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13C-{1H} NMR spectra of episulphide copolymers
European Polymer Journal, Vol, 12. p p 159 to 164, Pergamon Press 1976. Printed in Great Britain.
13C-{1H} NMR SPECTRA OF EPISULPHIDE COPOLYMERS C. CORNO and A. ROGGERO Snam Progetti, Direzione Ricerca e Sviluppo, Via Fabiani no. 1, 20097 S. Donato Milanese, Milano, Italy (Received 16 June 1975)
Abstrae~Ethylene sulphide-isobutylene sulphide and propylene sulphide-isobutylene sulphide copolymers have been prepared using anionic catalysts and investigated by t3C-I tH', NMR spectroscopy• The carbon-13 NMR spectra are assigned in terms of diad and triad sequences. There is discussion of the effects of mono- or dimethyl substitution in the ~, /3, 7 or 6 positions on the chemical shift of the main chain carbon atoms. It has also been shown that for isobutylene sulphide, as for propylene sulphide, under the influence of an anionic catalyst, there is a normal ring opening only at the primary carbon atom.
INTRODUCTION The assignment and discussion of 13C-{ 1H I N M R spectra of ethylene sulphide-propylene sulphide (ES-PS) copolymers obtained with anionic catalysts [-1] has been previously reported [2]. It was shown that 13C-NMR gives information on the distribution of the monomeric units and the tacticity in terms of diads and triads. These investigations have now been extended to two other episulphide copolymers, viz. the ethylene sulphide~isobutylene sulphide (ES-IBS) and the P ~ I B S copolymers. The aim is to obtain, through quantitative measurements, information not only on the sequence distribution but also on the mechanism of the anionic polymerization of episulphides. In this paper the assignment and discussion of the spectra of these compounds are presented; the quantitative aspects will be reported later. The procedure followed for the assignment of the spectra is essentially that used for the E S - P S copolymers. The main points were the following: (1) E S - P S copolymers can be generated by methyl substitutions on arbitrary non-adjacent carbon atoms in a poly-(ethylene sulphide). All the carbon atoms of a long unsubstituted chain are essentially equivalent; substitution on this chain induces a shift of the resonance line of the neighbouring carbon atoms. (2) The separation of the signals into their various triads can be predicted, since the monomers have three atoms in the main chain. (3) A limitation to the number of possible cases is provided by the "normal" ring opening of PS at the primary C atom; therefore this monomer is enchained in only one way: CH3
the effect on C* due to the methyl substitution in 6 is to be expected, at least in principle, to be different from that due to the methyl substitution in 6'. This means that the C* lines in the triads PEE and EEP, and likewise in the triads P P E and EPP, are expected to be distinguishable. Using these principles, it has been possible to interpret and assign all 14 peaks of the spectrum of the E S - P S copolymers (Table 1). The effect of methyl substitution, at various positions, on the chemical shift of the main chain carbon atoms has also been determined, and gives the following values in ppm: c~ = 7'9, fi = 7'5, Y = - 1 ' 4 , 6 = 0"8 and 6' = 0-1, where the assignment of the values of 6 and 6' was made indirectly noting Boileau's results on the PS methyl orientation effect [5]. The same scheme can also be applied to ES-IBS and PS-IBS copolymers.
EXPERIMENTAL Polymers
The copolymers were obtained using previously reported catalysts [1]. All copolymerizations were performed in glass vessels, under an atmosphere of nitrogen, at room temperature over a few hours. Ethyl bromide was used to terminate the copolymerization to avoid any unstable mercaptan chain-ends. The copolymers were precipitated with methanol, followed by filtration and drying for 24 hr in vacuo at 45°.
13C-NMR measurements All 13C-NMR spectra were run using 12 mm dia sample tubes containing ca. 15% solutions (w/v) of copolymer. The ES-IBS copolymers were dissolved in a 30% solution of C6D 6 in o-dichlorobenzene (o-DCB) at 50 or 90 '~. The PS IBS copolymers were dissolved in the same solution at 25 ° or in CDC13. Either C6D6 or CDC13 was used as the internal lock. The experimental peak positions are reported in ppm ~2H2--1CH--S -• from TMS, used as the internal reference in all cases except as already known from the chemistry of the episul- for the spectra recorded at 90 °. In this case, the chemical phides [3. 4], and confirmed by analysing the shifts were measured from C6D6 and are reported in ppm 13C-NMR spectrum of poly (PS), which showed prac- from TMS, assuming the chemical shift of C6D6 to be 127.96 ppm. The 13C-NMR spectra were recorded on a tically no head-to-head or tail-to-tail linkages [2]. Varian XL100 spectrometer operating at 25.14 MHz using (4) In a sequence of the type the F.T. Mode. The carbon spectra were all proton noise --C~C--S---C--C*--~-C--C-decoupled except where spectral analysis required only partial proton-decoupling. 159
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C. CORNOand A. ROGGERO Table 1. Line assignments in the 13C NMR spectrum of ES-PS copolymer Chemical* shift 41.76 41.63 41.50 41.31 41.02 40.83 40.41 38.90 33.83 33.67 33-04 32.92 31.60 20-98
Carbon
Monomer
CH CH CH CH CH CH CH 2 CH2 CH 2 (II) CH 2 (II) CH 2 (II) CH2 (II) CH 2 (I) CH 2 (I) CH3
P P P P P P P P E E E E E E P
Assignments sequence PPP syndiotactic EPP syndiotactic PPP isotactic EPP isotactic PPE EPE EPP + EPE PPP + PPE PEP EEP PEE EEE EEE + EEP PEE + PEP in all the sequences
* ppm from TMS. RESULTS AND DISCUSSION (a) ES-IBS copolymers A typical spectrum of an ES-IBS copolymer (in o-DCB at 90 °) is shown in Fig. 1. Before applying the assignment scheme used for the ES-PS copolymer, it is necessary to check the "normal" opening of the episulphide ring of the IBS under the effect of an anionic catalyst. This was checked in 13C-NMR spectrum of poly (IBS). As can be seen (Fig. 2) only three peaks at 45.06, 41.77 and 28.60 ppm are present and are assigned, from the off-resonance decoupled spectra, to the quaternary, methylene and methyl carbons, respectively. The absence of non-regular linkages is therefore confirmed. Noting this fact, eight triads can now be considered. If vE is the chemical shift of the methylene carbon in poly (ES), the chemical shifts for the C~ and C2 carbons resulting from di-methyl substitution in the eight possible triads are reported in Table 2. By considering the spectrum in Fig. 1, the assignment of each peak to one of the three possible types
45
of carbon atoms (methyl, methylene and quaternary) must now be achieved. Here both T1, the relaxation time measurements, and the off-resonance decoupled spectra were used. The T 1 measurements, although obtained by the progressive saturation technique and therefore not very accurate, allowed the assignment of the peaks at 45.06, 45.83 and 46.61 ppm to the quaternary carbon. The relaxation times, T1, of these peaks are respectively 4.4, 5-6 and 5.7 sec, while all the other peaks have a T1 less than 2.5 sec. Off-resonance decoupled spectra were used to distinguish between the methyl and the methylene peaks. The two peaks at higher field (28.60 and 28.28 ppm) were assigned to the methyl carbons since these two signals appeared as two quartets in the off-resonance decoupied spectra; all other peaks were due to methylene carbon atoms. It was now possible to assign these peaks to the various sequences according to the scheme in Table 2. The resonance, re, of the poly(ES) found for ES-PS copolymers is 32'92 ppm. In the present case, this resonance is found at 33.30 ppm and the small differ-
I
I
I
40
35
30
ppm from TMS
Fig. 1. 13C-{tH} spectrum (25.14 MHz) of ethylene sulphide-isobutylene sulphide copolymer in o-dichlorobenzene-C6H6 (70/30) solution at 90°. Mole fraction of ethylene sulphide Me = 0.56.
13C-{1HI NMR spectra of episulphide copolymers
I
I
I
45
40
~5
I 30
ppm from TMS
Fig. 2. 13C-{1H} spectrum of poly(isobutylene sulphide) in o-dichlorobenzene-C6D6 (70/30) solution at 90°. Polymer prepared with anionic catalyst.
161
trum of poly (IBS), it can be deduced that the resonance of the homopolymeric sequence falls at 45.06 ppm. By analysing the spectra of copolymers having different compositions, it was possible to assign the peak at 46.61 ppm to the alternating triad EIE, by observing that this resonance diminished differentially when the ES content was lowered. The EII and IIE triads overlap at 45.83 ppm. It is worth noting that the same 6 and 6' substitution effect, derived from the overlapping of these two triads, has never been observed before in episulphide copolymers even if this is not very surprising. As will be seen later, this finding only fits for the quaternary carbon. By considering the C2 carbon of ES, the peak at 33.30 ppm is obviously due to the homopolymeric sequence. Downfield, two signals at 35-14 and 35.02 ppm are still to be assigned. The relative position of the ES C2 carbon is quite similar to that found in the ES-PS copolymer, where four peaks at 32.92, 33.04, 33.67 and 33"83 ppm were found and assigned to the ES C2 carbon in the triads EEE, PEE, EEP and PEP, respectively. In this case only three peaks are evident, since in the peak at 33.30 ppm, a splitting is hardly distinguishable. However, analogous to the ES-PS copolymer, the following assignments can be derived: Vc~(EEE) = 33.30 ppm
ence can be attributed to difference in,.recording temperature. The temperature of 90° in this case was chosen since it allows the resolution of some peaks which overlap at room temperature. Only one line upfield of vE due to a methylene carbon is present at 29'45 ppm. Taking into account the expected negative value of 7 and the small value of E, this peak is attributed to C1 in the IEE and IEI sequences. The signal of C1 in the EEI sequence overlaps that of the EEE sequence at 33.30 ppm. As far as C1 in the triads centred on IBS is concerned, in the spectrum of poly-(IBS) C1 line of the homopolymeric sequence is found at 41.77 ppm. Because of the negative value of 7 and low value of E, the peak at 47"2 ppm (the only one belonging to a methylene carbon downfield of 41.77 ppm) is assigned to C1 in the EIE and EII triads, whilst the peak of the sequence IIE at 41.77 ppm overlaps with that of the III sequence. The quaternary carbon, C2, of IBS can now be considered. There are three peaks at 45.06, 45.83 and 46-61 ppm due to this carbon atom. From the specTable 2. Di-methyl substitution in ES-IBS triads Triads
C1
EEE EEl lEE EIE lie Eli IEI III
vE VE+ e2e) VE-[- 72 VE+ f12 v~ + f12 ~- "~2 vr: + f12 + e2 vz + y : +e2 vE +f12+72+E2
C2 vF VE+ 62 YE-[- 6'2 I¥ -[- ~2 VE-{- 0(2 -[- 6'2 vE + ~2 + 62 vE +62 +6", vE +0~2 +62 +6~
* The subscript 2 of the parameters indicates substitution with two methyls on the same carbon atom. Ip.f
12/3
(
Vc2 (EEI or I E E ) = 33"30 ppm Vc2(IEE or E E I ) = 35-02 ppm Vc2(IEI) = 35-14 ppm. The ambiguity between the signals of the EEI and IEE triads in this case cannot be resolved without employing model molecules. According to the findings of the ES-PS copolymers however, it seems more reasonable to assign the line at 35"02 ppm to the EEI triad. Consequently, the IEE peak overlaps that of the homopolymeric sequence at 33.30 ppm. In this spectrum two other signals upfield are present, due to the IBS methyls, where four peaks should be expected. Indeed, for IBS methyls, four peaks are found, as will be seen later in the PS-IBS copolymer, when CDCI 3 is used as solvent. However, only two peaks are present when o.DCB is used, since one of the two triads PII or IIP overlaps the homopolymeric and the other the alternating one. The assignment of the signals at 28,60 and 28.28 ppm, respectively to the III and EIE triads, is evident both from the comparison with the homopolymeric spectra and from analysis of copolymers having different compositions. As usual, the problem remains as to which of the two EII and IIE triads overlaps the homopolymeric sequence and which overlaps that of the alternating one. According to ES~PS copolymer results (2), the overlaps I I I + EII and I I E + EIE are proposed. Spectral assignment allows the values ~2,/32, 72, 62 and 6~, due to the di-methyl group substitutions on the same carbon atom, to be obtained. These values for c~2 and/~2 are: ~2 = 13.3 ppm
f12 =
13"8 ppm.
For 72, 6z and 6~ different values are obtained, depending on which monomeric unit (IBS or ES) they
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C. CORNOand A. ROGGERO
operate. In fact the values obtained are: for C of ES
for C of IBS
72 = -3-8 ppm 52 = -I-]-7 ppm 6~ = +0'1 ppm
72 = - 5.4 ppm 62 = - 0 . 8 ppm 6~ = - 0 . 8 ppm
different distribution also around the S---C bond is to be expected. The change of the 72 values could be explained since in the chain
CH3
These different values are probably due to conformational effects. It is indeed very likely that conformations around the S--CHz bond different from trans are practically forbidden in the II sequence because of the high steric hindrance of the CH3
---~--S---CH2--
~H3 the through space interaction between the methyls and methylene carbons is dependent only on the relative conformation on the C - - S bond. (b) PS-IBS copolymers
group. On the contrary, gauche conformations could be possible in a IE sequence as the steric hindrance is reduced. Since the distribution of the conformations around a bond clearly affects that of the adjacent bond, a
The assignment of 13C-NMR spectra of the PS~IBS copolymer is simplified by the results obtained from the ES-PS and ES~IBS copolymer spectra. For the PS-IBS copolymer, spectra were run in both CDCI 3 and o.DCB-benzene solutions (see Figs. 3 a a n d b). The main differences lie in the IBS methyl region. In fact in CDC13, the four expected peaks,
(a)
J
45
I
J
:55
40
:50
ppm f r o m
I
I
25
20
TMS
(b)
45
]
i
]
I
]
40
35
30
25
20
#1)rn from ]-MS
Fig. 3. (a) 13C-{IHI spectrum (25.14 MHz) of propylene sulphide-isobutylene sulphide copolymer in CDCIa solution at 25°. Mole fraction of propylene sulphide Me = 0,40. (b) 13C-{IH} spectrum (25.14 MHz) of propylene sulphide-isobutylene sulphide copolymer in o-dichlorobenzene-C6D6 (70/30) solution at 25°. Mole fraction of propylene sulphide My = 0.40,
13C.
{1H} NMR spectra of episulphide copolymers
for the triads Ill, PIP. PII and IIP, are present. The assignment of the peaks at 28.22 and 28"06 ppm to the III and PIP sequences, respectively, was easily derived from the relative intensities of the peaks in the copolymers having different compositions. The two peaks either side of the above-mentioned peaks at 28.34 and 27.94 ppm had the same intensity and were attributed to the PII and IIP triads. However, in o.DCB only two peaks are observed, since the triad IIP (or PII) overlaps that of PIP and PII (or IIP) overlaps that of III. For the PS methyl in both cases only two peaks are present, and their assignment is relatively straightforward: vcN,lPPP) ~ vcH,(IPP or PPI) = 20-74 ppm vcH.,llPI) = vcH.,(PPI or IPP) = 20.98 ppm. No other important differences are found between the spectra run in CDC13 and o.DCB-benzene for the main chain carbon atoms, except small differences in their chemical shifts. Only the spectra in o.DCBbenzene will be discussed to be consistent with the other copolymers, following the usual scheme where C~ is the IBS or PS methylene carbon and C2 is the quaternary or rnethine carbon. In Table 3, the substitutions for the eight possible triads are reported, subscripts 1 and 2 indicating the substitution with one or two methyl groups on the same carbon atom, respectively. Values for the substitution effect reported in Table 3 were previously obtained from ES PS and ES-IBS copolymers. These values provide a guide for the spectra assignment even when, as shown by the existence of the double values for 7.,. ,52 and 6'~ in ES-IBS copolymers, they cannot be used directly, since the small predictable changes depend on the considered sequence. Some features are, nevertheless, maintained and can be summarized as follows: (1) all contributions, in absolute value, of di-methyl substitution are greater than the corresponding mono-methyl substitution. (2) The 7 values are negative. (3) The E values are negligible. This last observation predicts only four signals for C~. relative to the reported substitutions. The peaks for the homopolymeric sequences are known from the other copolymer spectra. In fact: Vcu~(PPP) = 38.86 ppm Vc.~(III) = 41-20 ppm. Table 3. Mono and di-methyl substitution in PS-IBS triads Substitution Triads
CI
C2
PPP PPI
/~1 + 71 + el* fll + 71 ~- E2
:q + 61 + 6'1 ~1 -1- 62 -t- 61
IPI PIP
Ill + 72 + ~, []2 + 7~ + ~
c~l + 62 + 6'2 ~2 + 81 + 6]
PII
[¢2 + 7~ + ~2 B2 + 72 + e~ [t2 + 7_, + E,
~2 + 62 + 6] ~2 + 61 + 6'2
IIP Ill
~2 + 62 + 6~
* The subscripts l and 2 of the parameters indicate, respectively, substitution with one or two methyls on the same carbon atom.
163
Since ,?_,, > ),~ and ? is negative, one peak upfield at 38.86 ppm for the IPI triad and another downfield at 41.20 ppm for the PIP triad are to be expected. In fact, two other peaks, due to methylene carbons. at 36.07 and 44-57 ppm, respectively, are present and can be attributed to the previously mentioned triads. Values for 7 in the alternating sequence can be derived as follows: in IBS PS ]'2 = -4"3 ppm (intermediate value between -5.8 in the II sequence and -3-8 in the EI sequence); in PS-IBS 71 = - 2 . 2 ppm (slightly higher than the value of - 1 . 5 found in the PE and PP sequence). For C2, three peaks can be assigned to the quaternary IBS carbon; viz. 44.82, 45.46 and 46-06 ppm. respectively; the relative position of these peaks is quite analogous to that found for the ES-IBS copolymers. and the derived assignment, confirmed by copolymers with different monomer content, is: v% (III) = 44-82 ppm vc2(IIP and PII) = 45-46 ppm v% (PIP) = 46'06 ppm. The PS methine carbon in the homopolymeric sequence, as found before both from the E ~ P S copolymer and the PS homopolymer, lies at ca, 41.6 ppm and, in this case, overlaps the IBS methylene peak in the homopolymeric sequence. Another peak belonging to the methine carbon is found downfield at 42"06 ppm. The situation is quite analogous to that observed for the methyl group: only two of the four predicted peaks are found. The assignment of this peak to the IPI triad and the overlap of PPI (IPP) with IPI or IPP (PPI) with PPP is obvious. CONCLUSIONS
Several features of the effects of substitution of H atoms by methyl groups on the chemical shifts of the carbon atom in the chain: --S--C--C--S--C--42--S--C~C--S--can be derived from the assignment of the ~3C-NMR spectra of ES-PS, ES-IBS and PS-IBS copolymers. First. the decreasing effect of the two methyl substitutions, 72, in the sequence II, IP and Ig is noteworthy. This fact can be explained on the basis of different distributions of the conformations around the C--S and S---CH 2 bonds. In effect a progressive lowering of steric hindrance in the diads II, IP and IE is to be expected. A second aspect can be readily shown when the following chain is considered ~g
--C--C 1--S--C--C--S--C--C2--S-where
CH3
I
C* = *CH2, *CH
/ and
CHa
*C
\ CH~
It has been found that both C* and its methyl substituents (excepting only the C* of IBS) are affected by the mono- and di-methyl substitution on C1 quite
164
C. CORNOand A. ROGGERO
differently than by that on C2, since one of the two effects (probably that due to the substitution on C2) is almost negligible. For atactic poly (PS), it has been shown that the orientation of the methyl group bonded to C2 affects C* quite differently from that of the methyl group bonded to C r Therefore the orientation effect is similar but quantitatively lower than the substitution effect. The quantitative aspects of the sequence distribution will be treated in a subsequent publication. Nevertheless, it should be emphasized that such quantitative aspects facilitate a complete elucidation of the spectra. Finally, it must be admitted that the ambiguity for all the triads ABB and BBA has not been resolved
even when, as previously mentioned, reasonable hypotheses can be advanced. From a quantitative point of view, however, this is not a limiting factor, since the two signals have the same intensity.
REFERENCES
1. A. Roggero, A. Mazzei, M. Bruzzoue and E. Cernia, Adv. Chem. Ser. in press. 2. C. Corno, A. Roggero and T. Salvatori, Europ. Polym. J. 10, (7), 525 (1974). 3. M. Sander, Chem. Rev. 66, 297 (1966). 4. N. W, Schwartz, J. org. Chem. 33, 2895 (1968). 5. S. Boileau, H. Cheradame, W. Lapeyre, L. Sousselier and P. Sigwalt, J. Chim. phys. 70, (6) 879 (1973).
Sommario--Sono descritti ed interpretati in termini di diadi e triadi gli spettri laC-~1H~ NMR dei copolimeri etilensolfuro-isobutilensolfuroe propilensolfuro-isobutilensolfuropreparati con catalizzatori di tipo anionico. Sono stati determinati gli effetti sul chemical shift degli atomi di carbonio della catena principale derivanti dalla sostituzione di uno o due atomi di idrogeno con metili in posizione c~, fl, ), e 6. Si 6 inoltre dimostrato che sotto l'influenza di catalizzatori anionici vi ~ per l'isobutilensolfuro come per il propilensolfuro, solo una "normale" apertura delranello, sull'atomo di carbonio primario.
13C-{1H} NMR SPECTRA OF EPISULPHIDE COPOLYMERS C. CORNO and A. ROGGERO Snam Progetti, Direzione Ricerca e Sviluppo, Via Fabiani no. 1, 20097 S. Donato Milanese, Milano, Italy (Received 16 June 1975)
Abstrae~Ethylene sulphide-isobutylene sulphide and propylene sulphide-isobutylene sulphide copolymers have been prepared using anionic catalysts and investigated by t3C-I tH', NMR spectroscopy• The carbon-13 NMR spectra are assigned in terms of diad and triad sequences. There is discussion of the effects of mono- or dimethyl substitution in the ~, /3, 7 or 6 positions on the chemical shift of the main chain carbon atoms. It has also been shown that for isobutylene sulphide, as for propylene sulphide, under the influence of an anionic catalyst, there is a normal ring opening only at the primary carbon atom.
INTRODUCTION The assignment and discussion of 13C-{ 1H I N M R spectra of ethylene sulphide-propylene sulphide (ES-PS) copolymers obtained with anionic catalysts [-1] has been previously reported [2]. It was shown that 13C-NMR gives information on the distribution of the monomeric units and the tacticity in terms of diads and triads. These investigations have now been extended to two other episulphide copolymers, viz. the ethylene sulphide~isobutylene sulphide (ES-IBS) and the P ~ I B S copolymers. The aim is to obtain, through quantitative measurements, information not only on the sequence distribution but also on the mechanism of the anionic polymerization of episulphides. In this paper the assignment and discussion of the spectra of these compounds are presented; the quantitative aspects will be reported later. The procedure followed for the assignment of the spectra is essentially that used for the E S - P S copolymers. The main points were the following: (1) E S - P S copolymers can be generated by methyl substitutions on arbitrary non-adjacent carbon atoms in a poly-(ethylene sulphide). All the carbon atoms of a long unsubstituted chain are essentially equivalent; substitution on this chain induces a shift of the resonance line of the neighbouring carbon atoms. (2) The separation of the signals into their various triads can be predicted, since the monomers have three atoms in the main chain. (3) A limitation to the number of possible cases is provided by the "normal" ring opening of PS at the primary C atom; therefore this monomer is enchained in only one way: CH3
the effect on C* due to the methyl substitution in 6 is to be expected, at least in principle, to be different from that due to the methyl substitution in 6'. This means that the C* lines in the triads PEE and EEP, and likewise in the triads P P E and EPP, are expected to be distinguishable. Using these principles, it has been possible to interpret and assign all 14 peaks of the spectrum of the E S - P S copolymers (Table 1). The effect of methyl substitution, at various positions, on the chemical shift of the main chain carbon atoms has also been determined, and gives the following values in ppm: c~ = 7'9, fi = 7'5, Y = - 1 ' 4 , 6 = 0"8 and 6' = 0-1, where the assignment of the values of 6 and 6' was made indirectly noting Boileau's results on the PS methyl orientation effect [5]. The same scheme can also be applied to ES-IBS and PS-IBS copolymers.
EXPERIMENTAL Polymers
The copolymers were obtained using previously reported catalysts [1]. All copolymerizations were performed in glass vessels, under an atmosphere of nitrogen, at room temperature over a few hours. Ethyl bromide was used to terminate the copolymerization to avoid any unstable mercaptan chain-ends. The copolymers were precipitated with methanol, followed by filtration and drying for 24 hr in vacuo at 45°.
13C-NMR measurements All 13C-NMR spectra were run using 12 mm dia sample tubes containing ca. 15% solutions (w/v) of copolymer. The ES-IBS copolymers were dissolved in a 30% solution of C6D 6 in o-dichlorobenzene (o-DCB) at 50 or 90 '~. The PS IBS copolymers were dissolved in the same solution at 25 ° or in CDC13. Either C6D6 or CDC13 was used as the internal lock. The experimental peak positions are reported in ppm ~2H2--1CH--S -• from TMS, used as the internal reference in all cases except as already known from the chemistry of the episul- for the spectra recorded at 90 °. In this case, the chemical phides [3. 4], and confirmed by analysing the shifts were measured from C6D6 and are reported in ppm 13C-NMR spectrum of poly (PS), which showed prac- from TMS, assuming the chemical shift of C6D6 to be 127.96 ppm. The 13C-NMR spectra were recorded on a tically no head-to-head or tail-to-tail linkages [2]. Varian XL100 spectrometer operating at 25.14 MHz using (4) In a sequence of the type the F.T. Mode. The carbon spectra were all proton noise --C~C--S---C--C*--~-C--C-decoupled except where spectral analysis required only partial proton-decoupling. 159
160
C. CORNOand A. ROGGERO Table 1. Line assignments in the 13C NMR spectrum of ES-PS copolymer Chemical* shift 41.76 41.63 41.50 41.31 41.02 40.83 40.41 38.90 33.83 33.67 33-04 32.92 31.60 20-98
Carbon
Monomer
CH CH CH CH CH CH CH 2 CH2 CH 2 (II) CH 2 (II) CH 2 (II) CH2 (II) CH 2 (I) CH 2 (I) CH3
P P P P P P P P E E E E E E P
Assignments sequence PPP syndiotactic EPP syndiotactic PPP isotactic EPP isotactic PPE EPE EPP + EPE PPP + PPE PEP EEP PEE EEE EEE + EEP PEE + PEP in all the sequences
* ppm from TMS. RESULTS AND DISCUSSION (a) ES-IBS copolymers A typical spectrum of an ES-IBS copolymer (in o-DCB at 90 °) is shown in Fig. 1. Before applying the assignment scheme used for the ES-PS copolymer, it is necessary to check the "normal" opening of the episulphide ring of the IBS under the effect of an anionic catalyst. This was checked in 13C-NMR spectrum of poly (IBS). As can be seen (Fig. 2) only three peaks at 45.06, 41.77 and 28.60 ppm are present and are assigned, from the off-resonance decoupled spectra, to the quaternary, methylene and methyl carbons, respectively. The absence of non-regular linkages is therefore confirmed. Noting this fact, eight triads can now be considered. If vE is the chemical shift of the methylene carbon in poly (ES), the chemical shifts for the C~ and C2 carbons resulting from di-methyl substitution in the eight possible triads are reported in Table 2. By considering the spectrum in Fig. 1, the assignment of each peak to one of the three possible types
45
of carbon atoms (methyl, methylene and quaternary) must now be achieved. Here both T1, the relaxation time measurements, and the off-resonance decoupled spectra were used. The T 1 measurements, although obtained by the progressive saturation technique and therefore not very accurate, allowed the assignment of the peaks at 45.06, 45.83 and 46.61 ppm to the quaternary carbon. The relaxation times, T1, of these peaks are respectively 4.4, 5-6 and 5.7 sec, while all the other peaks have a T1 less than 2.5 sec. Off-resonance decoupled spectra were used to distinguish between the methyl and the methylene peaks. The two peaks at higher field (28.60 and 28.28 ppm) were assigned to the methyl carbons since these two signals appeared as two quartets in the off-resonance decoupied spectra; all other peaks were due to methylene carbon atoms. It was now possible to assign these peaks to the various sequences according to the scheme in Table 2. The resonance, re, of the poly(ES) found for ES-PS copolymers is 32'92 ppm. In the present case, this resonance is found at 33.30 ppm and the small differ-
I
I
I
40
35
30
ppm from TMS
Fig. 1. 13C-{tH} spectrum (25.14 MHz) of ethylene sulphide-isobutylene sulphide copolymer in o-dichlorobenzene-C6H6 (70/30) solution at 90°. Mole fraction of ethylene sulphide Me = 0.56.
13C-{1HI NMR spectra of episulphide copolymers
I
I
I
45
40
~5
I 30
ppm from TMS
Fig. 2. 13C-{1H} spectrum of poly(isobutylene sulphide) in o-dichlorobenzene-C6D6 (70/30) solution at 90°. Polymer prepared with anionic catalyst.
161
trum of poly (IBS), it can be deduced that the resonance of the homopolymeric sequence falls at 45.06 ppm. By analysing the spectra of copolymers having different compositions, it was possible to assign the peak at 46.61 ppm to the alternating triad EIE, by observing that this resonance diminished differentially when the ES content was lowered. The EII and IIE triads overlap at 45.83 ppm. It is worth noting that the same 6 and 6' substitution effect, derived from the overlapping of these two triads, has never been observed before in episulphide copolymers even if this is not very surprising. As will be seen later, this finding only fits for the quaternary carbon. By considering the C2 carbon of ES, the peak at 33.30 ppm is obviously due to the homopolymeric sequence. Downfield, two signals at 35-14 and 35.02 ppm are still to be assigned. The relative position of the ES C2 carbon is quite similar to that found in the ES-PS copolymer, where four peaks at 32.92, 33.04, 33.67 and 33"83 ppm were found and assigned to the ES C2 carbon in the triads EEE, PEE, EEP and PEP, respectively. In this case only three peaks are evident, since in the peak at 33.30 ppm, a splitting is hardly distinguishable. However, analogous to the ES-PS copolymer, the following assignments can be derived: Vc~(EEE) = 33.30 ppm
ence can be attributed to difference in,.recording temperature. The temperature of 90° in this case was chosen since it allows the resolution of some peaks which overlap at room temperature. Only one line upfield of vE due to a methylene carbon is present at 29'45 ppm. Taking into account the expected negative value of 7 and the small value of E, this peak is attributed to C1 in the IEE and IEI sequences. The signal of C1 in the EEI sequence overlaps that of the EEE sequence at 33.30 ppm. As far as C1 in the triads centred on IBS is concerned, in the spectrum of poly-(IBS) C1 line of the homopolymeric sequence is found at 41.77 ppm. Because of the negative value of 7 and low value of E, the peak at 47"2 ppm (the only one belonging to a methylene carbon downfield of 41.77 ppm) is assigned to C1 in the EIE and EII triads, whilst the peak of the sequence IIE at 41.77 ppm overlaps with that of the III sequence. The quaternary carbon, C2, of IBS can now be considered. There are three peaks at 45.06, 45.83 and 46-61 ppm due to this carbon atom. From the specTable 2. Di-methyl substitution in ES-IBS triads Triads
C1
EEE EEl lEE EIE lie Eli IEI III
vE VE+ e2e) VE-[- 72 VE+ f12 v~ + f12 ~- "~2 vr: + f12 + e2 vz + y : +e2 vE +f12+72+E2
C2 vF VE+ 62 YE-[- 6'2 I¥ -[- ~2 VE-{- 0(2 -[- 6'2 vE + ~2 + 62 vE +62 +6", vE +0~2 +62 +6~
* The subscript 2 of the parameters indicates substitution with two methyls on the same carbon atom. Ip.f
12/3
(
Vc2 (EEI or I E E ) = 33"30 ppm Vc2(IEE or E E I ) = 35-02 ppm Vc2(IEI) = 35-14 ppm. The ambiguity between the signals of the EEI and IEE triads in this case cannot be resolved without employing model molecules. According to the findings of the ES-PS copolymers however, it seems more reasonable to assign the line at 35"02 ppm to the EEI triad. Consequently, the IEE peak overlaps that of the homopolymeric sequence at 33.30 ppm. In this spectrum two other signals upfield are present, due to the IBS methyls, where four peaks should be expected. Indeed, for IBS methyls, four peaks are found, as will be seen later in the PS-IBS copolymer, when CDCI 3 is used as solvent. However, only two peaks are present when o.DCB is used, since one of the two triads PII or IIP overlaps the homopolymeric and the other the alternating one. The assignment of the signals at 28,60 and 28.28 ppm, respectively to the III and EIE triads, is evident both from the comparison with the homopolymeric spectra and from analysis of copolymers having different compositions. As usual, the problem remains as to which of the two EII and IIE triads overlaps the homopolymeric sequence and which overlaps that of the alternating one. According to ES~PS copolymer results (2), the overlaps I I I + EII and I I E + EIE are proposed. Spectral assignment allows the values ~2,/32, 72, 62 and 6~, due to the di-methyl group substitutions on the same carbon atom, to be obtained. These values for c~2 and/~2 are: ~2 = 13.3 ppm
f12 =
13"8 ppm.
For 72, 6z and 6~ different values are obtained, depending on which monomeric unit (IBS or ES) they
162
C. CORNOand A. ROGGERO
operate. In fact the values obtained are: for C of ES
for C of IBS
72 = -3-8 ppm 52 = -I-]-7 ppm 6~ = +0'1 ppm
72 = - 5.4 ppm 62 = - 0 . 8 ppm 6~ = - 0 . 8 ppm
different distribution also around the S---C bond is to be expected. The change of the 72 values could be explained since in the chain
CH3
These different values are probably due to conformational effects. It is indeed very likely that conformations around the S--CHz bond different from trans are practically forbidden in the II sequence because of the high steric hindrance of the CH3
---~--S---CH2--
~H3 the through space interaction between the methyls and methylene carbons is dependent only on the relative conformation on the C - - S bond. (b) PS-IBS copolymers
group. On the contrary, gauche conformations could be possible in a IE sequence as the steric hindrance is reduced. Since the distribution of the conformations around a bond clearly affects that of the adjacent bond, a
The assignment of 13C-NMR spectra of the PS~IBS copolymer is simplified by the results obtained from the ES-PS and ES~IBS copolymer spectra. For the PS-IBS copolymer, spectra were run in both CDCI 3 and o.DCB-benzene solutions (see Figs. 3 a a n d b). The main differences lie in the IBS methyl region. In fact in CDC13, the four expected peaks,
(a)
J
45
I
J
:55
40
:50
ppm f r o m
I
I
25
20
TMS
(b)
45
]
i
]
I
]
40
35
30
25
20
#1)rn from ]-MS
Fig. 3. (a) 13C-{IHI spectrum (25.14 MHz) of propylene sulphide-isobutylene sulphide copolymer in CDCIa solution at 25°. Mole fraction of propylene sulphide Me = 0,40. (b) 13C-{IH} spectrum (25.14 MHz) of propylene sulphide-isobutylene sulphide copolymer in o-dichlorobenzene-C6D6 (70/30) solution at 25°. Mole fraction of propylene sulphide My = 0.40,
13C.
{1H} NMR spectra of episulphide copolymers
for the triads Ill, PIP. PII and IIP, are present. The assignment of the peaks at 28.22 and 28"06 ppm to the III and PIP sequences, respectively, was easily derived from the relative intensities of the peaks in the copolymers having different compositions. The two peaks either side of the above-mentioned peaks at 28.34 and 27.94 ppm had the same intensity and were attributed to the PII and IIP triads. However, in o.DCB only two peaks are observed, since the triad IIP (or PII) overlaps that of PIP and PII (or IIP) overlaps that of III. For the PS methyl in both cases only two peaks are present, and their assignment is relatively straightforward: vcN,lPPP) ~ vcH,(IPP or PPI) = 20-74 ppm vcH.,llPI) = vcH.,(PPI or IPP) = 20.98 ppm. No other important differences are found between the spectra run in CDC13 and o.DCB-benzene for the main chain carbon atoms, except small differences in their chemical shifts. Only the spectra in o.DCBbenzene will be discussed to be consistent with the other copolymers, following the usual scheme where C~ is the IBS or PS methylene carbon and C2 is the quaternary or rnethine carbon. In Table 3, the substitutions for the eight possible triads are reported, subscripts 1 and 2 indicating the substitution with one or two methyl groups on the same carbon atom, respectively. Values for the substitution effect reported in Table 3 were previously obtained from ES PS and ES-IBS copolymers. These values provide a guide for the spectra assignment even when, as shown by the existence of the double values for 7.,. ,52 and 6'~ in ES-IBS copolymers, they cannot be used directly, since the small predictable changes depend on the considered sequence. Some features are, nevertheless, maintained and can be summarized as follows: (1) all contributions, in absolute value, of di-methyl substitution are greater than the corresponding mono-methyl substitution. (2) The 7 values are negative. (3) The E values are negligible. This last observation predicts only four signals for C~. relative to the reported substitutions. The peaks for the homopolymeric sequences are known from the other copolymer spectra. In fact: Vcu~(PPP) = 38.86 ppm Vc.~(III) = 41-20 ppm. Table 3. Mono and di-methyl substitution in PS-IBS triads Substitution Triads
CI
C2
PPP PPI
/~1 + 71 + el* fll + 71 ~- E2
:q + 61 + 6'1 ~1 -1- 62 -t- 61
IPI PIP
Ill + 72 + ~, []2 + 7~ + ~
c~l + 62 + 6'2 ~2 + 81 + 6]
PII
[¢2 + 7~ + ~2 B2 + 72 + e~ [t2 + 7_, + E,
~2 + 62 + 6] ~2 + 61 + 6'2
IIP Ill
~2 + 62 + 6~
* The subscripts l and 2 of the parameters indicate, respectively, substitution with one or two methyls on the same carbon atom.
163
Since ,?_,, > ),~ and ? is negative, one peak upfield at 38.86 ppm for the IPI triad and another downfield at 41.20 ppm for the PIP triad are to be expected. In fact, two other peaks, due to methylene carbons. at 36.07 and 44-57 ppm, respectively, are present and can be attributed to the previously mentioned triads. Values for 7 in the alternating sequence can be derived as follows: in IBS PS ]'2 = -4"3 ppm (intermediate value between -5.8 in the II sequence and -3-8 in the EI sequence); in PS-IBS 71 = - 2 . 2 ppm (slightly higher than the value of - 1 . 5 found in the PE and PP sequence). For C2, three peaks can be assigned to the quaternary IBS carbon; viz. 44.82, 45.46 and 46-06 ppm. respectively; the relative position of these peaks is quite analogous to that found for the ES-IBS copolymers. and the derived assignment, confirmed by copolymers with different monomer content, is: v% (III) = 44-82 ppm vc2(IIP and PII) = 45-46 ppm v% (PIP) = 46'06 ppm. The PS methine carbon in the homopolymeric sequence, as found before both from the E ~ P S copolymer and the PS homopolymer, lies at ca, 41.6 ppm and, in this case, overlaps the IBS methylene peak in the homopolymeric sequence. Another peak belonging to the methine carbon is found downfield at 42"06 ppm. The situation is quite analogous to that observed for the methyl group: only two of the four predicted peaks are found. The assignment of this peak to the IPI triad and the overlap of PPI (IPP) with IPI or IPP (PPI) with PPP is obvious. CONCLUSIONS
Several features of the effects of substitution of H atoms by methyl groups on the chemical shifts of the carbon atom in the chain: --S--C--C--S--C--42--S--C~C--S--can be derived from the assignment of the ~3C-NMR spectra of ES-PS, ES-IBS and PS-IBS copolymers. First. the decreasing effect of the two methyl substitutions, 72, in the sequence II, IP and Ig is noteworthy. This fact can be explained on the basis of different distributions of the conformations around the C--S and S---CH 2 bonds. In effect a progressive lowering of steric hindrance in the diads II, IP and IE is to be expected. A second aspect can be readily shown when the following chain is considered ~g
--C--C 1--S--C--C--S--C--C2--S-where
CH3
I
C* = *CH2, *CH
/ and
CHa
*C
\ CH~
It has been found that both C* and its methyl substituents (excepting only the C* of IBS) are affected by the mono- and di-methyl substitution on C1 quite
164
C. CORNOand A. ROGGERO
differently than by that on C2, since one of the two effects (probably that due to the substitution on C2) is almost negligible. For atactic poly (PS), it has been shown that the orientation of the methyl group bonded to C2 affects C* quite differently from that of the methyl group bonded to C r Therefore the orientation effect is similar but quantitatively lower than the substitution effect. The quantitative aspects of the sequence distribution will be treated in a subsequent publication. Nevertheless, it should be emphasized that such quantitative aspects facilitate a complete elucidation of the spectra. Finally, it must be admitted that the ambiguity for all the triads ABB and BBA has not been resolved
even when, as previously mentioned, reasonable hypotheses can be advanced. From a quantitative point of view, however, this is not a limiting factor, since the two signals have the same intensity.
REFERENCES
1. A. Roggero, A. Mazzei, M. Bruzzoue and E. Cernia, Adv. Chem. Ser. in press. 2. C. Corno, A. Roggero and T. Salvatori, Europ. Polym. J. 10, (7), 525 (1974). 3. M. Sander, Chem. Rev. 66, 297 (1966). 4. N. W, Schwartz, J. org. Chem. 33, 2895 (1968). 5. S. Boileau, H. Cheradame, W. Lapeyre, L. Sousselier and P. Sigwalt, J. Chim. phys. 70, (6) 879 (1973).
Sommario--Sono descritti ed interpretati in termini di diadi e triadi gli spettri laC-~1H~ NMR dei copolimeri etilensolfuro-isobutilensolfuroe propilensolfuro-isobutilensolfuropreparati con catalizzatori di tipo anionico. Sono stati determinati gli effetti sul chemical shift degli atomi di carbonio della catena principale derivanti dalla sostituzione di uno o due atomi di idrogeno con metili in posizione c~, fl, ), e 6. Si 6 inoltre dimostrato che sotto l'influenza di catalizzatori anionici vi ~ per l'isobutilensolfuro come per il propilensolfuro, solo una "normale" apertura delranello, sull'atomo di carbonio primario.