CONFORMATIONS OF ISOTACTIC POLYALKYL ACRYLATES
1059
Conformations of Isotactic Polyalkyl Acrylates in Solution Determined by Nuclear Magnetic Resonance
by Tsuneo Yoshino, Yoshikazu Kikuchi, and Jiro Komiyama Basic Research Laboratories, Toyo Rayon Company, Kamakura, Kanagawa, Japan
(Received Auguat BPI, 1966)
Nuclear magnetic resonance spectra of isotactic polymethyl acrylate-@-dl and isotactic polyisopropyl acrylate-@-dlshowed two triplets corresponding to two kinds of p protons. From the peak separations in the respective triplets, the conformations of the polymers dissolved in various solvents were determined by using the trans and gauche coupling constants obtained from trimethyl cis-hexahydrotrimesate. Probabilities of the polymers having trans, gauche, and the other gauche forms were found to be about 0.5 and in the ranges of 0.3-0.2 and 0.2-0.3, respectively, depending on the solvent used.
Introduction Much information about dimensions and shapes of polymers in solution has been obtained by means of light scattering, viscosity measurement, etc. It may be preferable to interpret these data in terms of microscopic structures of polymers. X-Ray diff ractionl and infrared spectroscopy2 have been used to determine the conformation in solids but have not been used successfully for similar studies on polymers in solution. Since Bovey, et u Z . , ~ applied nmr spectroscopy for the determination of the configurations (stereoregularity) of polymers in 1959, much work has been done in the field. However, only a few nmr studies are reported4 on the conformations (isomeric forms caused by the rotation about single bonds) of polymers. The aim of the present work is to investigate the conformation of isotactic polymethyl and polyisopropyl acrylates in solution by nmr spectroscopy, which is expected to be a powerful tool to determine the conformation since the spin-coupling constant between CY and p protons depends upon the dihedral angle of the CH bonds. I n the isotactic acrylate polymer, there are two sets of equivalent p protons, and they give a complicated spectrum due to the coupling between geminal @ protons as well as between a and p protoms Elimination of the coupling between geminal 0 protons is desired to obtain the unequivocal and accurate value of the
coupling constant between CY and 0 protons. Deuteration of one of the geminal @ protons is a method to meet this purpose6 since the spin-coupling constant becomes 0.15 when one of the protons is substituted with a deuteron. The observable vicinal coupling constants of the deuterated polymer are averaged with weight over the t, 9, and g' forms shown in Figure 1 since the rotation about the single bond is rapid as far as nuclear magnetic resonance is concerned. Probabilities of the polymers having these three forms can be estimated from the observed vicinal coupling constant if the coupling constants in trans and gauche arrangements are known, The trans and gauche coupling constants are obtainable from the spectrum of trimethyl cishexahydrotrimesate as model compound, in which the rotation about the single bonds of the ring is a negligible process.
(1) G . Natta and P. Corradini, J . Polymer Sci., 39, 29 (1959) (2) G . Natta and G. Zerbi, Ed., ibid., C7, 3, 224 (1964). (3) F. A. Bovey, G. V. D. Tiers, and G. Filipovich, ibid., 38, 73 (1959). (4) F. A. Bovey, et al., J. Chem. Phya., 42, 3900 (1965); T. M. Conner and K. A. McLauchlan, J . P h y s . Chem., 69, 1888 (1965). (5) C. Schuerch, et al., J . Am. Chem. SOC.,8 6 , 4481 (1964). (6) Schuerch, et aL,6 interpreted the complicated spectrum of nondeuterated isotactic polyisopropyl acrylate regarding that the two vicinal coupling constants are equal to each other. Analysis of the simplified spectra of deuterated polymers, however, gave unequal values for these vicinal coupling constants as seen in a later part of this paper.
Volume 70, Number 4
April 1966
1060
T. YOSHINO, Y. KIKUCHI, AND J. KOMIYAMA
Experimental Section
C
Preparation of Monomer and Model Compounds. (1) Methyl Acrylate-@-dl. A mixture of acetylene and deuterium chloride, prepared from thionyl chloride and heavy water, mas passed through a column packed with active charcoal treated with an alcoholic solution of mercuric chloride. The reaction temperature was 100-180". The resulting gas mixture was trapped after it was passed through sodium hydroxide solution and water. Unreacted acetylene was removed from the mixture by distillation. The portion distilled out in a temperature range of - 17 to - 10" was collected, passed through sodium hydroxide solution and water, and then dried with calcium chloride. Vinyl chloridetrans-@-dlthus obtained was introduced under stirring to a flask containing tetrahydrofuran, newly polished magnesium, a small quantity of ethyl bromide, and a trace of iodine. After completion of the reaction, the Grignard solution thus obtained was poured onto a mixture of Dry Ice and ether and was then hydrolyzed to give acrylic acid-P-dl. By refluxing with methanol, sulfuric acid, and a trace of hydroquinone, the acid was converted to methyl acrylate-@-dl,which was proved not to contain any other isomers and impurities by nnir inspection. The molar ratio y of the trans-p-dl to cis-@-dlisomer was found to be by nmr measurement. (2) Isopropyl Acrylate-P-dl. Acrylic acid-p-dl was refluxed with isopropyl alcohol, sulfuric acid, hydroquinone, and benzene, which mas added to remove the water formed through the reaction by making use of the fact that benzene makes an azeotropic mixture with isopropyl alcohol and water. The nmr spectrum of the isopropyl acrylate-p-dl showed that it did not contain any other isomers and impurities except benzene and that y = 3//z. (3) Dimethyl cis- and trans-Hexahydroisophthalates. Isophthalic acid in glacial acetic acid was hydrogenated over freshly prepared rhodium oxide catalyst a t 4050" (130-90 atm). cis- and trans-hexahydroisophthalic acids thus obtained were separated through their calcium salts.' The acids were esterified with diazomethane. (4) Trimethyl cis-Hexahydrotrimesate. Trimesic acid in ethanol was hydrogenated in the same way as mentioned above. The resulting hexahydrotrimesic acid was recrystallized from water. The nmr spectrum proved it to be pure cis-hexahydrotrimesic acid. The acid was esterified with diazomethane. Polymerization. (1) Isotactic Polymethyl Acrylate@-cZl. Lithium aluminum hydride (0.14 g) was introduced to an ampoule, which was then immersed in a Dry Ice-methanol bath. After the air in the ampoule The Journal of Physical Chemistry
c 1-Form
C
H
g-Form
C
H
g-Form
( X = C O O C H 3 , COOCH(CH3)z)
Figure 1. Isomeric forms due to rotation about a single bond.
was replaced by nitrogen, toluene (25 ml) was added to the ampoule. It was kept standing for 10 min, and then a mixture of methyl acrylate-@-dl (8 ml) and toluene (23 ml) was added with a syringe through a rubber tube connected with the ampoule. After it was sealed, the ampoule was kept in the Dry Icemethanol bath for 20 hr. The polymerization mixture was poured onto 100 ml of methanol containing 10% of hydrochloric acid, and polymer was extracted with chloroform. After the chloroform was removed, the polymer was again dissolved in benzene and freeze dried. (2) Isotactic Polyisopropyl Acrylate-P-d1. A toluene solution (8 ml) of phenylmagnesium bromide (8 mole % of monomer) was placed under nitrogen in an ampoule cooled with Dry Ice-methanol. A mixture of monomer (1 ml) and toluene (5 ml) was added by a syringe to the ampoule, which was then kept in a Dry Icemethanol bath for 24 hr. Polymer precipitated by petroleum ether (bp 30-70") was dried after washing with dilute hydrochloric acid and water. Nmr Spectral Measweiizent. A Varian HR-100 spectrometerwas employed torecord the spectraof monomer, polymer, and model compounds. hleasurement was done at room temperature, and the concentration of all solutions measured was about 0.1 g/ml.
Nmr Spectra and Interpretation Isotactic Polymethyl and Polyisopropyl Acrylate-@-dl. In Figure 2 is shown a typical spectrum of the CY and @ protons of isotactic polymethyl acrylate-@-dl.* The spectrum of isotactic polyisopropyl acrylate-@-dl is not shown here since it resembles very closely the spectrum given in Figure 2. The lowest field signal corresponds to the a protons, and the remaining two triplets correspond to the @-protons. The lower field triplet corresponds to the P protons (HA)oriented (7) G. A. Haggis and L. N. Owen, J . Chem. Soc., 399 (1953). (8) Here "isotactic" is referred t o the placement of the COOIE groups. The polymers prepared from the nionomers with y = 3/2 are considered t o be almost random with respect to the C H D groups: T. Yoshino and K . Kuno, J . Am. Chem. Soc., 87, 4404 (1965). However, it may be realized that the placement of the C H D groups is not concerned with the conclusions in Lhis paper.
COXFORMATIONS OF ISOTACTIC POLYALKYL ACRYLATES
1061
three axial methoxycarbonyl groups can be neglected because of larger steric hindrance. Since geminal, trans, and gauche coupling constants are known to be about 12, 12, and 3 cps, respectively,’O the equatorial and the axial methylene protons (HA and HB) and the methine protons (Hc) are expected Hc HA HE to give two triplets (each with 3 cps peak separations) separated by 12 cps, a quartet with peak separations of about 12 cps, and three triplets (each with 3 cps peak separations) separated by 12 cps, respectively. It is expected that the axial methylene protons appear a t higher field by several tens of cycles than the equatorial protons” and that the methine protons, which 75 80 85 ?-value are located near the negative methoxycarbonyl groups, Figure 2. Spectrum of the CY a n d the p protons of isotactic appear at lower field than the axial methylene protons. polymethyl acrylate-&dl in methyl formate. On the basis of this consideration, the observed signals of the trimethyl cis-hexahydrotrimesate were trans to the (Y protons (Hc) for the hypothetical trans assigned as shown in Figure 4. The present assignzigzag skeletal conf~rmation.~The higher field one ments are also supported by the decoupled spectra is the signal of the other p protons (HB). The 0 proshown in Figure 5. tons couple mainly with the vicinal a protons, and more Several solvents were employed for the spectral distant couplings may be neglected. The steric measurements of this compound. The coupling conrelation of any of the p protons to one of the vicinal JAC, and JBC mere obtained from these stants JAB, (Y protons is just the same as the relation to the other spectra, regarding the signals of HA, HB, and HC as vicinal cr proton when averaged over all conformations. the A and B parts of the ABC2 systemlZ and the C We may, therefore, interpret each of the P-proton part of the A2BnC system, respectively. The values multiplets as the A part of the ACZ system or the B obtained are shown in Table 11. It may be noted part of the BCz system, and then the separation of that the solvents have influence on the chemical shifts the outer peaks of the triplet is shown to be equal to but little on the spin-coupling constants. or (JBc)) Dimethyl trans-Hexahydyoisophthalate. The two protwice the vicinal coupling constant ((JAc) in the second-order approximation. trons HI and Hz of the methylene group located beI n Table I are listed the chemical shifts ( u ) of the tween (Y carbons are expected to give a single mulP protons and the spin-coupling constants of the tiplet because chemical shifts of H1 and H? are averpolymers dissolved in various solvents. In all of the aged by rapid inversion of the ring, and the inverted measurements it may be noted that the peak separaform is the same as the original one. The triplet at tions in the HA triplet are larger than those of the H B 7 8.01 in the observed spectrum (Figure 6) was ascribed triplet. to HI and Hz because the position is nearly the same as the mean of the chemical shifts of the axial and the equatorial methylene protons of trimethyl cis-hexaTable I : Chemical Shifts (in 7 Values) and Spin-Coupling hydrotrimesate. The spectrum of H1 and Hz was Constants (in cps) of B Protons of Isotactic analyzed as the A part of the AzBz system. The Polyalkyl Acrylates-P-dl separation of the outer peaks of the triplet is equal to ( J I t 3. 3J’,)/2, where J‘, is the coupling constant Polymer Solvent u.4 CB (JAG) (JBC) between the A and B protons in the axial-axial arrange7.2 5 . 9 Polymethyl acrylateB-dl
Chloroform Methyl formate Benzene
8.07 8.10 7.86
8.45 8.39 8.33
8.1 7.4
5.6 5.5
Polyisopropyl acrylate-
Chloroform Methyl formate Benzene
8.15 8.11 7.77
8.50 8.43 8.22
7.2 7.5 7.7
5.9
P-di
5.3 6.0
Trimethyl cis-Hexahydrotrimesate. Of the two conceivable conformations shown in Figure 3, the one with
(9) T. Yoshino, Yi. Shinomiya, and J. Komiyama, J . Am. Chem. Soc., 87, 387 (1965). (10) H . S. Gutowsky and C. Juan, J . Chem. Phvs., 37, 120 (1962). (11) L. XI. Jackman, “Application of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry,” Pergamon Press Ltd., London, 1959, Chapter 7 . (12) J. A. Pople, W. G. Schneider, and H. J . Bernstein, “Highresolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y., 1959, Chapter 6.
Volume YO, ,%‘umber I+ April 1966
T. YOSHINO, Y. KIKUCHI, AND J. KOMIYAMA
1062
HB
I
HB
HA
\-j X\
&
1
+
H
H H
HC
HC (a)
Figure 3. Conformations of trimethyl cis-hexahydrotrimesate.
HA, d e c o u p l e d f r o m HE
Ha HA
HC
zJAc
(b)
HE, decoup1,ed f r o m ha
(c)
Hc, d e c o u p l e d f r o m HE
2JAC
I
I 8.0
75
I
8.5
7-value
Figure 5. Decoupled spectra of trimethyl cCs-hexahydrotrimesate.
Figure 4. Spectrum of the methylene and the methine protons of trimethyl cis-hexahydrotrimesate.
ment, and J ' , is that in the axial-equatorial or equatorial-equatorial arrangement.
HI
I
H
Conformations of Isotactic Acrylate Polymers in Solution The observed coupling - constants (JAc) and (JBc) may be expressed in terms'of the probabilities (P,, P,, and Put)that the chain has the t, g , and g' forms, and the coupling constants J , and J , between a! and P protons in trans and gauche arrangements, respectively
+ PgJ, + P,J, PtJ, + PgJt + P,lJ,
(JAc) = PtJt
(1)
(JBc)=
(2)
P,
+ P , + POI = 1
p, P,! = [Jt
Jo)/(Jt
- Ju)
(4)
= ( ( J B c )- J o > / ( J t
- Jv)
(5)
+ J , - ((JAc)+ ( J B c ) ) I / ( J ~
- Jh
(6)
The values of J , and J , may be transferred from trimethyl cis-hexahydrotrimesate: J, = JBCand J , = JAC. Substituting the values of (JAc) and (JBc) (Table I) into eq 4-6, one obtains the probabilities P , , P,, and P g l(Table 111). The t form has the largest probability of around 50%, the g form about 30%, and the g' form about 20% except for polymethyl acrylate-@-dl in benzene and The Journal of Physical Chemistry
I
I
I
8.0 15 8.5 Z - v a l u e Figure 6. Spectrum of the methylene and the methine protons of dimethyl trans-hexahydroisophthalate.
(3)
Transforming these equations, one obtains the expressions for the probabilities
Pt = ((JAc) -
1
~
Table I1 : Chemical Shifts (in I Values) and Spin-Coupling Constants (in cps; Signs Are Neglected) of Trimethyl cis-Hexahydrotrimesate Solvent
UA
UB
ac
Chloroform Methyl formate Methanol
7.69 7.79 7.74
8.45 8.54 8.55
7.51 7.46 7.46
JAB"
14.9 14.6 13.3
JAC
JBC
3.1 3.3 3.3
12.1 12.3 12.0
' The value of JABis less accurate than those of JACand JBC because the former was obtained from the large separation JAB 2 J B C between the outermost peaks of the HB signal.
+
polyisopropyl acrylate-P-dl in chloroform. It should be emphasized that the g' form for which little attention has been paid has rather larger probability.
CONFORMATIONS OF ISOTACTIC POLYALKYL ACRYLATES
Table I11 : Conformations of Acrylate Polymers Dissolved in Various Solvents Polymer
Polymethyl acrylateB-di Polyisopropyl acrylate-p-dl
iolvent
Chloroform Methyl formate Benzene Chloroform Methyl formate Benzene
Pt
0.48 0.53 0.47 0.46 0.47 0.49
1063
Table IV: Outer Peak Separation (in cps) of the Triplet of Dimethyl Glutarate PC
0.33 0.29 0.25 0.24 0.29 0.30
Discussion In the preceding section it was mentioned that the probability of the polymers having the g’ form is rather large. This was also confirmed by the following experimental results. The nmr spectra of dimethyl glutarate and its solutions in benzene, methyl formate, carbon tetrachloride, and chloroform were observed. The separations of the outer peaks of the triplet corresponding to a protons were measured and listed in Table IV. The (J”, separation is given by P ’ , ( J ” , - J“,)/2 3J1‘,)/2, where P‘, is the proba#bility that the chain CH2-CH2-CH2--COOCH3has the trans zigzag form, and J”, and J”, are the coupling constants in trans and gauche arrangements, respectively. This expression has the maximum value J ” , J“, when P‘, = 1. The observed value of 14 cps in Table IV gives the lower limit of J”, J”, since P ’ , is smaller than unity in reality. The probability of finding the g’ form of the polymer is given by eq 6. The value of (JAc) ( J B C ) is, for example, 12.9 cps for isotactic polymethyl acrylate-/%&in benzene. The denominator may be about 9 cps, the error of which is at most 1 cps. By
+
+
+
+
+
Proton
Carbon
Pa’
0.19 0.18 0.28 0.30 0.24 0.21
LY
Separation
14.0
Chloroform
Methyl formate
Benzene
tetrachloride
13.6
13.9
12.3
13.6
+
+
J”,, P,, is concluded assuming J , J , = J“, to be larger than 0.1. Further support for the presence of the g’ form is this. If one neglects the g’ form, (JAc) (JBc) is equal to J , J,, which is considered to be independent of solvent species as stated before, in contrast to the observed change of (JAc) ( J w ) with solvents. Calculation of the probabilities was made by using the values of spin-coupling constants obtained from the model compound. Transferability of coupling constants will be discussed below. The observed values of the spin-coupling constants of cis-hexahydrotrimesic acid in methanol, JAC = 3.2 cps and JBC = 12.4 cps, are very close to those of trimethyl ester in methanol (Table 11). This indicates that the slight change of the structure has little influence on the trans and gauche coupling constants. The similar situation was found for the trans-hexahydroisophthalic acid. The separation of the triplet at r 8.03 of the acid in methanol was 5.8 cps. This should be compared with 5.9 cps of the ester in methanol. Besides these, the values of trans and gauche coupling constants in the trimesate are very close to those1° of [2.2]metacyclophane obtained by Gutowsky and Juan ( J , = 12.3 cps, J , = 3.2 cps, and J o t = 4.0 CPS)
+
+
+
Volume 70,Number 4 April 1966
