Study of the Microstructure of Ter;rahydrofuranS,3 Dimethyloxetane Copolymers by

13C-NMR Spectroscopy.

J. GUZMAN and E. RIANDE, Instituto de Plhticos y Caucho (CSIC), Madrid-28006, Spain, and M. RICO and J. SANTORO, Instituto de

Estructura de la Materia (CSIC), Madrid-28006, Spain

synopsis

The microstructure of tetrahydrofuran (A>3,3 dimethyloxetane (B) copolymers was studied by 'T-( 'H J-NMR spectroscopy. Only the methyl carbons corresponding to the 3,3 dimethyl- oxetane unit appear as a singlet, whereas the other carbons present a more complicated spectral pattern than it would be expected if L effects were negligible. The assignment of the resonance signals allowed the determination of the values of the probabilities of the different triads, which were in good agreement with those obtained from the reactivity ratios.

INTRODUCTION The cationic copolymerization of heterocyclic monomers has been widely

studied in order to obtain a better knowledge of the mechanism of cationic homopolymerization itself. The issues involved in the cationic copolymer- ization of these monomers refer specially to the nature of the propagating species and to the influence of the depropagation reactions on both the copolymer composition and the microstructure of the copolymers 0btained.l Random copolymerization occurs whenever the propagating species have similar reactivity; for example, random copolymers were obtained from copolymerization reactions of isobutylene oxide and 1,3 dioxolane? but only homopolymers were obtained from the copolymerization of tetrahydrofuran and 1,3 di~xolane.~ On the other hand, the depropagation reactions can play an important role in the copolymerization of some of these ~ys t ems .~ .~

The influence of temperature, initiator, and depropagation reactions on both the composition and the microstructure of copolymers obtained by ring-opening polymerization of several oxetanes have been reported else-

Particularly, the copolymerization tetrahydrofuran (A)-3,3 di- methyloxetane (B) was recently studied8 and the microstructure of the copolymers, determined from kinetic results, was analyzed by comparison with that obtained from 'H-NMR spectroscopy. It was shown that, appar- ently, the copolymerization obeys the classical Mayo-Lewis equation with no significant influence of the depropagation reactions. Since 'H-NMR spec- troscopy gave only dyad probabilities, we have considered it interesting to study in this work the microstructure of these copolymers by W-NMR spectroscopy in order to obtain a better knowledge of the kinetic mechanism of the copolymerization tetrahydrofuran-3,3 dimethyl-oxetane.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 23,2283-2289 (1985) 0 1985 John Wiley & Sons, Inc. CCC 0360-3676/85/0822837$04.00

2284 GUZMAN ET AL.

EXPERIMENTAL

Copolymerization reactions were carried out in high vacuo, at 0°C by using two different initiator systems: BF3 - OEt,-epichlorhydrin and acetyl hex- afluoroantimonate. Details of the copolymerization procedures, purification of the starting materials, and characterization of the copolymers were given elsewhere.*

The 1% { 1H 1 -NMR spectra of the copolymers were recorded at room tem- perature on a Bruker HX-90-E and a Bruker WM-360 operating at 22.63 and 90.55 MHz, respectively. Both deuterated benzene and chloroform were used as solvents and tetramethylsilane as internal reference. Quantitative spectra were obtained using long repetition times and inverse-gated decou- pling techniques in order to eliminate the nuclear Overhauser enhance- ment.

RESULTS AND DISCUSSION

W-NMR Spectroscopy of the Copolymers

The 22.63 MHz W-NMR spectrum of the copolymers presents several resonance signals, which were assigned by considering the spectrum of the parent homopolymers. Thus, the resonance signals corresponding to the methylene and oxymethylene carbons of the tetrahydrofuran structural unit appear centered at 26.6 ppm (one line) and 71.0 ppm (two lines), re- spectively. On the other hand, the resonance of the carbons of the 3,3 dimethyloxetane structural unit appears centered at 22.3 (one line), 36.4 (three lines), and 77.2 ppm (two lines), corresponding to the methyl, qua- ternary, and oxymethylene carbons, respectively. The fact that the reso- nance of the quaternary carbons gives rise to three lines indicates long- range effects as a consequence of the presence of a different number of methyl groups in E position with respect to the analyzed carbon. This prompted us to study the 13GNMR spectra of the copolymers using a higher field spectrometer. We recorded the spectra at 90.55 MHz using both de- uterated benzene and chloroform as solvents. In the spectra obtained at this frequency in chloroform solutions, only the methyl group appears as a singlet. The other carbons present a more complicated spectral pattern than that observed at 22.63 MHz. The assignment of the signals can be made by considering the six different triads (Scheme 1) which can be found in the copolymers tetrahydrofuran-3,3 dimethyloxetane. Four lines between 70.64 and 71.31 ppm are observed for the oxymethylene carbons corre- sponding to the tetrahydrofuran structural unit. The expanded part cor- responding to this region is shown in Figure 1. The signals at lower and higher field are attributed, respectively, to the carbons a'" and a; the other two signals at 70.73 and 71.25 ppm are assigned, respectively, to the carbons a' and a" in the triads AAB or BAA. The presence of two methyl groups in 6 position-increases the chemical shift of the analyzed carbon for about 0.6 ppm. Furthermore, carbons a and a' as well as a" and a'" give rise to close-spaced separate lines, suggesting that the long-range effects of the vicinal units extend over three further carbons in the chain.

MICROSTRUCTURE OF COPOLYMERS 2285

a b b a AAA-O-CH,-CH,-CH,- CH, - 0- CH, - CH,- CH,- CH,- 0- CH,- CH,- CH,- CH, -

7% a' b b' 8.

AAB- 0- CH, - CH, - CH, - CH, - 0 - CH, - CH, - CH, - CH, - 0 - CH,- C - CH, - I

CH3 7H3 b b ' 7H3

BAB- 0 - CH, - C - CH, - 0 - CH, - CH, - CH, - CH, - 0 - CH,- C - CH, - I I

CH3 CH3 CH3 7H3 7H3 c dl c

BBB - 0 - CH, - C - CH, - 0- CH, - C - CH, - 0 - CH, - C - CH, - I I I

CH3 CH3 CH3 CH3 7H3

C. dl C'

ABB - 0 - CH,- CH, - CH, - CH, - 0 - CH, - C - CH, - 0- CH, - C - CH, I I

CH3 CH3 C.l, d l CH3 C,.,

ABA - 0 - CH, - CH, - CH, - CH, - 0 - CH,- C- CH, - 0- CH, - CH, - CH, - CH, - I

CH3 SCHEME I

The signals at 26.48, 26.54, and 26.61 ppm are attributed, respectively, to the methylene car'mns labeled b" + b'", b and b'. This assignment is unambiguous since it has been made by considering the change in intensities of the resonance signals with the composition of the copolymers and has been crosschecked with a similar analysis of signals in other regions. When the mole fraction of tetrahydrofuran in the copolymers is very low, only

71.31

Fig. 1. Expanded W-NMR spectra in the 70-72 ppm region for two tetrahydrofuran3,3 dimethyloxetane copolymers in which the mole fractions of tetrahydrofuran are 0.34 (top) and 0.53 (bottom).

2286 GUZMAN ET AL.

two peaks are observed at 26.48 and 26.61 ppm, the former being of higher intensity than the latter. However, in increasing the tetrahydrofuran con- tent, the signal at 26.54 ppm appears, as can be seen in Figure 2.

The quaternary carbons, labeled d, d', and d!', corresponding to the 3,3 dimethyloxetane structural unit, give rise to three lines. The expanded part corresponding to these carbons is shown in Figure 3, where it can be seen that the intensities of the resonance signals at lower field (36.79 ppm) increase as the content in 3,3 dimethyloxetane increases. Therefore, the signals at 36.35,36.57, and 36.79 ppm are attributed to the central carbons d'), d , and d in the triads ABA, ABB, and BBB, respectively. The differences observed for the chemical shifts are a consequence of the presence of dif- ferent numbers of methyl groups in E position with respect to the analyzed carbon. For instance, carbon d presents four methyl groups in E position, whereas carbon d" presents none. Therefore each methyl group in E position increases the chemical shift of these quaternary carbons for about 0.11 ppm. This contrasts with the effect of two CH3 groups substitution in E

position on the methylene carbons b" and b!', which can be quantified as - 0.06 ppm. By accepting that the ultimate cause of these substitution shifts is the bond polarization induced by steric effects, it is to be noted that in the former case (d carbons) charge shifts on the pertinent carbon have to be transmitted along the bonds of its own CH3 group substituents, whereas in the latter a more straight C-H bond polarization occurs. A quantitative interpretation of these substituent shifts would require a knowledge of sterically crowded conformational populations, which is out of the scope of the present work.

26.54

1 26.61

Fig. 2. Expanded 1%-NMR spectra in the 26-27 ppm region for three tetrahydrofuran-3.3 dimethyloxetane copolymers in which the mole fractions of tetrahydrofuran are, respectively, 0.71, 0.49, and 0.18 (from left to right).

MICROSTRUCTURE OF COPOLYMERS 2287

id'

- 36 o p m

Fig. 3. Expanded 'SCNMR spectra in the 36-37 ppm region for three tetrahydrofuran-3b dimethyloxetane copolymers in which the mole fractions of 3,3 dimethyloxetane are, respec- tively, 0.66, 0.47, and 0.29 (from top to bottom).

Finally, in the spectra obtained in deuterated chloroform solutions at 90.55 MHz, the resonance signals of the oxymethylene carbons labeled c, c', c", and c'Ir appear overlapped with the signals corresponding to the solvent; the spectra of the copolymers obtained in deuterated benzene did not show a resolution so good as that obtained in deuterated chloroform; however, the fact that carbons c, c', c'l, and c'll resonate at different fre- quencies (78.50, 78.36, 77.95, and 77.83 ppm, respectively) indicates again long-range effects of the substituents of the vicinal units.

Microstructure of the Copolymers The analysis of the W-NMR spectra of the tetrahydrofuran-3,3 di-

methyloxetane copolymers allows the determination of the probabilities of the different triads, contrary to what would be expected by considering that c effects are negligible. In Table I are shown the different values of the probabilities obtained for the triads indicated in Scheme 1. Quantitative

2288 GUZMAN ET AL.

TABLE I Experimental Triad Probabilities for Tetrahydrofuran (Ah3.3 Dimethyloxetane (B)

Copolymers

X A f s RAAA) R U B ) RBAB) RBBB) RABB) RABA)

1.85 0.82 0.03 0.06 0.09 0.60 0.20 0.02 3.35 0.71 0.04 0.14 0.11 0.40 0.26 0.05 4.25 0.66 0.04 0.17 0.13 0.30 0.30 0.06 7.65 0.51 0.10 0.24 0.15 0.17 0.25 0.09 8.54 0.47 0.17 0.25 0.11 0.12 0.23 0.12

18.22 0.29 0.45 0.23 0.03 0.05 0.14 0.10

a xA: monomer ratio A/B in the feed mixture. f s : mole fraction of monomer B in the copolymer.

determinations have been made by averaging the values obtained with all the carbons in which there are splitting in their resonance signals. The good agreement obtained in all the cases suggests a proper assignment of the resonance signals to the different triads.

The copolymerization tetrahydrofuran (A)-3,3 dimethyloxetane (B) at 0°C may be described by the following kinetic scheme5.*:

-A+AK"'-U+ k-aa

- A + +B?i?!L,-m+

-B+ + B k - B B +

- B + + A AL, -BA+ where k , are the propagation rate constants for homopropagation (k, and kbb) and cross propagation (kab and kba) and k - , is the depropagation rate constant for tetrahydrofuran. The second and third reactions may be con- sidered almost irreversible because of the high strain of the 3,3 dimethyl- oxetane ring, whereas in the last reaction the irreversibility is due to the fact that the rate of ring opening of the four-membered ring is higher than the back cyclization rate of tetrahydrofuran. The first reaction is considered to be reversible because the depropagation rate constant in the homo- polymerization of tetrahydrofuran is not negligible at 0"C.9J0 From the dyad probabilities obtained by 'H-NMR spectroscopf and using the analytical method developed by Yamashita et a1.,6 the values of the reactivity ratios rA = k,/K& and rB = kbb/kba were found to be 0.12 f 0.03 and 7.6 f 0.3, respectively. In order to test if this kinetic scheme is adequate for describing this copolymerization, the triad probabilities were calculated from the reac- tivity ratios and the results compared with those obtained by 'QNMR spectroscopy. For instance, the values of the triad probabilities RBBB) can be calculated using the equation

where xA is the monomer ratio A/B in the feed mixture, and f a is the mole

MICROSTRUCTURE OF COPOLYMERS 2289

Fig. 4. Experimental and calculated (-) triad probabilities for the tetrahydrofuran3L dimethyloxetane copolymers as a function of the mole fraction of 3,3 dimethyloxetane in the copolymers. (0, Cn, and (0) represent, respectively, the experimental values of P(BBB), P(ABB), andP(ABA).

fraction of monomer B in the copolymer. In Figure 4 the calculated and experimental values of the triad probabilities P(BBB), P(BBA), and P(ABA) are shown as a function of the mole fraction of 3,3 dimethyloxetane in the copolymers; good agreement is obtained, so that the proposed kinetic scheme can be considered adequate. However, it is necessary to say that good agree- ment is also obtained by considering the copolymerization tetrahydrofuran- 3,3 dimethyloxetane as an ideal random copolymerization, since the differ- ence between calculated and experimental triad probabilities are negligible.

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7. M. Bucquoye and E. J. Goethals, Eur. Polym. J., 14, 323 (1978). 8. L. Gamdo, J. Guzmh, E. Riande, and J. de Abajo, J. Polym. Sci. Polym. Chem. Ed., 20,

9. P. Dreyfuss and M. P. Dreyfuss, Ado. Polym. Sci., 4, 528 (1967). 10. K. Matyjaszewski, S. Slomkowski, and S. Penczek, J. Polym. Sci. Polym. Chem. Ed., 17,

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69 (1979).

Received January 5, 1985 Accepted January 30, 1985