R. J. Mazza Medway ond Maidstone College of Technology Chotham, Kent, England
Free Radical Polymeriza'tion of Styrene A radiotracer experiment
Although polymer chemistry is now receiving more attention in many undergraduate counes, the range of experiments available in this field is still rather limited. In this experiment 1,l'-azobisisobutymnitrile-'4C is used to initiate the polymerization of styrene a t 60°C. Rates of reaction are measured gravimetrically and number average molar masses, Ma,are calculated from the specific activities of the polymers and of the initiator, which are determined by liquid scintillation counting. Polvmerization kinetics and viscometric characterization bf polystyrene have been described previously by Bradbuw in a phvsical chemistw experiment ( I ) . ~ o w e i e r ,in this paper we desciihe an alternative appmach in which emphasis is placed on the application of radioactive lahelling in polymer analysis (2). The experiment is also worthwhile because the students. workine.. in pairs. become accluainted with the hand. ling df radioactive materials, vac;um techniques, liquid scintillation counting, and gas-liquid chromatography (elc). .- . Most universities and colleges, nowadays, have laboratories suitably equipped for handling and counting radioisotopes, and Ayrey and Mama have shown that the channels ratio method of quench correction is applicable even when a single photomultiplier tube is used for liquid scintillation counting (3). Moreover, the relatively simple equipment is not very expensive.
Theoretical The thermal decomposition of 1,l'-azohisisohutymnitrile, (AZBN) follows first-order kinetics and the rate constant, kd, at 6 0 T is 1.20 X 10-5 s-1 (4). The 2-cyano-2pmpyl radical produced is capable of initiating the freeradical polymerization of a vinyl compound. In the absence of chain transfer reactions, the mechanism of the reaction is a chain process involving three distinct steps: initiation, propagation, and termination (5). The rate of initiation, R,, is given by the relation
where [I] is the initial concentration of the initiator, n is the number of free radicals produced from each molecule of initiator, and f is the efficiency of the initiator, i.e., the fraction of radicals which initiate polymerization since some radicals are consumed in wasteful side reactions. Rr can he calculated via eqn. (2)
where R, is the rate of polymerization and 0 , the mean kinetic chain length, is obtained from the specific activity of the polymer, a,, (d.p.s. g-I), the specific activity of the initiator, at, (d.p.s. mole-') and the molar mass of the monomer, m, according to the expression
This calculation allows for the fact t h a t two free radicals are produced from each molecule of AZBN. Thus, in eqn. (I), n = 2 and hence f can be calculated. It is now generally accepted t h a t termination is a bimolecular process involving two polymer radicals and leading t o either combination (eqn. (4)) or disproportionation h n . (5)) H H
bh bh Polystyrene radicals are known t o terminate solely by combination u~ t o 80°C (6) and hence the number average molar mass, Me, of t h e polymer can be calculated from the ratio of the specific activities of t h e initiator and of the polymer: (7.8)
where a, is expressed in d.p.s. per gram. T h e method depends on the absence of chain transfer processes and, for termination exclusively by combination, upon the incorporation of two labelled initiator fragments in each poly-mer molecule. If transfer reactions are absent, a plot of 1/Pn against R , should yield a straight line of insignificant intercept since kinetic analysis predicts t h a t the depepdence of the number average degree of polymerization, P,, upon the rate, R,, is given by eqn. (7) (5)
where transfer constants are defined by Cv = ktrMlkp Cs = kr,,slkp
CI = kri.JkD
and
k, = k,,
+ k,d
(eqns. (4) and ( 5 ) ) , and M = monomer, S = solvent, I = initiator. The experiment is open ended in t h a t 1) The radiochemical purity of the initiator, which is pmvided for
the students, can he cheeked bv reverse isotone dilution analvsis (2). 2) The number average molar mass of the polymer may he determined viscometrically and used to obtain the number of initiator fragments, n, per macromole from eqn. (8) -
n = 2a,M,/a,
(8)
so that the mode of termination can he established and compared with the literature. 3) Different methods of quench correction can be used, e.g. the internal standard method or the channels ratio method (3, 9, 10). In fact, the calibration of the liquid scintillation counter and the reverse isotope dilution analysis of the initiator, if de-
sired, can be carried out during the inactive periods, ex., during the kinetic runs or when the monomer is being distilled immediately before use. Experimental
All manipulations involving radioactive substances were earned out in deep, stainless steel trays covered with soft, absorbent paper to absorb any spillage, and the students wore disposable plastic gloves. (Arbrook Products, Ethicon Ltd., Edinburgh). Materials Azobisisobutyronitrile. Although the compound is available commercially ( I ) , it was prepared by the author by the method of Thiele and Heuser
&N
6~
Hydrazobisisobutyronitrile. Hydrazine sulfate (6.3 g) and sodi-
um cyanide (4.1 g) were dissolved in warm water (35 ml) and then added to acetone (5.0 g). The mixture was shaken and kept in the stoppered flask for 3 da with periodic shaking; then the aqueous solution was filtered through a glass sinter and the residual hydrazo compound washed with water (3 X 25 ml) and dried over phosphorus pentoride (9.2 gin 63%yield). Azobisisobutymnitrile. The dry hydrazo compound (9.2 g) was dissolved in ethanol (25 ml), 5 M hydrochloric acid (25 ml) was added, and then the solution was cooled to -2°C. Bromine water was added dropwise, with shaking, until a permanent yellow color was obtained. The colored, aqueous solution was removed through the glass sinter, the product washed with water (3 X 40 ml), and dried over phosphorus pentoxide to give azobisisobutyronitrile. This was recrystallized twice from ether, twice from ethanol, and twice from benzene. Finally, it was dried by pumping on a vacuum line far -24 hr, mp 102-103'C in 68%yield (6.26 g). ArobisisobutyronitriIe-"C. Labelled AZBN was also prepared in advance by the author. However, the compound could he prepared by the students, under supervision, as a separate experiment. For details of the method and of the apparatus, the reader is referred to the work of Bevineon and coworkers (8). Styrene.1 laboratory grade, was shaken with 10% aqueous solution of sodium hydroxide to remove inhibitor, washed with water (3 X 50 ml), and dried over calcium sulfate. The filtrate was distilled under reduced pressure in a stream of nitrogen and then further purified by fractional distillation. A pure fraction had bp 48"C/35 mm Hg: glc analysis revealed no impurities. The styrene was stored under nitrogen at -10°C and distilled in uacuo at room temperature immediately before use. Gas-Liquid Chromatography (GLC) Glc analyses were performed on a Perkin-Elmer F 11chromatagraph using a 1 m X Y, in.-metal column with a swadge lock and packed with Celite, and either 10% Apiezon L grease or 10% silicone oil as the stationary phase. The column temperature was 150°C.
Polymerization Procedure Polymerizations were carried out in Pyrex tubes of capacity -20 ml and with a narrow neck suitable for flame sealing. Immediately before use, the monomer was distilled at room tempera-
* This can be purified in advance by a member of the laboratary staff.
Polymerization Conditions and Results
ol = 1.74 X
107d.~-l mole-,;
[MI = 8.39 mole 1-L
, Volume 5.7, Number 7, July 1975 / 477
Figure 2. Plat of t h e of polymerization. -2.0
Figure 1. D e p e n d e n c e tration.
of
-2-4 Lor111
t h e rate
-22
of polymerization on
So = initiator
reciprocal d e g r e e
of polymerization
a g a i n s t t h e rate
sample count rate - backgmund count rate mass of sample
concenResults and Discussion
Rates of polymerization were calculated from ture and at a pressure
