Polymerization of t-butyl thiirane. II. Polymerization of enantiomerically enriched monomer....

Polymerization of t-Butyl Thiirane. 11. Polymerization of Enantiomerically Enriched

Monomer. Mechanism of Stereoelective Process

PHILIPPE DUMAS, NICOLAS SPASSKY, and PIERRE SIGWALT, Laboratoire de Chimie Macromole'culaire, Associe' au C.N.R.S., Uniuersite'

Pierre et Marie Curie, 4, place Jussieu 75230-Paris, Cedex 05, France

Synopsis

The kinetics of enantiomerically enriched t- butyl thiirane was studied. The stereoelectivity and the kinetic behavior are not changed when a monomer of an initial enantiomeric composition lower than a defined limit value is used. When the enantiomeric composition is higher than this value, the stereoelectivity increases. In agreement with kinetic results, a mechanism is proposed involving, in a first step, the complexation of monomer on preexisting sites of the initiator with formation of highly selective chiral active species on which propagation occurs.

INTRODUCTION

In a previous article we studied the stereoelective polymerization of racemic t-butyl thiirane.l We established that the overall election obeys a kinetic law of second order-eqs. (1) and (2)-and it was possible to obtain the kinetic equations of consumption of each antipode-eqs. (3) and (4).

These equations correspond to the case of a racemic monomer. The aim of this study was, first, to see if the set of general equations established

in the preceding article is still valid when an initial monomer with an enantiomerically unbalanced composition is used. Second, we wanted to see if the stereoelectivity is influenced by the optical purity of the initial monomer, as shown to be the case for methyl thiirane and methyl oxirane.2 We conclude by proposing on the basis of the results obtained a mechanism of stereoelective polymerization for t- butyl thiirane.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17,1595-1604 (1979) 0 1979 John Wiley & Sons, Inc. 0360-6376/79/0017-1595$01.00

1596 DUMAS, SPASSKY, AND SIGWALT

EXPERIMENTAL RESULTS AND DISCUSSION

In the course of our study we found that three cases have to be considered separately, depending on the enantiomeric composition of the initial monomer. Such a distinction is related to the limiting value of the optical purity of the monomer at complete conversion that resulted from the second-order equation found in the study performed with the racemic monomer.

This limit value in bulk at 20°C corresponded to 78% optical purity, i.e., S/R = [89]/[11] distribution.

Monomer Mixtures Having an Optical Purity Below the Limit Value

In this first part of the work we used monomers of about 40% optical purity, i.e., S/R = [70]/[30] composition, in the same experimental conditions as before-polymerization in bulk at room temperature and same initiator concentration.

The experimental data obtained are given in Table I and Figure 1, curve 2. When the initial monomer is not racemic-[R]o # [S]o-eq. (2) is no longer

If one starts from the integrated equation convenient, but another relationship can be established.

and replaces [R] and [S] by their values in function of conversion x and optical purity of unreacted monomer, also:

[R] = [R]o (1 + “) (1 - x ) “0 + is10

[S] = [S]O (1 - 5) (1 - x ) “0 + E l 0

a 0 2[RIo

a 0 2[SIo one finds finally the complete equation valid for initial monomers of any enantiomeric composition:

[Rlo + [SIO “0 + is10 (5) 2FIo

+ - PR - PR

(1 + a/ao)(l- X ) (1 - a/ao)(l- X ) 2[R]o - 1

This kinetic law may be verified by either plotting a/ao vs. x (Fig. 1) or using

TABLE I Stereoelective Polymerization of S(-)-t -Butyl Thiiranea

Initial Residual monomer Time Yield monomer Polymer

ag (neat, dm) a i / a o b (hr) (%) ag (neat, dm) a/a$ [a18 c

-18 0.43 22 36 -26.0 0.62 -4 -15.8 0.38 12 20 -20.1 0.48 -2 -15.8 0.38 66 65 -29.0 0.69 + 35 -15.8 0.38 89 81 -31.0 0.74 +4a

a Optical purity close to 4046. Polymerizations run in bulk at 20OC. Initiator concentration (C/M) -8% in moles.

Calculated for a0 = 42’ (neat, dm). In CHC13 solution (C - 0.4 g/100 ml).

POLYMERIZATION OF t -BUTYL THIIRANE. I1 1597

Fig. 1. Stereoelective curves for the polymerization of t-butyl thiirane in bulk a t 20°C. Dependence on optical purity of initial monomer: (A) curve 2; (I) curve 3; ( 0 ) curve 4.

a linear relationship by plotting 1/(1 + cu/ao)(l - x ) vs. 1/(1- cu/ao)(l - x ) (Fig. 2).

In the latter case the slope of the line gives the stereoelectivity ratio p ~ , which was found to be equal to 8.1 at 20°C. From the intercept a t the origin one can also calculate the p~ value, which is equal to 8.2, in good agreement with the previous value.

The overall order for the consumption of the monomer is equal to 1, as in the case of the racemic monomer. The overall rate of consumption is, however, slightly smaller than for the racemic mixture, and this may be simply explained by the fact that the mixture was enriched in S antipode and the initiator chose preferentially the R antipode. From the experimental data d o 1 0 = f ( x ) and x = f ( t ) it is possible to determine the variations of [S] = f ( t ) and [R] = f ( t ) . The corresponding curves are given in Figure 3.

The consumption of the R antipode, preferentially chosen, obeys a kinetic law of first order with a time of half-reaction of 17 hr. This corresponds to the same kinetics and the same rate constant as in the case when the initial mixture used is racemic.l The consumption of the S antipode follows complex kinetics. It is roughly of first order for conversions up to 40% ( t d 30 hr), but afterwards the order changes continuously.


30

20

10

0

Fig. 2. Linear relationship corresponding to eq. (5) in stereoelective polymerization of t-butyl thiirane in bulk at 20°C: (m) initial monomer, optical purity = 0.38; (A) initial monomer, optical purity = 0.81.

It is interesting to follow the variation of [SI2/[R] as a function of time, as shown in Figure 4. The rate of consumption of the S antipode follows a zero-order law until 400h conver~ion-[S]~/[R] is constant and approximatively equal to 2 (molar fractions unities)-and then it decreases. This behavior confirms that d [Sl ldt is constant and zero order only when the parameter [S I2 / [R] is constant.

Therefore one finds again the same type of kinetic scheme as for the racemic mixture.

--- d[R1 - k[R] d t

and

with k"[SI2/[R] = k' in some limited cases.

POLYMERIZATION OF t-BUTYL THIIRANE. I1 1599

\ \

q. '\

4

10 100

Fig. 3. Rates of consumption of both antipodes in stereoelective polymerization of levorotatory t-butyl thiirane in bulk at 2OOC: initial monomer, optical purity = 0.38.

A

Fig. 4. Variation of [S]Z/[R] ratio depending on conversion in stereoelective polymerization of levorotatory t-butyl thiirane (38% optically pure) in bulk at 2 O O C .


Monomer Mixtures Having Optical Purities Close to the Limit Value

In the previous case we studied the kinetics of initial mixtures of monomer the optical purities of which were far from the limit value established for racemic monomer. It was found that the stereoelective ratio did not vary (Figs. 1 and 2) and therefore the limit value was also identical, i.e., (a /ao) l im = 0.78 [ag l im = -32.7' (neat, dm)].

We have thus seen that when starting with a monomer having an optical purity close to this limit value, i.e., by using a monomer of ag = -30' (neat, dm), optical purity = 0.71, phenomena follow the same law as shown in Figure 1, curve 3. In fact, the previously established limit value (-32.7') is reached at 50% conversion, and then the optical activity of the unreacted monomer keeps a constant value, slightly higher (-33') at higher conversions (Table 11). Owing to the accuracy of a0 and p~ values, determined with an error of f5%, this new value is in good agreement with that found in the previous cases and must be considered a true limit value, i.e., (a/ao)li, = 0.79.

This result supports again the second-order kinetic scheme for the stereo- election of t - butyl thiirane.

Monomer Mixtures Having Optical Purities Higher Than the Limit Value

We have polymerized a monomer with an optical activity ah5 = -34' (neat, dn), alao = 0.81, slightly higher than the limit value. If this monomer had obeyed the same kinetic law, with the same stereoelectivity ratio, as before, we could have expected a decrease of the optical activity of the monomer in order to reach the same limit value.

The experimental data show, however, a continuous increase of the optical purity with conversion (see Table I11 and Fig. 1, curve 4). The kinetics could roughly fi t a second-order equation with a stereoelectivity ratio P R = 21, which is much higher than the value p~ = 8 previously observed (Fig. 2).

Therefore, in this particular case, when ai > abm, the behavior of the stereoelective process is different and seems to be in some way similar to that of methyl thiirane for which the stereoelectivity ratio tripled its value when monomers of 55% optical purity were used.2

TABLE I1 Stereoelective Polvmerization of SI-)-t-Butvl Thiiranes


ag (neat, dm) a i / a o b (days) (%) a:: (neat, dm) a/a$ [a19

-30 0.72 1 17 -32.2 0.76 +90 -30 0.72 6 54 -33.0 0.79 +120 -30 0.72 21 72 -33.0 0.79 +125

a Optical purity close to the limit value. Polymerizations run in bulk a t 20°C. Initiator concentration (C/M) -8% in moles.

Calculated for a0 = 42O (neat, dm). In CHC13 solution (C - 0.4 g/100 ml).


TABLE I11 Stereoelective Polvmerization of S ( - ) - t -Buts1 Thiiranea


a3 (neat, d m ) ai/aob (days) (%) (neat, dm) a/a$ [a]E c

-34 0.81 3 12 -34.5 0.82 +163 -34 0.81 5 20 -36.0 0.86 +162 -34 0.81 26 70 -37.5 0.87 +137

*Optical purity higher than the limit value. Polymerizations run in bulk at 20 "C. Initiator concentration (C/h4) - 8% in moles.

Calculated for a0 = 42' (neat, dm). In CHC13 solution (C - 0.4 g/100 ml).

Mechanism of Stereoelective Polymerization

From the results of stereoelective polymerization of racemic and enantiomerically enriched t - butyl thiirane we have obtained the following information.

The resulting polymers have an isotactic structure and can be fractionated into a pure polyenantiomer (poly-R) and a racemate (poly-R + poly-S). This suggests the existence of two types of stereospecific active centers present in unbalanced amount.

The rates of polymerization of both antipodes obey different kinetic laws. Molecular weights rise rapidly to a limit value, for less than 4% yield, and do

not change significantly afterwards, which suggests either a predominant transfer to monomer or a quasistationary state. The latter is more likely, since transfer to monomer has been generally found absent in anionic polymerization of thiiranes.

The stereoelectivity ratio does not seem to depend on the enantiomeric composition of the initial monomer, which is in favor of an equilibrium complexation between the monomer and the catalyst before the incorporation of the next molecule of monomer.

This equilibrium would also be in agreement with the strong temperature effect observed in the second-order stereoelectivity c o n ~ t a n t . ~

The most simple mechanism fitting all these experimental data is the following:

KR C + R (C,R) (6)

(C,S) + s % ((2,s) poly-s (10)

(11) k t

(C,S) poly-s + (C,S) + poly-s


where C is the initiator system ZnEtz/(-)-3,3-dimethyl-1,2-butanediol (l:l), KR and Ks are the complexation constants of R and S antipodes on some sites of this initiator, and (C,R) and (C,S) are these complexes, which reasonably could be taken as the true active centers of the stereoelective polymerization.

It is necessary to suppose that the complexation constants KR and K s are much higher than 1 in order that all the active centers should be present at the begin- ning of polymerization.

This explains also that the stereoelectivity is constant during all the polymerization and does not depend on monomer concentration. k R and ks are the propagation constants of both enantiomers on the respective active centers (C,R) and (C,S). Finally, eqs. (8) and (11) should represent the uncoupling of polymer chains from (C,R) poly-R and (C,S) poly-S.

As suggested by Inoue et aL4 reaction (8) might be due to alcoholic groups present in the initiator mixture. In the conditions of preparation of our catalyst it seems excluded that free 1,2-diol is present in the catalyst, but other species such as EtZnOR*OH may be present. In the standard preparation conditions that we have used the ratio of alkylalcoxy versus dialcoxy species

E:tZnOR*OH/Zn-0-R*

is close to 0.24 (ref. 5). The proportion of active species with respect to the concentration of initiator

introduced-the efficiency of the initiator-is of the order of lo+. Therefore enough alkylalcoxy species could be present to produce the transfer reaction. As a result of reaction (8) or (11) pure polyenantiomeric chains are liberated.

From the equilibrium equations (6) and (9) one obtains [C,Rl[SI - & [C,SI[Rl - Ks

Rates of consumption of both enantiomers are (10):

-- -d[R1 - ~R[R][C,R]

-d[S1 - ks[S][C,S]

dt

dt --

(12)

deduced from eqs. (7) and

which gives, combining with (12), K R ~ R with p~ = - ---- d [Rl K R ~ R [RI2

d[SI Ksks PI2 Ksks -

According to the experimental results, the overall order in monomer is constant during the whole polymerization, which means that the overall amount of active centers remains constant:

(15) Using eqs. (12) and (15), it is possible to determine the concentrations of active

[C,RI + [CSI = [Cal

centers:


By combination of (13) with (16) and (14) with-(l7) one obtains the rates of consumption of the two antipodes.

In order for eqs. (18) and (19) to be identical with the experimental eqs. (3) and (4), one should have K&] >> Ks[S]. In such a case

d[R1 - k ~ [ c ~ ] [ R ] = k[R] dt

The hypothesis that KR[R] >> Ks[S] seems to be true even at high conversions according to the experimental results. A t such conversions (90%) [S]/[R] in monomer is approximatively equal to 7, which means that KRIKs must be of the order of 100.

As p~ = 8 for polymerizations in bulk, one must consequently have ks/kR close to 10.

Thus, it appears that the R type of active centers (C,R) are present in great excess but that their reactivity is lower than that of the centers of opposite chi- rality.

CONCLUSION

The behavior of enantiomerically unbalanced monomer mixtures in stereoelective polymerizations is similar to that of racemic mixtures. The ratio of the rates of consumption of both antipodes obeys a second-order kinetic law. The stereoelectivity ratio is not changed if the enantiomeric composition of initial monomer is lower than that of the limit value found at complete conversion for the racemic monomer, i.e., (oLI~O)~;, = ( ~ R - I ) / ( P R + ~ ) ; when also is higher than this value, the stereoelectivity increases.

Kinetics of consumption of each antipode were established, and it was found that the preferentially chosen enantiomer obeys a first-order law in monomer consumption and the other antipode obeys a complex law that is zero order in some conditions.

The results obtained with racemic and enantiomerically enriched monomers allowed us to propose a mechanism that is in agreement with the kinetics. In a first step a reversible complexation occurs between monomer and preexisting sites of the initiator. This gives an unbalanced amount of active species of both chiralities, which are highly selective. Next comes the propagation step on active species. These two successive steps explain the second-order relationship found.

The authors thank Dr. M. Litt for stimulating discussions.


References

1. Ph. Dumas, N. Spassky, and P. Sigwalt, J . Polym. Sci. Polym. Chem. Ed., 17,1583 (1979). 2. M. Sepulchre, C. Coulon, N. Spassky, and P. Sigwalt, IUPAC First International Symposium

3. Ph. Dumas, N. Spassky, and P. Sigwalt, J. Polym. Sci. Polym. Chem. Ed., 17,1605 (1979). 4. S. Inoue, T. Tsuruta, and N. Yoshida, Makromol. Chem., 79,34 (1964). 5. A. Deffieux, M. Sepulchre, and N. Spassky, J . Organomet. Chem., SO, 311 (1974).

on Ring-opening Polymerization, Jablonna, 1975, Preprints, p. 80.

Received November 15,1977

Polymerization of t-butyl thiirane. II. Polymerization of enantiomerically enriched monomer....

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