New Copper Complexes as Potential Smoke Suppressants for Poly

Downloaded by UNIV MASSACHUSETTS AMHERST on September 17, 2012 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch021

Chapter 21

New Copper Complexes as Potential Smoke Suppressants for Poly(vinyl chloride): Further Studies on Reductive Coupling Agents *

Robert D. Pike , William H. Starnes, Jr.*, Jenine R. Cole, Alexander S. Doyal, Peter M . Graham, T. Jason Johnson, Edward J . Kimlin, and Elizabeth R. Levy Department of Chemistry, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187-8795

Complexes of Cu(I) with hindered arylphosphites and salts of Cu(II) with phenylphosphonates and -phosphinates were prepared and shown to cross-link poly(vinyl chloride) (PVC) in pyrolysis studies. The use of hindered phosphites prevented the facile hydrolysis of the potential P V C additives. Complexes of Cu(I) with thioureas, nitrogen-bridged aromatics, thiophosphate, and phosphine sulfide ligands were also prepared and considered for use as additives far P V C .

© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

267

268


Introduction and Background

Polyvinyl chloride) (PVC) is a polymer of immense commercial importance, especially in long-term applications, e. g., use in construction. As a result, the behavior of P V C materials during fires is of importance. The thermal decomposition pathways for P V C are well-established (1-4). Pyrolysis of P V C causes dehydrochlorination reactions that generate polyene segments and HCI gas, as shown in Scheme 1 (5). The presence of as linkages within the polyene segments

Scheme 1 Smoke Benzene Vapor Phase cis,trans Polyene PVC

Condensed Phase Cross-Linked Polymer

All-trans Polyene

Char

allows cyclization that produces benzene and other aromatic hydrocarbons. These hydrocarbons serve as vapor-phase fuel, thereby generating smoke. The production of cross-linked polymer is thought to be essential to char formation, which serves to keep most of the mass in the condensed phase. Polymer additives based on high-valent metal and metalloid compounds have been widely used to help suppress smoke in P V C . Such compounds are usually strong Lewis acids, and, as a result, they catalyze Friedel-Crafts reactions, which provide the desired polyene aoss-linking action. Unfortunately, under conditions of high enthalpy input, these additives can also promote the cationic cracking of the polymer char (4-6). Char cracking is known to produce vapor-phase aliphatic hydrocarbons, which function as superior fuels (5), burning efficiently and aiding flame spread. Hence, Lewis-acid additives that limit smoke-producing polyene cyclization reactions through cross-linking catalysis can actually increase flame in large fires. Research in our laboratory (4) has demonstrated that the promotion of thermal P V C aross-linking can be accomplished through the use of low-valent metal compounds. These findings were initially spurred by the known reductive coupling of allylic halides by zero-valent metal powders (7-9) and low-valent metal complexes (10-13) according to reaction 1 (X = C l , Br, I). (Oxidation of the metal


269 2 R C H = C H - C H X + M(0) -> R C H = C H - C H - C H - C H = C H R + M X 2

2

2

2

(1)

serves as the thermodynamic driving force for this reaction.) It is probable that similar reductive coupling occurs during the Œoss-linking of P V C by low-valent metals, as indicated in Scheme 2. Although allylic chloridefimctionaUtiesare


Scheme 2

dehydrochlorination direction

+ Cu(0)

- CuCl

2

produced in P V C through thermal dehydrochlorination, a low concentration of these defect structures is actually present in the virgin polymer (3). Since allylic chloride groups are found at the termini of growing polyene chains in dehydrochlorinating PVC, the dehydrochlorination process is expected to be blocked by reductive coupling at those sites. Our research has shown that freshly generated zero-valent metal films induce the coupling of allylic chloride model compounds for P V C . Similarly, slurries of freshly generated metal cause the gelation of P V C (an indication of cross-linking). The current focus of our research has been on identifying metal compounds that will produce zero-valent metal at elevated temperatures. Such compounds


270


would be good additive candidates, since they would produce the active CTOss-linking agent only under pyrolytic conditions. Herein, we report new copper complexes that are potential smoke-suppressant additives. Copper complexes are promising for several reasons. Copper has three common oxidation states, all of which he close together in energy. Copper(0) is relatively stable, often more so than Cu(I). This stability difference leads to disprorxjrtionation reaction 2. Copper(II) is also readily reduced. As indicated for 2CuO)^Cu(II) + Cu(0)

(2)

copper(H) oxalate in reaction 3, oxidizable ligands can drive this process. Another CuC O -»Cu(0) + 2 C O 2

4

(3)

2

advantage of copper(I) among the transition metals is that its compounds are usually colorless. This property is observed because the metal has a d electronic configuration and, therefore, is not subject to electronic transitions within the dshell. Finally, copper compounds are of minimal toxicity (copper is an essential trace metal for most organisms), and they are inexpensive. The favorability of reactions such 2 and 3 is dependent upon the temperature and the use of supporting ligands. Thus, through judicious choice of copper complexes, we hope to develop smoke-suppressant additives that will release copper metal at temperatures well above those of polymer processing, but below those of a full-scale fire. 10

Experimental

General All syntheses were carried out under a nitrogen atmosphere. All ligands, CuS0 *5H 0, CuCl »2H 0, Cu(C 0 ), and Cu(0 CH) were used as received. The CuCl and CuBr werefreshlyrecrystallized from aqueous HCI or HBr. The CuC0 was freshly prepared from CuS0 *5H 0 and Na CO . Preparation of P(OPh) (14,15) and the nitrogen-bridged (16,17) complexes of CuBr and CuCl are reported elsewhere. Details concerning PVC gelation testing, flame atomic absorption spectroscopy (AAS), and thermogravimetric (TGA) analyses are reported elsewhere (4,16). 4

2

2

3

2

2

4

2

4

2

2

3


2

a

271 Preparation of [CuCl(BPP)] and Other C o p p e r ® Complexes Copper(I) chloride (1.00 g, 10.1 mmol) and tris(2,4-di-M>utylphenyl) phosphite (6.54 g, 10.1 mmol) were suspended in 50 m L of CHC1 . The mixture was stirred at 25 °C under N for 0.5 h, during which time it became less cloudy. After filtration, the solvent was removed under vacuum to leave a colorless oil, which solidified to a white solid under high vacuum (6.35 g, 8.51 mmol, 84%). Other complexes of C u C l and CuBr with B P P (in CHC1 ), M B P O P (in C6lkl B H T P D (in CH CN), C P D (in C H C N ) , Dmtu (in C H C N ) , and Etu (in H 0 ) were prepared similarly (see Charts 1 and 3 and Tables I and III). 3

2


3

3

3

3

2

Preparation of [Cu(H P0 )2] and Other Copper(D) Complexes 2

2

Phosphinic acid, H P 0 (1.7 m L of 50% aq. solution, 1.08 g, 16.4 mmol), was added dropwise to a suspension of C u C 0 (1.00 g, 8.09 mmol) in 15 m L of H 0 at 0 °C. The resulting blue solution yielded crystals upon the addition of 100 m L of E t O H and cooling to - 5 °C. The precipitate was collected by filtration, washed with acetone and ether in succession, and dried under vacuum (1.18 g, 6.07 mmol, 75%). Other Cu(II) complexes of H P 0 , P h P 0 " , HPhP0 ", and P h P 0 " were prepared similarly (see Table IV). 3

2

3

2

2

3

2

3

2

2

2

Preparation of [Cu(S2P(OEt) )] 2

The dithiophosphate NH4S P(OEt) (2.56 g, 12.6 mmol) was dissolved in 50 m L of H 0 at 25 °C, and C u C l (1.25 g, 12.6 mmol) was added. Aqueous N H was added until the green color was discharged and a white precipitate remained suspended in a blue solution. The mixture was stirred overnight, and the white product was collected via filtration. It was washed with H 0 , EtOH, and ether in succession and vacuum-dried (1.04 g, 4.18 mmol, 79%) (18). 2

2

2

3

2

Preparation of [C«(S2P(OEt) )(SPPh ) ] 2

3

2

A suspension of Cu(S P(OEt) ) (0.200 g, 0.804 mmol) and Ph PS (0.473 g, 1.61 mmol) in 50 m L of C H C 1 was heated under reflux for 15 min. The resulting colorless solution was concentrated to a volume of 5 m L under vacuum. A white powder was precipitated by the addition of ether and vacuumdried (0.487 g, 0.582 mmol, 72%). 2

2

2

3

2


272 Preparation of [(CuCl(SPPli ) ] 3

3

Addition of C u C l * 2 H 0 (0.386 g, 2.26 mmol) to a suspension of Ph PS (2.00 g, 6.79 mmol) in E t O H gave a green solution. A solution of S n C l in E t O H was added until the green color was discharged, and a white precipitate formed on cooling overnight. It was washed i n succession with E t O H and ether and vacuum-dried (1.48 g, 1.50 mmol, 66%) (19). 2

2


3

2

Results and Discussion

Copper(I) Phosphite Complexes Previous results obtained i n our laboratory indicated that although copper(I) complexes of both triphenyl phosphite and triphenylphosphine are potent gel formers in pyrolyzing P V C , the phosphine complexes are unacceptable for use, owing to their promotion of dehydrochlorination (4). Unfortunately, P(OPh) also has the problem of being subject to facile hydrolysis. For these reasons, new Cu(I) complexes were prepared by using commercially available hindered phosphites (Chart 1) (20). Copper analysis confirmed the product stoichiometrics indicated in Table I. Although the structures of the new copper(I) phosphite complexes have not yet been determined, proposed structures are shown in Chart 2. The relatively small P(OPh) is known to produce "cubane" tetramers with C u X 3

3

Table I. Copper(I) Organophosphite Complexesί Prepared Complex Cu, Theory Cu, Expt. Color 15.1% white 15.5% CuCl(P(OPh) ) 13.6 14.0 CuBr(P(OPh) ) white CuCl(BPP) 8.3 white 8.5 CuBr(BPP) 7.9 white 8.0 CuCl(MBPOP) 9.8 white 9.3 CuBr(MBPOP) 9.2 white 8.7 (CuCl) (BHTPD) cream 15.6 15.3 tan 13.4 (CuBr) (BHTPD) 13.8 CuCKCPD) 6.6 white 6.7 CuBr(CPD) 6.4 6.8 white 3

3

2

2

Dec. Point 285 °C 251 226 238 220 230 235 248 269 269


273


Chart 1

bis(2,4-dicumylphenyl)pmaerythritol diphosphite (Doverphos S-9228) (75). However, the bulky monophosphites (P = BPP, M B POP) may form dimers (lacking the "dashed" bonds, Chart 2). The two pentaerythritol diphosphite complexes (PP = B H T P D , CPD) have differing stoichiometries. Thus, [(CuX) (BHTPD)] forms polymeric chains, as shown in Chart 2, possibly with the C u X cross-linking shown. The bulkier diphosphite complexes [CuX(CPD)] are probably dimeric but may link into sheet structures, as shown. As can readily be seen from Table Π, the hindered phosphites form more hydrolytically stable complexes with copper. These data are in accord with results for the free phosphites (20). Thus, coordination to copper apparently does not greatly affect the hydrolytic stability of the phosphites. 2


274 Chart 2


[CuXP]; X = Br, Cl; Ρ = P(OPh>3, BPP, MBPOP

Xu-

7*

7

u

[(CuX)2PP]; X = Br, Cl; PP = —c< ) c u — [ p p ] — c i f

— C < )Cu 1

BHTPD

jpu—fppl-

( r ë l — C < ^Cu [PP|—

t

[CuX(PP)J; X = Br, Q; PP = CPD

Or /

/ Cu28 days CuBr(BPP) >28 days CuCl(MBPOP) >28 days CuBr(MBPOP) >28 days (CuCl) (BHTPD) >28 days (CuBr) (BHTPD) >28 days CuCKCPD) >28 days CuBr(CPD) >28days 3


3

2

2

Chart 3

S HN

%

/

NH

Ethylenethiourea (Etu)

4,4-Dipyridyl (Bpy)

MeHN'

"NHMe

iV^V'-Dimethylthiourea (Dmtu)

Pyrazine (Pyz)

PrT

\"^Ph Ph SPPh

1,4-Diazabicyclo[2.2.2]octane (DABCO)

3

EtO

\ S" OEt

S P(OEt) 2

2

Hexamethylenetetramine (HMTA)

The Cu(I) products with bridging nitrogen ligands, only a few of which are listed (Bpy, Pyz, D A B C O , and H M T A ) , form inorganic networks (16,17) and might be expected to show fairly high thermal stability as a result However, this was not the case. Moreover, the complexes that incorporated conjugated ligands (e. g., Bpy and Pyz) were colored because of metal-to-ligand charge transfer.


276 Table m. Copper(I) Sulfur and Nitrogen Complexes Prepared Complex Color C"?Jhe?y... Cu, Expt. Dec. Point CuBr(Etu) white 25.9% 26.1% 160 °C CuCl(Dmtu) white 20.7 22.0 160 CuCl(SPPh ) white 6.5 6.5 225 Cu(S P(OEt) ) white 25.5 25.3 187 Cu(S P(OEt) XSPPh ) white 7.6 6.8 180 CuBr(Bpy) red 21.2 20.8 215 CuCl(Pyz) red 35.5 36.4 125 (CuCl) (DABCO) It. brown 41.0 40.0 161 (CuCl) (HMTA) sandy 37.6 37.4 180 2


3

3

2

2

2

2

3

2

2

2

Copper(II) Salts Copper(II) compounds have the disadvantage, for their use as polymer additives, of being colored. However, simple anhydrous salts of Cu(II) do not usually have high molar absorptivities. In combination with oxidizable ligands, thermal reductive elimination reactions, such as that shown in equation 3, can occur and thereby form low-valent copper. A series of copper(II) salts with oxidizable ligands was synthesized. The resultant compounds are listed in Table IV. Anions derived from both

Cu, Theory 37.3% 25.8 32.8 18.4 12.8 41.4 35.2

Complex Color pale blue Cu(HP0 )«1.5 H 0 pale blue OKPhPOsM.S H 0 Cu(H P0 ) v. pale blue Cu(HPhP0 ) v. pale blue Cu(Ph P0 ) pale blue-violet pale blue Cu(C 0 ) pale blue Cu(0 CH) »1.5 H 0 Decomposes over several days at 25 °C. 3

2

2

2

2

2

2

2

2

2

2

2

2

4

2

2

Cu,Expt._ Dec. Point 37.5% 126 °C 344 26.2 55 33.0 99 18.1 13.3 299 300 41.0 34.7 199 a

a

phosphonic acid (H3PO3, Ρ(ΠΙ)) and phosphinic a d d ( H P 0 , P(I)) were chosen, since these low oxidation states are subject to oxidation, which produces salts of phosphoric acid ( H P 0 , P(V)). It can readily be observed that replacement of the nonacidic P - H with P - P h conferred significant stability enhancement to the resulting salt. Thus, the phenylphosphonate and diphenylphosphinate salts are the most promising i n terms of stability. The previously 2

3

2

4


277 reported (4) oxalate and formate salts are also listed in Table I V for comparison.


Prospects for PVC Additives Many of the compounds listed in Tables I, HI, and IV have been tested for their promotion of the thermal cross-linking of P V C (as judged by gel yield). The desired behavior for a promising additive candidate is the formation of a high gel yield without significant sample mass loss during the pyrolysis (4). Some results are listed in Table V . Table V. Gelation and Mass Loss Results Additive Complex (10% by mass added to P V C ) Control CuCl(P(OPh) ) CuBr(P(OPh) ) CuCl(BPP) CuBr(BPP) CuCl(MBPOP) CuBr(MBPOP) (CuCl) (BHTPD) (CuBr) (BHTPD) CuCKCPD) CuBr(CPD) CuCl(SPPh ) Cu(S P(OEt) ) Cu^PiOEt^XSPPh^ CuBr(Bpy) (CuCl)2(HMTA) Cu(PhP0 )»1.5 H 0 Cu(Ph P0 ) Cu(C 0 ) Cu(0 CH) »1.5 H 0 3

3

2

2

3

2

3

2

3

2

2

2

2

2

2

4

2

2

Number of Trials 2 2 2 3 4 3 4 4 4 4 3 2 2 2 2 2 4 3 2 2

P V C G e l Yield, l h @ 190 °C

New Copper Complexes as Potential Smoke Suppressants for Poly

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