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Langmuir 1997, 13, 5056-5060
Polymer Matrix Influence on the Kinetics of Some Fundamental Inorganic Colloidal Reactions Rina Tannenbaum Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received February 24, 1997. In Final Form: July 3, 1997X Among the most useful methods for the preparation of pure metal powders is the thermolysis of metal carbonyl complexes in hydrocarbon solutions. Zero-valent cobalt particles are obtained by the decomposition of Co2(CO)8. The reaction is primarily governed by diffusion, which is strongly dependent on the viscosity of the solution. In a solution containing polystyrene, the viscosity is directly proportional to the concentration of the polymer. To study the variation of the rate constants of this colloidal reaction as a function of the solution viscosity, we examined various polystyrene solutions of different molecular weights and concentrations. The reaction rate increases considerably at polymer content below and at the critical coil overlap concentration. Above this concentration, as the polymer coils become entangled and contracted, the decreased mobility of the molecules due to higher viscosity and lower diffusion rate lowers the reaction rate. The influence of the molecular weight of the polystyrene on the reaction kinetics had a “catalytic” effect on reaction rates and was most dramatic with a MW(avg) of ∼120 000. We find that in the dilute polymer regime, below the PS coil overlap threshold, the polymer chains can provide the necessary support for the aggregation of the cobalt particles. Also, in the dilute regime, the mobility of the cobalt molecules is not hindered due to the low solution viscosity. However, there is evidently a critical molecular weight at which the “catalytic” effect of the polymer is at its maximum. These results will be discussed, and possible mechanisms for the polymer-enhanced colloid reactions will be offered.
1. Introduction Thermolysis of transition metal carbonyl complexes in solution under an inert atmosphere is a well-known technique for the preparation of pure metal powders.1,2 The thermal decomposition of Co2(CO)8 to metallic cobalt and carbon monoxide under an inert atmosphere at temperatures ∼90 °C proceeds in two steps:3-10 the first step is the equilibrium decomposition of Co2(CO)8 to Co4(CO)12, and the second step is the irreversible decomposition of Co4(CO)12 to Co(0) clusters, where COg is released in both steps.
2Co2(CO)8 h Co4(CO)12 + 4COg
(I)
Co4(CO)12 h Co4 + 12COg f Co(0) k
(II)
The thermal decomposition reaction of cobalt carbonyl complexes in solutions and in polymer matrices (referred to here as solid state) has been thoroughly documented.3-10 The mechanistic aspects of the solid state decomposition were similar to those found with the solution decomposition, as concluded from the type of intermediates formed in the reaction in both cases and from the nature of the end products. The kinetic aspects, however, were quite different for the two media, since the rate constant found for the solid state reaction was two orders of magnitude X
Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) Emerson, G. F.; Ehrlich, K.; Giering, W. P.; Ehntholt, D. Trans. N. Y. Acad. Sci. 1968, 30, 100. (2) Emerson, G. F.; Mahler, T. E.; Koehlar, R.; Petit, R. J. Org. Chem. 1964, 29, 3620. (3) Ungva´ry, F.; Marko´, L. J. Organomet. Chem. 1974, 71, 283. (4) Bor, G.; Dietler, U. K. J. Organomet. Chem. 1980, 191, 295. (5) Mirbach, M. F.; Saus, A.; Krings, A. M.; Mirbach, M. J. J. Organomet. Chem. 1981, 205, 229. (6) Gavrilova, V. M.; Gankin, V. Yu; Rugkovkii, D. M.; Trifel, A. G. Gidroformilirovanie; Kuimya: Leningrad, 1974; pp 114-118. (7) Natta, G.; Ercoli, R.; Castellano, S. Chim. Ind. (Milan) 1955, 37, 6. (8) Ungva´ry, F.; Marko´, L. Inorg. Chim. Acta. 1970, 4, 324. (9) Bor, G.; Dietler, U. K.; Pino, P.; Poe¨, A. J. Organomet. Chem. 1978, 154, 301. (10) Baev, A. K. Izv. Vyssh. Uchebn. Zaved. 1974, 17, 1750.
S0743-7463(97)00199-6 CCC: $14.00
smaller than the one found for the solution reaction.11,12 Hence, embedding cobalt carbonyl complexes in a polymeric matrix (high molecular weight polystyrene) significantly slowed down the decomposition process. The decomposition of Co2(CO)8 to Co4(CO)12 and further to metallic Co is a complex process which is comprised of several mechanisms. As a first step, the removal of a CO group from the coordination sphere of Co2(CO)8 generates an electron-deficient complex, Co2(CO)7, which dimerizes to yield Co4(CO)12 with the release of an additional 2 molecules of CO. This reaction is primarily governed by the mobility of the molecules through the viscous medium and their ability to diffuse toward each other. The solid state polystyrene system provides a medium with a considerable increase in viscosity,13-18 and diffusional barriers, thus giving rise to lower reaction rates. In the second step, the removal of one or more CO groups from the coordination sphere of Co4(CO)12 generates several types of electron-deficient complexes, which in turn may react to form cobalt complexes with high nuclearity (highorder clusters). As the CO/Co ratio decreases, the solubility of the high-order clusters also decreases, and the third and most significant step of the decomposition becomes a colloidal reaction.19-21 The growth of the cobalt particles in this last step may be described by the following (11) Tannenbaum, R.; Flenniken, C. L.; Goldberg, E. P. J. Polym. Sci., Polym. Phys. Ed. 1987, 25, 1341. (12) Tannenbaum, R.; Goldberg, E. P.; Flenniken, C. L. In MetalContaining Polymeric Systems; Carraher, C., Pittman, C. U., Sheats, J., Eds.; Plenum Press: New York, 1985. (13) Ball, J. M.; Carr, J.; Penrose, O. Commun. Math. Phys. 1986, 104, 657. (14) Flory, P. J.; Fox, T. G. J. Am. Chem. Soc. 1951, 73, 1904. (15) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (16) Graessley, W. W. Adv. Polym. Sci. 1974, 16, 1. (17) Bird, R. B.; Curtis, C. F.; Hassager, O.; Armstrong, R. C. Dynamics of Polymeric Liquids: Vol. 2, Kinetic Theory; Wiley: New York, 1st ed., 1976; 2nd ed., 1987. (18) deGennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (19) Spouge, J. L. J. Phys. A: Math. Gen. 1983, 16, 767. (20) Ziff, R. M.; Stell, G. J. Chem. Phys. 1980, 73, 3492. (21) van Dongen, P. G. V.; Ernst, M. H. J. Phys. A: Math. Gen. 1985, 18, 2779.
© 1997 American Chemical Society
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nucleation and growth reactions:
cluster stepwise growth: Coj + Co1 f Coj+1
(III)
cluster aggregation: Coj + Cok f Coj+k
(IV)
cluster dissociation: Cok f Coi + Coj
(V)
These types of reactions may be highly facilitated by the presence of large macromolecules in the reaction solutions, since these macromolecules can provide the necessary solid state support and microenvironment which would constitute the driving force for these reactions. The study of the variation of the rate constants of reactions I and II as a function of the concentration of the polystyrene moiety in the cobalt carbonyl solutions22 shows that since reaction I is primarily governed by diffusion, the reaction rate decreases with the increase in polystyrene content above the critical coil overlap concentration. Reaction II, on the other hand, being a colloidal reaction, is greatly facilitated by the presence of the polymer, and the reaction rate increases considerably at polymer content below and at the critical coil overlap concentration. Previous reports23,24 describe the stabilization of colloidal cobalt particles in dilute polymer solutions, by adsorption of the polymer to the metal particles to form a film that separates the particles sufficiently to keep Van der Waals forces below thermal energy levels. Above the critical coil overlap concentration, as the polymer coils become entangled and contracted, the decreased mobility of the molecules due to higher viscosity and lower diffusion rate has the effect of lowering the reaction rate of reaction II as well. The effect of polymer concentration on the decomposition kinetics of cobalt carbonyl complexes in polymer solutions was previously examined with polystyrene of MW(avg) ) 250 000.22 The decomposition kinetics of the two carbonyl precursors, Co2(CO)8 and Co4(CO)12, were examined. The presence of the polystyrene molecules in the reaction medium was most notable in reaction II, and hence we will concentrate on studying this particular reaction. Hence, in this paper we examine the effect of the variation of molecular weights and concentrations of polystyrene on the kinetics of reaction II, which seems to be the one “catalyzed” by the presence of polystyrene in the reaction solution. 2. Results and Discussion The qualitative identification as well as the quantitative determination of cobalt carbonyl species in polymer solutions was carried out by infrared spectroscopy. The carbonyl absorption bands of cobalt carbonyls, both terminal and bridging, are very sharp and well contoured in the 1800-2200 cm-1 region25-27 and permit an accurate (22) Tannenbaum, R. Inorg. Chim. Acta 1994, 227, 233. (23) Hess, P. H.; Parker, P. H., Jr. J. Appl. Polym. Sci. 1966, 10, 1975. (24) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel Publishing Company: New York, 1986 and pertinent references therein. (25) Noack, K. Helv. Chim. Acta 1962, 45, 1847. (26) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: New York, 1975. (27) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988; p 1034.
evaluation of concentration, without solvent spectral interference (which can be eliminated in any case by proper spectral manipulations). The infrared spectra of Co2(CO)8 and Co4(CO)12 have two main absorption regions of interest: (a) the absorption bands in the region between 2020 and 2070 cm-1, which correspond to the terminal CO groups in both complexes,28-31 and (b) the 1858 and 1867 cm-1 absorption bands, which correspond to the bridging CO groups both in Co2(CO)8 and Co4(CO)12.28-31 In polymer solutions, there is a general broadening of the spectrum (relative to the spectrum of pure hydrocarbon cobalt carbonyl solutions) of Co2(CO)8, that results in the formation of only two terminal carbonyl bands of approximately equal intensity at 2030 and 2065 cm-1 and only one bridging carbonyl band at 1858 cm-1. The same broadening of the spectrum is observed also in the spectrum of Co4(CO)12, resulting in the formation of only one terminal carbonyl band at 2058 cm-1, with the preservation of the bridging carbonyl band at 1867 cm-1. The effect of the polystyrene present in solution is much more evident with the terminal carbonyl bands than with the bridging carbonyl bands. This is probably due to a lessening of the number of degrees of freedom for the vibrations of the terminal carbonyls in the polystyrene solutions, while the vibrations of the bridging carbonyls are relatively more restricted in both pure hydrocarbon and polymer solutions. The quantitative analysis of carbonyl concentration was based upon the intensity variations of the 1867 cm-1 band. The bridging region was chosen as the analytical reference for the following reasons: (1) The spectrum of the terminal carbonyl region is much too complex for an effective quantitative analysis. However, Co4(CO)12 has a bridged structure exclusively, and therefore, changes in the intensity of its 1867 cm-1 band reflect changes in its absolute concentration. (2) The general broadening of the spectrum in polystyrene solutions causes overlap of some of the bands in the terminal carbonyl region, and hence these bands cannot be used for analytical purposes. The 1867 cm-1 band, on the other hand, is isolated, and even though it is considerably broader in the solid state, it can still be used for quantitative analysis. A crucial element of the quantitative analysis of the cobalt carbonyl solutions is the determination of the adequate molar absorption extinction coefficients for the 1867 cm-1 band. Some extinction coefficient values for the bridging carbonyl infrared absorption bands of Co4(CO)12 in hydrocarbon solvents have been reported in the literature.25,32,33 For dilute solutions, ∼10-1 to 10-3 M, these values are
1867 ) 10 440 L‚mol-1‚cm-1 When Co4(CO)12 was embedded in a polymeric matrix such as polystyrene to form solid films 0.5-0.8 mm thick (i.e. the solid state case11), the extinction coefficient values were two orders of magnitude lower than those in solution. For concentrations of cobalt carbonyls in the films of 1030 wt %, these values are (28) Bor, G.; Marko´, L. Spectrochim. Acta 1959, 15, 1747. (29) Cotton, F. A.; Monchamp, R. R. J. Chem. Soc. 1960, 1882. (30) Bor, G.; Marko´, L. Spectrochim. Acta 1960, 16, 1105. (31) Bor, G. Spectrochim. Acta 1963, 19, 1209. (32) Wiltzius, P.; Haller, H. R.; Cannell, D. S.; Schaeffer, D. W. Phys. Rev. Lett. 1983, 51, 1183. (33) Daoud, M.; Cotton, J. P.; Farnoux, B.; Jannink, G.; Sarma, G.; Benoit, H.; Duplessix, R.; Picot, C.; deGennes, P. G. Macromolecules 1975, 8, 804.
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Tannenbaum
1867 ) 795 L‚mol-1‚cm-1 It is very likely that the solid state imposes some restrictions on the free movement of the carbonyl groups of Co4(CO)12, which results in the considerable lowering of the extinction coefficients of this complex.11 The same phenomenon has also been observed with other carbonyl complexes.12 Since the main thrust of this study is to understand the influence of the molecular weight of polystyrene in the viscous solutions on the kinetics and mechanism of the carbonyl decomposition process (reaction II), it is important to determine the values of the extinction coefficients of the 1867 cm-1 band in solutions of various molecular weights and increasing viscosities. Figure 1 shows the variation of the extinction coefficients of the 1867 cm-1 band as a function of the concentration (expressed in weight percent) of polystyrene present in the solution for several molecular weights of polystyrene. For dilute polystyrene solutions, up to 1.5 wt %, the extinction coefficients of the 1867 cm-1 band are similar those of to the pure hydrocarbon solution without the polymer. The largest variation is observed for the semidilute regime, 2-20 wt % polystyrene. The rate of the decrease for 1867 is -376.85 L‚mol-1‚cm-1‚(wt %)-1. In reaction II, the first step consists of the loss of a CO group to form an electron deficient complex. This first step is energetically unfavorable and is considered as the rate-limiting step,9,19 and therefore the reaction may be approximated to be of first order. Hence, the rate constant becomes kdec and the overall rate expression is
ln X ) kdec‚t
Figure 1. Variation of the extinction coefficient of the 1867 cm-1 carbonyl bridging band of Co4(CO)12 as a function of the concentration (expressed in wt %) of polystyrene present in solution for several molecular weights (details for solution content are shown in Table 1).
(VI)
where X ) [Co4(CO)12]t. Figure 2 portrays the kinetics of reaction II in solutions of polystyrene of various molecular weights and various concentrations. A closer examination of the polystyrene solutions for which kdec reaches its maximal value reveals that the polystyrene content corresponds to its critical concentration in dilute solutions, C*, a concentration which marks the onset of the coil overlap process between the polymer chains in solution.18,32-34 For polystyrene concentrations [PS] < C*, the polymer coils have infinite dilution radii and do not overlap, while, for [PS] ) C*, the coils begin to overlap, having reached the overlap threshold. At this critical concentration C*, the intrinsic viscosity, [η], corresponds to the relationship18 C*‚[η] ) 1. The intrinsic viscosity is a measure of the polymer’s ability to increase solution viscosity in the absence of intermolecular effects, i.e., at infinite dilution. The intrinsic viscosity of polystyrene in toluene may be calculated according to the Mark-Houwink experimental relation: [η] ) KMa, where K and a are specific constants for various polymer/solvent combinations.35 Table 1 lists the coil overlap concentrations, C* (wt %), for the MW(avg) chosen for this study. The dependence of kdec(max) (at C*) on molecular weight is shown in Figure 3. The rate coefficients for reaction II increase with polymer chain size at lower molecular weights but decrease at higher molecular weights. The “catalytic” effect which the presence of polystyrene has on reaction II is most dramatic with a MW(avg) of ∼110 000. A comprehensive three-dimensional description of the dependence of kdec on [PS] and on MW(avg) is shown in Figure 4. It is clearly seen that the “catalytic” effect of (34) Graessley, W. W. Polymer 1980, 21, 258. (35) Brandrup, J., Immergut, E. H., Eds. Polymer Handbook, 2nd ed.; Wiley: New York, 1989.
Figure 2. Rate coefficients of reaction II, kdec, as a function of the concentration (expressed in wt %) of polystyrene, for solutions containing polystyrene at various concentrations and various molecular weights.
the PS presence in solution is optimized at the C* concentration of ∼110 000 MW polystyrene. Under these conditions, the various effects which govern reaction II allow for optimal and fastest reaction rates. The role of polystyrene in determining the reaction kinetics depends on the interactions between polystyrene and the forming cobalt clusters. The viscosity of the polystyrene solutions in which reaction II takes place increases with polymer concentration. In a system where there are no interactions between the polymer and the clusters, the mobility of the clusters is thereby reduced with increasing viscosity. Aggregation will be diffusionlimited, and the average cluster size will be smaller than that in pure solvent. This reduction in cluster size as a function of polymer concentration is in agreement with some of our preliminary results. The reaction rate should, for the same reason, also decrease monotonically with polymer concentration. However, as our results show, the effect of polymer concentration and molecular weight on the reaction rate is nonmonotonic and cannot, therefore, be explained by viscosity effects alone.
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Table 1. Preparation of Polystyrene Solutions of Various Concentrations and Various Molecular Weights (and Hence Various Viscosities) To Be Used in the Experiment wt (g)
wt (%)
MW ) 60 000
MW ) 90 000
MW ) 120 000
MW ) 180 000
MW ) 250 000
0.065 0.108 0.173 0.216 0.324 0.648 1.083 1.731 2.160 3.240 6.480 21.600 43.200 86.400 129.600 172.800 183.600
0.03 0.05 0.08 0.10 0.15 0.30 0.50 0.80 1.00 1.50 3.00 10.00 20.00 40.00 60.00 80.00 85.00
4.33 × 10-6 7.20 × 10-6 1.15 × 10-5 1.44 × 10-5 2.16 × 10-5 4.32 × 10-5 7.20 × 10-5 1.15 × 10-4 1.44 × 10-4 2.16 × 10-4 4.32 × 10-4 1.44 × 10-3 2.88 × 10-3 5.76 × 10-3 8.64 × 10-3 1.15 × 10-2 1.22 × 10-2
2.88 × 10-6 4.80 × 10-6 7.67 × 10-6 9.60 × 10-6 1.44 × 10-5 2.88 × 10-5 4.81 × 10-5 7.69 × 10-5 9.60 × 10-5 1.44 × 10-4 2.88 × 10-4 9.60 × 10-4 1.92 × 10-3 3.84 × 10-3 5.76 × 10-3 7.68 × 10-3 8.16 × 10-3
2.16 × 10-6 3.61 × 10-6 5.76 × 10-6 7.21 × 10-6 1.08 × 10-5 2.16 × 10-5 3.60 × 10-5 5.77 × 10-5 7.21 × 10-5 1.08 × 10-4 2.16 × 10-4 7.21 × 10-4 1.44 × 10-3 2.88 × 10-3 4.32 × 10-3 5.76 × 10-3 6.12 × 10-3
1.44 × 10-6 2.40 × 10-6 3.84 × 10-6 4.81 × 10-6 7.20 × 10-6 1.44 × 10-5 2.40 × 10-5 3.84 × 10-5 4.80 × 10-5 7.20 × 10-5 1.44 × 10-4 4.81 × 10-4 9.60 × 10-4 1.92 × 10-3 2.88 × 10-3 3.84 × 10-3 4.08 × 10-3
1.04 × 10-6 1.73 × 10-6 2.76 × 10-6 3.45 × 10-6 5.18 × 10-6 1.04 × 10-5 1.73 × 10-5 2.76 × 10-5 3.45 × 10-5 5.18 × 10-5 1.04 × 10-4 3.45 × 10-4 6.90 × 10-4 1.38 × 10-3 2.07 × 10-3 2.76 × 10-3 2.93 × 10-3
Figure 3. Maximal rate coefficients of reaction II, kdec(max) (at C* for each molecular weight of polystyrene), as a function of polystyrene molecular weight.
Figure 4. Semiquantitative three-dimensional plot of kdec as a function of polystyrene concentration (expressed in wt %) and polystyrene molecular weight (due to software constraints, 3-D Excel 4.0, note that the MW axis is not to scale).
We therefore suggest that the polymer adsorbs on the cluster surface via weak coordination interactions between the π-orbitals of the arene groups on polystyrene and the d-orbitals of the forming cobalt clusters. This is consistent with previous studies of metal-polymer interactions,36,37 (36) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole Publishing Company: Monterey, CA, 1985. (37) Kostandinidis, F.; Thakkar, B.; Chakraborty, A. K.; Potts, L.; Tannenbaum, R.; Tirrell, M.; Evans, J. Langmuir 1992, 8, 1307-1317 and pertinent references therein.
which reveal that, under certain conditions (typical for each metal-polymer pair), there are specific coordination bonds which can develop between an electron-deficient metallic species and an organic moiety in the polymer chain containing π molecular orbitals. In other systems, low molecular weight molecules have been shown to adsorb on the metallic particles being formed, and the thickness of the adsorbed layer was calculated by using a combination of TEM and STM measurements.38 In reaction II, the mechanism of decomposition implies the existence of such electron-deficient cobalt species, and therefore it is most likely that they will interact with the π-system of the phenyl rings in the polystyrene molecule.12,24 Such an interaction creates a driving force which prompts the cobalt molecules (at the onset of the reaction) or forming cobalt clusters to seek the proximity of the polystyrene and promote the adsorption of the polystyrene onto the forming clusters. As Fleer and Scheutjens show,39,40 when the concentration of the adsorbed layer is high, the polymer provides steric repulsion between surfaces. At low coverage, however, the polymer induces attraction between surfaces due to bridging. The attraction is maximal at a finite polymer concentration. The concentration of the adsorbed layer increases with the solution concentration,41,42 until it reaches a plateau at high solution concentrations. It is presumed that, at high polymer concentrations, all available reaction sites on the cluster surface are occupied by the adsorbed polymer, and hence no additional polymer chains can “stick” to the surface. At a given solution concentration, the concentration of the adsorbed layer was shown to increase monotonically with chain molecular weight.41,42 On this basis, it is possible to analyze the dependence of kdec on the polymer concentration in solution and on the polymer molecular weight. The rate of reaction II is a function of the cobalt concentration associated with the polymer chains, the mobility of the cobalt species in solution, and the ability of cobalt particles within a particular chain and on different chains to interact with each other. As [PS] increases, the cobalt concentration associated with the polymer increases (we assume that (38) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (39) Fleer, G. J.; Scheutjens, J. M. H. M., J. Colloid Interface Sci. 1986, 111, 504. (40) Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1985, 18, 1882. (41) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1980, 84, 178. (42) Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sci. 1982, 16, 341. (43) Tannenbaum, R.; Dietler, U. K.; Bor, G. Inorg. Chim. Acta 1988, 154, 109.
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Figure 5. Normalized rate coefficients of reaction II, kdec/ kdec(max), as a function of the normalized polystyrene concentration, [PS]/C*, for each corresponding molecular weight.
the number of cobalt clusters which are not interacting with the polymer decreases with the increase in polystyrene concentration), but the mobility of the cobalt species decreases due to the increase in solution viscosity and lower diffusion rates, resulting in the behavior shown in Figure 2. Furthermore, as the MW(avg) of polystyrene increases, the concentration of the cobalt species interacting with the polymer chains increases, but the radius of gyration and the excluded volume of each chain increase as well, thus rendering interactions between the chains more difficult, limiting the possibility of interactions between cobalt clusters associated with different chains, and resulting in the behavior shown in Figure 3. We may therefore conclude that there is an optimal solution concentration at a given chain MW or an optimal chain MW at a given solution concentration at which the attraction between clusters, and, hence, the reaction rate, is maximal. An interesting insight into the mechanism of decomposition can be obtained by the normalization of the results, i.e., their expression as the ratio of kdec and kdec at C* (kdec/kdec(max)) as a function of the ratio of polystyrene concentration and C* ([PS]/C*) for that particular molecular weight, as shown in Figure 5. The data points lie on a “universal curve”, which implies that the “catalytic” influence which the presence of polystyrene in the solution has on reaction II is a general characteristic of this type of cobalt cluster/polystyrene system. 3. Experimental Procedure 3.1. Preparation of Polystyrene Solutions. Polystyrene (Aldrich Chemicals, MW(avg) ) 250 000, powder form; 3.24 g) was dissolved in 250 mL of toluene (Fluka Chemical Corp., bp 110 °C, d ) 0.867), to produce a 0.518 × 10-4 M solution corresponding to its critical concentration C* of ∼1.5 wt %. Other solutions are listed in Table 1. The toluene used in these solutions was dried by molecular sieve pellets (Matheson, Coleman, and Bell) and redistilled under an N2 stream. The solutions were mixed in a 500 mL round-bottom flask for 24 h at room temperature. The
Tannenbaum more concentrated solutions (polystyrene concentrations above 40 wt %) were refluxed under N2 at 90-100 °C to ensure homogeneity. All solutions were either used immediately or stored under a nitrogen atmosphere. 3.2. Decomposition of Cobalt Carbonyl Solutions. The solution decomposition of Co4(CO)12 in an inert atmosphere (reaction II) was performed in a closed system according to the classical stability diagram of cobalt carbonyls as a function of temperature and p(CO).9 Freshly prepared Co4(CO)12 crystals (1.287 g), synthesized by the method of Natta et al.,7 were dissolved in 50 mL of toluene (Fluka Chemical Corp., d ) 0.867), corresponding to a 4.51 × 10-2 M solution. The solution was placed in a 100 mL three-neck round-bottomed flask flushed with carbon monoxide. The middle neck was equipped with a reflux condenser, a side neck with a rubber stopper, and the other neck with a thermometer. The rubber outlet was also used for sampling. The reflux condenser was equipped with a gas inlet-outlet glass fitting device. The outlet part was connected to a tube whose other side was inserted securely into the vent. These special precautions had to be taken in order to flush away carbon monoxide gas formed during the decomposition reactions. Then the flask was placed in a controlled heated oil bath and the decomposition reaction carried out for approximately 48 h at 90 °C under a continuous N2 stream. Stirring was essential throughout the reaction time. During the reaction period, samples were removed with a syringe at various time intervals, and their infrared spectra were recorded in NaCl liquid cells with toluene as a reference. The decomposition reactions performed in solutions containing polystyrene were identical to the procedure described so far, but instead of using pure toluene as the solvent, various polystyrene solutions were used (Table 1), depending on the desired polymer concentration and molecular weight. For polystyrene solutions of 40 wt % and above, some heating with vigorous stirring was necessary to ensure homogeneity of the solutions. The values of the extinction coefficients for the 1858 and 1867 cm-1 bands in polymer solutions of increasing viscosity were obtained by recording the infrared spectra of the polystyrene solutions containing varying amounts of cobalt carbonyls, in a manner similar to the method described elsewhere.39 The solid state decompositions and extinction coefficient evaluation were performed using the techniques described in a previous publication.11 The intrinsic viscosities [η] of polystyrene and the cobalt-polystyrene compositions were determined using an Ubbelohde OB viscosimeter and used to calculate the MW(avg) of the polystyrene moieties. 3.3. Infrared Spectroscopic Measurements. Infrared spectra were recorded on an IBM-IR-44 Fourier transform spectrometer using a demountable liquid cell purchased from Crystal Laboratories and equipped with a 0.5 mm spacer and new NaCl windows. The cell chamber was purged with dry nitrogen for at least 0.5 h before interferrograms were collected. The resolution was 0.5 cm-1, and 3000 scans were taken for both the sample and the reference solutions. The empty, clean, and dry liquid cell was used as the background. Upon completion of each interferrogram, the sample in the cell was removed by vacuum suction and the cell thoroughly flushed with toluene. When samples contained viscous solutions (over 40 wt % polystyrene), the liquid cell was disassembled, the NaCl crystals and spacer were washed in toluene, and then the cell was reassembled using the same spacer. Spectral subtraction was performed using a subtraction factor of 1.00 in order to avoid possible spectral distortions.
Acknowledgment. The author thanks Prof. Matthew Tirrell, Prof. Phil Pincus, and Prof. Nily Dan for enlightening discussions. The financial support of the University of Minnesota Foundation is also acknowledged. LA9701992
