Termination Rate Coefficients of Butyl Acrylate Free-Radical

Free-radical termination kinetics of butyl acrylate (BA) homopolymerization in supercritical CO2 (scCO2) and in bulk have been studied by single pulse...
0 downloads 0 Views 86KB Size
Download PDF
3338

Ind. Eng. Chem. Res. 1999, 38, 3338-3344

Termination Rate Coefficients of Butyl Acrylate Free-Radical Homopolymerization in Supercritical CO2 and in Bulk Sabine Beuermann,* Michael Buback, and Claudia Schmaltz† Institut fu¨ r Physikalische Chemie, Universita¨ t Go¨ ttingen, Tammannstrasse 6, 37077 Go¨ ttingen, Federal Republic of Germany

Free-radical termination kinetics of butyl acrylate (BA) homopolymerization in supercritical CO2 (scCO2) and in bulk have been studied by single pulse-pulsed laser polymerization (SP-PLP) at temperatures and pressures up to 120 °C and 2500 bar, respectively. Polymerization induced by a single laser pulse is monitored via microsecond time-resolved near-infrared (NIR) spectroscopy. From individual SP-PLP experiments performed at several stages during the course of a polymerization, the ratio of termination to propagation rate coefficients, kt/kp, is obtained as a function of monomer conversion. With kp being available from pulsed laser polymerization in combination with size exclusion chromatography (PLP-SEC) experiments in both solution of scCO2 and in bulk, the kt/kp data yield individual (chain-length averaged) kt. Irrespective of polymerization pressure and temperature, kt is larger in scCO2 than in bulk polymerizations. At the highest CO2 content of 46 wt %, this enhancement amounts to about 350%. The activation volumes of kt are similar for polymerizations in scCO2 and in bulk, whereas the activation energies are clearly dissimilar: In solution of scCO2 an apparent activation energy is found which is negative. In bulk polymerization, on the other hand, EA(kt) is positive. The experimental observations indicate that thermodynamic contributions strongly affect termination rate in scCO2 solution. Introduction co-workers1

reported the In 1992, DeSimone and benefits of polymerization in supercritical (sc) CO2. Their article induced an enormous interest in this area of research. Attempts to synthesize polymers with welldefined properties in the heterogeneous phase of scCO2 have been particularly successful.2 Less attention has been paid to free-radical polymerization of common monomers, such as styrene and the (meth)acrylates, in the homogeneous phase of scCO2. So far, mostly propagation rate coefficients, kp, have been studied for a few such monomers, applying the International Union of Pure and Applied Chemistry (IUPAC)-recommended pulsed laser polymerization-size exclusion chromatography) (PLP-SEC) technique.3 For methyl methacrylate (MMA)4 and for styrene5 pulsed laser polymerizations carried out under reaction conditions that yield low molecular weight (10 000-20 000 g/mol) material resulted in almost identical kp values for reactions in solution of scCO2 and in bulk. Our studies into kp of MMA and of butyl acrylate (BA) at PLP conditions that yield high molecular weight material,6,7 on the other hand, revealed a decrease in kp upon increasing CO2 concentration, e.g., at CO2 contents greater than 40 wt %, kp is 40% less than the bulk value. Recent kp data for MMA polymerization (to higher molecular weight material) in CO2 by Quadir et al.,8 turned out to be smaller than the corresponding bulk values, too. Studying the temperature dependence of kp for BA polymerizations in CO2 showed that the activation energy is identical with the corresponding bulk value.7 * To whom correspondence should be addressed. Telephone: + 49-551-393138. Fax: + 49-551-393144. E-mail: [email protected]. † Present address: BAYER.AG, 51368 Leverkusen, Germany.

The effects seen for kp in scCO2 were assigned to the poor solvent quality of CO2 for conventional polymers.6,7 These thermodynamic arguments should be less important for low molecular weight (MW) polymer. In the case of a high MW polymer, the poor solvent quality of CO2 may result in more tightly coiled polymer molecules. This situation should be associated with local monomer concentrations (at the site of the propagating radical) that are smaller than the overall monomer concentration of the system.6,7 Such an explanation is consistent with arguments that have been presented to explain the solvent dependence of copolymerization kinetics: preferential solvation of the propagating radical and the existence of “local” monomer concentrations at the freeradical site.9-11 The termination process should be affected by solventinduced changes of polymer coil characteristics to a much larger extent than the propagation reaction. Therefore, it seemed interesting to study the termination rate coefficient, kt, of BA in both solution of scCO2 and in bulk. Thermodynamic contributions to termination kinetics have been discussed previously by O’Driscoll and co-workers.12-14 They assigned observed variations in kt to changes in solvent quality originating from properties of the polymeric product. Polymerization in poor solvents leads to an enhancement in kt. The reported effects were, however, relatively small, of the order of a few percent. Single pulse-pulsed laser polymerization (SP-PLP) experiments allow for the determination of kt/kp across wide ranges of T and p and up to high degrees of monomer conversion.15 The technique has already been used to study bulk free-radical termination kinetics of ethene16-18 and of several acrylates.19-21 Recently, Buback and Kowollik succeeded in applying the SP-PLP method to MMA bulk polymerization22 as well as to

10.1021/ie9901933 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/22/1999

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3339

binary and ternary bulk copolymerizations of acrylatemethacrylate systems.23,24 The SP-PLP experiment consists of monitoring polymerization induced by a single laser pulse, typically of 20-ns width, via online IR/NIR spectroscopy with a time resolution of microseconds. The experimentally obtained monomer conversion vs time traces are fit by eq 1, yielding the two coupled parameters kt/kp and kt‚c0R.

cM(t) c0M

-kp

) (2‚kt‚c0R‚t + 1) 2kt

(1)

c0M is the monomer concentration before applying the single laser pulse; cM(t) is the monomer concentration at time t (after applying the laser pulse at t ) 0); and c0R is the primary free-radical concentration induced by a particular laser pulse. The derivation of eq 1 was detailed by Buback and Schweer.17 It should be noted that kt refers to the termination rate law (eq 2):

dcR ) -2‚kt‚c2R dt

(2)

With kp known from independent PLP-SEC experiments, kt is obtained directly from each individual kt/ kp value. kt refers to the narrow conversion range of the particular single-pulse experiment, which typically extends over no more than 2% of monomer conversion. Repeated SP-PLP experiments at several stages during a polymerization to higher conversion allow the determination of the dependence of kt on overall monomer conversion. Throughout the subsequent text, termination rate coefficients kt refer to chain-length averaged quantities. During each single-pulse experiment, the free-radical chain length increases linearily with time t (unless chain-transfer reactions come into play). Using a single chain-length independent kt to represent termination behavior appears to be a reasonable assumption under the specific polymerization conditions of the present work, as is demonstrated by the adequate fit of experimental monomer conversion vs time traces by eq 1. This equation takes only a single chain-length independent kt into account. The finding, however, does not prove that kt is chain-length independent, as during most of the time-resolved observation period, the free radicals are of moderately large or even large size. Under such conditions, a weak chain-length dependence is not detected easily. The aim of this present paper is to provide kt values from SP-PLP for BA polymerizations in scCO2 and in bulk. The dependence of kt on monomer conversion will be investigated for various CO2 contents at polymerization pressures and temperatures up to 2500 bar and 120 °C, respectively. Experimental Section Butyl acrylate (>99%, stabilized with 0.0015 wt % hydroquinone monomethyl ether, Fluka) is distilled over K2CO3 under reduced pressure to remove the inhibitor. BA, which is used in the bulk experiments, is freed from oxygen by several freeze-pump-thaw cycles. Under an argon atmosphere, the photoinitiator, 2,2-dimethoxy-2phenylacetophenone (DMPA, 99% Aldrich-Chemie), is added to the monomer, and the mixture is poured into

Figure 1. Experimental traces of relative monomer concentration vs time t (after applying an excimer laser pulse) of BA polymerizations at 40 °C/1000 bar at 38% of monomer conversion from preceding polymerization. The open circles refer to solutions containing 27 wt % CO2, the filled circles to a polymerization with 10 wt % CO2.

an internal cell,25 which is inserted in the optical highpressure cell. Pressure is applied by a manually driven pressure generator using n-heptane as the pressuretransmitting medium. For polymerization in scCO2 (grade 4.5, Messer Griesheim), BA is mixed with the photoinitiator and then freed from oxygen by passing CO2 through the mixture for 20 min. The setup used for preparing the homogeneous monomer-CO2 mixtures and for filling the reaction mixture into the optical high-pressure cell has been described previously.6 After reaction pressure and temperature have been reached, the high-pressure cell is disconnected from the mixing setup and introduced into the sample compartment of a Fourier transform (FT) IR/NIR spectrometer (Bruker IFS 88) to measure the absolute BA concentration. The procedure of deriving BA concentrations, for experiments in bulk and in CO2, from NIR absorbance spectra in the 61006250 cm-1 region, has been detailed previously.6 The laser pulsing is carried out in the SP-PLP setup in which the laser-induced change in relative BA concentration is measured with a time resolution in the microsecond range. The SP-PLP setup consists of a Lextra 50 excimer laser (Lambda Physik) with a pulse width of 20 ns operated on the XeF-line at 351 nm, a 75-W tungsten halogen lamp (General Electric) powered by two batteries (12 V, 180 A/h), a BM 50 monochromator (B&M Spectronic), and a detector unit equipped with a fast InAs detector (EG & G, Judson) with a 2-µs time resolution. The samples are irradiated with excimer laser light at single-pulse energies of approximately 1 mJ. The resulting monomer conversion is monitored by on-line NIR spectroscopy at approximately 6170 cm-1. A few “true” SP experiments are co-added to obtain one SP trace with reasonable signal-to-noise quality. Fitting this time-resolved signal to eq 1 yields kt/kp (and kt‚c0R as the second parameter). After conducting a few such SP experiments, absolute BA concentration is measured via FT-IR spectroscopy. The cycle of time-resolved SP experiment and absolute FTIR measurement of BA concentration is repeated until the polymerizing system becomes inhomogeneous or the photoinitiator is consumed. Results and Discussion In Figure 1 the relative change in monomer conversion, cM(t)/c0M, is plotted as a function of time t (after

3340 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

justified because the acrylate monomers show a pronounced family-type behavior.29 The influence of CO2 on propagation rate has been investigated previously.6,7 As pointed out in these articles, PLP-SEC experiments yield the product of kp and cM. The observed reduction of this product can be assigned to either kp or cM or to changes of both quantities. To account for the reduced propagation rate in the calculation of kt, we decided to assign the observed effect entirely to a reduction in kp. With this assumption, the relative (with respect to bulk kp) propagation rate coefficient for BA polymerization in solution of CO2 has been fitted to the empirical relation (eq 3)21: Figure 2. Conversion dependence of kt/kp for BA polymerizations carried out at 40 °C/1000 bar in bulk (triangles) and in solution of CO2 (46 wt %) (squares).

applying the laser pulse at t ) 0) for two BA polymerizations at 40 °C and 1000 bar, but at different CO2 contents, 10 and 27 wt %. c0M refers to the monomer concentration before the laser pulse is applied. Both signals were measured at a monomer conversion of 38% (from preceding laser-induced polymerization). The lines in Figure 1 are fits of the experimental data to eq 1. Conversion Dependence of kt/kp and of kt. Figure 2 gives experimental log(kt/kp) values, as derived from fitting SP-PLP concentration vs time traces to eq 1, for BA bulk and solution (containing 46 wt % CO2) polymerizations at 40 °C and 1000 bar. Within the entire conversion range of these experiments, bulk kt/ kp is clearly below the corresponding solution (in CO2) data. In both systems, an initial increase in kt/kp with conversion is seen which extends up to monomer conversions of about 10%. This enhancement is more pronounced, by about a factor of 3.5, in CO2 solution. Upon further polymerization, kt/kp slightly decreases in bulk polymerization, whereas kt/kp stays almost constant in the solution polymerization, at least up to the maximum experimental conversions of about 60%. The initial increase in kt/kp during bulk BA polymerization was not reported in the early study by Buback and Degener,26 which probably is associated with the rather low free-radical concentrations that were applied in the previous study. Under such conditions, inhibition by impurities can interfere and may translate into an overestimate of kt. Within the present investigation, it was carefully checked that the weak initial increase of kt/kp is real. After the initial conversion period, above 10% monomer conversion, no major difference in conversion dependence was seen between the bulk polymerization kt/kp data of the present and of the previous study.19,26 The subsequent discussion of the bulk polymerization rate data will be based exclusively on the results of this work. With kp being known, the termination rate coefficient was obtained immediately from kt/kp. Bulk kp values of BA at ambient pressure were determined by PLP-SEC experiments up to 30 °C. These data were extrapolated to the temperature of the SP-PLP experiment using the Arrhenius relation given by Beuermann et al.27 The pressure dependence of BA bulk kp has not yet been determined. To estimate kp for higher p, the activation volume ∆Vq(kp) of BA has been identified with the arithmetic mean value of the measured methyl acrylate (MA) and dodecyl acrylate (DA) activation volumes, ∆Vq(kp, MA) ) - 11.2 cm3‚mol-1 and ∆V q(kp, DA) ) - 11.7 cm3‚mol-1, respectively.28 This procedure appears to be

(

)

cM ) exp 4.2‚ - 5.1 + 0.59 kp(bulk) cM(bulk) kp

(3)

(0.3 e cM/cM(bulk) e 1) cM(bulk) and cM are the initial (zero conversion) monomer concentrations, at polymerization temperature and pressure, in bulk and in solution polymerization, respectively. Equation 3 has been determined from PLPSEC experiments at -1 °C/1000 bar and 11 °C/200 bar. The derived kp values are assumed to be constant across the entire conversion range.30 kp values from eq 3 are used to calculate kt from the SP-PLP kt/kp data measured in solution of CO2. It has been verified that the alternate procedure, in which kp is identified with kp(bulk) and the change in propagation rate is assigned exclusively to a local monomer concentration, cM,loc, differing from overall monomer concentration, cM, yields almost identical kt data. Figure 3 shows the conversion dependence of kt for BA polymerizations at 40 °C/1000 bar in bulk and at various CO2 contents, between 10 and 46 wt %. For the entire conversion range under investigation, kt in solution of CO2 is higher than in bulk. In the initial stage of the polymerization reactions, however, this difference is small. The trends seen in the kt vs monomer conversion data are close to what was observed for the variation of experimental kt/kp values with conversion (Figure 2). In bulk polymerization, an initial increase in kt is observed, followed by a decrease toward higher conversion. The initial increase in kt is more pronounced in CO2 solution and extends to conversions greater than 10%. The subsequent decrease in kt becomes weaker toward higher CO2 content and, at the largest CO2 concentration, a plateau value is observed that extends up to the highest experimental conversion. The largest value of kt is found for the highest CO2 concentration. The plateau value of kt for the BA polymerization at 46 wt % CO2 is by about a factor of 4 above kt at zero conversion. At first sight, the increase in kt with monomer conversion and thus with increasing viscosity of the reaction medium seems to conflict with diffusioncontrolled termination. Similar, although smaller effects have been reported in the past, however. For the polymerization of MMA and of styrene a decrease in overall polymerization rate, rp, in the initial stage of the reaction has been described by several groups.12,13,31-33 This reduction in rp is too large to be assigned to initiator and/or monomer consumption. In experiments with MMA, North and Reed31 showed that the observed

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3341

Figure 3. Conversion dependence of kt for BA polymerizations carried out at 40 °C/1000 bar in bulk and in solutions containing different amounts of CO2. Each plot contains data from between two and four individual SP-PLP experiments.

reduction in rp may be attributed to an increase in kt. They support this argument by measurements of the mutual diffusion coefficient, Dmut, which increases with polymer concentration in the binary polymer-solvent system. The observation is explained by a reduction in coil size, which is associated with a higher segment density of the coil. A larger concentration gradient between the inner and outer regions of the coil is assumed to be responsible for a faster diffusion of the free-radical chain end to the surface of the coil thus, favoring termination.12,13,31,32 Mahabadi and O’Driscoll12 estimated diffusion coefficients for poly(methyl methacrylate) at different conversions, and they successfully tested their model against the experimental data from North and Reed. Kent et al.34 presented a different explanation. They assumed that the extent and the duration of overlap between two free-radical chain ends control termination rate. The overlap is diminished by effective repulsion between two polymer coils. At low polymer contents, this repulsion is reduced by background polymer where polymer-polymer interactions become more important compared with polymer-solvent interactions, giving rise to a more substantial overlap and a longer time of overlap of the reactive chain ends. These effects result in an increase of kt in the early period of free-radical polymerizations. The theories of both North and Reed31 and Kent et al.34 are based on the assumption that the mechanism of diffusion control of kt is segmental diffusion (kSD). That kt values are higher for polymerizations in CO2 solution than in bulk can be understood by segmental diffusion models, too.13,34 Both explanations rest on the poor solvent quality that CO2 offers to poly(butyl acry-

late). According to Kent et al.,34 in a poor solvent polymer, polymer interactions become more important than polymer-solvent interactions, which results in better and longer overlap and thus in higher kt. Dionisio et al.13 assumed that coil size is smaller in a poor solvent, which leads to a larger concentration gradient between inner and outer regions of the coil. Consequently, the termination rate is enhanced. The solubility of polyBA in scCO2 is limited.35 Polymerizations of BA in homogeneous fluid phase of CO2 may only be carried out up to fairly large degrees of monomer conversion and thus to high polymer content (see Figure 3) because of the cosolvent action of the monomer. During the course of the reaction, polymer concentration increases and the cosolvent (BA) content simultaneously decreases. With respect to solvent quality, these changes go into the same direction. On the basis of solvent quality arguments, particularly strong effects must be expected for free-radical polymerizations in solution of CO2. This is obviously what happens in the initial polymerization period. The enhancement in kt is rather pronounced at high CO2 content, and the absolute value of kt at zero conversion conditions is not very sensitive toward the CO2 content. This indicates that the observed effects are not caused by specific interactions of either free radicals or of the BA monomer with CO2, but are influenced by thermodynamic contributions. The kt behavior at higher degrees of monomer conversion is not overly surprising. The decrease of kt in bulk polymerization is due to the significant increase in viscosity, whereas the plateau value observed at the highest CO2 concentration is characteristic for a solution polymerization at moderate viscosity.

3342 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

Because of the importance of thermodynamic (solvent quality) contributions to termination kinetics, it is not surprising that the purely kinetic model for kt, which takes segmental, translation, and reaction diffusion into account and which has been used to fit the conversion dependence of kt in bulk polymerizations,36 is not applicable to solution polymerizations in CO2 without modification. Because the required additional information, preferably from independent thermodynamic, structural, and dynamic investigations, is not yet available, however such a modification and subsequent fitting will not be carried out in the present article. Activation Parameters of kt/kp and of kt. The temperature and pressure dependence of kt/kp and kt were studied for BA polymerization in bulk and in solution of CO2 at high CO2 contents, between 36 and 47 wt %. The kt/kp and kt data that were considered for the determination of activation energies, EA, and of activation volumes, ∆Vq, are the maximum value around 10% conversion in bulk polymerizations and the plateau value, between 15 and 35%, in the experiments in solution of CO2. The pressure dependence of kt/kp was examined at 40 °C. From an extended set of SP-PLP experiments at CO2 concentrations between 36 and 47 wt %, the variation of kt/kp with CO2 weight fraction (at 40 °C/ 1000 bar) is adequately represented by eq 4.21

log(kt/kp) ) 2.93 + 0.015xwt,CO2

(4)

(27 ( 3) cm3‚mol-1 and ∆V q(kt/kp) ) (24 ( 4) cm3‚mol-1 agree within experimental accuracy. These ∆Vq(kt/kp) are close to the corresponding data of other acrylate monomers. Kurz20 reported ∆Vq(kt/kp) values of (27 ( 2) cm3‚mol-1 and (32 ( 3) cm3‚mol-1 for the MA and DA bulk homopolymerizations, respectively. The temperature dependence at 1000 bar of kt/kp for polymerizations in bulk and in CO2 (42 wt %) may be represented by eqs 8 and 9, respectively:

log(kt/kp) ) (1.14 ( 0.44) + (556 ( 146)T-1/K-1

(8)

(BA bulk polymerization at 1000 bar; 10 °C e Θ e 60 °C)

(40 °C, 1000 bar, 36 e xwt,CO2/wt % e 47) By this relationship, kt/kp data that have been measured in the CO2-rich regime are shifted to a common CO2 content of 42 wt % CO2. The resulting correction term (eq 5) is also used at other experimental pressures and temperatures to calculate kt/kp for the reference condition of 42 wt % CO2 from the experimental SPPLP values, (kt/kp)exp:

log(kt/kp) ) log(kt/kp)exp + 0.015(xwt,CO2 - 0.42)

Figure 4. Variation of termination rate coefficient kt with pressure for BA polymerizations at 40 °C carried out in solution of CO2 at 42 wt % (circles) and in bulk (triangles). For details see text.

(5)

(36 e xwt,CO2/wt % e 47) The pressure dependence at 40 °C for polymerizations in bulk and in CO2 (42 wt %) may be represented by eqs 6 and 7, respectively:

log(kt/kp) ) (3.38 ( 0.07) (4.57 ( 0.49)10-4p/bar (6) (BA bulk polymerization at 40 °C; 200 bar e p e 2000 bar) log(kt/kp) ) (3.82 ( 0.152) (4.00 ( 0.83)10-4p/bar (7) (BA solution polymerization at 42 wt % CO2; 40 °C; 1000 bar e p e 2500 bar) Activation volumes ∆V q for kt/kp may be calculated from the linear fits in eqs 6 and 7 according to: ∆V q ) - R‚T‚(d lnk)/d p). The resulting values for polymerizations in bulk and in CO2 (42 wt %), ∆V q(kt/kp) )

log(kt/kp) ) -(0.133 ( 0.412) + (1110 ( 142)T-1/K-1 (9) (BA solution polymerization at 42 wt % CO2; 1000 bar; 40 °C e Θ e 120 °C) The activation energies (at 1000 bar) for polymerization in bulk and in solution of CO2 (42 wt %) are significantly different: EA(kt/kp) ) - (11 ( 2) kJ‚mol-1 (bulk) and EA(kt/kp) ) -(21 ( 3) kJ‚mol-1 (CO2). A previously reported value37 for EA(kt/kp) of BA at 30% conversion and ambient pressure is close to zero, but this number appears to be too large, probably due to a significant scatter on the data of this earlier work which has been carried out on pure BA without any photoinitiator. Kurz20 reported as activation energies for MA and DA bulk homopolymerizations at 1000 bar: EA(kt/kp) ) -(8 ( 2) kJ‚mol-1 (MA) and EA(kt/kp) ) -(12 ( 2) kJ‚mol-1 (DA) with these data referring to the fairly extended initial polymerization range where kt/kp of MA and DA is independent of conversion. This literature data are in excellent agreement with EA(kt/kp) of BA from the present study. Use of the kp(p,T) values of BA allows determination of the pressure and temperature dependence of kt from kt/kp(p,T). The subsequent numbers again refer to about 10% conversion in bulk polymerization and to the plateau value between 15 and 35% in the solution (in CO2) polymerizations. The pressure dependence of kt at 40 °C is shown in Figure 4. The circles and triangles represent the data measured in solution containing 42 wt % CO2 and in bulk, respectively. kt for polymerization in scCO2 is

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3343

Figure 5. Temperature dependence of kt for BA polymerizations at 1000 bar carried out in solution of CO2 at 42 wt % (circles) and in bulk (triangles). For details see text.

higher than in bulk. The data are fit by straight lines:

log(kt/(L‚mol-1‚s-1)) ) (7.74 ( 0.01) (2.67 ( 0.49)10-4p/bar (10) (BA bulk polymerization at 40 °C; 200 bar e p e 2000 bar) log(kt/(L‚mol-1‚s-1)) ) (8.01 ( 0.15) (1.96 ( 0.83)10-4p/bar (11) (BA solution polymerization at 42 wt % CO2; 40 °C; 1000 bar e p e 2500 bar) The activation volumes of ∆Vq(kt) are (16 ( 3) cm3‚mol-1 in bulk and (12 ( 5) cm3‚mol-1 in CO2. There is no indication of any significant difference in activation volume for BA free-radical polymerizations in bulk and in CO2. The bulk ∆V q(kt) value of BA is close to the corresponding activation volumes of kt in MA bulk polymerization, ∆V q(kt, MA) ) (16 ( 3) cm3‚mol-1, and in DA bulk polymerization, ∆V q(kt, DA) ) (20 ( 5) cm3‚mol-1, that have been measured by Kurz.20 The temperature dependence of kt at 1000 bar is shown in Figure 5. The circles and triangles again refer to data measured in solution containing 42 wt % CO2 and in bulk, respectively. A remarkable difference is seen between the bulk and solution data in that kt decreases with temperature in BA solution polymerization, whereas the ordinary behavior, an increase with T, is observed in bulk polymerization. Equations 12 and 13 represent the straight line fits given in Figure 5.

log(kt/(K‚mol-1‚s-1)) ) (8.41 ( 0.48) (292 ( 144)T-1/K-1 (12) (BA bulk polymerization at 1000 bar; 10 °C e Θ e 60 °C) log(kt/(K‚mol-1‚s-1)) ) (7.10 ( 0.411) + (225 ( 142)T-1/K-1 (13) (BA solution polymerization at 42 wt % CO2; 1000 bar; 40 °C e Θ e 120 °C) The activation energy for BA bulk polymerization is: EA(kt) ) (6 ( 3) kJ‚mol-1. This number is close to the activation energies determined by Kurz20 for the MA

and DA bulk homopolymerizations (also at 1000 bar): EA(kt, MA) ) (8 ( 3) kJ‚mol-1 and EA(kt, DA) ) (3 ( 3) kJ‚mol-1. Moreover, the activation energy for kt of bulk styrene polymerization at 1000 bar has been determined to be of similar size, 5 kJ‚mol-1.38 Such small positive activation energies are consistent with assuming termination to be diffusion controlled. This view is further supported by the large activation volumes observed in bulk polymerization. From the log kt vs T-1 data in Figure 5, the activation energy of the termination rate coefficient for BA polymerization in solution of CO2 at 1000 bar is calculated to be EA(kt) ) -(4 ( 4) kJ‚mol-1. This apparent activation energy may be caused partly by the occurrence of lower molecular weight polymeric species at lower temperature. They may be associated with a higher kt. As no similar effect is seen in bulk polymerization, however, it seems more likely that a thermodynamic “solvent quality” effect plays a major role. Lowering the temperature should decrease solvent quality and thus increase kt for the same reasons that have been presented to explain the clear enhancement with conversion of kt in the initial polymerization period (see Figures 3 and 4). The experimental observations are strongly indicative of such a thermodynamic (solvent quality) contribution to termination kinetics. The detailed mechanism of diffusion control, e.g., by translational or by segmental motions, however is, not yet clear. It appears to be a matter of priority to find out whether differences, between bulk and solution (in CO2) polymerization, of similar size also will occur with other monomers. Furthermore, whether the kt values for bulk and solution polymerization of BA (or of another monomer) become identical at high temperatures as suggested by the straight lines in Figure 5 should be investigated. Moreover, the kinetic studies should be accompanied by light-scattering experiments to check whether modifications of solvent quality result in changes of coil size. Conclusions Because of the cosolvent action of the monomer, the free-radical polymerization of butyl acrylate may be carried out in solution of CO2 up to monomer conversions of about 60%. The CO2 content significantly influences termination rate. At identical conditions of monomer conversion, polymerization temperature, and polymerization pressure, kt is always higher for polymerization in CO2 than in bulk. This difference in kt largely increases with monomer conversion. During solution polymerization, monomer is replaced continuously by CO2, which is a poor solvent for poly(butyl acrylate). The variation of kt with conversion and with temperature indicates solvent quality effects that contribute to termination kinetics. Such effects were discussed by O’Driscoll14 more than 20 years ago. They seem to be particularly pronounced in BA free-radical polymerization with CO2 acting as the (poor) solvent material. Acknowledgment Financial support of this study by the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm: “U ¨ berkritische Fluide als Lo¨sungs- und Reaktionsmittel”) and by the Fonds der Chemischen Industrie is gratefully acknowledged.

3344 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

Literature Cited (1) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science 1992, 257, 945. (2) Schaffer, K. A.; DeSimone, J. M. Chain Polymerizations in Inert Near- and Supercritical Fluids. Trends Polym. Sci. 1995, 3, 146. (3) Buback, M.; Gilbert, R. G.; Hutchinson, R. A.; Klumperman, B.; Kuchta, F.-D.; Manders, B. G.; O’Driscoll, K. F.; Russell, G. T.; Schweer, J. Critically evaluated rate coefficients for free-radical polymerization. 1. Propagation rate coefficients for styrene. Macromol. Chem. Phys. 1995, 196, 3267. (4) v. Herk, A. M.; Manders, B. G.; Canelas, D. A.; Quadir, M. A.; DeSimone, J. M. Propagation Rate Coefficients of Styrene and Methyl Methacrylate in Supercritical CO2. Macromolecules 1997, 30, 4780. (5) Beuermann, S.; Buback, M.; Isemer, C.; Lacı´k, I.; Wahl, A., to be submitted for publication. (6) Beuermann, S.; Buback, M.; Kuchta, F.-D.; Schmaltz, C. Determination of Free-Radical Propagation Rate Coefficients for Methyl Methacrylate and n-Butyl Acrylate Homopolymerizations in CO2. Macromol. Chem. Phys. 1998, 199, 1209. (7) Beuermann, S.; Buback, M.; Schmaltz, C. Pressure and Temperature Dependence of Butyl Acrylate Propagation Rate Coefficients in Fluid CO2. Macromolecules 1998, 31, 8069. (8) Quadir, M. A.; DeSimone, J. M.; v. Herk, A. M.; German, A. L. Pulsed-Laser Polymerization of Methyl Methacrylate in Liquid and Supercritical Carbon Dioxide. Macromolecules 1998, 31, 6481. (9) Kuchanov, S. I.; Russo, S. Quantitative Theory of FreeRadical Copolymerization Allowing for the Phenomenon of Preferential Sorption. Macromolecules 1997, 30, 4511. (10) Kratochvil, P.; Strakova´, D.; Stejskal, J.; Tuzar, Z. Preferential Solvation in Polymer-Monomer-Solvent Systems and Its Potential Effect on the Kinetics of Polymerization. Macromolecules 1983, 16, 1136. (11) Harwood: H. J. Structures and Compositions of Copolymers. Makromol. Chem., Macromol. Symp. 1987, 10/11, 331. (12) Mahabadi, H. K.; O’Driscoll, K. F. Concentration Dependence of the Termination Rate Constant During the Initial Stages of Free Radical Polymerization. Macromolecules 1977, 10, 55. (13) Dionisio, J.; Mahabadi, H. K.; O’Driscoll, K. F.; Abuin, E.; Lissi, E. A. High-Conversion Polymerization. IV. A Definition of the Onset of the Gel Effect. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1891. (14) O’Driscoll, K. F. Pressure dependence of the Termination Rate Constant in Free Radical Polymerization. Makromol. Chem. 1977, 178, 899. (15) Buback, M.; Hippler, H.; Schweer, J.; Vo¨gele, H.-P. Timeresolved Study of Laser-Induced High-Pressure Ethylene Polymerization. Makromol. Chem., Rapid Commun. 1986, 7, 261. (16) Schweer, J. Bestimmung individueller Geschwindigkeitskoeffizienten radikalischer Polymerisationen aus Laserpulsexperimenten mit Infrarot-Spektroskopischer Umsatzdetektion im Mikrosekunden-Bereich, University of Go¨ttingen, Germany, 1988. (17) Buback, M.; Schweer, J. Chain-length dependence of FreeRadical Rate Coefficients. 1. Termination Rate Coefficient in Pure Ethylene Polymerization. Makromol. Chem., Rapid Commun. 1988, 9, 699. (18) Buback, M.; Schweer, J. Conversion and Chain-Length Dependence of Rate Coefficients in Free-Radical Polymerization. Z. Phys. Chem. N. F. 1989, 161, 153. (19) Degener, B. Laserinduzierte radikalische Polymerisation von Butylacrylat in einem weiten Umsatzbereich. Ph.D. Dissertation, University of Go¨ttingen, Germany, 1992. (20) Kurz, C. H. Laserinduzierte radikalische Polymerisation von Methylacrylat und Dodecylacrylat in einem weiten Zustandsbereich. Graphikum Verlag: Go¨ttingen, 1995. (21) Schmaltz, C. Radikalische Polymerisation von Butylacrylat in fluidem Kohlendioxid bis zu hohem Druck; Klaus Bielefeld Verlag: Friedland, 1998.

(22) Buback, M.; Kowollik, C. Termination Kinetics of Methyl Methacrylate Free-Radical Polymerization Studied by TimeResolved Pulsed Laser Experiments. Macromolecules 1998, 31, 3211. (23) Buback, M.; Kowollik, C. Termination Kinetics in FreeRadical Bulk Copolymerization: The Systems Dodecyl AcrylateDodecyl Methacrylate and Dodecyl Acrylate-Methyl Acrylate. Macromolecules 1999, 31, 1445. (24) Buback, M.; Kowollik, C., Termination Kinetics in freeradical polymerization: The systems methyl acrylate-butyl acrylate-dodecyl acrylate and methyl methacrylate-butyl methacrylate-dodecyl methacrylate. Macromol. Chem. Phys. 1999, 200, 1764. (25) Buback, M.; Hinton, C. In High-pressure Techniques in Chemistry and Physics: A Practical Approach; Holzapfel, W. B., Isaacs, N. S., Eds.; Oxford University Press: Oxford, 1997. (26) Buback, M.; Degener, B. Rate Coefficients for Free-Radical Polymerization of Butyl Acrylate to High Conversion. Makromol. Chem. 1993, 194, 2875. (27) Beuermann, S.; Paquet, D. A., Jr.; McMinn, J. H.; Hutchinson, R. A. Determination of Free-Radical Propagation Rate Coefficients of Butyl, 2-Ethylhexyl, and Dodecyl Acrylates by Pulsed-Laser Polymerization. Macromolecules 1996, 29, 4206. (28) Buback, M.; Kurz, C. H.; Schmaltz, C. Pressure Dependence of Propagation Rate Coefficients in Free-Radical Homopolymerizations of Methyl Acrylate and Dodecyl Acrylate. Macromol. Chem. Phys. 1998, 199, 1721. (29) Buback, M. Free-Radical Polymerization Kinetics in Extended Ranges of Temperature, Pressure, and Monomer Conversion. Macromol. Symp. 1996, 111, 229. (30) Buback, M.; Gilbert, R. G.; Russell, G. T.; Hill, D. J. T.; Moad, G.; O’Driscoll, K. F.; Shen, J.; Winnik, M. A. Consistent Values of Rate Parameters in Free Radical Polymerization Systems. II. Outstanding Dilemmas and Recommendations. J. Polym. Sci., Polym. Chem. Ed. 1992, 30, 851. (31) North, A. M.; Reed, G. A. Diffusion-Controlled Termination during the Initial Stages of Free Radical Polymerization of Methyl Methacrylate. Trans. Faraday Soc. 1961, 57, 859. (32) Horie, K.; Mita, I. Triplet Probe Study of the Excluded Volume Effect on the Rates of Diffusion-Controlled Intermolecular Reactions between Chain End Groups of Polystyrene. Macromolecules 1978, 11, 1175 . (33) Horie, K.; Mita, I. Diffusion and Reaction Rates of EndLabeled Polystyrenes in Semidilute Solution Studied with Luminescent Triplet Probe. Macromolecules 1981, 14, 1428. (34) Kent, M. S.; Faldi, A.; Tirrell, M.; Lodge T. P. Concentration Dependence of the Rate of Polymer-Polymer Reactions in the Dilute Regime. Macromolecules 1992, 25, 4501. (35) McHugh, M. A.; Rindfleisch, F.; Kuntz, P. T.; Schmaltz, C.; Buback, M. Cosolvent Effect of Alkyl Acrylates on the Phase Behaviour of Poly(alkyl acrylates)-Supercritical CO2 Mixtures. Polymer 1998, 39, 6049. (36) Buback, M. Free-Radical Polymerization up to High Conversion. A General Kinetic Treatment. Makromol. Chem. 1990, 191, 1575. (37) Buback, M.; Degener, B.; Huckestein, B. Conversion Dependence of Free-Radical Polymerization Rate Coefficients from Laser-Induced Experiments. 1. Butyl Acrylate. Makromol. Chem., Rapid Commun. 1989, 10, 311. (38) Buback, M.; Kuchta, F.-D. Termination Kinetics of FreeRadical Polymerization of Styrene over an Extended Temperature and Pressure Range. Macromol. Chem. Phys. 1997, 198, 1455.

Received for review March 16, 1999 Revised manuscript received May 17, 1999 Accepted May 18, 1999 IE9901933