RATES OF POLYMERIZATION OF ACRYLATES AND METHACRYLATES IN EMULSION SYSTEMS

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1 RATES OF POLYMERIZATION OF ACRYLATES AND METHACRYLATES IN EMULSION SYSTEMS K. G. ~/ICCURDY AND K. J. LAIDLER Department of Chemistry, University of Ottawa, Ottawa, Canada Received October 3, 1963 ABSTRACT The microcalorimeter has been used to obtain rates of polymerization of a number of monomers in emulsion systems. The rates with the acrylates are consistently higher than those with the methacrylates, and this is attributed to less steric hindrance. Low rates found with monomers containing a hydroxyl group are explained as due to solvation in the initial state. In both the acrylates and methacrylates the rates go through a maximum as one goes up the series methyl: ethyl: butyl: hexyl. This is discussed in terms of inductive and steric effects. INTRODUCTION The preceding paper (I) describes a microcalorimetric study of a number of vinyl polymerizations in emulsion systems. Several investigators (2-6) have described procedures for determining rate constants on the basis of calorimetric measurements, and in particular Baumgartner and Duhaut (7) and Lueck, Beste, and Hall (8) have developed methods that are especially suitable for our technique using the Tian-Calvet microcalorimeter. Lovering and Laidler (9) have recently applied the Baumgartner-Duhaut procedure to the deternlination of alcohol-isocyanate rate constants. The inethod of Lueck, Beste, and Hall (8), which is very similar, applies specifically to the differential method, which is the one employed in the present work. EVALUATION OF THE RATE CONSTANTS Kinetics of Emulsion Polymerization The kinetic laws applicable to emulsion polymerization have been worked out in detail, and the present brief account covers only the essential points. Soap solutions such as those used in the present work, in which cetyltrimethylammonium bromide is the emulsifying agent, have been shown by McBain (10) and Harkins (11, 12) to contain micelles consisting of layers of oriented soap molecules. When substances such as the monomers used in the present investigation are added to such a soap solution they are "solubilized", which means that some of the monomer penetrates the micelles. Soine of the monomer also remains as suspended droplets in the aqueous phase. Harkins (13) proposed that in emulsion systems there are two principal loci of polymerization. Initially, when there is not much polymer present, the micelles contain largely monomer, and because the radii of the micelles are much smaller than those of the droplets the micelles present a much larger interfacial area. Most of the polymerization therefore takes place in these micelles. The process is that the free radicals generated by the initiation in the aqueous phase are captured by the micelles, and that monomer is supplied to the growing radicals froin the droplets in the surrounding solution. The micelles therefore grow larger and soon consist largely of polymer particles with soap adsorbed on them. After 2 to 3% of the monomer has been converted into polymer the micelles have in fact largely disappeared, and have been replaced by polyiner particles. From this point on, therefore, the locus of polymerization is the polymer particles. Canadian Journal of Chemistry. Volume 42 (1964)

2 826 CANADIAN JOURNAL OF CHEMISTRY. VOL A quantitative formulation of this theory was put forward by Smith and Ewart (14). Applied to the later phase of the process, when polymerizatioll is occurring in the polymer particles, the number of which has become constant, the treatment may be outlined as follows. On the average there are 1014 particles per cubic centimeter and in a typical experiment 1013 radicals are produced per cubic centimeter per second; if all of these enter the polymer particles a particle will acquire a radical once in about 10 seconds on the average. Once a radical has entered a particle it proceeds to grow by adding monomer units at a rate equal to the rate constant for the propagation process, k,, multiplied by the lnononler concentration [MI in the particles. If a radical enters a particle that already contains a radical it will at once combine with it, and from this it follows that at any time one-half of the polymer particles will contain one radical and the other half no radical. If N is the total number of polymer particles the rate of polymerization is therefore given by This relationship has been confirmed (15-19) for a number of systems, and the k, values derived are in reasonable agreement with those obtained in other ways. Relationships between the number, N, of particles and the concentrations of emulsifier and initiator have been derived by Smith and Ewart (14) and by Medvedev (20), whose model is a little different from that outlined above. The essential point as far as the present investigation is concerned is that the monomer concentration is sufficiently low (0.2 M) that all of the monomer is solubilized. The monomer concentration in the particles is therefore proportional to the overall monomer concentration. The experiments were all carried out at an emulsifier concentration of 17, and an initiator concentration of lop3 148; this ensures constancy of the number N of particles. Since this number remains constant throughout a run the kinetics will be first-order. THE THERhJAL METHOD FOR DETEKMINISG RATE CONSTANTS The principle of the method used has been described by Lovering and Laidler (9). It is sufficient here to note that the extent of reaction at time t is measured by the area of the e.m.f. vs. time curve up to that point, plus a contribution due to the loss of heat from the cell to the calorimeter block; this is determined by a separate measurement in which electrical heat is supplied to the cell. In this way the amount reacted can be determined at various times, and the rate constant calculated in the usual way. This \\as carried out at various times, covering the initial part of the reaction, up to 50% completion. A graphical method was employed for obtaining the first-order rate constants. RESULTS Fig. 1 shows a typical plot used for the determination of a rate constant; it relates to experiment 1 (cf. Table I) for methyl methacrylate. Table I gives the rate constants calculated on the basis of a number of completely different experiments, under identical experimental conditions, for the polymerization of methyl methacrylate. The values are seen to be reasonably constant. Table I summarizes also the averaged rate constants, and standard deviations, for all of the polymerizations studied. DISCUSSION Comparison with Preuious Results Rate constants previously derived for the polyinerizations of the monomers ernployed in the present work are derived from bulk polymerization studies; they are second-order

3 McCURDY.4ND LAIDLER: RATES OF POLYMERIZATION TIME (seconds) Fig. 1. A typical first-order plot, for the polymerization of methyl methacrylate. TABLE I Rate constants for the polymerization of methyl methacrylate and all compounds studied Polymerization of methyl methacrylate All polymerizations studied (T = 25" C) - Expt. NO. Rate constant X 103 (seccl) Monomer Rate constant X lo3 (sec-1) WIethacrylic acid Methyl methacrylate Ethyl methacrylate n-butyl methacrylate n-hexyl methacrylate Acrylic acid Methyl acrylate Ethyl acrylate n-butyl acrylate Hydroxyethyl methacrylate 2-Hydroxypropyl methacrylate rate constants, and relate to those of the present work through N, the concentration of particles. The values agree provided that N is 1015 to 1016 particles per cc, and this is the value of N obtained in a number of investigations of emulsion polymerization (15-19, 21). The previous kinetic studies of bulk polymerization are consistent with the results of the present work as far as general trends are concerned. Thus in the methacrylate series previous work shows the maximum in rate constant in going from methyl to butyl. Some previously obtained rate constants are: methyl methacrylate at 24' C: 310 liter mole-i sec-l, (22) n-propyl methacrylate at 30' C: 467 liter mole-i sec-i, (23) n-butyl methacrylate at 30' C: 362 liter mole-i sec-l. (24) For the acrylates the only previously reported values are: methyl acrylate at 30' C: 769 liter mole-' sec-l, (25) n-butyl acrylate at 25' C: 13 liter mole-i sec-l. (26) Our value (cf. Table I) for methyl acrylate is lower than that for n-butyl acrylate. There is evidence, however, that the previously reported value (25) for methyl acrylate is too high on account of autoacceleration (27). Also, the previous value for n-butyl acrylate (26) is lower than that for n-butyl methacrylate (24), which is contrary to expectation on steric grounds.

4 828 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, 1964 Interpretation of Results The propagation rate constants are seen from Table I to be consistently higher for the acrylates than for the corresponding methacrylates. As discussed in the preceding paper (I), with reference to heats of poly~nerization, there is considerably less steric hindrance to polymerization in the case of the acrylates than in the methacrylates, and this may well explain the difference in rates. In the acrylate series the rate constants increase from acrylic acid to methyl acrylate to ethyl acrylate, but then decrease on going to butyl acrylate. The behavior in the methacrylate series is similar: the apparent deviation for methacrylic acid and methyl methacrylate is within the experimental error. There is a decrease from ethyl to butyl to hexylmethacrylate. These trends can be explained in terms of inductive effects and steric effects. The increases in rates from methyl to ethyl are attributed to the greater electrondonating power of the ethyl group as compared with methyl. It would appear that the increased electron-donating power facilitates the opening of the double bond, and in this way increases the rate of addition of the monomer to the polymer radical. The steric difference between methyl and ethyl is probably unimportant. In the series ethyl: butyl: hexyl the inductive effects will be very similar, and the steric effects will now become predominant, leading to a progressive decrease in rate. This steric effect is envisaged as simply arising froin the physical size of the monomers; as this size is increased there is more interference between substituent groups, a bulky monomer having greater difficulty in approaching a polymer radical. Hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate both exhibit low rates of polymerization, and this is to be related to their low heats of polymerization (1). The monomers will be more solvated than the activated complexes, and the free energies of activation will therefore be higher than with the monomers not containing hydroxyl groups. - ACKNOWLEDGMENT The work was supported in part by the Defence Research Board under Grant One of us (K. G. McC.) is indebted to the National Research Council for a Scholarship. REFERENCES K. G. MCCURDY and K. J. LAIDLER. Can. J. Chem. This issue. H. HARTRIDGE and F. J. W. ROUGHTON. Proc. Roy. SOC. A, 104, 376 (1923). R. P. BELL and J. C. CLUNIE. Proc. Roy. Soc. (London), 212, 16 (1952). N. H. RAY. J. Chem. Soc (1960). P. A. H. WYATT. J. Chem. Soc (1960). W. J. ALBERTY and R. P. BELL. Trans. Faraday Soc. 57, 1942 (1961). P. BAUMGARTNER and P. DUHAUT. Bull. Soc. Chim. France (1960). C. H. LUECK. L. F. BESTE. and H. K. HALL. T. Phvs. Chem (1963).., E. G. LOVER~NG and K. J.'LAIDLER. Can. J. Chem: 40, 31 (1962). J. W. MCBAIN. Advances in colloid chemistry. Vol. I. Interscience Publishers Inc., N.Y. p W. D. HARKINS. J. Chem. Phys. 13, 381 (1945). W. D. HARKINS and R. S. STEARNS. J. Chem. Phys. 14, 215 (1946). W. D. HARKINS. 1. Am. Chem. Soc ( W. V. SMITH and K. H. EWART. T. ~hek. ~hvs. 16: 592 (1948). W. V. SMITH. f. Am. hem. ~oc.~70, 3695 (1948). ' W. V. SMITH. J. Am. Chem. Soc. 71, 4077 (1949). M. MORTON, P. P. SANATIELLO, and H. LANDFIELD. J. Polymer Sci. 8, 111 (1952). M. MORTON, P. P. SANATIELLO, and H. LANDFIELD. J. Polymer Sci. 8, 215 (1952). M. MORTON. P. P. SANATIELLO. and H. LANDFIELD. T. Polvmer Sci ( S. S. MEDVEDEV. ~nternational symposium on mac~omol~cular chemistry.' ~e&amon Press, N.Y p J. G. BRODNYAN, J. A. CALA, T. KONEN, and E. L. KELLEY. J. Colloid Sci. 18, 73 (1963).

5 MCCURDY AND LAIDLER: RATES OF POLYMERIZATION M. H. MACKAY and H. W. MELVILLE. Trans. Faraday Soc. 45, 323 (1949). 23. G. M. BURNETT, P. EVANS, and H. \V. MELVILLE. Trans. Faraday Soc. 49, 1105 (1953). 24. G. M. BURNETT, P. EVANS, and H. W. MELVILLE. Trans. Faraday Soc. 49, 1096 (1953). 25. M. S. MATHESON, E. E. AUER, E. B. BEVILACQUE, and E. J. HART. J. Am. Chem. Soc. 73,5395 (1951). 26. H. W. MELVILLE and A. F. BICKEL. Trans. Faraday Soc. 45, 1049 (1949). 27. C. H. BA~IFORD, IV. G. BARB, A. D. JENKINS, and P. F. ONYON. The kinetics of vinyl polymerization by radical mechanisms. Butterworth Scientific Publications, London p. 82.

THERMOCHEMICAL STUDIES OF SOME ACRYLATE,4ND METHACRYLATE POLYMERIZATIONS IN EMULSION SYSTEMS

THERMOCHEMICAL STUDIES OF SOME ACRYLATE,4ND METHACRYLATE POLYMERIZATIONS IN EMULSION SYSTEMS K. G. AICCURDY AND K. J. LAIDLER The Department of Chemistry, University of Ottawa, Ottawa, Canada Received