Chapter 8
Fire and Polymers II Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/27/18. For personal use only.
Reductive Coupling Promoted by Zerovalent Copper A Potential New Method of Smoke Suppression for Vinyl Chloride Polymers J. P. Jeng, S. A. Terranova, E. Bonaplata, K. Goldsmith, D. M. Williams, B. J. Wojciechowski, and W. H. Starnes, Jr. Applied Science Ph.D. Program and Department of Chemistry, College of William and Mary, Williamsburg, VA 23187-8795 When they are exposed to activated forms of copper metal at moderate temperatures, allylic chloride models for structural segments in PVC experience rapid reductive coupling and thereby are converted into mixtures of diene hydrocarbons. The activated copper that promotes this process can be either (a) a slurry formed by the reduction of CuI·P(Bu) with lithium naphthalenide or (b) a film produced by the thermal decomposition of copper(II) formate. Both the slurry and, apparently, the Cu° formed by pyrolysis also cause the crosslinking of PVC itself. Moreover, at 200 °C, the crosslinking of solid polymer samples is promoted by copper powder of very high purity (99.999%). Since PVC crosslinking causes smoke suppression, the results of this study suggest that Cu°promoted reductive coupling will tend to inhibit smoke formation when the polymer burns. 3
Many compounds of copper are well-known to be effective smoke suppressants for polyvinyl chloride) (PVC) (1). Even so, their commercial use for that purpose apparently has not been extensive. Available evidence shows that, in general, copper additives cause "early crosslinking" of the polymer during its thermolysis (2). This process tends to inhibit smoke by retarding the formation of volatile aromatics such as benzene that can burn in the vapor phase (3). However, the crosslinking chemistry of copper compounds is not understood completely and is controversial for that reason. Metal-containing additives for PVC frequently retard both smoke and flame by acting as Lewis-acid catalysts for reactions that lead to crosslinking (2-4). Unfortunately, when the Lewis acidities of such additives are high, these compounds also tend to cause char breakdown when high temperatures are reached (5). This cationic cracking process (4) may enhanceflamespread enormously by generating volatile aliphatic fragments that burn with great facility (4,5). 0097-6156/95/0599-0118S12.00/0 © 1995 American Chemical Society
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Figure 1 summarizes results obtained in a study of the cracking reaction (4) that was carried out with the Lewis-acid smoke suppressant (2—6), Mo0 , and with the much stronger Lewis acid, Mo0 Cl , that can be formed in situ from Mo0 and the HCI that is generated by PVC thermolysis. Note that both of the organic substrates gave major amounts of low-molecular-weight alkanes, which are wellknown to be excellent fuels. Copper compounds are, in general, not strong Lewis acids. Thus is it not surprising that previous work has revealed significant differences between their behavior and that of Mo0 in reactions with organic substances that are models for PVC (2,6). Other studies have suggested that the high activity of copper-containing smoke suppressants may derive, in part, from unusually high crosslinking-tocracking ratios that result from the presence of Lewis acids that are weak (7,8). A further mechanistic possibility is that copper additives promote the crosslinking of PVC by reductive coupling (9). Such a process can be represented by the redox cycle shown as equations 1 and 2 (4,9), in which all of the metal ligands are not 3
2
2
3
3
w+1
2RC1 + 2Cu" -> R-R + 2Cu Cl n+1
2Cu Cl + -CH=CH-
-CH=CC1- + HCI + 2Cu"
(1) (2)
depicted. Operation of this mechanism would be especially attractive on technological grounds, because it does not require Lewis acidity and thus avoids the problem of cationic char cracking. No conclusive evidence has been reported for the reductive coupling of PVC by copper additives. However, it has been known for some time that, in burning PVC, higher-valent copper is reduced to Cu° (9,10), which should be produced in a highly active state and thus should be especially effective in reaction 1. The present chapter reports the use of activated copper metal to effect the reductive coupling of model organic chlorides and the crosslinking of the polymer. Our results suggest that the reductive coupling of PVC is indeed a viable process that is worthy of further study within the context of smoke suppression and fire retardance. Experimental Section General. Copper(II) formate (Pfaltz & Bauer) and the other starting materials were either used as received or purified by conventional methods. The PVC (Aldrich; inherent viscosity, 1.02) contained no stabilizers or other additives. Activated copper slurry was prepared by reducing the CuI-P(Bu) complex in THF or ether with a 10 mol % excess of lithium naphthalenide, according to a published method (11,12). Compounds not obtained from commercial sources were synthesized by standard procedures. All reactions were carried out under argon, and reaction products were identified from their GC retention times and mass spectral cracking patterns, using pure reference substances for comparisons as required. The GC/MS analyses were performed with a Hewlett-Packard System (Model 5988A) equipped with a fiisedsilica capillary column containing dimethyl- and diphenylpolysiloxane in a ratio of 95:5. 3
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M0O2CI2 300 °C
MoO 260
C H 2 6
5 2
(major) + C ^tLj (major) 3
8
+ > C alkanes (major) and alkenes (minor) 6
+ alkylaiOmatics (minor) + oxygenated products (traces) M0O2CI2 f 300 °C I
Figure 1. Nature and relative abundance of the products formed in reactions of molydenum additives with models for structures in PVC (4).
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Reactions of Activated Copper Slurry with Model Organic Chlorides. A model organic chloride (10 mmol) or a mixture of two chlorides (5 mmol of each) was added via syringe to a copper slurry (8 mL) prepared by reducing 10 mmol of CuI-P(Bu) . After 5 min of reaction at constant temperature, the mixture was subjected to GC/MS analysis. 3
Reactions of Activated Copper Film with Model Organic Chlorides. Activated copper film was obtained by decomposing copper(II) formate (1.50 g, 9.8 mmol) at 200 °C for 2—3 min. When the flask containing the film had cooled to room temperature, a model organic chloride (9.8 mmol) or a mixture of two chlorides (4.9 mmol of each) was injected via syringe. The reaction mixture was subjected to GC/MS analysis after a reaction time of 1 min. Reactions of Activated Copper Slurry with PVC in Solution. Activated copper slurry in THF (15 mL), prepared by reducing 20 mmol of CuI-P(Bu) , was added by syringe to a refluxing solution of PVC (2.00 g, 32 mmol of monomer units) in the solvent selected (35 mL for THF, 50 mL for the other solvents). In all of the experiments except those where THF was the only solvent used, the THF was allowed to boil off rapidly at the start of the reaction period. After the chosen reflux interval, the insoluble material was isolated by filtration or décantation, washed thoroughly and repeatedly with several portions of concentrated ammonium hydroxide (in order to remove copper salts), and subjected to Soxhlet extraction with THF for 24 h. The resultant insoluble polymer was dried under vacuum for 24 h at 60 °C. Its weight was then determined and used to calculate the degree of gelation that had occurred. 3
Dehydrochlorination Rates and Gel Contents of Thermally Degraded Samples of Solid PVC. By using an agate mortar and pestle, a 1.00-g sample of PVC was intimately mixed with 0.10 g of a copper additive. The mixture was subjected to dehydrochlorination at 200±2 °C under flowing argon, and the rate of acid evolution was monitored by acid-base titrimetry performed with a Brinkmann Metrohm 702 SM Titrino apparatus. According to the procedure described above, degraded samples were extracted with hot THF, dried, and weighed in order to determine their gel percentages. The weights were corrected for the presence of insoluble copper species. Results and Discussion Reactions of Activated Copper with Model Organic Chlorides. Activated copper slurry was obtained from the Li+C^Hg" " reduction of CuI-P(Bu) (11,12). In our hands, it was found to effect the rapid reductive coupling of a wide variety of simple organic halides (chlorides, bromides, iodides) in THF or ether at temperatures ranging from 0 to 67 °C. In general, the results of these experiments were consistent with those obtained in a similar study that was reported (12) while our work was in progress. Allylic chloride structures occur in undegraded PVC and also are formed when the polymer experiences degradation (Starnes, W. H., Jr.; Girois, S. Polymer 7
3
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Yearbook, in press, and references cited therein). Thus we were especially pleased to find that the copper-promoted coupling of allylic chlorides was quite facile under the mild conditions that we used. Some representative results obtained with allylic chlorides appear in the third column of Table I. Table I. Reductive Coupling Products Obtained from Allylic Chlorides and Activated Copper yields," % chloride(s) product(s) Cu" slurry" Cu" film 3-chloro-l-butene 19±2 20±2 8 b
C
H
1 4
frïws-4-chloro-2-pentene
CioH
1:1 (mol/mol) mixture of 3-chloro-l-butene and ira/zs-4-chloro-2-pentene
QH Q>H C 10^18
18
1 4
16
36±3
24±2
24±2 30±1 23±2
26±1 12±3 24±2
SOURCE: Reprinted with permission from ref. 13. Copyright 1994. GC area percentages based on amount(s) of starting chloride(s); values shown are averages derived from duplicate runs. ^Mixtures of several isomers. Values obtained from experiments performed in THF at ca. 65 °C. fl
c
Control experiments showed that our organic halides could be reductively coupled by lithium naphthalenide when no copper slurry was present. However, reductive coupling also was effected (albeit in lower yields) by slurries that had been freed of any unchanged lithium naphthalenide by repetitive washing with fresh solvent. The yield reductions observed in these cases may have resulted primarily from decreases in slurry activity (11,12) caused by the washing process. Encouraged by these findings, we then performed similar experiments with copper films that had been generated by the pyrolysis of copper(II) formate (14). These films also were effective coupling promoters, as is shown by the data in the fourth column of Table I. Reaction of Activated Copper Slurry with PVC in Solution. The ability of the copper slurry to cause gelation of PVC was studied by using solutions of the polymer in the solvents that are listed in Table II. Gel contents were obtained by Table II. Gel Contents of PVC Treated with Cu° Slurry in Solution solvent temp, C time, h gel,** THF 66±2 2.0 79±2 92±2 2.0 anisole 155±2 2.0 88±2 174±2 o-dichlorobenzene 90±2 0.5 phenyl ether 257±2 w
SOURCE: Reprinted with permission from ref. 13. Copyright 1994. "Mean values obtained from duplicate runs.
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determining the weight of the polymer that was not dissolved by hot THF, and control runs were used to show that no gelation occurred when the copper slurry was absent. The high gel percentages reported in the table are striking in view of the rather low molar concentrations of polymer and copper that were used. Also noteworthy are the relatively minor effects of temperature upon the extent of gelation at the three lowest temperatures studied. We have no direct information about the chemical nature of the crosslinks that were formed in the experiments of Table II. However, in view of the results of our model-compound studies, it certainly seems reasonable to believe that a major role in the crosslinking process was played by reductive coupling. Reaction of PVC with Copper Additives in the Solid State. Preliminary work in our laboratory had shown that a highly purified form of copper powder (nominal purity, 99.999%; supplied by Aldrich Chemical Co.) was able to promote the reductive coupling of model allylic chlorides (Jeng, J. P., unpublished observations). We therefore wished to compare the gelation effectiveness of this form of copper with that of the copper resulting from copper(II) formate pyrolysis. The requisite experiments were carried out with solid samples, and the results are exemplified in Figure 2. They show that both of the additives caused rapid gel formation which, in the case of the formate, evidently was brought about by the Cu° formed in situ. Rates of acid evolution were determined with solid PVC samples under the conditions used to acquire the data that are plotted in Figure 2. Figure 3 shows some rate curves that typify those obtained. The initial rapid rate observed with copper(II) formate may have resulted from the evolution of formic acid formed by decomposition of the additive, but that point was not established conclusively. Nevertheless, it is apparent that the formate caused no significant rate enhancement after ca. 20—40 min and that the commercial copper powder caused a modest rate reduction throughout the reaction period. Those observations are not in accord with Lewis-acid catalysis (which would have increased the rate (4)), but the effect of the powder is consistent with the mechanism of equations 1 and 2, which predicts a rate diminution caused by the coupling of allyl groups (4). However, we regard this evidence for that mechanism as being suggestive instead of decisive. It is interesting to note that copper(II) formate has been suggested to have potential utility as a thermal stabilizer for vinylidene chloride copolymers (75). The effects of the thermal decomposition of this additive on its stabilizing properties are, at present, unclear. Conclusions Activated forms of Cu° cause the reductive coupling of allylic chlorides that are models for structures in PVC. The coupling occurs quite rapidly under very mild conditions and has been observed with (a) a copper slurry formed by the reduction of CuLP(Bu) with lithium naphthalenide and (b) a copper film resulting from the pyrolysis of copper(II) formate. Both the slurry and, apparently, the film cause the crosslinking of PVC itself. In the case of the slurry, this process has been observed with solutions of the polymer in various solvents at temperatures ranging from ca. 66 to 257 °C. In reactions of copper(II) formate with solid PVC, accelerated crosslinking occurs at the decomposition temperature of the additive (200 °C) and 3
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100
time, min Figure 2. Gel contents of solid PVC samples degraded at 200±2 °C under argon: • , no additive; O, with Cu° powder (purity, 99.999%); · , with Cu(II) formate. See text for details. (Reproduced with permission from ref. 13. Copyright 1994 Business Communications Co.) 4.00
time, min Figure 3. Acid evolution rate curves for the degradation of solid PVC samples at 200±2 °C under argon: A , no additive; O, with Cu° powder (purity, 99.999%); · , with Cu(II) formate. See text for details. (Reproduced with permission from ref. 13. Copyright 1994 Business Communications Co.)
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thus is believed to result from the presence of copper metal formed in situ. Moreover, at this temperature, the crosslinking of solid polymer samples is promoted by a commercial form of copper powder that has a very high purity (99.999%). The crosslinking of PVC is well-known to cause smoke suppression. Thus the results of the present study suggest that copper-promoted reductive coupling will tend to prevent the formation of smoke during PVC combustion. Acknowledgment We thank the International Copper Association for partial support of this work. Literature Cited 1. 2.
14.
Kroenke, W. J. J. Appl. Polym. Sci. 1981, 26, 1167. Lattimer, R. P.; Kroenke, W. J.; Getts, R. G. J. Appl. Polym. Sci. 1984, 29, 3783. Starnes, W. H., Jr.; Edelson, D. Macromolecules 1979, 12, 797. Starnes, W. H., Jr.; Wescott, L. D., Jr.; Reents, W. D., Jr.; Cais, R. E.; Villacorta, G. M.; Plitz, I. M.; Anthony, L. J. In Polymer Additives; Kresta, J. E., Ed.; Plenum: New York, NY, 1984; ρ 237. Edelson, D.; Lum, R. M.; Reents, W. D., Jr.; Starnes, W. H., Jr.; Wescott, L. D., Jr. In Proceedings, 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; ρ 807. Wescott, L. D., Jr.; Starnes, W. H., Jr.; Mujsce, A. M.; Linxwiler, P. A. J. Anal. Appl. Pyrolysis 1985, 8, 163. Starnes, W. H., Jr.; Huang, C.-H. O. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1989, 30 (1), 527. Huang, C.-H. O.; Starnes, W. H., Jr. In Proceedings, 2nd Beijing International Symposium on Flame Retardants; Geological Publishing House: Beijing, China, 1993; ρ 168. Lattimer, R. P.; Kroenke, W. J. J. Appl. Polym. Sci. 1981, 26, 1191. Lattimer, R. P.; Kroenke, W. J. In Analytical Pyrolysis: Techniques and Applications; Voorhees, K. J., Ed.; Butterworths: Woburn,MA,1984; ρ 453. Ebert, G. W.; Rieke, R. D. J. Org. Chem. 1988, 53, 4482. Ginah, F. O.; Donovan, T. A., Jr.; Suchan, S. D.; Pfennig, D. R.; Ebert, G. W. J. Org. Chem. 1990, 55, 584. Starnes, W. H., Jr.; Jeng, J. P.; Terranova, S. A.; Bonaplata, E.; Goldsmith, K.; Williams, D. M.; Wojciechowski, Β. J. In Proceedings, 5th Annual BCC Conference on Flame Retardancy; Business Communications Co.: Norwalk, CT, 1994; in press. Kőrösy, F. Nature 1947, 160, 21.
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Howell, Β. Α.; Rajaram, C. V. J. Vinyl Technol. 1993, 15, 202.
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