Ind. Eng. Chem. Res. 1998, 37, 3567-3574
3567
Investigation of Methacrylate Free-Radical Depropagation Kinetics by Pulsed-Laser Polymerization Robin A. Hutchinson,* Donald A. Paquet, Jr., Sabine Beuermann,† and John H. McMinn Central Research and Development, E. I. du Pont de Nemours and Company, Inc., P.O. Box 80101, Experimental Station, Wilmington, Delaware 19880-0101
An extensive study of free-radical depropagation kinetics has been performed for several methacrylates at high temperatures in bulk and solution using a pulsed-laser technique. As expected from theory, the relative importance of depropagation on rate increases with increasing temperature and with decreasing monomer concentration. The temperature dependence of n-dodecyl methacrylate depropagation has been quantified. The resulting estimates for enthalpy (-50 to -60 kJ/mol) and entropy (-105 to -127 J/(mol‚K)) of polymerization, although highly correlated, are in good agreement with available literature data. Moreover, the best-fit values (-53.8 kJ/mol and -113 J/(mol‚K)) provide a good representation of high-temperature propagation/depropagation kinetics measured for n-butyl, cyclohexyl, isobornyl, and 2-hydroxypropyl methacrylates, suggesting that a single set of universal values may be employed when modeling high-temperature polymerizations of all methacrylates. Introduction Although most often considered irreversible, chain growth in free-radical polymerization is a reversible reaction: kp
Pn + M 98 Pn+1 kr
Pn+1 98 Pn + M where Pn represents a growing radical of length n and M the monomer. The effective forward propagation rate coefficient, denoted here by keff p , is given by
keff p ) kp - kr/[M]
(1)
The exothermic, exentropic nature of most free-radical polymerization dictates that for a given monomer concentration there exists a ceiling temperature above which chain growth will not proceed. Not only does the effective polymerization rate dramatically decrease as this temperature is approached but so does the polymer molecular weight since transfer and other side reactions continue at unabated rates. For many typical polymerization systems and conditions, depropagation does not occur to any appreciable extent. With monosubstituted ethylene monomers, for example, other side reactions become dominant well before the ceiling temperature is reached. However, for some 1,1-disubstituted ethylene monomers, it is possible * To whom correspondence should be addressed. Present address: DuPont European Technical Centre, P.O. Box 50, CH-1218 Le Grand Saconnex, Geneva, Switzerland. Telephone: +41-22-7176742. Fax: +41-22-7176622. E-mail:
[email protected]. † Present address: Institut fu ¨ r Physikalsiche Chemie, Universita¨t Go¨ttingen, Tammannstrasse 6, D-37077 Go¨ttingen, Germany.
to polymerize at conditions where the effects of the reverse reaction cannot be neglected. R-Methylstyrene, with a bulk monomer ceiling temperature of 60 °C, is the classic example of such a monomer.1 Methacrylate monomers also exhibit depropagation, although at much higher temperaturessceiling temperatures reported for [M] ) 1 mol/L are in the range of 200-210 °C.2 This temperature is the point at which the net rate of chain growth is zero. For methacrylate polymerizations run at low monomer concentrations (i.e., starved feed semibatch operation or batch polymerizations at high conversion), depropagation may have a significant effect on rate and polymer properties at temperatures as low as 120 °C.3 Understanding and modeling the effects of depropagation on reaction rate and polymer molecular weight are necessary to ensure a uniform polymer product. The difficulty lies in finding reasonable estimates for depropagation kinetics, which are related to the enthalpy (∆H) and entropy (∆S) of polymerization as shown:
∆H ) Ep - Er
(2)
∆S ) R ln(Ap/Ar) + R ln [M]
(3)
E and A are the activation energies and frequency factors of the forward and reverse rate coefficients expressed in the usual Arrhenius form:
kp ) Ap exp(-Ep/RT)
(4)
kr ) Ar exp(-Er/RT)
(5)
Following previous literature convention, ∆Sls is used to denote the experimental situation where liquid monomer is converted to polymer in solution. The superscipts “b” and “o” are adopted to indicate bulk ([M]b) and unit ([M])1) monomer concentrations, respectively:
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3568 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
-∆Sbls ) R ln(Ap/Ar) + R ln [M]b
(6)
-∆Sols ) R ln(Ap/Ar)
(7)
Enthalpies of polymerization are generally determined from reaction calorimetry; values for typical alkyl methacrylates are in the range of -50 to -60 kJ/mol.2,4 The entropy is more difficult to measure. The only measurements for methacrylates found in the literature date back to the 1950s: ∆Sols values of -115 to -125 J/(mol‚ K) have been estimated for methyl methacrylate5,6 and for ethyl methacrylate.7 Considering the industrial importance of high-temperature methacrylate polymerizations, it seems beneficial to examine depropagation kinetics using the pulsed-laser experimental technique introduced by Olaj et al.8 This method provides a reliable measure of keff p through analysis of polymer molecular weight distributions (MWD) produced by pulsed-laser polymerization (PLP). In the technique, a monomer system with photoinitiator is exposed to periodic laser flashes. Each flash generates a new population of radicals, with the radical concentration decreasing between flashes due to radical-radical termination. At the end of the time period between flashes (t0), the radicals which have escaped termination have propagated to a chain length DP0, given by the simple equation:
DP0 ) keff p [M]t0
(8)
where [M] is the monomer concentration. When the next flash arrives, the remaining radicals are exposed to a high concentration of newly generated radicals, which leads to a greatly increased probability for their termination. Thus, the formation of dead polymer molecules with length close to DP0 is favored. The best estimate for DP0 is the inflection point on the PLPcontrolled peak of the polymer MWD,8-10 readily determined by locating the maximum in a plot of the derivative of the distribution. With a measure of DP0, keff p can be calculated from eq 8. The PLP/MWD technique has proven to be a direct and reliable method for measuring homopolymer kp values. Measurements from many laboratories show excellent agreement and have been combined to provide benchmark kp data sets for styrene11 and methyl methacrylate.10 We have used the PLP/MWD technique previously to measure kp values between 10 and 110 °C for a variety of methacrylates in bulk;12,13 these results are in good agreement with other recent PLP studies.14-16 In all of these previous publications, reaction conditions were such that no depropagation occurred (keff p ) kp). Therefore, the reported experimental values and Arrhenius parameters correspond to the forward rate coefficients defined by eq 4. In this work, we extend the experimental range, measuring methacrylate keff p values to higher temperatures (up to 180 °C) in bulk and at reduced monomer concentrations in solution. Through eqs 1-5, the kinetics of depropagation can thus be quantified. Experimental Section The experimental pulsed-laser setup is similar to the one described previously.9,12 A Quanta-Ray pulsed Nd: YAG GCR-190-100 laser with a harmonic generator
produces light of wavelength 355 nm at pulse energies up to 70 mJ/pulse and a half-height pulse width of 6 ns; pulse repetition rates (10-100 Hz) were controlled with a digital delay generator (Stanford DG-535). Monomer samples of 2-4 mL (either bulk or solution) containing benzoin photoinitiator (1-5 mmol/L) in a round quartz cell were allowed to reach temperature in a thermostated aluminum block heater and then were pulsed with the laser energy for a time sufficient to allow low levels of conversion (
