagents reagents as asdescribed described earlier earlier were werealso used also in used another in patent another to prepare patent tocompound prepare 39 (Figure 16) and was subsequently oxidized [78]. Next, esterification with phenols or Materialscompound 2017, 3910, 784(Figure 16) and was subsequently oxidized [78]. Next, esterification with phenols16 of 31 orpolyhydroxybenzenesMaterials polyhydroxybenzenes 2017, 10, 784 was was performed performed to todevelop develop seve severalral flame flame retardants retardants and and one one such such example 16 of 31 is compound 40 (Figure 1717)) (obtained from bisphenol A). No further information on characterization, compound 40 (Figure 17) (obtained from bisphenol A). No further information on characterization, flameflame retardancy or applications werewere provided.provided. flame retardancy or applications were provided.

Figure 17. Chemical structure of bisphosphonate 40. Figure 17. Chemical structure of bisphosphonate 40. Friedel–Crafts reactions can also be performed using aluminum chloride as Lewis acid. It was Friedel–Crafts reactions can also be performedperformed using aluminum chloride as Lewis acid. It was reported that dichlorophenylphosphine sulfide reacts with two equivalents of toluene in the presence reported that dichlorophenylphosphine sulfidesulfide reactsreacts withwith twotwo equivalentsequivalents ofof toluenetoluene in the presence of AlCl3 catalyst to give 41a (Scheme 5, 84% yield) [79]. The compound was further oxidized to give of AlCl 3 catalystcatalyst to to give give 41a41a (Scheme(Scheme 5,5 ,84% 84% yield) yield) [79]. [ 79 The]. The compound compound was wasfurther further oxidized oxidized to give to BCPPO (41b, Scheme 5). Several polycondensations of this flame-retardant monomer and giveBCPPO BCPPO (41b, ( 41bScheme, Scheme 5).5 ).Several Several polycondensations polycondensations of of this this flame-retardant flame-retardant monomermonomer andand terephthalic acid (TPA) with bisphenol A were performed using different TPA/41b ratios, yielding terephthalic acid (TPA) withwith bisphenolbisphenol AA werewere performedperformed usingusing differentdifferent TPA/TPA/41b ratios,ratios, yieldingyielding six different polyarylates. Films were prepared from these phosphorus-containing polyarylates for six differentdifferent polyarylates.polyarylates. Films were prepared fromfrom thesethese phosphorus-containingphosphorus-containing polyarylates for further characterization. Mechanical properties of the polymers were evaluated, and it was found further characterization.characterization. Mechanical Mechanical properties properties of of the the polymers were evaluated, and it was foundfound that tensile strength and elongation at break decreased with introduction of 41b in the polyarylate. that tensile strength and elongation at break decreased with introduction of of 41b in the polyarylate. Crystallinity was lost when 41b was incorporated in the polymer. Glass transition temperature, Td5% Crystallinity was lost whenwhen 41b41b waswas incorporatedincorporated inin thethe polymer.polymer. GlassGlass transitiontransition temperature,temperature,T Td5%d5% temperature and char yield followed the same positive trend when phosphorus content was temperature andand char char yield yield followed followed the samethe same positive positive trend whentrend phosphoruswhen phosphorus content wascontent increased, was increased, showing tangible evidence that 41b introduction can improve the thermal stability of the showingincreased, tangible showing evidence tangible that evidence41b introduction that 41b introduction can improve can the improve thermal the stability thermal of stability the resulting of the resulting polymer. To confirm improved flame retardancy of the new materials, LOI value was polymer.resulting Topolymer. confirm To improved confirm flameimproved retardancy flame ofretardancy the new materials, of the new LOI materials, value was LOI measured value andwas measured and was shown to increase from 30.7% to 34.5% with increasing phosphorus content. wasmeasured shown and to increasewas shown from to 30.7% increase to 34.5% from with30.7% increasing to 34.5%phosphorus with increasing content. phosphorus Further studycontent. of Further study of the thermal decomposition behavior of the polymers concluded that increasing the theFurther thermal study decomposition of the thermal behavior decomposition of the behavior polymers of concluded the polymers that concluded increasing that the increasing proportion the of proportion of compound 41b in the polymer increased the char formation, indicating that compound compoundproportion 41bof compoundin the polymer 41b in increased the polymer the char increased formation, the char indicating formation, that compoundindicating 41bthat actscompound in both 41b acts in both the gas and condensed phase. Finally, an additional goal of this work was to study the41b gas acts and in both condensed the gas phase. and condensed Finally, an phase. additional Finally, goal an of thisadditional work was goal to of study this thework solubility was to study of the the solubility of the synthesized polyarylenes. Good solubility was achieved in a wide selection of synthesizedthe solubility polyarylenes. of the synthesized Good polyarylenes. solubility was Good achieved solubility in a wide was selectionachieved of in organic a wide solventsselection for of organic solvents for formulations with 20% or more of compound 41b. This was attributed to the formulationsorganic solvents with for 20% formulations or more of compoundwith 20% or41b more. This of was compound attributed 41b to. theThis increase was attributed of free volume to the increase of free volume induced by the introduction of bulky pendant phenyl units in the chain. It inducedincrease of by free the introductionvolume induced of bulky by the pendant introduction phenyl of units bulky in pendant the chain. phenyl It was units thus in concluded the chain. inIt was thus concluded in this work that introduction of compound 41b enhanced the thermal stability thiswas workthus concluded that introduction in this work of compound that introduction41b enhanced of compound the thermal 41b enhanced stability while the thermal improving stability the while improving the polyarylates’ processability. polyarylates’while improving processability. the polyarylates’ processability.

Scheme 5. Oxidation of 41a to 41b. Scheme 5. Oxidation of 41a to 41b.. DAP-Cl (compound 42a, Figure 18), a precursor to the nitrogen-containing DAPO (compound DAP-Cl (compound 42a, Figure 18), a precursor to the nitrogen-containing DAPO (compound 42b, Figure 18), was synthesized in 95% yield using 2-aminobiphenyl, PCl3 and AlCl3 as catalyst 42b, Figure 18), was synthesized in 95% yield using 2-aminobiphenyl, PCl3 and AlCl3 as catalyst [80,81]. It was reported that DAPO exists in the solution in a tautomeric equilibrium with its [80,81]. It was reported that DAPO exists in the solution in a tautomeric equilibrium with its corresponding phosphonamidic acid [82]. Another DOPO derivative, DOPS ( compound 42c, Figure corresponding phosphonamidic acid [82]. Another DOPO derivative, DOPS ( compound 42c, Figure 18), was synthesized by reacting DOP-Cl with hydrogen sulfide at room temperature (89% yield) [83]. 18), was synthesized by reacting DOP-Cl with hydrogen sulfide at room temperature (89% yield) [83]. Epoxy phenol novolac resins were functionalized with DAPO and DOPS, followed by a curing step. Epoxy phenol novolac resins were functionalized with DAPO and DOPS, followed by a curing step. Phosphorus content in these formulations ranged from 0.6 to 1.6 wt %. DSC measurements showed Phosphorus content in these formulations ranged from 0.6 to 1.6 wt %. DSC measurements showed that DAPO-modified resins had higher Tg compared to the reference DOPO-modified resins. This that DAPO-modified resins had higher Tg compared to the reference DOPO-modified resins. This was attributed to the presence of an amine group that can also act as a cross-linking reagent, resulting was attributed to the presence of an amine group that can also act as a cross-linking reagent, resulting in a more rigid network. On the contrary, DOPS-modification induced a sharp decrease of Tg, which in a more rigid network. On the contrary, DOPS-modification induced a sharp decrease of Tg, which can result in lower crosslinking density due to the bulky sulfur atom. Flame retardancy of these can result in lower crosslinking density due to the bulky sulfur atom. Flame retardancy of these modified resins was investigated and compared to DOPO-modified resins. At 1.6 wt % P loading, modified resins was investigated and compared to DOPO-modified resins. At 1.6 wt % P loading, DOPS- and DAPO-modified resins achieved UL 94 V0 rating. Char yield in TGA experiments reached DOPS- and DAPO-modified resins achieved UL 94 V0 rating. Char yield in TGA experiments reached

Materials 2017, 10, 784 17 of 32

DAP-Cl (compound 42a, Figure 18), a precursor to the nitrogen-containing DAPO (compound 42b, Figure 18), was synthesized in 95% yield using 2-aminobiphenyl, PCl3 and AlCl3 as catalyst [80,81]. It was reported that DAPO exists in the solution in a tautomeric equilibrium with its corresponding phosphonamidic acid [82]. Another DOPO derivative, DOPS ( compound 42c, Figure 18), was synthesized by reacting DOP-Cl with hydrogen sulfide at room temperature (89% yield) [83]. Epoxy phenol novolac resins were functionalized with DAPO and DOPS, followed by a curing step. Phosphorus content in these formulations ranged from 0.6 to 1.6 wt %. DSC measurements showed that DAPO-modified resins had higher Tg compared to the reference DOPO-modified resins. This was attributed to the presence of an amine group that can also act as a cross-linking reagent, resulting in a more rigid network. On the contrary, DOPS-modification induced a sharp decrease of Tg, which can result in lower crosslinking density due to the bulky sulfur atom. Flame retardancy of these modified resinsMaterialsMaterials was 2017 2017 investigated, 10,, 10784, 784 and compared to DOPO-modified resins. At 1.6 wt % P loading, DOPS-17 andof17 31 of 31 DAPO-modified resins achieved UL 94 V0 rating. Char yield in TGA experiments reached 35% for DAPO35%35% for for andDAPO DAPO 32% and forand 32% DOPS 32% for for DOPS based DOPS based resins. based resins. Nitrogen-phosphorus resins. Nitrog Nitrogen-phosphorusen-phosphorus synergistic synergistic synergistic effect effect waseffect was considered was considered considered to beto tobe the bethe reason the reason reason behind behind behind increased increased increased char char yield, char yield, andyield, and was and was also was also presented also presented presented as a possibleas asa possiblea possible explanation explanation explanation for their for for slowertheirtheir slower decompositionslower decomposition decomposition rate. rate. Sulfur-phosphorus rate. Sulfur-phosphorus Sulfur-phosphorus synergy synergy synergy hasn’t hasn’t hasn’t yet beenyet yet been extensively been extensively extensively studied studied studied so noso so conclusionnono conclusion conclusion could could could be be drawn. bedrawn. drawn. Increased Increased Increased char char char yield yiel yield indicate indicated indicate a a high high a high probability probability probability thatthat that DOPSDOPS DOPS andand and DAPO DAPO maymay work work in condensedin condensed and and gas gas phase, phase, whereas whereas DOPODOPO DOPO actsacts acts mainlymainly mainly inin gasingas gas phase.phase. phase.

FigureFigure 18. 18.DOPO DOPO analogues. analogues.

TwoTwo similarsimilar similar compoundscompounds compounds were were preparedprepared prepared usingusing using aluminumaluminum aluminum chloridechloride chloride and and phosphorusphosphorus phosphorus trichloride trichloride reagentsreagents and and di- di-p-tolyletherp-tolylether [84] [84 [84]] or ordiphenylamine diphenylamine [85], [85 [85],], affording affording DPPO DPPO (compound (compound 42d 42d,, 91% 91%, 91% yield) yield) yield) andand DPPA DPPA (compound(compound (compound 42e 42e,, 61%61%, 61% yield),yield), yield), respectivelyrespectively respectively (Scheme(Scheme (Scheme6 6).). Compound6).Compound Compound 42d 42d was was then then oxidized oxidized with hydrogen peroxide or reacted with formaldehyde, affording DPPO-OH and DPPO-CH2OH, with hydrogen peroxide or reactedreacted withwith formaldehyde,formaldehyde, affordingaffording DPPO-OHDPPO-OH andand DPPO-CHDPPO-CH2OH, respectively.respectively. These These two two intermediates intermediates along along with with unmodified unmodiﬁed unmodified 42d 42d were were then then used used to functionalizetofunctionalize functionalize epoxyepoxy novolac novolac resin resin DEN DEN 438 438 by bytheir their reaction reaction with with the the epoxy epoxy groups, groups, resulting resulting in inDPPO-OH-DEN, DPPO-OH-DEN, DPPO-CH2OH-DEN and DPPO-DEN. The same procedure was applied to DOPO, affording DOPO- DPPO-CH2OH-DENOH-DEN and and DPPO-DEN. DPPO-DEN. The The same same procedure procedure was applied was applied to DOPO, to DOPO,affording affording DOPO- OH, DOPO-CH2OH-DEN and DOPO-DEN. Each flame-retardant prepolymer was prepared with DOPO-OH,OH, DOPO-CH DOPO-CH2OH-DEN2OH-DEN and DOPO-DEN. and DOPO-DEN. Each flame-retardant Each ﬂame-retardant prepolymer prepolymer was prepared was prepared with withphosphorusphosphorus phosphorus content content content ranging ranging ranging between between between 0.5% 0.5% and andand 2.0%. 2.0%.2.0%. 4,4 4,44,4′-Diaminodiphenylmethane0-Diaminodiphenylmethane′-Diaminodiphenylmethane (DDM) (DDM) (DDM) was was usedused as as hardener hardener during during the the curing curing step. step.

SchemeScheme 6. Synthetic Synthetic6. Synthetic route route for for phosphine phosphine oxides oxides DPPO DPPO ( 42d (42d) and ) and DPPA DPPA ((42e42e (42e).). ).

Compound 42e was directly reacted to epoxy groups of DGEBA resin (via the phosphorus and Compound 42e was directly reacted to epoxy groups of DGEBA resin (via the phosphorus and nitrogen atoms) to obtain DPPA-DEN prepolymers [85]. The phosphorus content of the formulations nitrogen atoms) to obtain DPPA-D DPPA-DENEN prepolymers [[85].85]. The phosphorusphosphorus content of the formulations variedvaried from from 0 to 0 3.20to 3.20 wt wt %. %.Curing Curing was was then then pe rformedperformed using using DDM DDM or or3,5-di 3,5-diethyltoluene-2,6-diamineethyltoluene-2,6-diamine (DETDA)(DETDA) as ashardeners. hardeners. High High level level flame flame retardant retardant properties properties were were achieved achieved for for all allthe the DOPO- DOPO- and and DPPO-basedDPPO-based epoxy epoxy resins. resins. At Ata 0.81 a 0.81 wt wt % phosphorus% phosphorus content, content, all allof themof them acehieved acehieved UL UL 94 94V-0 V-0 rating. rating. ForFor this this same same phosphorus phosphorus content, content, DOPO-based DOPO-based resins resins exhibited exhibited LOI LOI value value between between 31% 31% and and 31.5%, 31.5%, whereas,whereas, for for DPPO-based DPPO-based resins, resins, LOI LOI values values ranged ranged between between 27.4% 27.4% and and 28.3%, 28.3%, both both considered considered really really goodgood values values for for flame flame retard retardants.ants. In Incase case of ofcompound compound 42d 42d, PO, PO radicals radicals were were shown shown to toform form in inthe the gaseousgaseous phase, phase, indicating indicating the the gas gas phase phase mode mode of actionof action [84] [84]. DPPA-DEN. DPPA-DEN resins resins exhibited exhibited lower lower flame flame retardantretardant performance, performance, needing needing a pha phosphorusosphorus content content of of3.20% 3.20% to toattain attain UL UL 94 94V-0 V-0 rating rating as aswell well achievingachieving LOI LOI values values 28.5% 28.5% (DDM-cured) (DDM-cured) and and 29.3% 29.3% (DETDA-cured). (DETDA-cured). Thus, Thus, it wasit was concluded concluded that that DPPO-,DPPO-, DPPA- DPPA- and and DOPO-functionalization DOPO-functionalization proved proved to tobe bebeneficial beneficial in inimproving improving flame flame retardants retardants in inepoxy epoxy resins. resins. 4-carboxyphenylpheny4-carboxyphenylphenylphosphiniclphosphinic acid acid (43 (,43 Figure, Figure 19) 19) was was prepared prepared by byaddition addition of oftoluene toluene to to dichlorophenylphosphinedichlorophenylphosphine and and anhydrous anhydrous aluminum aluminum chloride chloride followed followed by byhydrolysis hydrolysis and and the the oxidationoxidation reaction reaction [86], [86], achieving achieving a yielda yield of of33.1%. 33.1%. Its Itssolubility solubility in inwater water was was investigated investigated from from 26.7026.70 °C °Cto to94.52 94.52 °C °Cand and experimental experimental data data was was co rrelatedcorrelated using using a second-order a second-order polynomial polynomial with with an an

Materials 2017, 10, 784 18 of 32 varied from 0 to 3.20 wt %. Curing was then performed using DDM or 3,5-diethyltoluene-2,6-diamine (DETDA) as hardeners. High level flame retardant properties were achieved for all the DOPO- and DPPO-based epoxy resins. At a 0.81 wt % phosphorus content, all of them acehieved UL 94 V-0 rating. For this same phosphorus content, DOPO-based resins exhibited LOI value between 31% and 31.5%, whereas, for DPPO-based resins, LOI values ranged between 27.4% and 28.3%, both considered really good values for flame retardants. In case of compound 42d, PO radicals were shown to form in the gaseous phase, indicating the gas phase mode of action [84]. DPPA-DEN resins exhibited lower flame retardant performance, needing a phosphorus content of 3.20% to attain UL 94 V-0 rating as well achieving LOI values 28.5% (DDM-cured) and 29.3% (DETDA-cured). Thus, it was concluded that DPPO-, DPPA- and DOPO-functionalization proved to be beneficial in improving flame retardants in epoxy resins. 4-carboxyphenylphenylphosphinic acid (43, Figure 19) was prepared by addition of toluene to dichlorophenylphosphine and anhydrous aluminum chloride followed by hydrolysis and the oxidation ◦ reactionMaterialsMaterials 2017 2017 [86, ],10, 10, achieving 784, 784 a yield of 33.1%. Its solubility in water was investigated from 26.701818C of of to31 31 94.52 ◦C and experimental data was correlated using a second-order polynomial with an absolute averageabsoluteabsolute deviationaverage average ofdeviation deviation 2.1%. These of of 2.1%. solubility2.1%. These These measurements solubili solubilityty measurements measurements were considered were were of considered interestconsidered because of of interest interest of the importantbecausebecause of of rolethe the ofimportant important water in role therole industrialof of water water in productionin the the indu industrialstrial and production purification production and ofand compound purification purification43 .of Beingof compound compound a reactive 43 43. . phosphorus-containingBeingBeing a areactive reactive phosphorus-containing phosphorus-containing monomer, compound monomer, monomer,43 can compound compound be used as43 43 acan co-monomercan be be used used as as fora aco-monomer manufacturingco-monomer for for functionalmanufacturingmanufacturing polyesters. functi functionalonal polyesters. polyesters.

FigureFigure 19. 19. Chemical Chemical structure structure of of compound compound 43 43.. .

4.2.4.2. General General Substitution Substitution Reactions Reactions CompoundCompound 44 44 (Figure (Figure 2020) 20)) was was reportedreported reported toto to bebe be synthesizedsynthe synthesizedsized byby by addingadding adding chlorodiphenylphosphinechlorodiphenylphosphine chlorodiphenylphosphine toto aa a mixturemixture mixture ofof of pp -bromostyrene-bromostyrenep-bromostyrene andand and butyllithium,butyllithium, butyllithium, followed followed by by an an oxidation oxidation step, step, resulting resulting in in 65% 65% yieldyield [[87].87 [87].]. Cationic Cationic copolymerization copolymerization was was investigated investigated with with soybean soybean oil oil for for different different ratios ratios of of styrene styrene toto substance substance substance 44 4444, ,using, usingusing divinylbenzene divinylbenzene as as as crosslinker. crosslinker. crosslinker. Due Due Due to to a to aproblem aproblem problem of of heterogeneity ofheterogeneity heterogeneity before before before and and andafterafter after the the thepolymerization polymerization polymerization reaction, reaction, reaction, an an analternative alternative alternative phosphorus-containing phosphorus-containing styrene styrenestyrene substance substancesubstance waswas investigated.investigated. This This alternative alternative compound compound underwent underwent cross-metathesis cross-metathesis with with a a vegetable-oil vegetable-oil derivative derivative andand was was copolymerized, copolymerized, affording affording homogeneous homogeneous thermosetth thermosetermoset polymers. polymers. Thermal-oxidativeTherma Thermal-oxidativel-oxidative degradation degradation waswas investigated investigated by by TGA, TGA, showing showing that that 1 1 wtwt wt %% % phosphorusphosphorus phosphorus incorporationincorporation incorporation improved improved the the thermalthermal thermal stabilitystability of of the the thermosetthermoset thermoset slightly.slightly. slightly. Condensed Condensed ph phasephasease action action was was inferred inferred from from the the increased increased char char formationformation observed observed in in the the TGA. TGA. Phosphorus-free Phosphorus-free reference reference polymer polymer achieved achieved 19.2% 19.2% LOI LOI value, value, and and phosphorus-containingphosphorus-containing polymerpolymer polymer waswas was reportedreported reported toto to achieveachieve achieve aa amaximummaximum maximum LOILOI LOI valuevalue value ofof of 24%.24%. 24%.

FigureFigure 20. 20. Phosphine Phosphine oxide oxide derivative derivative 44 44 and and phosphonate phosphonate derivative derivative 45 45.. .

1,4-bis(diethylphosphoro)benzene (45, Figure 20) was synthesized from the reaction of 1,4- 1,4-bis(diethylphosphoro)benzene1,4-bis(diethylphosphoro)benzene ( 45,, Figure Figure 20)20 )was was synthesized synthesized from from the the reaction reaction of 1,4- of dibromobenzene with triethyl phosphite [88]. Compound 45 was used in battery electrolytes 1,4-dibromobenzenedibromobenzene with with triethyl triethyl phosphite phosphite [88]. [88 ].Compound Compound 4545 was usedused inin batterybattery electrolytes electrolytes compositionscompositions to to enhance enhance flame flame retardancy retardancy properties properties and and to to reduce reduce impact impact of of possible possible thermal thermal runaway. runaway. Phosphorus-carbonPhosphorus-carbon bonds bonds formation formation were were reported reported with with good good yields yields for for substitution substitution reactions reactions involvinginvolving triethyl triethyl phosphite phosphite and and cyanuric cyanuric chloride. chloride. These These compounds compounds are are considered considered very very efficient efficient asas charring charring agents agents due due the the presence presence of of nitrogen nitrogen in in the the triazine triazine ring ring and and thus thus expected expected to to exhibit exhibit P-NP-N synergism. synergism. InIn a arecent recent work work [89], [89], diethyl diethyl phosphite-functionalized phosphite-functionalized cyanuric cyanuric chloride chloride (86.1% (86.1% yield) yield) was was dimerizeddimerized using using neopentyl neopentyl glycol glycol as as a abridging bridging agent, agent, yielding yielding compound compound 46 46 (Figure (Figure 21, 21, 92.5% 92.5% yield). yield). TheThe target target flame flame retardant retardant can can bind bind covalently covalently to to co cottontton fabrics fabrics by by reaction reaction of of the the cellulose cellulose hydroxyl hydroxyl groupsgroups via via a asubstitution substitution reaction. reaction. The The best best curing curing method method was was established established from from several several trials trials and and waswas applied applied to to treat treat cotton cotton fabric fabric samples samples with with 10 10 to to 30 30 wt wt % % of of the the flame flame retardant. retardant. TGA TGA analysis analysis showedshowed an an improvement improvement of of thermal thermal stability stability and and incr increaseease of of the the char char yield yield above above 30% 30% for for all all treated treated samples,samples, indicating indicating the the condensed condensed phase phase mode mode of of action action of of the the flame flame retardant. retardant. During During vertical vertical flammabilityflammability testing, testing, char char length length became became shorter shorter wi withth increasing increasing flame flame retardant retardant content, content, and and a aB-1 B-1 ratingrating was was achieved achieved for for samples samples treated treated with with more more than than 20 20 wt wt % % flame flame retardant. retardant. Improved Improved flame flame retardancyretardancy was was confirmed confirmed from from LOI LOI tests, tests, which which showed showed a aLOI LOI value value of of 30.3% 30.3% and and 18% 18% for for 30 30 wt wt % % ofof flame flame retardant retardant treated treated fabric fabric and and untreated untreated fa fabric,bric, respectively. respectively. Washing Washing with with water water of of treated treated cottoncotton showed showed a arapid rapid decrease decrease in in LOI LOI values. values. Init Initialial LOI LOI of of 28.4% 28.4% dropped dropped to to 22% 22% after after 10 10 times times

Materials 2017, 10, 784 19 of 32 compositions to enhance flame retardancy properties and to reduce impact of possible thermal runaway. Phosphorus-carbon bonds formation were reported with good yields for substitution reactions involving triethyl phosphite and cyanuric chloride. These compounds are considered very efficient as charring agents due the presence of nitrogen in the triazine ring and thus expected to exhibit P-N synergism. In a recent work [89], diethyl phosphite-functionalized cyanuric chloride (86.1% yield) was dimerized using neopentyl glycol as a bridging agent, yielding compound 46 (Figure 21, 92.5% yield). The target flame retardant can bind covalently to cotton fabrics by reaction of the cellulose hydroxyl groups via a substitution reaction. The best curing method was established from several trials and was applied to treat cotton fabric samples with 10 to 30 wt % of the flame retardant. TGA analysis showed an improvement of thermal stability and increase of the char yield above 30% for all treated samples, indicating the condensed phase mode of action of the flame retardant. During vertical flammability testing, char length became shorter with increasing flame retardant content, and a B-1 rating was achieved for samples treated with more than 20 wt % flame retardant. Improved flame retardancy was confirmed from LOI tests, which showed a LOI value of 30.3% and 18% for 30 wt % of flame retardant treatedMaterials fabric 2017, 10 and, 784 untreated fabric, respectively. Washing with water of treated cotton showed a rapid19 of 31 decrease in LOI values. Initial LOI of 28.4% dropped to 22% after 10 times washing indicating not allwashing flame retardants indicating reacted not all with flame the cellulose.retardants Such reac flameted with retardants the cellulose. primarily Such act inflame the condensed retardants phaseprimarily in cellulose act in the based condensed materials. phase in cellulose based materials.

FigureFigure 21. 21.Triazine Triazine derivatives derivatives46 46and and47 47. .

AA reactionreaction ofof threethree equivalentsequivalents ofof triethyltriethyl phosphitephosphite withwith cyanuriccyanuric chloridechloride resultedresulted inin 2,4,6-trisubstituted2,4,6-trisubstituted 1,3,5-triazine 1,3,5-triazine (86.1% (86.1% yield). yield). OneOne of of the the diethyl diethyl phosphate phosphate groups groups on on the the triazine triazine ringring was was then then substituted substituted by by morpholine, morpholine, producing producing another another substituted substituted triazine-based triazine-based compound compound47 (Figure47 (Figure 21, 92.5% 21, 92.5% yield) [yield)90]. An [90]. oligomer An oligomer was synthesized was synthesized by transesteriﬁcation by transesterification reaction of compoundreaction of compound 47 with pentaerythritol in 93.6% yield. It was then incorporated in polypropylene at 47 with pentaerythritol in 93.6% yield. It was then incorporated in polypropylene at 10 and 30 wt %, which10 and respectively 30 wt %, which achieved respectively UL 94 V-2 achieved and V-0 UL ratings, 94 V-2 while and V-0 LOI ratings, values while of 23.4% LOI and values 28.9% of were23.4% observedand 28.9% for were the sameobserved formulations, for the same respectively. formulations A polymer, respectively. formulation A polymer was formulation further prepared was further with prepared with 30 wt % ammonium polyphosphate/pentaerythritol/47 (ratio 3:1:2), which achieved a 30 wt % ammonium polyphosphate/pentaerythritol/47 (ratio 3:1:2), which achieved a higher LOI higher LOI of 29.4% and UL 94 V-0 rating. This improvement in flame retardancy was attributed to of 29.4% and UL 94 V-0 rating. This improvement in ﬂame retardancy was attributed to the possible the possible synergistic effect between the three additives. synergistic effect between the three additives.

4.3.4.3. Addition Addition Reactions Reactions RecentlyRecently in in the the literature, literature, a a large large amount amount of of research research work work can can be be found found on on addition addition reactions reactions of of DOPODOPO derivatives derivatives on onα α,β,β-unsaturated-unsaturated ketones. ketones. This This is is mainly mainly due due to to the the versatility versatility and and high high yields yields allowedallowed by by this this type type of of reaction. reaction. DOPO DOPO addition addition on on 1,4-benzoquinone 1,4-benzoquinone is is one one of of the the most most common common usesuses of of this this addition addition reaction. reaction. DOPO-BQ DOPO-BQ (compound (compound48a 48a,, Scheme Scheme7 )7) was was synthesized synthesized following following a a previouslypreviously reported reported method method in 93%in 93% yield yield [91] as [91] an intermediateas an intermediate leading toleading six ﬂame to retardantsix flame additivesretardant foradditives lithium for ion lithium batteries. ion They batteries. were They then characterizedwere then characterized for their thermal for their properties thermal usingproperties TGA using and TGA and DSC [92]. From these six compounds, the compound 48b (Scheme 7) that achieved a T◦ d5% DSC [92]. From these six compounds, the compound 48b (Scheme7) that achieved a T d5% of 210 C wasof 210 selected °C was for selected further ﬂamefor further retardant flame evaluation. retardant Itevaluation. was mixed It with was conventionalmixed with conventional Li-ion electrolytes, Li-ion electrolytes, with weight fractions ranging from 2.5 to 25 wt %. Self-extinguishing time (SET) measurements were performed, showing a sharp decrease in SET, whereas ionic conductivity underwent a 50% decrease when increasing the weight fraction of the additive from 2.5 wt % to 25 wt %. The flame retardant efficiency of the electrolyte formulations was confirmed, but lower battery performances are to be expected due to lower ionic conductivity results. Char formation indicated a condensed phase action. Studies on actual battery performances along with electrochemical properties are underway.

Scheme 7. Synthesis of DOPO based phosphinate derivative 48b from 48a.

Compound 48a was reacted with acryloyl chloride to obtain a phosphorus-containing acrylate monomer (compound 49, Figure 22) [93]. Various concentrations (0 to 20 wt %) of this compound were introduced into the formulation of unsaturated polyester resins synthesized by radical

Materials 2017, 10, 784 19 of 31 washing indicating not all flame retardants reacted with the cellulose. Such flame retardants primarily act in the condensed phase in cellulose based materials.

Figure 21. Triazine derivatives 46 and 47.

A reaction of three equivalents of triethyl phosphite with cyanuric chloride resulted in 2,4,6-trisubstituted 1,3,5-triazine (86.1% yield). One of the diethyl phosphate groups on the triazine ring was then substituted by morpholine, producing another substituted triazine-based compound 47 (Figure 21, 92.5% yield) [90]. An oligomer was synthesized by transesterification reaction of compound 47 with pentaerythritol in 93.6% yield. It was then incorporated in polypropylene at 10 and 30 wt %, which respectively achieved UL 94 V-2 and V-0 ratings, while LOI values of 23.4% and 28.9% were observed for the same formulations, respectively. A polymer formulation was further prepared with 30 wt % ammonium polyphosphate/pentaerythritol/47 (ratio 3:1:2), which achieved a higher LOI of 29.4% and UL 94 V-0 rating. This improvement in flame retardancy was attributed to the possible synergistic effect between the three additives.

4.3. Addition Reactions Recently in the literature, a large amount of research work can be found on addition reactions of DOPO derivatives on α,β-unsaturated ketones. This is mainly due to the versatility and high yields allowed by this type of reaction. DOPO addition on 1,4-benzoquinone is one of the most common uses of this addition reaction. DOPO-BQ (compound 48a, Scheme 7) was synthesized following a previously reported method in 93% yield [91] as an intermediate leading to six flame retardant additives for lithium ion batteries. They were then characterized for their thermal properties using

TGAMaterials and2017 DSC, 10, 784[92]. From these six compounds, the compound 48b (Scheme 7) that achieved 20a T ofd5% 32 of 210 °C was selected for further flame retardant evaluation. It was mixed with conventional Li-ion electrolytes, with weight fractions ranging from 2.5 to 25 wt %. Self-extinguishing time (SET) measurementswith weight fractions were rangingperformed, from showing 2.5 to 25 a wt sharp %. Self-extinguishing decrease in SET, time whereas (SET) measurementsionic conductivity were underwentperformed, a showing 50% decrease a sharp when decrease increasing in SET, whereasthe weight ionic fraction conductivity of the additive underwent from a 50% 2.5 decreasewt % to 25when wt increasing%. The flame the retardant weight fraction efficiency of the of additivethe electrolyte from 2.5 formulations wt % to 25 wtwas %. confirmed, The flame but retardant lower batteryefficiency performances of the electrolyte are to formulations be expected wasdue confirmed,to lower ionic but lowerconductivity battery results. performances Char formation are to be indicatedexpected duea condensed to lower phase ionic conductivityaction. Studies results. on actual Char battery formation performances indicated along a condensed with electrochemical phase action. propertiesStudies on are actual underway. battery performances along with electrochemical properties are underway.

SchemeScheme 7. SynthesisSynthesis of of DOPO DOPO based based phosphinate phosphinate derivative 48b from 48a.

Compound 48a was reacted with acryloyl chloride to obtain a phosphorus-containing acrylate Compound 48a was reacted with acryloyl chloride to obtain a phosphorus-containing acrylate monomer (compound 49, Figure 22) [93]. Various concentrations (0 to 20 wt %) of this compound monomer (compound 49, Figure 22)[93]. Various concentrations (0 to 20 wt %) of this compound wereMaterials introduced 2017, 10, 784 into the formulation of unsaturated polyester resins synthesized by radical20 of 31 were introduced into the formulation of unsaturated polyester resins synthesized by radical copolymerization.copolymerization. AA highhigh LOILOI valuevalue ofof 24%24% was was achieved achieved for for formulations formulations containing containing≥ ≥55 wtwt %% ◦ ◦ phosphorus-containingphosphorus-containing monomer.monomer. T Td5%d5% peakedpeaked at at260 260 °C inC air, in air,at 300 at °C 300 in nitrogen,C in nitrogen, for the for highest the highestflame flameretardant-containing retardant-containing formulation, formulation, and andits peak its peak heat heat release release rate rate (PHRR) (PHRR) was reducedreduced toto 288.7288.7 W/g. W/g. ItsIts flameflame retardantretardant propertiesproperties werewere confirmedconfirmed bothboth in in condensed condensed and and gas gas phase, phase, due due to to charringcharring and and flame flame inhibition inhibition phenomena, phenomena, respectively. respectively.

FigureFigure 22. 22.DOPO DOPO based based phosphinate phosphinate derivatives derivatives49 49and and50 50. .

SubstanceSubstance48a 48awas was also also reacted reacted with with acetic acetic anhydride, anhydride, affording affording50 50(Figure (Figure 22 22,, 75% 75% yield) yield) [ 94[94].]. SubstanceSubstance 5050was was thenthencopolymerized copolymerized withwithp -acetoxybenzoicp-acetoxybenzoic acid,acid, terephthalicterephthalic acidacid andand thethe condensationcondensation product product of of terephthalic terephthalic acid acid with with 1,8-octanediamine, 1,8-octanediamine, producing producing a a new new flame flame retarded retarded thermotropic liquid crystal polymer. DSC experiment showed lower Tg and Tm for it compared to thermotropic liquid crystal polymer. DSC experiment showed lower Tg and Tm for it compared tousual usual liquid liquid crystal crystal polymers, polymers, namely, namely, 127 127 °C◦C and 147147 ◦°C.C. ItIt adoptedadopted nematicnematic liquid liquid crystalline crystalline behaviorbehavior in in the the temperature temperature range range 180–330 180–330◦ C.°C. Thermal Thermal stability stability of of the the polymers polymers was was investigated investigated by by TGA and Td5% was estimated at 385◦ °C, showing satisfying thermal stability and char yield, allowing TGA and Td5% was estimated at 385 C, showing satisfying thermal stability and char yield, allowing forfor very very good good flame flame resistance. resistance. DiglycidylDiglycidyl ether ether of of bisphenol-A bisphenol-A epoxy epoxy resin resin (DGEBA) (DGEBA) was was modified modified by by compound compound48a 48a, affording, affording thethe phosphorus-containing phosphorus-containing epoxy epoxy resin resin51 51(Figure (Figure 23 23))[95 [95].]. Four Four cured cured epoxy epoxy resin/polyhedral resin/polyhedral oligomericoligomeric silsesquioxanesilsesquioxane (POSS)(POSS) hybridshybrids werewere synthesized,synthesized, andand allall samplessamples containedcontained 22 wtwt %% phosphorusphosphorus but but with with varying varying silicon silicon content content (from (from 0 0 to to 4 4 wt wt %). %). The The thermal-oxidative thermal-oxidative stability stability of of the the polymerspolymers was was evaluated evaluated by TGA.by TGA. With With increasing increasing the silicon the silicon content, content, TGA analysis TGA showedanalysis ashowed decrease a decrease of Td5% and an increase of Td50% and char yield, which was attributed to the phosphorus- of Td5% and an increase of Td50% and char yield, which was attributed to the phosphorus-silicon synergism.silicon synergism. Further flameFurther retardant flame re propertiestardant properties were estimated were estimated by PCFC, by giving PCFC, evidence giving evidence that PHRR that PHRR decreased and occurred at higher temperatures with increasing silicon content. Tg of the polymer was shown to increase with increase in phosphorus and silicon content. SEM and XPS were used to analyze char residues and results obtained reinforce hypotheses on phosphorus-silicon synergistic impact on epoxy resins flame retardancy.

O P OH OH OH O O O O O O O O

O O O O n OH m 51 Figure 23. DOPO-BQ (48a) functionalized DGEBA resin 51.

Similar to the previous study, different epoxy resin 51 were synthesized from reaction of DGEBA and compound 48a in different molar ratios [96] and several dicyanate esters were used as curing agents. TGA data under nitrogen showed evidence that degradation temperature and char yield for the resin increased with the increasing phosphorus content, showing a condensed phase mode of action. UL 94 V-0 rating was attained for formulations containing around 2 wt % phosphorus, when UL 94 V-1 rating was achieved for the resins containing around 1 wt % phosphorus, the curing dicyanate ester having no major influence. Potential applications as flame retardant laminates or protective coatings were suggested for these resins.

Materials 2017, 10, 784 20 of 31 copolymerization. A high LOI value of 24% was achieved for formulations containing ≥5 wt % phosphorus-containing monomer. Td5% peaked at 260 °C in air, at 300 °C in nitrogen, for the highest flame retardant-containing formulation, and its peak heat release rate (PHRR) was reduced to 288.7 W/g. Its flame retardant properties were confirmed both in condensed and gas phase, due to charring and flame inhibition phenomena, respectively.

Figure 22. DOPO based phosphinate derivatives 49 and 50.

Substance 48a was also reacted with acetic anhydride, affording 50 (Figure 22, 75% yield) [94]. Substance 50 was then copolymerized with p-acetoxybenzoic acid, terephthalic acid and the condensation product of terephthalic acid with 1,8-octanediamine, producing a new flame retarded thermotropic liquid crystal polymer. DSC experiment showed lower Tg and Tm for it compared to usual liquid crystal polymers, namely, 127 °C and 147 °C. It adopted nematic liquid crystalline behavior in the temperature range 180–330 °C. Thermal stability of the polymers was investigated by TGA and Td5% was estimated at 385 °C, showing satisfying thermal stability and char yield, allowing for very good flame resistance. Diglycidyl ether of bisphenol-A epoxy resin (DGEBA) was modified by compound 48a, affording the phosphorus-containing epoxy resin 51 (Figure 23) [95]. Four cured epoxy resin/polyhedral oligomeric silsesquioxane (POSS) hybrids were synthesized, and all samples contained 2 wt % phosphorus but with varying silicon content (from 0 to 4 wt %). The thermal-oxidative stability of the polymers was evaluated by TGA. With increasing the silicon content, TGA analysis showed a Materials 2017, 10, 784 21 of 32 decrease of Td5% and an increase of Td50% and char yield, which was attributed to the phosphorus- silicon synergism. Further flame retardant properties were estimated by PCFC, giving evidence that decreasedPHRR decreased and occurred and occurred at higher at temperatures higher temper withatures increasing with siliconincreasing content. silicon Tg ofcontent. the polymer Tg of wasthe shownpolymer to was increase shown with to increaseincrease inwith phosphorus increase in and phosphorus silicon content. and silicon SEM and content. XPS were SEM used and toXPS analyze were charused residuesto analyze and char results residues obtained and reinforce results hypothesesobtained reinforce on phosphorus-silicon hypotheses on synergistic phosphorus-silicon impact on epoxysynergistic resins impact ﬂame on retardancy. epoxy resins flame retardancy.

O P OH OH OH O O O O O O O O

O O O O n OH m 51 Figure 23. DOPO-BQ ( 48a) functionalized DGEBA resin 51.

Similar to the previous study, different epoxy resin 51 were synthesized from reaction of DGEBA Similar to the previous study, different epoxy resin 51 were synthesized from reaction of DGEBA and compound 48a in different molar ratios [96] and several dicyanate esters were used as curing and compound 48a in different molar ratios [96] and several dicyanate esters were used as curing agents. TGA data under nitrogen showed evidence that degradation temperature and char yield for agents. TGA data under nitrogen showed evidence that degradation temperature and char yield for the the resin increased with the increasing phosphorus content, showing a condensed phase mode of resin increased with the increasing phosphorus content, showing a condensed phase mode of action. action. UL 94 V-0 rating was attained for formulations containing around 2 wt % phosphorus, when UL 94 V-0 rating was attained for formulations containing around 2 wt % phosphorus, when UL 94 V-1 UL 94 V-1 rating was achieved for the resins containing around 1 wt % phosphorus, the curing rating was achieved for the resins containing around 1 wt % phosphorus, the curing dicyanate ester dicyanate ester having no major influence. Potential applications as flame retardant laminates or having no major influence. Potential applications as flame retardant laminates or protective coatings protective coatings were suggested for these resins. wereMaterials suggested 2017, 10, 784 for these resins. 21 of 31 Previously synthesized compound 48a [91] and DPPO-HQ, (compound 52, Figure 24) (obtained by reactingPreviously compound synthesized42d compound(Scheme6 48a) with [91] andp-benzoquinone) DPPO-HQ, (compound [84,97] were52, Figure used 24) as (obtained diols to formby reacting aromatic compound polysulfones 42d (Scheme (PSU) by 6) nucleophilic with p-benzoquinone) aromatic substitution [84,97] were polycondensation used as diols to form with 4,4aromatic0-difluorodiphenylsulfone polysulfones (PSU) (DFDPS) by nucleophilic [98]. Two polysulfonesaromatic substitution were thus obtained,polycondensation having around with 54,4 wt′-difluorodiphenylsulfone % phosphorus content. Both(DFDPS) exhibited [98]. significantlyTwo polysulfones higher were Tg than thus phosphorus-free, obtained, having bisphenol around A-based5 wt % phosphorus PSU (reference), content. and Both a two-step exhibited decomposition significantly higher was observed Tg than phosphorus-free, instead of one-step bisphenol for the referenceA-based PSU polymer (reference), in TGA. and TGA a andtwo-step Py/GC-MS decompos dataition were was used observed to formulate instead a hypothesis of one-step concerning for the thereference thermal polymer degradation in TGA. process TGA and of Py/GC-MS these formulations. data were used It was to alsoformulate concluded a hypothesis that DOPO-based concerning PSUthe thermal combined degradation gaseous process and solid-state of these formulatio flame-retardingns. It was mechanisms, also concluded whereas that DPPO-basedDOPO-based PSU favoredcombined solid-state gaseous and action. solid-state Thus, DOPO-based flame-retardin PSUg mechanisms, was considered whereas as a DPPO-based slightly better PSU in termsfavored of flamesolid-state retardancy. action. Thus, DOPO-based PSU was considered as a slightly better in terms of flame retardancy.

Figure 24. Phosphine oxide derivative 52.

Following the same reaction principle as describeddescribed earlier, DOPO can also react with 1,4-naphthoquinone to give DOPO-NQ (compound 53a,, Scheme 88,, 70%70% yield)yield) [[99],99], which was then functionalized by trimellitic anhydride chloride and 4-aminophenol to give a bisphenol derivative (compound 53b, SchemeScheme8 8,, 71.8% 71.8% yield) yield) [ [100].100]. SeveralSeveral liquidliquid crystallinecrystalline co-poly(esterco-poly(ester imide)s were prepared by polycondensation of an aromatic diaciddiacid chloride with various ratios of the compoundcompound 53b and aliphatic diols. Thermal analysis of the polymerspolymers was studied by TGA, and it was observed that all polymers followed a two-step decomposition process. The least stable polymer exhibited a Td5% of 340 °C, whereas the most stable showed a Td5% of 395 °C. The thermal degradation behavior of the polymers was investigated by FTIR-TGA and SEM-TGA analysis. Phosphorus-nitrogen synergy is expected to contribute to flame retardancy, suggesting gas and condensed phase activity for the phosphorus compounds. Solubility in various solvents was investigated. Liquid crystalline properties were confirmed by XRD and DSC analysis.

Scheme 8. Synthetic route for DOPO based phosphinate 53b.

In a second paper [101], the same group synthesized three different poly(ester imide)s by solution polycondensation of equimolar compound 53b (Scheme 8) and aromatic diacids (either terephthalic acid, isophthalic acid or 4,4′-dicarboxydiphenylether) yielding PEI-1, PEI-2 and PEI-3, respectively. TGA experiments were run and concluded that these polymers exhibited 55% char and were stable up to 375 °C. Their Tg were respectively measured at 226 °C, 200 °C and 190 °C, making them good candidates for thermoforming. Energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of phosphorus in the char, proving condensed phase action. Liquid crystalline properties were confirmed by WAXD, optical properties and DSC studies. Diphenylphosphine oxide was reported to react with benzoquinone to give compound 54 (DPO- BQ, Figure 25, 78% yield) [102]. Its thermal stability was shown to be slightly better than the reference

Materials 2017, 10, 784 21 of 31

Previously synthesized compound 48a [91] and DPPO-HQ, (compound 52, Figure 24) (obtained by reacting compound 42d (Scheme 6) with p-benzoquinone) [84,97] were used as diols to form aromatic polysulfones (PSU) by nucleophilic aromatic substitution polycondensation with 4,4′-difluorodiphenylsulfone (DFDPS) [98]. Two polysulfones were thus obtained, having around 5 wt % phosphorus content. Both exhibited significantly higher Tg than phosphorus-free, bisphenol A-based PSU (reference), and a two-step decomposition was observed instead of one-step for the reference polymer in TGA. TGA and Py/GC-MS data were used to formulate a hypothesis concerning the thermal degradation process of these formulations. It was also concluded that DOPO-based PSU combined gaseous and solid-state flame-retarding mechanisms, whereas DPPO-based PSU favored solid-state action. Thus, DOPO-based PSU was considered as a slightly better in terms of flame retardancy.

Figure 24. Phosphine oxide derivative 52.

Following the same reaction principle as described earlier, DOPO can also react with 1,4-naphthoquinone to give DOPO-NQ (compound 53a, Scheme 8, 70% yield) [99], which was then functionalized by trimellitic anhydride chloride and 4-aminophenol to give a bisphenol derivative

Materials(compound2017, 1053b, 784, Scheme 8, 71.8% yield) [100]. Several liquid crystalline co-poly(ester imide)s22 were of 32 prepared by polycondensation of an aromatic diacid chloride with various ratios of the compound 53b and aliphatic diols. Thermal analysis of the polymers was studied by TGA, and it was observed that all polymerspolymers followed a two-steptwo-step decompositiondecomposition process. The least stable polymer exhibited a ◦ ◦ Td5% ofof 340 340 °C,C, whereas whereas the the most stable showed a Td5% ofof 395 395 °C.C. The The thermal thermal degradation behavior of thethe polymerspolymers was was investigated investigated by FTIR-TGAby FTIR-TGA and SEM-TGAand SEM-TGA analysis. analysis. Phosphorus-nitrogen Phosphorus-nitrogen synergy issynergy expected is expected to contribute to contribute to ﬂame to retardancy, flame reta suggestingrdancy, suggesting gas and gas condensed and condensed phase activity phase activity for the phosphorusfor the phosphorus compounds. compounds. Solubility Solubility in various insolvents various wassolvents investigated. was investigated. Liquid crystalline Liquid crystalline properties wereproperties conﬁrmed were byconfirmed XRD and by DSC XRD analysis. and DSC analysis.

Scheme 8. Synthetic route for DOPO based phosphinate 53b.

In a second paper [101], the same group synthesized three different poly(ester imide)s by In a second paper [101], the same group synthesized three different poly(ester imide)s by solution solution polycondensation of equimolar compound 53b (Scheme 8) and aromatic diacids (either polycondensation of equimolar compound 53b (Scheme8) and aromatic diacids (either terephthalic terephthalic acid, isophthalic acid or 4,4′-dicarboxydiphenylether) yielding PEI-1, PEI-2 and PEI-3, acid, isophthalic acid or 4,40-dicarboxydiphenylether) yielding PEI-1, PEI-2 and PEI-3, respectively. respectively. TGA experiments were run and concluded that these polymers exhibited 55% char and TGA experiments were run and concluded that these polymers exhibited 55% char and were stable were stable◦ up to 375 °C. Their Tg were respectively measured◦ at ◦226 °C, 200 ◦°C and 190 °C, making up to 375 C. Their Tg were respectively measured at 226 C, 200 C and 190 C, making them good them good candidates for thermoforming. Energy-dispersive X-ray spectroscopy (EDX) confirmed candidates for thermoforming. Energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of the presence of phosphorus in the char, proving condensed phase action. Liquid crystalline phosphorus in the char, proving condensed phase action. Liquid crystalline properties were confirmed properties were confirmed by WAXD, optical properties and DSC studies. by WAXD, optical properties and DSC studies. Diphenylphosphine oxide was reported to react with benzoquinone to give compound 54 (DPO- Diphenylphosphine oxide was reported to react with benzoquinone to give compound 54 BQ,Materials Figure 2017 25,, 10 78%, 784 yield) [102]. Its thermal stability was shown to be slightly better than the reference22 of 31 (DPO-BQ, Figure 25, 78% yield) [102]. Its thermal stability was shown to be slightly better than the ◦ referencecompound compound 48a, having48a, havinga Td5% of a T315d5% °Cof 315and Ca slower and a slower decomposition decomposition in air. in The air. Thecompound compound 54 was54 wasthen then reacted reacted to toepoxy epoxy resin, resin, followed followed by by curing withwith 4,44,40-diaminodiphenylsulfone′-diaminodiphenylsulfone toto achieve achieve thermosetthermoset resins resins having having 1 1 and and 2 2 wt wt % % phosphorus. phosphorus. TGA TGA investigations investigations showed showed few few differences differences betweenbetween the the two two formulations formulations containing containing various various concentrations concentrations of of compound compound54 54,T,d5% Td5%, being, being around around 391391◦ C°C in in nitrogen nitrogen and and 384 384◦ C°C in in air. air. Polymer Polymer formulation formulation with with 2 2 wt wt % % phosphorus phosphorus achieved achieved a a UL UL 94 94 V-0V-0 rating rating and and LOI LOI of of 32.4%, 32.4%, whereas whereas the the one one containing containing only only 1 1 wt wt % % phosphorus phosphorus reached reached a a V-1 V-1 rating rating andand a a LOI LOI value value of of 29.2%. 29.2%. Good Good flame flame retardance retardance was was achieved achieved at at low low phosphorus phosphorus loadings, loadings, although although TGATGA results results showed showed little little improvements improvements compared compared to phosphorus-free to phosphorus-free epoxy epoxy resins, exceptresins, forexcept higher for charhigher yields, char indicating yields, indicating a condensed a condensed phase mode phase of mode action. of action.

FigureFigure 25. 25.Chemical Chemical structures structures of of compounds compounds54 54and and55 55. .

AA recent recent patent patent reportedreported thatthat DOPODOPO additionaddition onon 3,33,30′,5,5,5,50′-tetramethyldiphenoquinone gave gave a ayellow yellow compound compound 5555 (Figure(Figure 25)25) in 73%73% yieldyield [[103].103]. The aimaim ofof thisthis workwork waswas toto synthesize synthesize phosphorus-containingphosphorus-containing compoundscompounds withwith lowlow stericsteric hindrance hindrance for for application application in in ﬂame flame retardant retardant epoxyepoxy resins. resins. According to previous studies, a disubstituted phosphorus compound 56a and a monosubstituted phosphorus compound 56b (Figure 26) were obtained as a mixture using modified separation and purification steps. Melting points for these compounds were 258–260 °C and 184–187 °C, respectively [104]. Further characterizations were performed and were compared with other phosphorus compounds described later in the text.

Figure 26. Phosphonate derivatives 56a and 56b.

4.4. Michaelis–Arbuzov Rearrangement In the same work [104], phosphonoterephthalic acid derivative 56c (Figure 27) was synthesized using a Michaelis–Arbuzov rearrangement from a previously published synthesis protocol [105]. Compounds 56d and 56e (Figure 27) syntheses were also based on a previous study from the same group (exhibiting m.p. of 296–298 °C and 310 °C, respectively). The syntheses involved iodoester reagents with palladium catalyst in a yield of around 80%. PU foams were prepared with compounds 56a to 56e according to two different processing methods: Preps (reactive flame retardant copolymerized during PU foam formation) and Blends (10 wt % of flame retardant additive used with already prepared PU). These foams were analyzed for thermal and flame retardant behavior using TGA and PCFC, respectively. It was found that compound 56e did decrease the heat release and inhibited melting and flowing of the PU in the Blend form. In the Prep form, the addition of this compound did not affect the heat release. The resulting PU formulation also had a two-step thermal degradation and increased char yield at 800 °C (neat PU around 9% and PU with 10 wt % flame retardant around 29%). For compound 56d, the Prep form didn’t affect either the heat release, but, in the Blend form, it induced a significant decrease of the THR. One possible explanation of this

Materials 2017, 10, 784 22 of 31

compound 48a, having a Td5% of 315 °C and a slower decomposition in air. The compound 54 was then reacted to epoxy resin, followed by curing with 4,4′-diaminodiphenylsulfone to achieve thermoset resins having 1 and 2 wt % phosphorus. TGA investigations showed few differences between the two formulations containing various concentrations of compound 54, Td5%, being around 391 °C in nitrogen and 384 °C in air. Polymer formulation with 2 wt % phosphorus achieved a UL 94 V-0 rating and LOI of 32.4%, whereas the one containing only 1 wt % phosphorus reached a V-1 rating and a LOI value of 29.2%. Good flame retardance was achieved at low phosphorus loadings, although TGA results showed little improvements compared to phosphorus-free epoxy resins, except for higher char yields, indicating a condensed phase mode of action.

Figure 25. Chemical structures of compounds 54 and 55.

A recent patent reported that DOPO addition on 3,3′,5,5′-tetramethyldiphenoquinone gave a

Materialsyellow2017 compound, 10, 784 55 (Figure 25) in 73% yield [103]. The aim of this work was to synthesize23 of 32 phosphorus-containing compounds with low steric hindrance for application in flame retardant epoxy resins. AccordingAccording to to previous previous studies, studies, a a disubstituted disubstituted phosphorus phosphorus compound compound56a 56aand and a a monosubstituted monosubstituted phosphorusphosphorus compoundcompound 56b56b (Figure 26)26) were were obtained obtained as as a amixture mixture using using modified modiﬁed separation separation and ◦ ◦ andpurification puriﬁcation steps. steps. Melting Melting points points for these for compou these compoundsnds were 258–260 were °C 258–260 and 184–187C and °C, 184–187 respectivelyC, respectively[104]. Further [104 characterization]. Further characterizationss were performed were and performed were compared and were with compared other withphosphorus other phosphoruscompounds compounds described later described in the latertext. in the text.

FigureFigure 26. 26.Phosphonate Phosphonate derivatives derivatives56a 56aand and56b 56b. .

4.4.4.4. Michaelis–Arbuzov Michaelis–Arbuzov Rearrangement Rearrangement InIn the the same same work work [ 104[104],], phosphonoterephthalic phosphonoterephthalic acid acid derivative derivative56c 56c(Figure (Figure 27 27)) was was synthesized synthesized usingusing a a Michaelis–Arbuzov Michaelis–Arbuzov rearrangementrearrangement fromfrom aa previouslypreviously publishedpublished synthesissynthesis protocolprotocol [105[105].]. CompoundsCompounds56d 56d andand 56e (Figure(Figure 27)27) syntheses syntheses were were also also base basedd on ona previous a previous study study from from the same the samegroup group (exhibiting (exhibiting m.p. of m.p. 296–298 of 296–298 °C and◦ C310 and °C, 310 respectively).◦C, respectively). The syntheses The syntheses involved involvediodoester iodoesterreagents with reagents palladium with palladium catalyst in a catalyst yield of in around a yield 80%. of aroundPU foams 80%. were PU prepared foams with were compounds prepared with56a compoundsto 56e according56a to 56eto accordingtwo differe tont two processing different processingmethods: Preps methods: (reactivePreps flame(reactive retardant flame retardantcopolymerized copolymerized during PU during foam PUformation) foam formation) and Blends and (10Blends wt % (10of flame wt % retardant of flame retardantadditive used additive with usedalready with prepared already prepared PU). These PU). foams These were foams analyzed were analyzed for thermal for thermal and flame and flameretardant retardant behavior behavior using usingTGA TGAand PCFC, and PCFC, respectively. respectively. It was It wasfound found that thatcompound compound 56e 56edid diddecrease decrease the theheat heat release release and andinhibited inhibited melting melting and and flowing flowing of the of thePU PUin the in theBlendBlend form.form. In the In thePrepPrep form,form, the theaddition addition of this of thiscompound compound did didnot notaffect affect the heat the heatrelease. release. The resu Thelting resulting PU formulation PU formulation also had also a two-step had a two-step thermal thermaldegradation degradation and increased and increased char yield char at yield 800 °C at 800(neat◦C PU (neat around PU around 9% and 9% PU and with PU 10 with wt 10% wtflame % retardant around 29%). For compound 56d, the Prep form didn’t affect either the heat release, but, in flameMaterials retardant 2017, 10 around, 784 29%). For compound 56d, the Prep form didn’t affect either the heat release,23 of 31 but,the inBlend the Blendform,form, it induced it induced a significant a significant decrease decrease of ofthe the THR. THR. One One possible explanationexplanation ofof this this phenomenon phenomenon was was the the incomplete incomplete incorporation incorporation of the of flamethe flame retardant retardant in the in PU the foam. PU foam. Compound Compound56c showed56c showed the same the flame same retardantflame retardant behavior behavior as 56e foras PU56e preparedfor PU prepared by Blend byas Blend wellas as in wellPrep asforms. in Prep Theforms. previously The previously mentioned mentioned phosphonoterephthalic phosphonoterephthalic acid derivatives acid derivatives56a and 56b 56ahave and shown56b have similar shown efficiencysimilar inefficiency decreasing in decreasing the total heat the releasetotal heat in both release processing in both methods.processing It methods. was also foundIt was thatalso theyfound didn’tthat inhibitthey didn’t melting inhibit and melting flowing and ofthe flow PU.ing In of conclusion, the PU. In conclusion, compounds compounds56c and 56e 56cshowed and 56e the showed best flamethe retardancybest flame retardancy properties ofproperties all the investigated of all the investigated structures. structures.

FigureFigure 27. 27.Chemical Chemical structures structures of compoundsof compounds56c 56cto 56eto 56e. .

4.5.4.5. [1,3]-Sigmatropic [1,3]-Sigmatropic Rearrangement Rearrangement TwoTwo polyphenols polyphenols were were transformed transformed into into corresponding corresponding phosphates phosphates using using diethyl diethyl phosphite phosphite followingfollowing Atherton–Todd Atherton–Todd reaction reaction [106 [106].]. Further Further treatment treatment with with lithium lithium diisopropylamine diisopropylamine allowed allowed breakagebreakage of aof P-O a P-O bond bond and migrationand migration of the of phosphorus the phosphorus to create to a create new P-C a new bond. P-C This bond. process This known process known as [1,3]-sigmatropic rearrangement yielded two different compounds, 57a (62% yield, m.p. 169–171 °C) and 57b (65% yield, m.p. 216–217 °C), and both were obtained as white crystals. Their structures are shown in Figure 28. Although no flame retardant tests were performed, it was concluded that these phosphorus-containing polyphenols could be introduced as flame retardant monomers in high-performance polymers including PEEK, PEES, PEK, PES or PC.

Figure 28. Phosphonate derivatives of polyphenols 57a and 57b.

According to the same mechanism, the phosphonate product (diethyl-[2-hydroxy-5-vinylphenyl) phosphonate (compound 58a, Scheme 9) was synthesized starting from diethyl-(4- vinylphenyl)phosphate and obtained in a yield of 82% [107]. Diethyl phosphite was then used to react with the hydroxyl group using Atherton–Todd reaction, yielding compound 58b (Scheme 9, 85.7% yield). Compound 58b was copolymerized with styrene using three different compositions (5, 10 and 20 wt % of monomer 58b) yielding copolymers that Tg decreases from 101 to 95 °C when flame- retardant monomer mass fraction is increased in the polymer formulation. TGA showed a two-step degradation behavior explained by the presence of diethyl phosphate and diethyl phosphonate, allowing the main degradation step to happen at a temperature at least 60 °C higher than the neat polystyrene. Copolymer incorporation lowered the temperature of the first decomposition step but increased the temperature of the second degradation stage. The char yield increased from 1.1% to 7.3% with increasing the phosphorus-containing monomer, showing evidence of a condensed phase mechanism. PCFC measurements followed a similar trend, the total heat release decreasing from 40.1 to 38.2 kJ/g with increasing phosphorus content. As a conclusion, efficient flame retardant vinyl polymers were obtained by copolymerizing phosphorus-functionalized styrene with styrene units.

Materials 2017, 10, 784 23 of 31

phenomenon was the incomplete incorporation of the flame retardant in the PU foam. Compound 56c showed the same flame retardant behavior as 56e for PU prepared by Blend as well as in Prep forms. The previously mentioned phosphonoterephthalic acid derivatives 56a and 56b have shown similar efficiency in decreasing the total heat release in both processing methods. It was also found that they didn’t inhibit melting and flowing of the PU. In conclusion, compounds 56c and 56e showed the best flame retardancy properties of all the investigated structures.

Figure 27. Chemical structures of compounds 56c to 56e.

4.5. [1,3]-Sigmatropic Rearrangement Two polyphenols were transformed into corresponding phosphates using diethyl phosphite Materials 2017, 10, 784 24 of 32 following Atherton–Todd reaction [106]. Further treatment with lithium diisopropylamine allowed breakage of a P-O bond and migration of the phosphorus to create a new P-C bond. This process asknown [1,3]-sigmatropic as [1,3]-sigmatropic rearrangement rearrangem yieldedent two yielded different two compounds, different compounds,57a (62% yield, 57a m.p. (62% 169–171 yield, m.p.◦C) and169–17157b (65% °C) and yield, 57b m.p. (65% 216–217 yield, m.p.◦C), 216–217 and both °C), were and obtained both were as whiteobtained crystals. as white Their crystals. structures Their arestructures shown inare Figure shown 28 .in Although Figure 28. no ﬂameAlthough retardant no flame tests wereretardant performed, tests were it was performed, concluded it that was theseconcluded phosphorus-containing that these phosphorus-containing polyphenols could polyphenols be introduced could as be ﬂame introduced retardant as flame monomers retardant in high-performancemonomers in high-performance polymers including polymers PEEK, including PEES, PEK, PEEK, PES PEES, or PC. PEK, PES or PC.

FigureFigure 28. 28.Phosphonate Phosphonate derivatives derivatives of of polyphenols polyphenols57a 57aand and57b 57b. .

AccordingAccording to to the the same same mechanism, mechanism, the the phosphonate phosphonate product product (diethyl-[2-hydroxy-5-vinylphenyl) (diethyl-[2-hydroxy-5-vinylphenyl) phosphonatephosphonate (compound(compound58a ,58a Scheme, Scheme9) was synthesized9) was synthesized starting from starting diethyl-(4-vinylphenyl) from diethyl-(4- phosphatevinylphenyl)phosphate and obtained and in a obtained yield of in 82% a yield [107 of]. 82 Diethyl% [107]. phosphite Diethyl phosphite was then was used then to used react to with react thewith hydroxyl the hydroxyl group usinggroup Atherton–Toddusing Atherton–Todd reaction, reaction, yielding yielding compound compound58b (Scheme 58b (Scheme9, 85.7% 9, yield). 85.7% Compoundyield). Compound58b was 58b copolymerized was copolymerized with styrene with using styrene three using different three compositionsdifferent compositions (5, 10 and (5,20 10 wt and % 20 wt % of monomer 58b) yielding copolymers that Tg decreases from ◦101 to 95 °C when flame- of monomer 58b) yielding copolymers that Tg decreases from 101 to 95 C when flame-retardant monomerretardant mass monomer fraction mass is increased fraction is in increased the polymer in the formulation. polymer formulation. TGA showed TGA a two-step showed degradation a two-step behaviordegradation explained behavior by theexplained presence by ofthe diethyl presence phosphate of diethyl and phosphate diethyl phosphonate, and diethyl allowingphosphonate, the mainallowing degradation the main step degradation to happen step at ato temperature happen at a at temperature least 60 ◦C higherat least than60 °C the higher neat polystyrene.than the neat Copolymerpolystyrene. incorporation Copolymer incorporation lowered the temperaturelowered the temperature of the first decomposition of the first decomposition step but increased step but theincreased temperature the temperature of the second of degradationthe second degradation stage. The charstage. yield The increasedchar yield from increased 1.1% tofrom 7.3% 1.1% with to increasing7.3% with the increasing phosphorus-containing the phosphorus-containing monomer, showing monomer, evidence showing of a evidence condensed of phasea condensed mechanism. phase PCFCmechanism. measurements PCFC measurements followed a similar followed trend, a similar the total trend, heat releasethe total decreasing heat release from decreasing 40.1 to 38.2 from kJ/g 40.1 withto 38.2 increasing kJ/g with phosphorus increasing content. phosphorus As a conclusion,content. As efficient a conclusion, flameretardant efficient vinylflame polymersretardant werevinyl polymers were obtained by copolymerizing phosphorus-functionalized styrene with styrene units. Materialsobtained 2017 by, 10 copolymerizing, 784 phosphorus-functionalized styrene with styrene units. 24 of 31

Scheme 9. Synthesis of the phosphorus-containing vinyl monomer 58b from 58a.

4.6. Phospho-Fries Rearrangement A phosphorusphosphorus version version of theof the Fries Fries rearrangement rearrangemen wast reported was reported by treating by tetraethyl-1,2-phenylenetreating tetraethyl-1,2- phenylenebis(phosphate) bis(phosphate) with a mixture with of a butyllithium mixture of andbutyll lithiumithium diisopropylamide and lithium diisopropylamide to produce diphosphonate to produce diphosphonatecompounds 59 (Figurecompounds 29, 67% 59 (Figure yield) [29,108 67%]. This yield) procedure [108]. This was procedure a modiﬁed was version a modified of a synthesis version ofreported a synthesis in a reported previous in publicationa previous publication [109] explaining [109] explaining that phosphate that phosphate esters in esters presence in presence of an oforganolithium an organolithium compound compound form anionsform anions that undergo that undergo phosphorus phosphorus migration migration from oxygen from oxygen to carbon. to carbon.Compound Compound59 was 59 an was intermediate an intermediate to four to four lithium lithium salts: salts: three three boron-centered boron-centered anions anionsand and one phosphorus-centered. Pyrolysis combustion flow calorimetry (PCFC) results showed evidence that lithium boron phosphorus complexes reached high char yield (up to 49.60%), which lowers the peak heat release rate temperature below 312 °C. These high char yield values indicate condensed phase activity, and could even indicate phosphorus-boron synergism. TGA analysis completed this study by proving these new lithium salts were stable up to 200 °C. It was concluded these new compounds should be effective flame retardants for lithium ion batteries thanks to their reduced flammability, either as electrolyte additives or as possible alternative to existing lithium salts.

Figure 29. Chemical structure of compounds 59 and 60.

4.7. Grignard Type Reactions Grignard reagents represent an additional method of P-C bonds synthesis in which the nucleophilic carbon can react as a base. 5-bromo-2-fluorobenzotrifluoride was added to magnesium turnings, forming the associated organomagnesium compound. Phenylphosphonic dichloride was then added to yield compound 60 (Figure 29) as a pale yellow solid (80% yield) [110]. Polymerization reactions were carried out using an equimolar ratio of compound 60 and different diphenol derivatives. Six novel poly(arylene ether phosphine oxide)s were obtained, with Tg in the range of 173–252 °C, and Td5% values being between 457 °C and 490 °C. PCFC investigation showed great influence of the diphenol structure, and the peak heat release rate varied from 184 to 67 W/g. Films were prepared, resulting in transparent, brittle or flexible films depending on the diphenol monomer used. The mechanical properties of the flexible films were evaluated, tensile stress reaching up to 54 MPa, Young’s modulus up to 0.97 GPa and elongation at break up to 47%. This study brought to light compounds with interesting thermal stabilities, and choosing the right diphenol monomer can allow to finely tune the poly(arylene ether phosphine oxide) properties. In a similar approach, diphenylphosphinic chloride was reacted with 4- (trifluoromethyl)phenylmagnesium bromide to yield compound 61a (Scheme 10) [111] having a melting point of around 90.5–91.2 °C. This product underwent nitration followed by hydrogenation to give the amino derivative 61b (Scheme 10). This phosphorus-containing diamino compound can be used to prepare polyimides, polyamides that exhibit very good fire retardancy, thermal stability,

Materials 2017, 10, 784 24 of 31

Scheme 9. Synthesis of the phosphorus-containing vinyl monomer 58b from 58a.

4.6. Phospho-Fries Rearrangement A phosphorus version of the Fries rearrangement was reported by treating tetraethyl-1,2- phenylene bis(phosphate) with a mixture of butyllithium and lithium diisopropylamide to produce diphosphonate compounds 59 (Figure 29, 67% yield) [108]. This procedure was a modified version of a synthesis reported in a previous publication [109] explaining that phosphate esters in presence Materials 2017, 10, 784 25 of 32 of an organolithium compound form anions that undergo phosphorus migration from oxygen to carbon. Compound 59 was an intermediate to four lithium salts: three boron-centered anions and one phosphorus-centered.phosphorus-centered. Pyrolysis Pyrolysis combustion combustion flow flow calorimetry calorimetry (PCFC) (PCFC) results results showed showed evidence evidence that that lithiumlithium boron boron phosphorus phosphorus complexes complexes reached reached high high char char yield yield (up (up to to 49.60%), 49.60%), which which lowers lowers the the peak peak heatheat release release rate rate temperature temperature below below 312 312◦ C.°C. These These high high char char yield yield values values indicate indicate condensed condensed phase phase activity,activity, and and could could even even indicate indicate phosphorus-boron phosphorus-boron synergism. synergism. TGA TGA analysis analysis completed completed this this study study byby proving proving these these new new lithium lithium salts salts were were stable stable up up to to 200 200◦ C.°C. It It was was concluded concluded these these new new compounds compounds shouldshould be be effective effective flame flame retardants retardants for for lithium lithium ion ion batteries batteries thanks thanks toto their their reducedreduced flammability,flammability, eithereither as as electrolyte electrolyte additives additives or or as as possible possible alternative alternative to to existing existing lithium lithium salts. salts.

FigureFigure 29. 29.Chemical Chemical structure structure of of compounds compounds59 59and and60 60. .

4.7.4.7. Grignard Grignard Type Type Reactions Reactions GrignardGrignard reagentsreagents representrepresent anan additionaladditional methodmethod ofof P-CP-C bondsbonds synthesissynthesis inin whichwhich thethe nucleophilicnucleophilic carbon carbon can can react react as as a a base. base. 5-bromo-2-fluorobenzotrifluoride 5-bromo-2-fluorobenzotrifluoride was was added added to to magnesium magnesium turnings,turnings, forming forming the the associated associated organomagnesium organomagnesium compound. compound. Phenylphosphonic Phenylphosphonic dichloride dichloride was was thenthen added added to to yield yield compound compound60 60(Figure (Figure 29 29)) as as a a pale pale yellow yellow solid solid (80% (80% yield) yield) [ 110[110].]. Polymerization Polymerization reactionsreactions were were carried carried out usingout using an equimolar an equimolar ratio of ratio compound of compound60 and different 60 and diphenol different derivatives. diphenol ◦ Sixderivatives. novel poly(arylene Six novel etherpoly(arylene phosphine ether oxide)s phosphine were obtained,oxide)s were with obtained, Tg in the with range Tg ofin 173–252the rangeC, of d5% ◦ ◦ and173–252 Td5% °C,values and beingT values between being 457 betweenC and 490457 °CC. and PCFC 490 investigation °C. PCFC investigation showed great showed influence great ofinfluence the diphenol of the structure, diphenol andstructure, the peak and heatthe peak release heat rate release varied rate from varied 184 from to 67 184 W/g. to 67 Films W/g. wereFilms prepared,were prepared, resulting resulting in transparent, in transparent, brittle orbrittle flexible or flexible films depending films depending on the diphenolon the diphenol monomer monomer used. Theused. mechanical The mechanical properties properties of the flexibleof the flexible films were films evaluated, were evaluated, tensile tensile stress reachingstress reaching up to 54 up MPa, to 54 Young’sMPa, Young’s modulus modulus up to 0.97up to GPa 0.97 andGPa elongationand elongation at break at break up toup 47%. to 47%. This This study study brought brought to to light light compoundscompounds with with interesting interesting thermal thermal stabilities, stabilities, and and choosing choosing the the right right diphenol diphenol monomer monomer can can allow allow toto finely finely tune tune the the poly(arylene poly(arylene ether ether phosphine phosphine oxide) oxide) properties. properties. InIn a similara similar approach, approach, diphenylphosphinic diphenylphosph chlorideinic waschloride reacted was with reacted 4-(trifluoromethyl) with 4- phenylmagnesium(trifluoromethyl)phenylmagnesium bromide to yield bromide compound to 61ayield(Scheme compound 10)[ 11161a] (Scheme havinga 10) melting [111] pointhaving of a aroundmelting 90.5–91.2 point of◦ aroundC. This product90.5–91.2 underwent °C. This product nitration underwent followed bynitration hydrogenation followed toby give hydrogenation the amino to give the amino derivative 61b (Scheme 10). This phosphorus-containing diamino compound can derivativeMaterials 201761b, 10, 784(Scheme 10). This phosphorus-containing diamino compound can be used to prepare25 of 31 polyimides,be used to polyamidesprepare polyimides, that exhibit polyamides very good that fire exhi retardancy,bit very thermal good fire stability, retardancy, mechanical thermal properties, stability, adhesionmechanical as properties, well as low adhesion dielectric as constant.well as low Possible dielectric applications constant. wouldPossible be applications adhesives for would metals, be semiconductoradhesives for metals, packaging semiconductor or optical materials.packaging or optical materials.

Scheme 10. Two-step amination of 61a toto yieldyield 61b..

5. Conclusions P-C bondbond creationcreation and and its its use use in in synthetic synthetic chemistry chemistry offer offer a wide a wide range range of possibility of possibility of design of design and developmentand development of exciting of organophosphorusexciting organophosphorus ﬂame retardants. flame Conventionalretardants. Conventional synthetic methodologies synthetic methodologies Michael addition, Michaelis–Arbuzov, Friedels–Crafts and Grignard reactions are the most commonly used in academics for development of P-C bonded flame retardants. Use of green reagents, catalytic reactions, microwave technology, photo initiated reactions, etc. are increasingly used by researchers to synthesize P-C containing organophosphorus compounds; however, these techniques are seldom utilized in development of flame retardant compounds. Though economics for these new routes for synthesis is important for future industrial exploitation, concerns for the environment and new regulations will help their acceptance in industrial exploitation. In our opinion, some synthetic strategies like Michael addition, Friedel–Crafts reaction and Michaelis–Arbuzov reaction can be industrially exploited in industry, taking in their advantages like higher yields, selectivity and simplicity of reaction. However, other reactions discussed in this review are also commercially feasible if the properties of the flame retardant significantly outweigh its cost of production. One can find a vast amount of literature in development of P-C containing flame retardants with detailed investigations of their flame retardant properties and potential applications in various polymers. In most cases, such flame retardants exhibit satisfactory to excellent fire performance in the polymers, and questions regarding their impact on their processability, mechanical performance, and long-term serviceability of the final products need more attention. As organophosphorus compounds are also toxic (used as pesticides, herbicides, insecticides, nerve gasses), one has to pay attention while developing new structures. Toxicity screening of the new compounds should be carried out early in their development to prevent accidents and potential issues relate to toxicity. Research on ways to hydrolyze P-C bonds will help reduce their potential environmental impact and biocompatibility.

Author Contributions: Sophie Wendels, Thiebault Chavez, Martin Bonnet, Khalifah A. Salmeia and Sabyasachi Gaan contributed to the writing of the review manuscript. Khalifah A. Salmeia and Sabyasachi Gaan led and structured the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

ABS Acrylonitril butadiene styrene AOPH Aluminium–organophosphorus hybrids BKZ-VB Swiss standart fire test (vertical specimen) CPPPA 4-Carboxyphenylphenylphosphinic acid DBU 1,8-Diazabicyclo(5.4.0)undec-7-ene DDM Diaminodiphenylmethane DETDA Diethyldiamino toluene DFDPS 4,4-Difluorodiphenylsulfone DGEBA Diglycidyl ether of bisphenol-A epoxy resin

Materials 2017, 10, 784 26 of 32

Michael addition, Michaelis–Arbuzov, Friedels–Crafts and Grignard reactions are the most commonly used in academics for development of P-C bonded flame retardants. Use of green reagents, catalytic reactions, microwave technology, photo initiated reactions, etc. are increasingly used by researchers to synthesize P-C containing organophosphorus compounds; however, these techniques are seldom utilized in development of flame retardant compounds. Though economics for these new routes for synthesis is important for future industrial exploitation, concerns for the environment and new regulations will help their acceptance in industrial exploitation. In our opinion, some synthetic strategies like Michael addition, Friedel–Crafts reaction and Michaelis–Arbuzov reaction can be industrially exploited in industry, taking in their advantages like higher yields, selectivity and simplicity of reaction. However, other reactions discussed in this review are also commercially feasible if the properties of the flame retardant significantly outweigh its cost of production. One can find a vast amount of literature in development of P-C containing flame retardants with detailed investigations of their flame retardant properties and potential applications in various polymers. In most cases, such flame retardants exhibit satisfactory to excellent fire performance in the polymers, and questions regarding their impact on their processability, mechanical performance, and long-term serviceability of the final products need more attention. As organophosphorus compounds are also toxic (used as pesticides, herbicides, insecticides, nerve gasses), one has to pay attention while developing new structures. Toxicity screening of the new compounds should be carried out early in their development to prevent accidents and potential issues relate to toxicity. Research on ways to hydrolyze P-C bonds will help reduce their potential environmental impact and biocompatibility.

Abbreviations

ABS Acrylonitril butadiene styrene AOPH Aluminium–organophosphorus hybrids BKZ-VB Swiss standart ﬁre test (vertical specimen) CPPPA 4-Carboxyphenylphenylphosphinic acid DBU 1,8-Diazabicyclo(5.4.0)undec-7-ene DDM Diaminodiphenylmethane DETDA Diethyldiamino toluene DFDPS 4,4-Diﬂuorodiphenylsulfone DGEBA Diglycidyl ether of bisphenol-A epoxy resin DIP-MS Direct insertion probe mass spectroscopy DOPO 10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide DPPO 2,8-Dimethyl-phenoxaphosphine-10-oxide DSC Differential scanning calorimetry EDX Energy-dispersive X-ray spectroscopy EVA Ethylene-vinyl acetate FPUF Flexible polyurethane foams FR Flame retardant FTIR Fourier Transform Infrared Spectroscopy HAPP Hexaallyltriphosphazene KMnO4 Potassium permanganate LOI Limiting oxygen index MCC Microscale Combustion Calorimetry m-CPBA meta-Chloroperoxybenzoic acid MMPP Methyl methylphenylphosphinate m.p. Melting point Materials 2017, 10, 784 27 of 32

N2 Nitrogen PA6 Polyamide 6 PC Polycarbonate PCFC Pyrolysis-combustion ﬂow calorimetry PEEK Polyether ether ketone PEK Polyether ketones PES Poly(ethersulfone) PHRR Lower peak heat release rate Poly(ethylene glycol) methacrylate/methyl methacrylate/diethyl PMMD allylphosphonate PN-TLCP Thermotropic liquid crystal polymer POSS Polyhedral oligomeric silsesquioxane PSU Polysulfones PU Polyurethane Py-GC-MS Pyrolysis-gas chromatography mass spectroscopy ROMP Ring opening metathesis polymerisation RPUF Rigid polyurethane foams SEM Scanning electron microscope SET Self-extinguishing time TCPP Tris(1-chloro-2-propyl) phosphate Tdx% Temperature for x% weight loss TEOS Tetraethyl orthosilicate TGA Thermogravimetric analysis THR Total heat release TPA Terephthalic acid TSR Total smoke release Standard for Tests for Flammability of Plastic Materials for Parts in Devices and UL94 HB Appliances (horizontal specimen) Standard for Tests for Flammability of Plastic Materials for Parts in Devices and UL94 VA or UL94 VB Appliances (vertical specimen) UV Ultraviolet WAXD Wide-angle X-ray scattering XPS X-ray photoelectron spectroscopy

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