1H and 13C Solution NMR of Polyimides Based on 4,4

Chapter 19

H and C Solution NMR of Polyimides Based

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 15, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch019

on 4,4-Diaminotriphenylmethane: Conformational Analysis Using NOE and Molecular Modeling 1

2

AntonioMartinez-Richa ,Ricardo Vera-Graziano , and Dmitri Likhatchev 2

1

Facultad de Química, Universidad de Guanajuato. Noria alta s/n. C.P. 36050. Guanajuato, Gto. México Instituto de Investigaciones en Materiales,UNAM,Apdo. Postal 70-360, Coyoacán, 04510,México,D.F.

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H and C-NMR spectra of a model compound and polyimides based on4,4'-diaminotriphenylmethanewere analyzed and assigned. NOE measurements in DMF for a representative polymer were recorded andinterpretedwith the aid of conformational analysis. Results indicate that conformational forces in the polymer lead to the formation of a round-shape geometry along the polymer chain that comprises five repeating units. Thisfindingis supported by the measured NOE's, which indicates proximity between nuclei that look far apart in the repeating unit. From molecular modeling and NOE's, internuclear distances were evaluated. Limitations of this approach are also discussed.

Aromatic Polyimides find widespread technological applications due to their unique combination of low dielectric constant, chemical resistance and high

242

© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H. N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


243 thermal and theimooxidative stability. (Ï-4) Polyirnides (Pis) can be Gonvemently prepared by the condensation polymerization of dianhydrides with examines. In general, polyirnides are synthesized by a multistep method through a polyamic acid or a polyamic ester mtermediate. Direct one-step high temperature polycondensation is an alternative route to the preparation of PI when they are soluble in organic solvents. We have previously reported that rx>lyimides obtained by one-step hightemperature polyeondensaaon of diarrimotriphenyln^than (DA-TPM) and aromatic dianhydrides yield flexible and tough films with good mechanical properties. (5--7) Properties of these polyirnides, named PI-TPMs, are mainly due to the homogeneity of the repeating units in the polymer backbone, which was demonstrated by UV-visible spectroscopy, CP-MAS C-NMR and WAXD analysis. (7,8) PI-TPMs are soluble in dimethyl formamide (DMF), so they are amenable to be studied by solution NMR in that solvent m this paper we report a study of the solution NMR spectra of a model compound and Ρί-TPMs obtained by the one-step route, in DMF-d . NÔË measurements afcied by molecular modeling proved to be a very valuable tool to understand the conformational details of this class of polymers in solution. It was found that a pentamer model satisfactorily explains the NOE values measured experimentally. The structure of the model compound and the polymers studied are shown in Figure 1. 13

7

Experimental.

Synthesis ofModei Compound Model compound were obtained by condensation of phthalic anhydrides with DA-TPM, as described elsewhere. (?)

Synthesis ofPolyirnides. The polyirnide films of PI-TPMs were prepared via polycondensation of DA-TPM with the aromatic dianhydrides by one step high temperature procedure in nitrobenzene. (5-7) Structures of PI-TPMs are shown in Figure l and the used nomenclature is based on the dianhydride moiety as PM-TPM, BSA-TPM, ÔDP-TPM and BZP-TPM.

r



244



Figure 1. Structures ofthe Model compound, Parent Polymer (PM-TPM) and the other PI-TPMs studied in this work.

2

X = S 0 BSA-TPM X=O ODP-TPM X=C=0 BZP-TPM


246 NMR Measurements. All the solution NMR measurements were performed at ambient probe temperature using 5 mm ad. sample tubes. Solution H and C NMR spectra in DMF-d (PI-TPM's) and CDC1 (Model Compound 1) were recorded using Varian Gemini 200 and Varian Unity Plus 300 spectrometers. Oxygen was removed from samples byfreeze-pumpthaw cycles. The chemical shifts are reported taking as a reference the chemical shift positions of the solvent with respect to TMS. For APT, COSY and C / H HETCOR experiments, standard pulses sequence were used. (9-11) For the long-range HETCOR experiment, J(C,C,H) was assumed to be 8 Hz. NOE Difference spectra (homonuclear ^{'H} decoupling experiments ) were recorded using the cyclenoe sequence. 1024 scans were recorded for each spectrum. l


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13

3

1 3

!

Theoretical calculations. Energy minimization, conformational searching and molecular mechanics were performed using Alchemy 2000 program system from Tripos loaded on a PC Pentium Computer. For molecular mechanics optimization, the MM3 Molecular Mechanics subroutine with a value of 4.7 for dimethyl formamide dielectric constant, a RMS gradient of 0.05 kCal/Â mol and a delta of energy of 0.001 kCal/Â was used.

Results and Discussion

l

n

Hand C NMR spectroscopy. 1 3

To assign *H and C spectra of model compound and Pi's, tentative chemical shifts were determined using substituent additivity rules. The APT spectrum allows to determine the number of protons attached to each carbon. C/*H chemical shift correlation experiment permits the assignment of the protonated carbons, while the non-protonated carbons connectivity is obtained from the long-range HETCOR diagram. The H NMR spectrum for BZP-TPM is presented in Figure 2. A discussion on the main features of this spectrum is given elsewhere. (12) This spectrum is the base for NOE measurements described below. 13

]


247 In Tables I and Π, the proton and carbon-13 chemical shifts for model compound and polyirnides are tabulated.

Table I. Ή NMR chemical shifts in solution (5H from TMS)*


Model I H-e H-g

7.95(^ = 6.0 Hz,.J^= 3.0Hz) 7.78

H-h n-j H-k H-n H-o H-p CH

7.4(J =8.5 Hz) 7.29 7.17 7.31 7.21 5.64 jk

Pl-TPM 8.48

ODP-TPM BZP-TPM SSA-TPM 7.68

8.31

8.69

n.d.

8.38 (Jgh= 7,5 Hz) 8.22

8.75^ = 8,5 Hz) 8.25

8.08 (Jgh= 8.0 Hz) 7.62 ( J - 7.56 ( J 8.1 Hz) 8.0 Hz) 7.43 7.46 n.d. 7.40 net. 7.30 n.d. 7.24 5.92 5.88 j k

hj

7.62 ( J - 7.53 (J*» 7.8 Hz) 8.5 Hz) 7.40 7.49 7.30 7.46 7.34 7.35 7.25 7.30 5.86 5.92 j k

* m some cases, chemical shifts for protons η, ο and ρ were resolved by 2-T> NMR. n.d. = non determined

Conformational Analysis. Conformation of model compound and polymers can be described using four or six torsion angles. Confonnational search was made by a combinatorial process on 2,985,984 conformera for six torsion angles and 20,796 for four torsion angles. 30 degrees increments were used, and the van der Waals scaling factor was 0.7. The method of conformational analysis searches the global minimum based on the value of total steric energy (which in turn is computed from the sum of compression, van der Waals and torsional energy). So, the final outcome is à family ôf erôifcHlnation& having populations according to à Bottzmann distribution. In order to detect structural differences among the computed geometries, 20 conformers with the lowest steric energies were analyzed in terms of intemuclear distances and torsion angles. The main variation observed is in the value of torsion angle τ , which indicates mat the phenyl ring conformational mobility of the triphenylmethane moiety is the less restrained This also implies that 5



248



-,

,

,

j

9.0

,

,

!

j

β.S

,

,

,

,

j

β.Ο

, r

— ,

,

n

7.5

j

ι

i

]

,

I II »°»P

!

j

7.Q

, η,

r

6.5

γ—j

,

,

,

,

6.0

ρ—,

I

,

CH

,

,

Figure 2. 300.1 MHz High-resolution HNMR spectrum (5.1-9.4 ppm region) of BZP-TPMïnàmt-άη

,

j

k


j

5

r

r

—,

250 I3

Table II. C NMR chemical shifts is solution (§c trou TMS)


Model I C-a C-b C-c C-d C-e C-f c-g C-h C-i C-j C-k C-l C-m C-n C-o C-p CH c=o

PI-TPM

167.3

167.5

131.7

133.7

123.8 134.4

123.0

130.& 126.3 130.2 143.3 142.9 129.5 128.5 126.7 56.2

131.4 128.0 130.4 145.7 144.5 130.0 129.3 124.8 57.5

ODP-TPM 167.1 166.9 135.4 128.1 114.4 161.» 125.6 126.6 131.2 127.8 130.8 144.4 144.2 129.9 129.2 123.8 56.2

BZP-TPM 166.0 167.9 132.9 135.7 124.6 142.7 136.7 123.4 131.1 127.* 130.4 144.6 144.1 129.9 129.2 124.4 56.4 194.1

BSA-TPM 166.4 166.2 133.8 135.2 125.5 146.4 136.9 123.3 130.8 127.8 130.3 144.6 143.9 129.8 129.2 127.2 56.2

intemuclear distances calculated between carbons and protons m, η, a and ρ and other atom(s) in the molecule have more variation (up to ±1Â). The other measured torsion angles and intemuclear distances in the conformers showed a very small variation (for intemuclear distances, the error is lesser than ±0.2Â ) The molecular structure for the lowest energy conformer of the PM-TPM and BZP-TPM repeating units obtained by this method is depicted in Figure 3 The torsion angles defining the chain conformation are also shown. The energy minima for repeating units and model compounds are close to the torsion angles reported in Table BI. Energy calculations revealed some different characteristics for the two Ar-X torsion angles (see Table Ε ) . Analysis of the torsion angles τι and τ% indicates that a deviation from coplanar geometry is present, and for X = SCb the orientation of sulfone group in the lowest energy conformer is almost perpendicular to the aromatic rings. As a consequence, there is a lack of conjugation between the phenyl ring and the X groups in all repeating units. Dihedral angles between rings and N-Ar bond of the triphenylmethane moiety are also non-planar, but values do not differ significantly from 0°. Deviation from planarity are more pronounced for torsion angles t , which 5



Figure Jl Molecular structure of the minimum-energy conformées for the PMTPM and BZP-TPM repeating units.


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Tabic ID. Torsion angles lor model compound and repeating mi its


ο

Torsion angle

t

3

T4 t5 t6

Model compound

8.5° -6.3° -59.7° 33.6°

PMTPM

9.8° -10.1° -58.3° -9.2°

x=o 40.3° 31.0° 9.2° 29.4° 35.7° 35.Γ

X = CO 15.9° 38.3° -9.5° 5.2 59.5° 5.9° e

x=so

2

82.2° 83.1° 15.2° -11.0° -60.4° -8.8°

correspond to the phenyl ring that do not bear a substituent in the para position. The non-planarity of die aromatic rings in T P M is due to steric hindrance (13)

NOE Difference Spectrum ofBZP-TPM. Nuclear Overhauser Enhancements (NOËs) are useful to understand the conformation and steric configuration of substituents along the polymer backbone. (14,15) NOEs depend in a complex way upon the distances from the observed nucleus to the nearest neighbor protons.



253 In Figure 4, NOE Difference spectra for BZP-TPM upon irradiation of the different protons are shown. Large homonuciear NOEs are observed in the difference spectra, and this are reported in Table IB. As expected* NÔE between neighbors atoms are present, but negative peaks near kradiated protons are due to partial safiaration by the used irradiation. Vectra obtained upon irradiation of rtfôtdns e, g and A show a positive NOE with protons j and k. There is a very small NOE interaction between protons e, g and k and proton n. When C H is irradiated, a positive NOE can be distinguished for protons f k and a small negative NOE with proton n. Interatomie distances derived from the molecule shown in Figure 1 (see Table IV) are longer than expected based on the observed NOEs. So, distances from protons e, g and h to proton η indicates that these atoms are very far from each other, so peaks are not expected in the NOE difference spectra, (16) In other words the measured experimental NOEs can not be explained based on the interatomic distancesfoundin the minimum energy conformer of the repeating unit. This means mat the real COnformation of the overall polymer in dmf can not be properly represented by the repeating unit, and a more realistic representation for PI-TPMs must be obtained. r

Refinement of the molecular model for the polyirnides. One approach to understand the molecular geometry of PI-TPMs in solution is to deterrnine the preferred conformation of a short-chain segment of the polymeric chain. With this in mind, the number of repeating units was increased up to five monomelic units and geometry optimization by Molecular Mechanics was undertaken. In Plate 1 we present the molecular geometry of a BZP-TPM pentamer obtained after energy minimization. It is dear that polymer chain shows a strong preference to coil up and adopt a round-shape geometry that is mainly a consequence of the TPM stereochemical influence. The pendant phenyl ring which is only attached to the C B carbon points outward and produce kinks in the polymeric backbone with the resulting round-shape form. Such a close pacldrig molecular ârfa^ement allows ί close proximity between some groups that look far apart in the monomeric unit. Internuclear distances observed for this structure are reported in Table IV. It is clear that internuclear distances between interacting protons predicted by this model are shorter than those measured in the repeating unit Measured NOEs depends upon many experimental parameters, such as decoupler power, duration of decoupler irradiation, presence of relaxation mechanisms other than dipolar, correlation times of the molecules and the effects of other nearby nuclei through cross-relaxation. (17,18) m that regard, NMR- distance geometry calculations are always approximate. In order to facilitate



aid

-ι / β·β

Figure 4. NOE difference spectra ofBZP-TPM.

M

Λλ

β . ζ β . ο

Τ

CH irradiated

H 'P irradiated

n

H irradiated

h

H irradiated

e

Η& irradiated


ppm

1/1

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Table IV· Atomic bond distances fop the stroetwe obtained after geometry optimization for BZP-TPM repeating unit andfera pentamer


Ο

Atomic bond distance (Â)

«=; 8.9,11.1 5.0,6.9 7.5, 8.8 9.8,11.7 7.2,9.4 7.2,7.9 9.6,10.2 7.4,8.8 4.3,6.1 4.8, 5.3 2.7,3.5

Η»-Η* Η -Η" 6

1

Ιΐ -»

Η*-Η* j

CH-H CH-H*

n=5 5.3,8.7 3.2,7.4 4.6, 8.1 7.9,10.2 5.0,7.9 7.1,9.4 6.1,8.2 3.5,6.4 3.5,4.3 2.9,3.9

Observed NOE (%) w 55 27 w 75 75 38 35 60 45 40

Bond distances from NOE w 3.4 3.8 w 3.2 3.2 3.6 3.6 3.3 3.5* 3.6

w - a weak signal was observed in the NOE difference spectrum * Taken as r = 3.5 À 0


256 comparison for data obtained from molecular modeling, internudear distance calculations were obtained based on the general equation: (16) l

NOE( H) = k r -

6

AB

where k is an empirically determined constant and TAB is the internuclear distance between protons A and B. Using a value of 3.5Â for the distance between methine and rf as a standard (το% a vame of 827 Â is recorded for k. In Table ΠΙ, the calculated distances from the experimental NOEs for the other interacting protons are tabulated. The computed distances compare better with the values obtained for the pentamer.


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Conclusions Based on one- and two-dirn^irMonsl NMR experiments, the chemical shifts observed in the proton and earbon-13 NMR spectra ôf PI-TPMs and model compound were assigned. Conformational energy calculations were used to explain the targe experimental NOEs for BZP-TPM in DMF-d . It was found that a model with five repeating units is more appropriate to explain the interaction through NOE present in the polymer chain. 7

Acknowledgment. The authors would like to thank Dr. Guulermo Mendo2a-Diaz (Unrvemdad de Guanajuato) for recording NOE Difference spectra and insightful comments.

References 1. Advances in Polyimides and low Dielectric Polymers; Sachdev, H.S.; Khojasteh; M.M.; Feger, C., Eds.; Proceedings of the 6 International Conference onPolyimides;Society of Plastics Engineers: Hopewell, NY 1997. 2. Polyimides: Synthesis, Characterization and Applications.,K.L.Mittal,Ed.; Plenum: New York, 1984; Vol. 1 and 2, 3. Sroog, C.E. Prog. Polym: Sci., 1991, 16, 561 th



257 4. Materials Science of High Temperature Polymers for Microelectronics; Grubbs, D.T.; Mita, I.; Yoon, D.Y., Eds.; Materials Research Society Symposium Proceedings, 1991; Vol. 227 5. Likhatchev, D.; Alexandrova, L.;Tlekopatchev,M . ; Vilar, R.; Vera -Graziano, R. J. Appl. Polym. Sci., 1995, 57, 37-44 6. Likhatchev, D.; Alexandrova, L.; Tlekopatchev, M . ; Martinez-Richa, Α.; Vera-Graziano, R. J. Appl. Polym. Sci., 1996, 61, 815-818 7. Martinez-Richa, Α.; Vera-Graziano, R. J. Appl. Polym. Sci, 1998, 70, 10531064 8. Likhatchev, D.; Chvalun, S.inref.1,pp 167-178 9. Bax, A.; Freeman R.; Morris,G.J.Magn. Reson., 1981, 42, 164 10. Bax, A.; Morris, G. A. J. Magn. Reson., 1981, 42, 501 11. Bax, A. J. Magn. Reson., 1983, 53, 517 12.Martínez-Richa,Α.; Vera-Graziano, R.; Alexandrova, L.; Likhatchev, D. in ref. 1, pp 191-219 13. March, J. Advanced Organic Chemistry, Wiley: New York, 3 edition, 1985; pp 149 14. Bovey, F.A.; Mirau, P. NMR of Polymers, Academic Press: New York, 1996 15. NMR Spectroscopy of Polymers; Ibbett, R.N., Ed.; Chapman and Hall: London, 1993. 16. Farrar, T.C. Introduction to Pulse NMR Spectroscopy, Farragut: Madison, 1989; pp 133-140 17. Neuhaus, D.; Williamson, M.P. The Nuclear Overhauser Effect, Wiley -VCH: New York, 2 edition, 2000 18. Duddeck, H.; Dietrich, W.; Tóth, G. Structure Elucidation by Modern NMR, Spinger: New York, 1998 rd

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1H and 13C Solution NMR of Polyimides Based on 4,4

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