Charles C. Price1
University of Pennsylvonio Philadelphia, 19174
I
Some Stereochemical Principles . from Polymers -
I 1
Molecular symmetry a n d molecular flexibility
The properties of polymers from ethylene, propylene, isobutylene, and the three related epoxides, can be used to illustrate several important basic principles relating chemical structure to properties. These include (1) concepts of asymmetry, or better molecular chirality, (2) the influence of chemical structure on molecular geometry and especially molecular flexibility, and (3) a clear view of entropy and how it can affect the properties of molecules. Let us examine the structure of these six polymers and their crystal melting points to see how the thermodynamics of melting can be at least qualitatively related to the chemical structure. First, at the melting point there is a true equilibrium between liquid and crystal. In other words, there is no change in Gibbs free energy ( G ) , so that, a t the melting point, T, AG, = 0 AG, = AH, - T,AS, AH, = T,AS, ?', = AH,IAS, Thus a high melting point will be favored by a high euthalpy change for crystallization (AH,"), i.e., very strong crystal forces, or a low entropy change for crystallization (AS,), i.e., relatively little disorder in the liquid state. fCH2CH2% mp 140" polyethylene
CCH,CH,O+ mp 65' poly(ethy1ene oxide)
polypropylene
poly(propy1ene oxide)
polyisobutylene
paly(isohutylene oxide)
Five of the above polymers are hard, crystalline plastics
(isotactic), while polyisohutylene is an amorphous, soft rubbery material which gives excellent crystalline X-ray diffraction patterns when highly stretched. Since the introduction of methyl in place of hydrogen greatly enhances the melting point for polyisohutylene oxide, what explanation can be offered for the opposite effect in polyisohutylene? First we must recognize that, for a linear polymer to fit into a regular, orderly crystal structure, the chain itself must assume an arrangement which makes this possible. For polyethylene, such an arrangement is not only possible, but is thermodynamically preferred (see figure). H H \ ,,'
1-2.54A-1
H
H
H
H
Extended (trans)
Bent (skew)
Careful study of the X-ray diffraction patterns for polyethylene has that the chain in the crystal is indeed in the extended (or linear) arrangement. In this form, the chains are readily packed together in a regular array necessary for crystallinity. The liquid molecules contain a number of bent (or skew) bonds, which represent a decree of disorder compared to the all-linear molecules i n t b e crystal. The liquid molecules are thus more disordered (i.e. higher in entropy) than the crystalline molecules. The very high melting plastic Teflon (polytetrafluoroethyh e , mp'- 325°C) has all hydrogens replaced by fluorine atoms. The AH, for Teflon (1370 kcal/mole) is actually lower than for polyethylene (1840 kcal/mole). The explanation for the high melting point is a much lower AS, (2.30 cal/mole "K compared to 4.53 cal/mole OK). This is interpreted as meaning that the Teflon chain is much stiffer, i.e., it bas much less chance of existing in the disorderly bent or skew conformation. But what of polyisohutylene? Why is i t not also a "stiffer" chain with a higher melting point? The explanation for this case requires a careful look a t the molecular geometry required for the extended conformation. H H
CH, k
~ ,CH, CH,
This extended trans conformation would place the adjacent eclipsed methyl groups on CI and CQa t a center-tocenter distance of 2.54 A. The sum of van der Waals radii of hydrogens on C2 and C4 (2.4 A) is readily accommodat0
W
120 ,180 240 XX) Angle of Rddion IDihedrol Angle),Degrees
Energy of trans and skew forms of polyethylene.
744
1 Journal of Chemical Education
360
1. Fnr nresentntions of this .-earlier .--r ~ -~ ~ -~ ~ bv~the- author ~ ~of asoeets ~ ~ subject see (a) J. CHEM. E D ~ C .36, , 160 (1959); (b) J. CHEM. EDUC., 42, 13 (1965); ( e ) "The Geometry of Molecules," McGraw-Hill Ca., New York, 1971. ~~~~
ed in the model. Incidentally, the sum of van der Waals radii of two fluorine atoms, 2.7 A, involves only a small overlap or interpenetration. However, the sum of van der Waals radii of two methyl groups, 4.0 A, is very much larger than the center-to-center distance of 2.54 A in the eclipsed conformation. Therefore the extended form would involve so much steric compression of interpenetration of van der Waals radii that i t would not he a preferred arrangement, as it is for polyethylene. The polyisohutvlene molecule thus nrefers a randomlv coiled conformation, which cannot fit into a regular crystal pattern. When ~ h h e r ypolyisohutylene is stretched, the molecules become extended in the direction of the stretch. They cannot become fully extended as for polyethylene, hut-can assume a nearly kxtended conformation with the methyl groups of CI and CJ staggered to minimize van der Waals repulsion. For example, one of the methyl groups on Ca will bisect the two methyl groups on C1. The methyl groups in the next unit will also he staggered in the same way. This molecule extended in this way will he essentially linear and can then fit into a crystal array, as shown by the beautifully sharp X-ray diffraction pattern which appears in stretched polyisohutylene. The repeat distance along the axis of the chain is not 2.54 A, as for polyethylene, hut is 18.5 .A. This indicates that eight staggered isohutylene units are required before one repeats the pattern. Thus, each unit is twisted with respect to its neighbors and the methyl groups describe a helix around the axis of the extended polymer molecule. As soon as tension is released on the polyisohutylene, it will return to its original unstressed dimension and its disorderly random coil geometry. In other words, the crystalline form is stable at room temperature only when stressed. This phenomenon can he understood by recalling When the polythe basic relationship T, = AH,/AS,. mer molecules are unstressed, in the highly disordered random-coil conformation, they have a high~entropy.On stretching, the random coils are unwound in the direction of stretch and assume a much more orderly array. Or, in technical terms, the entropy of the stretched ruhher is less than that of unstretched. In fact, i t has been amply demonstrated that the stored free energy in stretched rubber is mainly due to this change in entropy. AGStret,h= -TAS,t,,t,h
-
In other words, by stretching butyl rubber, we have decreased its entropy. This means that the entropy change (AS,) for stretched rubber to give crystalline ruhher is much smaller than for unstretched rubber. Since in the relationship T, = AH,/AS,, the entropy term appears in the denominator, we do in fact thus raise T, for a rubher by stretching it. Both natural ruhher and hutyl ruhher show this phenomenon; i.e., their unstressed T, is below room temperature, while for sufficiently stressed samples, the T, is aboue room temperature, so that these rubbers will crystallize on stretching, hut will "melt" on release of stress. When the stretched, crystslline ruhher is cooled sufficiently (i.e., if one cools the sample below the T, defined by AH,/AS,(unsrrefehedl) it will remain crystalline, even on relief of stress. In the poly(o1efin oxide) series, poly(isohutylene oxide) does not show the sharp drop in melting point exhibited by polyisohutylene. The explanation for this behavior is that the extra oxygen atom spaces the methyl groups further apart and thus prevents the strong overlap of 1,3-eclipsed methyl groups, which is such a dominant factor indicating a randomly coiled conformation for the polyisohutylene chain. In propylene and propylene oxide polymer, a new geometrical factor is involved. If one were to imagine converting an extended polyethylene chain into an extended polypropylene chain by replacing a hydrogen on every other carbon atom by methyl, one must face the question
of a choice of which hydrogen. (For isohutylene, there is no choice, since both hydrogens on every other carbon are replaced.) One possibility is to do the replacements in a purely random fashion, which gives a rubbery polymer with an atactic configuration. Another important possihility is to replace all the alternate hydrogens on the same side, to give the isotactic configuration, a crystalline polymer, mp 170°C.
Atactic (rubbery)
Isotactic, rnp 170'
Syndiotactic Professor Giulio Natta of Pisa received the Nobel prize for the discovery that certain organometallic coordination compounds catalyzed the formation of isotactic polypropylene, now an important industrial product. The atactic rubbery form of polypropylene had been known earlier. Inspection of the geometry of the isotactic polypropylene, as rewritten ahove, reveals it would suffer from van der Waals overlap of the eclipsed 1,3-methyl groups. The molecule relieves this interference by assuming a regular helical conformation with successive methyl groups radiating from the helical axis a t 120" angle to each other. The chain bonds are not all trans, as illustrated, hut regularly alternate between trans and skew. The important feature, to make a regular helix, is that each skew bond is twisted in the same direction. The reason for this follows from an examination of the projection formulas for polypropylene.
\
skew,
trans
skew,
If we have replaced the hydrogen on the left (in the trans conformation) by methyl, then the skew2 form ahove will involve more unfavorable interactions of bulky groups than either skew1 or trans. Thus the trans or skewl forms will have nearly equal energy and the chain will rotate only in the skew1 direction. This is a condition for forming a helix; i.e., i t must twist always in the same direction. For propylene oxide, the same configurational possihilities exist. In this case also, the random, atactic polymer does not crvstallize. It is rubbery and has found widespread commercial use, especially as the basis of polyurethane foam ruhber. In this case also, the isotactic polymer is readily available by the discovery of a large number of stereoselective catalysts. In the case of ~ronvleneoxide. the steric factors are refleeted in the monomer itself. ~ h i molecule s is asymmetric of chiral: i.e.. its mirror imaee cannot he suoerimoosed on it. The dhject and mirror image differ as dd right- and left-handed gloves.
. .-
CH,
0
H
0
These two molecules are present in exactly equal amounts in ordinary propylene oxide, hut special syntheses are Volume 50, Number 11. November 7973
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745
available to make either one pure. They are alike in most chemical and physical properties but do affect plane polarized light by twisting the plane of polarization in opposite directions. They are thus optically active with opposite signs of rotation and are referred to as right-handed or r e c t u s ( ~and ) left-handed or sinister (S).2 Base-catalyzed polymerization of the commercial RSmixture gives ata~icpoly(propyleneoxide) of 3000 molecular weight, which is a highly mobile oily liquid, valued for its ability to remain fluid even at very low Arctic temperatures ( - W C , e.g.). Incidentally, the fluidity of the polymer at low temperatures is explained by the very low barrier to rotation at the C-0 bond, as compared to C-C. .When we carried out a similar polymerization with either pure R- or pure S-propylene oxide, the product had the same molecular weight, but was a crystalline solid, mp -60". These molecules must have been isotactic, since they were made from monomer molecules all with the same confieuration. Incidentallv. .. this is not nossible for polypropylene, since the propylene molecule is not asvmmetric or chiral and there is onlv one kind of ~ nvl. r o.en; molecule.
l,4-palybutadiene cis-
f CH
mp I0C, rubbery --fCH,\
trans
H ,
H/~*\CH,+ mp 141j°C, R.D. 4.85 A
Another interesting case of the effect of molecular geometry on properties is that of poly(viny1 alcohol). This polymer, with hydroxyl groups on every other carbon atom, is made commercially by hydrolysis of polyvinyl acetate.
Poly (vinyl acetate) CH,COOH + Acetic acid Poly (vinyl alcohol)
Isatactic poly(propy1ene oxide) The property of chirality is always present when there are four different atoms or groups attached to a carbon atom. This condition is obviously met in both propylene oxide and its polymer. Actually the isotactic poly(propy1ene oxide) made from either R- or S-monomer is R- or S polymer and exhibits the phenomenon of optical activity when examined by plane polarized light. This is, however, not true for isotactic polypropylene. A careful look at the nature of the chiral atoms in isotactic polypropylene reveals that the difference between two of the groups is very subtle.
The poly(viny1 acetate) made commercially is an amorphous resin, exhibiting no crystallinity and with the atactic array of configurations at the pseudo-asymmetric atoms. However, the resulting alcohol can be crystallized, and X-ray diffraction studies show the chains to be in the same fully-extended conformation as in polyethylene. The hydroxyl groups will be eclipsed in the isotactic placements along such a chain. The much smaller size of the hydroxyl group does not bar this conformation with the hydroxyl groups eclipsed, since the sum of van der Waals radii for oxygen is only 2.8 A. On the contrary, in these placements the two hydroxyl groups are undoubtedly stahilized by being hydrogen-bonded to each other.
&
H/o\HP\H/o From the point of view of atom Ca, i t has attached to it a methyl group, a hydrogen atom, the polymer chain extending through C1, and the polymer chain extending through Cs. Thus a plane perpendicular to the chain axis through Ca is in fact a plane of symmetry. The molecule as a whole is therefore symmetrical. From the viewpoint of Cs, CI and C5 differ only in configuration, i.e., they are mirror images of each other. The atom 3 is called pseudoasymmetric; its configuration is important. For example, if we changed the configuration, we would introduce syndiotactic placements, a different geometry for the polymer, and different properties for it. Butadiene is an interesting example since it can be made in two structurally isomeric forms, 1,2- and 1,4-, each of which exist in geometrically isomeric forms. 1,2-palybutadiene --+CH&H+
I
nCH CHI
When poly(viny1 alcohol) is made by hydrolysis of poly(vinyl trichloroacetate), the polymer differs from the normal atactic poly(viny1 alcohol) in one significant property. The normal atactic material is water-soluble, while that from the trichloroacetate is insoluble in water. One explanation for this is that the latter polymer is mainly syndiotactic. This places the hydroxyl groups on adjacent atoms in the extended chains much further apart. They cannot now readily bond to adjacent hydroxyls in the same chain and therefore do so to hydroxyls in adjacent polymer molecules. These hydrogen bonds between hydroxyl groups on different chains serve to bond the chains much more firmly so that their separation by solvent, necessary to lead to solution, is energetically unfavorable. Another very important example of a major difference in properties arising from intermolecular versus intramolecular hydrogen bonding, also due to a configurational change in adjacent units, is the difference between cellulose and starch.
isotactic, mp 126"C,R.D. 6.5 A syndiotactic, mp 156'C,R.D. 5.14 A 2 T h e ~ edesignations are now advocated to replace the older symbols, D- (dextra)and L- (levo). 746
/ Journal of Chemical Education
a-1.4-Glucose unit (cellulose)
fl-1,4-Glucose unit (starch) Inspection of these structure units for cellulose and starch reveals that the change in configuration on carbon-1 transforms the essentially linear array of glucose units in cellulose into a strongly bent array in starch. This unit structure neatly accounts for the linear crystalline arrangement of cellulose chains in fibers of cotton, linen, etc. In this linear arrangement, hydrogen bonding of the hydroxyl groups on carbons-2, -3, and -6 will be largely to
hydroxyl groups in adjacent polymer chains, explaining the strong forces opposing dissolution (or melting) of cellulose. In starch, the bent units permit starch chains to form a helical arrangement with about six glucose units per turn of the helix. In this arrangement hydrogen bonding can occur hetween glucose units in adjacent coils of the same molecule. This intramolecular hydrogen bonding between hydroxyl groups is then not a bar to dissolution and starch is soluble in water. An amusing facet of the helical arrangement for starch is that this helix has a hole down the axis of the helix. Normally water molecules fill the hole, but molecular models show it has just the right size to accommodate one iodine molecule per turn of the helix. The intense blue color formed by starch-iodine is thus due to the behavior of iodine in this unusual environment, the central hole of the starch helix.
Volume 5 0 , Number 1 1 , November 1973
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