1130
Ind. Eng. Chem. Res. 1997, 36, 1130-1135
ESR Study on Peroxide Modification of Polypropylene Weixia Zhou and Shiping Zhu* Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
Chemical modification of isotactic polypropylene via a free-radical mechanism was studied using an on-line electron spin resonance spectrometer. Peroxides were used to provide primary radicals upon thermal decomposition at elevated temperatures for the generation of polymer backbone radicals. Four types of peroxides, tert-butyl perbenzoate, dicumyl peroxide, 2,5-dimethyl-2,5bis(tert-butylperoxy)hexyne-3, and benzoyl peroxide, at various concentration levels were employed. The ESR measurement demonstrated that the modification was accomplished via a free-radical mechanism. Both radical generation and termination were clearly observed. Peroxide type, concentration, temperature, and reaction time were the major factors that affected the radical concentration profiles. The radical concentration variations were used to estimate the radical termination rate constants. Introduction The commercial incentive for the chemical modification of commodity polymers by grafting, chain scission, long-chain branching, and cross-linking is the enhancement of the physical and chemical properties of these polymers and polymer mixtures (alloys and blends) and/ or the improvement of their processability. For example, polypropylene (PP) made by the conventional Ziegler-Natta process generally has a very high molecular weight (MW) and a broad molecular weight distribution (MWD). Chemical modification of PP is aimed to obtain PP with controlled rheology, a lower MW, and a narrower MWD. The application of extruders makes the modification an easy and preferable practice in terms of the productivity and economic factors (Xanthos, 1992; Lambla, 1994). Such chemical modifications are often implemented via free-radical mechanisms (Hamielec et al., 1991; Zhu and Hamielec, 1992). Peroxides are used as radical initiators. Upon thermal decomposition, peroxide molecules decompose into primary radicals. These primary radicals are so energetic that they can abstract atoms, generally protons, from polymer chains to give backbone radicals. The reactivities of polymer radicals are correlated with polymer chain structure. The polymer radicals may undergo chain scission, branching, and cross-linking as well as grafting in the presence of additives. It has been found that PP undergoes almost exclusively chain scission (Tzoganakis, 1988; Tzoganakis et al., 1988b; Suwanda et al., 1988a) and polyethylene (PE) favors branching and/or cross-linking (de Boer and Pennings, 1982; Tang et al., 1989). Polymer radicals are terminated by a bimolecular process, either disproportionation or combination. This free-radical mechanism has been suggested based on the modification products. Direct measurement and information about the modification kinetics are expected to provide a better understanding of the modification mechanism and an estimation of kinetic parameters. The advent of the modern electron spin resonance (ESR) technique has provided a powerful and direct method to measure radical types and concentrations. This technique has been successfully applied to the studies of free-radical polymerization, mechanical or * Author to whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (905) 525-9140, ext. 24962. Fax: (905) 521-1350. S0888-5885(96)00410-1 CCC: $14.00
irradiation degradation, and molecular motion (Ranby and Rabek, 1977; Kamachi, 1987; de Vries and Roylance, 1985; Shimada, 1992). However, a very low radical concentration unable to be detected by an ESR spectrometer and/or a high instability of radical intermediates severely impedes the extensive practice of ESR measurement. Most of the ESR studies to date were therefore conducted at very low temperatures, which were far from the reaction conditions of commercial interest. Recently, we successfully carried out an on-line ESR study on peroxide modification of isotactic polypropylene (iPP). This study demonstrated that the process was implemented via a free-radical mechanism. The radical generation and termination were clearly observed. Information of the radical concentration development during modification was obtained by analyzing the ESR spectra. Based on this information, polymer radical termination rate constants were estimated. In this paper, we present these results. Experimental Section iPP powder (KY6100, Shell) was mixed with peroxide at concentrations of 0.27, 0.22, 0.16, 0.11, 0.054, and 0.027 mol/kg of iPP. Four types of peroxides were employed. They were tert-butyl perbenzoate (TBPB, Aldrich), dicumyl peroxide (DCP, Aldrich), 2,5-dimethyl2,5-bis(tert-butylperoxy)hexyne-3 (L130, Akzo), and benzoyl peroxide (BPO, Aldrich). The chemical structures of these peroxides are shown in Chart 1. In order to mix iPP and peroxide uniformly, peroxide was dissolved in acetone prior to iPP addition. Acetone was evaporated by continuous stirring. The mixture was then dried in vacuum at room temperature for 2 days. The polymer sample was sealed into glass ampules of 5-mm outer diameter (3-mm inner diameter) for ESR measurements. The glass ampule filled with reactants was inserted into a TM110 cavity of a Bruker ER 100D ESR spectrometer. The spectrometer cavity was preheated and kept at a desired equilibrium temperature by a gas bath. Modification was implemented on-line to avoid changes in radical type and concentration during quenching. Three reaction temperatures, i.e., 160, 170, and 180 °C, were chosen to satisfy the low-temperature preference for the ESR measurements and the high industrial practice temperatures. The use of high peroxide concentration levels was also for the same purpose, i.e., to © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1131 Chart 1 O C
CH3 O
O
C
CH3
CH3 TBPB (tert-butyl perbenzoate)
CH3 C
CH3 O
O
C CH3
CH3
DCP (dicumyl peroxide) CH3 CH3
C CH3
CH3 O
O
C
CH3
CH3 C
C
CH3
C CH3
O
O
C
CH3
CH3
L130 (2,5-dimethyl-2,5-bis-(tert-butylperoxy)hexyne-3)
O C
O O
O
C
BPO (benzoyl peroxide)
obtain ESR signals with a high signal-to-noise ratio for satisfied quantitative analysis, especially at high temperatures which greatly lower the sensitivity of the ESR spectrometer. The reaction was initiated by thermal decomposition of peroxide at an elevated temperature. Radical spectra were recorded frequently to follow the process in detail. Since the reaction proceeded very quickly, half of ESR spectrum was normally recorded. Absolute radical concentrations were calibrated using 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH, Aldrich) dissolved in benzene at room temperature. Considering that ESR absorption is a function of the population difference between various unpaired spin levels and that this distribution is related to temperature, the temperature effect was compensated for (Poole, 1967). The ampule reactor in the ESR cavity was heated via thermal conduction through the glass wall. It therefore took a certain period of time for the inside reacting mass to reach the cavity temperature. To obtain this temperature development, the temperatures at two different positions inside the ampule reactor, inner wall and centerline, were measured using a method similar to that developed by Zhu and Hamielec (1991). Results and Discussion Figure 1A shows a series of ESR spectra recorded at different times during the course of modification. The reaction was carried out at 180 °C with 0.27 mol of TBPB/kg of iPP. Information about the radicals and their changes can be obtained directly from these ESR spectra. At the very beginning of the process, no polymer radicals were generated as shown by the lack of a signal. After a short time period, a radical signal started to appear. The growing intensity of the ESR spectrum revealed that more radicals were being generated. Since radical termination is a bimolecular reaction, the reaction became dominant when the radical concentration increased. The signal intensity therefore became weaker and finally disappeared. During the
Figure 1. ESR spectra recorded during the peroxide modification of iPP at 180 °C with the peroxide level (mol/kg of iPP): A, TBPB 0.27; B, DCP 0.27; C, L130 0.27; and D, BPO 0.33. The operation conditions were microwave frequency 9.39 GHz, modulation frequency 100 kHz, modulation amplitude 3.2 G, power 2 mW, gain 4 × 105, and sweep time 20 s.
reaction, the ESR signal normally exhibited changes only in the signal intensity except for the very initial stage. The observation of the ESR signals confirms that free radicals are involved in the modification process. In addition to TBPB, other peroxides, i.e., DCP, L130, and BPO, were also used. They all presented ESR spectra with the identical hyperfine structure, as shown in Figure 1B-D. This observation suggests that the exact nature of peroxide has little direct effect on the ESR spectra. It is therefore speculated that the signal carriers presenting the ESR spectrum are not related to the peroxide species but to the iPP backbone radicals. According to the early literature and well-accepted viewpoint (Zwolenik, 1967; Shida, 1968; Gilbert and Dobbs, 1972), the alkoxy radicals or primary radicals (RO•, R is a hydrocarbon group) generated via irradiation at low temperatures could not be detected by an ESR spectrometer, mainly due to their short lifetimes. At the high temperatures used in this work, it is even more difficult for an ESR machine to monitor these radicals. We also made an effort to observe these primary radicals by mixing a large amount of peroxide with various inert powders. Only a single-line peroxy radical (ROO•) signal (de Vries and Roylance, 1985) was observed. Since oxygen molecules were inevitably trapped in the iPP/peroxide reacting systems, a small number of peroxy radicals on the polymer backbone could be produced. This ESR spectrum was sometimes observed in our measurements but only at the very beginning of the reaction for a very short time period. The complicated hyperfine structure of the observed ESR spectra presented in Figure 1 is therefore believed to arise from iPP backbone carbon radicals.
1132 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
The successful on-line ESR observation verified the hypothesis that the peroxide modification process is implemented by a free-radical mechanism (Hamielec et al., 1991; Zhu and Hamielec, 1992). Peroxide is used only as a primary radical source. It generates primary radicals upon thermal decomposition at elevated temperatures. The primary radicals immediately undergo proton abstraction to produce carbon radicals on polymer backbones. The polymer backbone radicals then react to fulfill various modification purposes. The modification process becomes complete when the radicals are terminated. Referring to the structure of PP, primary radicals preferentially attack the tertiary protons (Russell, 1973; Dorn, 1985; Tzoganakis et al., 1988b) to introduce tertiary carbon radicals (A) on the PP backbone. ~
CH2
• C
CH2
CH
~
CH3
CH3 A
However, the hyperfine structue of the observed ESR spectrum in this work is remarkably different from that of the tertiary iPP radicals (A) generated by irradiation methods (Kusumoto, 1968; Ranby and Rabek, 1977; Ooi et al., 1975). The previous studies of irradiationgenerated iPP radicals also showed that the ESR spectrum experienced changes with the elevation of temperature and these changes were conceived to be due to the change in the polymer chain mobility (Kusumoto, 1968; Ooi et al., 1975). The significant difference of the ESR spectra observed at the high temperatures in this work compared to those low-temperature spectra is believed to be partly attributed to the phase change and the corresponding mobility difference of the polymer chains. It was proposed (Dorn, 1985; Tzoganakis et al., 1988b) that the mechanism of iPP modification involves the β-scission of tertiary radicals, presented as follows: ~
CH2
• C
CH2
CH
CH2
~
CH3
CH3 A
~
CH2
C
• CH2 + CH
CH2
~
Figure 2. Development of the ESR signal intensity during the peroxide modifications at 180 °C of 0.27 mol of TBPB, 0.27 mol of DCP, 0.27 mol of L130, and 0.33 mol of BPO per kg of iPP.
BPO as primary radical providers generated much weaker signals at the same conditions than those with TBPB and DCP. Therefore, only TBPB and DCP data are used for the quantitative analysis in this work. The spectrum intensity is known to be proportional to the radical concentration. Therefore, these intensity data can be calibrated and converted into radical concentration data which are very useful for a quantitative study of the modification kinetics. It should also be pointed out that the radical concentration decays rapidly. It is difficult to quench the reaction before ESR measurements. Our early attempts for off-line ESR measurements using the quench technique were fruitless. We therefore recommend the online approach. The radical concentration profiles during the course of modification can be used to estimate the kinetic rate constants. As mentioned earlier, the modification process starts with the thermal decomposition of peroxide. Polymer backbone radicals are thus generated. Some radicals may undergo secondary reactions, such as the β-scission of the tertiary radicals in this work. Since the radical concentration does not change during this process, the secondary reaction is not specified in the following analysis. Polymer radicals are terminated bimolecularly, either by combination or by disproportionation. Consequently, the two fundamental equations used to describe the process are as follows:
CH3
CH3 B
A secondary radical B is formed on the degraded chain. The experimental observation of the ESR spectrum could not elucidate this conversion. The hyperfine structure remains the same during the course of modification, as shown in Figure 1. This is possibly due to the short lifetime of radical A at the elevated temperatures (Hamielec et al., 1991). Therefore, it is assumed that the experimental ESR spectrum observed in this work is the combination of radical A and radical B, probably mainly radical B. Figure 2 shows the intensities of the ESR spectra presented in Figure 1 as a function of the reaction time. In spite of the variation of the peroxide type, the ESR spectra demonstrated the same tendency of the intensity changes. The intensity is zero at the initial time followed by a dramatic increase. After the maximum peak, it starts to decrease and finally approaches zero. The intensity also depends on the peroxide concentration and reaction temperature. Systems using L130 and
d[I] ) -Kd[I] dt
(1)
d[R•] ) 2fKd[I] - Kt[R•]2 dt
(2)
where t is the reaction time, f is the peroxide efficiency, Kd is the peroxide decomposition rate constant, [I] is the peroxide concentration. [R•] is the radical concentration, and Kt is the termination rate constant. The factor 2 accounts for the fact that one peroxide molecule produces two primary radicals. The experiments started at the moment when the glass ampule was inserted into the ESR spectrometer cavity. Figure 3 presents the temperature development at the inner wall and the centerline of the ampule reactor. It shows that it needs a certain period of time for the reactor to assume the cavity temperature. During this short period, as evident in Figure 2, there is a remarkable change in radical concentration. Tem-
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1133
Figure 3. Temperature profiles at the inner wall and centerline for the iPP-filled glass ampule reactor inserted into the TM110 cavity of a Bruker ER 100D ESR spectrometer. The cavity temperature was set to 170 °C. The points are experimental data, while the curves are the fitting results using eq 3. Table 1. Parameters for the Temperature Profiles at the Inner Wall and Centerline of the iPP-Filled Ampule Reactor T, °C
position
C1
C2
C3
160
center wall center wall center wall
434.0 437.2 442.0 445.9 451.9 456.3
143.2 151.1 148.1 160.6 147.5 177.2
0.038 0.051 0.028 0.052 0.043 0.052
170 180
perature consideration is therefore essential for the appreciation of radical concentration change. The temperature profiles, both at the wall and the centerline, can be adequately described by the following empirical correlation:
T(t) ) C1 - C2 exp(-C3t)
(3)
where the empirical parameters C1, C2, and C3 are related to the temperature changes and thermal properties of the reacting system. The temperature difference at the two positions is reflected in these constants. They are readily obtained by curve fitting and presented in Table 1. On account of the temperature variation during the modification process, the peroxide decomposition rate constant changes accordingly. Its temperature dependence obeys the Arrhenius law
Kd ) A exp(-E/RT)
(4)
where A is the frequency factor and E is the activation energy of decomposition in kJ/mol K. These parameters are provided by the peroxide manufacturers.
Kd,TBPB ) 9.25 × 1013 exp(-133.93/RT)
(4a)
Kd,DCP ) 9.24 × 1015 exp(-152.67/RT)
(4b)
Kd,L130 ) 1.9 × 1015 exp(-150.67/RT)
(4c)
Kd,BPO ) 2.86 × 1012 exp(-113.22/RT)
(4d)
Equations 1-4 can be solved numerically with the initial conditions of [I]t)0 ) [I]0 and [R•]t)0 ) 0 provided that all of the parameters are known. As shown in Figure 3, the temperatures at the inner wall and the centerline differ, even though they finally approach
Figure 4. Radical concentration vs time profile of 0.16 mol of TBPB/kg of iPP at 180 °C. The points are experimental data. The solid curve is calculated using the wall temperature, while the dashed curve is calculated using the centerline temperature.
similar values. Figure 4 presents the results using either the wall temperature or the centerline temperature as the bulk temperature of the reacting mass. It can be seen that the temperature profile has a significant effect on the radical concentration development. A comparison with the experimental results reveals that the use of the temperature at the inner wall gives a better fit, whereas the use of the centerline temperature delays the change in the polymer radical concentration. The suitability of using the wall temperature may be explained by the very fine nature of the iPP powder and tight packing. The iPP particles, several micrometers in diameters, ensures good contact with the reactor wall. It is therefore reasonable to assume that the powder near the wall can quickly reach the inner wall temperature. When peroxide molecules decompose, polymer radicals are generated and are detected by the ESR spectrometer instantaneously. After the establishment of the temperature variation, there are two parameters remaining to be estimated: one is the peroxide efficiency f and the other is the radical termination rate constant Kt. Keep in mind that Kt only depends on the temperature, while f can be a function of temperature, peroxide type, and concentration level. Figures 5 and 6 present the results of curve fitting against the experimental data at various conditions. The termination rate constants were found to be 3.1 × 106, 3.2 × 106, and 3.2 × 106 L/(mol s) for 160, 170, and 180 °C, respectively. The rate constant showed a weak function of temperature. Bimolecular radical termination is normally composed of three steps (Benson and North, 1962; Russell et al., 1988): first, two radicals migrate together by translational diffusion and; then reorient to obtain favorable conformation through segmental movement and finally overcome the energy barrier and terminate. Since the third step has a low activation energy, the termination process is generally diffusion controlled, which has a weak dependence of temperature. Table 2 present the peroxide efficiency data of TBPB and DCP as a function of temperature and peroxide concentration. These f values are about 0.5 within the range of 0.4-0.6, which are lower than 0.6-0.8 reported for PP degradation by Suwanda et al. (1988b), Tzoganakis et al. (1988a,b), and Ryu et al. (1991/1992), who used very low peroxide concentrations and estimated their efficiencies from MWD data. A slight increase of the efficiency with decreasing peroxide concentration can also be noticed in Table 2. This concentration dependence can be explained by the so-called cage effect
1134 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
Figure 5. Radical concentration vs time profiles of TBPB/iPP. The reaction temperatures are A, 180 °C; B, 170 °C; and C, 160 °C. The peroxide concentration levels are 0.27, 0.22, 0.16, 0.11, 0.054, and 0.027 of mol/kg of iPP. The points are experimental data. The curves are calculation results. Table 2. Rate Constants of Radical Termination (Kt) and the Peroxide Efficiency (f) as a Function of Temperature (T), Peroxide Type (TBPB, DCP), and Concentration (C)
T, °C 160 170 180
Kt
C, mol/kg iPP
3.1 × 106 TBPB/iPP DCP/iPP 3.2 × 106 TBPB/iPP DCP/iPP 3.2 × 106 TBPB/iPP DCP/iPP
f 0.27 0.22 0.16 0.11 0.054 0.027 0.55 0.46 0.45 0.45 0.50 0.43
0.53 0.48 0.50 0.47 0.53 0.45
0.53 0.50 0.44 0.45 0.48 0.50
0.50 0.52 0.44 0.45 0.43 0.55
0.52 0.50 0.50 0.50 0.50 0.53
0.55 0.52 0.55 0.50 0.54 0.55
(Bamford, 1985). However, there seems to be no strong correlation of the peroxide efficiency with reaction temperature. Conclusion The on-line ESR technique was demonstrated to be a useful tool for the kinetic studies of polymer modification systems. For the first time, the successful observation of ESR spectra during the peroxide modification of
Figure 6. Radical concentration vs time profiles of DCP/iPP. The reaction temperatures are A, 180 °C; B, 170 °C; and C, 160 °C. The peroxide concentration levels are 0.27, 0.22, 0.16, 0.11, 0.054, and 0.027 of mol/kg of iPP. The points are experimental data. The curves are calculation results.
iPP elucidated that the reaction process was accomplished via a free-radical mechanism. The distinct polymer radical generation and termination rates were shown directly by the intensity change of the ESR spectra. The polymer radical concentrations were determined by the experimental conditions: peroxide type, concentration level, reaction time, and temperature. The radical termination rate constants were estimated via kinetic modeling. Acknowledgment Financial assistance from the Ontario Center for Materials Research (OCMR) and the Canadian Natural Sciences and Engineering Research Council (NSERC) is appreciated. Literature Cited Bamford, C. H. Radical Polymerization. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New York, 1985; Vol. 13, p 789.
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1135 Benson, S. W.; North, A. M. The Kinetics of Free Radical Polymerization Under Conditions of Diffusion-Controlled Termination. J. Am. Chem. Soc. 1962, 84, 935. de Boer, J.; Pennings, A. J. Crosslinking of Ultra-High Molecular Weight Polyethylene in the Melt by Means of 2,5-Dimethyl-2,5Bis(tert-butyldioxy)-3-Hexyne 2. Crystallization Behavior and Mechanical Properties. Polymer 1982, 23, 1944. de Vries, K.; Roylance, D. Electron Spin Resonance. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New York, 1985; Vol. 5, p 687. Dorn, M. Modification of Molecular Weight and Flow Properties of Thermoplastics. Adv. Polym. Technol. 1985, 5, 87. Gilbert, B. C.; Dobbs, A. J. Electron Spin Resonance Studies of the Decomposition of Organic Peroxides. In Organic Peroxides; Swern, D., Ed.; Wiley Interscience: New York, 1972; Vol. 3, p 271. Hamielec, A. E.; Gloor, P. E.; Zhu, S. Kinetics of Free Radical Modification of Polypropylene in ExtruderssChain Scission, Crosslinking and Grafting. Can. J. Chem. Eng. 1991, 69, 611. Kamachi, M. ESR Studies on Radical Polymerization. Adv. Polym. Sci. 1987, 82, 207. Kusumoto, N. ESR Studies of Polypropylene with Different Stereospecificities. J. Polym. Sci., Part C 1968, 23, 837. Lambla, M. Reactive Extrusion: A New Tool for the Diversification of Polymer Materials. Macromol. Symp. 1994, 83, 37. Ooi, T.; Shitsubo, M.; Hama, Y.; Shinohara, K. E.s.r. Study of γ-Irradiated Isotactic and Atactic Polypropylene. Polymer 1975, 16, 510. Poole, C. P. Electron Spin Resonance: A Comprehensive Treatise on Experimental Techniques; Interscience: New York, 1967. Ranby, B.; Rabek, J. ESR Spectroscopy in Polymer Research; Springer-Verlag: New York, 1977. Russell, G. A. Reactivity, Selectivity, and Polar Effects in Hydrogen Atom Transfer Reactions. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; p 275. Russell, G. T.; Napper, D. H.; Gilbert, R. G. Termination in FreeRadical Polymerizing Systems at High Conversion. Macromolecules 1988, 21, 2133. Ryu, S. H.; Gogos, C. G.; Xanthos, M. Parameters Affecting Process Efficiency of Peroxide Initiated Controlled Degradation of Polypropylene. Adv. Polym. Technol. 1991/1992, 11 (2), 121. Sheldon, R. A.; Kochi, J. K. Pair Production and Cage Reactions of Alkyl Radicals in Solution. J. Am. Chem. Soc. 1970, 92, 4395. Shida, T. Electron Spin Resonance and Optical Studies of t-butyl Peroxide Ions Produced by γ-Irradiation. J. Phys. Chem. 1968, 72, 723.
Shimada, S. ESR Studies on Molecular Motion and Chemical Reactions in Solid Polymers in Relation to Structure. Prog. Polym. Sci. 1992, 17, 1045. Suwanda, D.; Lew, R.; Balke, S. T. Reactive Extrusion of Polypropylene I: Controlled Degradation. J. Appl. Polym. Sci. 1988a, 35, 1019. Suwanda, D.; Lew, R.; Balke, S. T. Reactive Extrusion of Polypropylene II: Degradation Kinetic Modeling. J. Appl. Polym. Sci. 1988b, 35, 1033. Tang,Y.; Tzoganakis, C.; Hamielec, A. E.; Vlachopoulos, J. Peroxide Crosslinking of LLDPE During Reactive Extrusion. Adv. Polym. Technol. 1989, 9, 257. Tzoganakis, C. Peroxide Degradation of PP during Reactive Extrusion. Ph.D. Thesis, McMaster University, Hamilton, Canada, 1988. Tzoganakis, C.; Vlachopoulos, J.; Hamielec, A. E. Production of Controlled Rheology Polypropylene Resins by Peroxides Promoted Degradation During Extrusion. Polym. Eng. Sci. 1988a, 28, 170. Tzoganakis, C.; Vlachopoulos, J.; Hamielec, A. E. Controlled Degradation of Polypropylene. Chem. Eng. Prog. 1988b, Nov, 47. Xanthos, M. Reactive Extrusion: Principles and Practices; Hanser: New York, 1992. Zhu, S.; Hamielec, A. E. Termination of Trapped Radicals at Elevated Temperatures during Copolymerization of MMA/ EGDMA. Polymer 1990, 31, 1726. Zhu, S.; Hamielec, A. E. Heat Effect for Free Radical Polymerization in Glass Ampoule Reactors. Polymer 1991, 32, 3021. Zhu, S.; Hamielec, A. E. Kinetics of Polymeric Network Synthesis via Free Radical Mechanisms. Makromol. Chem., Macromol. Symp. 1992, 63, 135. Zwolenik, J. J. Photolytic Generation and Kinetic Electron Spin Resonance Spectrometry of Cumylperoxy Radicals. J. Phys. Chem. 1967, 71, 2464.
Received for review July 10, 1996 Revised manuscript received September 17, 1996 Accepted September 17, 1996X IE960410P
X Abstract published in Advance ACS Abstracts, February 15, 1997.
