INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1931
781
POLYMERIZATION AS RELATED TO PAINT AND VARNISH Papers presented before the Division of Paint and Varnish Chemistry at the S l s t Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931
Macromolecules and Micelles in Organic Polymers’ S. E. Sheppard EASTMAN KODAKCOMPANY, ROCHESTER, N. Y.
I
N T H E synthesis of crystallizable compounds of moderate molecular weight and, ultimately, assignable constitution, organic chemists frequently obtain a residue or byproduct in the form of tars, resins, and insolubles. An earlier generation was content to dismiss these messes as %chmieren,” occasionally stating that they were formed by polymerization. This process, when limited to dimerization or trimerization, has been studied rather carefully both from the point of view of structural chemistry and from that of the phase rule. One may distinguish two principal types, direct or addition polymerization and condensation or intermolecular polymerization. I n the first no displacement of atoms is involved; in the second displacements are effected and certain groups, such as HzO, Clp, NH,, etc., eliminated between pairs of molecular units. The application of these concepts to such processes as resinification was rather slow. One finds “the value of cryoscopy for investigations on technical varnishes” indicated by Scheiber and Nouvel in 1923 (9). The following results, obtained in this laboratory, deal with the blowing of linseed oil, and illustrate the growth of molecular weight obtained by boiling-point depression in naphthalene. Table I-Polymerization HEATEDAT 315’ C. Minutes 0 (ram oil) 20 40 60 80
a n d Cryoscopy MOL. WT.
760 963
1143 1326 1057
The data indicated dimerization followed by insolubilization in the solvent. Colloid-Chemical Conceptions
The colloid chemist, in dealing with the high-molecular organic substances, has tended to regard molecular weights and molecular individuality with considerable skepticism. He replaced these by the more empirical concepts of “particle size” and “micellar structure,” derived mainly from the study of inorganic suspension colloids. The formation of gels, in polymerization and condensation processes, was assimilated to the coagulations of inorganic suspensoids. But while the phenomena in the latter case were observed in aqueous systems, and could be associated with ionic interchange and electrostatic neutralization of charged particles, the phenomena connected with polymerization-the drying of oils, for example-were in non-aqueous media, and could only by violence be regarded from the electrostatic point of view. The last ten to fifteen years have seen an intensified interest in the constitution and mode of formation of so-called “highmolecular’’ compounds, among which may be mentioned the Received April 9, 1931.
rubber hydrocarbons (polyprenes), cellulose and cellulose derivatives, starches, proteins of naturally occurring substances, and synthetic resins (plastics) of laboratory production. Common to this group are certain properties which render difficult the determination of their constitution. First, they cannot generally be volatilized without decomposition (volatilization of rubber in a very high vacuum has been reported), nor, from such solutions as are possible, can they usually be crystallized (ezceptis mcipiendis-e. g., albumin, cellulose acetate), This means that conventional methods of molecular-weight determination are excluded. From certain limiting measurements of osmotic pressure, conclusions were drawn of very large molecular weights, but the suspicion that residual traces of highly active impurities of low molecular weight were present made them rather unreliable. Only recently, in the case of proteins by SBrensen and by Adair, have osmotic measurements given results which, both by internal critique of the method and by comparison with values determined by an independent method, can be regarded as significant. The most important contribution from the colloid-chemical side has been precisely this independent method of molecular-weight determination for large molecules. It is the method of hypercentrifugal analysis of Svedberg. As applied to the proteins, it has established two facts (Table 11): That many proteins form solutions of isodisperse particles (of singular molecular weight), and that many proteins give molecular weights which are integral multiples of the molecular weight of egg albumin. Table 11-Molecular Weights of Proteins (Svedberg et al.) PROTEIN MOLECULAR WEIGHT Erre albumin 34.500 = 1 .x, 34..500 .. ,- .. HG&oglobin 68;ioo = i.97 x 34,500 Serum albumin 67,500 = 1.95 X 34,500 Serum globulin 103 600 = 3 00 X 34 500 Phycocyan 105:500 = 3:06 X 34:500 Phycoerythrin 208.000 = 6.02 X 34.500 Limulus-hemocyanin 1,760;OOO Helix-hemocyanin 5,005,000
This method is commencing to be applied to other highmolecular systems-e. g., of cellulose-and is of great value. Certain other indications or estimations of molecular weight, based on indirect methods, which cannot yet be regarded as definitive, will be mentioned later. It suffices at present to note that molecular-weight methods indicate a very large real or apparent molecular weight. On the other hand, the constitutional analysis of these substances-e. g., hydrolysis of proteins, fractionation of amino acids, hydrolytic breakdown of cellulose and starch-had indicated that they could be regarded as compounded of multiples of a small individual group, or building unit. Symbolically, one could cellulose; express them by such shorthand as (C&O&,,
(C&),,, caoutchouc (yolypreue); aud R(NliltCO),,X, protein. The specific evidence that cellulose, for example, could be regarded as a multiple of glucosan (anhydro glucose) residues, caoutchouc of isoprene, and so on, must be passed over.
diagram of cellulose could be consititently interpreted in terms of parallel oriented chains of unit groups held by primary valences. The basis cell is periodically reproduced, irrespective of the size of the molecular entity, whose periodically recurring groups form the units of the cell. Figure 2 illustrates this conception. In such a case the molecular entities, consisting of atom groups held by primary valences, u'ere termed "primary valence chains." From the organic cliemical side Staudinger, long a protagonist of the validity of orthodox organic structure theory for rubber and the like, brought forward similar evidence bearing on synthetic polymm of oxymethylene (formaldehyde): HCHO
+-O-CH,-~~~O-CH2-(O--CHn),0-CH2polyoxymethylenes
Staudiiiger termed such primary valence chains "macromolecules." His evidence that by synthesis (polymerization) a large number of model substances may be built up, giving valuable analogies to the natural or bio-colloids, is very full and suggestive. By ta!&g an unsaturated hydrocarbon such as styrol, CSHS.CH=CH2, a series of different polymers can be obtained by varying the conditions of treatment. Thus: TEBATMXNT
PRODUCT
Short heat treatment at high temperaturer (rapid polymerizatipn) Long hesting (Stsndmg) at low temperatures (slow polymerization) Treatment with catalystp (rapid polymerization)
Hard, brittle m a s s ~ s Very tough, tenadous. SlassY masse9 Hard, britrle mo51~s
The following data illustrate these conclusions: AV. &soL. wr.-
'rBxPBnmsn
TBBAIMSNT
".
"r22 hours in benzene 50 hours in benzene
Figure 1-X-Ray Diifractlm Spectra of Three Types of Auregarlon
But with acceptance of this composition, a great cleavage of opinion existed as to the mode of union of the units. Among organic chemists the theory divided into: (a) primary valence combination, (b) secondary valence or association, and (c) coordination in the solid state of units incapable of free existence (in vapor or solution). One may associate with these views the names of Staudinger as principal protagonist of the primary valence combination, vf Hess and of Pringshe& of the association theory, and of Bergmann of the coordination thesis.
X-Ray Investigation The difficulty or impracticability of obtaining externally crystalline substances has been largely evaded by resort to x-ray spectroscopy. Figure 1 illustrates types of aggregation in their x-ray diffraction spectra-liquid, crystalline powder, and fiber diagram. The fiber diagrams of certain materialscellulose of ramie, stretched rubher----could be attributed to crystallites similarly orientated along the fiber axis. Moreover, their analysis permitted derivation of a unit or basis cell of definite dimensions. Such a cell could only contain a finite small number of elementary building units of the material-such as cellulose, 4 glucosaus-and the tendency was a t one time to regard this as supporting the view that the chemical molecular individuals of these large-molecular compounds actually consisted of relatively small molecules, thus favoring the associationists. Contradiction of this from several sources was soon forthcoming. Sponsler (15) in this country showed that the fiber
3,400 5,800 >20 wo >20:000
240
180
3 weeks in sunshine
2% "cars an,ontsneousi*
Deduced from vixosity~conceotralioncurves according t o a relation propored by Staudinzer.
The variation of average molecular weight at early stages of synthetic polymerization suggests, as proposed by Staudinger, that series of homologous polymers are produced, so that any such product is a mixture of polymers of different degree. This could be confirmed by fractionation experiments, and is of importance in considering the natural nusynthesized colloids, such as cellulose, its derivatives (which are already partly artificial), rubber, and the like. The most probable type of polymerization is one-dimensional, a chain, of which an explicit anatomy may be illustrated Styrol
PolyStyl(l1
-_-
0
16
0
C~N*,CH--CHI 4-CIr-CH-C~i--CHp-CH--CH*-+ Structural Binding ""it zroup Peptide Polypeptide -NII-CHa.CH*---CO 4-NH.CH*.CO ~ N I I ~ C H S ~ C H B . C O ~ N H ___c-Id
Binding S""P
Structural "nit
Paly~inylalcohol Vinyl alcohol CHFCHOH 4 -CH-CI%-CH-CII-
I
OH
I
OH
It will be observed that this conception of ohaiii formation leaves the ends unsaturated. Staudinger supposes that these ends may (a) become oxidized or otherwise converted to saturated groups; (b) attach molecules of solvent; or (c) combine with each other so that ring formation occurs.
IXDUSTRIAL AND ENGINEERILVGCHEMISTRY
July, 1931 Example (a): HO-CHz-0-(
CH-O).-CHI-OH Polyoxymethylene dihydrate (paraformaldehyde) CHL!Oz-CHz-O-( CHZ-O)~-CHZ.O~C.CH~ Polyoxymethylene diacetate
Example ( c ) :
I
CHs
I
CH3
I
1
CHZ-OH=CH-CH~CH~-C=CH-CH~ Polyprene ring form
The nature of the end groups may be decisive for certain properties of the material, such as chemical reactivity. The effect of an end group is most simply exhibited in the case of normal paraffins of high molecular weight, and derivatives such as monocarboxylic acids and primary alcohols. They all have similar solubilities, melting points, and physical properties, but the carboxyl and primary alcoholic end groups confer chemical reactivity lacking in the hydrocarbons. The production of two-dimensional and three-dimensional valency chains is evidently possible on structure chemistry grounds, but much less probable than the unidimensional ones. Staudinger and Bruson ( 1 5 ) , however, obtained by polymerization of cyclopentadiene a product, about hexamolecular, which formed two-dimensional leaflike molecules, which in spite of relatively low molecular weight was completely insoluble, in contrast to the linear polymerization products. Graphitic acid has been assigned a two-dimensional structure. The best ascertained three-dimensional valence structure is diamond (carbon), but Bakelite and glyptal resins have been assigned such constitutions by K. Meyer, and lignin by Freudenberg. I n all these cases radioscopic investigation has given, and promises to give, most valuable help. I n many instancese. g., polyoxymethylene-the average degree of polymerization can be determined with considerable precision from the diminution in diameter of the innermost diffraction rings arising from multiplanar reflections. The extent to which condensation polymerization, termed by Carothers (2) “polyintermolecular condensation,” may occur has been discussed very fully in two interesting papers. For the immediate production of regular two-dimensional macromolecules, two functional (active) groups must get into operation, and also with equal reaction velocities. These products must be stable, and formation of five- or six-membered rings excluded. Carothers points out here that the assumption that the condensation polymerization proceeds by primary excision (anhydrization, etc.) of the original molecules, followed by their addition polymerization, is not justified. Hence, the mechanism of addition polymerization is probably not so simple as has been assumed. Reaction Mechanism of Polymerization Roughly speaking, we have apparently rather simple experimental conditions for polymerization reactions. These are heating, say, in sealed tubes, addition of catalysts, exposure to ultra-violet light, or exposure to visible light in presence of photochemical catalysts. But it has already been noted that acceleration of the reaction by heat diminishes the degree of polymerization, and the same is true of catalysis, but generally to a less degree. One may note here the significant fact that many of the effective catalysts-e. g., benzoyl peroxide and metallic sodium-are either substances concerned in autoxidation or autoxidizable; further, that polymerizations in paint films (drying oils, etc.) are concomitant with autoxidations. On the other hand, ultra-violet light is frequently a very effective polymerizing agent. From Backstrom’s researches ( 1 ) the autoxidation processes are known to be chemiluminescent. It is this factor which is correlated with their high energy of reaction. “The energy
783
present in a newly formed molecule of the oxidation product is often sufficient, when transferred to a molecule of the autoxidizable substance, to cause the latter to be excited to one of the energy levels responsible for the ultra-violet absorption spectrum of the substance in question.” It is concluded that the thermal reaction is sometimes followed by a photochemical reaction. If we consider that one condition for polymerization is activation of certain linkages or functional groups, we may suspect that absorption of ultra-violet light is an important activating mechanism. The following dissociation-energy values of a number of typical organic linkages have been deduced from the Raman effect:
.......................
Linkage Dissociation energy, kg-cal. per mol
......... . . .... . ...... . . .
C-Hal.
OH
N-H
C:C
C:C
C.0
92
120
98
166
125
203
The excitation and predissociation energies will be considerably less-i. e., not in the extreme ultra-violet (140 kg-cal. e 2000 A,), but in the near ultra-violet (72 kg-cal. == 4000 A.).
I “p,
c
9
-aw Figure 2-Lattice
a d
Structure of Cellulose, according t o Mark a n d Meyer
The connection between autoxidation processes and polymerization appears to receive a reasonable explanation on this basis. It appears that the spectroscopic and photochemical study of polymerization reactions should give valuable results in conjunction with the organic chemist’s functional group interpretations. Molecular a n d Micellar S t r u c t u r e s
In spite of what has been said in regard to the linear macromolecule or valency chain, we meet little direct evidence of these as individuals, but are mostly concerned with their behavior in groups. It is as solids, and to a lesser degree as solutions, that these polymers are of the greatest practical interest. The formation of films in the paint, varnish, and lacquer technologies of moldable plastics depends very greatly upon the group behavior of these macromolecules. What do we know about it? So much is already known, or asserted, that we can consider the subject only very briefly here. First, paradoxically, let us mention evidence for the individual or ungrouped existence of macromolecules. It is natural to expect this only under conditions of greatest rarefaction or dilution. It occurred
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
to the writer in 1927 (10) that if solutions of a given polymer were progressively diluted in a volatile solvent and the thickness of the film which this formed on another liquid were determined, the limiting thickness might show whether, and what, molecular limits were reached. Keenan (IO, 12) in the writer’s laboratory, was able to show that the cellulose esters, rubber, and gelatin gave films on mercury from dilute solutions which were of the same order of magnitude as the dimensions of molecular units, or atom groups composing them. Either these solutions had dissociated these polymers to such unit groups-which disagreed with osmotic data-or the thickness of linear or planar macromolecules was being measured. These results were independently confirmed about the same time by Katz and Sanwel (4), and it may be concluded that a t certain limiting dilutions in given solvents these highmolecular substances are present as individual (macro) molecules. The limit at which they are relatively out of each other’s spheres of influence can be ascertained to some extent by viscosity and plasticity measurements. The writer has found, for example, that a t concentrations below 0.20 per cent a solution of cotton cellulose in cuprammonium gives purely viscous flow, like a true liquid, whereas above this plasticity develops, indicating interference of some kind. I t is interesting to note that Stamm’s (14) hypercentrifugal analysis of cuprammonium alpha-cellulose showed this to be monodisperse, with a molecular weight of 40,000 * 5000 (copper-free). Such particles would have 200 to 260 glucosan units. Above 0.20 per cent concentration the diffusion constant indicates intermolecular action or interference. The nature of the interference is a subject of much controversy, both generally and in specific cases. Herzog ( 3 ) and Meyer and Mark (8) have given evidence, from x-ray studies, that in cellulosic fibers the chains are aggregated in more or less similarly oriented crystallites, of about 50 mp linear dimension. Herzog has claimed evidence from diffusion experiments that in solutions both of cellulose in cuprammonium and of cellulose derivatives in organic solvents the colloid is only dispersed down to micelles of the magnitude of these crystallites. On the other hand, the evidence from the thin film spreading and that from x-ray studies of swelling and sorption (Hess and Trogus) are that such solutions involve intermicellar cleavage down to the primary valence chains at sufficient dilution. This is the view taken by Staudinger and recently admitted, below certain concentrations, by Mark and Fickentscher (6). The great increase of viscosity (- plasticity) of lyophile colloid sols of tjhe cellulosic-rubber-protein type with concentration has been attributed to solvation (6). It is remarkable, however, that gelatin sols, on hydrolysis, show enormous decrease of viscosity, without any appreciable change of specific density ( I I ) , i. e., without change of degree of hydration. This is more consistent with a mere monomolecular solvate layer, the viscosity phenomena being dependent upon the macromolecular length as suggested by Staudinger. The region of action of a long macromolecule is suggested by Staudinger to be
where n = degree of polymerization D = length of structural unit d = width of structural unit
This is equivalent to a cylinder of revolution about the long axis, whereas it might be equally suggested that a sphere approaching r ( 12 X f ) 3 is equally probable, supposing the macromolecule to be quite rigid.
Vol. 23, No. 7
Quantitatively, we still have much to learn here. It may be agreed that a t certain limits of concentration micellar solutions or, in Staudinger’s nomenclature, association colloids or gel solutions are found. These solutions are generally tending to form actual jellies and then to synerese. Individual study of such solutions over wide ranges of concentration, solvent composition, and temperature is required before explicit statements can be made. I n the solid state we meet the natural high polymers or eucolloids, as mainly fibrous substances (cellulose and many proteins) giving typical x-ray fiber diagrams. The fact that a fiber diagram can be developed in natural rubber by stretching to very high elongations has been much debated. Whether a process of crystallization or a mere realignment of a preexistent heterogeneous net structure occurs is undecided. The facts that stretching gelatin jellies before drying allows a similar approach to fibrous state as in collagen to be developed and that similar phenomena are found on stretching cellulose ester gels indicate that a common basis of orientation of long molecules or micelles is operative. Further evidence in this direction has been obtained from study of the birefringence of these materials by McNally and the author (7). The strength of these materials may be regarded as dependent upon two types of chemical forces-the molecular or primary valence forces, and the intermolecular or secondary valence (van der Waals’) forces. For the first, kind we can take, in the equation Force =
energy distance
-
the values of dissociation energy of different chemical linkages already discussed on spectroscopic evidence. These energies are of the order of 70,000 to 200,000 kilogram-calories, according to the linkage, and to some extent variable for the same linkage according to the molecular group constitution. The distance here will be of the order 3 to 4 A. Mark (6) has calculated on this basis that, for the oxygenbridge linkage, and reckoning in cellulose 4.10Ia primary valence chains per square millimeter, a thread of such chains of infinite length would have a strength of 800 kg. per sq. mm. Actually, while cotton shows some 30 kg. per sq. mm., flax gives 100 kg. per sq. mm., evidently still much below this theoretical limit. But such fibers consist of crystallites of chain length about 100 glucosan units. We must turn to the secondary or intermolecular forces, since it is not so much the strength of the chain as the drag of cohesion of the chains on each other which will determine the strength. Calculation of this for complete orientation of the chains gives again a value of about 200 kg. per sq. mm., which is still considerably higher than the highest experimental values for fibers. But a completely non-oriented arrangement would give only one-third of the maximum value. Experimentally it is found, with regenerated cellulose (cuprammonium, viscose) and cellulose esters that the strength in a given direction increases with orientation in that direction, as demanded by the model. Consonant with this, such stretched or oriented material becomes greatly reduced in strength perpendicular to the axis of stretch, or “splits” easily. The problems covered by the term “polymerization” are among the most interesting and practically important in the whole field of chemistry. Theoretically they invite us to consider compounds built up, as Graham suggested, by idiochemical affinity, to molecular units of giant size by processes which are of the greatest importance both in vivo and in vitro. Practically, the solutions of the problems offer guiding paths in the chemistry of waxes and oils, of rubber, of cellulose, of leather and gelatin, of silk, of molded plastics, of paints and varnishes.
I.VD USTRIAL AND ENGINEERING CHEMISTRY
July, 1931
Literature Cited (1) Backstrom, M e d d . 1.elenskapsakad. Nobelinsf., 6, No. 15 (1925). (2) Carothers, J . A m . Chem. Soc., 51, 2548, 2560 (1929). (3) Herzog, 2. angew. Chem., 41, 534 (1928). (4) Katz and Sanwel, ivafur~'issenscha~feilen, 16, 592 (1928). (5) Mark, Melliands' T e x f i l h e r . , Mannheim, No. 9 (1929). (6) Mark and Fickentscher, Kolloid-Z., 49, IS5 (1929). (7) McNally and Sheppard, J . Phys. Chem., 34, 165 (1930).
785
(8) Meyer and Mark, Ber.. 593 (1928). (9) Scheiber and Nouvel, 2 . anpew. Chem., 41, 353 (1923). (10) Sheppard, Nature, 121, 9S2 (1928). (11) Sheppard and Houck, J . Phys. Chem., 34, 273 (1930). (12) Sheppard, Nietz, and Keenan, IND.END.CHEM.,21, 126 (1929). (13) Sponsler and Dore, Colloid Symposium Monograph, p. 174, Chemical Catalog, 1926. (14) Stamm, J . A m . Chem. Soc., 51, 304 (1930). (15) Staudinger and Bruson. A n n . , 447, 97 (1926).
Polymerization us. Association and Condensation' Eugene C . B i n g h a m a n d Laurence W. Spooner GAYLEYCHEMICAL LABORATORY, hFAYETTl3
OME years ago Bingham and Harrison (3) devised a method for measuring the association of liquids, but they did not suggest how association is related to constitution and their method remained in obscurity. Recently Bingham and Darrall (1) were afforded the opportunity to study twenty-three of the isomeric octyl alcohols. The results proved that association depends to a high degree upon the constitution and in a manner which is readily understandable. I n the first place, there are certain polar groups which acCO, GI, centuate the association, notably OH, COO, "2, and SH. On the other hand, hydrocarbon residues are very little associated, and therefore they tend to lower the association of the molecules in which they occur depending upon the magnitude and the constitution of this hydrocarbon residue. The more exposed a polar group is, the higher will be the association; and the more fully it is surrounded by alkyl groups, the lower will be the association. The above conceptions are so simple and natural that it is surprising that they have not already become established. There is also a considerable mass of evidence to prove them:
S
(1) In the f i s t place, the normal compound with the polar group a t the end of the chain is the most highly associated. (2) .Lengthening the chain decreases the association, probably without exception. ( 3 ) Bringing the polar group toward the center of the chain further decreases the association. (4) An is0 grouping in the hydrocarbon residue nearly invariably brings about a decrease in association. ( 5 ) The decrease in association is still further accentuated by a further clustering of the hydrocarbon residues around the polar group. (6) In ten classes of compounds thus far investigated, it has been found that the addition of a methylene group lowers the association approximately 9 per cent. (7) In the class of esters it is found (Z), as might have been expected, that methyl formate is the most associated of all esters; furthermore, methyl esters are all associated, as are all formates, but all the esters of the higher alcohols united t o acids above butyric acid appear to be very slightly associated. The theory of protection enables one to estimate the association of the various esters very precisely. The effect of a methylene group is of course quite different, dependent upon whether it is placed in the acid or the alcoholic residue, but it can be exactly calculated. (8) I n the aromatic compounds, like the cresols, the presence of an alkyl residue in the ortho position to a polar group is most effective in lowering association, as would be expected, and in the para position it is least effective. (9) On the other hand, a second polar group in the ortho position should be less effective in raising the association than in the para position, but data are here lacking. (10) By the same reasoning one can explain why maleic acid is less associated than fumaric acid, (11) Finally, in an aryl compound having two alkyl groups, as in the xylenes, the theory explains on the basis of protection why para-xylene is less associated than ortho-xylene. More evidence from the aryl compounds is being sought. 1
Received April 9, 1031
COLLEGE, E a s T o N , P A .
There is not opportunity here to give all the data upon which these conclusions are based, but it is expected to have shortly the associations of some three hundred liquids a t a variety of temperatures and fluidities, and for these substances the associations may be calculated by a series of simple formulas suited to the various classes of compounds. The method has already served to correct certain of the data of the literature, and there are still some compounds, such as ethyl palmitate and propyl propionate, which need verification, but in general the calculated associations are as satisfactory as could be expected. To be able to measure the association will doubtless be important, but it is much more valuable to he able to calculate the association from the known composition through the relationships established between chemical composition and association for the various classes of compounds. It now becomes possible for the first time to write the formula of a substance which will have a given fluidity at a certain temperature. Thus there is answered the age-old query of Democritus, "Why is water a liquid?" and it is no longer necessary for the student of organic chemistry to learn "by heart" that hydracrylic acid is "a thick, sirupy liquid." But even wider uses, not hitherto utilized, open out before us. Let us confine ourselves to very simple examples in demonstrating this use of the viscometer. If acetaldehyde were associated in the liquid state to three molecules, the absolute temperature required to produce a fluidity of 100 rhes would be 3 X 159.2 = 477.6' K. 3CH3. CHO 159.2' K.
(CHI. CHO)a 477.6' K.
The actual temperature as obtained by considerable extrapolation is 191' K., so that acetaldehyde is only associated to a slight degree (n = 1.20). I n the process of polymerization of acetaldehyde, the compound C6HI203is formed with the loss of three double bonds and the gain of a ring grouping. It has already been proved that n double bond or ring grouping has the same effect upon the flow as if hydrogens were present. The paraldehyde should have the same temperature for a fluidity of 100 rhes as a straight-chain compound equally unassociated having the formula of CeH1403, which is 276" K. As a matter of fact, the observed temperature is 301' K., so that even paraldehyde is somewhat associated (n = 1.09), but this is 176'K. lower than would have been the case had acetaldehyde been merely associated into trimolecules. If polymerization is less effective than association in lowering the fluidity, there is a third method for bringing molecules together which lowers the fluidity still less, as in the familiar condensation. Two molecules of acid condense with elimination of water:
+
2CzFsCOOH +(CzHaC0)zO Hz0 2 X 232.2 K. = 321.3' K. f 143.1' K. a t 200 rhes
