Makromol. Chem. 178,609-616 (1977)

Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, t 13 Japan

Short Communication

13C NMR Study of Tacticity of Poly((RS)-rerr-butyloxirane) Obtained by Polymerization

Initiated with Potassium tert-Butoxide

Akira Sato, Tsuneo Hirano, and Teiji Tsuruta

(Date of receipt: Ju ly 14, 1976*’)

The polymerization of (RS)-tert-butyloxirane, [(RS)-tert-butylethylene oxide], ((RS)-tert-BuEO) by potassium tert-butoxide (tert-BuOK) was reported to give a crystalline polymer differing in configuration from that produced by a coordination catalyst”. This crystalline polymer was proposed to have roughly equal numbers of isotactic and syndiotactic diad placements from ‘H NMR study2’ and from the polymer degradation3’. The origin of the crystallinity was ascribed to a stereoblock structure of iso- and syndiotactic sequences2’ or to a regular alternating sequence of iso- and syndiotactic placemen ts2. ’).

Our results4’ of the asymmetric-selective copolymerization of (R) - and (S)-tert-BuEOs in bulk with tert-BuOK, however, cannot be interpreted by either of these proposed microstructures of the crystalline polymer. Thus, we studied the microstructure of the polymer by 13C NMR spectroscopy and discuss here the stereoregulation pattern of the bulk polymerization of tert-BuEO with tert-BuOK.

In the present study, six different samples were prepared, the polymerization conditions of which are listed in Tab. 1 . The preparation methods of ( R ) - and (RS)-monomers and the polymerization and fractionation procedures are given in our previous reports4,”.

Revised manuscript of October 19, 1976.

609

Tab

. 1.

Prep

arat

ion

cond

ition

s of

var

ious

pol

y(te

rt-bu

ty1o

xira

ne)s

Sam

ple

Mon

omer

In

itiat

or”)

(mol

-%)

Solv

ent

Tem

p.

Tim

e Y

ield

(v

olum

e ra

tio

in “

C

in d

ays

in %

to

mon

omer

)

Ab’

(RP

re

rt-B

uOK

(0,

62)

in b

ulk

60

30

98

Bd

.”

(RS)

te

rt-B

uOK

(0,

20)

in b

ulk

80

36

99

cfs)

(R

S)

tert

-BuO

K (

0,74

) , D

MSO

(0,6

5: 1)

80

28

10

0

F”

(RS)

Z

nEt2

/H20

, 1 :0

,8 (1

0,O

) be

nzen

e (3

: 1)

80

5 34

DX)

(RS)

A

IEt,/

H,O

, 1 :

1 (7

,3)

benz

ene

(3 : 1

) 80

16

61

(R

)-ric

h“

tert

-BuO

K (

1,24

) in

bul

k 60

23

85

E

h.e

) tert

-Bu=

tert-

buty

l; E

t=et

hyl;

num

ber

ratio

s ar

e m

ole

ratio

s.

b,

Met

hano

l-ins

olub

le f

ract

ion:

mp

144”

C, [z

]:&,=

- 1

7,4

(1,9

0 g/d

l in

ben

zene

) and

[x]

:&=3

2,3

(1 $3

g/d

l in

cyc

lohe

xane

). c,

[XI;’

of

the

(R

)-m

onom

er w

as -

20,4

(n

eat)

and - 1

8,l (

1,83

g/d

l in

ben

zene

), (O

bs. [X

I;’ fo

r 1

dm (

neat

) - 16

,62,

cf: /

it.6

) - 16

,6).

d,

Met

hano

l-ins

olub

le f

ract

ion:

mp

56°C

. e,

T

he c

rude

pol

ymer

and

the

met

hano

l-ins

olub

le f

ract

ion

show

ed t

he s

ame

13C

NM

R s

pect

ra.

‘) M

etha

nol-i

nsol

uble

fra

ctio

n: m

p 27

-29°

C.

g,

The

com

plic

ated

I3C

NM

R s

pect

rum

of

this

sam

ple

sugg

ests

the

pres

ence

of a

bnor

mal

link

ages

suc

h as

hea

d-to

-hea

d an

d ta

il-to

-tail

h,

Met

hano

l-ins

olub

le f

ract

ion:

mp

63°C

. i,

T

he (R

)/(S)

mol

e ra

tios

in t

he s

tart

ing

mon

omer

and

in t

he re

cove

red

mon

omer

wer

e 76

/24

and

59/4

1, re

sp.

j) T

he b

enze

ne-in

solu

ble

frac

tion

was

use

d fo

r th

e X

-ray

pow

der

diff

ract

ion

mea

sure

men

t: m

p 14

8-14

9°C

.

sequ

ence

s. T

he li

nes

corr

espo

ndin

g to

the

se a

bnor

mal

lin

kage

s ar

e le

ft un

assi

gned

.

? c “0

3 3: vl

1

Pa =i 0

pi 5 a j

I3C NMR Study of Tacticity of Poly((RS)-tert-butyloxirane)

The melting point and X-ray diffraction pattern show that our Sample B is identical to that prepared by Price et ~ l . ' , ~ ' under similar polymerization conditions, allowing us a comparative discussion on the microstructure. It is to be noted from Fig. 1, however, that the X-ray patterns of Sample A (poly((R)-tert-BuEO), isotactic as discussed below) is quite similar to that of Sample B (methanol-insoluble fraction of poly((RS)-tert-BuEO) prepared with tert-BuOK)') and is different from that of Sample F (isotactic poly((RS)- tert-BuEO) prepared with Zn(C2H5)2/H20 (1 :0,8) catalyst).

A Fig. I . X-ray powder dif- fraction spectra of poly(tert- butyloxirane)s, Sample A, Sample B and Sample F (cf. Tab. 1). Sample A and Sample B are freeze-dried polymers from the benzene solution, and Sample F is the benzene-insoluble pre- cipitate. The spectra were obtained with Nickel-fil- tered CuK, radiation

10 12 14 16 18 20 22 24 26 2 0 in degrees

The I3C NMR spectra of Samples A and B are given in Fig. 2. All the chemical shifts hereafter are given in Hz with plus values for upfield shift from the center line of CDC13. Assignment of 13C NMR signals of Sample A (Fig. 2a)) was made by the gated method**) as reported previously5'. Each carbon signal in Fig. 2a) is a sharp singlet and no extra signal due to irregular structures can be observed, indicating that Sample A obtained in 98% yield from (R)-tert-BuEO by the bulk polymerization with tert-BuOK is configura-

* I Since the tacticities of Samples A and B are different as is shown below from I3C NMR spectra, the similarity in these two X-ray patterns suggests that the packing of the bulky turf-butyl side groups is the determinant factor in overall packing of the polymer molecules regardless of the tacticities of the main chains.

* * I Random noise irradiations to the protons were discontinued for the FID (free induction decay) signal acquisition period to obtain Nuclear Overhauser Enhancement with the proton-carbon couplings.

61 1

A. Sato, T. Hirano, and T. Tsuruta

1

tionally homogeneous (i.e. isotactic), and that the amounts of head-to-head and tail-to-tail sequences are too small, if any, to be detected by 13C NMR. It follows that the bulk polymerization of tert-BuEO with tert-BuOK proceeds almost exclusively to form head-to-tail sequences. Thus, the multi-peak signal for each carbon, except for the methyl carbon, in Fig. 2b) should be the reflection of the various tactic sequences in the polymer chain of Sample B.

Chemical shift in Hz

J

1 "1

L

In the tert-butyl quaternary carbon region (1 057 to 1063 Hz), two peaks were observed, as is shown in Fig. 3c), except for Sample A. Compared with the isotactic signal for Sample A, peak 10 (Fig. 3) should be the signal

612

I3C NMR Study of Tacticity of Poly((RS)-tert-butyloxirane)

of the quaternary carbon in the isotactic sequence. The amounts of isotactic diad and triad in a completely atactic polymer should be 50% and 25%, respectively. The relative amount of the area of peak 10 of Sample C, considered to be atactic by Price et d3', is 25% of the total area of peaks 10 and 11. Therefore, peak 10 was assigned to the quaternary carbon in triad ii and peak 11 to those in triads is+si+ss. Here, i denotes a mode of diad placement in which the adjacent tert-butyl groups in Fischer projection drawing are on the same side of the main chain, and s denotes that in which they are on opposite sides of the main chain.

In the methine carbon region (-287 to -270 Hz), (Fig. 3a)), Sample B shows three to four peaks, which therefore are to be assigned to the

Sample

A

Fig. 3. Pulsed Fourier transform 13C NMR spec- tra (25,03 MHz) of poly((R)- tert-butyloxirane) and poly- ((RS)-tert-butyloxirane), Samples A, B, C, D and E (cf. Tab. I) , in the 1&30 g/dl solutions in CDClj at 5@55 "C; operating condi- tions as in Fig. 2. a): methine carbon region, b): methylene carbon region and c): quaternary carbon region. The peak heights of methine, methylene and quaternary carbon spectra for a sample are scaled to 2, 4 and 1 times, resp. For numbering of signals see text

?, I

i 1

-287 -280 -270 57 117 1057 1063 Chemical shifts in Hz from CDCl3

61 3

A. Sato, T. Hirano, and T. Tsuruta

triad tacticity. Compared with the signal of Sample A, peak ,2 is the signal of the methine carbon in triad ii. The areas of peaks 1 and 3 were equal for all samples, so that peaks 1 and 3 are the methine signals from heterotactic triads, is and si. Two kinds of heterotactic triad should be distinguished for a poly(alky1oxirane) which contains a true asymmetric carbon atom in each monomeric unit, but in this case we have no decisive experimental data to identify peak 1 (or 3 ) to be is or si. The area of peak 2, assigned above as ii triad, is always larger than the expected value calculated from the triad peaks in the quaternary carbon region, indicating the inclusion of peaks coming from other than ii triad. The relative areas of peaks 1, 2 and 3 of Sample C (more atactic polymer), for example, are 25%, 50% and 25%, respectively. Accordingly, peak 2 should contain the ss triad peak, and in fact this ss peak was observed in the spectrum of Sample D by about 3 Hz upperfield than the ii peak. Peaks 1, 2, and 3 were, therefore,

Tab. 2. Relative areas of 13C NMR peaks of poly((RS)-tert-butyloxirane), Sample B

Triads (from quaternary carbon and methine carbon peaks): 11 0,32 is + si 0,48 ss 0,20

:. Pj,=O,43, P, j=0,55"'

Tetrads :

Peak no. Tetrad Obsb' assignment

Calc."

iii iis + sii isi + sis iss or ssi ssi or iss sss

0,18 0,30 0,24 0,lO 0,lO 0,08

0,18 0,28 0,23 0,ll 0,l 1 0,09

a) Roughly estimated errors for Pi and P, , are f0,02 (or 5%). b, Peak area ratio determined by using a Du Pont 3 I0 curve resolver.

Peak area ratio calc. with Pi ,=0,43 and P, i=O,55 according to first-order Markov statistics.

614

13C NMR Study of Tacticity of Poly((RS)-teut-butyloxirane)

assigned to the signals of heterotactic 1 (is or si), isotactic (ii) plus syndiotactic (ss), and heterotactic 2 (si or is) triads, respectively, as shown in Fig. 4a).

As far as triad tacticity is concerned, the relative amount of ii, is+si, and ss triads can be determined from the quaternary carbon and methine carbon signals according to the assignments mentioned above. From these data, the conditional probabilities according to the first-order Markov statistics such as P,li (probability of the formation of isotactic bond (i) at the syndiotactic (s) growing chain end) can be calculated. The results for Sample B are given in Tab. 2.

In the methylene carbon region (57 to 117 Hz) (Fig. 3b)), six peaks, at most, were observed, which might be assigned to tetrad tacticity peaks. As compared with Sample A, peak 4 can be assigned to the methylene carbon in an iii tetrad sequence. Sample D, considered to contain less i-unit, gave four peaks 6, 7, 8 and 9 in high amounts, so that these peaks are ascribed to the methylene carbons in the s-unit rich tetrads. Sample E, considered to contain a high amount of i-unit, showed a high peak (4), two medium peaks (5 and 6) and three small peaks (7, 8 and 9), so that peaks 4, 5 and 6 are assigned to i-unit rich tetrads. The atactic sample, Sample C, consistently gives two higher peaks (5 and 6) and four nearly equal peaks (4, 7, 8 and 9). From comparison of the methine spectra of Samples C, D and E, peak 9, the smallest in the spectrum of Sample E, should be sss, and peak 6, high in those of Samples C, D and E, should be composed of two peaks, one is to be assigned to the i-unit rich tetrad and the other to s-unit rich tetrad. These considerations lead to a tentative tetrad assignment of the methylene signals, as given in Fig. 4b). This tetrad assignment is supported by the agreement between the observed peak areas for Sample B and the calculated areas from the Pi,, and P,:i values obtained from the triad peaks of the quaternary and the methine carbons as is shown in Tab. 2.

According to the assignments discussed above, poly((RS)-tert-BuEO) ini- tiated with tert-BuOK in bulk (Sample B) has such a triad tacticity that ii = 0,32, is + si = 0,48 and ss = 0,20, and, therefore, the conditional probabilities were calculated as Pi,, = 0,43 (i.e. Piji = 037) and Psli = 035 (i.e. P,,, = 0,45). The result that Piji z Psii indicates no penultimate effect to be operative in the stereoselection in the chain growth. The result that Pili > Pi,, and P,li > Psi, indicates some preference in the persistence of the i-sequence, resulting in the formation of relatively short s-sequence. These results from 13C NMR for poly((RS)-tert-BuEO) obtained with tevt-BuOK initiator in bulk are not compatible with isotactic-syndiotactic alternating placement^^^^' nor iso- and

615

A. Sato, T. Hirano, and T. Tsuruta

a ) -0-

2

b) -CHz-

5

I I 1 I I I I I v

is i i s s orsi iii iis isisis ssi sss IS sii iss or .

S/

Fig. 4. Tacticity assignments of the rnethylene and methine carbon signals of poly((RS)- rerf-butyloxirane), Sample B (cf. Fig. 3). a) : triads for the methine carbon and b): tetrads for the methylene carbon. All curves were resolved by using a Du Pont 310 curve resolver

syndiotactic stereoblock placements2’. In comparison with the non-stereoselec- tive incorporation of (R)- and (S)-monomers, as in the case of methyl~xirane’.~), these statistical parameters for the terr-BuOK initiated bulk polymerization of (RS)-tert-BuEO suggest that polymerization occurs under stereoregulation due to the interaction among the growing chain end, K + counter cation and the incoming monomer*). In this case the bulky side group, i.e. tert-Bu, seems to play an important role.

C. C. Price, H. Fukutani, J . Polym. Sci., Part A-1, 6, 2653 (1968) 21 H. Tani, N. Oguni, J. Polym. Sci., Part B, 7, 803 (1969) 3 , C. C. Price, M. K. Akkapeddi, B. T. DeBona, B. C. Furie, J . Am. Chern. SOC. 94,

4, A. Sato, T. Hirano, T. Tsuruta, Makrornol. Chern. 176, 1187 (1975) 5 , A. Sato, T. Hirano, T. Tsuruta, Makrornol. Chern. 177, 3059 (1976) ‘) M. Sepulchre, A.-M. Sepulchre, Bull. SOC. Chim. Fr. 1973, 1164 ’l T. Tsuruta, J. Polym. Sci., Part D, 6, 179 (1972) *l a) N. Oguni, K. Lee, H. Tani, Macromolecules 5, 819 (1972); b) T. Uryu, H. Shimazu,

3964 (1 972)

K. Matsuzaki, J. Polym. Sci., Part B, 11, 275 (1973)

* I In addition, participation of the growing chain end moiety in the stereoregulation was observed when the (R) / (S ) mole ratio in the starting monomer was not equal to one4).

616