Nov., 1960
STERIC ORDERAND DIELECTRIC BEHAVIOR IN POLYMETHYL METHACRYLATES 1701
STERIC ORDER AND DIELECTRIC BEHAVIOR I N POLYMETHYLMETHACRYLATES BY HERBERT A. POHL, ROBERT BACSKAI AND WILLIAM P. PURCELL Princeton University Plastics Laboratory, Princeton, N . J. Received May 11, 1060
The effects of steric order along the chain of polymethyl methacrylate upon the molar polarization, the infrared absorption spectrum, and the X-ray diffraction were compared. Steric order was seen to affect all these parameters. This is in accord with known data for diastereomers. Solution measurements of the dielectric properties showed the isotactic material to have the highest molar polarization, the shortest mean relaxation time, and the narrowest spread of relaxation times. The bulk isotactic material showed the fewest infrared absorption peaks, and as judged by X-ray analysis, the greatest ease of crystallization. The syndiotactic material in dilute solution showed the lowest molar polarization, the longest mean relaxation time, and the greate8t spread of relaxation times. The bulk syndiotactic material showed the most infrared absorption peaks and an intermediate ease of crystallizability. The atactic material exhibited dielectric and infrared absorption behaviors intermediate to the other polymer species, and exhibited the lowest ease of crystalliza.bility. The study of the structure of the various stereospecific form of polymethyl methacrylate indicates that there are 18 most probable conformations possible. By taking account of the effect of steric hindrance and the resultant moment of each conformation, the root mean square dipole moment per monomer was calculated for the several species. Reasonable agreement of calculated and observed moments was found.
Introduction
be detectable by polarization measurements of high polymers which are polar. l2 Polymethyl methaSince the early demonstrations of marked effects on the crystallizability of polymers by control of crylate polymers were chosen because they are the catalytic conditions during polymerization, polar and are synthesizable in isotactic, atactic and numerous attempts have been made to additionally syndiotactic forms. characterize stereospecificity in high polymers. Experimental Natta and co-workers’ have investigated polymers Results on dilute solution dielectric measurements of the of varied tacticity with the aid of X-ray crystallog- several polymer species were obtained over a wide frequency raphy and melting point studies on the solid ma- ran e. These results were correlated with measurements ilms using infrared spectra and X-ray diffraction terial. They showed that solution viscosities were of onDielectric Polarization Measurements at 1000 Cycles little help. Danusso and Moraglio2 found that Per Second.-The dielectric capacitance cell used for the solution viscosity-molecular weight relations were lo00 c.p.5. measurements waa described earlier.l*Ja It relatively insensitive to tacticity, but that notice- has a volume of 400 ml., and a capacitance of 380 mmf. It is made mainly of monel and quartz. Capacitance able differences existed in the second virial coeffi- measurements were made with i t using a General Radio Their results were corroborated by Ang.a cients. Type 716-BS3 Capacitance Bridge. After appropriate Kinsinger and Wessling4 recently indicated that amplification, an oscilloscope was used to determine balance differences in the “Flory (0) temperature” and the on the bridge. The measuring cell waa maintained at 30 f 0.05’ during the runs. A Robertson pycnometer’s entropy parameter existed for stereoisomeric poly- was used to assist in the determination of the solution styrenes. Hydrolysis rate studies have also been densities. Calculations of the molar polarization in these made in an attempt to find distinguishing character- dilute solution measurements were made using a modified istics of stereospecificity.6s6 Krigbaum, Carpenter form of the Debye molar polarization equation.18 The value of 24.55 ml./mole for the molar refractivity, R , and Newman’ studied isotactic and atactic poly- per monomer unit of the PMMA was calculated from atomic styrene, and from observed differences in intrinsic refractivities The molar orientation polarization per viscosity and virial coefficient data suggested that monomer unit, po, was calculated (neglecting “atomic rethe isotactic molecule occupied a larger volume in fractivity”) for these relative measurement purposes as dilute solution. Po Ps - R In the present work, use has been made of dielec- where Pz is the molar polarization per monomer unit obtric behavior in dilute solution. The method has served for the PMMA in the solution measurements. The been used in the pasts-I0 on small organic mole- reproducibility of the Pg values was f 0.5 ml./mole. The apparent dipole moment per monomer unit, hpp, was calcules, particularly on diastereomers, molecules culated from the POvalues as which contain pairs of asymmetric carbon atoms. 0.01281 X 10-18 (Po X T)’/z p8pp Debye and Bueche” have emphasized the significance of polymer configuration on the dipole where T is the absolute temperature, degrees Kelvin. Dielectric Measurements at High Fre uencies.-The moment. Stereostructural differences should thus (1) G. Natta, Aneew. Chem., 68, 393 (1956); J . Polymer Sci., 16, 143 (19%). (2) F. Danusso and hforaglio. ibid., 24, 161 (1957). (3) F. Ang, ab&., 25, 126 (1957). (4) J . B. Kinsinger and R. A. Wessling, J . Am. C h m . Soc., 81,2908 (1959). ( 5 ) H. Moravctz and E. Gaetjens, J . Polymer Scd., 38,526 (1955). (6) B. Boteler Y T.G. Fox, Mellon Institute Tech. Report No. 1, Office of Naval Research Contract No. 2693(00) 1959. (7) W. R. Krigbaum, D. K. Carpenier and 5. Newman, THISJOURNAL, ea, 1586 (1958). (8) A. Weissberger, J . Oro. Chem., 8 , 245 (1937-38). (9) S. Winstein and R. E. Wood, J . Am. Chem. Soe., 62, 548 (1940). (10)V. Ramakrishmsn, Kolloid Z., ISB, 30 (1953). (11) P. Debye and F. Bueche, J . Chem. Phys., 19,680 (1951).
real (e’) and imaginary ( 6 “ ) parts of the dieyectric constant were measured a t 10, 25 and 50 cm. wave lengths. Dilute benzene solutions (0.4-1.0 wt. %) were measured at 30’. A resonant cavity ap aratus which haa a cell coupled to a signal generator detector by magnetic loo 8 was employed. A moveable plunger changed the egective length of the cavity and resonant lengths, corresponding
ansa
(12) R. Bacshai and H. A. Pohl, “Stereospecificity and Electric Polarization in High Polymers,” Plastics Laboratory Technical Report 558. October 1, 1959; J . Polymer Sci., ra, 151 (1960). (13) H. A. Pohl. M. E. Hobbs and P. M. Gross. Ann. N . Y. Acad. Sei., 40, 389 (1940). (141 H. A. Pohl, M. E. Hobbs and P. M. Gross, J . Chem. Phys., 9, 408 (1941). (15) Q. R. Robertaon, Ind. Eng. Chem., Anal. Ed., 11, 464 (1939).
H. A. POHL,R. BACSKAI AND W. P. PURCELL
1702
to values of one-half wave lengths, were observed at power peaks. The real part of the dielectric constant was calculated from the wave length in the dielectric while the loss was obtained by measuring the widths of the resonant peaks a t one-half the maximum power. The apparatus and measuring techniques have been thoroughly described elsewhere Calculations.-The complex dielectric constant is defined 2s” =
- ie”
€1
where e * is the complex dielectric constant E‘ is the real part of the dielectric constant E” is the loss factor i is the 4 1 : Debye €*
=
+
Em
EO
- Em
where e m is the optical dielectric constant eo is the static dielectric constant o is the angular fre uency r is the dielectric reqaxation time Cole and Cole developed the useful empirical relationship18
where 7 is the most probable relaxation time a! is the distribution parameter with values between 0 and 1 To calculate ro, one plots E” vs. E’. Generally a semicircle intersecting the abscissa a t the values of e, and EO is obtained. The radius of the circle drawn through the center from the em point makes an angle of a r l 2 w i t h the abscissa. The relaxation time can then be calculated from the relation
where u is the distance on the Cole and Cole plot between and the experimental point, and u is the distance between the point and e m . I n the present paper in which dilute solutions were employed, slopes (a’ = &’/&, u’‘ = &”/dc where c = concentration) were used in place of absolute values as in studying pure substances, and the differential method, as indicated below, was employed. €12’ €l’(XI/X12)2 €12” 2 2~12‘K12 where is the wave length in the cavity K is the absorption index subscript 1 corresponds t o the pure solvent subscript 12 corresponds to the solution A full description of the method is given in detail in the paper by Pitt and Smyth.16 Higasi, Bergmann and Smvth have recently developed a method1* for determining the upper and lower limits of the relaxation time distribution from values of T O and a.
Vol. 64
Results The polarization measurements made a t low f requency are summarized in Table I. TABLE I THE AVERAGEMOLARPOLARIZATIOXS A N D MOMENTS PER MONOMER UNITOF PMM STEREOISOMERS IN DILUTESOLUTIONS IN BENZENE AT 30.0°*2 Polymer
Initiator
Pr, ec./ mole
cc./ mole
X 10‘8
2007, isotactic 2007, isotactic 2008, atactic 2008, atactic 2011, syndiotactic 2011, syndiotactic 2006, atactic 2006, atactic
CsHsMgBr CeH5MgBr B~202 Bz202 W benzoin W benzoin Rz202 Bz202
65.33 65.48 57.69 57.69 56.51 56.95 60.92 60.92
40.8 41.0 33.2 33.2 32.0 32.4 36.4 36.4
1.425 1.428 1.285 1.285 1.261 1.269 1.346 1.346
+ +
Po.
uspp
Isotactic material was observed t’ohave the highest molar polarization and average dipole moment. The syndiotactic material was observed to have the lowest molar polarization and moment, while the atactic material (a copolymer of isotactic and syndiotactic chain segments) exhibited intermediate values. The polarization data obtained a t higher frequencies are shown in Fig. 1as graphs of the specific dielectric loss us. the specific dielectric constant, in the manner of Cole-Cole plots. The geometric construct’ionsfor estimating the relaxation times is indicated. Table I1 gives the observed data, Table I11 contains the results of the theoretical analysis of that data in terms of the critical wave length Am, the most probable relaxation time, TO, the relaxation constant, a, and the lower and upper relaxation times of the systems as calculated from Higasi’s theoryl9 as discussed earlier.
EO
=
71
=
rOe-*‘2
r2 =
where A is related to a: in the following way tan
[I(1 - -13 a:)
a = ..i tan-’sinh
(.4/2)
(16) D. -4. Pitt and C. P. Smyth, J. A m . Chem. Soc., 69,582 (1959). (17) C. P. Smyth, “Dielectric Behavior and Structure,” McGrawHill Book Co., New York, N. P., 1955, p. 54. (18) K. 8. Cole and R. H. Cole, J. Chem. Phgo., 9, 341 (1941). (19) K. Higasi, K. Bergmann and C. P. Smyth, T H IJOURSAL, ~ 64, 880 (1960).
TABLE I1 RESULTS OF MICROWAVEMEASUREMENTS OF PMMA POLYMERS Polymer
10 cm. a’ a,/
a’
25 em.
a,,
a‘
50 cm. a“
Static ao
2007, isotactic 0.48 0.33 0.75 0 . 6 2 1.12 0.96 2.88 2008, atactic .64 .26 0.72 .36 2.35 2011, syndiotactic .60 .17 .69 .26 .88 .34 2.28
TABLE I11 DERIVED RELAXATIONTIMESFOR PMMA POLYMERS Am,
ern.
2007, isotactic 2008, atactic 2011, syndiotactic
x
70
x
10’0,
see.
01
71
1010,
see.
88.7 270
4.7 14.3
0.09 .21
1.8 2.7
323
17.1
.47
0.6
x
72
10’0, see.
12.7 76.0 512
The infrared spectra of the polymers used in the dielect’ric measurements showed considerable differences. They are mentioned here mainly for identification purposes and to demonstrate the change of some physical properties other than dielectric properties with changing stereoregularity. A Perkin-Elmer Model 21 double beam recording
Nov., 1960
STERICORDERAND DIELECTRIC BEHAVIOR IN POLYMETHYL METHACRYLATES 1703
infrared spectrophotometer with NaCl optics was used to obtain infrared spectra. Samples were prepared by casting benzene solutions of the polymer onto a XaC1 disc, whereupon the solvent was completely evaporated in vacuo. Our results generally confirm t'he data of Miller, et U Z . , ~ ~in the case of the at,actic and isotactic polymers. I n addition, we were able to detect several other small relative intensity differences between absorption peaks of the syndiotactic and atactic polymer which had not yet been described.12 Examinat.ion of the ease of crystallizability, as judged by X-ray spectra of films which had been treated with a semi-solvent (methanol) to prom0t.e crystallization, showed the isotactic material to be the most readily crystallizable, the syndiotactic bo he less readily crystallizable, and the atactic material to be very little crystallized under the experimental conditions used. X-Ray spect,rograms were made using thin films of the samples which had been treated with boiling methanol (a semi-solvent) for 20 min. to promote crystallization, dried as a thin film in air a t 50' for 15 hours, and annealed for 24 hours a t 50' between ferrotype plates under mild pressure. A recording Norelco Geiger Counter X-ray Spectrometer, Type S o . 12021, having an Fe a-source with a t,hin Mn Fllt,er was used.
Discussion We may briefly summarize the experimental results as showing serial order in properties as one goes from isotact.ic through atactic to syndiotactic as : (1) decreasing average molar polarizat,ion per monomer unit, or, equivalently, decreasing average dipole moment per monomer unit; (2) increasing mean relaxation times ( 7o values) ; (3) increasing spread of relaxation times ( a values, and 71 vs. 7 2 values). As judged from the ease of cryst'allizability, the isot,actic material appears to be more easily crystallizable than the syndiotactic, and it in t'urn more easily crystallizable than the at,actic material. It is possible to t#alkabout interpreting these observed differences among the three polymer species in terms of hindrance. There are two types of hindrance to be distinguished here; hindrance to rot.ation about ma.in chain bonds, and hindrance to positioning of chain-attached groups. It is possible. for example, to suggest that t'he above results arise from a differing degree of freedom of rotation about the main chain bonds. Study of tahemolecular structure of the isotactic and of the syndiotactic forms of P1LII1IA,particularly on using models, indicates that the isotactic form possesses the great,er degree of free rotation about, the CH2 group joining the assymmetric carbons of successive monomer u n i k Z 1 Greater freedom of rotation in the isotact,ic PMRiA material would lead to a ( 2 0 ) R. G. J. 3Iiller, B. Mills, P. A . Small, A. Turner-Jones and D. G. M. Wood, Chemistry &Industry, 1323 (1958). (21) It is interesting to note in this connection t h a t Krighaum, Carpenter and Newman, using intrinsic viscosity and virial coefficient data, concluded that their fractions of isotactic polystyrene O C C U P Y a volume some 2 5 3 0 % larger than that occupied by an atactic chain of equivalent molecular weight when either is in dilute solution. This can be interpreted as meaning that the main chain bonds are more hindered in rotation in the ieotactio form than in the atactio form.
0.8
I-
(2011)
674 1.6
2.0
1 -2.4
I 2.8
FIG. I bEI/bf2.
Fig. 1.-Diagram of the relation of the partial molar quantities, be"/bfi and be'/8f2 for isotactic (sample 2007), atactic (sample 2008) and syndiotactic (sample 2011) polymethyl methacrylates, from determinations made in dilute.solutions of the polymers in benzene a t various frequencies.
shorter relaxation time than in the case of the syndiotactic, as observed. It would also explain that a narrower range of relaxation times occurs in the isotactic PMMA material, for the average length of field oriented segments would be shorter and the spread in size of detectable relaxing units would be narrower than in the syndiotactic material with its relatively stiffer bonds. Greater freedom of rotation in the isotactic arrangement would also be expected to permit greater ease of crystallizability, despite the similarity of perfection along the chains of either isotactic or syndiotactic forms. This is consonant with the observed X-ray data. It is not logically adequate to speak, however, as though almost completely free rotation existed in the G-C bonds of organic polymer chains. There is abundant evidence to show that considerable restricted rotation is the general rule as ably reviewed and demonstrated by Mizushima,22and by Debye and Bueche." The energy barrier height for rotation is in the order of 3 kcal./mole, much higher than room temperature average energy. It must be concluded that rotation about C-C bonds is normally highly restricted even in such simple molecules as ethane, and that it is too much of an oversimplification to say that, for example, the polarization differences observed between isotactic and syndiotactic PMMA arise because one species has more free rotation about the C-C bonds while (22) 9. Mizushima, "Structure of Molecules and Internal Rotation," Acad. Press, Inc., New York, N. Y., 1954.
1704
H. A. POHL, R. BACSKAI AND W. P. PURCELL
(a) (b) Fig. 2.-Selected steric conformations of the repeating units in isotactic and syndiotactic polymethacrylates: (a) a conformation showing two monomeric units, (d-d), isotactic arrangement; (b) a conformation showing two monomeric units, (Z-d), s diotactic arrangement. Key. A,A' = -CHr of first a n g e c o n d monomer units, respectively. B,B' = -CHI of first and second monomer units, respectively. C C' = -COOR of first and second monomer units, respectiveiy.
e
C'
A
the other has less. Accordingly the preceding explanation, strictly in terms of free rotation about C-C bonds, while helpful perhaps, is to be regarded as inadequate. An additional factor, such as that of hindrance to posit,ionirig of chain-attached groups appears to be required. Weissberger studied the dipole moments of optically active and inactive diastereomers.8 Le F e ~ r considered e ~ ~ the differing moments between the optically active and meso forms of several diastereomers to arise from a hindrance of free rotation. (23) R. J. W. LeFevre, "Dipole Moments," Methnea L Co., Ltd., London, 1918, p. 98.
He predicted the optically active forms to have high moments and the meso forms to have low moments because of the inability of the dipoles in trans conformations to compensate completely in the optically active form, whereas the trans conformation of the meso form could compensate. The argument appeared to apply to the then available data except for diethyl tartrate, but since then other diastereomeric compounds measured show contrarily sized moments of the two forms,B negating his conclusions. Mizushima22 suggested that the exception shown by diethyl tartrate to Le Fevre's hypothesis might be due to the presence of hydrogen bonding in that molecule. It occurred to us that the existence of differing specific steric repulsions arising among the various steric forms might more satisfactorily account for the observed differences in the average moments. I n view of the formidable difficulties in assigning degrees of steric repulsion factors for the various conformations the following analysis is presented as a suggestive method. Fortunately, there are many conformations in which the isotactic and syndiotactic forms may lie, so that the individual preciseness of assignment of degree of steric hindrance to each conformation can be somewhat masked in the statistics of handling many in the final calculation of the average resulting moment. The Estimation of Average Moment in Stereospecific Vinyl Polymers.-The following simple and approximate method has been used here to obtain a prediction of the relative polarizations and average dipole moments of stereospecific vinyl polymers. It consists in essence of considering the successive monomer units pair by pair as they may lie in space, with the aid of molecular models. Each pair can be shown to exist in 18 most probable conformations. Among these 18 conformations the probability of each conformation was examined with the aid of molecular models and the moment of the paired monomers calculated. The relative average polarization of the chain was calculated from a weighted average of the pair moments. For the specific case of polymethyl methacrylate the configurations of the isotactic and syndiotactic forms may be written
C'
Fig. 3.-Diagram of the 18 more probable conformations of pairs of monomer units of polymethacrylates: (a) diagrams of pairs of monomer units, in isotactic arrangements; (b) diagrams of pairs of monomer units in syndiotactic arrangements. The identity of the attached groups is that shown in Fig. 2.
VOl. 64
-CHz-
A
CHa -CH2-
L o
I
2 I
c=o LCHa
dd-configuration
O-cHs
&O -CHz-C-CHZI
AH,
CHa
L I
-0
(!)-CHI Id-configuration
In more fully spatial representation, we may represent the essential differences in the configuration as with the aid of Fig. 2. As a shorthand method of representing the 18 different interlapping conformations of the two asymmetric carbon tetrahedral assemblies, the system shown in Fig. 3 may be used. Conformations other than the gauche, in which the direct overlapping of attached groups is present, are regarded for the present as too improbable to include in the calculation. The moment of a monomer pair, wp, is
INFRARED STUDIESOF AMINE-HALOGEN INTERACTIONS
Nov., 1960 PP = 2PO
cos (V2)
(1)
where po is the moment of the monomer molecule, and 6 is the angle between the average moments of the monomer units as directed along the asymmetric carbon-carboxyl carbon bond. To evaluate the average polarization of the whole polymer, we assume as a first approximation that the average moment pave per monomer unit of the ensemble of monomer pairs in the polymer chain is the root mean square of the weighted moments of the pairs n
n f i pia
(Pave)’
1
= ___ = 42fi
fi COS’
P’
(‘~Si/2)
1
5 fi 1
where (‘n” is the number of different conformations possible to the given polymer species (e.g., 9 for the isotactic or the syndiotactic species), pi is the moment of the ith pair and the fi’s are the probabilities of the individual pair conformations of moment pi as shown in shorthand form in Fig. 3 above. The values of fi were 0 5 fi S 1. Inspection of the molecular models in the form of pentamers (e.g., Fisher-Taylor-Hirschfelder models) permitted an estimate of the relative probability of the various conformations and the dipoledipole angles of the monomer pairs. The results of this analysis are recorded in Table IV. The values of 2fl c0s2(6/2)/2fi for the two species, calculated from the above data are: 0.767 and 0.570, for the isotactic and syndiotactic, respectively. The values for the polymer average moments, pave = po ( 2 f i c0s2/(6/2))’/2/Zfi, using the value uo = 1.69 Debyes, as for a similar simple ester, methyl pr0pi0nate.l’~are 1.48 and 1.27 Debyes, respectively. The observed moments for the isotactic and syndiotactic species, expressed as the
1705
TABLE IV IN THE CONFORMATIONS OF ISOTACTIC AND STERICFACTORS SYNDIOTACTIC POLYMETHYL METHACRYLATE Conformation symbol
Isotactic species, d-d Dipole-dipole angle, fi degrees
I I1 I11 IV V VI VI1 VI11 IX
Syndiotactic species, 14 dipole-dipole angle, fi degrees
50 45
140
160 120 120
60 100 115 120 55
50
105 160 60 20
55 175
apparent moment per monomer unit were 1.43 and 1.265 Debyes, respectively. This is in the order and magnitude expected for such molecules when calculation of the average moment is made as above using consideration of the steric repulsions and basic moment for the ester units. The closeness of the calculated to the observed values of the average unit moments is regarded as rather fortuitous, but the calculation is expected to show the relative sizes of the two species’ moments to a fair approximat ion . (estimated precision, 3 ~ 5 %in the ratio
Pave pave
(d (1 - 4 ”’>
-
The method of calculation given has had a measure of success in predicting the relative moments for the isotactic and syndiotactic forms of several other related polymers. It is planned to describe this more fully in a forthcoming discussion of those polymers. Acknowledgment.-The authors wish to acknowledge with appreciation the helpful discussions with Dr. Keniti Kigasi.
INFRARED STUDIES OF AMINE-HALOGEN IXTERACTIONS’ BY RALPHA. ZINGAROAND W. B. WITMER Department of Chemistry of the Agricultural and Mechanical College of Texas, College Station, Texas Received Mav 1 1 , io60
The marked changes which are brought about in the 1000 cm.-l region of the infrared spectrum of pyridine upon the addition of iodine have been found to be generally characteristic for amine-halogen solutions. Infrared studies on a series of solutions made up from five different halogens and inter-halogens in pyridine and various pyridine derivatives, all reveal corresponding infrared shifts. Several new solid amine-halogen complexes have been isolated, and, in every case, the frequency shifts observed in the solutions can be correlated with infrared bands characteristic of the solids. Comparison of the spectra with those of substituted benzenes gives a reasonable interpretation of the data.
Introduction I t has been recently demonstrated2 that the marked changes which are obeerved in the infrared spectrum of pyridine upon the addition of iodine3 can he correlated with the spectra of solid compleyeq of the tyne (Py1)X and ~ P ~ ~ J These ~X. solids possess infrared bands which hare the same (1) Presented at the Southwest Regional Meeting of the American Chemical Society, Baton Rouge, Louisiana, December 4, 1959. (2) R. A . Zingaro and W. E. Tolberg. J . Am. Chem. SOC..81, 1353 (19.59). (3) D. L. Glusker and H. W. Thompson, J. Chem. Soc., 471 (1955).
location, and which are of the same intensity as the new bands which are found in the infrared spectrum of pyridine following the addition of iodine. The present investigation represents an extension of these studies and includes a variety of solutions made up of different halogens and interhalogens in a number of pyridine derivatives. The amines were chosen so that both steric and electronic effects could be observed. The fundamental purpose of this study was to determine whether the rather profound infrared shifts which are observed in iodinepyridine solution could be observed as a phenom-
