Flame retardant polymeric materials achieved by incorporation of styrene monomers containing both nitrogen and phosphorus

Polymer Degradation and Stability 97 (2012) 2611e2618

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Flame retardant polymeric materials achieved by incorporation of styrene monomers containing both nitrogen and phosphorus Adina Dumitrascu*, Bob A. Howell 1 Center for Applications in Polymer Science, Central Michigan University, Mt. Pleasant, MI 48859-0001, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 December 2011 Received in revised form 4 January 2012 Accepted 4 January 2012 Available online 27 July 2012

Organophosphorus compounds have a long history as effective flame retardants for polymeric materials. There is increasing interest in developing new phosphorus flame retardants as a consequence of the increasing worldwide regulatory pressure on the use of organohalogen flame retardants which are persistant in the environment, tend to bioaccumulate, and pose potential health risks for the human population. It has been widely suggested that the flame retardant activity of phosphorus compounds may be enhanced by the presence of nitrogen, sulfur, silicon and a few other elements. This has been tested by examining the flamability behavior of copolymers derived from styrene monomers containing phosphorus moieties in comparison to those containing similar monomers in which both phosphorus and nitrogen are present. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Flame retardance Styrenic Phosphorus Phosphoramidate Phosphonate

1. Introduction Poly(styrene) is a commodity plastic encountered in daily life since it is used in numerous applications from computer housings to components for cars and airplanes, to toys and many household appliances and building insulation (expanded polystyrene foam). As with many other polymers, poly(styrene) suffers from high flamability and lack of charrability which limits its use for many applications. Flame-retardant additives such as halogenated aromatics or organic and inorganic phosphorus compounds are commonly used to impart fire retardance to styrenic polymers. However, additive systems have the disadvantages of promoting undesirable changes in physical and mechanical properties of the polymer and of poor durability in long term use. Moreover, halogenated flame retardants have come under scrutiny because of perceived environmental and toxicological issues and there are movements to discontinue the use of halogen containing flame retardants [1,2]. One of the most promising alternatives to halogenated flame retardants seems to be organophosphorus compounds that can be chemically attached directly to the polymer chain. Several reports on the effect of the incorporation of phosphorus-containing * Corresponding author. Tel.: þ1 989 832 5555; fax: þ1 989 832 5560. E-mail addresses: [email protected] (A. Dumitrascu), bob.a.howell@ cmich.edu (B.A. Howell). 1 Tel.: þ1 989 774 3582; fax: þ1 989 774 3883. 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.012

styrenic comonomer units on the flame retardance of poly(styrene) have revealed significant increases in limiting oxygen index (LOI) together with increases in char yields on combustion of the polymers [3e8]. However, although some copolymers showed good flame retardant performance, no commercially valuable systems has come yet from this research [1]. It was noticed that in order to obtain UL-94 V-0 rating with commercial phosphorus FRs for styrenic plastics including ABS and HIPS, the additional incorporation of halogen-based phosphorus derivatives (e.g. aliphatic chlororethyl phosphonate), or a charring agent along with phosphorus FRs is inevitable [9]. Another very promising lead to flame-retard styrenics is the use of nitrogenephosphorus compounds which are known to evolve little toxic gases or vapors as well as low levels of smoke during combustion and to be more convenient from a recyclability standpoint [9,10]. Among this type of compounds, the phosphoramidates are particularly promising since they showed a great potential as FRs for various polymeric systems, can be easily manufactured from relatively inexpensive raw materials with high yields, are less volatile as compared to analogous phosphates, have good thermal stability and exhibit enhanced char formation during the burning process [11,12]. The development of newer phosphoramidate compounds became even more important due to possible phosphorusenitrogen synergism phenomena. Encouraged by the recent findings, a series of four phosphoramidate-based styrene monomers was synthesized and fully characterized. Each monomer was homopolymerized in solution to

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provide a series of four new styrenic homopolymers. The thermal properties and fire behavior of this series were compared with an analog series of phosphorus-based styrenic homopolymers lacking the nitrogen atom.

O R1

P

H

CH2Cl

NaH

O

THF

THF CH2

R2

P

R1

R2

2. Experimental O

2.1. Materials R1

Diethylphosphite (98%), diphenyl phosphite (w85%), 4-tertbutylcatechol (TBC) and sodium hydride (NaH) as 60% dispersion in mineral oil were purchased from the Aldrich Chemical Company and dimethyl phosphite (98%) from Alfa-Aesar, and used as received. 4-(Chloromethyl)styrene (min. 90%), 4-aminostyrene and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were supplied by TCI America and used as received. Styrene (99%, Sigma Aldrich) was passed through a column of basic alumina prior to use, in order to remove polymerization inhibitors. 2,20 -Azobis(isobutyronitrile) (AIBN) (Sigma Aldrich), which was used as a radical initiator, was recrystallized from methanol. All solvents and other common reagents were obtained from usual laboratory suppliers and were purified prior to use by standard methods. In general, reactions were carried out in a dry three-necked, round-bottomed flask fitted with a Liebeg condenser bearing a gas-inlet tube, a magnetic stirring bar, and a pressure-equalizing dropping funnel. All glassware were flame-dried and allowed to cool under a stream of dry nitrogen prior to use. Chromatography was accomplished using ultrapure synthetic silica gel SilaFlash P60 (230e400 mesh silica) in a column of appropriate size, and combinations of hexane and ethyl acetate as eluents. Silica-coated (fluorescence UV254) Whatman plates (Whatman Ltd.) were used for thin layer chromatography (TLC). 2.2. Monomer synthesis The phosphorus- and phopshorus and nitrogen-containing monomers synthesized are: dimethyl-(4-vinylbenzyl)phosphonate (M1), diethyl-(4-vinylbenzyl)phosphonate (M2), diphenyl-(4vinylbenzyl)phosphonate (M3), 4-[(6-Oxido-6H-dibenz[c, e] [1,2] oxa-phosphorin-6-yl)]methyl styrene (M4), phosphoramidic acid N-(4-vinyl-phenyl)-dimethylester (M5), phosphoramidic acid N-(4vinyl-phenyl)-diethylester (M6), phosphoramidic acid N-(4-vinylphenyl)-diphenylester (M7) and 4-[(6-Oxido-6H-dibenz[c, e] [1,2] oxaphosphorin-6-yl)]amino styrene (Fig. 1). The details of the monomer synthesis are given below (Fig. 2).

H2C

R1 P O

R2

P

CCl4 ,

NH2 O

H Et3N

R2

NH

P

R1

R2

Fig. 2. Synthesis of styrene monomers containing flame-retarding moieties.

2.2.1. Dimethyl-(4-vinylbenzyl)phosphonate (M1) [13,14] Sodium hydride (0.55 g, 0.024 mmol) as a suspension in 60% mineral oil was added to 20 mL dry THF and the suspension was stirred at room temperature, under nitrogen for 0.5 h. A solution of dimethyl phosphite (2.6 g, 0.024 mol) and 10 mL THF was added slowly and the mixture was allowed to cool at 0e5  C and stirred for 0.5 h 4-(Chloromethyl)styrene (3 g, 0.02 mol) and TBC (100 mg, 0.6 mmol) in 25 mL of THF were added dropwise. The reaction mixture was stirred at ambient temperature for 2 h and then at the solvent reflux for 12 h. The salt which formed during the reaction was removed by filtration. The solvent was removed from the filtrate by rotary evaporation at reduced pressure. The residual material was dissolved in 100 mL of methylene chloride. The solution was washed, successively, with two 100-mL portions of distilled water and 100 mL of saturated aqueous sodium chloride solution. The solution was dried over anhydrous magnesium sulfate and the solvent was removed by rotary evaporation at reduced pressure. The residue was purified by flash chromatography (eluant: ethyl acetate/hexane 50/50, v/v). The pure product was obtained as a pale yellow oil (4.02 g, yield 90.13%):IR (thin film, cm1) 3003 (w, AreH), 2909 (m, CH2), 1630 (m, vinyl), 1515 (C¼Caromatic), 1410 (m, PeC), 1250, vs (P]O), 1030 (vs, PeOeC); 1H NMR (300 MHz, CDCl3, d) 2.75 (d, 2H, JPH ¼ 21.7 Hz, CH2eP); 3.1 (d, 6H, JPH ¼ 10.89 Hz, CH3eO); 5.09 (d, 1H, JHH ¼ 10.8 Hz, CH]CH2); 5.42(d, 1H, JHH ¼ 17.6 Hz, CH]CH2); 6.61e6.71 (dd, 1H, cis JHH ¼ 10.8 Hz, trans J ¼ 17.6 Hz, CH]CH2); 6.95e7.33 (m, 4Haromatic); 13 C NMR (75.46 MHz, CDCl3, d) 33.5 (d, JPC ¼ 127.8 Hz, CH2eP), 53.3 (d, JPC ¼ 6.8 Hz, CH3eO); 113.6 (1C, CH]CH2), 130.04, 131.10 (d, J ¼ 9.8 Hz, CH]CH2), 126.15, 126.25 (d, JPC ¼ 3.45 Hz, 1Caromatic), 130.5, 136.3 (2C aromatic); 31P NMR (121.48 MHz, CDCl3, d) þ 34.5; GCeMS: calcd. for C11H15O3P: 226, found: 226.

HN

P O

R1 R2

R1= R2 = OCH3 Dimethyl-(4-vinylbenzyl)phosphonate (M1)

R1= R2 = OCH3 Phosphoramidic acid, N-(4-vinyl-phenyl)-dimethylester (M5)

R1= R2 = OC2H5 Diethyl-(4-vinylbenzyl)phosphonate (M2)

R1= R2 = OC2H5 Phosphoramidic acid, N- (4-vinyl-phenyl)-diethylester (M6)

R1= R2 = OC6H5 Diphenyl-(4-vinylbenzyl)phosphonate (M3)

R1= R2 = OC6H5 Phosphoramidic acid, N- (4-vinyl-phenyl)-diethylester (M7)

R1, R2 = DOPO 4-[(6-Oxido-6H-dibenz[c, e][1, 2]oxaphosphorin-6-yl)]methyl styrene (M4)

R1, R2 = DOPO 4-[(6-Oxido-6H-dibenz[c, e][1, 2]oxaphosphorin-6-yl)]amino styrene

Fig. 1. The monomers synthesized and used in homopolymerization.

(M8)

A. Dumitrascu, B.A. Howell / Polymer Degradation and Stability 97 (2012) 2611e2618

2.2.2. Diethyl-(4-vinylbenzyl)phosphonate (M2) [8,13e23] The synthesis of this compound was performed according to reported procedures [13e23] as previously described [8]. 2.2.3. Dipheyl-(4-vinylbenzyl)phosphonate (M3) [8,17,24] The synthesis and characterization of this compound were adapted from reported procedures [17,24] as previously described [8]. 2.2.4. 4-[(6-Oxido-6H-dibenz[c, e] [1,2]oxaphosphorin-6-yl)]methyl styrene (M4) Sodium hydride (2.0 g as 60% dispersion in mineral oil, 0.051 mol) in 50 mL THF was stirred at 0e5  C under nitrogen for 0.5 h. DOPO (10 g, 0.046 mol) dissolved in 150 mL THF was added dropwise and the reaction mixture was stirred at room temperature for 2 h and then a solution of 4-(chloromethyl)styrene (7.76 g, 0.051 mol) and TBC (200 mg, 1.2 mmol) in THF (50 mL) was added slowly. The resultant mixture was stirred at solvent reflux under nitrogen for 48 h. The white salt which formed during the reaction was removed by filtration through Cellite and a portion of the solvent (approximately 100 mL) was removed by rotary evaporation. To the remaining solution, 100 mL of hexane was added and the white crystals which formed were separated by filtration. The crude product was purified by flash chromatography (eluant: ethyl acetate/hexane 70/30, v/v) to afford 9.8 g (63.7% yield) of white crystals, mp 127.6  C (DSC) [Lit. 24 : mp 56  C ]: IR (thin film, cm1); 1628 (m, vinyl group); 1238 (s, PeOeCaryl); 1206, 1118 (s, m, P]O and PeOeC, respectively), 1477 (PePh); 1H NMR (300 MHz, CDCl3, d) 3.27e3.44 (m, 2H, PeCH2) 5.17e5.20 (d, J ¼ 10.8, 1H, CH]CH2); 5.62e5.68 (d, J ¼ 17.6, 1H, CH]CH2); 6.56e6.65 (dd, 1H, CH]CH2 cis JHH ¼ 10.8 Hz, trans JHH ¼ 17.6 Hz); 6.98e7.02 (m, 2Haromatic); 7.14e7.26 (m, 4Haromatic); 7.31e7.44 (m, 2Haromatic); 7.62e7.74 (m, 2Haromatic); 7.81e7.89 (m, 2Haromatic); 13C NMR (75.46 MHz, CDCl3, d) 36.13, 37.34 (d, JPC ¼ 91 Hz, CH2eP); 113.7 (CH]CH2); 120.2, 120.3 (d, JPC ¼ 9.8 Hz, 1Caromatic from DOPO), 121.9, 122.1 (d, JPC ¼ 11 Hz, 1Caromatic from DOPO, CH2eP(O)eCaromatic), 122.9 (1Caromatic from DOPO) 123.4, 123.6 (d, JPC ¼ 11 Hz, 1Caromatic from DOPO), 124.4 (1Caromatic from DOPO), 124.5 (1Caromatic from DOPO), 125.1 (1Caromatic from DOPO), 126.1, 126.2 (d, J ¼ 3.5 Hz, 1Caromatic), 128.1, 128.3 (d, JPC ¼ 13.3 Hz, 1Caromatic from DOPO), 129.4, 129.5 (d, JPC ¼ 8.6 Hz, 1Caromatic from DOPO), 129.9, 130 (d, JPC ¼ 6.3 Hz, 1Caromatic from DOPO), 130.5 (1Caromatic from DOPO), 130.6, 130.7 (d, J 9.8, 1Caromatic, ]CaromaticeCH2e) 133.3, 133.4 (d, JPC ¼ 2.3 Hz, CH]CH2), 135.8, 135.9 (d, JPC ¼ 6.9 Hz, 1Caromatic, ]CaromaticeCH]CH2), 136.2 (1Caromatic from DOPO), 136.3 (1Caromatic from DOPO), 149.4, 149.5 (d, JPC ¼ 8.1 Hz, 1Caromatic from DOPO, eP(O)e OeCaromatic); 31P NMR (121.48 MHz, CDCl3, d) þ34.5; GCeMS: calcd. for C21H17O2P: 332, found: 332. 2.2.5. Phosphoramidic acid, N-(4-vinyl-phenyl)-dimethylester (M5) A stirred mixture of dimethyl phosphite (3.3 g, 0.03 mol) and 30 mL of carbon tetrachloride was allowed to cool to 0  C, in a nitrogen atmosphere. 4-Aminostyrene (3.0 g, 0.025 mol) and TBC (100 mg, 0.6 mmol) dissolved in 25 mL THF were then added and the mixture was stirred for 1 h. Triethylamine (TEA) (3 g, 0.03 mol) in 10 mL of carbon tetrachloride was added dropwise and the resulting solution was stirred overnight. After approximately 20 h, TLC (eluant: ethyl acetate) indicated that 4-aminostyrene had been completely consumed. The triethylammonium chloride formed in the reaction was removed by filtration and washed with 30 mL of carbon tetrachloride. The organic solution was washed successively with two 100-mL portions of distilled water and 100 mL saturated aqueous sodium chloride solution. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed by rotary evaporation at reduced pressure. The residue was purified by column chromatography using a mixture of ethyl acetate:hexane

2613

(50:50, v/v) as eluant to provide 5.1 g of phosphoramidic acid, N-(4vinyl-phenyl)-dimethylester (89.5% yield) as a colorless liquid: IR (thin film, cm1) 3200 (NeH), 1610 (s, vinyl group); 1025, 1400 (s, m PeOealkyl); 1520 (yC]C in the phenyl ring); 1167 (P]O stretching); 1230, 840 (m, vs, yCeN and yPeN); 1H NMR (300 MHz, CDCl3, d) 3.7 (d, JPH ¼ 11.4 Hz, OCH3); 5.12 (d, 1H, JHH ¼ 10.8 Hz, CH]CH2); 5.61 (d, 1H, JHH ¼ 17.6 Hz, CH]CH2); 6.59e6.68 (dd, 1H, cis JHH ¼ 10.8 Hz, trans J ¼ 17.6 Hz, CH]CH2); 6.75 (d, JPH ¼ 9, NH); 6.96e6.99 (dd, 2Haromatic); 7.28e7.31 (dd, 2Haromatic); 13C NMR (75.46 MHz, CDCl3) 53.10e53.15 (d, JPC ¼ 5.2 Hz, CH3 from phosphoramidate P(O)CH3); 112.0 (CH]CH2); 117.25e117.34 (d, JPC ¼ 6.79 Hz, CaromaticeP); 127.2, 131.4, 136.1, 139.2 (s, 4Caromatic); 31P NMR (121.48 MHz, CDCl3, d) þ6.6; GCeMS: calcd. for C10H14NO3P: 227, found: 227. 2.2.6. Phosphoramidic acid, N-(4-vinyl-phenyl)-diethylester (M6) [25] A stirred solution of diethyl phosphite (2.1 g, 0.015 mol) in 30 mL of carbon tetrachloride was cooled to near 0  C (ice bath). A solution of 4-aminostyrene (1.5 g, 0.012 mol) and TBC (100 mg, 0.6 mmol) in 25 mL of carbon tetrachloride was added and the mixture was stirred for 1 h at 0  C. A solution of triethylamine (1.53 g, 0.015 mol) in 10 mL of carbon tetrachloride was added dropwise and the mixture allowed to warm to room temperature and stirred overnight. The same work-up as for the monomer M5 was performed and the crude product was purified by column chromatography (eluant: ethyl acetate/hexane 50/50e100/0, v/v) to provide 2.8 g of the product (87.5% yield) as a viscous, slightly yellow liquid: IR (thin film, cm1) 3182 (NeH), 1612 (s, vinyl group); 1025, 1397 (s, m PeOalkyl); 1515 (yC]C in the phenyl ring); 1299, 1165 (P]O stretching); 1229, 840 (m, vs, yCeN and yPeN); 1H NMR (300 MHz, CDCl3, d) 1.28e1.3 (two dt, 6H, CH2CH3); 4.03e4.2 (multiplet, 4H, PeCH2); 5.10e5.14 (dd, 1H, CH]CH2); 5.58e5.64 (dd, 1H, CH]CH2); 6.58e6.68 (dd, 1H, cis JHH ¼ 10.8 Hz, trans J ¼ 17.6 Hz, CH]CH2); 6.83 (d, JNeH ¼ 9.7, 1H, NH); 6.97e7.01 (m, 2Haromatic); 7.26e7.31 (m, 2Haromatic); 13C NMR (75.46 MHz, CDCl3, d) 16.01, 16.11 (d, JPC ¼ 7.54 Hz, CH3 from from phosphoramidate group); 62.64, 62.71 (d, JPC ¼ 5.2 Hz, CH2 from phosphoramidate group); 111.73 (CH] CH2); 117.15, 117.25 (d, JPC ¼ 7.5 Hz, CaromaticeP); 127.11, 131,2 136.15 139.6 (s, 4Caromatic); 31P NMR (121.48 MHz, CDCl3, d) þ3.44; GCeMS: calcd. for C12H18NO3P: 255, found: 255. 2.2.7. Phosphoramidic acid, N-(4-vinyl-phenyl)-diphenylester (M7) A solution of diphenyl phosphite (2.57 g, 0.11 mol) in 250 mL of carbon tetrachloride was cooled to 0  C under a nitrogen atmosphere with stirring. A solution of 4-aminostyrene (1.2 g, 0.01 mol) and TBC (100 mg, 0.6 mmol) in 25 mL of THF was added and the mixture stirred for 0.5 h. A solution of triethylamine (1.1 g, 0.011 mol) in 25 mL of carbon tetrachloride was added dropwise over a period of 1 h. The reaction mixture was allowed to warm to room temperature and stirred until 4-aminostyrene was completely consumed (TLC: ethyl acetate/hexane 50/50, v/v). Following a similar work-up as in the case of monomer M6, 3.4 g of pure compound (96.6% yield) was obtained after purification by column chromatography (ethyl acetate/hexane ¼ 50/50, v/v), as a viscous colorless liquid, which solidified upon standing at 0  C: IR (thin film, cm1) 3200 (br, s, NH), 1628 (s, vinyl group); 1026, 1397 (m, s, PeO-aryl); 1299, 1162 (s, m, P]O); 1235, 840 (m, vs, yCeN and yPeN). 1H NMR (300 MHz, CDCl3, d) 5.20 (d, 1H, JHH ¼ 10.9 Hz, CH] CH2); 5.70 (d, 1H, JHH ¼ 17.6 Hz, CH]CH2); 6.66e6.76 (dd, 1H, cis JHH ¼ 10.8 Hz, trans J ¼ 17.6 Hz, CH]CH2); 7.11e7.37 (m, 4Haromatic); 7.66 (d, JNeH ¼ 10.8, 1H, NH); 13C NMR (75.46 MHz, CDCl3, d) 112.07 (CH]CH2); 117.94, 118.05 (d, JPC ¼ 8.1 Hz, 1Caromatic), 120.2, 120.27 (d, JPC ¼ 3.8 Hz, 1Caromatic); 125.25, 127.1, 129.66, 131.57, 136.11, 138.67 (6Caromatic); 150.06, 150.15 (d, JPC ¼ 6.3 Hz, PeOeCaromatic); 31 P NMR (121.48 MHz, CDCl3, d) 5.1; GCeMS: calcd. for C20H18NO3P: 351, found: 351.

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2.2.8. 4-[(6-Oxido-6H-dibenz[c, e] [1,2]oxaphosphorin-6-yl)]amino styrene(M8) DOPO (2.16, 0.01 mol) was dissolved in a mixture of 25 mL of carbon tetrachloride and 15 mL of THF and the solution was cooled to 0  C under a nitrogen atmosphere with stirring. 4-Aminostyrene (1.31 g, 0.011 mol) in 10 mL CCl4 was added at once. A solution of triethylamine (1.1 g, 0.011 mol) in 15 mL of CCl4 was added over a period of 10 min and the reaction mixture was then stirred at room temperature for approximately 40 h. To the suspension formed during the reaction, 75 mL distilled water was added and the organic layer was separated and washed with two 50-mL portions of distilled water and 50 mL of saturated aqueous sodium chloride solution. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed by rotary evaporation at reduced pressure. The residue was purified by column chromatography using a mixture of ethyl acetate:hexane (75:25, v/v) as eluant to afford 2.9 g of 4-[(6-oxido-6H-dibenz[c,e] [1,2]oxaphosphorin-6-yl)]amino styrene (87.9% yield) as a colorless liquid which solidified upon standing in the freezer: IR (thin film, cm1) 3138 (br, NeH), 1610 (s, vinyl group); 1513 (yC]C in the phenyl ring); 1297, 1150 (m, m, P]O stretching); 1219, 839 (vs, m, yCeN and yPeN); 1H NMR (300 MHz, CDCl3, d) 5.04 (d, J ¼ 10.8, 1H, CH]CH2); 5.5 (d, J ¼ 17.6, 1H, CH]CH2); 6.46e6.56 (dd, 1H, CH]CH2 cis JHH ¼ 10.8 Hz, trans JHH ¼ 17.6 Hz); 7.1 (d, JNeH ¼ 8.7, 1H, NH); 7.26e8.09 (m, 12Haromatic); 13C NMR (75.46 MHz, CDCl3, d); 111.9 (1C, CH]CH2); 118.23, 118.32 (d, JPC ¼ 9.91 Hz, 1Caromatic styrene), 120.75, 120.84 (d, JPC ¼ 6.3 Hz, 1Caromatic from DOPO), 121.72, 121.88 (d, JPC ¼ 11.5 Hz, 1Caromatic from DOPO); 122.63 4 (1Caromatic from DOPO); 123.72, 123.88 (d, JPC ¼ 11.9 Hz, 1Caromatic from DOPO); 124.8 (1C, ] CaromaticeCH]CH2), 124.65, 124.96 (d, JPC ¼ 23 Hz, 1Caromatic from DOPO); 126.94 (1Caromatic from DOPO), 128.45, 128.64 (d, JPC ¼ 14.9, 1Caromatic from styrene), 130.13, 138.25 (d, J ¼ 9.7 Hz, 1Caromatic from DOPO), 130.45 (1Caromatic from DOPO), 131.47 (1Caromatic from DOPO), 133.13, 133.16 (d, JPC ¼ 2.3 Hz, 1Caromatic from DOPO), 135.99 (1C, CH] CH2); 136.79, 136.89 (d, JPC ¼ 6.9 Hz, 1Caromatic from DOPO), 139.15 (eP(O)eNHeCaromatic); 149.61, 149.70 (d, JPC ¼ 6.9 Hz, 1Caromatic from 31 DOPO, eNHeP(O)eCaromatic); P NMR (121.48 MHz, CDCl3, d) þ34.5; GCeMS: calcd. for C21H17O2P: 332, found: 332. 2.3. Synthesis of homopolymers In a typical preparation a solution in N,N-dimethylacetamide (DMAC) of monomer (40% wt) and initiator (0.5% mol AIBN/mol monomer) was placed in a Pyrex polymerization tube, sealed under nitrogen, and heated in an oil bath for a period of 3 h at 70  C. Subsequently, the tube was transferred to an oven and kept at 110  C for 16 h, to provide a viscous solution of polymer. The polymer was separated by precipitation in water. The crude polymer was dried and re-dissolved in DMAC and then precipitated by the addition of hexane. The polymer was washed repeatedly with hexane then dried at 15 torr and 50  C for 24 h. The purity of the polymers was assesed by 1H NMR spectroscopy which showed no traces of monomer. 31P NMR (121.48 MHz, d) of the polymers: HMP1 (CDCl3): þ30.2; HPM2 (CDCl3): þ5.9; HPM3 (CDCl3): þ21.1; HPM4 (CDCl3): þ34.4; HPM5 (DMSO-d6): þ7.9; HPM6 (DMSOd6) þ3.9; HPM7 (DMSO-d6): 5.2; HPM8 (DMSO-d6): þ9.4. The two series comprising eight homopolymers of phosphorus and phosphorusenitrogen based styrenic monomers are illustrated in Table 1. 2.4. Analytical techniques Nuclear magnetic resonance (NMR) spectra were obtained using 10%e25% solutions in deuterochloroform and a Varian Mercury 300 MHz spectrometer. Proton and carbon chemical shifts (d)

Table 1 Phosphorus- and phosphorusenitrogen-based styrenic homopolymers.

n

H2C

n

R1 O

P R2

O

Homopolymer code name R1]R2]OCH3 R1]R2]OC2H5 R1]R2]OC6H5 R1, R2]DOPO

HPM1 HPM2 HPM3 HPM4

R1

HN

P

R2

Homopolymer code name R1]R2]OCH3 R1]R2]OC2H5 R1]R2]OC6H5 R1, R2]DOPO

HPM5 HPM6 HPM7 HPM8

were reported in parts-per-million (ppm) with respect to tetramethylsilane (TMS) as internal reference (d ¼ 0.00 ppm). 31P NMR spectra were referenced relative to the signal for triphenylphosphate at d : 18.0. Infrared (IR) spectra were obtained using thin films between sodium chloride plates or solid solutions (1%) in anhydrous potassium bromide (as discs) and a Nicolet MAGNA-IR 560 spectrometer. Absorptions were recorded in wavenumbers (cm1), and absorption intensities were classified in the usual fashion as very weak (vw), weak (w), medium (m), strong (s), and very strong (vs) relative to the strongest band in the spectrum. Mass spectra were obtained using a HewlettePackard 5890A gas chromatograph/mass spectrometer (GCeMS) with an ionizing potential of 70 electron volts and temperature programmed elution into the spectrometer inlet (90e200  C). Melting points were determined by differential scanning calorimetry (DSC) using a PYRIS Diamond DSC (PerkinElmer). All samples were analyzed in a constant nitrogen purge of 50 mL/min at a heating rate of 5  C/min. Thermogravimetric analysis (TGA) was performed using a TA Instruments 2950 Hi-Res TGA instrument interfaced with the Thermal Analyst 2100 control unit. Most generally, a heating rate of 10  C/min was used. Samples (5e10 mg) were contained in a standard platinum pan. The sample compartment was purged with dry nitrogen at 50 mL/min during analysis. TA Universal Analysis software was used for data analysis. Glass transition temperatures (Tg) were determined using a DSC Q 2000 (TA Instruments). The samples were contained in standard aluminum DSC pans. Samples were heated from 20 to 200  C in a nitrogen atmosphere at a rate of 10  C/min. The Tgs were obtained from the second heating cycle, using TA Universal Analysis software. Pyrolysis combustion flow calorimetry (PCFC) was performed using a Govmark microscale combustion calorimeter [26e29]. Small polymer samples (1e10 mg) were dried for at least 8 h at 75  C in a convection oven and held in a desiccant chamber until testing. The samples were heated at a constant rate (1  C/s) from 150 to 700  C. The volatile pyrolysis products generated during the temperature ramp were swept from the pyrolyzer into the combustion chamber (900  C) by nitrogen gas flowing at 80 cm3/min to which was added 20 cm3/min of pure oxygen. Deconvolution of the oxygen consumption signal was performed during the test, and the heat release rate was calculated in watts per gram of sample.

A. Dumitrascu, B.A. Howell / Polymer Degradation and Stability 97 (2012) 2611e2618

Fig. 3. Polymerization of styrene monomers containing flame-retarding moieties.

3. Results and discussion Phosphorus compounds are commonly used as flame retardants for polymeric materials. It has widely been suggested that the effectiveness of these flame retardants is enhanced in the presence of a nitrogen-containing compound. To directly assess the impact of the presence of nitrogen on the effectiveness of phosphorus compounds as flame retardants in styrene polymers, two series, one containing only a phosphorus moiety and the second containing both phosphorus and nitrogen, have been prepared, characterized, and evaluated for potential flame retarding effect. Monomers were prepared from 4-chloromethylstyrene or 4aminostyrene as illustrated below: Polymers were obtained from AIBN-initiated radical polymerization of the monomers (Fig. 3).

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The chemical structures of the polymers were characterized spectroscopically. The infrared of phosphorus-containing copolymer contains additional specific absorptions not present in the poly(styrene) spectrum at 1200e1275 cm1 (very strong, eP]O), 1116 and 967 cm1 (ePeOePh) and 1030 cm1 (ePeOeC), reflecting the presence of the phosphonate groups pendant to the polymer chain and peaks at 1000e1250, 870 due to yCeN, yPeN absorptions. In the phosphoramidates series, the presence of a broad absorption at about 3200 cm1 is noticed, due to the NeH stretch (Fig. 4). The 1H NMR spectra (Fig. 5) the following characteristic peaks may be noted: d z 1.4 (PheCHeCH2); d z 1.8, (PheCHeCH2), aromatic protons from former vinyl segments, at d z 6.5 and d z 7.05; methyl protons (PeCH2eCH3) from diethylphosphonate groups at d z 1.2e1.25; methylene protons (PeCH2eCH3) from phosphonate groups at 4e4.25 ppm; benzylic PheCH2eP signals at d z 3e3.4, aromatic protons from diphenylphosponate residues at 7.2e7.3 ppm and signals at d z 4 due to NH protons from phosphoramidate pendants. The glass transition temperatures (Tg) for these polymers (Table 2) are generally higher that for poly(styrene). This may be reflective of strong intermolecular interactions between the polar groups pendant to the polymer mainchain. Thermal decomposition of the polymers bearing only phosphorus-containing pendants is depicted in Fig. 6. As can be seen, the onset temperature for degradation is higher for the polymers with phosphorus-containing pendant groups than for unsubstituted poly(styrene). Further, the residue from degradation of the phosphorus-containing polymer is significantly greater (16e27% of the initial sample mass) than for the degradation of unsubstituted poly(styrene) (0%). The degradation of the polymers bearing pendants groups containing both phosphorus and nitrogen is depicted in Fig. 7. Again, the onset temperature for degradation of the polymers with phosphorus/ nitrogen containing pendant groups is generally higher than that for unsubstituted poly(styrene). Degradation of these polymers

Fig. 4. Infrared spectra of polystyrene and homopolymers HPM3, HPM4 and HMP6.

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Fig. 5. 1H-NMR spectra of polystyrene (CDCl3) and homopolymers HPM3 (CDCl3), HPM4 (CDCl3) and HMP6 (DMSO-d6).

also leads to significant levels of residual material (25e40% of the initial sample mass). Not only is the level of residue much higher than that for unsubstituted poly(styrene) but is significantly greater than that for degradation of polymers with pendant groups containing only phosphorus. Based on these simple observations, it would seem that the beneficial impact of phosphorus on the

Table 2 Glass transition temperatures for poly(styrene) and phosphorus- and phosphorusenitrogen-containing styrenic polymers. Polymer

Tg [ C]

Polymer

Tg [ C]

HPM1 HPM2 HPM3 HPM4 PS

122.4 130.2 64.3 153.4 102

HPM5 HPM6 HPM7 HPM8 PS

131.5 132.4 136.1 126.2 102

Fig. 6. Thermal degradation of styrene polymers bearing phosphorus-containing pendant groups.

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Fig. 7. Thermal degradation of styrene polymers bearing pendant groups containing both phosphorus and nitrogen.

Table 3 Thermal degradation data for the polymers from phosphorus- and phosphorusenitrogen-containing monomers. Sample

Tonset [ C]

Mass loss [%]

Residue at 600  C [%]

Sample

Tonset [ C]

Mass loss [%]

Residue at 600  C [%]

HPM1 HPM2 HPM3 HPM4 PS

254 293 412 416 384

64.2 69.9 84.4 83.6 100

26.9 20.2 15.6 16.3 0

HPM5 HPM6 HPM7 HPM8 PS

206 220 391 426 384

63.6 59.2 68.7 72.8 100

36.4 40.8 31.3 26.2 0

thermal stability of poly(styrene) is strongly enhanced by the presence of nitrogen. Data for the degradation of both sets of polymers are displayed in Table 3. A reflection of the impact of the presence of the pendant groups on the combustion of the polymers is provided by the data from pyrolysis combustion flow calorimetry [30,31]. Plots of heat release rates versus time for polymers are displayed in Figs. 8 and 9. First, it is apparent that the presence of phosphorus in the polymer dramatically alters heat release rate for combustion of the polymer. As may be seen from the plots in Fig. 9, this impact is enhanced by the presence of nitrogen in the phosphorus/nitrogencontaining pendant groups.

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Fig. 9. Heat release rate for the combustion of styrene polymers bearing phosphorus/ nitrogen-containing pendant groups.

Table 4 Heat release (HR) values for the combustion of the homopolymers from phosphorus- and phosphorusenitrogen-containing monomers. Homopolymer

HR [kJ/g]

Homopolymer

HR [kJ/g]

HP1 HPM2 HPM3 HPM4 PS

25.8 27.9 28.3 26.6 42.4

HPM5 HPM6 HPM7 HPM8 PS

14.02 22.6 22.9 25.23 42.4

Heat release values for the polymers are collected in Table 4. The heat release values for the combustion of polymers containing both phosphorus and nitrogen are only marginally smaller than those for the polymers containing only phosphorus. This would suggest that the influence of phosphorus on the combustion of styrene polymers is little impacted by the presence of nitrogen. 4. Conclusions Two sets of styrene monomers, one containing a phosphorus moiety as a substituent and the other containing a substituent with both phosphorus and nitrogen components, have been prepared, characterized, and used to generate a set of polymers containing, in the first instance, only phosphorus and, in the second, both phosphorus and nitrogen. Both the thermal degradation and the combustion characteristics of the polymers have been examined. Thermogravimetry suggests that the presence of nitrogen enhances the impact of phosphorus on the stability of the polymers (high onset temperature for degradation and significantly higher levels of residue after decomposition). Polymers containing both phosphorus and nitrogen display slightly depressed combustion heat release rates compared to those containing only phosphorus. References

Fig. 8. Heat release rate for the combustion of styrene polymers bearing phosphoruscontaining pendant groups.

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