Edward 1. McCaRery' Lowell Technological Institute Lowell, Massachusetts 01850
Kinetics of Condensation Polymerization Preparation o f a polyester
condensation polymerization (1) is characterized by the stepwise or progressive conversion of a monomer with two reactive end groups to higher molecular-weight homologs which, themselves, retain two reactive end groups. Condensation polymerization can be accompanied by the formation of a low-molecularweight by-product, as with polyesterification
measurement of the quantity of water produced, and it will be confirmed by titration of the unreacted acid. Because the temperature of the reaction must be increased periodically to produce high molecular-weight product within the limits of the laboratory period, one is presented with an excellent opportunity to study the rate of polyesterification a t several temperatures, thereby, to evaluate the energy of activation for the process.
or, as with polyurethane formation, the polymer can be the only product of the reaction OCN(CH2).NC0 + HO(CHn)aOH (OCN). . . +oCNH(CH,).NHCoo(CH1)boff. . . H
Experimental
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Kinetically, polyesterifications of either type should be identical to esterification provided that both the increase in molecular weight accompanying the formation of the polyester does not alter the reactivity of the end groups, and the increase in viscosity associated with polymerization does not cause the process to become diffusion controlled. The first objection was set aside by Skrabal and Singer (8) who demonstrated that the influence which one carboxyl group exerts upon the other in dibasic acids rapidly disappears as the number of methylene groups separating the carboxyl groups increases. Flory's (3) work on the kinetics of polyester formation dispelled the second. The reaction-rate constant for polyesterification can be calculated conveniently from the equation (4) :
when an external catalyst such as p-toluenesulfonic acid is used. Because the quantities 5, (the numberaverage degree of polymerization) or 111-p (where p is the extent of reaction) are dimensionless, klco assumes the dimensions of time-'. The pseudo-second-order rate constant, k', and the actual rate constant, k (where k = kf/[catalyst]), can be written.in more conventional terms if both co and [catalyst] are expressed as mmoles/g. The progress of the reaction will be determined by 1 The material in this article is derived from the author's forthcoming book: "Laboratory Preparation for Macromolecular Chemistry," to be published by the McGraw-Hill Book Company and is used with the permission of the publisher. The total charge should he restricted to about 250 g. Therefare, with the higher molecular-weight glycols, the charge must he reduced proportionately. 'The half mole of glycol is added at this point if anhydrides rtre used in order to moderate the relatively vigorous anhydrideglycol reaction. A long-stem thermometer also is satisfactory.
A 1-1four-necked reaction kettle (see figure) provided with a heating mantle is charged with 35 ml of decalin and with 1 mole of the prescribed dibasic acid2 (adipic, fumaric, maleic, ~hthalic, or succinic acids); or if ananhydride (maleic, phthalic, or succinic) is to be employed, the charge should contain 0.5 mole of glycolS (ethylene, diethylene, triethylene, tetraethylene, or other polyethylene glycols) in addition to the anhydride and decalin. The rheostat should be adjusted to permit maximum safe heating of the mantle, and the charge should be preheated while the rest of the apparatus is being assembled. The reaction kettle is fitted with a mechanical stirrer, a 25-1111 graduated distillation-trap topped by a condenser, and a nitrogen inlet tube. A thermoApparatus for poly- couple wire contained in a glass sleeve e5ter formotion. is connected to a pyrometer and is positioned so that the wire will make contact with the reaction m i x t ~ r e . ~The distillationtrap should be filled with decalin to permit a reasonably constant weight to be maintained in the flask as the reaction proceeds. One mole of glycol (or the additional 0.5 mole of glycol if an anhydride is the coreactant) is introduced into the reaction kettle along with 0.17 g (1 mmole) of p-toluene-snlfonic acid. If the acidic comer is unsaturated, 0.1 g of hydroquinone also should be added a t this point. The temperature of the reaction should be permitted to rise rapidly until reflux commences; usually this will occur somewhere between 130'-180°C. Thereupon, the temperature should be maintained constant until one fourth of the total water of reaction has been collected in the distillation trap. The water level in the Volume 46, Number 7, January 1969
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trap should be recorded at 1-min intervals during this period. If the water-decalin interface is indistinct, a few drops of bromthymol blue indicator solution may be added to the trap; the indicator will become concentrated at the interface. After the first quarter of the water of reaction has been collected, the temperature of the reaction is permitted to rise quickly about 10" where the temperature again is maintained. The quantity of water evolved again should be recorded a t 1-min intervals. After about one half of the water has been collected, a glass tube is inserted into the kettle and about 2 g of the hot resin sample is withdrawn and is deposited into a small, weighed flask. The nitrogen-inlet tube is replaced and the temperature of the reactiou is permitted to rise an additional 10". The reaction is conducted isothermally a t this new temperature until about three quarters of the water has distilled. Meanwhile the exact weight of the withdrawn resin is determined by difference, and the resin is diluted with 10 ml of acetone. After the sample has dissolved, i t is titrated with 0.8 N methanolic IiOH solution to a phenolphthalein end-point! The extent. of reaction as is determined by tit,ration should be compared with the value calculated from the evolved water of reaction. Before the temperature again is increased, a second sample of resin is withdrawn and titrated. The tem-
perature then is raised an additional lo0, and the quantity of water evolved a t the usual 1-minintervals is observed until the rate of reaction perceptibly has diminished. The polyesterification reaction can be completed a t the highest temperature attainab1e.O Formulations which contain unsaturated-acid comers cannot he permitted to react to their theoretical end points since side reactions cause crosslinking and its consequence-gelation. If the reaction mixture thickens significantly in a short time and it ascends the stirrer shaft, additional glycol should be introduced quickly to permit ester-interchange reactions which will serve to inhibit the crosslinking reaction of the decomposing resin and will facilitate the removal of the resin from the kettle. R4easurements made beyond the gel point are meaningless; the partially gelled resin should he discarded. The decalin-polyester mixture will separate upon being cooled to room temperature. A characteristic resin sample will be obtained merely by decanting the supernatant decalin layer. Four different pseudo-second-order rate constants (k') and four actual rate constants (k)can be determined from the data. Only those data secured isothermally should be included in the calculations. The energy of activation and the Arrhenius A factor for each polymer prepared also can be evaluated. Literature Cited
6Because of the presence of hydroquinone, the typical phenolphthalein end-point will not be observed. An easily discernible change from light yellow to violet-brown or to chocolate will take place. "ometimes it will be necessary to raise the temperature as high as 250°C, hut this temperature should be considered as an upper limit. Any intermediate temperature at which rapid reflux is maintained and decomposition is absent will be satisfactory.
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Journal of Chemical Education
(1) MARK,H. and WHITBY,G . S., (Editors), "Collected Works of Wallace Hume Carothers on High Polymeric Substances," Interscience (division of John Wiley & Sons, Inc.), New York. 1940. - ~-~ -~ (2) SKRABAL, A. AND SINGER, E., M n a l s h . , 41, 339 (1920). (3) FLORY,P. J., J. Am. Chem. Soe., 61, 3334 (1939). (4) FWRY, P. J., "Principles of Polymer Chemistry," Cornell U. Press, Ithaca, N. Y., 1953.
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