Polystyrene Kinetics by Infrared An Experiment for Physical and Organic Chemistry Laboratories Heather Andrews-Henry Augusta College, Augusta, GA 30910
Polymerizations fascinate the students. This experiment gives the undergraduate experimentalist the opportunity to determine the kinetics of the free radical ~olvmerization of polystyrene. The experimental portion ofthi; laboratory is small com~aredto the data analvsis. Students com~lete a successfulbrganic synthesis, interpret infrared spekra, and use computer software to plot and calculate statistical parameters. Most important, they use the data they collect to determine the kinetics of the reaction. Theory The polymerization of olefinic monomers usually follows a mechanism that involves the chain reaction of free radicals. This mechanism is different from that involved in condensation polymerization. The basic steps common to all free radical polymerizations are initiation (formation of free radicals) propagation (continuation of the reaction wthout a c h w ~ win the number of free radical31 remination ehminntion of free radicals
where R represents the initial free radical and M the monomer. Chain ~ r o ~ a e a t i o continues n via
.
A
-
RM. + M + RM%. RM,. RM,.
+ M + RM,. + M + RM,.
RM;.+M+RM.,+,
If vi= k%[R.I[MI, the rate constant kz is assumed to be the same for each reaction, independent of chain length. The rate of the propagation steps is much faster than that of the chain initiation steps due to the high reactivity of the free radical. The monomer is assumed to be the only species capable of reacting with a growing polymer chain.' In this experiment, the polymerization of styrene to polystyrene is initiated by the photodecomposition of AIBN (2,Y-azodiisohutyronitrile):
Chain initiation is often achieved by a n initiator. An initiator is a trace substance that produces a steady supply of free radicals to the polymerization. Firsborder production of free radicals from the initiator is given by u; = k,[Al where [A1 is the initiator concentration. Free-radical polymerization can also he initiated photochemically. The reaction rate for photochemical initiation is given by u; = k'Z
The chain propagation reactions are then given by
where I is the light intensity. Chain propagation begins when the free radical formed in the initiation step reacts with the double bond of the monomer.
'Tanford, C. Physical Chemistry of Macromolecules; Wiley: New York, 1963.
Volume 71 Number4 April 1994
357
The recombination steps terminate the poljymerization with rate constant kt a t rate u, given by
-1 . 782 cm w~th7 min illumination
where x is the molecularity of the reaction. Assuming that the concentration of the free radical is small and constant (d[Rlldt = 0, the steady state approximation), the rate2 is found to be rate =
k,Mk,[AI~Nf%yrenel kt
Combiningthe rate constants, we get a simple first-order equation. rate = k[.41~~1~[styrenel
Students should determine that this reaction follows simple first-order kinetics. Typical values of the rate constants3 a t 80 'C are ki = 1.5 x lo4 s-' (for AIBN)
k, =
lo3 K's - ~
kt =
lo7 K's-' at 80 'C
Calculation of k from these typical values gives an expected result on the order of k = 4 x lo3 M-' 6.' Schulz, Dinglinger, and Husemann found that the degree of polymerization for the thermally initiated reaction is determined by the rate constants and is not a function of time. This result is expected, because free radical polymerization gives a completely polymerized product a t all times during the reaction? Experimental A 1.84 x lo3 M solution of initiator was prepared by weighing out 0.2504g of AIBN (MW is 136.20 g/mol) into a 100-mLvolumetric and bringing to volume with CCld. The spectrum of pure styrene is measured either with a NaCl 'Moore, J. W.; Pearson, R. G. Kinetics and Mechanism: Wiiey: New York, 1981. 3Burnetl. G. M.; Melville, H. W. Chem. Rev. 1954, 54,225. 4Schulz, G. V.: Dingliger, A,; Husemann, E. 2.Phys. Chem. 1939, 839,385.
358
Journal of Chemical Education
0 2 4 6 8 10 12 14 16 TIME (MINUTES) Styrene polymerization kinetics cell or an attenuated total reflectance (ATR) trough using a dispersive or FT instrument. The results presented here were done using an ATR trough equipped with a ZnSe crystal and a Nicolet 800 FTIR. Ameasured quantity (about 1mL) of pure styrene is placed on the window or ATR crystal. (For styrene, the MW is 104.15 g/mol, and the density is 0.9060 g/mL). The AIBN is then added. This reaction mixture is exposed for several minutes to a 500-WUV lamp. (Illumination for 7 min gave good results, although 2 min also produced reliable data.) Spectra are collected every minute. Sample Results The results from a class of physical chemistry students are shown for illustration. The students were not told in advance what band would give the best results, and different bands were chosen by differentlab groups. Recall that the plot of in [AliIAJ versus time for a first-order reaction yields a linear plot with slope equal to -k. The natural log of the absorbance a t 782 cn-' (C-H out-of-plane bending vibration) is plotted against time in the figure. The line in the figure has a slope of 4.00374. Hence, the student's experimentally determined value of k is 3.74 x lo3, which agrees with the expected value of 4 x
