Nuclear magnetic resonance studies of polydienes: 1. 13C n.m.r. of 1,4-polybutadiene obtained by π-allyl nickel trifluoroacetate catalysts

Nuclear magnetic resonance studies of polydienes: 1. '3C n.m.r, of 1,4-polybutadiene obtained by n-allyl nickel trifluoroacetate catalysts F. Conti and A. Segre Istituto di Chimica delle Macromolecole, Nucleo di Roma, c/o Istituto Chimico, Universit~ di Roma, Roma, Italy

and P. Pini and L. Porri Istituto di Chimica Organica Industriale, Universit~ di Pisa, Pisa, Italy (Received 16 July 1973)

z3C n.m.r, spectra of 1,4-polybutadienes catalysed by ~r-allyl Ni trifluoroacetate systems show cis-trans sequence distribution. The assignment of different peaks is obtained both by comparison with model compounds as well as by comparing samples with different cis:trans ratios. All the observed peaks are assigned in terms of triads of steric configuration and a quantitative analysis of triad content is possible. Information on the structure of the so-called 'equibinary' polybutadienes is also reported.

INTRODUCTION Proton magnetic resonance has proved to be of great interest for the study of the microstructure of diolefin polymers, cis- and trans-l,4-Polybutadiene, and natural and synthetic cis- and trans-l,4-polyisoprene have been investigated by this method and accurate quantitative determinations of the cis: trans ratios and of the amount of minor structural units (1,2- or 3,4-) have been obtained 1-5. However, it provides very little information on the distribution of the various isomeric structural units in the polymer chainsa. laC nuclear magnetic resonance (n.m.r.) seems to be particularly attractive for investigating these microstructural features of diolefin polymers. Recently, Mochel has investigated by laC n.m.r, a n-BuLi catalysed polybutadiene and has reported data concerning the isomeric structural unit distribution in the polymer chains7. We began a systematic study on the structure of various diolefin polymers obtained by transition metal catalysts. This paper reports the results of an investigation by laC n.m.r, of 1,4-polybutadienes obtained by ~r-allyl nickel trifluoroacetate, with or without additional ligands. Catalysts of this type are knowns to give polymers consisting almost exclusively of cis-l,4 and trans-l,4 units, in almost equal amounts, but scarce information is at present available concerning the distribution of the structural units in the polymer chains. EXPERIMENTAL All laC n.m.r, spectra were run on 12mm samples containing ~10~o solutions (w/v) of polymer in 1,4-

dioxane (20~o D8+80~o Ha). Experimental peak positions are reported in ppm from tetramethyl silane (TMS), used as an internal standard. The spectrometer used was a Varian XL 100 operating in the Fourier transform (FT) mode. The FT conditions were as follows: Spectral width (Hz) Acquisition time (sec) Pulse width (/zsec) K transients Data length Signal enhancement (sec) FT length Hertz/point

5000-1000 0.8-2 30-45 2000 8192-4096 0.40 8192-4096 1.25-0.50

Polymer preparation ~r-Allyl nickel trifluoroacetate, rr-C3Hs-Ni-OCOCFa, and 2,6,10-dodecatriene-12-yl nickel trifluoroacetate, rr-C1eH19-Ni-OCOCFa, used as polymerization catalysts, were prepared according to the methods reported in the literature9, z0. The polymerizations were carried out following the procedure already described 11. The percentages of cis-l,4, trans-l,4 and 1,2-units of the polybutadienes were determined by infra-red analysis, according to the method of Morero et al. 12, on polymer solutions in carbon disulphide, or on thin films prepared by evaporating polymer solutions in carbon disulphide. A Perkin-Elmer model 225 spectrophotometer was used. The polymerization conditions and the infra-red analysis of the polymers that have been subsequently investigated by 13C nuclear magnetic resonance are summarized in Table 1. POLYMER, 1974, Vol 15, January 5

Nuclear magnetic resonance studies of polydienes (1): F. Conti et aL Table 1 Polymerization of butadiene by =-allyl nickel trifluoroacetate catalysts*

I.r. analysis of the polymers$

%

%

%

Run

Catalyst systemt

Solvent

cis

trans

1,2

1

~r-Cz2H19-Ni-OCOCFa+

heptane

51

49

CFaCOOH 2

~r-Cz2H19-Ni-OCOCFs+

heptane

51

49

--

3

CFaCOOH ~-CsHs-Ni-OCOCFs+

heptane

72

25

3

4

CFsCOOH ~-CaHs-Ni-OCOCFs+

heptane

55

45

--

5

CFsCOOH =-CsHs-Ni-OCOCFs

benzene

57.5

38

4.5

* Polymerization temperature: runs 1, 2, 4--20°C; run 3, 55°C; run 5, 70°C 1"CFaCOOH/nickel complex: 20 $ Analyses were performed on the CHsOH-insoluble polymers for runs 1, 2, 3, 5. Crude polymer from run 4 was extracted with methyl ethyl ketone (MEK) and i.r. analysis were performed on the residue to MEK extraction

33-2-~,L

27.9-~.

Figure la Aliphatic range zaC n.m.r, spectrum of sample 2 (see Table I)

130"25 (130"10 130"85.. "~

130"75--

RESULTS AND DISCUSSION Spectra relative to different 1,4-polybutadienes show peaks in two different regions as shown in Figures la and lb for one of the samples. In the range of the aliphatic carbons two intense resonances, at 33.2 and 27.9ppm respectively, are present in every spectrum. In some of the samples a few minor additional peaks are present. In the range of the vinylenic carbons, four main peaks (Figure lb) at 130.85, 130.75, 130.25 and 130.10ppm are always present, whose relative intensity is different for the various samples. A few minor peaks are also observable in some samples. On the basis of the data reported in previous papers 7, as, 14, it is possible to assign the two peaks at 33.2 and 27.9 ppm respectively to the methylenic carbons directly bonded to a trans or a cis double bond. The minor multiplicity of the resonance peaks observed for the aliphatic carbons with respect to the vinylenic carbons, shows that the first ones are sensitive only to the configuration of the directly bonded double bond. This result is in full agreement with the conclusion of Roberts et al. la for compounds which could be considered as models for our polymers. It is, however, in disagreement with the conclusions that Mochel 7 derived from the study of a n-BuLi catalysed polybutadiene and of some model compounds (cis, trans, trans-l,5,9-cyclododecatriene; cis, trans-4,8-dodecadiene ; 3,7-decadiene). Mochel's conclusions are that the effect of cis and trans structures of the adjacent unit is much more pronounced on the aliphatic carbons than on the olefinic carbons. It is to be noted, however, that Mochel does not take into account the peaks at 130.85 and 130.10ppm, also present in the spectrum of the polybutadiene of his work. Furthermore, he uses as model compound cis, trans, trans-l,5,9-cyclododecatriene, although it has already been recognized that alkenes which include cyclic structures do not fit as well into the additivity rules is. With regard to the 13C n.m.r, spectrum of cis, trans-4,8-dodecadiene, it is possible to calculate the chemical shift of the various aliphatic carbons on the basis of the additivity coefficients, reported by Roberts et al. za, and by Grant and Paul 15. Results of this calculation are reported in Table 2, together with experimental frequencies as obtained from the spectra of ref. 7. The calculated values are essentially in agreement with Mochel assignments, except for the inversion of the assignment of the signals due to C3 and C7. Our assignment is in agreement with the insensitivity of aliphatic 13C chemical shifts with regard to the configuration of double bonds in/3 position Is. This insensitivity is also supported by the small chemical shift difference (0.14ppm) of carbons in position I1 and 2 Table 2 Calculated and observed aliphatic carbon chemical shifts for cis, trans-4,8-dodscadiene Shift* (ppm)

J Figure lb Table 1)

6

Vinylenic range laC n.m.r, spectrum of sample 2 (see

POLYMER, 1974, Vol 15, January

Carbon

Calc.l"

Exp.$

Assignment as from ref. 7

3 6 7 10

30'4 32'9 27.4 24'9

30.8 33.2 27.5 25"3

7 6 3 10

* From TMS t According to refs 13 and 15 $ From ref. 7

Nuclear magnetic resonance studies of polydienes (1): F. Conti et al. of e/s, trans-4,8-dodecadiene z6, and by the coincidence of the signals due to /3 aliphatic carbons in mixtures of cis- and trans-2-heptene and of cis- and trans-2octene 17. A quantitative determination of the relative content of cis and trans units in 1,4-polybutadienes follows from the assignment of the main peaks at 33.2 and 27.9ppm. The assignment of the vinylenic main peaks at 130.85, 130.75, 130.25, 130.10ppm can be made on the basis of the results obtained from trans-l,4- and cis-l,4polybutadienez4, as well as by considering the relative intensities of the peaks in our samples having different cis:trans ratios (Figure 2). The peak at 130.75 can be assigned to ttt sequences, while that at 130.25 can be assigned to ccc sequences. The equivalence by symmetry of the vinylenic carbons of the central unit both in the ccc and in the ttt triads (Table 3) has already been pointed out t4. The ctc and tct triads have C~ and Cs equivalent by symmetry. Considering, as generally admitted, that there is an additivity of the neighbour's contribution, we attribute the peaks at 130.85 and 130.10ppm to the ctc and tct triads respectively, being the most different from the already assigned ccc and ttt triads. In the asymmetric triads cct, tcc, ttc and ctt, Ct and Cs are not equivalent. Let us consider, for instance, the cct triad in which C1 is in a configurational situation with respect to its first neighbours similar to that of C1 and C2 of the ccc unit and in which Cz is in a configurational situation with respect to its first neighbours, similar to that of CI and Cz of the tct unit. It follows that the signals due to C1 and C2 of the cct sequences may have chemical shifts similar to the ones found for ccc and tct units respectively. Analogous considerations hold for the other asymmetric triads. The resultant assignment is summarized in Table 4. Experimental data for the polybutadienes examined, based on the above assignments, are presented in Table 5. Samples 1 and 2 consist exclusively of cis-l,4and trans-l,4 units, in a ratio very close to 1. This is in

~,~-130"2S

,3c7s

130,10

Figure 2 Table I)

Vinylenic range I3C n.m.r.spectrum of sample 5 see)

Table 3

Triads of 1,4-polybutadienes

c\

/c--c

ccc

C

C

% cct

/c--c\ ~C I

/c-%

--C--C

C

2

/c--c,~ C

cj ,

/C--C~"C

Ct'

c/

c

C

\c

2

C ttt

\C

2

C=c/C--C~c

tct

\c /c

/c--c.

C'

c/

c/c C

--C I

tcc

C

2

~C

/C--%__

C

/c--c\ ctt

C=C

c/

\c

ctc

C=C

C/

c

2

c/'

~C__C /

ttc

c

C

C

/c--c.

C

i

\C---C

2

/-c\

C

\c

/c c c/

2

~C__C/'

Table 4 Olefinic carbon chemical shift assignment for 1,4-polybutadienes

Exp. frequency (ppm from TMS)

Triad assignment

130-85

Cz, Ca+ Ca + Cz

130"75

C1, Ca+ Cz + Ca

130"25

C1, Ca+ Ca + Cz

130" 10

C1, ~.'~2+ Ct + c~

ctc

ttc

ctt

ttt

ttc

ctt

ccc

tcc

cct

tct

tcc

cct

full agreement with the data reported by Teyssi6 et al. 8-11, who defined these polymers as 'equibinary polybutadienes'. With regard to the distribution of the cis and trans units in the polymer chains the laC n.m.r, spectra permit to exclude a regular distribution of the type ctctctc, since in that case only two peaks, at 130.85 and 130.10, should be present, with only very weak or absent peaks at 130.25 and 130.75. In the spectrum of sample 1 (and to a lesser degree of sample 2) the four peaks in the olefinic region are almost of the same intensity. This finding could be rationalized on the basis that sequences of the type ccttccttcc are present in that polymer. However, regularity bands have been found to be absent in the infra-red spectrum of the solid polymer, in the temperature range from -180 ° to +100°C, and, in addition, no crystallinity has been detected by X-ray examination. This seems to indicate that sequences of the type ccttccttcc, if present, are rather short. A distribution consisting of short

POLYMER, 1974, Vol 15, January

7

Nuclear magnetic resonance studies of polydienes (1): F. Conti et aL Table 5

Relative intensities of zaC resonance peaks Methylenic z3C resonances 1,2 units laC resonances Vinylenic zaC resonances

33"20

27'90

(ppm) (ppm) 130-85 (ppm) Sample

ctc+½ttc+}ctt

1 2 3 4 5

25"4 24' 0 12"6 23"3 Ng-0

130.75 (ppm)

130.25 (ppm)

23"3 21"2 10"6 20"3 28'2

25"4 26"6 61 "4 31 "5 51.5

130.10 (ppm)

ttt+½ttc+½ctt ccc+½tcc+½cct tct+½tcc+½cct trans 25"9 27"4 15"2 24'0 ,,,11-5

sequences ccc and ttt, interspersed by sequences ctc and tct, seems more probable, on the basis of the 13C n.m.r, spectra as well as of the infra-red and X-ray examinations. In the olefinic portion of the spectra of samples 3, 4 and 5 the peak at 130.25 is the most intense, which is indicative of a predominance of the ccc, tcc and cct triads in these polymers. This is consistent with the higher percentage of cis units in these polymers, in agreement with the infra-red analysis (Table 1). As a conclusion we may say that the catalyst system zr-C12Hzg-Ni-OCOCFa+CFaCOOH (Table 1, runs 1 and 2) is able to give 'equibinary' polybutadienes, but these do not consist of regular sequences ctctct, as proposed in earlier studies s. With regard to the peaks of minor intensity in the olefinic region of the spectra of some of the samples examined, those at 112.4 and 145-2ppm (samples 3 and 5) can be attributed to C= and Cp of the 1,2 units, in agreement with Mochels's assignment: 6 -

-

ell2

,y -

-

CH

-

-

I ~CH II aCH=

Analogously, some of the minor peaks in the region of the aliphatic carbons are most probably due to the methylene (C~) and methine (Cr) carbons of the 1,2 units. Mochel tentatively assigned 7 the peaks at 44.2 and 38-6ppm present in the spectra of his polybutadienes to the methine carbons (Cr) of the 1,2 units which are linked to trans-l,4 and cis-l,4 units, respectively. Similarly, the peaks at 30.7 and 25.4ppm were assigned to the methylene carbon (C~) connected to a trans or a cis unit, respectively. Our findings do not seem fully in agreement with these assignments, since in one of our samples (sample 3) the peak at 38.6ppm is missing. For this reason there is the possibility that Cr and C~ give only single peaks, at 44.2 and 25.4

8

CH2

POLYMER, 1974, Vol 15, J a n u a r y

49 46 24 43 38

CH2

cis 51 54 76 57 62

112.40 (ppm)

145.26 (ppm)

44.20 (ppm)

(ppm)

absent absent present absent present

absent absent present absent present

absent absent present absent present

absent absent present absent present

C=

C#

C~

25.40

C#

respectively, independently of the stereochemisty of the next neighbour unit. As a consequence, the weak peaks at 38.6, 36.2, and 30.7ppm present in the spectra of some samples, independently of the 1,2 unit content, might be due to satellites (z3C-laC couplings) or to possible crosslinks, or also to the presence of oligomers. ACKNOWLEDGEMENTS The financial support of the Consiglio Nazionale delle Ricerche is gratefully acknowledged. We thank Mr A. Seritti for the assistance in preparing the polymers. The authors are deeply indebted to Dr F. Werhly of Varian AG, Zug (Switzerland), for running the n.m.r. spectra. REFERENCES 1 Chen, H. Y. Analyt. Chem. 1962, 34, 1134 2 Chen, H. Y. Analyt. Chem. 1962, 34, 1793 3 Worsfold, D. J. and Bywater, S. Can. J. Chem. 1964, 42, 2884 4 Stehling, F. C. and Bartz, K. W. Analyt. Chem. 1966, 38, 1467 5 Chen, H. Y. J. Polym. Sci. (B) 1966, 4, 891 6 Bovey, F. A. 'High Resolution NMR of Macromolecules', Academic Press, New York, 1972, p 224 7 Mochel, V. D. J. Polym. Sci. (A-l) 1972, 10, 1009 8 Dawans, F. and Teyssir, Ph. Ind. Eng. Chem. (Prod. Res. Develop.) 1971, 10, 261 and referencescited therein 9 Dawans, F., Marechal, J. C. and Teyssir, Ph. J. Organometal. Chem. 1970, 21, 259 10 Durand, J. P. and Teyssir, Ph. J. Polym. Sci. (C) 1968, 6, 299 11 Durand, J. P., Dawans, F. and Teyssir, Ph. J. Polym. Sci. (A-I) 1970, 8, 979 12 Morero, D., Santambrogio, A., Porri, L. and Ciampelli, F. Chim. Ind. (Milan) 1959, 41, 758 13 Dorman, D. E., Jautelat, M. and Roberts, J. D. J. Org. Chem. 1971, 36, 2557 14 Duch, M. W. and Grant, D. M. Macromolecules 1970, 3, 165 15 Grant, D. M. and Paul, E. G. J. Am. Chem. Soc. 1964, 86, 2984 16 Mochel, V. D., Lawson, D. F. and Farrar, T. C. J. Am. Chem. Soc. 1962, 94, 6202 17 Le Roy Johnson, F. and Jankowsky, W. C. 'Carbon-13 NMR Spectroscopy', Wiley, New York, 1972