Isomerism in Vinyl and
elated Po
r5
C. E. SCHILDKNECHT, S. T. GROSS, AND A. 0. ZOSS General Aniline & F i l m Corporation, Easton, Pa.
Some cases of isomerism in high polymers prcpared from pairs of different monomers are reviewed in relation to the more difficult problems of isoinerisrn in vinyl polymers prepared from a single monomer. Branching, head-to-tail against head-to-head addition, and several types of spatial isomerism are discussed. Evidence is presented of spatial isomerism in polj \in31 isobutyl ethers and poly\inyl rnethS1 ethers obtained under widely different conditions of polymerization. Differences in d and 1 sequence OP the OR groups, as well as liinlis stabilized by restricted rotation, m a y contribute to the shape of the chain molecules and to the contrasting physical properties observed in the polyvinjl ethers of similar degree of polymerization. There is proposed a classification of vinj 1 and related polymers in which the symmetry of local chain segments is emphasized. The slow polyphase type of ionic polymerization appears to be unique in giving comparatively well ordered polymers of the type CHzCHX, where X is a large substituent.
I
T WAS suggested in the earlier daysof vinyl polymer research,
notably by Ostromislensky, that several different types or species of polymerizates might result from a single monomer depending on the conditions of polymerization ( d 5 ) , Staudinger and others taught that vinyl polymerizations gave in each case a mixed homologous series of products, having an average molecular weight and a molecular weight distribution. Since that time, variations in the properties of vinyl polymers obtained from a given monomer have been interpreted primarily in terms of average chain length or average molecular weight and by molecular weight distribution. Among the many studies relating physical properties with molecular weight, that of Douglas and Stoops on fractions of vinyl chloride-vinyl acctate interpolymers is a notable example (IO). These investigators showed that sonie mechanical properties can vary markedly with average molecular weight, but that in the high molecular weight range, the differences in properties with further increase in molecular weight became progressively smaller. Although certain studies of vinyl polymer solutions have suggested differences in molecular structure not well explained by molecular weight, this work in general has not been conclusive. Recent investigations of ethylene high polymers by Fox and Martin (16) and by Bryant (5), and of polyvinyl isobutyl ethers in this laboratory have directed attention to the possibilities of structural isomerism in vinyl and related high polymers (39). I n addition to a general discussion of isomerism in vinyl and related polymers i t is the purpose of this paper to present further evidence of isomerism in polyvinyl alkyl ethers. Two types of polyvinyl methyl ethers have been prepared having contrasting physical properties. I n connection with the interpretation of our experimental results, several suggestions regarding thc st.ructure and properties of vinyl and related polymers have been made, and a classification is proposed emphasizing monomer symmetry and the symmetry of local segments of macromolecules.
Differences in local chain order in polyvinyl isobutyl ethers of similar degree of polymerization have been shown to produce relatively great differences in comparative rubberlike and plastic properties such as tack, extensibilitv, hardness, and breakdown on milling (28). Although it has not been proved that branching and variation from head-to-tail orientation are absent, it is believed that stereoisomerism related to that discussed by Iluggins (17) and Mark (18) may be the principal cause for isomcrisin in polyvinyl isobutyl ethers and polyvinyl methyl ethers. ISOMERIC POLYMERS FWORZ DIFFERENT MOiXOMERS
Before considering differences in branching, head-to-tail orientation, and stereoisomerism which may result from the polymerization of a single monomer under different conditions, attention is called to polymers which show isomerism as a result of their formation from chemically different monomers. Table I gives examples in which the hydrogen atoms have been omitted from the skeletal formulas as a simplification. These isomeric polymers give some insight into the relation of local chain structure to physical properties. The observations apply to polymers of high molecular weight., where differences in properties with change in molecular weight me of secondary importance. End groups such as attached peroxide catalyst residues are neglected, and straight structures with head-to-tail addition are assumed. Staudinger pointed out that branched hydrocarbons are more readily soluble in hydrocarbon solvents than are relatively straight chained hydrocarbons of similar molecular weight (SI). Commercial polyisobutylenes, Tab12 I, A , are soluble in benzene a t room temperature, while the highest molecular weight hydrocarbons obtained by the Fischcr-Tropsch synthesis do not dissolve readily in benzene a t room temperature. The softer more rubberlike propert,ies of polyisobutylenes can be att'ributed largely to side methyl groups. In Table I, B and C, an isobutyl group on a side substituent is exchanged for an n-butyl group. It has been pointed out, for example, by Keher that polyisobutyl acrylates are harder. and less rubberlike than poly-n-butyl acrylates (24). This case has similarity to the differences in properties between vinyl isobutyl ether high polymer and vinyl n-butyl ether high polymer of similar degrees of polymerization and when the same method of polymerization is used (28). Baker in 1944 observed that the isobut,yl group is capable of better packing than the n-butyl group in polyvinyl butyl ethers ( 2 ) . The differences in properties between the high polymers of methyl methacrylate and ethyl acrylate are well known. Methyl and ethyl acrylate polymers are flexible, extensible, and rather rubberlike a t room temperatures (f27). The fact that mcthyl methacrylat,ehigh polymers are rigid solids of relatively high softening point may result from the contribution of the alpha methyl group toward chain symmetry by balancing to a degree the COOCH:, group. I n the acrylate series increasing the lengt'h of the alkyl group attached t o the carboxyl group leads to increased softness, until a minimum brittle point is reached in the case of poly-n-octyl acrylate (27). Compare ( I d ) .
1998
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1949 TABLEI. A.
(C4Hs)n
latter dissolves in chloroform or in methyl ethyl ketone (21, 23). ISOMERIC POLYMERS FROM DIFFERENTMONOMERS The concentration of the two polar chlorine atoms a t the farther Polyisobutylene Paraffin hydrocarbon distance from the main carbon chain necessitates the use of a more polar solvent. Differences between electrical loss factorC/’ \ //
d’
C--c:
C \
/
C \C
C /
/ Poly-n-butyl acrylate
Polyisobutyl acrylate
/
c’\ /o\c/c\c/c /C-Cn 0
l i
Polyvinyl isobutyl ether
Polyvinyl n-butyl ether
/
/
c/’
c
0
c
/ Polymethyl tiiet,hacrylate /
Polyethyl acrylate
d\c /
Polyvinyl acetate
c/ \c/ /
0
\c/
/
c
b
2,B-Dichlorostyrene
/
0
c’
/No
\c-c
/
Polytnethacrylic acid
/
/
C’
\ c-c-c
/
/
\o
/
0
3,4-Dichlorostyrene
polymer
/
4 ‘/ c d c , /
G.
Polyvinyl P-phenyl butyrates:
1 and dl
/
c / o
c
c
/ /
i)l
1”
1999
H.
Polyvinyl chloride against chlorinated polyethylene
f,
Poiyvinylldene chloride against chlorinated polyethylene
Examples of side groups that are functionally different are shown in Table I, E,in the case of the isomeric polymers, polyvinyl acetate, and polymethacrylic acid. Polyvinyl acetate is relatively soft while dry polymethacrylic acid is a rigid solid of high melting point. The attachment of a side group through an oxygen atom with the nearly right angle bend is associated with softness both in vinyl alkyl ether polymers and in polyvinyl esters. The relatively exposed polar groups in polymethacrylic acid may contribute to the rigidity of these polymer chains. Dichlorostyrene isomers show some interesting differences in their polymers notably in solubility and in electrical properties. From the six dichlorostyrene monomers, the polymers of all are soluble in benzene except the 3,4-dichlorostyrene polymer. The
frequency curves for polymers from 2,5-dichlorostyrene and 2,6dichlorostyrene have been recorded (38). Polyvinyl 6-phenyl butyrates differing in optical activity were prepared by Marvel and co-workers ($0). Thompson and Torkington have observed that the infrared vibrational spectra of polyvinyl chloride and polyvinylidene chloride are different from those of chlorinated polyethylenes having similar percentages of chlorine (37). ISOMERISM IN POLYMERS OBTAINED FROM A SINGLE MONOMER
Before the high order of magnitude of the molecular weights of vinyl high polymer was appreciated, cyclic or ring structures were considered as well as linear structures. The ring formula accounted for the low degree of remaining unsaturation observed (39). With present day evidence that vinyl macromolecules are predominantly long chains, their physical properties seem best explained in terms of the configurations and constituents of chain segments and their interaction. Of the possible types of local chain isomerism, branching, variation from head-to-tail orientation, and stereoisomerism are considered below. There has been little evidence until recently that BRANCHING. addition polymers, different in structure and properties, could be obtained from a single monomer under different conditions of polymerization. Branching in high polymers has been much d i e cussed, but there has been no general method of establishing the extent of branching. However, in the case of high molecular weight paraffin hydrocarbons different degrees of branching have been firmly established. Bamberger and Tschirner observed the formation of polymethylene in solutions of diazomethane in dry ether a t room temperature (8). The phenomenon was observed by von Pechmann about the same time (26). The reaction was catalyzed by surfaces of unglazed porcelain, platinum, or sodium. Bamberger and Tschirner suggested t h a t diazomethane dissociated giving methylene which formed polymethylene of correct analysis (CH2),. Polymethylene would not dissolve in ordinary organic solvents, but dissolved in boiling cumene and in boiling pyridine. The reprecipitsted, dried polymethylene was a chalklike powder having a melting point of 128” C., somewhat higher than polyethylene plastics. It is probable that this polymethylene or polyethylene is a relatively straight chained hydrocarbon. Fischer and Tropsch obtained a mixture of hydrocarbons of high molecular weight from carbon monoxide and hydrogen over contact catalysts at 10 atmospheres for long periods of time ( I S ) . A t room temperature these gave a hard mass which became half liquid on heating. By difference in melting point, fractions having relatively high degrees of branching were separated, yielding what was regarded as linear high molecular weight p a r a h hydrocarbons. They were soluble in hot benzene, xylene, or chloroform, but not readily soluble cold. The relatively straight chained Fischer-Tropsch hydrocarbon solids are brittle a t ordinary temperatures in contrast t o polyisobutylenes and polyethylene plastics. Branching in polyethylenes (commercial polythene plastic) was first disclosed by Fox and Martin (16). They found an unexpectedly large proportion of methyl groups representing chain ends indicated by infrared absorption. The fractions from commercial polythenes, obtained by solvent extraction with carbon tetrachloride, contained a greater proportion of methyl groups than the material which was relatively insoluble in carbon tetrachloride. Bryant (6) has made physical and chemical studies of polyethylenes which reveal branching isomerism with corresponding differences in crystallinity in samples obtained under different
*
2000
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
condit,ions of polynierisation, apparently modifications of the high pressure process of Fawcett et al. ( 1 1 ) . Bryant found that the side chains average a t least 12 to 20 carbon atoms in length. These studies indicate that branching isomerism can produce differences in softening point, solubility, mechanical properties, transparency, and in x-ray diagrams. Although Staudinger and eo-workers suggested that braiiching occurred in polystyrenes and in polyvinyl chlorides, their evidence was indirect and the physical properties of branched against straight chain fractions were not compared except in solution. I n polystyrenes more branching was indicated at higher temperatures of polymerization and this was used to explain the larger differences betTTeen the osmotic molecular weight and viscosity molecular weight with rise in temperature of polymerization (34). Singer found some support for this idea froin a study of streaming birefringence of polystyrene fractions in solut,ion (80). From viscosity and molecular weight studies Staudinger and Schneiders advocated that polyvinyl chlorides are more or less branched (35). The fact that many polyvinyl chlorides are 0.5 t,o 1.0% low in chlorine has been attributed to splitting out of hydrogcxn chloride at points of branching. VARIATION FROM HEAD TU T A I L ORIBKT.4TION. (larothers SUggested that in the formation of vinyl polymers from compounds of the type CH2=CHX, occasional inversions of the order of the units probably occur (8). This was advocated as a reason why some of these polymers, such as polystyrene, art: amorphous. The work of Marvel and eo-workers has shown that head-to-tail addition is favored in such vinyl polymers as polyvinyl chlorides and that head-to-head addition, although more unusual, is found in the haloacrylates (19). Small deviations from head-totail order, which may be sufficient to affect chain shape and properties, probably would not have bceri detected by t,h[b chcmical methods employed. Staudinger and Steinhofer favored the head-to-tail or 1,s formula for polystyrene based on their identification in the cracking products of 1,3-diphenylpropane, 1,3,5-triphenylpentane and other compounds with alternating substituents (55). Using entirely different conditions of polymerization Midgley and coworkers obt'ained 1,4-diphenylbut8ane as a n intermediate, which suggested a head-to-head, tail-to-tail arrangement ( 2 3 ) . Flory and Leutner have recently reported indications of small amounts of head-to-head arrangement in polyvinyl alcohols (14). STEREOISOMERISM IN HIGHPOLYMERS. Staudinger proposed that the lack of crystallinity in polystyrenes might result from the great number of possible stereoisomers ( 3 2 ) . Rlarvel has called attention also to the possibility of local chain isomerism resulting from the alternating assymetric carbon atoms in polymers of the type CH2-CHX and CH2CRIRz (19). It seems improbable t,hat optically active polymer chains or chain segment.s could result from nonactive monomer. However, a n isomerism involving differences in the distribution of the substituents in d and 2 positions on the sides of the chain seems possible. See Figure 1, A to 11. Because of the two possible positions of each OR group wit,h respect to the planes of the C-C chain, regular alternation on each side or a random distribution is possible, In 1943 Alfrey and co-workers reported differences in solubility and k' values for polystyrene fractions of similar molecular weight, but obtained from polymerizations at different temperatures (1). They concluded t h a t differences existed in the internal structure
of the macromolecules and attributed the effect to branching. However, Huggins pointed out that the effects observed by AlfreY and co-workers might be better attributed to differences in the regularity of distribution of the phenyl groups on the two sides of the carbon-carbon chain (17). Greater regularity would be expected in polymers prepared a t lower temperatures. Differences in the properties of the polystyrene fractions were not further disclosed and the evidence was insufficient for deciding betwrw the two theories.
Vo1.;41,
No. 9
Figure 1. Possible Arrangements of Alkoxy Groups in Isomerism of Polyvinyl Alkyl Ethers Atoms behind the plane of t h e paper are represented by small letters and arrows: atoms i n front of t h e plane of t h e paper are represented by large letters and large arrows. A = regular alternating distribution of OR groups B = diagram of alternating distribution C = random distribution of OK groups D = diagram of random distribution E = bending of carbon-carbon chain as a result of three successive OR groups on t h e same side of t h e chain t.' = a kink stabilized by restricted rotation G = two kinks stabilized bv restrieted rotation giving a mpandering ohain
ISOMERISM IN POLYVINYL ALKYL ETHERS
POLYVIXYL ISOBUTYL ETHERB.Evidence of isomerism in polyvinyl isobutyl ethers prepared llnder different conditions of polyby merization was first reported from this laboratory in a Schildknecht, zoss, and McKinley ($9). A more complcte studg of the physical properties of both types of polymers covering a range of degrees of polymerization has been reported by the present authors together with Davidson and Lambert (28). Both 01 the polymerization reactions used with vinyl isobutyl ether occur a t low temperatures with polar catalysts. The more rubhcrlikc.
September 1949
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
2001
amorphous polyvinyl isobutyl ether abbreviated PVI-R is obtained by nearly instantaneous or flash polymerization using gaseous boron fluoride as catalyst. The more crystalline polyvinyl isobutyl ether, PVI, is prepared by a slow growth of the polymer about the catalyst, boron fluoride etherate, in a separate phase. Some of the more significant properties of the two types of polyvinyl isobutyl ethers are summarized in Table 11; x-ray patterns of unstretched and stretched polyvinyl isobutyl ethers made by the two types of polymerization are shown in Figure 2.
ISOMERIC POLYVINYL METHYLETHERS.It was found possible to prepare two types of polyvinyl methyl ethers, again using Friedel-Crafts catalysts, but with other conditions different from those employed for the two types of polyvinyl isobutyl ethers. Some properties of the two polyvinyl methyl ethers are shown in Table 111. The amorphous, balsamlike polymer is prepared by a method similar to that for Igevin M40, a German commercial product. The crystalline, hazy, solid polyvinyl methyl ether is a new experimental product obtained by an activated polymerization at lower temperatures than used for Igevin M. Although a number of samples of both types have been prepared having specific viscosities in the same range, the authors have not yet covTABLE11. SOMECONTRASTING PROPERTIEB OF ISOMERIC ered as wide a range of degrees of polymerization as for the polyPOLYVINYL ISOBUTYL ETHERS vinyl isobutyl ethers. The two types of polyvinyl methyl ethers Rubberlike Crystalline can be dissolved and reprecipitated without change. The aqueType (PVI-R) Type (PVI) ous solutions of both show cloud points on heating to near 35' C. Polymerization Flash with BFz Slow with BFa etherate Viscosityrange prepareda 2 . 2 to 4 . 6 0 . 4 to 9 . 1 X-ray diagrams of unstretched thin layers of the two types of Gentle Steeper slope polymers are shown in Figure 3. Dissolving in methanol and re60 to 80 12 to 15 precipitating by the addition of hot water did not change the - 22 - 18 Rubberliked Nontacky sharp rings in the x-ray diagram of the crystalline polyvinyl Rubberlike Partial cold draw methyl ether. Reprecipitation did not change the degree of haziInsoluble Soluble B O O c. Breaks down Little breakdown or change ness in the crystalline polymer. The translucence or haziness is in tack believed to be inherent in the crystallinity of the polymer as in a Values of '2 for solutions of 0.10 gram per 100 ml. of benzene a t 25' C. the cases of polythene and polyvinyl fluoride. C
using Ubbelohde viscometer. b Shore A2 durometer instantaneous readings a t 25' C. C Temperatures at which sheets cracked on bending; some variation with degree of polymerization was observed. d Similar to milled unvulcanized natural rubber.
The two types of polyvinyl isobutyl ethers have shown no tendency to change one into the other over a period of 2 years a t room temperature. Neither milling at room temperature nor heating a t 100 O C. brings about a transformation of type. However, it was found that prolonged heating in solution can cause a small increase in crystallinity in the rubberlike type judging from x-ray diagrams. This experiment was carried out as follows:
To 7.6 grams of the plwticizer, dibutyl sebacate, there were added 2.0 grams of rubberlike polyvinyl isobutyl ether and 0.10 gram of N-(p-hydroxypheny1)morpholine a stabilizer. A relatively large amount of the morpholine derivative was used to prevent polymer breakdown on heating. The mixture was in a glass tube and was heated for hours at I7O0 during this time the polymer dissolved. The highly viscous contents were diluted with 50 ml. of toluene and the polymer then was recipitated by adding 200 ml. of methanol. After drying at 59" C. in vacuum, sheets were molded between cellophane, and x-ray diagrams of the stretched films were obtained before and after the heat treatment in the lasticizer. The heated sample showed a greater degree of crystaylinity, but only a partial change toward the crystalline nontacky type of polyvinyl isobutyl ether was indicated. As a result of the antioxidant stabilizer present degradation of the polymer to sticky liquid and volatile products was avoided during the heat treatment.
c.;
I
Crystalline, unstretched
Crystalline, stretched
Figure 2.
POLYVINYL METHYL ETHERS TABLE 111. ISOMERIC German General Aniline Crystalline Igevin M 40 PVM Type Physical form a t Balsamlike vis- Balsamlike vis- Form stable solid 2 5 O C. cous liquid cous liquid 0.43 0.36 Viscosity, 0.41 C Very sticky Not stick or tacky Tack or stickiness Very sticky Clear Trsnslucent Transparency Clear Viscosity of solutions oontaining 1.00 gram per 100 ml. of benzene a t 25' C. using Ubbelohde viscometer; viscosities are not critical but represent typical samples.
' 2
The isomeric polyvinyl methyl ethers differ in behavior from the two types of polyvinyl isobutyl ethers in stability and in several other interesting respects. When films of the more crystalline polyvinyl methyl ether are melted or deposited from solution, as long as a week a t room temperature is required for the material to return to a nontacky, translucent, solid state. After the crystalline type polymers were heated a t 140' to 150' C. they became more reluctant to crystallize on standing a t room temperature. APPLICATION OF THEORIES TO 1SOMERISM IN POLYVINYL ALKYL ETHERS
As expected in both the isomeric polyvinyl isobutyl ethers and polyvinyl methyl ethers, the polymer formed a t the lower temper-
Rubberlike, unstretched
X-Ray Patterns of Polyvinyl Isobutyl Ethers
Rubberlike, stretched
2002
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cr, stalline
Balsam type
Figure 3. X-Ray JJatterjls of Thin Layers of polyvinJ] Methyl Ether
ature exhibits a higher degree of chain regularity. Both types ot vinyl isobutyl ether high polymers when removed froin antioxidants readily break down even at room temperature to softer low polymers with the elimination of free isobutyl alcohol. This fact together with the low temperatures of polymerization make rearrangements in the monomer before polymerization unlikely. In the case of methyl vinyl ether a migration of the double bond before polymerization would be impossible. A number of the differences in properties might be explained b> greater branching or irregularities in head-to-tail against head-tohead addition in the rubberlike polyvinyl isobutyl ether and in the balsamlike polyvinyl methyl ether. Against the occurrence of long branches is the fact that, although the rubberlike polyvinyl isobutyl ether is readily degraded by milling a t room temperature, i t does not thereby become more crystalline. If branching were an important difference one might expect greater differences in molding temperatures, brittle points, and solubility in common solvents than actually are observed for polyvinyl isobutyl ethers of similar specific viscosity. Against the occurrence of many relatively short branches is our interpretation of electrical data previously reported (28). Unless there were uniform branching throughout the polymer, a sizable fraction of more crystalline material would be expected in the rubbery polyvinyl isobutyl ether. Fractionation experiments failed to reveal such a hardrr. nontacky fraction. STEREOISOMERlSZl O F THE ROTATIOhAL AND D I TYPES
Many of the data seem capable of being explained by a more orderly arrangement of the polar OR groups along the molecules of the more crystalline tvpes of polyvinyl isobutyl ether and polyvinyl methyl ether. Associated with the more random arrangement of the polar groups in the amorphous or iubberlike polymers will be kinks, bends, or spiral shapes of thr polymer chains in contrast to the stiffer straighter chains of the crystalline polymers. The following points seem to support the stereoisomerism theories:
hindrance to free rotation about the C-C
Vol. 41, No. 9
Some t>pes of spatial arrangement of the alkoxy groups on segments of molecules of polyvinyl isobutyl ether and polyvinyl methyl ether are represented diagrammatically in Figure 1. Headto-tail addition and absence of branching are assumed. B and B represent the regular alternating distribution of alkoxy groups along chain segments. The smaller arrows and smaller letters represent groups lying behind the plane of the paper, while the larger arrows arid Letters represent groups on the nearer side of tht. plane of the paper. The segments C and D represent a more irregular arrangement in which two successive OR groups are both in the plane behind the paper. I n Fischer-Hirschfelder models successive isobutoxy groups in 1,3positions on the same side of the chain are shown to be badly crowded. Segment E represents a possible bending of the chain segment to accommodate three successive OR groups on the same side, Randomness in the diatribution of alkoxy groups and deviat,ions from chain straightness both can contribute to rubberlike properties aiid absence of crystallinity. It seems less probable that all of the OR groups in a macromolecule could occur on the same side of the chain and thic condition is not represented. Segment F represents a stabilized kink in a polyvinyl isobuty! ether chain resulting from addition of a monomer unit in a diffcrent orient,ation. The rotation to give a straight chain is restricted by the alkoxy group, but is facilitated by heating in a solvent, or when the restricting group is small such as OCHI. The segment represented in G has two kinks. These may be removed by the, rot,ation of either end or bp the rotation of the small middle sert,ion. The entanglement of chains having kinks stabilized by rcstrickd rotation may play an important role in the rubberlike extensibility and retraction of the amorphous type of vinyl isobutyl ether high polymer. Rotation in high polymers may require considerably more energy of activation tJhan in smaller organic. molecules, since relatively long segments must act togethcr. Rotation cannot change the dl arrangement in the planar zigzag. In vinyl alkyl ether polymers, where the alkyl group is joined t,ci the main carbon chain through an oxygen atom with an angle of approximately log", t,here is also restricted rotation about the bond joining the OR group to the chain. The alkyl groups therefore can have different orientat>ions. FTovieve~,t,his effect docs not
chain
3. Rotation provides for a degree of change in chain structure on heating at temperatures at
crystalline polymer to have stiffer chains while the rubberlike polyvinyl isobutyl ether rhains seem more nearly randomly kinked (88).
Pol: acrylonitrile
Polyviny pyrrolidone
Figure 4. X-Ray Patterns of Stretched Polymerfi
September 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
seem so important as the distribution of the polar oxygen atoms along the main carbon chain and stabilized kinks in determining the physical properties. The occurrence of rotational isomerism in normal organic compounds outside the high polymer field makes more plausible the occurrence of isomerism of this type in vinyl polymers containing bulky side groups. Restriction of rotation about C-C single bonds by bulky substituent groups was discussed by Bischoff in 1891 (4, 7 ) . A dynamic hypothesis was proposed in which those configurations are most favored in which all of the substituent groups of the molecule are mutually least hindered in their vibrations. Isomerism in substituted diphenyls and in other ring compounds is well known. The tcrms cis and trans have been suggested for rotational isomers where rotation about a single C-C bond is restricted by bulky substituents and where no double bond is present (36). That the more ordered condition is not that of greatest possible entropy is suggested by our experiments in which heating the crystalline polyvinyl methyl ether impairs its ability to crystallize. Thus an equilibrium condition intermediate between the rubberlike and crystalline forms may be assumed on heating at high temperatures especially in the presence of active solvents. CLASSIFICATION OF VINYLAND RELATED POLYMERS. A final argument for stereoisomerism in the polyvinyl alkyl ethers is advanced in connection with a proposed classification of vinyl and related polymers. This classification is based largely on monomer and local chain symmetry and the size of the substituent side groups, but polarity and mode of polymerization are also taken into account. The addition polymers considered are all derived from substituted ethylenic monomers having a terminal =CHI group (with the exception that fluorine can replace hydrogen in the terminal methylene). CLASSI. Symmetrically substituted monomers giving in the stretched polymer relatively sharp x-ray fiber patterns: CHzCHz Polyethylene CHzCHF Polyvinyl fluoride (fluorine and hydrogen atoms are nearly equivalent) CHzCXz Polyvinylidene chloride and polyisobutylene CFlCFz Polytetraffuoroethylene CLASS11. Unsymmetrical monomers having two substituents, X , and Y , different in size or polarity. Polymers ordinarily give
none or very poor fiber patterns: CH2CXY Methyl methacrylate and a-methyl styrene polymers CLASS111. Monomers having a single relatively small substituent. Stretched polymers give diffuse or imperfect fiber patterns : CH,CHX Polyvinyl chloride, polyvinyl bromide, polyacrylonitrile
.
CLASSIV A. Monomers having a single large substituent and when peroxide catalyzed polymerization or ionic polymerization of the flash or bulk types are used. The polymers are relatively noncrystalline and do not give sharp x-ray fiber patterns: CH,CHX Polystyrene, polyvinyl acetate, polymethyl acrylate, polyvinyl pyrrolidone Polyvinyl alkyl ethers (rubberlike and balsam types). CLASSIV B. Monomers having a single large substituent, but where a relatively high degree of order can be obtained by special polymerization technique-for example slow polyphase ionic method. CHzCHX Crystalline type polyvinyl isobutyl ether Crystalline type polyvinyl methyl ether X-ray data for ethylenic polymers have been reviewed by Fuller (16), Baker (Ba), and Bunn ( 6 ) . Polyvinyl fluoride has been investigated recently (9). A number of the x-ray diagrams of poly-
mers in addition to polyvinyl ethers have been examined in this laboratory. Two of the cases in which doubt existed were those of polyacrylonitrile and polyvinyl pyrrolidone; the samples of these were obtained by peroxide catalyzed polymerizations (Fig-
2003
ure 4). The sample of polyacrylonitrile showed only partial ordering of the chains after stretching, but no clear fiber diagram. The sample of polyvinyl pyrrolidone was the material of low molecular weight employed in a synthetic blood plasma in the German Army under the name Periston. No evidence of crystallinity or fiber pattern was found in this sample of polyvinyl pyrrolidone. Polyvinyl alcohol has been omitted from our classification since it is not directly derived by polymerization of an ethylenic monomer. Although some possible exceptions exist, the above classification shows that in general the ability of ethylenic polymers to give sharp x-ray fiber diagrams depends principally on local symmetry as affected by the size and arrangement of the substituents in the monomer. Both polyisobutylene and polyvinylidene chloride are capable of giving sharp fiber diagrams although the methyl groups and chlorine atoms are very different. If great randomness in head-to-tail against head-to-head orientation existed in these polymers, one would not expect the polymers always to give fiber diagrams. Poor fiber diagrams are associated with bulky substituents in the monomers, which makes stereoisomerism more likely as a determining factor rather than deviation from head-totail orientation. It is quite possible that special methods of polymerization will be found by which higher degrees of order can be obtained in polymers of class 11, class I11 and class IV A. The classification brings out in the cases of vinyl isobutyl ether and vinyl methyl ether polymers the differences in structure which are attributed to the different methods of carrying out the polymerizations. The slow ionic polymerization methods seem unique in giving relatively well ordered polyvinyl alkyl ethers in spit,e of the single large side group of the monomer. Both in polyvinyl esters and in polyvinyl alkyl ethers models show that the bend through the oxygen atom by which the side group is attached to the main chain, contributes to hindering rotation in addition to the bulk of the substituent group. ACKNOWLEDGMENT
The assistance of P. W. Kinney, of the Central Research Laboratory, Easton, Pa., in this work is acknowledged. The authors wish to thank C. E. Reed, Office of Technical Services, Washington, D. C., who furnished the sample of Periston polymer used in this work. LITERATURE CITED
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RECEIVED September 3, 1948.
Presented as a p a r t of the High Polymer ~ \ M E R X C A N CHEMICAL SOCIETY, Kew
Distillation of an Indefinite Number of Components JOHN R. BOWLM-4N Mellon Instrtute, P i t t s b u r g h , P a .
The fundamental equations of distillation theory are generalized to apply to systems formally composed of an infinite number of Components. The results apply to systems of any number of components, because the elements of the general infinite set are arbitrary and not necessarily all different. Compositions are designated by functions of the form x ( a ) , where ol is the classical relative volatility; the functions x have the property that the amount of material in the compositions they represent, having a! between a and a 4- dol, is xda. All concentrations are therefore of the form xda, and material balances are accordingly set up on a differential basis. Siiice material balances and idealized equilibrium relations are linear and homogeneous in the concentrations, however, the factors da di\ide out, and the fnndamental equations of distillation on the present basis are formally similar to the classical ones.
E“
THE theory of multicomponent separation processes, com-
positions are classically specified by a discrete set of numbers, usually the mole fraction concentrations of the components. Where many components are concerned, this leads to large systems of equations, which often become unmanageable. Though all mixtures, theoretically, contain only a finite number of components, or molecular species, many of the formulas governing multicomponent processes can be simplified by regarding all systems as cornposed of an infinite number of components, not necessarily all different. Mathematically, the transition to a continuum of components is essentially a formal one. Systems of algebraic equations, as the number of variables increases, pass into single integral equations a t the limit, and, similarly, systems of differential equations become integrodifferential equations. Such limiting relations involve explicit integration of the dependent variable and are amenable to mathematical analysis and computational devices which are generalizations of those for finite simultaneous systems ( 3 ) . Representation of a multicomponent composition as a continuum merely requires that the set of numbers corresponding to the components be replaced by a function of a single variablefor example, the true boiling point curve is such a specification of compositions, irrespective of the number of components present. As another example, in the conventional designation of mole
fraction concentrations as z(j) , where i runs through the integers from 1 to n, i can be regarded as a continuous variable, with the condition that it designate the mth component if it has any value in the range, m - 1 < i < m. With this symbolism, the conventional notation is scarcely changed, and little advantage is gained. The principal innovation introduced is that summations must be replaced by integrals-that is, n
A third and most useful example is differential representation of composition. \J7ith it, several new generalized equations of performance can be derived. DIFFEREYTIAL REPRESENT4TlON OF COMPOSITIONS
)There the number of components is infinite, their correlation with unit intervals of the component-designating variable is unsatisfactory, and some or all of the concentrations become infinitesimal. For such problems, recourse must be had to the true boiling point curve function or some scheme of designation derived from it. Actually, the reciprocal of the slope of the true boiling point curve is mathematically more convenient than the function itself. With a slight extension of the definition of 5,the composition can be designated by the function ~ ( a ! ) where , the mole fraction havda: is xda. In effect, ing relative volatilities in the range a to 01 a: now serves as a component designating variable; the correlation is reasonable, inasmuch as association of components with intervals of i was a wholly random labeling, while each component always carries a definite ci. The physical interpretation of the variable x as the reciprocal of the slope of the true boiling point curve has little practical importance. The practical importance of x is always as the product xdci changes the differential of concentration. I n conventional notation, the relative volatility, a , is usually taken t o be the ratio of the vapor pressure of the component t o that of a selected key component. For purposes of symmetry, however, no key component is selected, and the relative volatilities are taken to be the ratios of the vapor pressures of the components to the total pressure a t some fixed temperature. The gen-
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