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

Transcription

1 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 October ABSTRACT 1%-ith the use of a Tian-Calvet microcalorimeter, the heats of polymerization of a number of monomers have been measured in aqueous emulsion systems: the emulsifying agent was cetyltrimethylammonium bromide, and initiation was by Fenton's reagent (Fe++ + HzOe). The monomers employed were acrylic acid, methyl, ethyl, and butyl acrylates, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, methacrylic acid, and methyl, ethyl, butyl, and hexyl methacrylates. The heats of polymerization lie in the range from 13 to 19 kcal per mole, the experimental standard deviations being 40 to 110 cal per mole. A steady increase was observed for acrylic acid and its CHI, C?Hs, and C4Hg esters, and for methacrylic acid and its CHI! CzHs, C4H9, and C6H13 esters. This change is attributed to a steady increase in the proportion of head-to-head, tail-to-tail polymers, leading to a decrease in steric hindrance. Abnormally low heats observed with hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate are attributed to the solvation of the monomer, less solvation being possible in the polymer as a result of shielding. INTRODUCTION During the past few years a considerable amourlt of atterltion has been devoted to heats of polymerization, including those of monosubstituted (CH2=CHX) and 1,l-disubstituted (CH2=CXY) ethylenes. Estimates of heats (-AH) of polymerization on the basis of bond and group energies lead to values close to 20 kcal per mole (1). Heats of polymerization of the monosubstituted ethl-lenes agree with this value, but the disubstituted ethylenes have heats that are less by 3 to 9 kcal per mole. This difference has been attributed (2, 3) to the energy of repulsion between the neighboring side-units in the polymer formed. The vinyl polymers exist predominantly in the head-to-tail arrangement, and in polymers formed from 1,l-disubstituted monomers there is steric hindrance involving side-groups or neighboring monomer residues. I'aluable support for this point of view has been obtained by Baxendale and Madaras (4) in a study of heats of copolymerization. The work done so far has shown that the heats evolved in the pol>-merization of the methacrylates (1,l-disubstituted ethylenes, H2C=C(CH3)COOR) are significantly lower than with the acrylates (monosubstituted ethylenes, H2C=CHCOOR). No systematic study of the effect of substituents has, however, been carried out on these compounds. This has been done in the present investigation, the object of which was to learn more about the magnitude of the effects of steric hindrance. The polymerizations were carried out in aqueous emulsions, the emulsifying agent being cetyltrii~~ethylammonium bromide. Initiation was brought about by Fenton's reagent: this produces OH radicals, Fe++ + Hz02-4 Fe+++ + OH- + OH, and these initiate polymerization by adding on to double bonds. EXPERIMENTAL Calorimetry The heats of polymerization were measured using a Tian-Calvet n~icrocaloriineter that has been described in previous publications from this laboratory (5, 6); the amplification and integration circuits have been Canadian Journal of Chemistry. Volume 42 (1964) 818

2 McCURDY ASD LAIDLER: THEKMOCHEMICAL STUDIES 819 described in detail by Attree and co-workers ('7). The instrument was calibrated electrically by passing a known current through a measured resistance for a definite period of time. As a check the heat of solution of anhydrous lithium sulphate was measured; the value obtained, -6.54f 0.1 kcal per mole, is in satisfactory agreement with the value of kcal per mole reported in the N.B.S. tables (8). The calories reported in this paper are defined calories, equal to joules. Maferials The monomers employed in this work were kindly supplied by the Rohm and Haas Company, and had been purified. They were all reported to be over 99% pure except for 2-hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate, which were both Y6.5y0 pure. \Then received, the monomers contained a few parts per million of inhibitor. Just prior to polymerization of a monomer the inhibitor was removed by distillation under reduced pressure of nitrogen. The middle fraction of the distillate was collected and kept under refrigeration until used. The emulsifying agent, cetyltrimethylamn~onium bromide, was an Eastman Kodak product; it was purified by recrystallizing it twice from acetone. The ferrous sulphate and hydrogen peroxide were reagentgrade chemicals. The materials used in the volumetric determination of the monomers were Fisher Certified Reagents. Reaction Cell Assembly A special cell was designed for measuring the heats of polymerization, and is shown in Fig. 1. The function Fig. 1. 'The reaction cell assembly. of the apparatus is to add the initiator to the reactants and to bring about gentle stirring. The cell A is constructed completely of glass, and it is connected to the upper part B by means of a 19/38 ground glass joint C. The monomer solution is contained in A, while the initiator is held in a 1-in1 syringe, D. The syringe needle E passes through the ground glass joint, as does a small diameter glass tube F and a glass tube G carrying electrical leads; these are sealed in with epoxy resin, and the glass tube ends in a bulb H, which contains a calibrated resistor immersed in oil. This bulb is held submerged in the middle of the liquid in the reaction cell. The tube F is used for stirring, small regular alterations in pressurc causing the liquid to oscillate through a short distance in the tube. The plunger of the syringe D was extended by a glass rod J that extended out of the top of the assembly; in this way the initiator solution could be injected into the reaction cell by merely depressing the plunger.

3 820 CANADIAN JOURNAL OF CHEMISTRY. VOL Experimental Procedure The general procedure used for measuring heats of polymerization was as follows. The two syringes in the reaction assembly were loaded with the appropriate amount of hydrogen peroxide to give a 10-3 M solution after injection into 10 ml of solution in the reaction cell. An oxygen-free solution was prepared containing 1% of emulsifier and N with respcct to sulphuric acid and M with respect to ferrous sulphate. Part of this solution (10 ml) was placed in one of the reaction cells, to be used as a blank. Monomer was introduced into the remainder of the solution and 10 ml of this placed in the other reaction cell. Both cells were allowed to stand in the calorimeter block long enough for thermal equilibrium to be attained. To initiate the polymerization in the cell containing monomer, the syringe plungers were then depressed. At the same time stirring was begun (and continued through the experiment) and the recording mas started. When the reaction had essentially stopped, the heat evolved was estimated, and approximately the same quantity of electrical heat was developed in the reaction cell. At the conclusion of this calibration the exact heat evolved was calculated from the ratio of the two curve heights on the recorder that was connected to the integrator circuit. Immediately after the completion of the calibration, the amount of monomer in the initial solution and in the final partially polymerized solution was determined by titration. The titration procedure was a modification of that described by Mitchell et al. (9). Ten milliliters of bromide solution was placed in each of three stoppered iodine-number flasks and the flasks were cooled in ice water. To one flask mas added 5 ml of initial monomer solution and 5 ml of chloroform. Into a second flask was introduced the partially polymerized solution from the reaction cell, which was rinsed twice with 2 ml of water and twice with 2.5 ml of chloroform. The contents of the other reaction cell was introduced in a similar manner to the third flask. Each iodine-number flask was placed on an oscillating shaker immediately after the addition of the solution. The flasks were shaken for 1-hour periods, and then again chilled in the water. A 25-ml portion of a 10% KI solution was then added to each flask, which was then shaken. The solutions were then titrated with sodium thiosulphate solution until the chloroform layer was clear. The amount of monomer polymerized can readily be shown to be given by no. moles polymerized = (N(NazSz03)/2000) (VB -2 Vmi+ V,,) where N(NazSz03) is the normality of the thiosulphate solution, TiB is the titer of the blank, Vm, is the titer of the initial monomer solution, and V, is the titer of the partially polymerized solution. Duplicate titrations of monomer solutions were in agreement with each other to within 1%. Titrations on weighed quantities of monomer showed that gravimetric and volun~etric determinations agreed to better than 1%. Special checks were made to confirm that neither the initiation reaction nor the stirring contributed to the heat measured. RESULTS A number of measurements, at least five, were made on each monomer. The results obtained with 2-hydroxypropyl methacrylate are shown in Table I. Most of the measurements, including all of those in Table I, were made with a fixed initiator concentration and emulsifier concentration, namely M initiator and l.oyo emulsifier. A number of measurements were made with different initiator and emulsifier concentrations, the range covered being from 0.5 to 2.0y0 emulsifier and 0.5X lop3 44 to 4X A4 initiator. No effect on the heat was observed. TABLE I Heats of polymerization of 2-hydroxypropyl methacrylate Initial Final Molar monomer monomer Amount Heat heat of Expt. conc. conc. polymerized Polylneri~atio~l evolved polymerization No. (mmoles/lo cc) (mmoles/lo cc) (mmoles/lo cc) (%) (cal) (kcal/mole) Mean = Standard deviation = 0.07

4 . McCURDY AND LAIDLER: THERMOCHEMICAL STUDIES Table I1 summarizes the final results for the various monomers. TABLE I1 Summary of heats of polymerization, T = 25.0" C Molar heat of Standard No. polymerization deviation Mononler measurements (kcal/mole) (kcal/mole) Acrylic acid Methyl acrylate Ethyl acrylate n-butyl acrylate 2-Hydroxyethyl methacrylate 2-Hydroxypropyl methacrylate Methacrylic acid Methyl methacrylate Ethyl methacrylate n-butyl methacrylate n-hexyl methacrylate DISCUSSION Comflarison with Previous Results Heats of polymerization have previously been reported for some of the monomers used in the present study. These heats relate to a number of different states of the monomer and polymer, and Dainton and Ivin (10) have suggested a notation, which is summarized in Table 111, for these states. Table IV summarizes the results obtained by Acrylic acid TABLE I11 ~ Summary of standard states Notation State of monomer State of polymer Gas Gas Liquid Liquid Solution Solution TABLE IV Gas (usually hypothetical) Condensed (liquid or amorphous solid) Condensed Solution in monomer Solution Condensed Molar heat of Temperature Monomer Standard states polymerization (kcal/mole) of polymerization Reference Methyl acrylate n-butyl acrylate Methacrylic acid Methyl methacrylate Ethyl methacrylate n-butyl methacrylate n-hexyl methacrylate

5 822 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, 1964 various previous workers, our own values being shown in parentheses. Our values' are best compared to the solution-solution values where available; they are measured at a different temperature (25.0' C) from those of previous studies, but the effect will be small. The agreement between our results and those of other workers is good in some cases (acrylic acid and ethyl methacq-late) but not in others. In the case of methyl acrylate we agree with the lc value of Tong and Kenyon (11), but the ss value of Evans and Tyrrall (12) is considerably higher than ours. Their estimated error of 1 kcal, amounting to 5y0, indicates a large degree of uncertainty in their results. Their value for methacrylic acid also differs considerably from ours, again being much higher. In the case of methyl methacrylate our value does not differ greatly from any of the several previous values, especially when the estimated errors are taken into account. The value of Tong and Kenyon (13) may be too low because they assumed complete polymerization. The results of Joshi (23, 24), for bulk polymerization, were obtained at 74.5' C. They are in fair agreement for methyl methacrylate and n-butyl methacrylate, but differ significantly from previous results, and from the results of the present paper, for the two monomeric acids. Interpretation of Results Addition of a free radical to a monosubstituted or 1,l-disubstituted ethylene may occur in one of two different ways: The former process leads to a head-to-tail (HT) polymer, the latter to a head-to-head, tail-to-tail (HHTT) polymer. The relative rates of the two processes depend on the energies of the corresponding activated complexes, and the following two factors are of primary importance: (I) In the radical R-CH2-CXIT- the substituents X and Y provide the possibility of resonance structures in which the unpaired electron resides on the radical, and this has the effect of stabilizing the radical. This resonance stabilization will also enter to some extent into the activated complex. KO such resonance stabilization is possible in the case of the radical R-CXY-CH2- or of its corresponding activated complex. This factor therefore favors the formation of HT polymers, and structural studies (14, 15, 16) in fact indicate that these are the most common. (2) Steric effects, on the other hand, tend to favor the HHTT arrangement, as is shown schematically in Fig. 2. There is more steric interference in the case of an HT polymer than in an HHTT polymer, and this is also true in the case of the activated complexes. A polymer radical in which the odd electron is on the -CH2- group (I and 11 in Fig. 2) will therefore prefer to add on to the -CHz end (I) rather than the -CXY end (11). Similarly a polymer radical with the odd electron on the CXY group will prefer to add on to the -CXY end of the monomer (IV) rather than to the -CH2 end (111). As a result, the HHTT polymers tend to be preferred if there is much steric hindrance. In agreement with this, Marvel and Cowan (17) have shown that the monomers C1 CHz=C / 'COOCH8 and CHFC / Br \ COOCHa 9

6 McCURDY AND LAIDLER: THERMOCHEMICAL STUDIES INITIAL STATE ACTIVATED STATE Fig. 2. Initial and activated states in polymerization. where considerable steric effects are expected, lead predominantly to the HHTT arrangement. These considerations lead naturally to an interpretation of the apparently paradoxical result that in both the acrylate and methacrylate series the change from methyl to ethyl to n-butyl leads to a steady increase in the heat of polymerization. It must be supposed that in these series the increasing steric hindrance leads to an increase in the proportion of HHTT polymer, so that in the products there is actually less hindrance with the more bulky substituents than with the smaller ones. 2-Hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate both exhibit lower heats of polymerization than do the monomers having no hydroxyl groups. The difference seems too large to explain on the basis of changes in the bond strengths, or by steric hindrance, brought about by the hydroxyl groups. The most likely explanation appears to be that in solution the hydroxyl group in the monomer is solvated, but that in the polymer chain the hydroxyl groups are only partially solvated. The heats of solvation of alcohols are in the range from 2.5 to 4 kcal per mole, so that the effects observed could be explained if only part of the solvation shell is removed on polymerization. It must be emphasized that since all of the effects discussed above are small, the expla~lations are necessarily tentative. Factors such as heats of e~nulsification of the monomers, or heats of wetting of the polymers, might also play a part. ACKNOWLEDGMENT The work was supported in part by the Defence Research Board under Grant One of us (K. G. NIcC.) is indebted to the National Research Council for a Scholarship.

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