Optically Active Poly[oxy(1-alkyl)ethylene]

13 Optically Active Poly[oxy(l-alkyl)ethylene] TEIJI TSURUTA

Downloaded by UCSF LIB CKM RSCS MGMT on November 19, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch013

Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, Japan 113

The optical rotation of poly(R-oxypropylene) has different signs at the sodium D line (589 nm) in different solvents. The specific rotation, [α] D , of poly(R-oxypropylene) is positive in cyclohexane and in chloroform, but negative in benzene and tri fluoroethanol. Since Price and Osgan (1) first reported this phenomenon, several attempts have been made to interprete this in terms of a collision complex formation (2) with the solvent or a change of conformation of polymer molecules in response to the nature of solvent. According to our knowledge, however, any decisive conclusion has not yet been drawn concerning the opti cally active behaviors of poly(R-oxypropylene). To solve this problem, a series of studies on optically active poly[oxy(l-alkyl) ethylene] has been carried out. It was found from these studies that the influence of solvent on the ORD spectra decreased with the increase in bulkiness of the alkyl substituent of the oxy ethylene unit (3),(4). The bulkiness of the alkyl substituent also exerts an enor mous i n f l u e n c e upon the nature o f s t e r e o s e l e c t i v e p o l y m e r i z a t i o n of alkyloxiranes. When the R , S - c o p o l y m e r i z a t i o n o f t - b u t y l -oxirane was c a r r i e d out s t a r t i n g with a monomer mixture c o n s i s t i n g o f R/S=76/24 using t-BuOK as initiator, R-monomer was found to be incorporated i n t o polymer chain p r e f e r e n t i a l l y over S-monomer (5). T h i s i s explained i n terms o f the growing chain c o n t r o l mechanism, i n which the c h i r a l s t r u c t u r e o f the growing polymer chain i s r e s p o n s i b l e f o r the s t e r e o s e l e c t i o n . A unique and s i g n i f i c a n t e f f e c t o f the bulky s u b s t i t u e n t has r e c e n t l y been found a l s o i n a c a t i o n i c o l i g o m e r i z a t i o n o f (R) - t - b u t y l o x i r a n e with boron t r i f l u o r i d e etherate as i n i t i a t o r , where a c y c l i c tetramer was formed i n an e x c e l l e n t y i e l d ( 6 ) . In the present review a r t i c l e , the author intends to d i s c u s s on the e f f e c t o f the bulky s u b s t i t u e n t on the p h y s i c a l p r o p e r t i e s o f p o l y ( a l k y l o x i r a n e ) as well as the mechanism o f s e l e c t i v e p o l y m e r i z a t i o n and o l i g o m e r i z a t i o n r e a c t i o n s .

178

1. Conformation and O p t i c a l Rotatory Behavior o f alkyl)ethylene]

Poly[0xy(l-

In Ring-Opening Polymerization; Saegusa, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

13. T S U R U T A

Optically

Active

Poly(oxy(l-alkyl)ethylene)

179

For the d i s c u s s i o n o f conformation o f poly(i?-oxypropylene) m o l e c u l e s , we have to c o n s i d e r stereochemistry with r e s p e c t to C-C and C-0 bonds along a polymer c h a i n , ( 0 - C H ( C H ) - C H ) n In order to get information on the r o t a t i o n a l isomers around the C-C bond, a deuterated poly(#-oxypropylene) was prepared s t a r t i n g from t r o n s - d e u t e r a t e d methyloxirane monomer ( 7 ) , ( 8 ) . The v i c i n a l c o u p l i n g c o n s t a n t s , |JAC|» between the methylene and the rnethine protons were found to be 5.3, 4.9 and 5.2 (Hz) i n cyclohexane, chloroform and benzene, r e s p e c t i v e l y . By assuming the standard values J = 2 . 6 (Hz) and 0 §=9.3 (Hz) reported i n a v a r i e t y o f 1 , 2 - d i o x y g e n - s u b s t i t u t e d propane d e r i v a t i v e s , i t was p o s s i b l e t o estimate the population o f the three r o t a t i o n a l isomers i n c y c l o hexane, chloroform and benzene, r e s p e c t i v e l y . 3

3

e


6 0

3

2

ϊ8

From the r e s u l t s s t a t e d above, the d i s t r i b u t i o n s o f the three r o t a t i o n a l isomers were estimated to be almost the same f o r poly( i?-oxypropylene) i n the three s o l v e n t s , the conformational d i f f e r ences being not so l a r g e as to e x p l a i n the s i g n d i f f e r e n c e i n the [α]χ> o f the oxirane polymer. Studies on the d i p o l e moment o f an isotactic poly(oxypropylene) suggested the d i s t r i b u t i o n o f r o t a t i o n a l isomers around C-0 bonds to be s c a r c e l y changeable i n benzene and i n cyclohexane as shown i n Table I. We t h e r e f o r e c a r r i e d out a s e r i e s o f s t u d i e s on the c i r c u l a r dichroism spectrum o f poly(#-oxypropylene) i n a number o f s o l v e n t s i n the vacuum u l t r a v i o l e t region under the cooperation with W.C. Johnson, Oregon S t a t e U n i v e r s i t y . In the c i r c u l a r dichroism(CD) spectra o f poly(i?-oxypropylene), two CD bands were observed f o r cyclohexane, a c e t o n i t r i l e , and t r i f l u o r o e t h a n o l ( T F E ) s o l u t i o n s . The CD spectrum was extended to 140 nm and three bands were measured i n a 1,1,1,3,3,3-hexafluoro-2-propanol(HFIP) solution. A Kronig—Kramers transform o f the two CD bands observed i n cyclohexane accounts f o r the observed p o s i t i v e ORD spectrum. In contrast, a t h i r d large and negative ORD band centered a t 155.5 nm i s r e s p o n s i b l e f o r the negative ORD spectrum observed i n HFIP. In the l a t t e r s o l u t i o n as well as i n benzene, the ORD spectrum was found to f i t the Drude one term equation with λ = 1 5 0 nm. In the l i g h t o f the r e s u l t s o b t a i n e d , i t i s most probable to conclude t h a t the i n t e r a c t i o n between polymer main chain and s o l vent molecules should be the major cause f o r the d i f f e r e n t s i g n o f ORD i n the two groups o f s o l v e n t i n the v i s i b l e r e g i o n . Since p o l y [ o x y ( l - a l k y l ) e t h y l e n e ] i s expected to possess lower degrees o f s o l v e n t i n t e r a c t i o n as the a l k y l - s u b s t i t u e n t becomes b u l k i e r , p o l y ( i ? - i s o p r o p y l o x i r a n e ) (3) and p o l y ( i ? - t - b u t y l o x i r a n e ) (4) were s y n t h e s i z e d and t h e i r o p t i c a l r o t a t o r y behaviors were examined i n a number o f s o l v e n t . As shown i n F i g . 1, the i n f l u e n c e o f s o l v e n t on the ORD s p e c t r a decreases with the i n c r e a s e i n b u l k i n e s s o f the a l k y l s u b s t i t u e n t o f the oxyethylene u n i t . The bulky t - b u t y l s u b s t i tuent seems to make the main chain r e l a t i v e l y " r i g i d " and reduces the a c c e s s i b i l i t y o f the main c h a i n i n the p r e f e r r e d conformation (perhaps a l o c a l h e l i x ) to the s o l v e n t molecule. 0


180

RING-OPENING POLYMERIZATION

In order to get independent information concerning the degree of r i g i d i t y o f p o l y [ o x y ( l - a l k y l ) e t h y l e n e ] molecule, p a r t i a l l y relaxed FT C NMR spectra were examined. The C s p i n - l a t t i c e r e l a x a t i o n times f o r the three polyoxiranes are l i s t e d i n Table II, i n which η denotes the number of hydrogen atom bound to the r e l e v a n t carbon atom. The b u l k i e r the a l k y l s u b s t i t u e n t , the smaller nTi values were o b t a i n e d , which was regarded as a conse quence of slower segmental motion owing to the enhanced r i g i d i t y of the macromolecule possessing b u l k i e r s u b s t i t u e n t s . Under the extreme narrowing c o n d i t i o n s , the c o r r e l a t i o n time, τ # , f o r each carbon atom was c a l c u l a t e d . Results obtained are shown i n Table 1 3

1 3


β

It can be s a i d from the values f o r τ # / τ £ Λ t h a t the methylene-carbon undergoes more r a p i d movement than the methine-carbon i n poly(oxypropylene), whereas the movement of the both carbon atoms synchronized i n the molecules o f p o l y [ o x y ( l - £ - b u t y l ) e t h y l ene), suggesting more f l e x i b i l i t y i n the poly(oxypropylene) c h a i n . β

2. R e g i o s e l e c t i v i t y i n the Ring-Opening Polymerization of t-Butyloxirane It was p r e v i o u s l y reported (5) that the bulk polymerization o f t-butyloxirane i n i t i a t e d with potassium t - b u t o x i d e (t-BuOK) proceeded according to the l i v i n g mechanism with the i n i t i a t o r e f f i c i e n c y being 100%. In order to get information on the s i t e o f bond cleavage during the polymerization p r o c e s s , (i?)-t-butyloxirane was p o l y merized i n bulk with the t-BuOK i n i t i a t o r . The C NMR spectrum o f p o l y [ ( i ? ) - t - b u t y l o x i r a n e ] i s given i n F i g . 2. Assignments o f the NMR s i g n a l s were made by the gated method. Each carbon s i g n a l i n F i g . 2 i s a sharp s i n g l e t and no e x t r a s i g n a l due to i r r e g u l a r s t r u c t u r e s can be observed, i n d i c a t i n g t h a t the sample of p o l y [ ( i ? ) - t - b u t y l o x i r a n e ] obtained i n 98% y i e l d from (i?)-t-butyloxirane by the bulk p o l y m e r i z a t i o n with t-BuOK i s c o n f i g u r a t i o n a l l y homogeneous ( i . e . , i s o t a c t i c ) , and t h a t the amounts o f head-to-head and t a i l - t o - t a i l sequences are too s m a l l , i f any, to be detected by C NMR. T h e r e f o r e , the bulk p o l y m e r i z a t i o n o f t - b u t y l o x i r a n e with t-BuOK was concluded to proceed to form h e a d - t o - t a i l sequences under the e x c l u s i v e c l e a v age a t e i t h e r o f the 0-CH bond (α-opening) o r the 0-CH bond (β-opening). The C NMR studies c a r r i e d out on the l i v i n g p o l y m e r i z a t i o n system o f ( f l ) - t - b u t y l o x i r a n e l e d us to conclude the β-opening to be o p e r a t i v e i n the propagation process o f t - b u t y l o x i r a n e . This c o n c l u s i o n was drawn on the basis o f the f o l l o w i n g o b s e r v a t i o n s . In the C NMR spectrum o f the l i v i n g system i n i t i a t e d by l a r g e r amount o f t-BuOK, several new s i g n a l s were observable along w i t h . t h e s i g n a l s which,were assigned p r e v i o u s l y to - C H - (74.5 ppm ), - Ô H - (89 ppm) and - C - (35.1 ppm) o f the i n t e r n a l u n i t s of poly [ ( f l ) - t - b u t y l o x i r a n e ] . 'New s i g n a l s a t 64 ppm (-CH -) and 73 ppm (-0-) i n the l i v i n g system were assigned to s t r u c t u r e [ 1 ] , 1 3

1 3

2

1 3

1 3

2

2


13.

TSURUTA

Optically

Table I

Dipole Moments of isotactic Solvent

Temp, i n °C

benzene


cyclohexane

Table II

Polymer

CH

in C D : 6

6

2.2 0.84 0.52

25 35 51 25 35 51

Poly(oxypropylene)

μ i n Debye 1.09 1.09 1.10 1.04 1.04 1.05

± ± ± ± ± ±

0.03 0.03 0.03 0.03 0.03 0.03

C S p i n - L a t t i c e Relaxation Times, nT

CH,

substituent C

Me isoPr t-Bu

181

Active Poly(oxy(l-alkyl)ethylene)

3.0 0.98 0.54 4.0

CH

CH

CH,

0.84 -

6.9 3.9 2.0

x

(sec)

substituent

CH,

CH 2.2 0.70 0.43

3.4 0.84 0.40

Cone.σα 10 \n/v%, a t 60°C i n C D g

1 2

0.70 3.4 -

CH, 6.9 3.9 2.2

: Cone.cal0 w/v%, a t 60°C



182

because these s i g n a l s were a l s o found i n the spectrum o f an oligomer which was obtained a f t e r the treatment o f the l i v i n g system with aqueous HCl s o l u t i o n . CH t 3 CH -C-0-CH -

Bu

0

Q

2

! 3 73

! 64

81

ppm

ppm

ppm ppm

C H


0-CH -CH-0K

9

[1]

83

[2]

The s i g n a l s at 81 ppm (-CH -) and 83 ppm (-CH-) observed i n the l i v i n g system were assigned to the s t r u c t u r e o f the growing chain end u n i t [ 2 ] , because they disappeared when the l i v i n g system was t r e a t e d with aqueous HCl s o l u t i o n . In accord with t h i s o b s e r v a t i o n , a s i g n a l a s s i g n a b l e to methine carbon o f s t r u c t u r e - C H ( £ - B u ) - 0 H was found a t 78 ppm i n the spectrum o f the oligomer. From these r e s u l t s , i t was unambiguously confirmed t h a t the bond cleavage o f t - b u t y l o x i r a n e takes place e x c l u s i v e l y at 0-CH bond during the p o l y m e r i z a t i o n process with t-BuOK as initiator. 2

2

3. E f f e c t of t - B u t y l S u b s t i t u e n t on the Polymerization o f t - B u t y l o x i r a n e

S e l e c t i v i t y i n the

It was p r e v i o u s l y reported (9) t h a t copolymerization study between i?- and S-monomer i s a useful tool f o r e l u c i d a t i o n o f the s t e r e o c o n t r o l mechanism. When the #,S-copolymerization of t - b u t y l oxirane was c a r r i e d out s t a r t i n g with a monomer mixture c o n s i s t i n g of i?/S=76/24 using t-BuOK as i n i t i a t o r , i?-monomer was incorporated i n t o polymer c h a i n p r e f e r e n t i a l l y over S-monomer (5). As the consequence, the o p t i c a l p u r i t y i n the recovered monomer became smaller than that o f the s t a r t i n g mixture i n the course of the i?,5-copolymerization ( F i g . 3 ) . This i s explained i n terms of the growing chain c o n t r o l mechanism, i n which the c h i r a l s t r u c t u r e of the growing polymer chain i s r e s p o n s i b l e f o r the s t e r e o s e l e c t i o n . (R)

-CH -CH{£-Bu)-0K

(S)

2

(i?)

^

„t-Bu

y

^ 7 ^ ^ - B u 0

(k

)

The curve f o r t - b u t y l o x i r a n e i n F i g . 3 was analyzed i n more d e t a i l by deviding the curve i n t o $ - s t a g e s . From experimental data a t every stage, i t was p o s s i b l e to c a l c u l a t e a parameter, α · , which i s d e f i n e d as f o l l o w s : 7


13. T S U R U T A Table III

Optically Active Poly(oxy(l-alkyl)ethylene) Ratio o f the C o r r e l a t i o n Times w / ^ " CH Carbons ** Substituent Me isoPr t-Bu


f o r CH, and

2

i

C0CI

3

Λ

90

Figure 2.

183

in C D 6

6

in CgD

0.76 0.87 0.97

f

2

12

0.67 0.83 1.1

ί

80

70

60

50

4-0

30

20

10

Tulsed FT C-NMR spectra (25.03 MHz) of P((R)-tert-butyloxirane) in the CDCl solution (10 w/v%) at 45°C 13

3

50

Conversion

100

(%)

Figure 3.

R-Content in unchanged mono mer vs. conversion


184



[dR/dS^CR/S]^ The parameter oy i s regarded as a measure of the s t e r e o s e l e c t i v i t y which i s a s s o c i a t e d with a growing polymer chian a t j - s t a g e . The parameter, ou, was found to become g r e a t e r as the conversion i n c r e a s e s . S i n c e the polymerization o f t-butyl oxirane i n i t i a t e d by t-BuOK was proved p r e v i o u s l y to form a l i v i n g system ( 5 ) , the above r e s u l t s i n d i c a t e that longer chain o f growing polymer to possess the g r e a t e r s t e r e o s e l e c t i v i t y , sudden increase o f oy-value being observed when the degree o f polymerization o f growing chain a t t a i n s about 20. T h e r e f o r e , we can conclude t h a t not only the c h i r a l s t r u c t u r e o f growing chain end but a l s o the c h i r a l second ary s t r u c t u r e o f the polymer chains should be r e s p o n s i b l e f o r the s t e r e o s e l e c t i o n o f incoming monomers. Methyloxirane behaves a b s o l u t e l y d i f f e r e n t way from t - b u t y l o x i r a n e , no change a t a l l i n the o p t i c a l p u r i t y o f the monomer phase being observed i n the course o f i?,5-copolymerization under s i m i l a r r e a c t i o n c o n d i t i o n s ( F i g . 3). I t was a l s o confirmed t h a t the main chain o f poly(methyloxirane) formed possesses randomly d i s t r i b u t e d R- and s-monomeric u n i t s with the same R/S r a t i o as t h a t i n the monomer phase r e g a r d l e s s of the conversion o f p o l y m e r i z a t i o n . These r e s u l t s i n d i c a t e t h a t no s t e r e o s e l e c t i o n takes place i n the i?,S-copolymerization o f methyloxirane with KOR i n i t i a t o r (9). C.C. P r i c e (10) reported the IsoSyn mechanism f o r the s t e r e o chemistry o f p o l y m e r i z a t i o n o f #5-t-butyloxirane i n i t i a t e d with t-BuOK. For the formation o f IsoSyn p o l y ( t - b u t y l o x i r a n e ) , -RRSSRRSSRRSSRRSS-, the f o l l o w i n g c o n d i t i o n s should be e s t a b l i s h e d

P

SR/R

>

P

RR/R

P

RR/R

P

RR/S

P

i/s

P

SR/R

P

s/i

P

SR/S

P

s/s

P

i/i

Penultimate Effect

where Ρ means a c o n d i t i o n a l p r o b a b i l i t y . The r e a c t i v i t y o f the terminal u n i t R i s expected to be d e f i n i t e l y c o n t r o l l e d by the penultimate u n i t . In order t o e l u c i d a t e the nature o f the growing chain c o n t r o l mechanism, a s e r i e s o f NMR s t u d i e s was c a r r i e d out with p o l y ( t b u t y l o x i r a n e ) which was prepared by the polymerization o f racemic monomer i n i t i a t e d by t-BuOK (11). Pulsed FT C-NMR spectra of the polymer are shown i n F i g . 4. By comparison with NMR spectra o f p o l y [ ( i ? ) - t - b u t y l o x i r a n e ] shown i n F i g . 2, i t was p o s s i b l e to 13


Optically

13. TsuRUTA

Active


185

estimate t r i a d population i n the p o l y [ ( 2 ? S ) t - b u t y l o x i r a n e ] as follows: ii(SSS+RRR) 32%; is+si[(SSR+RRS)+(SRR+RSS)] 48%; ss(SRS+RSR) 20%. Assuming the F i r s t Order Markov Chain mechanism, the f o l l o w i n g values a r e c a l c u l a t e d f o r Ρί/s and P / £ : s

£/ ( "s+si)/[2(ii)+(is+si)]=0.43 P /|=(1s+si)/[2(ss)+(is+si)]=0.55 p

=

1

s

s

These r e s u l t s suggest the terminal u n i t r e a c t s with an incoming monomer without any s i g n i f i c a n t i n f l u e n c e from the penultimate u n i t , because P /i was found almost equal At the very i n i t i a l stage o f the i?,S-copolymerization (R/S: 76/24) o f t - b u t y l o x i r a n e , any ordered secondary s t r u c t u r e has not y e t been e s t a b l i s h e d i n the growing polymer c h a i n , so that i t i s reasonable to use the value k ^ / k ^ (or k £ £ / k £ g ) l . 3 [obtained from 0.55/0.43 (see above)] as the r e a c t i v i t y r a t i o a t the e a r l i est stage o f the i?,S-copolymerization. S t a r t i n g with [R]o/[S] =76/24, α - v a l u e can be c a l c u l a t e d as f o l l o w s : t

0


s

s

d£Rl

[R] j f e O T +

=

d[S]

kgOT

[S] ^ [ R * ] + k ^ [ S * ]

d£Ri

=

d[S]

[R]

(*RR/*SS)

[S]

where k

f f î

[R*3/(S*]

+ T

[ R * ] / [ S * ] + (K; tt ) S

=k^, k ^ k ^

^ α

SR

[

R

]

[S]

and [ R * ] / [ S * ] = [ R ] o / [ S ]

0

The v a l u e , 1.14, f o r the α^· a t the i n i t i a l stage f a l l s i n the range of value which i s a n t i c i p a t e d from the experimental curve. Since the copolymerization was s t a r t e d with a mixture o f i?-content being 76%, there i s an enhanced chance to form i s o t a c t i c enchain ment, which w i l l r e s u l t i n the formation o f a c h i r a l secondary s t r u c t u r e . T h e r e f o r e , the p r e f e r e n t i a l i n c o r p o r a t i o n of f?-monomer w i l l be much more a m p l i f i e d i n the l a t e r stage as observed i n the experiment. In order t o get c l e a r e r p i c t u r e as t o the c h i r a l secondary s t r u c t u r e o f the growing chain end, the l i v i n g polymerization system [3] was studied by p a r t i a l l y relaxed (PR) FT C NMR i n terms o f the s p i n - l a t t i c e r e l a x a t i o n behaviors. 1 3

(3)

(2)

O)

Me, Me, C C r ι Me C-0-CH -(-CH—0—CHg—)^CH—0—CH

(4)

2

3

J

2

73 (Ti) (sec)

64

89

74.5

(1.3)

(0.8)

(0.35) (0.3)

89.1

2

él

-CH—OK 83 ppm

(0.15)(0.25)


[3]


186


The PRFT spectra o f the l i v i n g system [3] showed t h a t values o f the s p i n - l a t t i c e r e l a x a t i o n time ( T J f o r the carbon atoms l o c a t e d i n the growing end u n i t s , whose s i g n a l s appeared a t 83 ppm, 81 ppm and 89.1 ppm, were observed to be much s h o r t e r than T i values f o r other carbon atoms o f the l i v i n g system. T i values o f the methyl-carbons (1) and (2) were observed again to be s h o r t e r than those o f the methyl-carbons (3) and ( 4 ) . The observed T i values f o r the l i v i n g system make i t p o s s i b l e to c o n s i d e r that the segmental motion o f the u l t i m a t e and p e n u l t i mate u n i t s o f the growing chain are r e s t r i c t e d to a s i g n i f i c a n t extent. We i n t e r p r e t the r e s t r i c t e d segmental motion i n terms o f the formation of a r a t h e r r i g i d s t r u c t u r e i n which ether-oxygen atoms o f the growing end u n i t s are bound together through c o o r d i n a t i o n bonds with potassium c a t i o n . I f a l l of the asymmetric c e n t e r s i n v o l v e d i n the growing end u n i t s have the same sense o f s t e r e o isomerism, the growing end moiety w i l l e x h i b i t a s i g n i f i c a n t c h i r a l c h a r a c t e r around the counter c a t i o n K . We c o n s i d e r the nature o f the observed enhancement o f the s t e r e o s e l e c t i v i t y , oy, to be a s c r i b a b l e to the formation o f the c h i r a l secondary s t r u c ture around the potassium c a t i o n . Under the r e a c t i o n c o n d i t i o n s f o r the a s y m m e t r i c - s e l e c t i v e i ^ s - p o l y m e r i z a t i o n stated above, p r o b a b i l i t i e s f o r formation o f such c h i r a l s t r u c t u r e w i l l be r a t h e r low before the degree o f polymerization o f the growing chain a t t a i n s about 20. +

4. S p e c i f i c Formation o f C y c l i c Tetramer by C a t i o n i c O l i g o m e r i z a t i o n o f (i?)-t-Butyloxirane A unique and s i g n i f i c a n t e f f e c t o f the bulky s u b s t i t u e n t has r e c e n t l y been found a l s o i n a c a t i o n i c o l i g o m e r i z a t i o n o f t - b u t y l o x i r a n e with B F - 0 E t as i n i t i a t o r ( 6 ) . When b u t y l o x i r a n e was t r e a t e d with 5 mol% o f B F 3 ^ 5 t t , a snow-white s o l i d was formed i n 89% y i e l d (by weight). After purification, the c r y s t a l l i n e product (melting p o i n t 168°C) was analyzed. Iff, no e x i s t e n c e o f OH group; Molecular Weight (by vapor pressure osmometry), 410; A n a l , C a l c d : as a c y c l i c tetramer, C 71.95, H 12.07; Found: C 71.91, H 12.45. [aU 50.8 (C 1.10, i n C H i ) ; 53.8 (C 1.28, i n C H ) . From these r e s u l t s , the c r y s t a l l i n e product was concluded to be a c y c l i c tetramer of ( f f ) - t - b u t y l o x i r a n e . *H NMR and C NMR show t h a t the tetramer c o n s i s t s o f f o u r i d e n t i c a l monomeric u n i t s . T h e r e f o r e , the oxirane r i n g o f ( # ) - £ - b u t y l o x i r a n e must have opened e x c l u s i v e l y a t e i t h e r o f the CH-0 bond ( α - o p e n i n g ) or the CH -0 bond ( β - o p e n i n g ) . In order to o b t a i n information on the s i t e o f r i n g - o p e n i n g , a model r e a c t i o n o f ( f ? ) - t - b u t y l o x i r a n e with B F - 0 E t was c a r r i e d out i n the presence of t - b u t y l a l c o h o l i n one to one mole r a t i o to the o x i r a n e . A f t e r the unchanged a l c o h o l was removed, the r e a c t i o n mixture was examined by C NMR. Three s i g n a l s were observed a t 78 ppm, 73 ppm and 64 ppm 3

2

2

5

6

6

6

1 3

2

3

2

Ï 3


2

13. T S U R U T A

Optically

Active


187

besides the s i g n a l s assigned to the i n t e r n a l o x y ( l - t - b u t y l ) ethylene u n i t s . The f i r s t s i g n a l (78 ppm) was assigned to methine carbon of -CH-OH, while the second and t h i r d s i g n a l s to the s t r u c t u r e [ 1 ] , When a s i m i l a r r e a c t i o n was conducted i n the presence of methanol, s i g n a l s assignable to - C H - 0 C H group, i n s t e a d o f [ 1 ] , were observed a t 75 ppm and 58 ppm. From these r e s u l t s o f the model r e a c t i a n , i t was concluded t h a t the c y c l i c t e t r a m e r i z a t i o n o f ( i ? ) - t - b u t y l o x i r a n e with B F - 0 E t c a t a l y s t proceeds under the cleavage o f the CH -0 bonds ( β - o p e n i n g ) , which r e s u l t e d i n the formation of (2#, 5i?, 8i?, l l i ? ) ( 2 , 5 , 8 , l l - t e t r a - t butyl-1,4,7,10-tetraoxacylododecane with the r e t e n t i o n o f the c o n f i g u r a t i o n a t the asymmetric carbons. The s p e c i f i c formation o f the tetramer o f ( i ? ) - t - b u t y l o x i r a n e forms a sharp c o n t r a s t with the r e s u l t s obtained i n the r e a c t i o n of ( s ) - i s o p r o p y l o x i r a n e with B F - E t 0 , i n which the r e a c t i o n product was an o i l y compound c o n s i s t i n g o f a t l e a s t seven major compounds of low molecular weight. The bulky t - b u t y l s u b s t i t u e n t i s expected a l s o to cause more severe r e s t r i c t i o n i n the p o s s i b l e p r e f e r r e d conformation o f the c y c l i c tetramer i n comparison with the corresponding l i n e a r polymer, p o l y ( ( i ? ) - t - b u t y l o x i r a n e ) . The chemical s h i f t and c o u p l i n g constant values i n the *H NMR spectra o f the c y c l i c tetramer i n deuterated benzene and chloroform are l i s t e d i n Table IV. In Table IV, the s i g n a l s around δ 2.8 ppm were assigned to the methine proton He, those around δ 3.6 - 3.8 ppm and δ 3.8 4.0 ppm to Ηβ and H/\ methylene protons, r e s p e c t i v e l y . The Newman p r o j e c t i o n f o r the p o s s i b l e three conformers around the methylene-methine carbon bond i s shown i n F i g . 5. The assignment o f H/\ and Ηβ are made from J s and d e s h i e l d i n g e f f e c t of the ether oxygen atoms. On the b a s i s o f the Karplus-type dependency of J on d i h e d r a l angle (12), i t was concluded t h a t He should come approximately on the b i s e c t i o n plane of HA and Ηβ because both o f the observed J and J B were about 3 H (Table IV). T h e r e f o r e , the predominant conformation around the main c h a i n CH-CH bond should be G i n F i g . 5. The most p l a u s i b l e s p a t i a l s t r u c t u r e f o r the c y c l i c tetramer i s t h a t shown i n F i g . 6, where the main chain o f the tetramer predominantly takes a G+G+T conformation: 2

3

3

2


2

3

3

2

1

3

3

3

A

c

C

z

+

2

-0-CH(t-Bu)-CH 2

G G Τ According to t h i s s t r u c t u r e , H/\ should be more deshielded than Ηβ because two e t h e r oxygen atoms 0 ( i - 2 ) and 0(i+4) come c l o s e to Ηβ than Ηβ o f the methylene group ( £ ) , and so are assigned the H/\ and Ηβ protons to the observed two methylene s i g n a l s . The ORD spectra o f the c y c l i c tetramer i n benzene, i n cyclohexane and i n chloroform are shown i n F i g . 7. In c o n t r a s t to the corresponding l i n e a r p o l y ( a l k y l o x i r a n e ) s , the ORD curve o f the c y c l i c tetramer i n cyclohexane i s e x a c t l y the same as that i n benzene and very +

+



188


-CH

»

•

'

•

•

•

-200

I

0

I

200

I

I

400

1

» - — ι

600

800

1

«

1

'

1

3

—

1000 1200

Hz

Figure 4. FT C-NMR spectra (25.03 MHz) of ?((RS)-text-butyloxirane) initiated by t-BuOK in bulk. Solvent, CDCl (cone. 30 w/v%); Temp., 50°C. 13

s

Table IV

Solvent

Chemical S h i f t s and Coupling Constants f o r the C y c l i c Tetramer, (2i?,5i?,8i?,lli?)-2,5,8,11 - t e t r a - t e r t - b u t y l 1,4,7,1O-tetraoxacyclododecane, i n Deuterated Benzene and i n Deuterated Chloroform a t Various Temperatures Temp,°C

Chemical shift.appm Δ

C

6Ο 6Ο D

CDC1,

ό a

Α

Δ

Β

S

C

Coupling c o n s t a n t , Hz

I ABI I ACI I BCI 3J

3J

3J

30 50 75

3.70 3.76 3.81

3.64 3.68 3.71

2.66 2.71 2.75

11.7 11.8 11.7

2.6 2.7 2.6

3.0 3.3 3.3

30 50

3.90 3.90

3.78 3.79

2.83 2.83

12.0 11.9

3.0 2.9

3.2 3.3

Chemical s h i f t i s given i n ppm u n i t with plus value f o r downfield s h i f t (oscale) from the i n t e r n a l standard TMS.


Optically Active Poly(oxy(l-alkyl)ethylene)

13. T S U R U T A

'Bu

H

B

-t°-T-K H

3

H

c

A

a ' ά°' Hc

βίΓ Α

Η

Η

Βυ

H 'V^H B

0

**

H

H b 1J H

A

A


1

r

mp (

Optically Active Poly[oxy(1-alkyl)ethylene]

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